Nealio's Jumping Guide Uwe Hale on new rider question
Gearing Basics -- by Rich Rohrich Rich Rohrich speaks on AV Gas
Gordon Banks on 2000 XR400 Stuff Setting Suspension Sag
JC on required equipment for new riders 2 Stroke Porting by Eric Gorr

From: KTMNealio <>
Subject: Re: Jumping Technique
Date: Tue, 18 Jul 2000 10:05:43 GMT

If anyone knows about jumping around here, its probably me..

Nealio's jumping guide

First of all there are several types of jump faces than a person may
see. In order to understand how to take different types of jumps, we
must understand the different jumps themselves.

Jump Types
There are *basically* 4 types of jump faces (or at least 4 ways jump
faces behave). I refer to them as 'the normal jump face', 'the
roller', 'the compression jump', and 'the kicker'.

Your 'normal' jump face is what most track makers try to make. Its
your run of the mill, medium steepness, smooth jump face. The jump
face itself is usually about twice as long as your bike. They usually
don't do anything 'weird' to you (or your bike) when you go over them.
These are easy: even throttle, carry good speed going into it, stay
neutral on the bike, and you shouldn't need to pull up or push down on
the bars.

Your 'roller' type jump is one that doesn't have a "lip" at the top of
the jump face. Its usually the same size/dimensions as a normal jump
face, but the jump face itself is usually worn down. You can find
these faces on high-speed tabletops on big outdoor tracks. Jump faces
like these require you to carry a lot of speed, but you will need to
stay off the throttle on take-off. Keeping the throttle on will cause
your rear tire to keep driving and remain on the ground. This will
cause you to come off the take-off nose high or possibly loop out. You
should also stay neutral on the bike on take-off.

The 'compression' type jump is when you have a jump face that is
approximately the length of your bike and is very steep. Jumps like
these don't re-direct your bike into the air, but will bottom your
suspension and shoot you up abruptly. Jump faces like this require you
to be on the gas slightly at take-off. You should also let the front
of the bike come up to you; don't let the motion of the bike throw you
back. If you don't let the front of the bike come up to you, you will
go off the jump with your weight too far back and possibly loop out.
Too much gas will loop you out as well while not enough will cause you
to go nose-down. These type of jumps are fairly difficult to master.

Your 'kicker' type jump will kick your ass-end up when you go over it.
These are the type of jumps that you want use lots of throttle on take-
off. These type of jumps are the hardest to master due to the fact
that you are usually afraid of looping out and won't give it enough
gas. You will see many higher-level riding go off jump faces like this
sitting down and on the gas hard. This allows them to transfer more
weight over the rear-end of the bike and get more drive off the face of
the jump. It also allows the rider to pull back on the bars on take-
off to counteract the "kick" of the jump. If you watch supercross, you
will sometimes see riders doing what is called a "bump seat jump" into
whoops or rhythm sections. If they were to go into a jump like this in
the neutral position, it would most likely cause them to endo.

Jump Practice
I have found that finding a good jump that you are comfortable with is
key to mastering different jumping techniques.
For starters, you should practice going into jumps you are comfortable
with coasting in a higher gear. This way if you accidentally grab some
throttle, it *shouldn't* loop you out.
Once you feel confident about clearing the jump, you can practice going
in a gear lower at the same speed (which would put you closer to the
meat of the powerband). When you feel comfortable going off in the
powerband, you can work on how you land by either giving it slightly
more throttle on the face to keep your nose higher, or letting off
slightly to drop your nose a little.
Gassing it on the face of the jump allows you to counter-act a steep
jump face by keeping your front end up as you take off instead of
letting the jump kick your ass-end upward. Also gassing it on the face
will give you increased loft and distance when trying to clear a
difficult jump.
Chopping the throttle on the jump will allow you to bring the front end
down when going off a smooth, yet steep jump face. Also a slight
throttle chop will keep you lower in the air thus make you faster
(because you will have less hang time).

The Brake Tap & Panic Throttle
What the "brake tap" does is use the rotational inertia of the rear
wheel to bring the front end down. The "panic throttle" does just the
opposite: brings the front end up if its too low when you pin it in the
air. However, the panic throttle is not very effective compared to the
brake tap, but it will still make a difference.
The brake tap can be a very good tool in building your confidence and
allowing you to go off jump faces with more throttle. The most common
fear when jumping is that the jump will kick your ass end up and you
will endo when you land. The brake tap allows you to go off on the gas
and if the jump is a "kicker" you are set up fine. If the jump is a
normal jump face, you can hit the brake and you are still set up fine
for the landing. Going off the jump face with more throttle allows you
to gain more lift and distance when trying to clear a difficult jump,
but it will also cause you to fly nose-high. The brake tap allows you
to bring the front end back down.
To execute a brake tap, one must pull the clutch in in the air. If you
don't pull the clutch in, you will kill the engine and possibly wreck
when you land. If you have a lot of clutch drag (like I did with my
CR500) you risk killing the engine when you brake tap as well.

Body Position
For the most part you want to keep your body in the "neutral" position
(in the middle of the bike, arms bent, legs bent, etc). Pulling back
on the bars will only put your weight too far back and put you out of
position to land properly (possibly looping out). Leaning too far
forward will cause you to be out of position as well, but you risk
endo'ing instead. You should bend you knees when you take off, but not
so much that the seat hits you in the butt. This will cause you
to "eject" if the seat slaps you hard enough. Going off with stiff
legs is also a bad idea. If the bike does not go off the jump
completely square, stiff legs will throw your body off balance instead
of allowing you to absorb it with your legs.
A common mistake for beginning jumpers is to not squeeze the bike with
your legs while you are in the air. Squeezing the bike in the air with
your legs will keep you and the bike as one and give you more control
over the bike while its in the air. If you don't squeeze the bike you
will feel like you are coming off the bike in the air.

Well, that's my take on catching air. And remember to always keep the
shiny side up..

- Nealio

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From: "Rich Rohrich"    Back to Top 
Subject: Gearing Basics
Date: Thu, 16 Mar 2000 13:35:37 -0600

I posted this on Eric Gorr's website the other day and a self-proclaimed lurker in RMD sent me an e-mail asking me if it was OK for him to post it out here. I'll save him the trouble :-)

Gearing Basics -- by Rich Rohrich

The question: 

Rick Fuller asked an interesting question in the forums and it gave me the idea for this month's MotoTech article on Gearing.

Rick asked :

I read SOMEWHERE that 1st gear ratios are: WR400 2.416, YZ426 1.846. Since I only work with the 4 basic functions of math (+-x/), can you tell me what sprocket changes I would have to make on my yz426 to get the closest match to a WR400 1st gear, both c/s and rear sprockets separately? I have a 13t c/s but have not tried it yet, am wondering if I should try a 51 or 52t rear? 

An answer (sort of):

Gearing is really pretty simple if you take a step back and look at what is actually happening. Basically gearing just provides a way to reduce the rear wheel speed in relationship to the crankshaft speed. By reducing the rear wheel speed the gearing provides a mechanical advantage. The greater the speed reduction the greater the mechanical advantage.

The total speed reduction comes from three separate speed reductions. You have the Primary reduction which comes from the connection between the crankshaft and the clutch outer hub. If you look in the engine on most modern bikes you'll see they are connected directly together with straight or helical cut gears using a small gear on the crank and a large gear on the clutch. Some older designs used a small primary chain between these two gears. Vintage racers can no doubt tell you lots of horror stories about primary chains that have failed. On the 1998 YZ400 the crank gear has 21 teeth and the clutch gear has 62 teeth , for a primary reduction of 2.952 :1 (62 / 21 = 2.952) . Other dirt bikes will have similar ratios. The 1972 Hodaka Super Rat 100 uses a primary ratio of 3.71:1, while the dual sport Wombat of the same year uses a ratio of 2.75:1 . Street bikes tend to use much lower primary ratios (below 2.0 :1) to provide a higher top speed . Because the crank sprocket (drive sprocket) is smaller than the clutch sprocket the crank will turn more rpm than the clutch when they are connected together. So in the case of the YZ400, the crank will turn 2.952 rpm for each complete revolution of the clutch.

Once the clutch is turning it will transfer it's rotation to the counter shaft. There are two shafts in the transmission , the main shaft that is connected to the clutch inner basket. This is our second speed reduction. This transmission (or internal) speed reduction is variable depending on the gear that is selected. In this case the main shaft is the smaller or "drive" gear while the counter shaft has the larger or "driven" gear . The YZ400's first gear has a 14 tooth gear on the main shaft and a 27 tooth gear on the counter shaft for a 1.929 :1 ratio (27 / 14 = 1.929) . So now the main shaft is turning 1.929 rpm for each full rotation of the counter shaft. We have the counter shaft turning now, so if we hang a small sprocket on the end of it, loop a length of chain around it and connect it to a large sprocket on our rear wheel we can provide one more speed reduction that we'll call the secondary or Final reduction. If we use the YZ one more time we end up with a 14 tooth counter shaft sprocket or "drive" sprocket, working with a 49 tooth rear or "driven" sprocket, for a final reduction of 3.500 : 1 (49/14 = 3.500 ) . Now that we know all the individual speed reductions between the crank and the rear wheel we can figure out the Total reduction with some simple math .

Primary ratio * Internal ratio * Final ratio = Total reduction (ratio)

For the 98 YZ400 with stock sprockets in first gear that works out to : 2.952 * 1.929 * 3.500 = 19.930 ( Total reduction) Which means the crank will turn 19.930 rpm for each rotation of the rear wheel when the bike is in first gear. Now if we plug in your WR & YZ426 numbers for first gear we can see what changes we need to make. If we assume the WR and the 426 has the same Primary reduction as the 98 YZ400 (not sure if this is the case) the numbers look like this For the WR400 with 14/49 sprockets and a first gear ratio of 2.4126 we get : 2.952 * 2.416 * 3.500 = 24.962 ( Total reduction) For the YZ426 with 14/49 sprockets and a first gear ratio of 1.846 we get : 2.952 * 1.846 * 3.500 = 19.072 ( Total reduction) So the first two speed reductions on our virtual WR400 are 23.6 % lower than the YZ426. Now we know that we can't easily change the primary or the internal reductions, all we have left is the final drive ratio. If we use the first two ratios as a constant for each bike, figuring the gearing is just a matter of juggling the sprocket numbers: WR400 first gear constant : 2.952 * 2.416 = 7.132 YZ426 first gear constant : 2.952 * 1.846 = 5.449 Now we can use these constants to determine what final drive ratio we need. WR400 Total reduction / YZ426 constant = Final drive reduction we need 24.962 / 5.449 = 4.581 Now all we have to do is find a sprocket combination that provides close to 4.581 : 1 ratio . 55/12 gives us a ratio of 4.583, and 55/13 is 4.23 :1 . These are pretty low ratios, and I don't know if a 55 would even fit on the YZ.

So to get as close as possible to the WR you'll need to verify the primary ratio on the WR400 and the YZ426 and plug the numbers in to the original equation: Primary reduction (ratio) * Internal reduction (ratio) * Final reduction (ratio) = Total reduction (ratio) You can use this simple formula to determine the top speed of a bike as well. If we plug the fifth gear ratio (0.952) into the equation we get 2.952 * 0.952 * 3.500 = 9.836 (Total reduction) . If the YZ400 peaks at 11,000 rpm then we are reducing the rear wheel speed to : peak rpm / total reduction = rear wheel rpm or 11000 / 9.836 = 1118 rpm

Now we can figure out how far the bike went in one minute Using the equation rear tire circumference (inches) * rear wheel rpm = inches traveled / min we come up with 84 * 1118 = 93912 inches traveled in one minute. If we convert that to feet we get : 93912 / 12 = 7826 feet traveled in one minute.

Then converting that to miles/min gives us - 7826 / 5280 = 1.482 miles / min If the YZ400 has sufficient torque to turn 11,000 in fifth gear and we do the final conversion to miles per hour we get 1.482 * 60 or 88.93 mph. So next time some dessert racer tells you his YZ400 runs 100mph with stock gearing you can whip out your calculator and take him to school.

Rich Rohrich

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From: Gordon Banks <>        Back to Top 
Subject: 2000 Honda XR400R Stuff
Date: 27 Aug 1999 15:54:05 GMT

Stock Carb Specs:
Main Jet 142, Pilot Jet #52, Needle Clip in 3rd groove.

Right off the showroom floor, it ran fine, but definitely on the rich side. Removing the airbox snorkel without any re-jetting, however, made it run lean and overheat.

This information may or may not apply to earlier models. It was performed only to a 2000 model XR400R

RELIEVING THE 2000 HONDA XR400R MUFFLER BAFFLE (This is a very simple procedure, and one that can be almost as easily reversed. Best of all, though... it works.)

After carefully examing the stock baffle/spark arrestor, and running some flow numbers for the different areas involved with the numerous plates and baffles involved, I came to the
conclusion that the primary restriction to exhaust flow is the small final outlet, which has an i.d. (inner diameter) of only 20mm.

Without removing the baffle insert from the muffler, examine the exhaust end. Notice the actual outlet which measures 20mm i.d. (approx. 0.787"). Around this is the larger 'bright finish'
ring which appears to have no real function unless it's there to help prevent the rider from coming into contact with the actual outlet, which probably runs hotter. Down in between the 20mm outlet and the bright finish outer ring, there is room to drill one or more holes into the spark arrestor chamber, to
provide additional outlets, and thusly additional exhaust flow area. Holes drilled in this area will be 'inside' the spark arrestor chamber, so the spark arrestor function IS maintained.

On my baffle insert, the area to be drilled is just large enough to accept a #2 drill bit (0.21"). It may even accept a 1/4" bit (0.25"), but there are benefits to using the #2 bit, which I will explain shortly.

Since the stock 20mm (0.787") outlet provides a flow area of only 0.4862, and a 0.21" hole has a flow area of 0.0346 sq. in., each 0.21" hole adds 7.1% more flow area! Just three such holes will increase the exhaust flow area by over 21%!

First I ran the engine with the undrilled baffle, to get an up-close feel for the sound level at idle, and while revving the engine. After drilling one hole, I could barely hear any difference at all. After drilling a second hole, I could hear the difference, but it was slight. The third hole made a bigger difference, but still not objectionably loud. The fourth hole took it over my limit, however, adding a definite bark to the exhaust note. This, I decided, was a bit too loud. So, since I had use a #2 drill bit, which is approx 0.21" in diameter (it's supposed to be 0.221"), I was able to plug the 4th hole very simply by screwing in a 1/4x28 set screw, which I'd not be able to do had I drilled the holes with a 1/4" bit. This
effectively reverted back to having just three holes, and it also shows that I can plug them all with more 1/4x28 setscrews, to return to the stock sound level.

A brief test ride showed that the added three holes made the bike run a bit better, probably because it runs so rich when stock. It was now running leaner. But it definitely showed that the added 21.3% flow area was beneficial, and it had cost me nothing but a little time, and it's totally reversible by plugging the holes with set-screws.

It is NOT necessary to remove the insert when drilling each hole. The metal chips will fall either outside the muffler, or into the screened area of the spark arrestor. Once you have drilled the desired number of holes, you can then remove the insert and shake out the tiny pieces if you so desire. If you don't, they will eventuall fly out the exhaust outlet anyway, since the screened spark arrestor chamber
prevents them from falling down inside the main muffler.

I'd read about the header inlets being partially shut off by the welding that builds up when welding the 1" i.d. header pipes to the clamping flanges, so I examined mine. Simply loosen the clamp bolt where the header pipes assembly slides into the muffler, and then loosen and remove the four nuts (two per pipe) where the headers are clamped to the head. The muffler bolt and all four clamp nuts take a 12mm socket. Then the header pipes assembly slides forward into your hands.

On my 2000 model, the built-up welded area in each pipe was terribly restrictive! The remaining opening measured a rough 0.75", leaving a flow area of only 0.44 A 1" i.d. pipe has a flow area of 0.78, so the welding left only 58% of that! I started grinding down the built-up welds using
small grinding stones in my Dremel Moto-tool, but that was way too slow. I went to the hardware store and bought some inexpensive coarse grinding stones to fit in a 3/8" drill, and one 1" ball stone for finishing. I spent over 2-1/2 hours grinding away. I selected an 18mm socket, which has an outer
diameter of 0.944" (different sockets will vary in size, of course), as my size guide. Once the 18mm socket would slide into the header pipe, I quit, not wanting to remove too much of the weld, and weaken the joint. I then used the 1" ball grinding stone to finsih up. Since the stone itself wears
away faster than the weld material, I ground a little on each pipe, going back and forth between the two, until enough of the stone wore away to fit into the opening. This final touch didn't really make either opoening larger, but it did make them both the same size and round shape.

Since I started with a 0.75" opening, which had a flow area of only 0.44, and finished with a 0.944" opening, which has a flow area of 0.670, I achieved a gain of more than 52.5% in available flow area.

In one afternoon, I improved the flow characteristics of the stock exhaust system, and my total investment was under $10 (for some cheap grinding stones and one 1/4x28 set screw).
(I already had the electric drill and numbered drill bits.)

I removed the air box snorkel, and then used a scrap of aluminum window screen to cover the opening to keep out trash and clumps of mud. I then removed the stock air filter and support, the latter of which includes the backfire screen. I've read where Scott Summers advises NOT to remove this screen, but I've also read where others have found it beneficial to remiove it. I've removed it from my Honda 300 4x4, and I too felt an improvement in throttle response. So, noting that the backfire screen consists of two layers of screen, between which are trapped two more layers (actually a flattened screen 'tube'), I carefully cut away the outer layer of screen, and then removed the trapped inner piece, leaving only one, the inner layer, of the original four-layers of screen. I happen to like foam air
filters, so I'm sticking with the stock filter for now.

Despite what I've read elsewhere, it is NOT necessary to move or remove the subframe to remove the carburetor. After removing the seat and gas tank, I simply loosened the clamp holding the airbox channel to the carb, then loosened and removed the three bolts holding the intake manifold to the head, and slid the carburetor, with intake manifold, out the left side. After removing the carb from the intake manifold, I examined the composite rubber & plastic intake manifold. Using my Dremel
Moto-tool with a medium size sanding drum, I cleaned up the few ridges found inside, none of which were large enough to make any real difference anyway.

Now I'm waiting for my new jets to arrive. I plan to start with a 158 main and a #60 pilot, leaving the needle clip in the 3rd groove, and work from there toward achieving the right combination for my altitude (approx 750' above sea level) and riding style (wobbly).

I'll also be replacing the 15t drive sprocket with a 14t. For the terrain where I ride, the stock XR400R is geared a bit too high.

Constructive comments, advice, and recommendations from other XR400R owners will be appreciated.

GL Banks, Huntsville, AL

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From: Tyson Kamp <>        Back to Top 
Subject: Re: New rider Q: safety gear?
Date: 31 May 1999 19:06:49 GMT

-jc <> wrote:
> On 28 May 1999 21:29:21 GMT, Tyson Kamp <> wrote:


> My list with approximate "new" prices ...

> Helmet ................... $150-500
> Chest protector ..... $60-100
> Boots .................... $150-275
> Gloves .................. $25-35
> Goggles ................ $20-50

> Knee pads ............ $15-50
> Elbow guards ........ $15-35
> Kidney belt ............ $30-50
> Pants .................... $90-150
> Jersey ................... $20-50

> For a total of ......... $575-1295

> Buy all that you can afford .... the first five are required.

Thanks, this is _exactly_ what I was looking for!


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From: Uwe Hale <>        Back to Top 
Subject: Re: Newbie Questions
Date: Wed, 02 Jun 1999 04:42:33 GMT

DSchroed92 wrote:

> 1. What in the world is jetting?

It's the changeable orifices in the carburetor that meter fuel for different
throttle settings. The orifices are screw in brass thingys called jets.
Changing jets to make the motor run at the proper air/fuel ratio (~14:1 by
weight) is called jetting.

> 2. Could someone explain compression ratio for me in a way a moron would
> understand? (someone tried to explain it to me before but I didn't really
> understand)

Air gets drawn in the motor when the piston moves down in the cylinder,
enlarging the cylinder area and creating a vacuum. All the air in the cylinder
is compressed together when the cylinder moves back up. The difference between
the area when the piston is down and when the piston is up is called the
compression ratio. If your bike is a 250cc (cubic centimeters), it will draw
in 250cc s of air and fuel with the piston at the bottom. When the piston is
at the top, the air and fuel only takes up 25cc s of space. Compressing 250cc
into 25cc is a 10:1 compression ratio.

> 3. Given my size what would be a proper beginners sized bike?

You're large enough for anything. But a Honda XR400/250, Kawasaki
KDX200/KLX300, would have tame power suited for a beginner.

> 5. If you don't have a truck is there some other way to haul 2 bikes?

Put a trailer hitch on the car and get a small trailer.

> 6. What are the major differences in 2 stroke engines and 4 stroke engines?

Pictures are worth a thousand words. So check out: 

> 7. Are there any websites geared for beginners?

See #6 above.

> Any other knowledge that you think would be helpful would be appreciated.

You can search the archives at: 

enter in the forum field. In the search field enter any
words you want to find more info on. You can use Boolean operators & (and) |
(or) in the search field. Example: steel & bars & bend which gives you about
100 articles with those words.

Uwe Hale - 89 YZ250WR, 99 GasGas EC200

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From: Rich Rohrich <>        Back to Top 
Subject: Re: Performance question...
Date: Tue, 16 Mar 1999 22:28:08 -0600

Mark Cronk wrote:

> Just curious - are you getting your 110 octane by running av gas? if so
> BEWARE. Oh man I love telling people this - your av gas is a one piece motor
> waiting to happen. It is formulated for high altitudes and among other
> things it's formulation for aircraft engines means is has been optimized for
> 2,450 rpm, if I recall correctly.
> That is *way* off the 12,000 rpm little 2-strokes are capable of, and
> consequently the fuel can not atomize (there's that word again) fast enough
> and/or burn fast enough at 12,000 rpm and the engines seizes due to an
> improper/lean combustion mixture.

Setting aside the fact that Tmax is just out trolling for new fish, I haven't
annoyed anyone by going off on a psychotic rambling Avgas jag in quite a while.
So in the interest of shaking Harvey out of his self-imposed silence, let me
just say Mark is sort of correct here. Avgas will perform poorly at really
high rpm , but the seizures and rod bending is a combination of old wives
tales, and knuckleheads that can't jet worth a damn. Mark, if your friend
twisted a crank, he needs to look elsewhere.

The distillation curve of Avgas will have a great impact on the performance of
a high speed engine. The distillation curve or Volatility curve of a fuel
determines to a large degree the warm-up, transitional (on & off) throttle
response, and acceleration characteristics of an engine. Here's the simplified
version. A fuels distillation curve designates the maximum temperatures at which
various points between 10% and 90% of the fuel will be evaporated as well as the
maximum end point temperature. So for any Engine/Air Temperature combination
there is a minimum volatility that is required for proper running. Gasoline is
made up of different hydrocarbons, with different boiling points. By combining
these hydrocarbons together you get a Distillation/Volatility curve. Some
hydrocarbons boil(light ends) off at low temps some at much higher temps.
Depending on the intended application, a petrochemist will blend hydrocarbons to
get a curve that matches the temp, altitude, and acceleration characteristics
for the application. The problem with avgas as a race fuel is the fact it is
blended for an application where characteristics like acceleration and throttle
response are not high priorities. What's more important to the Avgas designer
is controlling mixture strength by eliminating the possibility of vapor lock and
making sure that light end hydrocarbon fractions don't boil off too early.

In an e-mail correspondence with Tim Wusz of Tosco/Union 76 race fuels he clued
me in on the power robbing possibilities of fuels with 90% boiling point over
280 degrees F in high rpm applications. I don't have the CRC distillation curve numbers in front of me, but I'm pretty sure that the minimum 90% boiling point for 100LL Blue Avgas is right around 280
degrees F, which in an engine that turns over 7000 rpm will likely make less
power than a fuel that has it's 90% point lower. My experience has been that
fuels with 90% boiling points that are too high usually tend to run a little
lean, and tend to run a little flat. I've always attributed this to large blobs
of fuel ending up in the chamber leeching heat and lowering cylinder pressure,
and just generally not contributing much to the process.

As you can see, by using straight Avgas or by mixing various types of fuel
together you are modifying a number of important fuel design parameters. You may
hit on a combination that works well , but more likely you'll have an engine
that doesn't detonate, but doesn't accelerate very well either. So avgas is
SAFE, but not a very good choice. It makes a very good base stock if you want to
play back yard petrochemist. A number of race fuels use Avgas as their base
stock, so it can be done, it's just difficult to hit on that magic combination
without a lot of testing.

Rich Rohrich
Applied Fluid Dynamics

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From: (SticksPop)         Back to Top 
Subject: Re: Setting shock sag
Date: 15 Mar 1999 03:15:00 GMT

Found this that may help:

Friction and Binding
* Compress the rear suspension several times. The bike should return to the same position within a 1/4" every time. If not there's binding in the linkage or swing arm pivots.

Shock Ride Height
* With bike on center stand and rear suspension totally extended, take measurement from center of the rear axle to a consistent point (seat mount, fender bolt or mark on the rear fender) straight up or slightly forward. Record this measurement.
* With rider aboard in normal seating position with full riding gear, have him bounce up and down several times then let it settle to it's rested position.
* Subtract this measurement from the fully extended measurement. Record this measurement. This will give you the Shock Ride Height.
* Shock Ride Height should be between 30% and 33% of total wheel travel. (Example: On 12" travel rear suspension, 30% would be 3 1/2"" and 33% of the travel would be 4".
* If this measurement is in excess of 33% then increase spring pre-load one
full turn at a time until its adjustment is brought within range. If this
measurement is less than 30% decrease the pre-load.

Free Sag
* Use the same measuring points as used to measure shock ride height, now
measure the sag of the rear suspension with no rider aboard by lifting and
pressing on the rear suspension to get a consistent resting point. Record
* The difference should be between 6% and 10% of the total wheel travel. On
12" of travel 6% will be 3/4". If Free Sag is less than 6% of the total
travel you'll need the next stiffer spring. If Free Sag is more than 10%
you'll need the next softer spring.

Too stiff or too soft?
* If Ride Height and Free Sag are set as recommended and hard bottoming or
excessive stiffness is felt, then a compression dampening adjustment is needed.
If a slight dampening adjustment doesn't cure the problem then switch to the
next stiffer/softer spring.
* If you do change springs always recheck the Shock Ride Height and Free Sag
before riding again.

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From: "Eric Gorr " <>        Back to Top 
Subject: Porting Tech Article
Date: Wed, 15 Dec 1999 11:23:24 GMT

An Overview of Basic Principles and Processes

The process of cylinder porting is a funny paradox. The people in the
market to buy it are looking for information and the people in the market
of selling it are hiding information on porting. So much myth and
misinformation is associated with this complex machining and metal
finishing process. Yet the tooling is easily available and the design of
the ports is actually quite straightforward with resources like computer
design programs. This article is an overview of how porting is performed
and how it can benefit your performance demands.

Two-Stroke Principles
Although a two-stroke engine has fewer moving parts than a four-stroke
engine, a two-stroke is a complex engine with different phases taking place
in the crankcase and in the cylinder bore at the same time. This is
necessary because a two-stroke engine completes a power cycle in only 360
degrees of crankshaft rotation, compared to a four-stroke engine, which
requires 720 degrees of crankshaft rotation to complete one power cycle.
Two-stroke engines aren't as efficient as four-stroke engines, meaning that
they don't retain as much air as they draw in through the intake. Some of
the air is lost out the exhaust pipe. If a two-stroke engine could retain
the same percentage of air, they would be twice as powerful as a
four-stroke engine because they produce twice as many power strokes in the
same number of crankshaft revolutions.
The following is an explanation of the basic operation of the two-stroke
1. Starting with the piston at top dead center (TDC 0 degrees) ignition has
occurred and the gasses in the combustion chamber are expanding and pushing
down the piston. This pressurizes the crankcase causing the reed valve to
close. At about 90 degrees after TDC the exhaust port opens ending the
power stroke. A pressure wave of hot expanding gasses flows down the
exhaust pipe. The blow-down phase has started and will end when the
transfer ports open. The pressure in the cylinder must blow-down to below
the pressure in the crankcase in order for the unburned mixture gasses to
flow out the transfer ports during the scavenging phase.
2.Now the transfer ports are uncovered at about 120 degrees after TDC. The
scavenging phase has begun. Meaning that the unburned mixture gasses are
flowing out of the transfers and merging together to form a loop. The
gasses travel up the backside of the cylinder and loops around in the
cylinder head to scavenge out the burnt mixture gasses from the previous
power stroke. It is critical that the burnt gasses are scavenged from the
combustion chamber, to make room for as much unburned gasses as possible.
That is the key to making more power in a two-stroke engine. The more
unburned gasses you can squeeze into the combustion chamber, the more the
engine will produce. Now the loop of unburned mixture gasses have traveled
into the exhaust pipe's header section. Most of the gasses aren't lost
because a compression pressure wave has reflected from the baffle cone of
the exhaust pipe, to pack the unburned gasses back into the cylinder before
the piston closes off the exhaust port.
3. Now the crankshaft has rotated past bottom dead center (BDC 180 degrees)
and the piston is on the upstroke. The compression wave reflected from the
exhaust pipe is packing the unburned gasses back in through the exhaust
port as the piston closes off the port the start the compression phase. In
the crankcase the pressure is below atmospheric producing a vacuum and a
fresh charge of unburned mixture gasses is flowing through the reed valve
into the crankcase.
4. The unburned mixture gasses are compresses and just before the piston
reaches TDC, the ignition system discharges a spark causing the gasses to
ignite and start the process all over again.
What is Porting?
Porting is a metal finishing process performed to the passageways of a
two-stroke cylinder and crankcases, that serves to match the surface
texture, shapes and sizes of port ducts, and the timing and angle aspects
of the port windows that interface with the cylinder bore.
The port windows determine the opening and closing timing of the intake,
exhaust, blowdown, and transfer phases that take place in the cylinder.
These phases must be coordinated to work with other engine components such
as the intake and exhaust system. The intake and exhaust systems are
designed to take advantage of the finite amplitude waves that travel back
and forth from the atmosphere. Porting coordinates the opening of the
intake, exhaust, and transfer ports to maximize the tuning affect of the
exhaust pipe and intake system. Generally speaking porting for more
mid-range acceleration is intended for use with stock intake and exhaust
systems. Porting for more high rpm power is intended for use with
aftermarket exhaust systems and especially clutching mods.

These are some common words and terms associated with porting.
Ports - Passageways cast and machined into the cylinder.
Ducts- The tube shape that comprises the ports.
Windows- The part of the port that interfaces the cylinder bore.
Exhaust Port- The large port where the burnt gasses exit the cylinder.
Exhaust Bridge- The center divider used on triangular shaped exhaust ports.
Sub-Exhaust Ports- The minor exhaust ports positioned on each side of the
main exhaust port.
Front Transfers- Transfer ports link the crankcase to the cylinder bore.
The front set (2) of transfers is located closest to the exhaust port.
Rear Transfers- The rear set of transfers are located closest to the intake
Auxilary Transfers- Some cylinders have a minor set of transfers located
between the front and rear sets.
Transfer Port Area Ratio- The area of the crankcase side of the transfers
divided by the area of the port window.
Boost Ports- The port or ports that are located opposite of the exhaust
port and in-line with the intake port. These ports are usually by-pass
ports for the intake or piston and sharply angled upwards to help direct
the gas flow during scavenging.
Port-Time-Area- A mathematical computation of the area of a port, divided
by the displacement of the cylinder, and multiplied by the time that the
port is open. The higher an engine revs the more time-area the port needs.
The higher the piston speed the less time available for the gas to flow
through the port.
Duration- The number of crankshaft angle rotational degrees that a port is
Opening Timing- The crank angle degree when the piston uncovers the port.
Crank Angle- Measured in units of degrees of crankshaft rotation. On a
two-stroke engine there are a total of 360 degrees of crankshaft rotation
in one power cycle.
Port Side angle- The side angle of a port measured at the window, from the
centerline of the bore with the exhaust port being the starting point (0).
Port Roof angle- The angle of the top of the port at the window.
Port Height- The distance from the top of the cylinder to the opening point
of the port.
Top Dead Center (TDC)- The top of the piston's stroke.
Bottom Dead Center (BDC)- The bottom of the piston's stroke.
Chordal Width- The effective width of a port, measured from the straightest
point between sides.
BMEP- Brake Mean Effective Pressure.
Loop Scavenging- Scavenging is the process of purging the combustion
chamber of burnt gasses. Loop scavenging refers to the flow pattern
generated by the transfer port duct shapes and port entry angles and area.
The gasses are directed to merge together and travel up the intake side of
the bore into the head and loop around towards the exhaust port.
Blow-Down- This is the time-area of the exhaust port between the opening
time of the exhaust and the transfers. When the exhaust port opens the
pressure blows down. Ideally to below the rising pressure of the gasses in
the transfer ports. Blow-down is measured in degrees of crank rotation and
Effective Stroke- The distance from TDC to the exhaust port height. The
longer the effective stroke the better the low end power.
Primary Compression Ratio- The compression ratio of the crankcase.
Secondary Compression Ratio- The compression ratio of the cylinder head.
Compression Waves- Pressure waves that reflect from the end of the intake
or exhaust system and return to the engine.
Expansion Waves- Pressure waves that travel from the engine and out to the

Tools of the Trade
There are two main types of tools used in porting, measuring and grinding.
Here is an overview of how these tools are used.
The basic measuring tools include a dial caliper, an inside divider, and an
assortment of angle gauges. The caliper is used to measure the port height,
the divider is used to measure the chordal width of the port, and the angle
gauges are used to measure the roof and side angles of the ports. Calipers
and dividers are available from places like Sears or industrial supply
stores. Angle gauges are fashioned from cardboard and specific to
individual cylinders.
The most common grinding tools are electric powered. They consist of a
motor, speed control, flexible drive shaft, tool handle, and tool bits. The
power of these motors ranges from 1/5th to 1/4th HP with a maximum rpm of
15,000. Popular manufacturers include Foredom, Dremel, and Dumor. Each
company sells a full compliment of accessories for all sorts of hobbyist
activities. The most popular source for cylinder porting tools and
accessories is CC Specialty in Tennessee (1-800-762-6995).
The tool handles and bits are the secret to porting. There are two types of
tool handles; straight and right angle. The straight tool handles are used
for machining the port ducts. The right angle tool handles are used to gain
access to the port windows from the cylinder bore. Over the years I've
tested hundreds of different tool bits and arrived at some simple materials
and patterns for finishing the different surfaces of a cylinder. The
materials of a cylinder range from aluminum as the base casting material,
to a cast iron or steel liner, or nickel composite plated cylinder bores.
Here are the basic tool bits used for porting; tungsten carbide works best
for aluminum, steel, and cast iron, stones are best for grinding through
nickel composite. The tungsten carbide tool bits are available in hundreds
of different patterns and shapes. The diamond pattern is the best
performing and the shape of the bit should match the corresponding shape of
the port. Stones, or mounted points as they are termed in industrial supply
catalogs, are available in different shapes and grits. The grits are graded
by the color of the stones. Gray being the most course and red being the
most fine. The finer the grit the faster it wears but the smoother the

Making Ports Bigger
Generally speaking, if you're trying to raise the peak rpm of the powerband
with an aftermarket exhaust system of clutching on a snowmobile, the ports
will probably need to be machined in this manner; widen the transfer ports
for more time-area and raise the exhaust port for more duration. Most OEM
cylinders have exhaust ports that are cast to the maximum safe limit of
chordal width. Often times widening the exhaust port will cause accelerated
piston and ring wear. In some cases the port will be widened so far that it
breaks through into the water jacket. Transfer ports should be widened with
respect to the piston ring centering pins. The ports should have a safe
margin of 2mm for the centering pin.

Making Ports Smaller
Ports are purposely made smaller for several reasons. One or more of the
ports could have been designed too big, or a well meaning tuner may have
been overzealous, or a customer may of asked for more that he could handle.
There are performance gains to be had from smaller ports, for high altitude
compensation or for more punch for trail and snowcross riding. The exhaust
ports can be made smaller by two ways, either by simply using a thinner
base gasket (Cometic makes graded gaskets) or by turning-down the cylinder
base on a lathe. The other method is by welding the perimeter of the port,
although that entails replating the bore. Transfer and intake ports can be
made smaller with the use of epoxy. Brand name products like DURO Master
Mend or Weld-Stick are chemical resistant, easy to mold to fit, and can
withstand temperatures of 400F. Master Mend is a liquid product and
Weld-Stick is a semi-dry putty material. The epoxy can be applied to the
roof of the ports to retard the timing and reduce the duration. It can be
applied to the sides of the transfers to reduce the time-area, and it can
be applied to the ducts to boost the primary compression ratio (crankcase

Porting for Big Bores
WISECO offers big bore piston kits for most popular snowmobiles. The
average increase in displacement is 50cc per cylinder. This requires that
the cylinder be over-bored 4-8mm. Because the ports enter the cylinder bore
at angles, when the bore size is increased all the ports drop in height.
The steeper the port angles the greater the port height will drop. Lower
port heights mean retarded timing and reduced duration. The exhaust port
gets narrower and the transfers get wider. A larger displacement cylinder
will require more port-time-area. Normally the exhaust port needs to be
raised higher than stock to compensate for the compression ratio. If you're
adding a set of performance pipes at the same time as the big bore, you'll
need to compensate the port timing to get the best gains from the pipes.
It's a complicated thing. Sometimes tuners use thicker base gaskets to
compensate for big bores, but ideally the port-time-area needs to be
calculated before any serious porting changes are made. If you are strictly
trail riding at high altitude, you can just have the cylinders bored and
replated because the porting will inherently change to suit that type of

Computer Design Software
The best-kept secret in high performance tuning is the use of computer
design software. These products became popular about six years ago when Tom
Turner adapted the SAE programming code and added in his own empirical data
from his career as a motorcycle drag racer and tuner. Tom's products are
named TSR software ( and available in MS-DOS format for
PCs. The programs cover individual engine components. The most popular
program for porting is PORTTIME 2000 and it sells for $200. It features
target specs for all sorts of vehicles including snowmobiles. Basically a
tuner types in engine spec dimensions like the bore and stroke plus all the
individual port measurements. The program runs mathematical calculations in
order to provide a simulation of how the porting changes will affect the
engine's powerband. TSR's programs will get a tuner 90% to the engines
potential. The next level of programs includes 1D and 3D gas dynamics
simulators. Dynomation is a 1D simulator that sells for about $500
( It enables tuners to combine all the engine components
together to simulate dyno runs on the computer so they can save time
machining metal and swapping parts on actual dyno runs. Dr. Gordon Blair
programmed the 3D simulator named Virtual Two-Stroke, the most well know
two-stroke engine researcher. His program is marketed through Optimum
( through lease programs. Priced at $12,000 a year,
it's intended for use by engine manufacturers.

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