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Technical Information

Chassis Tuning Wonder what things like camber, caster, toe in/out are? Read and learn.

Head Gear How to get a properly fitting helmet.

Jetting How to Jet your carb.

Shocks Theory on how they work, adjusting and setting them up.

Springs How they are rated, multi rate setups, formulae for figuring out rates.

Tire Information Charts on tire sizes and other useless info.


Setting Up A Pilot For Stadium Racing

After running a Pilot as a Stadium Lite for the past two years, taking second overall in the Pace Series both times (winning once each year) and winning the CORR World Championship Round at Crandon, WI a couple summers ago, I've decided to share some of my Pilot's setup secrets.

First, the heart and soul of my Pilot is ATV's Long Travel Suspension kits and the 500cc conversion. BUT, you still have to tune these to work up to maximum levels. The biggest gain to be made in getting a Pilot to work well for racing is in the suspension modifications.


Chassis tuning? I'm getting there, I'm getting there...

The shock settings that come with the Long Travel Kit are built around dune riding. The majority of the cars that end up with ATV's LT kit end up racing around in the dunes where the surface is for the most part smooth and slightly soft. A stiff spring and lighter damping rates are perfect for this setting. BUT, if you want to play on a track, be it a motocross track, out door off road track or a stadium track, you need to do some changes.

The spring rates that came with my LT kit were as follows: Front shocks - single 188 pound spring. Rear shocks - Dual rate setup with a 125 pound tender and a 150 pound main. These are too stiff for the off road stuff. I switched to a dual rate up front and used a 175 pound tender and a 260 pound main. On the rear I went down to a 100 pound tender and a 125 pound main. Drastically softer than what the car came with.

By going to the softer spring settings my damping was a lot closer, but it was still too soft on both compression and rebound. I ended up completely changing the shim stacks and jets a couple of times. Here's a mapping of the stacks and jets that I finally decided on and used for the Pace Series. (I unfortunately don't have the originally supplied shim data to compare against)


Recommended Shim Stacks and Shaft Jetting for ATV Racing's Long Travel Custom Axis Shocks

Front Compression Front Rebound Rear Compression Rear Rebound
.008 x .800 .010 x .700 .010 x .950 .015 x .800
.009 x .900 .010 x .800 .010 x 1.100 .015 x .950
.009 x 1.000 .008 x .900 .008 x 1.250 .015 x 1.100
.009 x 1.100 .008 x 1.000 .008 x 1.350 .012 x 1.250
.009 x 1.250 .008 x 1.100 .008 x 1.500 .006 x 1.3350
Front Jet 70 Rear Jet 86

If I were to run the Pilot on one of the large out door tracks, such as Crandon, I would probably go up on spring rate for both front and rear and leave the shock damping where it is, or go slightly stiffer on the compression damping. The reason for the spring changes are that the speeds are so much higher and the jumps are not near as radical compared to a stadium track.

Now that I've covered the basics on shocks and spring setup for a Long Travel Pilot, how about actually looking at chassis tuning?

So far, all I've done is cover the suspension and how it works. What about the tires and the way they contact the ground? Let's dive in.
Some key terms:
Ackerman Angle - The angle of the two steering arms which produce toe out on turns.
Alignment - The process of adjusting the position of the tires and steering axis to bring them to a specified, predetermined position.
Bump Steer - The amount of toe in or out that is induced as the suspension goes through it's travel.
Camber - The angle of the king pins from vertical as looking parallel to the chassis' longitudinal axis. What? How about the angle from vertical the king pin leans in or out towards the body of the car (as looking at the front or rear of the car).
Caster - The angle of the king pins from vertical as looking perpendicular to the chassis' longitudinal axis. There he goes again! More like the angle of the king pin from vertical as looking at the side of the car. I always remember caster as you "cast" a fishing rod. Positive Caster is when the top of the king pin (from vertical) leans towards the back and Negative Caster is when the king pin leans towards the front
Caster Offset/Trail - The distance in side elevation between the point where the steering axis intersects the ground and the center of tire contact. The offset is considered positive when the intersection point is forward of the tire contact center and negative when it is rearward.
Contact Patch - the part of the tire that actually touches the ground.
Scrub Radius - The distance between the center of the tire and the steering axis when measured at the road surface.
Toe - An angle of a tire, relative to straight ahead, if viewed from above.
Toe Angle - The actual amount that the tire differs from pointing straight ahead.
Toe In - A condition where both tires of an axle are positioned so they are closer together at the front than the rear.
Toe Out - A condition where both tires of an axle are positioned so they are closer together at the rear than the front.
Track - The lateral distance between the centers of tire contact of a pair of wheels.
Track Change - The change in wheel track resulting from vertical suspension displacements of both wheels in the same direction.


Now that I've tossed out a bunch of terms, how do they relate and what settings did I use? It's all pretty straight forward:

Toe Front/Rear* 1/4" toe out / 1/8" toe out
Caster Front/Rear* 3 degrees positive / 0 degrees
Camber Front/Rear* 3 degrees "top of tire in" (Negative I think) / 0 Degrees.

* ALL REAR MEASUREMENTS DONE WITH SHOCKS OFF AND SUSPENSION AT 1/2 TRAVEL.
When setting up the suspension, the first thing to do is make sure that the car is square (shocks on, set car on it's "butt" so it's pointing up). Measure on the left side from the center of the front lower ball joint to the bolt head on the lower left rear carrier - write it down. Do the same for the right side of the car. These are supposed to be the same. Next, measure diagonally from the left lower ball joint to the right rear carrier lower bolt head - write it down and then do the same the other way. These are supposed to be the same too - bet they aren't! Go about adjusting this until the cross measurements are the same and the side measurements are the same (you will end up with two sets of measurements - the cross and side measurements won't be the same as each other).

Now put the car back on it's wheels and block up the rear end. Pull off the shocks, pull the suspension to 1/2 travel and set the rear wheels so that they have no camber (wheels sit vertically) and you want 1/8" of toe out. This lets the suspension come to 0 toe (due to flex) when the drive train is under load. The Caster should be set so that at full extension, the upper ball joint on the rear carriers just barely touches the springs. As the suspension compresses, the ball joint will rotate away from the spring.

You'll notice that as you change the settings, EVERYTHING from the side to side measurements to the cross measurements will change on you. It's VERY frustrating and takes quite a bit of time to get done correctly. (Beer makes the time needed LONGER - and you have to go back and recheck it all later.)

The front end is pretty easy. Find (beg/borrow/steal) a set of turn plates and a caster camber gauge and follow the instructions. Do the caster first, then camber - go back and check caster as you adjust for camber and then finally do toe out - 1/4".
Toe OUT on the front end will help the car dive INTO the corner. Toe IN on the front will help the car come OUT of the corner. On the rear end, it's exactly the opposite.


A Dissertation on Front Ends - Sort of....
Long, Complicated and Probably Pretty Boring to Most People!

Let's start with the definitions: (you probably saw them at the top of this)
Ackerman Angle - The angle of the two steering arms which produce toe out on turns. This also can be considered the relationship of the angle between the two wheels as they go through the steering motion.
Alignment - The process of adjusting the position of the tires and steering axis to bring them to a specified, predetermined position.
Bump Steer - The amount of toe in or out that is induced as the suspension goes through it's travel.
Camber - The angle of the king pins from vertical as looking parallel to the chassis' longitudinal axis. What? How about the angle from vertical the king pin leans in or out towards the body of the car (as looking at the front or rear of the car).
Caster - The angle of the king pins from vertical as looking perpendicular to the chassis' longitudinal axis. There he goes again! More like the angle of the king pin from vertical as looking at the side of the car. I always remember caster as you "cast" a fishing rod.
Positive Caster is when the top of the king pin (from vertical) leans towards the back and Negative Caster is when the king pin leans towards the front .
Caster Offset/Trail - The distance in side elevation between the point where the steering axis intersects the ground and the center of tire contact. The offset is considered positive when the intersection point is forward of the tire contact center and negative when it is rearward.
Contact Patch - the part of the tire that actually touches the ground.
Scrub Radius - The distance between the center of the tire and the steering axis when measured at the road surface.
Toe - An angle of a tire, relative to straight ahead, if viewed from above.
Toe Angle - The actual amount that the tire differs from pointing straight ahead.
Toe In - A condition where both tires of an axle are positioned so they are closer together at the front than the rear.
Toe Out - A condition where both tires of an axle are positioned so they are closer together at the rear than the front.
Track - The lateral distance between the centers of tire contact of a pair of wheels.
Track Change - The change in wheel track resulting from vertical suspension displacements of both wheels in the same direction.

So, what's all this crap mean? Well, lots of stuff; but how does it relate to us? Let's see. (the majority of it - who cares - I'm not gonna bother with it.)

The original question was concerning trying to adjust ackerman to help in steering with our off road cars. Being that most of the time we're counter steering through corners we can toss most of the "normal" road course stuff out the window. (:

First - ackerman - well, anyone that's familiar with a Pilot's front end will realize right off the bat that trying to adjust ackerman is useless. Ackerman would be the LAST thing that you'd try to work on with a vehicle. I'll explain why.

First, if you've ever taken notice of a Pilot's front wheels as it goes through it's travel, you'll notice that there is a huge amount of bump steer. It's because of the relationship of four points on the suspension:
1) inner mounting point of the tie rod
2) outer mounting point of the tie rod
3) inner mounting point of the a-arms
4) outer mounting point of the a-arms

In a perfect world, the suspension and tie rods would all work as a parallelogram system. Upper and lower a-arms would be exactly the same length. The distance between the mounting points of the a-arms on the chassis and the mounting points on the spindles would be exactly the same length and the tie rods would be exactly the same length as the a-arms - and they would sit parallel to the a-arms.

As the suspension went thru it's travel, the system would stay parallel - there would be no toe change, no camber change, no caster change. All nice and tidy. But, that's not what we have.

To adjust for ackerman (in our fantasy, parallelogram suspension vehicle) the length of the tie rod would be changed - AND the mounting point for the tie rod would be changed too - if we just lengthened/shortened we'd induce toe. By doing this the relationship between the tie rod's pivot points and the suspension's pivot points would change. I'm assuming (we all know what that means) that if we lengthened the tie rod that the inner wheel (in the turn) would turn less - my thinking is that you'd have less horizontal travel as the arm moves through it's travel - more on that later - maybe... So, if we shortened the arm (and adjusted the mounting point to keep the toe that we started with) then the ackerman angle would be greater (the desired result that was spoken of in the previous posts). I'm either correct or completely backwards!

So, back to our suspension design.

The entire idea of a suspension design is to keep the contact patch as large as possible in all aspects of the suspension/wheel movement. Certain factors will make a huge difference in this.

King Pin Angle and Caster are probably the two biggest factors.

First; King Pin Angle - This is the line drawn through the spindle's pivot points (draw a line from the pivot point of the top ball joint thru the bottom ball joint and then project it on to the ground). This angle and the distance from the center of the tire is what determines scrub radius. This is the amount of pivoting the contact patch does around the king pin angle - if the line we drew ends up pointing directly to the middle of the contact patch, (assuming that we have no camber or caster yet - everything's set perfectly vertical so far) then we'd have no scrub radius. (I'm gonna call this the king pin's contact point - kpcp for now) The wheel would pivot right around the middle of the contact patch. This is a pretty desirable thing - so far....

BUT, you want to induce a bit of scrub - you want the contact patch to move around the king pin angle - it forces the tire to bite by inducing scrub. A bit is a good thing.

Now, let's toss in caster - by leaning the king pin back you make the contact patch move behind the kpcp. This gives the car steering stability and controls the bite of the contact patch. Think about trying to push a grocery cart - those casters follow the kpcp. It makes you the boss and not the wheels when you try to steer. Move the kpcp behind the contact patch and you have a wheel that's trying to be driven from behind - would be like trying to keep that caster on the grocery car from spinning around when you start pulling on it after you've been pushing it. Too much forward caster (I think it's called positive - never can remember which is which) and you start getting a contact patch that's overbearing and takes control - makes the car over steer and a bitch to steer.

Move the top of the king pin backward too far and then you end up swinging the contact patch around the kpcp - and you start inducing scrub and the car will develop a push. Usually the optimum setting is somewhere around 3 degrees of negative caster.

Now let's toss in Camber.

The main idea of camber is to keep the contact patch as large as possible in corners. - back to our perfect suspension. As the car chassis goes into a left hand turn, the chassis would roll about it's longitudinal axis to the right - now our tires are leaning to the right and we're turning left - contact patch would move towards the inside of the right tire and the outside of the left tire - problem is, since we're turning left, we want the outer (right) tire's contact patch to move towards the INSIDE of the tire because the tire's profile will roll and we're trying to load the tread with as even of a load as possible. Ever watch a CART car on an oval? They have the right side tire leaning in and the left side tire leaning out - so when the car is at full lateral load, the tires are actually sitting flat against the surface. Full contact patch has been achieved.

BUT WAIT (there's more!) We changed the length of the upper a-arm (shorter) to make camber - now our system's not a parallelogram anymore - hey, looks like we've induced a bit of ackerman, eh? Damn! So now we've got to go back and adjust the length of the tie rods to make up for the change.

Now comes the fun stuff....

Take a piece of paper and draw two circles - I'll wait - go get your paper, compass, ruler and pencil.

Draw a 5" circle and then using the same center point, draw a 7" circle. Done? Good. Now, draw a horizontal line thru the center point. Next draw a horizontal line two inches above and two inches below the first line.

Draw vertical lines thru the points where the upper and lower horizontal lines pass through each circle. Now, measure the distance on the center horizontal line from where the vertical line and the 5" circle cross the horzontal line (hey, it turns out to be exactly 1 inch! - just did it in ACAD) - do the same for the 7" circle - it's .6277".

So, if we have a shorter upper arm (the 5" circle) than the lower arm (7" circle), you can see that the HORIZONTAL TRAVEL of the upper arm is 30% greater than the lower arm. Guess what's just happened as the suspension goes through it's travel (the upper and lower horizontal lines indicate full compression and extension, respectively). It induces - anyone? - CAMBER!!! You can use this to design a system that makes the camber change as the suspension goes thru it's motion - and since the suspension on the outside of the car (during a turn) compresses - and the chassis rolls to the outside - we make the contact patch stay nice and big and under the tire instead of moving out towards the outside of the tire. Pretty cool, eh?

We're still assuming that the suspension arms are on parallel planes to each other and the tie rods. The change in camber is the same amount equally above and below the horzontal line. By varying the distance between the chassis mounting points you can design in the amount of induced camber thru the suspension travel. Make them wider than the spindle's mounting points and you move the caster changes down lower in the suspension's movement - move them closer together and you move it up. (or is it exactly the other way around? I'm not gonna bother trying to draw it out on ACAD - but I hope you get the picture).

Now, here comes the really fun stuff. We can make the CASTER change as the suspension goes thru it's travel too! (oooohhh, ahhhh)

Remember, we started out with a completely parallel system. Even the four mounting points of the a-arms on the chassis (each side) were all equal distance - but what happens if - (we're looking at the right side of the car now) our lower a-arm is horizontal and our upper a-arm is slightly tilted - the front mounting points of the a-arms are 5" apart and the rear are 5.5" apart. Now the arms move in different angular planes. Our lower a-arm's ball joint moves perfectly vertical. but the upper a-arm's ball joint now moves on a plain that's (hang on - more ACAD work) - uh I think it's 14 degrees from vertical. So, what's our spindle gonna do as we compress the suspension? (we're assuming that at full extension that the angle between the two ball joints is 90 degrees from horizontal. So, we've induced 14 degrees of caster into the suspension at full compression. What you do is make it so that as the suspension compresses, you loose a bit of caster - this keeps the contact patch behind the kpcp and keeps the front wheels from becoming over steering monsters when landing off of jumps or during braking and such.

Once again, to try to deal with ackerman with the huge amounts of changes in long travel suspension is damn near impossible. You have to get EVERYTHING ELSE set up first. By that time, ..whew...

The application that was mentioned in ackerman tuning was in vehicles that have very small (in comparison) suspension travel.

Now You Know. (:

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2/2/03