Suspension guide

[tfaith08]

Either way, thanks a ton for taking so much time to post all of this. This is awesome. For me, suspension means a hell of a lot more to me than speed.


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No problem at all. It’s probably the least discussed element in amateur racing and someone has to do it. I just happen to enjoy it.


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[tfaith08]

How to plan a chassis build

If you need more detail, let me know.

In order:

Pick tires based on ground clearance required

Pick knuckles. 9/10 will be your OEM ones.

Pick rims to get KPI where you want it

Pick arms for length to get the width you want

Pick axle length to match front width at ride heigh

Pick swingarm length

Choose shocks LAST

All the details are listed in previous posts


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[kawa110]

Getting ready to “setup” two bikes.

Both fireball EGC arms.

One bike is for duning, trailing. 20 inch rear and 21 fronts. -2 avenger swing arm.

Other desert racing. 22inch and 23 fronts. -1.5 fireball arm.

Both 19 1/4 elkas up front. Both oem rear shocks.

Toms of adjustment on the fireball arms.


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[tfaith08]

Getting ready to “setup” two bikes.

Both fireball EGC arms.

One bike is for duning, trailing. 20 inch rear and 21 fronts. -2 avenger swing arm.

Other desert racing. 22inch and 23 fronts. -1.5 fireball arm.

Both 19 1/4 elkas up front. Both oem rear shocks.

Toms of adjustment on the fireball arms.


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Not much else in the world that gets me excited quite like a desert racing build.

Lmk if you need help bud


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[kawa110]

Lol. Oh I do!


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[Metal_man_Rob]

Not much else in the world that gets me excited quite like a desert racing build.

Lmk if you need help bud


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Sorry to keep beating this subject up…

So I usually ride on very gnarly trails with sharp turns, ruts, rocks, tree roots, mud pits, streams, etc. I never really go beyond 3rd gear and most of my riding is top end of 2nd.

Today I got out on wide open flat trails, dirt roads, open fields, with long straight shots.

The handling with my usual riding terrain always seemed fine. Today I realized it was not fine but just unnoticeable with the terrain.

My steering is sharp and responsive and very predictable on the trails. But today, in the higher gears and the higher speeds of 5th and 6th, on straight shots, the steering was squirrelly. So I am definitely going to readjust the entire front end. And try to follow your advice as closely as possible.

What tools do you use for the caster angle measurements, camber, and KPI? The way I was doing it isn’t very accurate.


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[tfaith08]

Sorry to keep beating this subject up…

So I usually ride on very gnarly trails with sharp turns, ruts, rocks, tree roots, mud pits, streams, etc. I never really go beyond 3rd gear and most of my riding is top end of 2nd.

Today I got out on wide open flat trails, dirt roads, open fields, with long straight shots.

The handling with my usual riding terrain always seemed fine. Today I realized it was not fine but just unnoticeable with the terrain.

My steering is sharp and responsive and very predictable on the trails. But today, in the higher gears and the higher speeds of 5th and 6th, on straight shots, the steering was squirrelly. So I am definitely going to readjust the entire front end. And try to follow your advice as closely as possible.

What tools do you use for the caster angle measurements, camber, and KPI? The way I was doing it isn’t very accurate.


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Don’t feel sorry. The more information here, the better.

Biggest thing that you absolutely cannot overlook is to set tire pressure and get to ride height before you make any measurements.

KPI isn’t something you can adjust, it’s just something you have to know and live with. It does dictate a lot of other measurements.

After tire pressure and ride height are set, set caster very next. Don’t skip ahead. Then set camber, then toe. Then go back and forth with camber and toe until they’re where you want them.

Too low caster can make it feel squirrelly, but it typically only happens with a bump. Better way to say it is that the less caster you run, the smaller the bump it takes to kick the steering (even a small amount). This is why it’s critical to make sure you’re running a tire and run setup with low scrub radius.

Camber is pretty simple. Strap the front end down to about 3/4 compressed (or so, 2/3 is fine too) and turn the turn the tires close to full lock (full lock if you actually turn full lock while riding. Not many riders actually do). The outside tire should absolutely never go past the vertical plane. The top should still be leaned in just a touch.

Toe, just set it straight and turn it in a little at a time. People get a little too hung up on measurements and what not. It actually plays a little with Ackerman so I’ll try to add more on that later.


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[tfaith08]

In regards to the last paragraph, people get a little too hung up on setting toe. I like it just barely toed in enough to be manageable in a straight line at full speed.


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[Metal_man_Rob]

In regards to the last paragraph, people get a little too hung up on setting toe. I like it just barely toed in enough to be manageable in a straight line at full speed.


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Yeah I don’t really get squirrelly at all when hitting bumps, roots and rocks. It actually handles nicely in that regard.
Perhaps I have more toe than I think. It just gets weird at higher speeds but handles so nice at trail speeds. But the trails I ride on are more for the big 4x4 quads. Fun as heck on a sport quad though.


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[Metal_man_Rob]

In regards to the last paragraph, people get a little too hung up on setting toe. I like it just barely toed in enough to be manageable in a straight line at full speed.


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And I meant scrub radius, not KPI. Sorry. I think I have my camber set way too far too. That would affect the scrub radius too, wouldn’t it?


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[Metal_man_Rob]

And I meant scrub radius, not KPI. Sorry. I think I have my camber set way too far too. That would affect the scrub radius too, wouldn’t it?


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Nevermind. As I think about it, the scrub radius would stay the same, in general, as you adjust in or out. The angle would get tighter or larger but that line should always ran through the ball joking to the center of the contact patch. If I’m am picturing it correctly


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[tfaith08]

Nevermind. As I think about it, the scrub radius would stay the same, in general, as you adjust in or out. The angle would get tighter or larger but that line should always ran through the ball joking to the center of the contact patch. If I’m am picturing it correctly


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The scrub radius will change very, very slightly with more camber. The combination of KPI and camber is called included radius.

Too much camber can make it feel a little weird. I kind of like a touch more camber than most but I ride very recklessly.

Too much toe will feel very weird at speed and isn’t really important at lower speeds. I’ll set toe dead straight if I’m riding in tight and technical stuff.

The only real benefit of too much toe is that it can correct a setup with too much Ackerman. It tends to hurt a setup if you start really turning them in.


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[Metal_man_Rob]

The scrub radius will change very, very slightly with more camber. The combination of KPI and camber is called included radius.

Too much camber can make it feel a little weird. I kind of like a touch more camber than most but I ride very recklessly.

Too much toe will feel very weird at speed and isn’t really important at lower speeds. I’ll set toe dead straight if I’m riding in tight and technical stuff.

The only real benefit of too much toe is that it can correct a setup with too much Ackerman. It tends to hurt a setup if you start really turning them in.


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I’m guessing a combo of too much toe in and too little caster is the problem. I wanted responsive handling for the trails I ride on. They are tight and rough. But at speed the quad is twitchy and scary. Haha. A slight nudge and it wants to go off course so slight variations in terrain end up turning into large variations on the quad.


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[tfaith08]

I always mean to add more content as time permits but it never happens. Got a pretty good bit of time on my hands now so I guess I’ll just ramble for a bit about caster, camber, and random diagnostics.

Caster targets are pretty simple. There are a lot of misconceptions about caster within the off-road groups and the big companies don’t do much to explain the effects of caster within their installation guides. Not only that but some of the big names are just flat out wrong. I’m not going to name names but I will say that in the past year, I’ve found that I will only take my hat off to three companies: Roll Design, Laeger’s, and Texeria Tech.

As with all things suspension, it all boils down to the mathematics. One of my favorite things to say is that the glory of mathematics is that if you don’t like my math, you can do it yourself.

So... the only thing that matters with caster is trail. To find trail, follow the formula below:

*These values don’t have a commonly used term to denote the variable so a lot of the formulas that I may use have variations in naming but the math is the same.*

C = caster in degrees
S = spindle height in inches
CA = camber in degrees
T = trail in inches

S(SinC/cosCA) = TR

Spindle height is of important note here. How much trail you have is directly influenced by how the tire is compressed. Mark the tire and check rollout with your weight on it. Either put the quad on the ground and drive forward or put the same weight on the pegs and push it. To check rollout, mark the sidewall of the tire and mark the ground under that point, then roll forward and mark the second point it touches the ground and measure the distance between the marks on the ground. It’s best to do it 4-5 times and divide the distance the same number of times. This just makes for a more accurate measurement.

Once you do that, just divide by 2 pi.

You also have to factor camber into it since every degree of camber increases the steering axis length, which is what part of that formula above derives as the hypotenuse length.

To find target caster, follow the formula below:

WB = wheelbase in inches
R = rake in degrees
C = caster in degrees
RH = ride height in inches
MC = minimum clearance
E = error factor in degrees

MC should be +1” for TT and FT, 2” for desert, and 1.75” for all others.

E is an error factor for unforeseen circumstances. Rough terrain, add a degree. Smooth terrain, remove a degree. Average terrain, do nothing. New and expert riders (be honest with yourself) add a degree. Everyone else, so nothing.

SinC = (RH-MC)/(WB-sinR(RH-MC))

C+E = target


Best way to picture all of this is that when viewed from above, the steering axis contacts the ground (coining the phrase “steering axis contact” or SAC now) at a different point than the tire contact patch. The forward component is trail and is derived largely from caster, while the outward component is scrub radius and is derived largely by KPI, camber, and offset. Each affects the SAC while toe rotates the contact patch around the SAC.

If there is ever anything I’ve said about front suspension that I would call the most important, it’s the above paragraph. When most paper suspension setup guides that come with a suspension kit are written, they’re too simplistic. Yes, toe increases stability as per the LSR guide, the Walsh guide, etc. but only if the rest of your setup is within the correct parameters. Sometimes, parallel toe gives you more stability than some toe (and I can explain why in my sleep) and sometimes too much toe works out just fine somehow. Go look at the guide and all it says is that toe increases stability. Same with caster. Too much caster is going to make the quad a living nightmare in the rough stuff.

I’m not going to say who, but you can take the guides from more than one top company and put them side by side and the wording, step by step order, and even one particular punctuation error matches up identically across three of them.

I digress. What I’m trying to get at is that your circumstances on your quad and with your components and how you ride and where you ride all matter to setup. I’m just here to educate.


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[tfaith08]

Toe is probably the most complicated of all alignments to understand.

The absolute most simple way to explain it is that toe should be set to a conservative value AND that when possible, it should point directly at SAC.

In reality, this will almost never happen but when it does, it will handle like no other.

If you don’t understand any of the previous post, please go back and read it because it is important to understanding all of this.

If your scrub radius is too large, there is nothing you can do with toe to make it stable. Just point the tires slightly inward and hope for the best. There is no patch. You have to start swapping parts.

If the SAC falls outside the tire (negative scrub), toe in will make the quad wobble at speed and slight weight imbalance will make the quad push steer away from obstacles. If inside (positive scrub), it will still wobble at speed and the quad will steer into obstacles. These will persist no matter what the toe setting is and is one huge reason scrub radius matters so much.

If your SAC is wider than the contact patch, bias your toe more parallel. If narrower, bias inward.

The mechanic behind this is that the tire wants to align itself to where the SAC is between itself and the forces being introduced. If the scrub is positive, the tire has to rotate inward to make this happen, but the direction of the tire centerline wants to keep the tire going in the direction the tire is facing. This imbalance doesn’t cancel out except for sometimes at high speed (and only on smooth surfaces with little traction), but it will just bounce between the two far more often than not. It will bounce quickly and feel like tire shake.

I’d scrub is negative, the same mechanic is prevalent but the tire wants to turn outward while the forces acting on it want to make it fall inward. The shake will still occur all the same but it’ll just want to turn outward first. At the frequency at which this occurs, you don’t care which happens first; all you see is tire shake and darting back and forth.

In a sense, the tire is always searching for its natural point to turn. I like to use the term “hunting.”

Slip angles matter too. Without a picture to add, it’s probably best to google image search “tire slip angle” but the basic idea is that the contact patch has its own direction that varies from the overall tire direction. You want minimum slip angle but maximum slip angle capability. More pressure cuts down on slip angle but reduces capability. Less pressure increases both. I’m personally a fan of very high pressure because it gives good rider feel (which typically comes from positive scrub radius in power steering equipped setups.

More pressure also reduces contact patch variation and mobility which makes adjustments more noticeable and effective. Reduced pressure is the last step to achieving a good grip but is one of the first steps required in alignment and setup. Set it high, dial it in perfectly, then consider dropping pressure slightly if any (I never do).

Bigger tires also like a little more toe as well and can hide small issues with scrub radius.

Toe in also makes the quad bias the outside tire more in corners. People like to say it makes the quad turn lazily but Ackerman angles overcome this significantly. Parallel toe makes the quad want to drag the inside tire more which isn’t always bad. It will also make a quad track around high speed corners better. Too little toe and it will want to give the inside tire more say in what happens over higher speed obstacles. Try to remember that the quad is going to bias the outside tire in a corner, toe just dictates whether the inside tire is going assist in turning (less load = less slip angle capability) or assist in slowing (dragging).

When people say more caster makes the quad harder to steer, 3/4 of that is that the added caster uncovered a problem with the scrub radius and toe not playing well together. Yes, the inside tire is jacking the quad up but you notice little of that when hanging off the side in a corner AND all the weight is typically on the outside tire.

In a perfect world, the only toe required is what you need to correct for component flex and rolling slip angle. If you can’t get toe right, you probably need to go back to fixing the scrub radius.


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