When your vehicle is viewed from the side, the IC is a point in space where the upper and lower links converge. It is where we can measure the transferred force from both links and put just a single load point on the vehicle when under acceleration, a
point in space where forces from the converging upper and lower links are applied to the chassis. The only concern the Instant Center has on a vehicle is in respect to Anti-Squat. Some people design around the IC by saying the IC should be at the
water pump or some other point. The problem is, that only works as a comparison if the comparing is of two similar vehicles.
Anti-Squat is the amount of force applied to the links that will resist the rear squatting under acceleration. When you step on the gas (from either a standstill or while moving forward) to accelerate and the rear of the vehicle either drops down or props
upwards. By adjusting the location of the Instant Center you change the amount of AS. Seen another way, Anti-Squat is the amount of force placed on the links from the weight transfer of acceleration. When you accelerate weight is transferred from
the front axle to the rear axle. There are two things that will keep the rear suspension from compressing because of the extra load transfer. 1: the springs themselves, 2: Anti Squat. With a 0% anti-squat all of the weight transfer goes to compressing
the springs. If the springs are stiff they won’t compress much and if they are soft they will compress a lot. If you have 100% AS all of the weight transfer goes into the links of the vehicle and none of it goes to the springs. Because of this the
suspension will neither compress/squat nor extend/raise. If you have 60% AS, then 40% of the weight transfer will go into the springs and 60% will go into the links. If you have less then 0%, aka negative amount of Anti Squat, a.k.a. Pro Squat, the
rear suspension will squat more than the spring rate would infer.
To measure the AS of any linked vehicle you first need to know the height of the center of gravity followed by the wheelbase. From the side draw a line across the length of the vehicle at the height of the CG. Next draw a vertical line from the contact
patch of the front tire up to the line indicating the CG. Now draw a diagonal line from the contact patch of the rear tire to the intersecting point of the CG and the vertical line of the front tire. That line is the 100% AS line. Next draw a line
from the contact patch of the rear tire forward through the IC and continuing through the vertical line of the front tire, this would be your AS line. If your AS line intersects the front tires vertical line at 30% of the total distance from the ground
to the 100% line, then you have 30% AS. In other words, if your AS line intersect the front tires vertical line at 30” off the ground with a 40” CG, then you have a 75% AS. If the measured distance from the ground to this point is 20” then you have
50% AS. If it’s 10” then you have 25% AS.
Many desert guys run around 20-50%. Drag racers have been know to run all kinds of AS numbers from 50 to 140%. As rock crawlers, we’ve slowly been adjusting the amount of AS we use on our vehicles. Just a few years ago we ran numbers above 100% like the
drag racers. Today more common numbers are closer to 50-80%. For your home-brewed application an adjustable link bracket will help you dial in what is best for your needs. What’s the magic number? You tell us what works best for you and that’s the
magic number. Just remember the three contributing factors: height of center of gravity, tire size and wheelbase. To accurately measure the height of the CG please visit the links below.
An imaginary line running through intersecting points of a suspension, through which either the front or rear reacts, if the vehicle were held fixed. It is this line between these two points in which the suspension rotates about or moves around. In other
words it is the line your suspension swings from.
On a triangulated four-link, assuming both upper and lower links are triangulated, the converging point of the upper links just behind the axle and the converging point of the lowers form the needed two points in space to draw the intersecting line that
is the Roll Axis. On a suspension with a panhard bar, this line would be determined by drawing a line though the points that are the center of the panhard bar and the converging point of the lower two links. Generally speaking, a flat or slightly
negative angled sloped axis is desired. It is also this line or axis, which determines the amount of oversteer or understeer your axle has. If this slope rolls downhill toward the center of the vehicle you have a negative Roll Axis and your axle will
have understeer characteristics. If this slope rolls downhill away from the center of the vehicle then you have a positive angle and therefore oversteer characteristics.
Roll steer can be felt while traveling down most any road. The roll and swell of most roads can be felt as it forces the body to lean from side to side, even while travelling in a straight line. It is when the body leans that roll steer takes effect,
causing the axle’s angle in relation to the frame to change and thus have a steering type effect on the vehicle. Oversteer: your suspension causes your axle to oversteer a turn, meaning you turn more than the input of the steering wheel infers. Example:
while in a left-hand turn, the body will roll out of the turn to the right. If your rear axle has oversteer, it will turn outward and cause the rear end to come around quicker than it would have otherwise. When you want it, it’s great, but when you
don’t, it’s really annoying. A vehicle with solid axles front and rear will drive much nicer down the road with a little understeer.
Vehicles with oversteer typically wander in the lane, which makes for constant steering corrections while driving. If you find yourself with oversteer in your front suspension, then you can compensate by building understeer in the rear. If your Roll Axis
is downhill towards the center of the vehicle, regardless of front or rear suspension, then you have a negative angle or understeer. If your Roll Axis points downhill away from the center of your vehicle, then you have a positive angle or oversteer.
Many designers try to build a little understeer into their systems, because it seems to be more predictable and benign. It also has a tendency to track straighter and wanders less at speed. However, if you need to get around corners quickly, like
a CORR truck, you may find that you need oversteer from the rear suspension to help slide you around corners.
The Roll Center is the point through which the body wants to rotate about the axle, or the point that the axle supports the body laterally. It is again, an imaginary point directly above your axle that intersects the Roll Axis. In that regard it is just
like the IC but from a different perspective. It can also be thought of as Anti-Roll, if the Roll Center is at the Center of Gravity there will be no body roll and if the Roll Center is lower than the CG but higher than the ground, then some load
from cornering will go into the springs. If, however the Roll Center is on the ground, then all of the cornering load will go into the springs. In other words the higher the roll center the less body roll you get when on side hills and cornering.
However, with a high Roll Center you increase the amount of sideways movement applied to the body from the axle’s movement. This is mostly noticeable at high speeds while traversing cross-grain sections of desert. In other words, a high Roll Center
can push your truck sideways when one wheel compresses at higher speeds. In rock crawling, your axle will simply move a bit more to the compressed side then the other when articulated and you’ll never notice any sideways forces due to such a slow
overall speed. Desert guys tend to use a lower Roll Center, it may be because it’s easier to package or it may be because it reduces the lateral force on the truck. They tend to compensate for this lower roll center and body roll by adding additional
track width via wider axles, anti-sway bars and/or stiffer springs. However at slower speeds like rock crawling, a high Roll Center helps maintain stability without having to add additional components like anti-sway bars and stiffer springs.