Setup The Race Car II

Setting Up The Race Car Part 2

Handling the forces

Anti roll bar:

The anti-roll bar is a very important and essential part of a race car’s suspension. It’s purpose is to help take some of the roll resistance away from the springs. It helps limit body roll, suspension travel and limits severe camber change during cornering, while allowing a very easy method of adjusting roll stiffness and roll couple distribution.

How does an anti-roll bar work? It basically a torsion bar mounted transversely across the chassis. It only begins working when the car’s body rolls from the inside to the outside. It resists the roll by taking weight off the outside wheel and placing it back on the inside wheel, while keeping the roll angle to a minimum. This is done by deflecting the bar with equal amounts of force, to both wheels.

One of the most important uses on the anti-roll bar is to balance the chassis for roll couple distribution. How this works, lets suppose your car is pushing going into a left turn. Do you change to a softer RF spring, or change to a softer front anti-roll bar? The bar is easier to change and is the best choice. The softer RF spring means the car would dive more, toward the RF when subjected to hard braking loads.

When that happens, the front wheels are subjected to more camber change, because the wheels are traveling more in the bump position. Its better to have the spring rates just stiff enough to resist braking loads and let the anti-roll bar resist body roll.

Front sway bar:

A sway bar is known as an anti-roll bar. As stated above use the sway bar to control body roll in the front of the car during cornering. The bar acts as a third spring to balance the weight transfer during cornering.

We are allowed a total of 12 different choices for the front sway bar. The range of adjustments are 0″, 0.875″ to a maximum of 1.500″ in 0.062″ increments. The sway bar is measured by the diameter of the bar. The larger the diameter the stiffer the bar.

Changing the diameter of the bar we are able to adjust the amount of roll couple or weight transfer that occurs at the front of the car. Generally, the larger the bar the less the body roll up front. Less body roll the tighter the car becomes. Therefore the smaller the bar, the more body roll and the looser the car becomes. Fine tuning with sway bars is an easy way to compensate for roll couple or body roll.

How the Front Sway Bar effects the handling of the chassis:

The larger the bar the tighter the chassis.
The smaller the bar the looser the chassis.

Rear sway bar:

The purpose of the rear sway bar is the same as the front sway bar except it controls body roll at the rear of the car.

The rear sway bar adjustments range from a low of 0.000″ to a maximum of 1.000″. This differs by a half inch over the front sway bar, with 26 adjustments in 0.025″ increments. The larger the bar the stiffer the rear becomes. Making the rear stiffer, it has just the opposite effect that occurs on the front.

A larger rear sway bar will actually loosen the car up due to the fact the way the weight is being transferred at the rear. It is just the opposite of the way the weight gets transferred at the front of the vehicle.

Track bar:

Another name for a track bar is called the Panhard bar. This is just a long bar (or tube, usually) running across the chassis and parallel to the ground, joined at the frame on the right side and attached to the axle on the left.

There are two purposes for this device:

Provides lateral control (sideway movement) of the axle while cornering.

It establishes the rear roll center location.

The rear roll center is established where the centerline of the panhard bar intersects with the centerline of the vehicle. The roll center moves somewhat whenever either the axle travels vertically (hitting a bump) or when the car rolls in a turn.

The mounting points of the bar are both adjustable up and down which repositions the rear roll center. We are allowed to lower the bar as low as 7.00″ and raise it as high as 14.00″. Raising the track bar equally on both ends raises the rear roll center, which loosens up the car, while lowering it will tighten up the car. Another point to remember is the rear roll center directly effects the amount of body roll experienced in the car.

We can also angle the track bar. Increasing the bar angle (right side higher than the left) loosens the chassis during acceleration and in high banked corners while tightening it under braking. This in effect is lowering the roll center slightly during cornering and acceleration. It transfers the weight to the LR tire.

The opposite is used on dirt, because on dirt you need the weight transferred to the RR during acceleration, but could be tried on pavement. It would have the opposite effect.

Springs:

The springs primary purpose is to resist the amount of body roll when cornering and to keep the tyres in constant contact with the road surface.

We are allowed to choose springs from 450 lbs to 2500 lbs with 50 lb increments on the front and 150 lbs to 700 lbs in 25 lb increments in the rear.

Softer spring rates are preferred, but with it comes a greater tendency for more body roll, chamber change and the car bottoming out. Body roll can be kept under control by careful planning of the roll center heights. Another way to minimize chassis roll that soft springs permit is through the use of the anti-roll bars (sway bars). Chassis bottoming out is controlled by a ride height adjustment.

Changing roll center height and spring rates is a basic and convenient way to modify the handling of a race car. But there is more to it than that. The load transfer from the inside to the outside pair of tyres depends only on the radius and speed of the turn, the tyre track width of the vehicle and the height of its center of gravity. What the suspension does affect is the share of the transfer load taken by the front versus the rear tires.

The springs have to resist the tendency of the body to roll when cornering. Since the chassis does not twist much, the roll angle must be virtually the same on both ends of the car, so both sets of springs will be compressed the same amount. If we put a stiffer spring on one end, that end will resist the rolling more than the softer end and that end will loose traction first.

If we have identical springs, but change the height of the rear roll center, the end with the higher roll center will take more of the load transfer and will loose traction first. Just remember the stiffer end will have the tendency to loose traction first.

The lower the rear roll center (lowering of the track bar) creates a higher amount of overturning force which must be resisted with a higher spring rate on the outside tyres. This is without a change to the anti-roll bar.

A spring rate change is also required for banked tracks, higher than 15 degrees and 1/2 mile and longer in length. Its needed to compensate for the additional down force generated by the car and the tracks bank angle converting some lateral acceleration into down force. The steeper the banking the greater the down force. Just increase the RF & RR spring rates, with no change required for the anti-roll bars.

A good starting point for springs is 1000 lbs RF, 800 lbs LF and 250 lbs on each rear wheel.

Remember the corner force a tyre can deliver depends on how hard it is pressed to the ground. More vertical load yields more cornering force, but the connection isn’t perfectly proportional: every increase in vertical force yields a progressively smaller increase in side force.

As a result, a pair of tyres sharing a load can develop the greatest cornering power when they share the load equally. Any un-even distribution of load between them will reduce the cornering power of the pair, because the one that is more heavily loaded will gain less then the other loses.

Stiffer springs mean larger changes in wheel rates when hitting bumps or changes in direction, that would proportionally double the force on that tyre. This is another reason why going to softer spring is better.

In general stiffer front springs make the car tighter. Stiffer rear springs will loosen the car.

Let’s try to put it in it’s simplest form:

Softer LF makes car tighter.

Softer RR makes car tighter.

Softer RF makes car loose.

Softer LR makes car loose.

Stiffer RF makes car tighter.

Stiffer LR makes car tighter. It will also tighten from mid-corner through the exit.

Stiffer LF makes car loose.

Stiffer RR makes car loose.

Remember the diagonal corners as pairs. What one pair does the opposite pair will have an opposite effect. Just remember that a stiffer RF spring tightens the car, then a diagonally stiffer LR also equals a tighter condition. Now a stiffer RR & LF would have the opposite effect on the car.

Shocks:

Shocks are designed to control the up and down movement of the suspension springs. They dampen the oscillations of the spring, not absorb shocks as the description implies. A shock controls the speed at which the spring moves. Without shocks a car would handle like a boat in the water, swaying back and forth while moving up and down.

Understanding shocks and how they work will give you a major advantage over those that don’t. Controlling the chassis with the proper shocks is the key to getting through a corner smoothly and effortlessly. Shocks are used to help control handling problems and can even be used to induce desirable handling characteristics.

Shocks influence handling transitions – braking, acceleration and cornering. They do NOT control the amount of weight transfer in a corner. They will however, control how quickly the weight is transferred.

Shock absorber ratios such as 50/50 or 90/10, is the ratio control of the shock absorber unit as it is divided between Bump (the “down” or compression stroke) and Rebound (the “up” or extending stroke). A 50/50 ratio means that the damping control of the shock is equal in both directions. This is the most widely used control ratio of a shock absorber. A 90/10 shock would have 90 percent of its total damping control on compression and only 10 percent on extension. That means this is a shock that is very easy to pull out but very heavily controlled on compression.

If a shock has a “/” followed by another number indicates a multiple ratio shock. The number to the left of the slash (/) is the Compression (Bump) damping force and to the right is the Rebound (Extension) damping force. Example a “7/5” indicates 390 lbs of damping on the Compression stroke and 265 lbs of damping on the Rebound stroke.

The pounds damping force rating is another important parameter. Racing shocks are rated at their pounds of force traveling at 24 inches/second. Shocks used on WC teams have a valve designation rating from 1 through 9. The common valving designation of shock absorbers and their pounds of force rating are:

Designation Force Rating

1 100 lbs

2 135 lbs

3 175 lbs

4 220 lbs

5 265 lbs

6 320 lbs

7 390 lbs

8 475 lbs

9 560 lbs

Compression or Bump stroke:

A shock has a piston inside that compresses the shock oil. As a car goes into a corner, at the transition from throttle to brake, or lifting off the throttle, the rear weight from acceleration shifts back to the front of the car as it slows down. That weight compresses the front shocks. Likewise, as a car is exiting the corner under acceleration, the weight once again transfers to the rear of the car and compresses the rear shocks.

Rebound or Extension stroke:

Rebound is the opposite of compression. When the car is in transition to braking from acceleration, the weight transfers to the front shocks. The rear shocks are rebounding at that point. As you get back into the throttle, the weight shifts once again to the rear of the car and now the front shocks are rebounding as the rear shocks are compressing to accept that weight transfer.

To get the car to handle properly on entry and exit to a corner, the weight transfer must be balanced properly. Racing shocks are nothing more than timers. They time when you need the weight to shift. A softer shock setting either receives weight (compression) or it dumps weight (rebound), faster than a stiff shock does.

Remember the compression and rebound rules. If you desire to keep the rear weight in the rear for a longer amount of time, you could do many different things. You could stiffen the rebound of the rear shocks or stiffen the compression of the front shocks. Just remember these keys:

STIFF = SLOW weight transfer

SOFT = FAST weight transfer

All handling problems are nothing but wrong weight transfers for the speed you are going around the corner.

Before adjusting shocks, try to visualize how weight is being transferred at each corner upon deceleration, braking and acceleration. With a better understanding, you will have an easier time deciding which shock to adjust to help cure or smooth your corner transitioning problem properly.

Proper compression and rebound settings basically come down to what a driver feels more comfortable with. What works with one driver, might not necessarily be correct for another. This is due to the fact that different drivers have different driving techniques. Smooth throttle, brake and steering transitions will require slower shock travel because weight isn’t being transferred as quickly compared to those drivers that use abrupt throttle, brake and steering transitions.

The following is a guide to fine tune shocks.

LOOSE ENTERING CORNER: Momentarily reduces weight transfer from LR to RF.

Stiffer RF Compression.

Softer LR Rebound. May need to use a multiple ratio shock at LR of 5/3 or 6/4 (Compression/Rebound).

LOOSE – MIDDLE OF CORNER:

Softer LR Rebound.

Softer RR Compression.

LOOSE EXITING CORNER: Need to get weight to transfer quicker from RF to LR.

Softer RF Rebounds.

Softer LR Compression.

PUSHING ENTERING CORNER: Reduce rate of weight transfer from LR to RF.

Stiffer LR Rebound. May need to use a multiple ratio shock at LR of 4/6 (Compression/Rebound).

PUSHING – MIDDLE OF CORNER:

Softer LF Rebound.

PUSHING EXITING CORNER: Reduce rate of weight transfer from RF to LR.

Stiffer LR Compression. May need to use a multiple ratio shock at LR of 4/6 (Compression/Rebound).

Stiffer RF Rebound.

Shock tuning tips:

Always start with 50/50 shocks and get the chassis sorted out first. Fine tune with split shocks.

To get more weight transfer to the LR under acceleration, keep the Rebound lighter on the RF compared to the LF.

More Rebound on the LF & LR will LOOSEN car at corner entry.

Less Rebound on LF & LR will TIGHTEN car at corner entry.

Softer Rebound on LF & RF will create better TRACTION under acceleration.

If car skates across track when a bump is encountered the shocks are too stiff.

If hop occurs when a bump is encountered the shocks are too soft.

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