What Affects Grip While Racing?

Keeping Our Focus On Springs

In previous content (click here to view) we discussed the procedure for selecting a spring rate as establishing the starting point for the chassis tuning process. Staying on the topic of springs, it helps to understand how springs and sway bars affect the handling characteristics of our car. Springs and sway bars play the largest role in establishing the overall balance of the car. That is, the general oversteer/understeer characteristics of our car.

 

Roll Couple Distribution

Springs and anti-roll bars (sway bars) provide resistance to roll. Roll couple is the total amount of roll resistance (front/rear) provided by the springs and sway bars. Roll-couple distribution is the amount of roll resistance at the front relative to the rear. It is roll couple distribution that affects the balance of the car (tendency for oversteer/understeer). It may help to think of it as, springs determine where the weight is transferred during cornering, front relative to rear. The driver and other elements, such as shocks, determine the when.

 

In general,

• Less stiff front (springs/sway bars) and/or more stiff rear (springs/sway bars) will tend to make the car “oversteery“.
• More stiff front (springs/sway bars) and/or less stiff rear (springs/sway bars) will tend to make the car “understeery“.

 

As a rule, for springs, we want to run as soft as possible to help overall mechanical grip. Softer springs help the weight be more evenly distributed across all four tires. To a point though. More on this later. Now we want to drive the car. See how it feels. Without changing driving style to adapt to the car, have the driver drive the car and see how it feels. Does the car tend to understeer? Oversteer? This will help provide us a direction to make changes. Changing springs at the track may be a somewhat non-trivial activity so try and make changes using the sway bars. Bear in mind, stiffening sway bars will tend to unload the inner tire, lessening overall grip.

 

Remember, roll resistance changes are the key to finding a handling balance we can be happy with, so we want to spend some time and get this right. We attain a neutral balance when grip levels front and rear are equal.

 

What Affects Grip?

Next we turn our attention to overall grip. This is why we’re here. When on track what we desire is more exit speed upshifting sooner heading to our next brake zone. Being able to be on throttle sooner. All of our attention both as a driver providing inputs to the car, and chassis tuning is geared to this goal!

 

• So, is body roll bad? Depends.
• Weight transfer is what we want to avoid.

 

Many people think our goal with making mods to the car is to limit body roll. This is not necessarily true. Limiting body roll will come into play if we are on a high speed track and we are running aero mods. Keeping the body level (limiting body roll) will help our aero to do its job.

 

For aero sensitive vehicles with air dams/splitter, diffusers, rear wings, etc. stiffer springs allow for a lower ride height so less air passes under the vehicle. Also these springs will compress less when loaded (downforce) so there is less variation in ride height allowing the aero to do its job.

 

If springs are too stiff your tires will “skate” over bumps (leaving the track surface) and won’t be able to do their job correctly.

 

Body roll also negatively affects camber. We also desire a lower ride height (more on this later) which will require a slightly stiffer spring. So although we want to run as soft a spring as possible, in these instances we want to run stiff enough to maintain camber compliance and aero.

 

Limiting Weight Transfer

Tires make grip via the friction between the rubber molecules of  the tire compound and the contact patch with the road surface. It should be understood that if we press down on the tire vertically, this will increase that friction (i.e. increase grip). This is vertical load. Vertical load is the load the tires see, perpendicular to the track surface. Increasing vertical load increases overall grip. Adding aero is good because we add vertical load without adding to the weight of the vehicle.

 

Weight transfer (lateral load transfer) is what hurts our overall grip. Weight transfer occurs in cornering and also under braking and throttle. Weight transfer results in a change in the vertical load seen at the tire. In cornering, removing load from the inside tire and adding load to the outside tire. This hurts our overall grip so we want to limit weight transfer.

 

Weight transfer is based on these three elements only:

• Total weight of the vehicle.
• Track width
• Center of gravity (CG) and the force acting upon the center of gravity.

 

Reducing total weight of the vehicle goes without saying. We are always seeking ways to remove weight or “add lightness”. For track width, we can increase track width using spacers mounted at the wheel hub. Center of gravity is lowered when we remove weight, specifically removing weight that is high in the vehicle. We also lower our center of gravity by running a lower ride height. On a smooth track we want to run as low a ride height as possible without negatively affecting the suspension geometry. This will also require a slightly stiffer spring. A bumpier track will require a softer spring which will require a higher ride height.

 

Time for a key takeaway. Weight transfer is what we want to limit since it hurts our overall grip.

 

• Weight transfer is NOT caused by body roll. And reducing body roll will not reduce weight transfer.

 

Traction

Next we want to talk about traction which means getting as much rubber in contact with the track surface (contact patch). These are the elements below that affect the contact patch.

• Camber
• Toe
• Tire Pressure
• Roll Steer
• Bump Steer

 

Summary

In general, we want to run as soft a spring as possible. This helps mechanical grip and helps the weight be more evenly distributed across all four tires. This is what we want! Only run stiff enough to maintain a level chassis helping any added aero, and run stiff enough to minimize camber changes in cornering. Running a lower ride height (lower CG, desired! Lessens weight transfer) also needs stiffer springs.

 

Springs determine where the weight is transferred during cornering, front relative to rear.

 

Driver controls, when, timing and rate the weight is transferred.

 

Above all, have fun!

 

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