Most of us understand that tires make traction through friction between the rubber molecules at the tire’s contact patch and the road surface. And most of us understand that traction increases as vertical load on the tire increases, which is why aerodynamic downforce works so well. In addition, we understand that the tire will make more traction if the entire contact patch is equally loaded, which is why monitoring tire temperatures is useful. Within this basic knowledge are misconceptions and misinformation that can add confusion to an already difficult topic. Let’s see if we can shed some light on the subject.
We will not address tire design or construction, since no one I know can change those parameters anyway. Let’s start with the factors within the tire that affect traction. These are the only factors within the tire affecting traction:
- Basic tire design and construction
- Sidewall rigidity
- Tread rubber compound
- Tread design
- Tire size
Tire size, compound and possibly tread design are the only choices we have, and those are limited.
There are also factors we have control over. These include:
- Tire pressure
- Tire camber
- Tire toe-in (out)
- Camber change
Each of these items has an optimum setting that allows the tire to create maximum traction for a given set of circumstances on a given car.
Then there is the vertical load on the tire, which is crucial to understand, but also the most misunderstood element of tire traction. Traction increases as the vertical load on the tire increases. But it is very important to understand that the relationship between vertical load and traction is not linear. Being nonlinear means that if the load on the tire is increased, while the traction also increases, it does not increase as much as the load. This is a good time to more clearly explain traction in terms of pounds of force and vertical load on a tire.
One way to look at traction is in pounds of force. The most convenient way to do this is to look at the entire car as a whole, and measure the force that the tires create. Most automotive performance enthusiasts have heard the term g force. If a car accelerates at 1 g, and the car weighs 3,000 pounds, then the tires are producing 3,000 pounds of traction force. This applies to acceleration forward, braking (negative acceleration) and cornering (lateral acceleration).
A “good handling” performance car can produce a cornering force of about 0.99 g in a corner, about 1.01 g under braking and somewhere around 0.50 g acceleration in first gear. For a 3,500-pound car cornering at 0.99 g, the traction in pounds is 3,465 pounds (3,500 x 0.99 = 3,465). That is a lot of force from those four tire contact patches. Put an R-compound DOT tire on the same car and raise that force to 1.05 g of cornering force.
Vertical load is the load actually seen at the tire contact patch. This includes the weight resting on the tire contact patch plus any aerodynamic downforce. If the car creates any aerodynamic lift, then the vertical load on the tire will be less than the weight on the tire, since the car is lifting instead of being pushed down. Aerodynamic downforce is good because it increases traction without increasing the weight of the car. Let’s look more closely at this, since this is another area of some confusion.
Downforce is pretty much a traction freebee. It costs a little in acceleration at high speeds, and reduces top speed somewhat, but it adds no weight to the car. Adding weight to the car actually reduces the relative amount of traction compared with the total weight of the vehicle. For example, say a 3,000-pound car makes 3,000 pounds of cornering force at the limit. That’s 1 g of lateral force.
Let’s say we add 500 pounds to the car with nothing else changed, including the weight distribution. It’s easy to understand that the car will not accelerate as quickly because it weighs more and the engine is making the same horsepower. It is less obvious that cornering speed will reduce. Here’s why. The 500 pounds of weight adds 500 pounds of vertical load to the tires, but because the relationship between the vertical load increase and traction increase is not linear, the amount of traction increase will only be about 400 pounds.
That means the tires now make an additional 400 pounds of traction (400 x 1 g) for a total of 3,400 pounds of traction. This works out to a cornering force of only 0.97 g. This equates to a loss in cornering speed due only to the effect on the tires, not on the dynamics of the suspension. This is due entirely to the characteristic of tires where traction does not increase as fast as load. This non-linear relationship also becomes more significant as the design load of the tire is approached.
In other words, if a tire has a maximum load capacity of 2,000 pounds, but normally carries only 750 pounds, doubling the load to 1,500 is approaching the design limit. Here the traction may only increase by about half the extra load. If the design load is exceeded, the situation gets worse. While there is nothing you can actually do to a tire or suspension to change this non-linear relationship, there are plenty of factors you need to understand to minimize its effect and allow your car to create maximum possible traction.
These factors are crucial to maximize traction for each individual tire:
- Camber angle at the front
- Camber at the rear for cars with independent or adjustable rear suspension
- Tire pressure at each tire
- Toe settings front and rear; axle or axle housing squareness on solid axle cars
- Roll steer and axle squareness at the rear
- Bump steer
TIRE CONTACT PATCH
A bigger tire contact patch, all else being equal, means more traction, but bigger is not always faster. Sometimes a wider tire is slower because it increases rolling resistance too much and/or because the suspension cannot control the tire contact patch effectively (camber change, etc.). Whatever size tire you run, it is important to get as much of the tire contact patch working for you as you possibly can.
The uniform goal in every case is to have the entire tire contact equally loaded across the surface of the contact patch. If the entire contact patch is not equally loaded, you are not getting all the traction possible from that tire. If you look at the tire contact patch as a series of one-inch squares, one square compared to another acts just like one tire compared to another tire. Reducing the load on one square increases the load on another square. The square losing load loses traction more quickly than the other square gains traction from the increased load. In other words, the tire contact patch as a whole is making less traction than it could be if the contact patch were equally loaded over its entire area.
Once you have the entire tire contact patch at each corner working to its maximum traction potential, then the goal is to get all four tires creating the maximum amount of traction possible for the whole vehicle. To accomplish this, you must understand weight transfer. Many factors contribute to traction, like construction characteristics, design, peak slip angles and track conditions. The things controlled to some degree by the team, like weight distribution and chassis setup make all of the difference in handling. In motorsports, the team making best use of the potential traction at all four tires is the team with the best chance to go faster or to win.
TIRE SLIP ANGLE
The tire slip angle, which is actually the amount of twist in the tire sidewall that causes the tire contact patch to turn at a smaller angle than the wheel centerline — the difference is the slip angle — determines the lateral force of the tire. At a given slip angle, the tire will create the maximum cornering force. At a smaller slip angle, the tire will create less cornering force and the same holds true at greater slip angles. The goal for the driver is to keep the tire at the optimum slip angle for maximum cornering force at all times in a corner — not an easy task. The front tire slip angles vs. the rear tire slip angles determine the handling balance of the car. If they are equal, the car is neutral. If the front slip angles are bigger than the rears, the car will push or understeer. If the rears are greater, the car will be loose or oversteer.
Lighter is better. For competition purposes, most classes have minimum weight rules. You want to be at minimum weight. If there is no minimum weight rule, run as light as possible. There are two big reasons that minimum weight is important. First, the engine must accelerate the extra weight. Second is the factor we looked at above – tire traction vs. vertical load on the tire. If you add 500 pounds to the car to improve handling, you have added 500 pounds to the tire’s workload, but only about 450 pounds of additional traction force. That makes the car slower under braking and cornering. It’s not a good tradeoff.
During cornering, weight transfers from the inside to the outside, under braking from the rear to the front and under acceleration from the front to the rear. Weight transfer hurts overall vehicle traction. In cornering situations, weight moves off the inside tires to the outside tires. This changes vertical load on all four tires.
The inside tires lose vertical load while the outside tires gain vertical load. The inside tires lose traction while the outside tires gain traction. Sounds OK so far. But remember that the relationship between vertical load on a tire and the traction force of that tire is not linear. The weight coming off the inside tires causes them to lose traction faster than the outside tires gain traction from the new found additional vertical load. So the net total traction of the tires is reduced compared to the same situation if no weight transfer occurred. Since it is not possible to eliminate weight transfer in a corner, we at least want to minimize it so that the overall traction remains as high as possible.
Under braking, the same thing occurs, but is less pronounced. Under acceleration on a rear-drive car, weight transfer actually helps accelerate the car because the drive wheels are gaining traction while the tires losing traction are not driving the car — and the opposite is true for a front-drive car. Even though we gain some acceleration traction from more weight transfer, if you have to turn and slow down for corners, weight transfer hurts lap times, so our goal is to minimize weight transfer as much as possible.
There are only four factors that effect the amount of weight transferred. Nothing else affects the amount of weight transfer.
- The total weight of the vehicle — more weight means more weight transfer, all else being equal
- The force acting on the center of gravity — more force means more weight transfer
- The height of the center of gravity above ground — higher centers of gravity transfer more weight
- The track width (for cornering) or the wheelbase (for acceleration and braking) — narrower track widths or shorter wheelbase mean more weight transfer
Let’s look a little more closely at each of these. We have already discussed total weight. Since we want to run as light as possible, or at minimum weight, this is a constant factor we cannot change unless we make major changes to the vehicle. The traction force of the tires determines the force acting at the center of gravity. Reducing the traction or driving below the limits of tire traction are certainly contrary to our goal of getting around the track as fast as possible, so again, this is not really a factor we want to use.
Maximum track width is always set by rules in competition or by practical considerations, and unless you are running at very high speeds where aerodynamic drag is a big factor, you want to run the widest track width possible, so again this not a controllable factor.
However, the center of gravity, the point within the car where it, if suspended at that point, would be in perfect balance, can be altered. Maybe not much on some cars, but enough to affect performance. Simply keeping weight as low as possible in the car will lower the center of gravity, thus reducing weight transfer. This is very important to consider when modifying the suspension and lowering a car, which also lowers the center of gravity.
There are many misconceptions about weight transfer. Only the four items listed affect the amount of weight transfer. Body roll has a minimal effect and should not be considered a factor. Dive and squat are not factors. Neither is the phase of the moon. So do not be misled to believe that anything other than the four factors listed have an effect on the amount of weight transferred while cornering, braking or accelerating.
Roll couple is the total amount of roll resistance present in a car. Roll resistance is generated by the springs and antiroll bars. Stiffer springs and antiroll bars reduce body roll. Body roll is not necessarily a bad thing in itself, but it does cause some dynamics within the car that can hurt handling and overall performance. The most significant problems are increased camber change and aerodynamics.
Camber change causes the tire contact patch to become loaded unequally across the tire surface, even to the point that part of the tire contact patch loses contact with the road. This requires more negative camber to counteract, which can cause the same problem under straight-line braking — part of the contact patch is not in firm contact with the road. Reducing body roll will reduce this effect. Aerodynamically speaking, roll at the front allows more air under part of the car, causing aerodynamic lift and increasing aero drag. This is a big factor with stock-based competition vehicles and gets worse as speeds increase.
Body roll occurs when a car corners. Weight transfer is not caused by body roll, and reducing body roll does not reduce weight transfer. Weight transfer in corner occurs even with zero body roll, even with no suspension on the vehicle, like a go-kart.
Where roll couple is the total amount of resistance to body roll provided by the springs and antiroll bars at both front and rear, roll-couple distribution is the amount of roll resistance at the front relative to the amount at the rear. Changing the roll couple distribution balance of the car changes the handling balance of the car. If we increase the front roll resistance, the handling balance will change. If the car was neutral before the change, the car will now understeer. If the car understeered prior to the change, it will understeer more. If it oversteered, it will oversteer less after the change. The opposite effect occurs at the rear, where increasing rear roll resistance will increase oversteer or reduce understeer. The roll resistance can be increased by increasing either the spring rates or the antiroll bar rates, or both. This makes roll resistance changes the key to finding a perfect steady state handling balance. Adjustable antiroll bars allow fine-tuning the roll couple distribution, making setup much easier.
While body roll is not directly related to the amount of weight transfer during cornering, roll-couple distribution determines where the weight is transferred, front vs. rear, during cornering. Increasing front roll resistance forces more of the total weight being transferred to go to the front tires and less to the rear tires. Increasing rear roll resistance forces more of the weight being transferred to go to the rear tires. The load change on the tires, more at one end and less at the other, is what changes the handling balance. This works exactly the way you would expect based on the effects of vertical load on tire traction we discussed earlier.
The traction circle is a graphic representation of tire traction used for cornering, braking and acceleration. Tires can make traction in any direction: forward for acceleration, rearward for braking and laterally for cornering. The rubber molecules at the tire contact patch do not know or care which direction they work in. For this reason, all of a tire’s traction can be used in one direction, or part can be used for cornering and part for either braking or acceleration. Study the accompanying illustration to fully understand this principle.
Often overlooked as a racecar dynamics factor, the driver is actually a major factor. The driver controls when dynamic events occur based on when the driver uses one or more of the controls. And the driver determines, at least in part, how quickly dynamic events occur based on how fast and abruptly the driver uses the controls. Many drivers are too abrupt with control inputs and upset the dynamic balance of the chassis, which degrades overall vehicle performance. The term used by many racing instructors and coaches “slow hands” refers to gentle steering inputs. The same factor applies to brake and throttle applications.
BACK TO WEIGHT TRANSFER
Let’s take another look at our main topic: weight transfer. There is so much misinformation floating around about racecars, their setup and the dynamics affecting them that all of this can be confusing. While the concept of weight transfer is simple enough — since you can feel it while driving — the effects of weight transfer on the performance of a racecar are not simple at all. First, the bad information.
Body, or chassis, roll affects weight transfer and reducing body roll reduces weight transfer. This is actually true technically, but the effect of body roll on weight transfer is tiny, possibly less than 1 percent of the total. So reducing body roll even 100 percent would have very little effect on weight transfer. Body roll does have a big effect on camber change and aerodynamics however. Now the important stuff.
From a car modification perspective, getting the center of gravity as low as possible will improve tire traction by reducing weight transfer — so will running the widest track width under your rules. And of course, reducing the vehicle weight to the minimum also will reduce weight transfer.
Next is roll-couple distribution. From the weight-transfer-control perspective, it does not matter if you change spring rates or antiroll bar rates. It is the front vs. rear roll resistance that matters. So which is better, antiroll bars or springs? Good question. It depends on the smoothness of the track. Springs rates are — or should be — determined by the bumpiness of the track surface. This is done by calculating suspension frequencies. Next question!
Is it better to increase the rate at one end or lower it at the other? If everything is close, and the understeer/oversteer is not caused by a setup issue like tire pressures or camber angles being off, then the change should be fairly small. Look at tire temperatures, especially the average tire temperatures. If the temps are high for the tire compound (ask your tire company engineer) at one end, then soften that end. If one end is too cool, stiffen that end.
As a rule of thumb, if issues are in low-speed turns, balance the roll couple with bars and springs. If issues are only in high-speed turns, go with aero adjustments. But always get mechanical grip (tire traction at low speed) dialed in first, then work on high-speed cornering balance with aerodynamics.
NOW THAT YOU ARE OVERLOADED
That is a lot to absorb, but if you work on the basics, you will be very surprised at how well all this stuff actually works. And work is the key word here. It does take work, lots of it, as a matter of fact. But success on the track takes a major commitment and this is all part of that commitment. But it is fun, so go after it!
We found this video on YouTube uses a fish bowl full of water to demonstrate the effects of weight transfer.
We found this video from the folks at The National Science Foundation, which also highlights the details of weight transfer.