Cutting Edge Traction Techniques

Advanced Methods for More Forward Bite

Racers are forever trying to find more forward bite. The problem is greater for higher horsepower motored cars than with the crate classes, but nonetheless, everyone experiences times when they could definitely use more bite to help them use more throttle off the corners.

Over the years, the methods have changed somewhat, but the overall goal remains the same. Just what is that goal? It is to cause a mechanical change in the load distribution on the rear tires so that there will be a more equal loading of the two rear tires. It’s as simple as that.

We have known forever that the more equally loaded a pair of tires is, the more traction they will have. That is why cars running more left side weight go faster through the turns than ones with less left side weight. Because of load transfer, the left side tires always end up with much less load on them than the right side tires. We would need over 80 percent of static left side weight in order to end up with equally loaded tires through the turns.

Past Methods – Just to give you a little history lesson here to show how racers previously manipulated the loading on the rear tires coming off the turns, we’ll take you back to the mid to late 1990’s. Then several methods were used that may or may not apply to today’s racing. Many of the tracks were flat and the goal was to get off the corners better, not necessarily get through the middle well.

So, almost all of the teams of that day ran a softer right rear spring rate than what was in the left rear. Upon acceleration, the stiffer LR spring loaded that corner and cause a spike in the cross weight percent and that tightened the car. This also made the cars tight through the middle, but the teams mostly lived with that because they believed the greater gain was acceleration off the corners.

Another “trick” used was to stagger the mounting of the rear coil-over springs. If you placed the LR spring in front of the rear axle tube and the RR spring in the rear of the tube, and used a spring pull bar third link, as the rear end rotated on acceleration, the LR spring added load to that tire and the RR spring took load from the RR tire. This put more loading on the LR tire to make it more equally loaded in relation to the RR tire and then provided more bite. Again we had more equally loaded rear tires this way and it didn’t necessarily hurt the mid-turn handling.

In today’s racing, the use of softer RR spring rates and staggered spring placement methods are not applicable or useful for several reasons. First, with the bump setups being used, these methods will not allow us to reduce the rear roll enough when running a softer RR spring. We must run a stiffer RR spring rate, or one with more force, in today’s setups and we’ll tell you more about that later on.

As for spring stagger, many sanctions do not allow the use of a pull bar, so the effect of utilizing rear end rotation to load the LR tire does not work. Even the method of staggering the heights of the trailing arms that I have described in previous articles is of no use if you cannot use a pull bar.

The Problem With Today’s Setups – The problem we run into in today’s racing is trying to deal with setups using stiffer RR spring rates over the LR spring rate. When bump setups started being used, the front suspensions became very stiff, initially with bumps and large sway bars, and now more recently with just the bump stops and/or bump springs and smaller sway bars.

Again in previous articles, we have explained the concept of a balanced setup whereby the two ends of the car are made to work together in roll stiffness, or what we call equal roll angles. They must match up in order for the car to perform well through the turns.

So in order to accomplish that, we had to install rear spring rates where the RR spring was stiffer than the LR spring by whatever amount worked to balance the setup. Here was where the problem stated. If a softer RR spring makes better bite, the opposite will make the car have less bite, right? Yes, that is exactly what happens.

The problem that arose when we started running bumps and stiffer front suspensions is that when we matched the rear stiffness with the front, we lost grip off the corners. The following is one way in which many racers are solving that problem.

Today’s Solution To Loss Of Bite – In the following explanation, I’ll try to make this simple although the calculations used to show the process are somewhat complicated. I did those for you and I do not expect you to have to do them yourself. Just try to understand the process and build your rear suspension system in a way that will maximize rear bite not only with bumps setups, but also with soft conventional setups.

First off we need to understand what is happening in the rear of the car when we go through the turns, which is the first step in getting to accelerating off the turns. We have load transfer going on. For a typical asphalt late model car weighing 2800 pounds, we typically see, just in the rear, 560 or so pounds of load going from the left side of the car to the right side in a 1.8 G-force turn. The total vehicle load transfer is around 925 pounds.

Most rules allow 58 percent left side weight. That means that the left side tires carry 224 pounds more than the right side tires. But our rear load transfer is 550 pounds, so our LR tire ends up weighing much less than the RR tire.

Since more equally loaded tires have more traction, we need to get more weight onto the LR tire so that the rear tires will be more equally loaded, and we can do that. We just need to fool the car into thinking it has the higher spring rates it needs through the turns, and the softer rates it needs to get off the turns. It’s tricky, but it can be done.

Understanding Force Verses Spring Rate – Here is the beginning of understanding how to get what we need. First off, we need to get through the turns well. It’s not like the 1990’s where we accept a sacrifice; we don’t in today’s racing.

The stiffer RR spring rate serves to provide more force than the LR spring rate to reduce roll and make the rear stiffer to match the front stiffness of the bump setups. Force is the work a spring does when it is compressed.

If we think about using a greater force number in the RR corner to get the rear roll stiffness we need through the turns instead of a stiffer spring rate, we are then on the right track. By using this concept, we could ultimately be using 1990’s technology in the 2017’s.

Every spring will generate a certain force when compressed. A stiffer spring will generate a given force in less travel than a softer rate spring. If we compress a softer spring a greater amount, we can generate the same force as the stiffer spring.

How The Typical Stiff RR Spring Works – Let’s say we are running a pair of rear springs that are: LR = 150ppi and RR = 250ppi (ppi means Pounds Per Inch). If the RR spring was compressing 2.35 inches at normal ride height, then it is producing 588 pounds of force, sufficient to hold up the RR corner in a typical late model at 2800 pounds total with 58% left side weight and 50% rear weight.

Let’s assume this RR spring traveled 3.65 inches more through the turns. That is total of 6 inches of spring compression. If the spring rate is 250ppi and we travel 6.0 inches, the force needed to get the car through the turns in a balanced state is 1,500 pounds. Hold that thought.

Spring Pre-loading Technique – There is a technique called spring pre-load whereby we can pre-load a spring on a shock body to a certain force number. Then we can use a softer rate of spring and pre-load it a certain amount to achieve the desired force amount that will balance the car through the turns.

If we do this right, we can install a softer RR spring than before to gain loading on the LR tire off the turns. Then the two rear tires will be more equally loaded and provide more bite. Let’s see if that is possible.

For our example, let’s choose a 200ppi spring for the RR corner. In order for this spring to generate 1,500 pounds of force, it would have to travel 7.5 inches, something not possible. If we want it to travel more than the amount of the original 250ppi spring we could pre-load it by 600 pounds and then when it gained the additional 900 pounds of force needed at mid-turn, it would have then traveled an additional 4.5 inches. That is 0.85 inches more than the 250ppi spring would have traveled.

It is important to say here that you need special equipment to safely pre-load a spring with that much force. And I’m not saying that this is the amount of pre-load you will need. There are many possibilities. It’s just that with pre-load, we have the tools, using a force machine, to play with spring rates to better gain loading on the LR tire to become more equally loaded in the rear for more traction.

Pre-load Plus Bump – So, in the above example, we see where we can utilize a softer RR spring rate to achieve the same desired force at mid-turn. We are now balanced like before we pre-loaded the RR spring, but have a softer spring rate installed.

What we desired all along is more bite off the corners and we have only made it through the mid-turn section. We can now look at the acceleration phase of the turns to see where we can gain loading on the LR tire.

If the RR spring is something softer than when we did not pre-load, that corner as well as the whole rear of the car will travel more as the car transfers load from front to rear upon acceleration. In order to take the next step in gaining bite, we need for the rear to travel down, or squat, somewhat. We may want to reduce our Anti-squat by reducing the angle in the third link.

When we have made the car squat, we can use that motion to gain loading on the LR tire by installing a bump stop or spring in the LR shock. This is the rest of the story I am telling here but not necessarily exactly the way all racers are doing it.

If we have arranged the spring rates in the rear to less of a spring split, say going from a 150 LR spring and a 250ppi RR spring with a 150ppi spring split, to a 200ppi RR spring and a 50ppi spring split, we have made quite a difference in how the car will exit the turns under acceleration. But we still have a stiffer RR spring rate and what we need is a reverse spring split to gain loading on the LR tire.

If we install a bump at the LR corner, be it a stop or spring, we can gain spring rate at that corner while the car is accelerating off the turns. But we need to time the contact onto the bump just right or it will ruin our mid-turn handling.

If at mid-turn we are off the bump on the LR by ¼ inch and then the car squats ½ inch during initial acceleration off the turns, we gain spring rate and the force of whatever the bump spring rate is per inch divided by four. If our bump were rated at 400ppi, then we would gain 400ppi of spring rate and a total of 100 pounds of force.

Load Difference For Rear Tires – Let’s see what we need for load gain to really make equally loaded rear tires. For our example car with the numbers presented above, we have a 2800 pound car with 200 pounds of un-sprung front weight and 300 pounds of un-sprung rear weight. Typically we see 58% left side weight, 50-50% front to rear weight distribution, running a 10 degree banked track generating 1.8 G-force.

With a Center of Gravity height of 15.0 and a track width of 66 inches, this car will transfer a total of 564 pounds of rear weight from left to right through the turns. This represents a change in cross weight from whatever you run to a number closer to creating equally loaded rear tires. Gaining LR loading to achieve a number that will equal out the rear tire loadings may or may not be possible, but the closer you can get to that number, the more bite you will have.

Any force you can add to the LR tire will be added to the RF corner and that will increase the cross weight loading. Any force you can add to the LR corner will take force or loading off the RR tire as well as the LF tire. You move towards the higher cross weight needed to offset the weight transfer caused by the lateral G-forces and that makes the rear tires more equally loaded.

Which Route Will You Take – There are many ways to go about this and combinations of pre-load, spring split and LR bump rate and timing that will get the job done are many. We are not in any way or form telling you exactly how to do this and what numbers to use. Every car is different in weight distribution, motion ratios, front spring/bump stiffness, sway bar stiffness, rear spring rates, etc.

So, if we were to define numbers to use, some of you might benefit and some would be out to lunch so to speak. Take what we presented as a tool for thought and try to understand the concepts we explained. Apply those concepts to your car and experiment until you have gained bite while not hurting your mid-turn performance. Remember that any loading you can gain on the LR tire will help your bite off the corners.