Circle Track|May 2017

Dirt and Asphalt Comparison of Bump Setups

In the world of bump stops, bump springs or air bumps, there are two ways to set these up, using one bump or two at the front. For now we’ll not talk about the rear as that is another story all together. In asphalt racing the one-bump crowd generally will use the left front as the bump corner and in dirt racing it is the right front that has the one bump. Is it generally better to use one or two bumps? Let’s investigate that.

Let’s take a look at the dynamics of the front end on a race car and what the differences between running one bump or two is. We’ll mostly cover entry into the corner, and some of mid-turn. Exit off the corners is much less affected and is a separate event.

I am going into this discussion without a bias as far as my thinking is concerned, but that said, I have always advocated running two bumps on the front end when working with teams. As we go along here, I will either convince myself that using one is best, or that two are best. Let’s see how it goes.

Using One Bump 

 The influence on the handling between using one bump or two is most evident on entry to the corners. At steady state mid-corner there is still an influence, but much less. On entry to the corners, we are letting off the throttle, then braking, then letting off the brake and then entering mid-turn. During all of that, weight is transferred onto the front corners and loading the front springs and bumps.

If we have one bump up front, much of that load will find its way onto the corner that has the bump. It has to because that corner has a much stiffer spring rate due to the bump spring rate added to the ride spring rate. So, the corner that has the bump will load up.

If it is the left front, as on asphalt, then the LF and RR corners will gain load. We cannot load one corner without affecting the other three corners. Loading the LF and RR will necessarily take load away from the RF and LR corners. There is only so much loading to go around. The car doesn’t gain or lose weight, it just gets redistributed.

So if the RF and LR corners lose load, the cross weight that those corners represent goes down. And here is where it gets tricky. One would think lowering the cross weight will loosen the car. That is not necessarily true.

The amount the car is “loosened up” depends on how much cross weight is taken out of the car. We know from studying the dynamics of a race car that there are ranges of cross weight that will still make the car neutral in handling. A dirt car for example can run 50-75 pounds of LR, or 200-250 pounds of LR and still be neutral in handling.

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