Bumps – One Or Two?
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.
The same is true for asphalt cars. A car I have worked with can run either 52.3 or 58.0 percent and still be neutral in handling. For both types of cars, an even lower cross might be possible going from 52.3 to 46.6 or so.
Now let’s get back to our single bump car. If we are running dirt and bumping the RF corner, as we enter the corner, we increase the RF and LR corner loading and raise the cross weight. But what if we raise it the same as in our example to go to the next cross weight range? The handling might not change at all. Here is why that is possible.
I did some calculations and for a typical asphalt car, on braking we transfer about 310 pounds of weight from the rear to the front. If the LF bump corner were to take 75% of that load, that is 232 lbs., or 8.3 percent of cross weight based on a 2800 pound car.
If we are running a cross weight of say, 58%, then our cross on entry would be 58 minus 8.3 = 49.7%. If for this car the lower range were 52.3%, then we would definitely be loosening our car on entry, which might be the desired result. We are just in a different cross weight range and the percent of change varies with how hard we brake.
For dirt, it is the opposite scenario. We would be gaining 205 pounds at the front and if 75% of that were added to the RF, then that would represent a 5.9% increase in cross weight percent. At the low range of 50% cross weight (52lb. LR weight), we could be going to 55.9% cross weight, or 210 pounds of LR weight. That again puts us in a different cross weight range only higher in this example. Again, the change varies as the braking force changes.
At any rate, using one bump changes both our cross weight range and percent and depending on how hard we brake for how long, that percent change is different meaning our handling could be in a constant state of change.
Using Two Bumps – Now let’s see what happens when we have two bumps. With both front corners having bumps, it is like having more equal but much stiffer springs on the front. As we brake into the corner, load transfers onto the front end and is taken by the two corners. If the spring rates of the front corners are similar, then the load that is transferred will be more equally distributed.
If our goal is to affect our entry handling, then using two bumps won’t necessarily accomplish that goal. Because the load transfer is more equal, our cross weight percent stays the same as it was before entry braking.
If we need to tune entry handling, such as a car that is tight in, we need to find other ways to do that. And we have discussed many different ways to cause the car to be either tighter or looser into the corner while it is in transition.
We could use different rates of bump springs or bump stops for each corner to facilitate a change in handling, a stiffer RF to tighten the car or a stiffer LF to loosen the car. Those changes would cause much less change in cross weight percent and probably be more predictable than if we were running only one bump.
Mid-turn Handling One Verses Two – We need to get into the mid-turn handling aspect of this because there will be an influence on handling between the two scenarios. The changes in cross weight percent and the roll stiffness in the front will be different between the two scenarios.
The cross weight will change at mid-turn when using only one bump. For the dirt example, the car will be tighter and adjustments must be made statically so that it is not too tight. This is caused by load transfer laterally from turning left and more of that load ends up on the RF verses the RR tire.
The stiffer RF corner will also make the front more roll resistant therefore creating a tight setup condition. The large spring split, very stiff RF verses much softer LF, equals more roll resistance at the front. So, changes to the rear spring rates and/or rear J-bar height may be needed.
In our asphalt situation, the stiffer LF corner of the car using one bump causes the opposite effect as the dirt car description. The stiffer LF loosens the car through the middle by adding load to the LF corner due to the stiffer spring rate. This is most evident on banked tracks whereas on flatter tracks it might not be as much of an influence.
The large spring split of a stiffer LF verses the much softer RF spring rate promotes roll and lessens roll resistance. This car would be looser through the mid-turn segment and adjustments must be made to compensate.
We might lower the panhard bar, soften the RR spring, etc. in order to improve the mid-turn handling while using the single bump on asphalt. At any rate, changes must be made to compensate for the effect.
As for using two bumps, none of the above changes in weight distribution or roll resistance happens when using two bumps on the front. We don’t have the load distribution or cross weight change going on and we don’t have a change in the roll stiffness happening either.
So, Why Run Either – Now that we have examined the effects of the two situations, let’s go over the Pro’s and Con’s of each one and see which might be a better choice.
As for the single front bump setup direction, we can see there will be a lot of changes in weight distribution and roll stiffness going on. It is possible that you could utilize these changes to your advantage to improve corner entry and possibly corner exit.
To do that, you would need to do a lot of experimentation and/or understand exactly what is happening in your situation with your cars design. The teams that have the resources to do this testing might find an advantage, or not. I just cannot say right now with what I know.
For those who like to experiment, have at it. But remember that when you get this all figured out for one track, going to a different track that is designed differently will change your results. You would need to tune the car differently.
When using two bumps on the front, we see where there is much more consistency in the load distribution and roll stiffness. This consistency will not enhance transitional handling like improving turn entry or exit. You’ll need to find other ways to accomplish those if that is your goal. But the transition from track to track will be much more predictable with less overall setup changes needed.
So there you have it, information about running two bumps verses one and why. Only you can determine if either would suit your situation. There are teams doing well with both of these approaches, so we won’t necessarily push you in either direction. As for me, having thought this out just now, I would still go with simplicity and two bumps looks like it is simpler and easier to manage. But that is just my choice.
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