When setting up rear-wheel-drive cars, I sometimes hear people say that they use bump stops at the rear to gain traction.
I used to think, “If the rear hits the bump stops and that increases the rear suspension’s effective spring rate, wouldn’t that actually reduce rear tire grip?” I couldn’t really make sense of it.
But I think about it a little differently now, so I’d like to write about that today.
Why does increasing the rear spring rate reduce rear tire grip in the first place?
There is only one body, so when body roll tilts the car, the inside and outside tires at both the front and rear compress and extend at the same time.
When the rear spring rate is increased, the roll stiffness distribution shifts toward the rear, which means the rear takes a larger share of the lateral load transfer. As a result, the load lost from the inside rear tire and the load added to the outside rear tire both become larger.
Tires have a characteristic where the increase in grip becomes more gradual as the load increases. Because of this, the extra grip gained by the outside tire can’t make up for the grip lost by the inside tire, so the total grip from the two tires ends up decreasing.
That’s the basic idea.
But when a rear bump stop engages, the increase in effective spring rate only happens on the outside, right?
The inside stays the same.
I’ve never actually measured the load on each tire, but I suspect that when the rear bump stop is compressed, the effect is something like this: the load on the outside front tire decreases, and that load is transferred to the outside rear tire.
Lateral load transfer basically works by reducing the load on the inside tires and increasing the load on the outside tires. So regardless of how the balance between the front and rear is distributed, the total amount of load lost on the inside must equal the total amount of load gained on the outside, excluding longitudinal load transfer and things like aerodynamic effects.
When the outside rear bump stop compresses on corner exit and increases the effective spring rate, I think it’s safe to say that the vertical load on that tire increases more than usual. So if we ask ourselves, “Where did that extra load come from?” then it has to have been transferred from the outside front tire.
(Strictly speaking, the inside tire load is also affected to some extent as the rear roll center moves toward the area near the contact patch of the outside rear tire.)
The result is that the load on the inside rear tire doesn’t change very much, while the load on the outside rear tire increases. This means the total grip from the two rear tires increases.
If that’s what’s happening, then there’s no contradiction in saying that “adding bump stops to the rear improves traction.”
But this really depends on how much the bump stop is compressed.
For example, when the bump stop is fully compressed and the suspension can no longer compress any further even as the load increases, essentially becoming like a solid rod, the outside suspension can’t compress any further while the inside suspension tries to extend due to load transfer. The body roll motion effectively becomes centered around a point near the contact patch of the outside tire.
As the inside suspension extends and the body continues to roll, an angular difference develops between the stationary outside wheel and the ground. The only way to accommodate that angular difference is through deformation of the tire itself.
This causes the contact pressure to become concentrated toward the outside edge of the tire. Even though the vertical load on that individual tire is large, it can no longer generate enough grip. The end result is that the total grip from the two tires decreases.
This is what happens with some lowering springs that are driven with the bump stops almost constantly engaged because the ride height has been lowered too far.
And the edges of the tires wear heavily.
So if the bump stops are compressed too far, you won’t get the traction you wanted in the first place. To get good traction, you need to keep the setup at the point where the load on the outside tire is as large as possible without excessively concentrating the contact pressure toward the outside edge of the tire.
Also, if the effect of the bump stops is too strong, the outside front tire may not have enough load, causing understeer and slowing the car down.
Basically, you have to try different amounts of bump stop compression and find the sweet spot. Depending on the setup, it can actually have the opposite effect if you don’t.
Finding the right amount of compression is pretty difficult in a lot of ways, so setting up bump stops can be quite challenging.
If you’re only slightly compressing the tip of a soft urethane bump stop, you don’t have to think about it too deeply. But parts like that inevitably have to be fairly long, and on most lowered cars, I think there often just isn’t enough space for them.
So, once again, bump stop setup can be pretty difficult.
This applies to pretty much any part: it’s not enough to just install it and call it a day. Take a good look at the condition of the car and set everything up properly so the parts can actually do their job!