April 20, 2023 3 min read
Most cars these days are 15 to 20mm from the bump stops before you and your Wendy’s large fries and double bacon cheeseburger get inside. Almost universally, they’re used as an aid for the main springs and in play during most handling maneuvers. Your car was designed to be “bump stop active.”
You might not notice when you get into them…and that’s the point. So why are they there? And what about when you lower the car with lowering springs?
Put down that fast food and read on.
Bump stops are really just a type of spring. They act in parallel to your main springs, which means you add their spring rate to the rate of your main spring. Bump stops can be mostly linear when you start to compress them but will get much more progressive after that. For example, they might have a spring rate of around 100 lbs/in until they’re halfway compressed, after which their spring rate will ramp up.
So they add a little rate when you start to get into them and get much stiffer as your suspension uses more of its travel.
This stock BRZ above is using its bump stops right now in this corner.
That progressive ramp up to a stiff rate helps keep you from using up all your travel. And an interesting thing about bump stops is that they have a lot of hysteresis, meaning that their rate is stiffer in compression than extension. So unlike a coil spring, they squish a little differently than they expand back. This is important when looking at how they work with a shock’s rebound damping.
You get a little more support in corners, reduce bottoming the shock, and don’t need to run as much rebound damping as you would with stiffer main springs.
Bump stops can also provide platform for racecars with lots of aero. Downforce pushes the car down at high speeds, so a bump stop can be used to add additional support. At low speeds, the main springs are doing the work.
Bump springs, essentially a little coil spring where a bump stop would be, are often used for this and to provide a two stage linear spring set up. Bump springs do behave differently in that they’re usually linear and don’t have much hysteresis.
Right, and if you’re ½ inch or less from your bump stops to start and then you lower your car an inch? Well, instead of that softer initial rate of the bumpstop, starting in the stiffer part can cause serious problems. A lack of travel and the very stiff end of a bump stop leads to a lack of grip and poor ride control.
So can you just cut them? This gives you more room before engagement, but cutting a bump stop makes it stiffer and changes the ramp in stiffness. You also only have so much travel and you do need to protect your dampers.
This graph above shows force vs. travel for 3 bumpstops of the same material but different lengths. At 0.5 inches into each stop, the shortest one (red) is providing the most force.
Cutting a small part off a bump stop might be okay, but a replacement bump stop designed to work with the amount of travel you have and your main springs is usually the way to go. There are many options out there and some springs (such as RCE springs) include new bump stops.
For a car with aftermarket coilovers, it’s crucial to set ride height, bump/droop ratio, and bump stop engagement correctly. This is especially true for dual height adjustable coilovers since it’s easy to get it all wrong there.