Bump Stops, Bump Travel, and Lowering Springs
The modern bump stop is more properly called a jounce bumper. Made from urethane and cut to varying sizes, today's bump stops are an integral part of most factory suspensions. Knowing their effects on the suspension is as important- if not more so- than that of your main springs. This page explores their effects when installing springs that lower a production car.
Below is a stock strut from a 2005 Impreza WRX STI:
The strut has about 5.75 inches of total travel, but it also has a 2.25 inch bump stop (also called a jounce bumper) installed. This bump stop is very stiff to prevent the car from damaging itself under extreme stress, but this also makes the ride uncomfortable and a bit unpredictable if the bump stops engage in regular driving.
With only 3.5 inches of travel before the damper engages the bump stop, it's important to see how much of that travel is in bump travel and how much is in droop travel. Upon measuring, the Subaru Impreza WRX STI was found to have nearly 3.5 inches droop travel on the front wheels, meaning the car is resting just above its bump stops at its ride height. Any bump the car hits- or even the weight of the driver- will engage the bump stops.
As it turns out, this is quite common- many popular cars actually ride on their bump stops at their factory ride height. Manufacturers use long bump stops to create a very progressive spring rate. This allows the manufacturer to use much softer main springs for better overall ride quality. Usually, the bump stop itself is very progressive- starting out soft but increasing in resistance until it won't compress anymore.
Because bump stops are designed to be very progressive, the behavior of a car drastically changes if a person replaces the factory springs with springs that lower the vehicle's ride height. This is because lowering springs don't just put the car onto the bump stops at ride height- they put the bump stops past their initial soft phase.
Below is the measured resistance from the bump stop pictured above:
You can see these particular bump stops offer a somewhat linear resistance of 300lbs in the first inch. There is no very soft first progression to be found here- the main springs are 224lbs per inch, so the bump stops are roughly 33% stiffer than the main springs! The total spring rate here is:
224lb/in + 300lb/in = 524lb/in
Beyond an inch, though, the jounce bumper quickly becomes four times stiffer still! This is the range of the bump stop designed to prevent the car from damaging itself under extreme stress. Most bump stops become extremely stiff by half of their full height and reach their terminal compression (they won't compress further with any amount of load) at 75% compression. The particular bump stop measured follows these trends.
So, what do lowering springs do? On this car, most lowering springs lower the car about an inch. This means the bump stop has also compressed nearly an inch, and it is now supporting 300lbs per wheel of the vehicle's weight. Furthermore, the suspension will not compress further except with extreme forces since the spring rate in the bump direction is around 1600lb/in just from the bump stop alone. Below is a graph illustrating how the spring rate stiffens significantly (the slope of the line gets steeper):
What does such a car feel like? It'll depend on the road, tires, and more. When the car hits a bump, the suspension will not be able to absorb the bump by compression. Instead, large forces will be transmitted to both the tires and the chassis, putting stress on the tire and accelerating the chassis upwards into its droop range. The chassis has lots of travel in that direction (and the spring rate gets much softer), so the car will once again feel soft after a harsh initial impact. The car will then settle down onto its bump stops again until the next bump. This extra chassis movement means a change in the car's center of gravity and inertia, which can create unpredictably and instability that the driver must cope with.
Furthermore, because the effective spring rate will have risen, the vehicle's dampers may now be too soft. This, along with the tendency of bumps to be transmitted to the chassis, can create a feeling of bounciness. Because the chassis is heavy, movement of the chassis contains a lot of potential energy that must be dissipated by the damper. The extra movement means your dampers are working longer, which causes them to generate more heat. This heat generally leads to wear.
So, there are three huge drawbacks to this design. All are present to some degree on a factory car that rides on its bump stops, but all get much worse as bump travel is reduced via lowering springs.
- Harsh initial impact on bumps, even small ones
- Ride height raises over bumps, creating unpredictability
- Potential for faster wear on the chassis, bushings, tires, and dampers