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      11-11-2018, 10:04 PM   #8
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ashiohsiao's Avatar

Drives: 2012 F20 118i
Join Date: Oct 2018
Location: Taiwan

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Bump Stops

Many people only see the bump stop as a part of the suspension that protects the damper from bottoming out, but in my understanding of Shaik's concepts, using my own words, I see the bump stop as :
1. A part that protects the damper from bottoming out (duh...)
2. A second-stage suspension spring if bump stop only comes into contact when pushing hard
3. A secondary spring that adds to the actual spring rate if the suspension is designed to be riding on the bump stop

Most people know that front and rear spring rates affect how a car turns, and you can:
- Use stiffer rear springs to reduce understeer / increase oversteer
- Use softer rear springs to increase understeer / reduce oversteer
- Use stiffer front springs to increase understeer / reduce oversteer
- Use softer front springs to reduce understeer / increase oversteer

Let's talk about Point #3 first, if the car is designed to ride on the bump stops, your net spring rate is actually: coil spring rate + bump stop rate = net rate

Therefore, for a car riding on bump stops, you can achieve cornering behavior tuning not just by changing the actual springs, but also the bump stops.

Assuming your car currently understeers, you could potentially reduce understeer by changing the front bump stops to a set of softer bump stops, which would then give you less net spring rate, the same effect as using a set of softer front springs.

Point #2 is similar, but has two stages, which is what most BMWs have. Using the F20 as an example, the car is a bit tail-happy (slight oversteer tendency) most of the time, but when cornering very hard, you would notice that the car becomes quite understeer-y.

This is because when cornering very hard, the springs are compressed to the point that bump stops are being engaged, and since BMW uses a stiffer and longer bump stops in the front than the rear, your net spring rate for the front now becomes much higher than the rear, which then results in an increase in understeer tendency.

This is the reason why Shaikh recommended me to cut off a portion of the front bump stops, to delay the engagement of the front bump stops and to reduce the net spring rate when bump stops are engaged. And true enough, I haven't run into any understeering so far since I trimmed my bump stops.

However, there are two things to keep in mind when doing this:
1. You need to know how much of the bump stops you need to keep to allow the bump stops to still be able to protect the dampers from bottoming out
2. You should cut off the thicker (stiffer) section of bump stops, to allow the softer portion to cushion the impact when bump stops are engaged to prevent a sudden shift of resisting force during compression, which causes discomfort

Shaikh has a series of videos explaining about bump stops:

And there's also a video of him trimming off front bump stops and adding packers to the rear on a Mini Cooper:

Last edited by ashiohsiao; 11-12-2018 at 02:27 AM.