bumpstops, rallycross, and me

I am piecing together coilovers for my 1986 BMW 5 series rallycross car. The last bit I need help on is bumpstops. My current set of struts have internal bump stops and landing with a thud off of a hill or big rut isn't fun. I am not sure how much the stiffer springs will stop the car from riding on the bump stops. I have seen E28s with 600lb springs and I will be running a littler over half of that.

The used coilovers I bought did not come with any bump stops. So the obvious choice was to just get a set of ground control bump stops along with some other parts I need. I guessed a longer bump stop will allow a more progressive increasing spring rate. Then, I looked for some and got lost among the million choices on the internet.

Here are a few questions.

1. Do I want my bump stops to be on the harder end or the softer end? Put another way, the graphs on RE Suspension's website look pretty cool but I don't know which curve is best for my application.
2. Once I have a bump stop long enough to stop coil binding or shock damage, is longer better?
3. Does it make sense to get the ground control bump stops and a couple of puck style bump stops for more tunability?
4. My front struts have a 7/8" shaft and rear shocks have a 5/8" shaft. Most bump stops are specced for the outside diameter only. The ones that do have an inner diameter are generally closer to a 5/8" diameter. Can I drill out the hole to fit on my larger struts or does that compromise the foam?
5. Bump springs are the coolest things I have ever seen. Could someone give me $500?

1. For rallycross where you'll be using them a lot for soaking up huge bumps and landings, you'll want hard bump stops.

2. Depends on how much you want to ride on the bump stops. For rallycross you want lots of travel so you should probably stick to stock length or maybe even go shorter.

3. Depends on how much you want to spend (and how competitive you're trying to be )

4. You can drill them out but it will effectively make them softer - kind of like shaving down the diameter of a spring coil.

5. So you haven't seen hydraulic bump stops yet?

When I was playing with them my general feeling was longer and softer is better. You can seen on the RE charts that the rate ramps up pretty quickly for the short/hard stops. The idea is to smooth the transition between just the spring and the sudden stop that is coil/suspension bind. You can also make little pucks that snap on the shock shaft that will let you mess with the engagement point of the bump stops. I had fun tuning with those. Also remember that stacked bump spring rates work with the weird math of stacked springs. Once you go with more than one the world gets confusing.
I liked running a car on softer springs with long bump stops. I felt like it gave me better control over the small stuff while still keeping me from pounding my car into pieces when things rutted up.

GameboyRMH wrote: So you haven't seen hydraulic bump stops yet?

I've seen pneumatic ones, which are absolutely awesome, but I'm not sure that I've seen hydraulic based ones as of yet (I'm not doubting that they exist though)

I just like the idea of springs inside of my springs.

mazduece, I read through your post a few years ago with the removable spacers. Interesting stuff.

Some quick math based on re suspensions graphs. A 3" soft foam bump stop compressed down to 1.15" exerts the same amount of force as a 2.27" soft foam bump stop compressed down to 0.77". The 2.27" bump stop gives an extra 3/4" of travel without affecting the spring rate and an extra ~3/8" overall travel before fully compressing. Conversely, I would assume a hard landing would be somewhat softer with the longer bump stop because it is doing it's thing longer.

There are different opinions on the implementation of bumpstops. Some people want to use almost the full range of suspension as God and the engineers designed it and only use the bumps to avoid metal to metal. Others want to use them as an integrated part of the travel acting more like a progressive spring later in travel. I'm not sure either one is 'right', just different, and how your car behaves might dictate your path. I would sit down and figure out how much up travel you have and think about how much of your travel you are affecting with the bumpstop, what your spring rates are, and sort of guess from there.

^Right. I'm recommending the first setup for rallycross to maximize travel and handle huge bumps, the second setup makes sense on a track-driven car that has to run soft springs for some reason, but is probably not ideal for rallycross.

See, I prefer a soft car for rallycross. I really dislike stiff cars that skate over hard pack with bumps. Of course a lot of the depends on the surface you run on and whether you have an open diff or not. An LSD equipped car can run a bit stiffer, but I still like wheels on the ground. I think a lot of it is subjective.

I agree with a softly sprung car for rallycross, the 323 has 360lb in the front and 160lb in the rear. Use all the travel available and don't run an overvalued strut setup, allow the springs and struts to work in unison to soak up the surface keeps the chassis stable and power being put down. Stiff might prevent bottoming out, but it also jars the chassis and creates wheelspin when attempting to lay down power.