Quick bumpstop question (BMW content)

Preparing to swap my 128i over from Bilstein to Koni. The Bilsteins have internal bumpstops, so I bought new ones for the Konis that do not. The issue is that the car came with the Sport package suspension, and now is even lower on Eibachs (about 25mm), and the bumpstops I was able to get are for the non-Sport suspension. I'm guessing that if I install these as they are the car will basically be riding on them, which is of course double plus ungood, so the plan is to cut them. The questions are 1) how much should I remove (I can't find the spec lengths anywhere), and 2) does it matter if I just lop off the bottom, or should I cut it out of the middle (i.e., are the foam densities in modern bumpstops layered or uniform)?

Koni sells bump stops. 

Bump stops in shocks act like a helper spring. The ones you buy from Koni are somewhat cone shaped. So, when they first start compressing, you get little resistance, which builds to more resistance the more they are compressed.  Smith said to "tune" your suspension with them.  So, if you need a little less spring at almost full compression, trim them accordingly.

Do you have any idea how much bump travel is left with the car resting on the springs? Also, you say you have Eibach springs... ProKit? You may want to email Eibach and ask what they recommend. My latest Eibach spring purchase included four new bump stops to match the springs.

I emailed Koni and specifically asked about bumpstops - this is a direct quote from the response:

All of the factory suspension components will swap right over to the new KONI struts and shocks. During installation you will need to either reuse the originals or replace the upper strut mounts, upper and lower shock mounts, bumpstops, and dust covers.

The bumpstops I got are Sachs for the front and Rein for the rear (finding these outside of dealer channels was surprisingly difficult). The Sachs units are cone-shaped; if you look at this page you'll see the difference in the sport and non-sport pieces. Unfortunately, I could not find the sport kit anywhere in the US. The Rein rear units are integral to the dust shields, so I'll have to look at them more closely to figure out their shape.

I did not think to check with Eibach, but their website shows the Pro-Kit package for this application as springs only, and the installation instructions make no mention of bumpstops.

I think the reason Koni's response looks to be a bit less than helpful is because it's difficult to account for every single customer's choice of spring, and I'm sure they have canned responses so they don't drown their engineers in questions.  The reason I recommend asking Eibach is because it's more likely that the person monitoring their email will have access to this information as the length and stiffness of what bumpstops you should use is more dependent on spring rate and ride height than what dampers you're using.

Also, if you know how much bump travel you have while the car is at it's resting ride height you can better make your own educated guess. With that information you can even go looking for different length and stiffness bumpstops to tailor the car's handling to your liking. Of course there are different aftermarket bumpstops from a variety of companies, but you can also find what you want as equipped on different cars from the factory.

Just put the dampers in with no springs and flex the suspension up (with the tire on) until it looks like something is going to hit, and make your bumpstops start to engage 1-1.5" (shaft travel) before that.  That's what i'd do, anyway. 

I recently 'tuned in' the bumpstops on front of my lowered conversion van. I had someone bounce up and down on the door sill while i eyeballed it the first time, then i drove it, went back and messed with it, drove it, went back and messed with it, and i think at that point it was right where i wanted it.  That's a bit more tedious on struts, granted, but you probably don't necessarily have to remove the spring to trim the bumpstop if you REALLY don't want to. 

I believe the popular thing for lowered E8x/E9x, is to run E36 M3 front bump stops and E85 Z4M rear bump stops.

Driven5 said:

I believe the popular thing for lowered E8x/E9x, is to run E36 M3 front bump stops and E85 Z4M rear bump stops.

I found this last night after expanding my search to include the E9x platform; I saw no mention of it in the E8x stuff I looked at. Going to see about sourcing these, as it seems it might be better than chopping up what I have.

EDIT: Well, it seems better except that it ends up being $70 worth of foam, because nobody seems to offer these except for BMW. I don't know if I can live with myself spending that kind of money on four bits of molded foam....

Rock Auto has rear aftermarket bumpstops for $4.82 each. FCP Euro has fronts for $9.09. I think bumpstops act like springs and the shape impacts how they behave. This is the Uro bumpstop I mentioned from rock auto (guessed your 128i was a '08). I don't see any reason why you couldn't trim some of the right hand side off as suggested by others. Trimming the bumpstop would probably be not dissimilar to cutting springs and increase the rate.  If you're running a higher spring rate this would probably roughly approximate the changes you'd want.

In reply to Snrub :

Those are the stock bumpstops, not the M3/Z4M pieces Driven5 mentioned. I have new stock ones, but if you look at the upgrade parts (31332225377 front, 33507836826 rear - FCP Euro has pics in the listings), they are a lot shorter, and those are only available as BMW factory parts, with prices to match. At this point I think I'm just going to cut the stock ones to approximately the length of the upgrade parts and hope for the best.

Have you considered starting with 'non-genuine' OE stock E90 sport suspension bumpstops? Still cheap, but shorter and would require less cutting, which should help maintain more of their designed progressive characteristics. Also, it looks like the 335i xDrive front bump stops may also be a bit shorter yet than those on the RWD sport suspension. Not E36 M3 short, but not E36M3 priced either.