[anonymouse20910]

All,

I'm preparing to install an eibach pro kit tomorrow on my e90. I read the instructions, and it gives "Bump-stop Trimming Instructions", which instruct me to trim 25mm from the bottom of the rear bump stops.

I didn't realize this was something I had to do.

Does anybody have experience with this? Is this necessary?

Thanks in advance.

[scollins]

The problem is when you lower the car, you are also reducing the amount of suspension travel. Let's say you 50mm of suspension travel right now (just a made up figure...) and you drop the car by 25mm, you now have only 25mm of suspension travel before you hit the bump stops. Trimming 25mm off the bumpstops will regain most of that suspension travel. Those bump stops are there to keep the shocks from bottoming out as well.

At the same time however, the suspension geometry was designed to work best in a certain range of parameters. Lowering your car changes those parameters, and trimming the bump stops may cause the suspension to operate in a range that it wasn't designed for, and could cause potential problems.

I would assume that since the instructions are saying to trim the bumpstops, that amount of trimming is to regain some of the suspension travel and still protect the shocks from bottoming out. I would also assume that Eibach did their homework on the trim amount and tested the suspension to work in that increased range of upward motion. But you know what happens when you ASSume something....

[anonymouse20910]

Originally Posted by scollins

The problem is when you lower the car, you are also reducing the amount of suspension travel. Let's say you 50mm of suspension travel right now (just a made up figure...) and you drop the car by 25mm, you now have only 25mm of suspension travel before you hit the bump stops. Trimming 25mm off the bumpstops will regain most of that suspension travel. Those bump stops are there to keep the shocks from bottoming out as well.

At the same time however, the suspension geometry was designed to work best in a certain range of parameters. Lowering your car changes those parameters, and trimming the bump stops may cause the suspension to operate in a range that it wasn't designed for, and could cause potential problems.

I would assume that since the instructions are saying to trim the bumpstops, that amount of trimming is to regain some of the suspension travel and still protect the shocks from bottoming out. I would also assume that Eibach did their homework on the trim amount and tested the suspension to work in that increased range of upward motion. But you know what happens when you ASSume something....

Thanks for the knowledge. I really didn't understand the suspension set up of the BMW compared to my last 2 cars (subarus, one with inverted struts). Now I understand why the bump stops need to be cut. If I don't cut them, I would bottom out a lot sooner. BTW, I tested the eibach's on a few curvy roads today and the improvement in handling was noticeable, even with stock sport shocks. Good enough for me I guess.

[Ray]

This is exactly right.

The standard bump stop length simply minimizes the shaft travel

Originally Posted by scollins

The problem is when you lower the car, you are also reducing the amount of suspension travel. Let's say you 50mm of suspension travel right now (just a made up figure...) and you drop the car by 25mm, you now have only 25mm of suspension travel before you hit the bump stops. Trimming 25mm off the bumpstops will regain most of that suspension travel. Those bump stops are there to keep the shocks from bottoming out as well.

At the same time however, the suspension geometry was designed to work best in a certain range of parameters. Lowering your car changes those parameters, and trimming the bump stops may cause the suspension to operate in a range that it wasn't designed for, and could cause potential problems.

I would assume that since the instructions are saying to trim the bumpstops, that amount of trimming is to regain some of the suspension travel and still protect the shocks from bottoming out. I would also assume that Eibach did their homework on the trim amount and tested the suspension to work in that increased range of upward motion. But you know what happens when you ASSume something....

[iheartsole]

Originally Posted by scollins

The problem is when you lower the car, you are also reducing the amount of suspension travel. Let's say you 50mm of suspension travel right now (just a made up figure...) and you drop the car by 25mm, you now have only 25mm of suspension travel before you hit the bump stops. Trimming 25mm off the bumpstops will regain most of that suspension travel. Those bump stops are there to keep the shocks from bottoming out as well.

At the same time however, the suspension geometry was designed to work best in a certain range of parameters. Lowering your car changes those parameters, and trimming the bump stops may cause the suspension to operate in a range that it wasn't designed for, and could cause potential problems.

I would assume that since the instructions are saying to trim the bumpstops, that amount of trimming is to regain some of the suspension travel and still protect the shocks from bottoming out. I would also assume that Eibach did their homework on the trim amount and tested the suspension to work in that increased range of upward motion. But you know what happens when you ASSume something....

+2

[jeftrz]

OK, so I just installed my pro-kit and had the same question about whether or not to trim the rear bump-stops. I saw the "yes" above but...

I haven't done it yet because it appears that you have to take the shock out to get to the bottom or the bump stop. Either that, or use a Dremel tool to cut it off while still attached. Is there an easier way?

The car rides great without trimming, i.e. the rears don't seem to bottom out on normal crappy Chicago roads with pot-holes.
Should I do it anyway?

Do I care?

Part of me says if it ain't broke, don't "fix" it.

I'd like a second, third, etc...opinion

JT2

[mike3000fl]

i left mine in at first to see how often i bottomed out, i had an annoying squeek in the rear over speed bumps though so i went ahead and trimmed my bumpstops. removed wheel, cut around bump stop with an exacto knife half way up and then made a long slit so i could remove it from the shaft. car does not squeek now or bottom out as often. now to roll fenders....

be careful not to scratch up the shaft, it has to stay smooth so it seals correctly.