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      12-15-2020, 10:58 AM   #14
N8N
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Drives: 2009 E92 335i 6MT
Join Date: Sep 2012
Location: Sterling, VA

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Quote:
Originally Posted by Tambohamilton View Post
Yes, length and rate are the only difference between sport and n/s bump stops. They will both physically fit with sport or n/s struts/shocks/etc.

Generally, you should use shorter and/or softer bump stops if you want comfort. I know that sounds counterintuitive, but it's true (within reason). BMW actually calls the bump stops 'auxiliary springs', because they're there to ramp up the spring rate fast when the suspension is compressed beyond ride height. The bump stops are actually engaged at ride height in a stock setup. This is to give a degree of compliance and a degree of body roll resistance...you could argue that the stock setup does neither sufficiently well! So if you want to increase comfort (at the expense of some body control), you should soften or shorten the bump stops. Any stock bump stop (eg E36 M3 fronts) will perform the traditional duty of a bump stop sufficiently well. However, you should be very careful with bump stop tuning, because they can have a huge effect on chassis balance (understeer/oversteer characteristic). What's the correct answer? How long is a piece of string?
I have heard previously of relying on bump stops to provide progressive springing. I did not realize, if you are correct (I will look once the last corner is back on the ground - I did three corners over the weekend and did not finish yesterday as it was raining all day), although given what I'm seeing I have no reason to disbelieve you, that the bump stops are actually engaged at normal ride height. However, that would explain why everyone always asked me if I'd lowered my car (I hadn't; the one and only suspension mod was M3 strut rods when the originals wore out) and definitely at some point in the vaguely recent past the suspension on my car went from feeling a little underdamped to completely crashy and super loose. That's what prompted this exercise... well it turns out that only the driver's side front strut had anything resembling a bump stop on it. I'm guessing that's the only corner that had any damping left as well; I haven't taken the passenger side strut off yet but it looks the worst, with no bump stop at all, not even a little ring of busted foam left, and oil running down the housing. Both rears had minimal damping as well when I got them off (none had any residual gas pressure, or at least not enough to overcome seal drag), and both rear bump stops were 80% MIA.

I'm actually looking forward to driving this once I get it back together; hopefully this will wind the clock back a bit in terms of handling.

Things that I've learned:

1) BMW likes making you use All The Tools. I actually had to use Torx (cargo hooks in the trunk), E-Torx (rear lower shock mounts) and Allen sockets (shock shafts, upper and lower for rear and upper for front.)

2) You need to compress the front springs by the upper spring seat. There is no other way to get them off the struts.

3) related to above, I needed an impact gun and vice-grips (that's mole grips to you limeys) to get the upper front nut off.

4) Whoever told me to buy a pass through socket set was on point. I don't think I could have done this job without it. Even if I'd used stock Sachs struts, the old and new BMW upper nuts (I bought the whole installation kit) were different sizes and the Koni nut was a different size yet. A long length Allen bit socket would have been good for installing the strut bearings, although with the flex handle pass through ratchet I was able to get it done without.

5) the Koni front struts are way longer extended than the stock E92 (in the US, that automatically means sport suspension) front struts (more droop) so Sunday I found myself in a bit of a bind as I had not removed the tie rod end from the knuckle and suddenly my strut assembly was about 3/4" too long to fit under the fender lip. WHOOPS. TRE nut laughed at my M18 Fuel impact driver. No way to remove using hand tools with no strut attached. Ended up re-compressing the spring and then having the fun exercise of removing the spring compressors with the strut half installed and under the fender. Good times good times. At least I didn't have to re-install the old front strut and then disassemble it again to transfer the spring over.

6) related to 2) and 5) above, my old Harbor Freight spring compressors did not work. I had to use the loaners from Advance which appear to be the Powerbuilt branded version of OEMTOOLS 25553.