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Bilstein B8 shock question (spring perch/groove)

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SORIANOTRON
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Bilstein B8 shock question (spring perch/groove)

Post by SORIANOTRON » Thu Apr 05, 2018 12:35 am

Hey guys,

I currently have a set of KYB GAS-A-JUST shocks w/some B&G springs, they ride and respond pretty well. However, the rears just sit a little to high for my liking, even with the springs.

So now I am looking to grab a pair of Billy B8's to even it out since they have the adjustable grooves for the spring perch...

My question is, depending on where you decide to set the perch it will adjust the ride height and stiffness...so does that mean if I set the perch to the lowest groove (towards the ground) the ride height will be at is lowest and stiffest?

Was thread digging all night but I couldn't find a definitive answer.

pics if you have em!

Appreciate yall

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Brucey
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Re: Bilstein B8 shock question (spring perch/groove)

Post by Brucey » Thu Apr 05, 2018 6:55 am

unless the springs are designed to work with Bilstein dampers there is no guarantee that they will fit or be the right length. The lowest perch height will give the lowest ride height but it does not change what goes on inside the shock in any way. It of course cannot change the stiffness of the spring.

However the Bilstein dampers are designed to work with the OEM bump stops and the OEM dust boots. You need those if you want your shock absorbers to work properly and/or last very long.

The OEM bump stops are available in two lengths ( fit standard car or M635CSi) and if you car is lowered you want the latter type. If the car is lowered a non-standard amount the bump stops need to be trimmed to a non standard length and new grooves put into them in order to locate the dust boots correctly.

The bump stops in this system are not like others, which are just there to stop the suspension from bottoming out, these bump stops operate whenever the suspension is slightly compressed, i.e. on the outside wheels round every corner that is taken briskly. They provide extra spring rate, extra roll stiffness and extra compression damping and are absolutely fundamental to the car handling correctly. They are the magic ingredient that can give you a car that doesn't rattle your fillings on the highway, yet goes round corners properly too.

In bilstein front inserts, there is a similar bump stop that is hidden inside the base of the unit. If you don't similarly rework those bump stops to suit (a non-standard) ride height you will again get rather sub-optimal handling and ride.

FWIW I have driven a fair number of cars with a modified suspension that does not use the OEM bump stops in the intended fashion and IF you intended to spend most of your time on a racetrack you can get some real gains this way. But.... the gains are pretty small and the downside is that the ride quality is completely lost. IMHO, for any car that spends most of its time on normal roads, it is invariably not any kind of net improvement. In fact in most cases I'd describe it as a very expensive way of making your car uncomfortable and handle like crap.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

SORIANOTRON
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Re: Bilstein B8 shock question (spring perch/groove)

Post by SORIANOTRON » Thu Apr 05, 2018 8:47 am

Hey bruce, thanks for chiming in

I had read another post the other night where you had mentioned this about the bump stops. This give me a little more clarity, appreciate it.

I just replaced all 4 bump stops with what I believe are the M635CSi specific type (how can I verify this?), so are you saying if I want to install the B8's and use the lowest spring perch I will ALSO need to cut/shorten the stop as well? Would this be in relation to the distance the spring perch sits on the shock? How would one know how much to shave?

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Brucey
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Re: Bilstein B8 shock question (spring perch/groove)

Post by Brucey » Thu Apr 05, 2018 9:06 am

to retain something like the standard ride and handling, the bump stop length should be altered in relation to the ride height of the car (otherwise it would depend on the exact lengths and stiffnesses of springs etc...) . Ride heights (eg from wheelarch lip to hub centre, so that they are independent from tyre varations etc)) are listed on this site and in the workshop manual.

Failing this trim the bump stops so that you have some suspension travel that does not include the bump stop (eg so the highway ride isn't jiggly). About 1" to 2" would be OK for most folk, but you can go a bit more or a bit less (ideally matching to the way the front is set up) depending on how you want the car to feel and behave exactly.

BMW themselves varied the bump stop clearance a bit; for example the E28 M535i used slightly lowered suspension I think, but used standard length bumpstops at the rear.

BTW if you "just bought all four bumpstops from BMW" you didn't buy all M635CSi ones; external bumpstops are only fitted to front dampers that are the twin-tube type (eg Boge etc). All M635CSi models were fitted with bilstein front inserts/ bilstein rear shocks , both of monotube design. [E28 M5 models were different and used Bilsteins or Boge dampers. BTW the thickest ARBs were (I think) only fitted to those M5 models fitted with Boge shocks because they didn't bolster the roll stiffness at the front by using bump stops in the same way. ]

As I mentioned previously the Bilstein front inserts are fitted with internal (hidden) bumpstops. These are only available from Bilstein; BMW do not supply them. You can only change the bumpstop by partially disassembling the insert. These shocks do not accept an external bumpstop because the shaft is a much larger diameter (because the damper itself is inverted in the insert). Rear bilsteins use a BMW-supplied bumpstop.

Standard E28 and E24/2 bumpstops are ~85mm long front (production mark 1 124 449) and rear (production mark 1 124 573). M635CSi rear ones are shorter. All have a short length at the top which is a snug fit on the shock shaft; cut this away entirely and the whole thing will slide downwards and the protection tube won't do its job. You can sometimes recover this by packing the top 1" of the shock shaft (eg with tape) so that the bump stop is still a snug fit.
If you trim at the bottom, you will remove the part of the bumpstop that is most flexible and gives a smooth transition into the bump stop.
Thus for small trims I recommend trimming the bumpstop at the top and for larger trims expect to pack the top with tape and/or cut at the bottom (instead or as well) and if you trim at the bottom be prepared

a) to cut a fresh groove and/or make up the width so that the protection tube still fits and
b) for the transition into the bump stop to be more abrupt than normal.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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Brucey
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Re: Bilstein B8 shock question (spring perch/groove)

Post by Brucey » Thu Apr 05, 2018 9:46 am

also.... the stock bump stops have grooves in at the top; these are to allow any air trapped between the shock shaft and the bumpstop to escape as the shock compresses. If you trim the top of a standard bumpstop, it is important to recut these grooves else the bumpstop may burst.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

SORIANOTRON
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Joined: Tue Oct 31, 2017 7:58 pm
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Re: Bilstein B8 shock question (spring perch/groove)

Post by SORIANOTRON » Thu Apr 05, 2018 5:57 pm

Your the man bruce, appreciate your time.

1bhp
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Re: Bilstein B8 shock question (spring perch/groove)

Post by 1bhp » Sat May 12, 2018 3:40 pm

Brucey wrote:
Thu Apr 05, 2018 9:46 am
also.... the stock bump stops have grooves in at the top; these are to allow any air trapped between the shock shaft and the bumpstop to escape as the shock compresses. If you trim the top of a standard bumpstop, it is important to recut these grooves else the bumpstop may burst.

cheers
Hi everyone, new here... After years of looking I've finally found one I trust/ like and put some money into a '89 635! Now without further ado time to make some adjustments. All pretty standard and I'm sure that the info exists in various posts on here but after extensive trawling I can't pull it all together... Any help greatly appreciated.

Bilstein B8's - these aren't listed on their site under e24's but assume that's because they don't work with the standard ride height?
Using B8's & H&R springs means I'll need to source additional M635 bumpstops, front and rear or just rear?
I plan to renew strut mounts and poly bushes, anything else I need to do/ think of whilst this is being done?
Both f&r should fitted on the middle perches for standard ±30mm drop?

Please go easy, I have limited technical knowledge of these and for reference I won't be doing the work, the garage will!
cheers all. B
London
'89 635csi
'72 914
'96 850R

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