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Thread: Rebound Knock using Progressive Fork Springs

  1. #16
    Quote Originally Posted by frascati View Post
    It's pretty clearly a clunk that could not be produced by the spring slapping in this fashion.
    Check the preload anyway. I have heard this condition more than once. I could be wrong but it is easy to check.

    Also check that the required white spacers are in place at the bottom of the springs.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://web.bigbend.net/~glaves/

  2. #17
    Registered User AntonLargiader's Avatar
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    Is it happening at the end of the stroke (forks are topping out) or in the middle?
    Anton Largiader 72724
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    Virginia Motorrad Werkstatt BMW motorcycle service and repair in central Virginia

  3. #18
    Rally Rat 1074's Avatar
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    Springs

    My mistake. The correct part number for the :sport" spring is: 31 42 1 232 017.
    Boxerbruce

  4. #19
    Quote Originally Posted by AntonLargiader View Post
    Is it happening at the end of the stroke (forks are topping out) or in the middle?
    It would appear to be at the bottom of the stroke where (when) the forks are fully unloaded.

  5. #20
    Hi, I went back to Max two weeks ago to 'discuss' the clunk - Phil had two suggestions: remove the preload spacers or replace the springs with stock.

    1. I removed the preload spacers (he cuts them in half). When I unscrewed the nuts there was no tension at all on the nut...so I installed stock size spacers and it is def preloading the springs but made no difference, still clunks. Then tried no spacer, same clunk...

    2. may try stock springs next. does anyone have any good used stock springs?

    3. this is driving me nuts...


    thanks Chuck
    Last edited by crouthier; 07-08-2018 at 08:48 PM.
    C Routhier NY
    1972 R75/5
    1969 2002
    1982 528e

  6. #21
    While reconsidering all of this and getting on and off of the centerstand, slowly rolling over potholes, etc, I may also be dealing with some stiction. It's sooooo hard to get a good read on this.

    There is clearly bottoming out knock in cases where the forks are fully unloading. But in rolling the bike forward off of the center stand I'm also hearing a fainter clunk almost immediately after the front tire hits the ground. It seems to fit the feel and sound of stiction if I keep an open mind about it. I have a robust fork stabilizer brace over the fender that the previous owner also included in an effort to tone down the frame wobbles. I've taken great care, following any fork rebuilds, to install this brace last in order so that it can be shimmed and spaced before tightening so it doesn't impede the free travel of the forks. But I've never actually ridden the bike without it. That may be my next step.

    Hi, I went back to Max two weeks ago to 'discuss' the clunk - Phil had two suggestions: remove the preload spacers or replace the springs with stock.

    1. I removed the preload spacers (he cuts them in half). When I unscrewed the nuts there was no tension at all on the nut...so I installed stock size spacers and it is def preloading the springs but made no difference, still clunks. Then tried no spacer, same clunk...

    2. may try stock springs next. does anyone have any good used stock springs?

    3. this is driving me nuts...


    thanks Chuck

    You're very much not alone

  7. #22
    Registered User AntonLargiader's Avatar
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    Quote Originally Posted by frascati View Post
    There is clearly bottoming out knock in cases where the forks are fully unloading.
    We need to speak the same language here. When the forks are fully unloaded (wheel in the air) they are topped out. The extension is controlled by the rebound damping and the top-out bumper or spring. Even with new bumpers the old forks have more of a top-out knock than newer ones (which often have springs from the factory). On '70s bikes I very often feel a knock when they go on the centerstand, because that is when the forks top out.

    Bottoming out is when the forks are fully compressed/loaded. A bottoming knock is bad because it means there is no extra "last inch" anti-bottoming cushion. You can bend and break stuff that way. Newer forks have hydraulic bumpers to soften the bottoming, generally called bottoming cones or hydraulic stops.
    Anton Largiader 72724
    Tech articles - YouTube
    Virginia Motorrad Werkstatt BMW motorcycle service and repair in central Virginia

  8. #23
    Anton, thank you for clarity on language. When I push on my front end - it certainly feels like I am getting a knock (soft knock) or movement on bottom out and topping out, that is on both push down and rebound. when I hit a pot hole it fells like something is loose and knocking. The shop has agreed to take another look and replace springs with new original BMW units. I'm not convinced it's spring related but will see what happens.

    thanks for help.

    Chuck
    C Routhier NY
    1972 R75/5
    1969 2002
    1982 528e

  9. #24

    Progressives

    Quote Originally Posted by crouthier View Post
    Hi, I just had my forks rebuilt on my '72 R75/5 by a shop in CT. I got the bike back and the front end had a loud tactile rebound knock - the forks were tight and not dangerous...I brought it back and they inspected it, put heavier fork oil and said that nothing is amiss and that's just the way it is with the stiffer Progressive springs. I had a 73 /5 back in the day with Progressive springs and it never knocked.

    Has anyone ever had this experience with Progressive springs?

    They said only way to remedy is to install a $200 valve (of some sort) or replace with stock BMW springs (and I'd have to pay for the 4 hrs labor and the materials)...any thoughts?

    Are replacing springs beyond a competent home mechanic (I did my push rod seals no probs)?

    thank you Chuck
    As others have stated, Progressives are highly overrrated. I had multuple sets in a few Airhead's and they were always too stiff, worse ride quality and developed strange noises.

    I have stock BMW springs in my R90s and just yesterday I was out all day on the bike, smiling at how nice and compliant the ride is.

    Spend the money, get stock springs.

    RickR90s

  10. #25
    thanks Rick, that's the plan, can't wait to get the stock springs back in. Best Chuck
    C Routhier NY
    1972 R75/5
    1969 2002
    1982 528e

  11. #26
    Registered User
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    Plus 1 for the stock springs, no spacer. Light fork oil 5 or 7.5, 500 ml each leg. Definitely not a sport bike but so much less likely to break a fork tube which I have done twice trying to make a pig fly. My r90s dives a bit and has that rubber cow handling but it soaks up the road so much nicer without the stiff springs and preload spacers. 50 year old fork tubes should be treated like 50 year old knees, any extra stress is aking for trouble, that knock is being absorbed at the lower triple clamp, sooner or later its going to break,which sucks at speed.

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