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Thread: Rebound Knock using Progressive Fork Springs

  1. #1

    Rebound Knock using Progressive Fork Springs

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

  2. #2
    Liaison 20774's Avatar
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    Welcome to the forum, Chuck! Answering your last question first, no I don't think replacing springs is that big of a deal. I think in the case of the /5, you just need to remove the two caps and nuts that hold the springs in. Those nuts tend to be quite thin, so you might need to create a "tool" which is a socket with the inside chamfer ground off. If you have the stock tool kit, the flat wrench in there might do the job.

    As for the knock, how was the fork set up? There should be some sag. Generally, the fork should compression from completely unladen to you sitting on the bike about 1 to 1-1/2 inches. I'm not sure what the full suspension travel is for the /5, but I read in my Haynes that it is a little over 8 inches. When just sitting on the bike you need to be somewhere in the between. With much more travel being in the compression direction, you want more travel that way than fork extension. So, that is why you want to be nearer full extension.

    Also, think about that. Say you had 0 sag. It's easy to see that there would be no spring support for keeping the fork from banging back on the top once you hit a bump, get compression, and then rebound. Similarly, if you had all 8 inches of sag and you were sitting on the bottom stops, you can imagine what the feeling would be like on that first bump!!

    You can figure out what sag is by tying a zip tie to the front fork just above the slider with the bike sitting on its own weight. Then get on and look at where the zip tie moves to. That would be sag. You can modify this sag, especially if it's too much, by putting PVC spacers in above the springs...then put the caps back on.
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  3. #3
    Hi Kurt, Thanks! And thanks for the thoughtful response. I'm going to study how the springs work - I do have a Haynes - and will make a plan to figure out. It sounds like you are saying it's not really a Progressive vs. Stock spring thing but a set up thing, that it can be resolved with spacers...Sounds do-able.

    I really prefer to learn how to do it myself as I am usually more careful, and it seems to take weeks + months as the 'shop. It felt like the forks were one of 'things' that needs special knowledge. I feel I can tackle it.

    But let me ask this - the 'nuts' you are talking about are the top nuts I assume, as when I take a dust cap off I can see the spring - then remove that nut (and assorted washers, spacer, etc and instal the appropriate PVC spacer? Wasn't sure if I had to remove the sliders etc...

    Regards,
    Chuck
    Last edited by crouthier; 06-15-2018 at 01:01 AM.
    C Routhier NY
    1972 R75/5
    1969 2002
    1982 528e

  4. #4
    Rally Rat 1074's Avatar
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    Progressive springs

    My experience has been that these springs are totally overrated! They are way too stiff to begin with, bind inside the fork tube, and come packaged with spacers that are put in by people who have minimal knowledge of how the fork assembly should work!
    With any luck at all you can return them and get your money back! Order a set of OEM springs with the part number that ends in 013. They should have spots of white paint on one end. Yellow paint denotes the soft standard springs. Try that with 7.5 wt. fork oil at the factory specified level and make sure that the internal parts are in good shape. Let us know what you think.
    Boxerbruce

  5. #5
    Liaison 20774's Avatar
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    I, too, have heard about Progressives being a bit too much. Look at a few of the threads in the Similar Threads pane below.
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  6. #6
    Thanks Guys, yes, I'm not impressed and the knocking is such a distraction I don't enjoy it, I'm sorta sorry I did it. I'll take a look at the similar threads and will consider going back to stock.

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

  7. #7
    Registered User AntonLargiader's Avatar
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    If you're hearing the top-out knock while you are riding normally then you have too much preload. The fork is not staying in its operating range. If you only hear it when going on the centerstand that's different.

    There is a bumper that should take the worst of the knock out, but the diagrams for that fork are very confusing. You'd have to disassemble a lot to install it, if it's not there. Over the years there have also been short springs sold to do the same thing.
    Anton Largiader 72724
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    Virginia Motorrad Werkstatt BMW motorcycle service and repair in central Virginia

  8. #8
    I've had an R75/6 with Progressives and an R90S with Racetech springs, both worked well with my preference being the latter. A few thoughts: 1) If you have too little sag, it may be the case that the tech put in spacers that are too long. As Kurt says, easy enough to pop off the caps (bike on centerstand with the front raised so no weight on the front wheel) pull the spacers and cut them down. This is trial and error stuff so be prepared to do this more than once. 2) if that doesn't do it, there's a plastic bumper at the bottom of the fork slider. I've had good luck removing the sliders with the forks mounted -- remove the wheel, etc. and the springs provide enough tension to remove and replace the bolt that holds the slider on. Drain the fork oil first. The hardest part is figuring out which part to order. I'd call Max BMW or some other specialist (the guy at Bob's helped me get it but we were both struggling). 3) if there's too little sag, I think putting in heavier fork oil was a mistake. I had Progressives and heavier oil in the R75 but I was trying to make it handle like a sport bike and it was definitely stiffer than you'd want a daily rider.

    The '$200 valves' are Gold Valve emulators and work very well but, I think, are only worth it if a) you want to push the bike a bit or b) you enjoy projects of this sort. Here's a link to my experience installing them, which may help give a sense of how the forks come apart. At least it shows the bumpers.

    https://forums.bmwmoa.org/showthread...ght=gold+valve
    '61 Clubman's Gold Star, '13 690 Duke, '13 Daytona 675R, '14 Street Triple R, '17 1290 GT (gone but not forgotten: '76 R75/6, '84 R100, '76 R90S)

  9. #9
    Thanks for info. I don't hear the knock pulling it on the center stand. Before they put in the heavier oil, if you pushed on the handlebars standing still, you'd hear it, clearly. After the heavier oil riding over medium uneven pavement (which is to say ALL pavement) you'd get the knock 50% of the time...I will pull the nuts and investigate. I'm a pretty sedate rider and won't 'push' it, I just don't want the knock, it's very distracting and IMO should not be there at all...

    Thanks for responses, Chuck
    C Routhier NY
    1972 R75/5
    1969 2002
    1982 528e

  10. #10
    One other thought -- check everything else that might cause a noise here -- steering head bearings in particular but also the brake stay and whatnot. All unlikely but...
    '61 Clubman's Gold Star, '13 690 Duke, '13 Daytona 675R, '14 Street Triple R, '17 1290 GT (gone but not forgotten: '76 R75/6, '84 R100, '76 R90S)

  11. #11
    My forks also topped out when I had Progressive fork springs put into my R90S a couple decades back. I haven't bothered with them again.

  12. #12
    I can't imagine this being good. I would fix asap!!
    "The difference between death and taxes is that death doesn't change every time congress meets." - Will Rogers

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