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Rebound Knock using Progressive Fork Springs

crouthier

Member
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
 
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.
 
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
 
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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.
 
I, too, have heard about Progressives being a bit too much. Look at a few of the threads in the Similar Threads pane below.
 
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
 
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.
 
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.php?42105-Gold-Valve-Emulator&highlight=gold+valve
 
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
 
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...
 
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.
 
:wave
AAAAAAAAAAAAAAAAAAAAAAAAAAAAaaaaaaaaaaaaaaaaaaaaRRRRRGGGGGGGGGHHHHHHHHHH!!!!!!!!

Rebound Knock?

Really?

So this is what I get for finally deciding to join BWMMOA after all these years?

I bought my '83 R65 in '92. It had 12K on it. I'm not a hard core rider and Michigan doesn't have many great days to ride. So it's 2018 and the odometer reads 45K.

Within a week of owning the bike I was trying to track down a 'clunk' in the steering head. Always happened when the bike was travelling 5 to 10 mph and going off of a curb height drop or pothole etc. I adjusted the steering head bearings from too tight to too loose. Disassembled and reassembled forks half a dozen times. Rode without the tank (maybe it's contacting the frame). Checked engine mounts. Checked and checked and rechecked every brake caliper, triple clamp, handlebar, light, instrument, et al, mount. Check check check. Had it in twice to BMW dealership mechanics to discover this very problem. Dozens of forum threads over the years across the web. No one could ever help me with it.

This is bizarre really. Two separate BMW mechanics offered no clue, even though they verified the clunk in test rides. Over the years this has truly driven me to distraction and ruined a bit of the enjoyment of riding.

So the first day I join BMWMOA there is this thread. And a light blinks on in my head. Yeah. The previous owner told me he was obsessed with correcting the infamous tank slapper wobbles in certain sweeping turns at speed on this bike. New steering head bearings, fork brace, and yes... progressive springs. I remember him mentioning the progressive springs. The strange thing is that in all the times I inspected and rebuilt the forks in trying to track down this clunk I in fact suspected this very thing, of course, but could not find any point inside where metal actually meets metal when the fork rebounds fully. And now I still can't quite get my head around why returning to the OEM springs would remedy this. But if it does I'd be ecstatic.
 
I think the springs are "slapping" the inside of the fork tubes as they lose compression at full fork extension. When even slightly compressed the spring curves slightly against the inside of the fork tube and as long as still compressed the spring remains in that condition. But if at full extension the spring loses pre-load it can slap the opposite side of the fork tube. The very first thing I would do is to make sure there is sufficient spring preload with the forks fully extended. If not - add preload using a spacer or longer spacer.
 
I think the springs are "slapping" the inside of the fork tubes as they lose compression at full fork extension. When even slightly compressed the spring curves slightly against the inside of the fork tube and as long as still compressed the spring remains in that condition. But if at full extension the spring loses pre-load it can slap the opposite side of the fork tube. The very first thing I would do is to make sure there is sufficient spring preload with the forks fully extended. If not - add preload using a spacer or longer spacer.

It's pretty clearly a clunk that could not be produced by the spring slapping in this fashion.
 
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.
 
Springs

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