Rebound Knock using Progressive Fork Springs

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

Click to expand...

You're very much not alone

frascati said:

There is clearly bottoming out knock in cases where the forks are fully unloading.

Click to expand...

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, 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

Progressives

crouthier said:

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

Click to expand...

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

thanks Rick, that's the plan, can't wait to get the stock springs back in. Best Chuck

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.

Hi Finally - some closure on this issue. MAX agreed to take the forks apart - again - 3rd or 4th time, and this time they found shavings and admitted that this was NOT normal and was not a progressive spring issue. They ended up swapping out parts to isolate the knock and found that one fork tube and one slider were somehow causing the knock. They installed BMW springs and replaced those parts and now bike is compliant, tight and quiet on all but the most extreme potholes (where there is a slight knock) - thanks for all the input. Just in time for the cold weather here in NY!

Chuck

Good to hear.

crouthier said:

Hi Finally - some closure on this issue. MAX agreed to take the forks apart - again - 3rd or 4th time, and this time they found shavings and admitted that this was NOT normal and was not a progressive spring issue. They ended up swapping out parts to isolate the knock and found that one fork tube and one slider were somehow causing the knock. They installed BMW springs and replaced those parts and now bike is compliant, tight and quiet on all but the most extreme potholes (where there is a slight knock) - thanks for all the input. Just in time for the cold weather here in NY!

Chuck

Click to expand...

I know this an older post but I have the same issue on my 76 R90/6. Progressive springs. Do you know what parts they had to change besides the springs?

Hi, they changed out the fork tubes, installed a rebuild kit (bumpers, etc) and switched back to standard springs and spacers. However, I STILL get rebound knock, it's way softer and not on every single road imperfection. The bike rides fantastic so it's not quite an issue - I'd love to solve it some day, but have just learned to live with it. I plan on changing out the fork oil and have had suggestions that it's wise to add a bit more oil - I'd have to look at my notes for the exact extra amount...

This is my second /5 and my first never ever made a sound in the suspension. So, I guess I am not really sure what's still causing it. Spring slap on the sides? Good luck with your issue. Keep us posted.

C