Twin Shock Forks

[John King]

On my '79 R100RT the front fork occasionally give a loud 'clunk' when ride over uneven road surfaces at slow speed. Initially I put this down to the ATE callipers moving up and down on the mountings. However I have eliminated that movement and I still get the clunk.

I know that inside the forks there are rubber or plastic buffers and possibly other parts that wear but after having studied the exploded diagram I can't say for certain which they are. Can anyone help.

The bike has only done 44K miles since new (genuine) but I know the fork stanchions were replaced by the previous owner because they were rusty, I also have a sneaking suspicion that something has been left out on the re-assembly. Given the comparatively low mileage I would not think that the fork springs are worn to any degree, but has anyone else any different ideas? (There is no leak from the seals)

I will be using the bike to go over Europe this coming May and would appreciate and advice so I can get the work done beforehand.

[K.C.]

Yeh,my bikes and my mates bike do that ! I dont worry about it.

[boxerman]

A 'topping out' clonk was a known issue. There was a modification available to buy but I can't remember what it consisted of....

[John King]

Thanks for that I will leave well alone and go to do better things. The springs however do feel a little 'soggy' compared to those in my 83 R100RT what are your feelings about different rated springs to stiffen the front end up a little.

[SteveMac]

Hi, not wanting to Hijack your thread, but a quick Q, (sitting at work and not with my bike to measure myself)

The Nut on the top of the fork tube, what size socket is required?

Cheers

Steve

[SteveD]

The clunk.
There are a couple of things to check...
1. The fork top nuts are tight.
2. ..more likely, the dome nut over the steering head can wear. Remove it and line the outer part where it fits in with a slice of beer can as a spacer. Reinstall and test. If the clunk goes, then there's your problem. You can then leave it as is, or buy a new dome nut.

It's a cheap and easy test...but you should use a genuine German beer can.

[wulfrun]

I rebuilt a pair of ATE forks not so long ago, they did this before the rebuild and still did afterwards, though not as loudly. I found the buffers (the big doughnut-shaped ones in the fork bottom) were completely disintegrated. If yours have never been replaced, there's a chance they've gone the same way. Apperently it's common, they don't withstand 20-30 years immersion in oil. Mine came out with the oil, like a soft gritty-looking stuff!

[bmk12s]

There's also a rubber ring at the top of the forks, under the damper assembly on the damper rod, that hardens with age, that's more to do with the "topping" out of the forks maybe?

Recently rebuilt a pair of 78 forks without renewing this & still have the annoying topping out knock. I'll try the beercan mod though before stripping them down again... sigh....

Boxerman - I'd love to know what the mod was if you get a chance to delve into the memory archive :)

Stevemac - 36mm if memory serves

[John King]

The clunk.
There are a couple of things to check...
1. The fork top nuts are tight.
2. ..more likely, the dome nut over the steering head can wear. Remove it and line the outer part where it fits in with a slice of beer can as a spacer. Reinstall and test. If the clunk goes, then there's your problem. You can then leave it as is, or buy a new dome nut.

It's a cheap and easy test...but you should use a genuine German beer can.

I have heard of this being caused by the top nut. I will have to have a go and try to get the nut off. What size socket please?

I surmise that you are joking about the German beer can, but I have a sheet of thin mild steel not much thicker than a beer can and that would be easier to cut. What actually does the shim do to kill the noise?

[SteveD]

The dome nut wears over time. The beer can shim just tightens the fit up.
I use a big open end adjustable spanner on the dome nut...haven't measured it's width.

[John King]

The dome nut wears over time. The beer can shim just tightens the fit up.
I use a big open end adjustable spanner on the dome nut...haven't measured it's width.

I have just remembered the bike does not have a domed nut, it has a steering damper and the damper adjustment rod passes right through the headstock -Oooops!

The for top nuts a tight as well.

What I have noticed is, it is missing is a short strong spring (2" long) which fits on the damper rod and just below the damper adjustment knob. My '83 R100RT has one and that doesn't clunk at all. Were all models so fitted?

[John King]

Sorted. Reference the missing spring as per my last post.

I swapped the spring over from my other RT and then rode over all varieties of surfaces and the clunk has gone. So simple isn't it?