geoffm
16th March 2008, 19:00
I got a price from Robert Taylor to look over my BMW K100RS-16v forks and new springs, which was a fair price, unfortunately his friends Auntie Helen and Uncle Michael decided they needed my money more than Mr T. Which leaves me to do the best I can for no money...
The fork seals were suspected of leaking and the forks have always been soft, so a couple of weeks ago i pulled them apart when I was fixing the brakes. I have 2 spring sets - the standard ones, and some progressive full length ones - brand unknown but as the bike was originally UK based I suspect hagon or Renntech and these are in there now.
The first thing I discovered was that the forks are the later model (mid 91-93) ones (made by Mazocci) rather than the ones the VIN number date would suggest (04-91). This discovery cost me $20 for the wrong fork seals...
1. How can you tell if the bushes are worn? It was interesting when I had the forks apart, one side the bushes looked almost new, the others had worn through the anti friction coating. As far as I know they haven't ever been replaced, certainly not in the last 9 years and 40,000km. I priced up bushes from the UK (NZ BMW prices are a sick joke so I usually avoid buying in NZ) and this might happen in the future. There was a wee bit of slop - forks fully extended, springs out, etc it would have been 2-3mm arc at the top of the stanchion, although I didn't set it up with the dial gauge. How much is to much?
2. I used the standard weight oil (the Clymer manual doesn't specify it, calling for "BMW Comfort", but I believe it is 7.5wt - I used Motul 7.5wt). 420ml in each leg (fully disassembled with 142mm ai r gap - standard 160mm IIRC) and the front seems to behave itself over bumps although I need to take it for a good long thrash. It may be a bit "sproingy" when you push on the bars, but that doesn't seem very scientific. The front dives a lot under brakes however with a sharp pitching motion as the brakes are first applied, especially suddenly. Would this be oil weight to low, leading to not enough compression damping over the initial movement, or spring to light, or...?
I didn't have any 10wt otherwise I would have done it today to see if it made a difference. Any suggestions?
3 Static sag - with no preload, the static sag is 45mm, with 110mm fork travel (without looking up the manual which is downstairs). with 10mm preload (4 washers) it was 42mm. I made some 25mm spacers in the lathe from pipe, but there was no way I could get the mickey mouse cap back on the fork leg. It is a 30mm screw on cap with a very fine thread, and there was absolutely no way you could push it down and get the thread to engage. A speedbrace helped, but that is the main reason it has 10mm preload in it, not 25mm. At least with the LC, it uses a retaining clip, and you can use a gear puller to push the cap down.
Any tricks for getting this cap back on?
Any tricks for calculating the preload spacer size to get a required static sag, or is it a cut it and see thing?
Geoff
The fork seals were suspected of leaking and the forks have always been soft, so a couple of weeks ago i pulled them apart when I was fixing the brakes. I have 2 spring sets - the standard ones, and some progressive full length ones - brand unknown but as the bike was originally UK based I suspect hagon or Renntech and these are in there now.
The first thing I discovered was that the forks are the later model (mid 91-93) ones (made by Mazocci) rather than the ones the VIN number date would suggest (04-91). This discovery cost me $20 for the wrong fork seals...
1. How can you tell if the bushes are worn? It was interesting when I had the forks apart, one side the bushes looked almost new, the others had worn through the anti friction coating. As far as I know they haven't ever been replaced, certainly not in the last 9 years and 40,000km. I priced up bushes from the UK (NZ BMW prices are a sick joke so I usually avoid buying in NZ) and this might happen in the future. There was a wee bit of slop - forks fully extended, springs out, etc it would have been 2-3mm arc at the top of the stanchion, although I didn't set it up with the dial gauge. How much is to much?
2. I used the standard weight oil (the Clymer manual doesn't specify it, calling for "BMW Comfort", but I believe it is 7.5wt - I used Motul 7.5wt). 420ml in each leg (fully disassembled with 142mm ai r gap - standard 160mm IIRC) and the front seems to behave itself over bumps although I need to take it for a good long thrash. It may be a bit "sproingy" when you push on the bars, but that doesn't seem very scientific. The front dives a lot under brakes however with a sharp pitching motion as the brakes are first applied, especially suddenly. Would this be oil weight to low, leading to not enough compression damping over the initial movement, or spring to light, or...?
I didn't have any 10wt otherwise I would have done it today to see if it made a difference. Any suggestions?
3 Static sag - with no preload, the static sag is 45mm, with 110mm fork travel (without looking up the manual which is downstairs). with 10mm preload (4 washers) it was 42mm. I made some 25mm spacers in the lathe from pipe, but there was no way I could get the mickey mouse cap back on the fork leg. It is a 30mm screw on cap with a very fine thread, and there was absolutely no way you could push it down and get the thread to engage. A speedbrace helped, but that is the main reason it has 10mm preload in it, not 25mm. At least with the LC, it uses a retaining clip, and you can use a gear puller to push the cap down.
Any tricks for getting this cap back on?
Any tricks for calculating the preload spacer size to get a required static sag, or is it a cut it and see thing?
Geoff