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Fork Tube Oil Holes

The place to discuss the R 90 S as well as the R90S Worldnet archives!
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sblaylock
Posts: 189
Joined: Thu Jul 22, 2010 1:05 pm
Location: Victoria, BC. Canada

Fork Tube Oil Holes

Post by sblaylock »

This doesn't just apply to the R90S, but it's the bike I'm currently working on so I'll drop the question in here.

I have seen (read) multiple descriptions on which way the fork oil ports should face. Some people say they should face towards the back. Others say they should face towards each other because of fork flex. On my R90S I think the last person who rebuilt these forks read what I've read and decided to face the RH fork oil ports towards the LH fork and the LH oil ports towards the back in hopes of getting one right.

Anyways, does anyone know, definitively, which way they should face, or faced from the factory... or does it even matter?

Thanks,
Scott.
Scott Blaylock
Stoney Peak BMW Service
Victoria, BC. Canada
BMW Classic and Vintage Motorcycle Service and Restoration
service@stoneypeak.com

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schrader7032
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Re: Fork Tube Oil Holes

Post by schrader7032 »

I can't find anything on these holes and their alignment. It's been a long time since I saw the internals of my forks. Are the holes above the slider seals? If so, then I guess they're designed to let air out when the spring is being compressed...that air then goes through the roll pins in the bottom of the lower yoke on the steering stem. Maybe I'm getting that wrong.

Anyway, I remember when I did overhaul my forks and approached my answer from my engineering background. I suspect that the forks flex fore and aft as the front tire hits bumps in the road...the axle will move up as well as aft when all that happens. Either movement of the axle will tend to bend the forks. That puts the front facing and rear facing surfaces of the fork in tension and compression depending on the motion.

Thus, I ended up putting the holes facing each other. Because I didn't want a hole in a tension field...holes tend to be the points were cracks can start...ie, they create stress concentrations and increase stress locally. Half way around the fork tube from the front to the back reaches a point where the fork tube is in a zero bending stress region. To me that's the best place for the hole to be.

In the end, I suspect it doesn't make much of a difference.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.

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sblaylock
Posts: 189
Joined: Thu Jul 22, 2010 1:05 pm
Location: Victoria, BC. Canada

Re: Fork Tube Oil Holes

Post by sblaylock »

Hi Kurt,

The holes are below the slider seals and port oil through them as the dampner rod moves up and down.

The person that said to face them towards each other used the same logic as you. The flex being fore and aft could cause cracking.

I'll install them facing each other... assuming there isn't significant wear. That said, the X and Y axis are way out on them so the wear pattern will change once I get them back into spec.

Thanks for the help,
Scott.
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Scott Blaylock
Stoney Peak BMW Service
Victoria, BC. Canada
BMW Classic and Vintage Motorcycle Service and Restoration
service@stoneypeak.com

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