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Update
Posted: Sun Apr 26, 2020 11:39 pm
by skychs
The front end is apart. The fork sliders, anti dive springs and damper rods are out. I left the fork tubes in the upper tree. All the springs, rings and bushings are still in very good shape. The oil was dirty with a little grunge in the bottom but not too bad.
Overall I think everything is good to go. The new springs are much different than what was in the bike. I hope that helps.
Tomorrow I will put it all back together with some new O-rings and washers.
Replacing front fork springs
Posted: Sun Apr 26, 2020 11:44 pm
by skychs
1975 R90S
I rebuilt the front forks when I restored the bike two years ago. Not really knowing anything about these bikes I reinstalled all the parts I found when I took it apart. They included a generic spring, the anti dive spring in the bottom, a 1" PVC spacer at the top and 7.5 weight fork oil. Ive tried 5, 7.5 and 10wt fork oil. No real difference.
The plan is to go through the front suspension, remove the short anti dive spring in the bottom and install new Wilburs progressive springs.
chuck
R 90 S Forks
Posted: Sat May 02, 2020 11:39 am
by bmwsog
Let us know how it turns out. My 1975 bike needs the same treatment plus
head bearing re-packing/adjustment. A stronger Toaster Tan upper brace/tree is also a thought.
Springs
Posted: Tue May 05, 2020 9:55 am
by skychs
Let us know how it turns out. My 1975 bike needs the same treatment plus
head bearing re-packing/adjustment. A stronger Toaster Tan upper brace/tree is also a thought.
Much better. I took out the anti-dive springs in the bottom. Installed Wilburs springs and filled with 7.5 wt fork oil. I also reused a 1" PVC spacer in the top for preload. I may drop it down to a 5 wt oil but the ride is much better.
40 years ago I changed my
Posted: Tue May 05, 2020 10:26 am
by MikeL46
40 years ago I changed my fork oil to 5W-20 Mobil 1. No stiction and the fork seals have never failed. Never going back to plain Fork oil.
Mike
Fork oil
Posted: Tue May 05, 2020 10:54 pm
by skychs
40 years ago I changed my fork oil to 5W-20 Mobil 1. No stiction and the fork seals have never failed. Never going back to plain Fork oil.Mike
Mike ..... I have to ask. Why? Cheaper? Better performance? Im just curious.
With the BMW fork oil I had
Posted: Wed May 06, 2020 10:26 am
by MikeL46
With the BMW fork oil I had some stiction. And I preferred the lighter weight oil.
Probably, the first change was when the shop was closed and I needed to fill the forks. Used what I had on hand that was ~5wt. Worked so well that I never changed back or questioned the change. Mobil 1 is cheaper than fork oil.
Mike
oil
Posted: Wed May 06, 2020 5:48 pm
by skychs
With the BMW fork oil I had some stiction. And I preferred the lighter weight oil.
Probably, the first change was when the shop was closed and I needed to fill the forks. Used what I had on hand that was ~5wt. Worked so well that I never changed back or questioned the change. Mobil 1 is cheaper than fork oil.
Mike
Thanks for responding. Any benefits from using the multi weight oil?
chuck
I have to wonder what the
Posted: Wed May 06, 2020 7:13 pm
by schrader7032
I have to wonder what the different weights/viscosities are. 5wt in fork oil is likely not 5wt in engine oil. Multi weight oil changes characteristics due to heat. The base weight of the oil is 5wt and when warmed to operating temperature, it would be 20w. Doubt that the forks are getting anywhere near to an elevated temperature.
As a comparison Snowbum has a range of discussions on viscosities on this page:
https://bmwmotorcycletech.info/viscosity.htm
Viscosity is expressed as centistokes at a given temperature. Hard to make any meaningful and direct comparison because he doesn't list the fluids involved above. But he does have this which is centistokes at 100degC:
- Belray fork oil 10 (well known maker of fork oil) - 5.8
- engine oil grade 5 (I think that means 5w) - 3.8 to 4.1
So that seems to suggest that at 100degC, engine oil might be less viscous and flow better. But we'd need to know the readings at room temperature to really know.
oil
Posted: Thu May 07, 2020 10:41 am
by MikeL46
With the BMW fork oil I had some stiction. And I preferred the lighter weight oil.
Probably, the first change was when the shop was closed and I needed to fill the forks. Used what I had on hand that was ~5wt. Worked so well that I never changed back or questioned the change. Mobil 1 is cheaper than fork oil.
Mike
Thanks for responding. Any benefits from using the multi weight oil?
chuck
Probably not. It is possible the oil doesn't thin as much on a really hot day, but I don't ride hard when the weather is really hot (or cold).
Mike