Replacing front fork springs

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skychs
Posts: 359
Joined: Fri Nov 25, 2016 2:12 pm

Update

Post 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.
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User avatar
skychs
Posts: 359
Joined: Fri Nov 25, 2016 2:12 pm

Replacing front fork springs

Post 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

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bmwsog
Posts: 81
Joined: Wed Mar 19, 2014 2:53 pm

R 90 S Forks

Post 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.
LTC R Marriott--Current-1951 R-51/3
1962 R 69 S 1975 SS R-90 S
Past:1974 SS R 90 S, 74 R 90 , 77 R 100S, 1960 R 50/2,1973 R 75/5, 1977 R 75/7, 1977 R 100 , 1972 R 75 /5

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skychs
Posts: 359
Joined: Fri Nov 25, 2016 2:12 pm

Springs

Post 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.



MikeL46
Posts: 97
Joined: Fri Jan 01, 2016 1:58 pm

40 years ago I changed my

Post 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
67 R50/2 w/R100 engine/trans and Ural Sidecar
69 R60/2 76 R90S 78 R100RS
70 Triumph w/Spirit Eagle Sidecar

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skychs
Posts: 359
Joined: Fri Nov 25, 2016 2:12 pm

Fork oil

Post 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.

MikeL46
Posts: 97
Joined: Fri Jan 01, 2016 1:58 pm

With the BMW fork oil I had

Post 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

67 R50/2 w/R100 engine/trans and Ural Sidecar
69 R60/2 76 R90S 78 R100RS
70 Triumph w/Spirit Eagle Sidecar

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skychs
Posts: 359
Joined: Fri Nov 25, 2016 2:12 pm

oil

Post 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

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schrader7032
Posts: 7139
Joined: Fri Oct 27, 2006 3:00 am

I have to wonder what the

Post 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.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

MikeL46
Posts: 97
Joined: Fri Jan 01, 2016 1:58 pm

oil

Post 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
67 R50/2 w/R100 engine/trans and Ural Sidecar
69 R60/2 76 R90S 78 R100RS
70 Triumph w/Spirit Eagle Sidecar

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