Progressive springs question R100 GS

I posted this at ADV Rider, didn't get much. I thought I would post it here too.

I put some progressives in my 89 GS last week. I couldn't find any info from progressive, or from BMW, about how much, or what weight, of fork oil to use with those springs.

We went to a lighter oil, Aeroshell 4/5 weight, and put in a little less than stock - 390 cc's left, and 420 cc's right.

I like that the fork is stiffer than it used to be, but it feels kind of harsh, especially on washboard or small bumps. I'm wondering what anyone else used for fork oil, amount and weight with your progressives, or if someone has found a good recommendation on it.

Thanks

My Haynes as some way different numbers for fluid volume after rebuild - both sides at 470cc. For just a fluid change, it's right 440cc and left 410cc. Sounds like you have a lot in the right side.

What is Aeroshell 4/5...it's a mix of 4 and 5?

20774 said:

My Haynes as some way different numbers for fluid volume after rebuild - both sides at 470cc. For just a fluid change, it's right 440cc and left 410cc. Sounds like you have a lot in the right side.

What is Aeroshell 4/5...it's a mix of 4 and 5?

Click to expand...

No, it was called Aeroshell 4 something or other, I have it written down, but I was told that it is a 4 to 5 weight oil depending on who you talk to.

I thought we looked at a Haynes. Maybe not. I'll check that out. Thanks.

20774 said:

My Haynes as some way different numbers for fluid volume after rebuild - both sides at 470cc. For just a fluid change, it's right 440cc and left 410cc. Sounds like you have a lot in the right side.

Click to expand...

Sorry, that was my mindless typing. We put in 420 right, and 390 left. The idea was to reduce the amount of fluid to match the larger amount that a progressive spring displaces. Problem is, no one knows what that amount is. We guessed at 20 cc's per side.

Tom Cutter has posted quite a bit and his opinion is that the Progressives will tend to produce a harsh ride...I think he feels they are too stiff to start with. As far as volumes, his point was that BMW recommends a volume (for their springs) which should get you to the right fluid level. He suggested removing fluid will help with the stiffness but typically don't go less than 1.0 inch below the stock volume. Now if the Progressives bring more metal to the area such that the this throws things off, it's going to be a tough call. Interesting to know what the oil level was with the proper volume prior to replacing the springs...you could use that level as a starting point.

The mechanic that I was working with always just goes by a set number of cc's, at least on the GS, so we didn't measure. These springs were the full length of what had been stock springs and preload spacers. With the tighter windings, they definitely displaced more oil. He was confident that this was fine, and he's been at it for nearly 50 years, I was just curious about what others had experienced.

I like the new springs a lot on the road, in corners, and especially braking. They seem fine everywhere I have been so far but on something like washboard, so it's probably just the nature of the beast. He has progressives in an R100 S, but wasn't sure he would want them in his R80 G/S. He's not sold on everyone wanting/needing the stiffest shocks possible. I had read some good things, so thought I would give it a try.