Bad to switch from synthetic to mineral fork oil at Drain and Fill?

[jnclem]

I'm reluctant to even start this conversation, but I've found nothing by searching. I have an '89 R100GS that is overdue for a fork oil change. The last time I changed it, I used Silkolene 7.5, and liked it. I have an unopened bottle of Belray 7 on the shelf, and thought about using it. But, I've been warned that "mixing" the two could cause problems.

I'm pretty sure that this bike has had Aeroshell 4, Belray 10, and the Silkolene, just to name a few, in it over the years. Who knows before that. I can wait and find some more Silkolene, but is there really a problem with having residual amounts of these oils in the forks at a drain and fill?

 

[20774]

I can't imagine there would be a huge issue in this case. If you were to mix them in large ratios, like 50-50, then that might be an issue.

 

[jnclem]

I find this entire fork oil thing to be a bit confusing. I've read that BMW recommended 10 wt in one leg, and 15 in the other. I have tried 10 wt in both legs, and the bike felt like it was riding on 2X4's. Then, there is the fact that the different manufacturers "weight" claims have no resemblance to one another. I really don't know what would be best, but this 7.5 seemed to give me a pretty good ride. If I could find some in town, I'd just use it again.

 

[PGlaves]

There is so much misinformation that abounds on the Internet it is like listening to seventeen drunks sitting around a camp fire making stuff up.

"I heard that ..." I've been warned ..." I've read that ..."

Switching from a synthetic to a non-synthetic will do no harm. Any oil that would be harmful if touched by remnants of another oil would be destructive in and of itself. So if the synthetic didn't turn things to mush the draining it and filling with non-synthetic won't do damage.

As for viscosity, there is a reason that manufacturers provide specifications. They are not deep dark secrets known only by the few worthy, to be imparted selectively only to the needy. They are published.

Short of walking out to the shop and looking it up in my repair manuals - Haynes, Clymer, or Factory - my recollection is that 7.5 wt is specified. This can be obtained as BMW branded 7.5 wt oil, or approximated by a 50-50 mix of 5 wt and 10 wt fork oil. In any event I would recommend looking up the specifications as opposed to believing me since I didn't just look it up and am operating from memory.

 

[jnclem]

There is so much misinformation that abounds on the Internet it is like listening to seventeen drunks sitting around a camp fire making stuff up.

"I heard that ..." I've been warned ..." I've read that ..."

Switching from a synthetic to a non-synthetic will do no harm. Any oil that would be harmful if touched by remnants of another oil would be destructive in and of itself. So if the synthetic didn't turn things to mush the draining it and filling with non-synthetic won't do damage.

As for viscosity, there is a reason that manufacturers provide specifications. They are not deep dark secrets known only by the few worthy, to be imparted selectively only to the needy. They are published.

Short of walking out to the shop and looking it up in my repair manuals - Haynes, Clymer, or Factory - my recollection is that 7.5 wt is specified. This can be obtained as BMW branded 7.5 wt oil, or approximated by a 50-50 mix of 5 wt and 10 wt fork oil. In any event I would recommend looking up the specifications as opposed to believing me since I didn't just look it up and am operating from memory.

Thanks Paul,

I had checked my Haynes Manual prior to starting this thread. Chapter one refers you to chapter 7, and chapter 7 refers you to chapter 1. Chapter 7 does provide a list of recommended fork oil types, which range from 4 weight Aero, to 10 weight Spectro, none of which are available to me locally, hence my frustration.

The mysterious internet information I referenced on the 10 wt in one leg, 15 in the other, came from an obscure source called Snowbum, not 100% correct, but a pretty reliable source.

I think I can get a quart of 7 wt. Silkolene in town. That will be close enough.

 

[PGlaves]

Thanks Paul,

I had checked my Haynes Manual prior to starting this thread. Chapter one refers you to chapter 7, and chapter 7 refers you to chapter 1. Chapter 7 does provide a list of recommended fork oil types, which range from 4 weight Aero, to 10 weight Spectro, none of which are available to me locally, hence my frustration.

The mysterious internet information I referenced on the 10 wt in one leg, 15 in the other, came from an obscure source called Snowbum, not 100% correct, but a pretty reliable source.

I think I can get a quart of 7 wt. Silkolene in town. That will be close enough.

You are absolutely right - the specifications recited by Haynes and Clymer are as clear as chocolate syrup. BMW originally listed at least 10 European and other oils and both Haynes and Clymer regurgitated that list without clarity. The one which is recognizable for sure is Belray 5wt. Another, Shell Aero Fluid 4 is 4wt. Quantities listed are 430cc in the right leg and 410 cc in the left leg.

I think the 7wt will work well.

 

[jnclem]

You are absolutely right - the specifications recited by Haynes and Clymer are as clear as chocolate syrup. BMW originally listed at least 10 European and other oils and both Haynes and Clymer regurgitated that list without clarity. The one which is recognizable for sure is Belray 5wt. Another, Shell Aero Fluid 4 is 4wt. Quantities listed are 430cc in the right leg and 410 cc in the left leg.

I think the 7wt will work well.

Thanks again Paul. I was able to find some Silkolene 7.5 in town after all. I don't think the .5 makes any difference, its just that I thought it worked well last time. I normally do a drain and fill every year, which is usually just shy of 12,000 miles. I seem to have forgotten last year, so it had been a while. It made quite a difference. I didn't realize how sticky things had gotten. Now I'm thinking I need to add some spacers to increase preload. I'm going to measure and work on that tomorrow.

I did use the Shell Aero 4 once in this bike. My brother favors it in most of the Beemers he works on. For me it felt far too soft, so I tried Belray 10 weight - far too hard, so I've settled on the 7.5.

Now, off to Utah.

 

[pffog]

There is so much misinformation that abounds on the Internet it is like listening to seventeen drunks sitting around a camp fire making stuff up.
................

Quote of the week! Needs to be in you signature line!

 

[jnclem]

Quote of the week! Needs to be in you signature line!

My first thought was, "yeah, and I'm reading this ON THE INTERNET!!"

 

[chasmrider]

the reason BMW specifies different weight oil for each side is, one side deals with compression, and the other side ( right if I remember correctly ) deals with rebound. as far as I understand( I read on the internet ) the different weights just change the feel in your front end, higher the weight stiffer the feel.

 

[jnclem]

the reason BMW specifies different weight oil for each side is, one side deals with compression, and the other side ( right if I remember correctly ) deals with rebound. as far as I understand( I read on the internet ) the different weights just change the feel in your front end, higher the weight stiffer the feel.

I think you're right. Problem is, my manual says nothing about different weights in each side. I can only find that on Internet lore. I know my bike has had the same weights in both sides for at least the last 6 years. Prior to that , I don't know.

 

[PGlaves]

the reason BMW specifies different weight oil for each side is, one side deals with compression, and the other side ( right if I remember correctly ) deals with rebound. as far as I understand( I read on the internet ) the different weights just change the feel in your front end, higher the weight stiffer the feel.

I have never seen the difference in viscosity one fork to another in any BMW specifications and I've looked at and read many, many manuals. I don't doubt that some experimentation has taken place and that somebody liked the feel of the forks better if the weight was varied in those few models I can't remember any more which have compression damping in one leg and rebound in the other.

I have experimented with varying viscosity in matched forks and varying fork oil volumes to allow or limit dive. But if/when I write about those experiments they are described as such and not as Bavarian gospel.

 

[8ninety8]

So, when did BMW start using forks with one tube for compression, the other for rebound? Probably in another thread somewhere?

 

[PGlaves]

So, when did BMW start using forks with one tube for compression, the other for rebound? Probably in another thread somewhere?

I'm not at all sure. I vaguely recall that maybe on some model some time. I do remember some classic K bikes had all the damping in one leg with the other passively going up and down with it. My point was that I have never seen a BMW spec calling for different viscosities in the two legs: volumes yes, viscosities, no.

 

[20774]

My Haynes shows different volumes for the R80GS and R100GS, so it would seem that this the time period they had the different fork arrangements. Haynes also separates these GS models in terms of fork oil types. None of the GS oils are used in any of the other models.

 

[PGlaves]

My Haynes shows different volumes for the R80GS and R100GS, so it would seem that this the time period they had the different fork arrangements. Haynes also separates these GS models in terms of fork oil types. None of the GS oils are used in any of the other models.

I found the same - different volumes. But not different viscosities. And no specific mention of compression damping in one leg and rebound damping in the other. I'm still waiting for somebody who actually knows or has specifications for this split in the damping.

 

[20774]

Maybe Mr. GSGiant, Ian, can 'splain how it works!!

My SWAG is that this is because of the superior forks on these bikes. I can see on the older forks, you need to have compression and damping working in parallel on each side of the front end, otherwise the geometry gets out of shape and things bind up. But with a much stiffer and superior front end, why not specialize each fork to one task or the other. With the extra stiffness, the geometry is not affected and better front end action is the result.

 

[8ninety8]

I'm only guessing, but, if wheel and axel were removed, and the fork brace if so equipped, one sider would slide up easier than the other, and the other would slide down easier than the other? Vooella, you've got a split compression/rebound front end?

 

[20774]

Anton has a nice right up on the R100GS front forks. Near the bottom of the page, he lists different volumes as well as different weights of oil for each leg.

http://www.largiader.com/tech/gsforks/

Since the compression side has relatively larger holes, it would seem that a higher viscosity will work well enough on that side.

 

[PGlaves]

Anton has a nice right up on the R100GS front forks. Near the bottom of the page, he lists different volumes as well as different weights of oil for each leg.

http://www.largiader.com/tech/gsforks/

Since the compression side has relatively larger holes, it would seem that a higher viscosity will work well enough on that side.

OK, now we know it is the Airhead GS that has forks with the damping split - one for rebound and the other for compression. And we know that BMW specifies diffeering volumes - the same 470 cc at rebuild but differing volumes on fork oil change. And we are told that riders have found they like a heavier oil on one side compared to the other. Still no mention that BMW specified this - just that riders have found they like it.