Fork oil question

[James.A]

After quizzing a couple of experienced riders where I work, I will put the question to this esteemed forum. Is there any reason NOT to use 10w motor oil in the forks of my /5? I found 2 quarts of straight 10w Wolfs Head motor oil while rooting around in my shed last weekend.

[bprigge]

Might be a little thick if you live in a cool climate, resulting in a stiff front end.
Also, I understand that real fork oil has some kind of 'seal swell' agent in it that keeps the seals soft and leak free longer. Bruce

[flash412]

After quizzing a couple of experienced riders where I work, I will put the question to this esteemed forum. Is there any reason NOT to use 10w motor oil in the forks of my /5? I found 2 quarts of straight 10w Wolfs Head motor oil while rooting around in my shed last weekend.I have ALWAYS used 10wt fork oil. That is the cheapest performance-handling modification you can do on an airhead. The improvement in handling is significant. I have never met anyone who has gone back to "water" after running 10wt oil. Besides... if you find you DON'T like it, you haven't lost more than a little time to change it to the oil you were going to buy anyway.

But... I dunno for sure about MOTOR oil part of your question. Sure... why not. Maybe your seals won't last as long. Maybe there will be no difference. Maybe you'll never be able to tell anyway.

[lorazepam]

A really cheap guy at work used 15wt hydraulic fluid in his forks for years cause he could get it for free. It sure made the bike firm in the front, and his fork seals never seemed to go bad. I think I will stick with 10wt.

[James.A]

....I'm gonna try it. I figure I can get at 3 changes out of 2 quarts. If it works out, vintage 10w motor oil will become the new "must have" accessory.

[BMWRich58]

After quizzing a couple of experienced riders where I work, I will put the question to this esteemed forum. Is there any reason NOT to use 10w motor oil in the forks of my /5? I found 2 quarts of straight 10w Wolfs Head motor oil while rooting around in my shed last weekend.


...might have something to do with "foaming"....who knows..??......

[manicmechanic]

There are some "vintage" Japanese bikes that recommend ATF in the forks. FYI.

[James.A]

Ya never know what will emerge the first time you dump the oil out of a case or system. When doing the recent transmission change, a reddish colored oil came out of the driveline on that bike. It smelled like soiled diapers. A sure sign that it had been in there too long. Dumping the transmission oil on my friends non-shifting R75/6, all that came out was a half a coffee cup of sulfurous yellow paste.

The poop smell = bad.

[boofer]

Non-detergent oil probably won't foam.

Tim

[pmdave]

I've used 7W fork oil, and ATF, both with good results. Flash is probably right that 10W fork oil is a good choice, especially with a high-mileage bike.

While any oil of the correct viscosity should work, the small orifices in dampers, plus the air present, lead to foaming. And of course, foam works differently than liquid. Oils such as ATF and gen-u-wine fork oil are designed to resist foaming.

I've also seen some engine oils that foam much more than others, particularly that stuff in the yellow cans that reminds you of Pennsylvania. On my old Honda 450, there would actually be gobs of creamy foam in the cavities.

pmdave

[James.A]

How would foamy fork oil manifest to the rider?
BTW, I'm still gonna try it.

[manicmechanic]

Since the foam is considerably less viscous (thick) than the oil, it would flow through the damping orifices much easier. Damping would be reduced, and the forks would bounce more. I believe it was primarily for this reason that suspension manufacturers went to gas-charged shocks. They also went to the external and remote reservoir for better heat dissipation, as hot fluid flows easier, thereby reducing the damping.

[pmdave]

Plus, the foam (being lighter) rises to the top of the fluid. So, there would be sudden loss of dampening when the forks extend--say when the wheel bounces down into a pothole, or the bike gets lighter topping a rise.

Having a fork suddenly change it's damping characteristic is not helpful. If the tire can't maintain contact with the pavement, it loses traction. It doesn't have to be a tire flying off the ground, but any change in pressure of the tire against the road changes traction.

Or, to put it a different way, "the manifestation of bad suspension is the rider crashing."

pmdave :thumb

[James.A]

Thanks for the additional info. Bottom line is this....will 10w motor oil in the forks make my bike squirrelly? In the absence of convincincing testimony to the contrary, I am willing to be the test dummy. With a fork mounted windshield, in cross winds, changing lanes on our shattered Illinois highways is dicey. It is already squirrelly as it is.

[CGJohn]

call me capt dumas but i believe if u use a different type of oil other than what is recommended, it just might cause the seals or other parts to fail due to excessive pressure or the 10W motor oil you want to use may not be compatible with the plastic the seals are made of. There might be some kind of failiure (something to do with carbon compounds?) I'm not a chemist but i wish i knew one.... Would that be considered catastrofic failure? i would only use a different type of oil other than what is recommended only in an emergency type of situation. Let me know of your results. After all we are talking BMW, don't you think the masterminds have thought of this already? i think BMW has "been there, done that" and "got a new one". Keep the rubber side down CG John... make mine a double :drink :drink

[flash412]

call me capt dumas ... After all we are talking BMW, don't you think the masterminds have thought of this already? i think BMW has "been there, done that" and "got a new one".You apparently have the old BMW in mind. The NEW BMW spends a lot more money on marketing than they do on engineering. As soon as they find something that works, they sell it. Let the customers test it. Maybe improve it if it has a negative effect on warranty costs. If it breaks AFTER the warranty expires, leave it alone. Bugs that become apparent after the warranty expires are a profit center. (Thank Bill Gates "ethics" for that industrial paradigm.)

Point number two: Time does not stand still. What was once "true" may not be true thirty years later. Ex.: I doubt that you can find motor oil for sale anywhere that has the API requirement on it as stated in the /5 owner's manual.

[dlearl476]

FWIW, my '74 Bultaco owner's manual recomends motor oil (10wt, as a matter of fact) for the forks and I think foaming would be a much more significant factor in a dirt bike.

>It sure made the bike firm in the front, and his fork seals never seemed to go bad.

If you've ever seen what a broken hydraulic line does, especially to human flesh, you'd have to think that hydralic fluid is probably one of the most "rubber-friendly" substances around. It doesn't surprise me that his seals lasted forever. (Which is probably the best reason I would use fork oil instead of motor oil in MY forks. I use BelRay in the Bul)