Nose diving and Fork Oil

[jonathanrowley]

Another item I need a little info is Fork oil.

I filled them up to spec in 2014 when I built the bike but coming to a hot climate with the Bike and it seems to dive quite a bit on braking.

Any ideas - should I top up or use heavier oil?
Can you use Engine Oil?

Maybe a daft question, and I'm ready for the scolding!
But I just bought some 20w 50w for a change and service now its in the desert - think it deserves it.

Remember it's 110 degrees F here - all the time!

[thrang]

I would not use engine oil, but it might be worth while going to a heavy weight fork oil, and change it more regularly than those of us in colder countries need to.  

[wilcom]

Remember it's 110 degrees F here - all the time!

Mr. Roller.................... almost makes you fill chilly there in Phoenix, huh?

[georgesgiralt]

Two questions :
Are you the only one riding a bike here ?
Have you asked what construction people use as hydraulic oil on their jacks (and the difference from less hot climates)?
If you respond no to the first question ask you fellow motorist for the kind f oil they use and adapt to the BMW. When you know the difference in oil the construction people use in their jack, you will know in which way adapt the viscosity of your fork oil.
Two things to remember : You *need* an hydraulic fluid. not an engine fluid. BMW fork need quite low viscosity oil in their forks.

[Barry]

Increasing the viscosity of the fork oil will help but at the expense of a stiffer ride.

There is another thing to try at least in the first instance. The air space above the oil acts as a progressive pneumatic spring which increases the overall spring rate as the forks compress.  You can tune that pneumatic spring within the limits of acceptable oil level.  The oil level measured with a dipstick should be between 20mm and 50mm above the top of the damper piston.  Adding oil up to the 50mm level should reduce dive.   If it's still not good enough then try thicker oil.

Engine oil would be vastly too thick and would not work well in the forks. At your ambient temperature 20W50 is 175 cSt compared 30 cSt for a typical 10W fork.  

[jonathanrowley]

The oil level measured with a dipstick should be between 20mm and 50mm above the top of the damper piston.  Adding oil up to the 50mm level should reduce dive.  

Great - I'll take a look at that... see if I can find some Fork oil here... How big would the dipstick need to be...?

[Bob_Roller]

You can experiment with different weight oils and quantity, but if it doesn't work out for you, you may want to get Progressive Suspension springs for the forks .

No, it isn't 110 F, 43 C all year long here, but we have 5-6 months of 105-120 F, 41-49 C, we actually have a ' winter ', but it's mild compared to the rest of the northern areas of North America .

[Barry]

How big would the dipstick need to be...?  

Approx. 500 mm.  

I always use the dipstick out of my car.

[Tony Smith]

BMW  specified an obscure and no longer available Milspec aviation hydraulic fluid, I once spent an interesting hour with some engineers to discover that Dexron II is a direct allowable replacement for the no longer available fluid.

Sadly ATF of any variety is too light (IMHO) for use in R65 forks, but then I weigh a bit more than I should and live in a country not noted for the smoothness of its roads.

The next problem is that motorcycle shops want to sell you fork fluid by weight - there is no more meaningless measure of a fluid's suitability for use as fork fluid than its "weight". The only real way of comparing fluids is their Centistoke number, but a lot of manufacturers don't bother to quote one.

To give you an idea of the scope of the problem, Castrol fork 5 has a higher viscosity than Motul 15w fluid.


Now I happen to like Castrol fork fluid and it also happens to be available just about everywhere. Most people are very happy with Castrol Fork 5, but some (like me) who are fat like a varying blend between Fork 5 and fork 10 (in my case 50/50).


Next, fork dive is more a function of inadequate spring rates than damping, how old are your springs and what do you weigh?

I have heavy duty "progressive" springs meant for an R80ST in my forks and they each have a 1.5" hollow slug on top of the spring to preload them.

My advice is to set your "static drop" of your forks to around 1" by varying the preload on your springs (if they are in good condition) or using new springs if your old ones are sacked out. A good question to answer at this point is "how do I tell if my springs are sacked out?" - simple, if when you drop them down the fork tube they do not stick out over the top of the fork tubes, they are sacked.

You can 'reset" springs and if you have a local spring works it is an easy and relatively inexpensive job - give the springs and tell them how long you want the free length to be, they will heat the entire spring to red heat and then stretch it until it reaches the desired free length, then re-heat and quench in water.

A friend of mine has been resetting his own springs for years by doing them in sections of about 5" at a time using a little propane torch. I have always been skeptical, but nothing bad has happened to him in the 40 odd years I've known him.


Get your spring rate correct and then worry about fork fluid.

[Motu]

I use ATF for most bikes, and in the BMW, and sometimes hydraulic oil. ISO32 is SAE 10, about the same as ATF, ISO 46 is SAE 20.  They are actually 32 and 46 centistokes at 40c.

[jonathanrowley]

Next, fork dive is more a function of inadequate spring rates than damping, how old are your springs and what do you weigh?

Thanks Tony - I'm circa 100 kilos or 15 stone in English money!
Springs are probably 1981 so 30 years old I guess...

[jonathanrowley]

Get your spring rate correct and then worry about fork fluid.  

Maybe time for new springs then? before worrying about fluid levels, I just assumed it was fluid related.

Maybe I too should go Heavy Duty.

[Barry]

I'm 145lbs in the buff and the stock springs are just fine or based on static sag they are in fact a little too strong (or I'm too light). I would image stock springs are perfect for someone around  170 lbs.

When I first got my bike I fussed about dive but the truth is unless you do actually use very near full fork travel under extreme provocation then the forks aren't set up right. That's what the maximum fork travel is for to get used. If it doesn't bottom out when you attempt to do a stoppie then the spring rate is strong enough.  If the springs still measure to spec and you fill the oil to 50mm I'd be surprised if you can bottom the forks.

Put a zip tie on the stanchion to measure the static sag and also to measure how much travel you are using.