Fork Oil

[Poot]

Hi everybody,

I'm going to be draining out, flushing, and putting in new fork oil in my bike tonight.

My clymers manual says that I can use either 10, 15, 20 weight fork oil, or DEXRON ATF.

since I work at a lube shop, I managed to finagle some castrol syntec ATF (which ill be using to flush them out) and some synthetic Mobil 1 ATF.


Do you think this will cause seals to leak, and is it advisable to use ATF over fork oil?

I'll be draining anyways, but if its not a good idea, i could always drain again and add regular fork oil.


thanks,


poot

[bwringer]

ATF is awfully light -- last time I read anything about it, it's somewhere around 7wt.

It won't hurt anything, though. Your seals and fork innards will love it.

In the admittedly not-factory-fresh forks on my 60,000 mile GS850G, I usually use 20w fork oil mixed with 5% to 10% "motor honey" to make it a bit thicker. Motor honey is a great way to firm up damping on older forks.

[Poot]

what would be the difference in handling with lighter and heavier weights??

[Gary Lich]

I've been using ATF in the forks on all my bikes for 25 years with no problems.

[Poot]

sweet! thanks guys! (mmmmmm......mobil 1)

[Anonymous]

I've been using ATF in the forks on all my bikes for 25 years with no problems.

What is the benefits of using a lighter oil such as ATF??

Does it make the dampening faster or lighter?

[bwringer]

This is horrifically simplified, but...

The damping action is provided by squeezing fork oil through a small hole. Therefore, thicker oil will give you more damping and thinner oil will give you less.

Damping is the resistance to motion that keeps your front wheel from bouncing up and down like a pogo stick. Damping has nothing to do with the stiffness of the spring, BTW.

With thicker oil, a suspension component is less likely to bounce around in unwanted ways. The downside is that it may then be less comfortable, and may actually decrease grip if it's so stiff that the tire skips along rather than following the road surface.

Google for "motorcycle suspension theory" for much more...

[Anonymous]

Damping has nothing to do with the stiffness of the spring, BTW.

Not true. There are basically three damping types to consider:

1) High speed compression - This is damping during a high speed bump motion, where the fork is forced to move at high speed to acommodate a bump. Note you do not have to be moving fast, hitting a speed bump at moderate speeds would qualify. The "high speed" part refers to the fork motion speed, not the motorcycle speed. You want as little of this as possible. It is not affected by the fork spring rate.

2) Low speed compression - This is the damping that prevents a "wallowy" or vague feeling from the front end. This is set largely on personal preference and gives a solid "feel" to the front end. More is better, but eventually it becomes harsh and you back off a bit from there. This is not affected by the fork spring rate.

3) Rebound damping - This controls how the fork extends after compressing. It IS affected by spring rate and a stiffer spring requires more damping to control it. You want enough damping that if you hold your bike straight, hold the brakes and give the fork a good bounce, it goes down, then returns smoothly, slightly overextends past the static sag point and settles. If it bounces at all, or tops out, there is not enough rebound damping. Most damping rod forks NEVER have enough rebound damping no matter what you do to them. So using thinner oil is not a good idea. I would start with the stock weight or one heavier and try that, then move on if you feel you need it.

Mark