Fork oil level?

[Crisis management]

I am rebuilding my F4 race bike (that sounds way better than the usual description of the bike ) which is essentially a cobbled together collection of 125cc two stroke sized bits from various road bikes. It's called bucket racing in NZ and conducted on kart tracks, absolute mayhem but lots of fun.
Anyway, I thought I may apply some science to the suspension and am trying to find a better way of deciding the oil level in the front forks other than "that looks about right".
Anyone know of a formula, suggestion, hint, anything? The forks are damper rod, right way up type, definately low tech!

[LuciferMutt]

Are the forks Honda F4 forks? If so, simply start with Honda's spec for the oil level "air gap" which is measured down from the top of the fork tube with the cap off, springs/spacers removed, and the fork compressed all the way. If you use this spec as your starting point you can toy with it from there. Make sure you cycle the fork several times before taking your measurement to ensure the damper rod is full.

[Crisis management]

Sorry, I wasn't very clear on that... the racing class is F4, it's not a Honda F4. The forks were originally off a 1986 Suzuki NZS 250 S (a one year product).
I can probably track down such a measurement if I really wanted to try but am more interested in any formula for calculating the air spring component (controlled by the oil level height).

[racer1735]

Either purchase or go online and find the specs for your machine. It will give the fluid volume, as well as what weight fork oil to use.

Also get a graduated cylinder (large measuring cylinder), a turkey baster and your fork oil.

Remove the fork (loosen the top cap before removing the fork from the triple clamp), then remove the top cap completely. Pull out the spring, then drain the fork of its oil.

Pump the fork many, many times to help remove fluid. Then let it hang overnight to drain more. Pump a few more times and call it good.

There are no standard weights/viscosities for fork and suspension fluid; one manufacturer's 10w is anothers 15w.

A graduated measuring cup (such as one uses to do 2-stroke premix) will get you pretty darn close to the right amount of fluid in each leg. To get it to the right level, I take a simple cooking baster (turkey baster), and mark an appropriate point from the bottom. I then wrap a strip of duct tape around it, with the 'upper' edge of the tape aligning with the point I'd marked on the baster. The baster is clear, this way you can see if you are above that level or not? Simply squeeze the baster tube and insert the baster into the fork tube (tubes collapsed) until the top point of the tape aligns with the top of the fork tube. Then release the bulb and it will suck up any fluid. No fluid in the baster means you either haven't put in enough fluid or its at the right level already. Fluid in the tube is how much excess you need to remove. I do this a couple times just to make sure I'm at the right amount. Also, you don't want to stick the baster in, then squeeze the bulb or you end up with air bubbles in your fluid that will alter the fluid height.

Also, put the fork caps back on before you put the tubes in the bike and the tire back on...much, much easier that way.

Pretty easy stuff once you've done it. You can make minute adjustments in the ride characteristics by either raising or lowering the oil level (by just a few milimeters) or by ising a heavier or lighter fork oil.

[Crisis management]

Thanks, that's a very thorough description, unfortunately I do already know that stuff and what I am after is the theory about the air spring component / oil level height. Maybe I'm over complicating the whole thing but I'm interested in whether there is a calculable volume of oil / air or do I just go back to the old set a level and adjust from experience?

[chollo9]

I think it's down to you and try it and see what works. Even with a theory in hand, your bike, in your situation, is what matters. The best suspension and engine tuners in the world still operate using the time tested method--keep changing it until it gets worse or breaks and then go back one step!

You have the knowledge already, so have at it.

[anotherguy]

Oil level only influences the bottoming resistance in the last third of travel. If you are bottoming on bumpy corner entries try raising level 5MM at a time until you're satisfied.

Oil viscosity effects rebound. Thicker gives a slower more controlled rebound. If you find the bike not responding well on small bumps use a lighter oil.

Use this chart to determine your oil's viscosity. It rates oil in centistrokes (cSt) which is consistent from one oil to another where oil "weight" is not.

[Crisis management]

Thanks for the input guys, I will go back to the suck it and see method. It will be a couple of weeks before I can get onto the track but I will go with "set the oil level, high then reduce it in 5mm increments until I crash" methodology, if there is any learning to come from it I will post it up here.





Keep an eye on Face Plant for any updates!

[anotherguy]

What problems are you having? The oil level is only one part of the puzzle.

[Crisis management]

Originally Posted by anotherguy

What problems are you having? The oil level is only one part of the puzzle.

Not having any particular problems, simply interested in learning the how and why of suspension action and whether there are any "standard" settings for the oil / air volume as a starting point.
The race bike is simply a good tool to fiddle with as I have a standard set of conditions (one track) that I can adjust for and hopefully see the difference made.