Fork oil wt changes and damping?

[Huhwhye]

I'm looking to firm up the suspension on my old BMW GS and wanted to know how fork oil wt changes effects damping and if there is a way to predict or ballpark what I should use. Trial and error can be expensive.

For Example if I go from 10 wt to 15 wt what can I expect? Is there some kind of formula like going from 10wt to 15 wt will give my X% increase in rebound/compression or something like that?

On another note I looked at my owners manual and it lists several reccomended brands of fork oils. What I found interesting is that the wt was different between the brands. That makes sense, each company makes their 10wt a little different. So I guess when I make the change I should stick with one brand so I can tell if there was a true difference.

 

[KW]

Huhwhye, you have the right idea with a thicker fork oil makes the forks a bit stiffer. About the only way to get it sorted is to use the OEM fill as a base line and work up from there. The Suzuki's I had a few years ago did well with type F automatic transmission fluid. It was slightly heavier than the recommended stuff and a lot cheaper too.

 

[Huhwhye]

Thanks for the tip. That's a good point. Because fluid level can change damping too right? Have to keep that constant otherwise results won't be reliable.

 

[KW]

Yes you can play with the fliud level a little, just a little, you don't want to start popping fork seals. The base line I'm refering to is the weight of the OEM fliud it has in it now. If the fork oil has been in there for many miles/years just changing it to a fresh fill might help a lot.

 

[Adam Wade]

Changing the fluid level changes your spring rate, in essence. The air above the oil gets progressively compressed as your suspension gets compressed, so the spring rate goes up at something like an exponential rate; less oil means a softer "ramp" on the spring rate, and more oil means a steeper "ramp".

Changing viscosity in a conventional fork without compression or rebound adjustment increases BOTH compression and rebound damping. Unfortunately, this is generally not what you want. Let's assume that the stock damping was correct for the stock spring, given a rider of a certain weight on the bike. Now let's assume (as is probably the case) that you're heavier than the bike's suspension was set up for. So to keep suspension travel toward the middle of the stroke, you install a heavier spring. HOWEVER... compression damping works WITH the spring (slowing down the compression of the forks), but rebound damping works AGAINST the spring (slowing the return of the forks to full extension). But changing the oil viscosity changes both compression and rebound in the SAME direction.

If you have adjustable rebound damping, depending on its range of adjustment, you may be able to get both compression and rebound near ideal settings with a change in oil; however, the further you get from the stock spring rate, the less likely you are to be able to accomodate both with just a fluid change.

If you have a cartridge fork, not only is damping better (high speed vs. low speed; a simple orifice responds backwards of what you want, so cartridge-type forks use a stack of bendable washers to control damping oil flow), but you can change the washers in the shim stack to balance out compression vs. rebound damping for a given spring. With damper-rod (conventional) forks, you need to drill out or braze up orifices in the damper rod itself to change the ratio of compression to rebound damping.

To really "get it right", the best thing to do is to send your forks, or entire bike, to a reputable motorcycle suspension shop, where threy have years of experience to do things right the first time. I can give some recommendations in California if anyone wants.

 

[Huhwhye]

Good post. But just to complicate things even more on my bike the Right fork does Compression only and the Left fork does Rebound only. Some guys run different wt oil in the left from the right side. Also, there is no external adjustment on the forks.

From my research there is nothing that can be done to improve the rebound side fork. So on that side only spring rate and fluids can be changed to affect rebound. I guess that is why I asked the question. A suspension guy told me he could work the compression side to improve damping. But the rebound side can't be improved by drilling or brazing.

I am planning on running a much stiffer than stock spring. Factory springs are a joke.

 

[castor]

well, i have tried EVERYTHING in my '92 fatboy, can't seem to see any noticeable difference. i've used light medium and heavy fork oil, 15w50, 30,50 and 60 oil, synthetic and mineral, maybe they're just happy having SOME oil present...

 

[jkhawaii]

funny thing about fork oils is no one seems to be able to recommend the proper factory equivalent. sure you can send some out for analysis but they check vis at 212 deg or so
and with differing viscosity indexes one oil may be twice as thick at normal operating temperature than the other
one example is Belrays cross reference to Harley type E. they says to use there 20W, well there 20w at room temperature is
almost twice as thick (using my non scientific method of poring a small amount into a cocking tube end and watching rate of drips) and when put into the fridge over night there was another interesting find, the Type E did not thicken much at all! were the belray 20 was thicker than a 20w50 at room temperature

so that leads me to beleve that the Type E has a much higher
viscosity index. I also sent a Type E sample to Black Stone
and at 212 its vis was a 20w

 

[jkhawaii]

for weight changes you may need to check the oil viscosity yourself. I want to get some redline and ams and do some checks as well

 

[wileyE]

what jkhawaii said. I posted some here: http://theoildrop.server101.com/ubb/ultimatebb.php?ubb=get_topic;f=9;t=000372#000002

 

[Adam Wade]

Huh, I knew some bikes used compression only and rebound only fork legs, but I wasn't aware BMW did it years ago in their GS bikes. Live and learn.

Sure, if compression and rebound are each in their own fork leg, then change viscosity appropriately as needed. If you want to play around with oil levels to change the progression of the spring rate, I would seriously advise makign the same level changes in BOTH forks. While it is unlikely that you will have any front end twisting/bending problems from having different oil levels, it is possible that the fork tubes could cock slightly in the fork legs during suspension travel, increasing both stiction and fork bushing wear. if you use the same weight oils as before and leave a notable difference in fork oil levels between the legs, everything will suddenly feel very stiff, in comrepssion and rebound. That's the increase in stiction from the uneven spring rate between the fork legs. It's very similar to the way the front end feels when a front wheel gets tweaked a little bit from a drop and the fork tubes get slightly twisted in the triple clamps.