I was recently thinking about replacing the fork oil in my VFR 750. I heard some recommendations for 10w oil in the forks, so I thought why not use some 10w40 engine oil? 10w40 is 10w when its cold, no? Well it isn’t and I needed to learn something about oils.

I read on the VFR mailing list that people are using either ATF or fork oils. I assume that the needs of transmissions and forks are vastly different. If all these very different fluids can be used in forks with no massive problems, then it probably doesn’t matter all that much what fluid is in there as long as it provides the right damping without causing wear and corrosion. And I am fairly certain that specially labeled fork oil has a higher profit margin than engine oil because people buy it in smaller quantities. So any business would prefer that you buy specially labeled fork oil, even if it’s hardly different than other oils.

Although fork oil might be labeled as 10w, the 'w' is not the same as for engine oils. When you look at the viscocities (measured in centistokes at 40 deg C and 100 deg c) 10w40 engine oil is about 3 times as 'thick' as 10w fork oil even both are labeled with the same 'w' value. 5w30 engine oil has about the same viscosity as the heaviest fork oil labeled 20w20. There are also special 'cartridge fork oil' which have about the same viscosity as 5w fork oil. Synthetic Amsoil ATF has viscosity between the Spectro 10w and 15w fork oils.

So 10w40 engine oil should give 3x the damping of 10w fork oil. I assume damping is linear with viscosity but I'm not a mechanical engineer, so I am not sure. And 5w30 oil has the same viscosity as the heaviest fork oil. Then there is the question of variation in viscosity (and therefore damoing) over temperature. Suprisingly, Amsoil ATF keeps its viscosity more constant through temperature variations (higher viscosity index). I think this means that, only considering damping, ATF is better (more consistent damping over temp changes) than special fork oils.

You might think that fork oils handle the cold better than engine oils or ATF. Strangely it’s the other way around! The pour point (temp at which the oil stops flowing) for fork oils is *higher* for equivalent engine oils and much higher than ATF. So fork oils would lead to a harder suspension at low temp than equivalent engine oils or ATF. When you compare engine oils and fork oils of similar viscosity (i.e. 5w30 engine and 20w20 fork) the 5w30 has a pour point of -34 deg C and the 20w20 has -22 deg C. In comparison, Amsoil ATF synthetic has a pour point of -56 deg C. The pour point of 10w40 engine oil is about -29 deg C and for 10w fork oils about -32 deg C. So even though 10w40 engine oil is 3 times as thick as 10w fork oil, the both stop flowing at close to the same temp. So concerning thickening at cold temps, fork oil is worse than equivalent engine oil and much worse than ATF. I know it sounds weird, but these are just the numbers from the Spectro and Amsoil sites.

The other points made about fork oil is that is should have anti-foaming agents to make sure that damping is consistent over hard use. Oil that foamed up and filled with air isn’t going to dampen very well. How much degradation in performance depends (I assume) on use. Foaming and subsequent degradation of damping might only be a problem for rough use i.e. racing or motorcross etc. I can’t imagine that engine oil is going to massively foam up for normal street riding. The suspension doesn't work all that hard on the street, at least not compared to motorcross racing. IIRC some engine oils mention that they have anti-foaming agents, but there is the question of amounts. I don’t know if ATF has anti-foaming agents.

Another thing mentioned about fork oils is that they have special stuff called 'seal conditioners' to keep your fork seals flexible. I dont think engine oil is going to eat through the seals and I suspect that fork seals are make of the same material (anyone know this for sure?) as the multitude of seals in the engine and transmission. So I seriously doubt that putting engine oil or ATF in the forks is going to ruin the seals.

I still haven't decided what to do with my forks. I have my front lowered a bit because I prefer a bit of oversteer and so there is more weight on the front of the bike and the front suspension feels much too soft. I would like to have a 'tighter' front end feel, but maybe a 3x increase in fork oil viscosity is too much. Then again I am a curious type and might try it just to be obstinate. My current guess is that it's not going to destroy anything.

I got most of my info from:
http://www.spectro-oils.com/products.htm
http://www.amsoil.com/products/atf.html

Following up on some mails to the VFR list about the viscosity of fork oils vs motor oils vs ATF. Although I know that 10w40 engine oil is probably too thick for forks I decided to try it anyway as a part of an experiment. Just to see how it works out and maybe learn something about motorcycle suspension. With the original (unknown) fluid, I had problems with excessive brake dive, and occasionally (just once) problems with suspension bottoming from big bumps during braking. The fork oil that came out was clean, but there didn’t seem to be much of it. I didn’t measure how much came out each leg, but I doubt it was about 400 ml that my manual says should be in each leg. So it’s possible that the air gap on my old forks was too large. This would cause a linear, as opposed to progressive, suspension feel and may have been the reason for the excessive brake dive and bottoming.

From what I could read on the net about motorcycle suspension I see lots of advice to increase compression damping to counteract excessive brake dive. At first I found this difficult to believe, since damping only changes the rate of change of fork length. When you are at maximum braking, the forks are compressed but aren’t changing their length anymore. Compression damping might make maximum dive occur more slowly, but doesn’t change the amount of dive. But really the link between compression damping and fork dive is more complex. The reason why people say to increase compression damping to reduce brake dive is to change the balance between compression and rebound damping. If rebound damping is too high compared to compression, then the forks will ‘pump down’ on bumps during braking. In this case the damping is working against fork extension, but working less against fork compression. The result is compressed forks during braking.

Since I only changed my fork oil and didn’t change any valving, I assume that I have changed both compression and rebound damping. Since 10w40 is much thicker than fork oils I assume that I have equally increased my compression and rebound damping. The VFR doesn’t have separate compression/rebound damping adjustments so I cant adjust them separately. Since I want some fork dive back, I have to consider spring rate and the air gap.

With 10w40 in the forks and a correct air gap (177 mm IIRC), the bike tends to standup in corners much more than it used to. It had some of its understeer characteristics back from before I had raised the forks in the triple clamps. To get more weight on the front and get some oversteer back I raised the back with 2.7 mm shim under the shock and lowered the forks in the triple clamps even lower, to about 2 cm.

Even so there is a lot less brake dive. I have a feeling that I can’t brake as hard as before because of a lack of weight transfer to the front. I used to have the feeling that when I got hard on the brakes in a straight line the bike would squat down and allow me to brake even harder. With less brake dive, the bike’s center of gravity is higher during braking than before so I can’t break as hard without the fear of losing traction at the rear. The bike might stoppie easier, but that’s not what I am looking for.

There doesn’t appear to be excessive harshness over bumps in the sense of ripping my hands off the bars from bumps although it is clearly stiffer than it was before. So my initial suspicion is that damping with 10w40 in the forks isn’t mind-numbingly excessive. I would like to have more brake dive to help weight transfer to the front during braking. I’m not going to change my springs because its too much work and money but I may try to increase my air gap.

I’ll ride the bike a bit more and then change the air gap. I’ll try to keep the VFR list informed about my little experiment and share what I learn.

I realized that I still had my front preload at maximal from my previous attempts to reduce brake dive. When I reduced the preload I got a smoother ride and a bit more brake dive. I can now dial in exactly how much brake dive I want by changing my front preload. Although I am far from an expert I think the bike handles much better that it did with the original fork oil. It feels less sloppy while being reasonably compliant.

What I learned from all this is that if you use thicker fork oil you get more damping which allows you to reduce your fork preload to get the same brake dive. On every bike I have ever owned I have always had front preload at max. This is probably related to the fact that I also have always lowered the front end of my bikes to get a slight oversteer. Either that or the bikes I have had are all undersprung. Now with the thicker fork oil and correct air gap I actually have to back off my fork preload a bit.

If you are looking to do things ‘right’ then my research says that ATF is probably the best oil for forks assuming you can get it in the viscosity you need.

If you are looking to experiment I would say that 10w40 in the forks does not destroy the handling of the bike. That being said, I admit I am certainly no connoisseur of front suspension. The handling is definitely better than it was before, especially in sweeping corners. I seem to be getting used to the handling more and now hard braking doesn’t feel all weird and I have actually increased my preload to close to maximum again. Under hard braking the front-end dives much less that before but the lack of dive is IMHO not excessive. Hard braking and some well placed bumps might still bottom the suspension but I haven’t tried that yet. Of course the improvement in handling could be just as much an effect of having a correct air-gap instead of the (probably) too large air gap that the bike had.

Doug Eleveld (deleveld@dds.nl)