Shock & Fork Oil Question

8 replies to this topic
  • bigtooth

Posted June 03, 2006 - 06:03 AM


I have a '91 xr600r and I bought an extra shock off ebay that i'm trying out rebuilding. I have it appart and the seal head looks alright, so I was just going to change the oil and get it re-charged and see if what it feels like. My question is what oil (kind and weight) should I use? I am 220lbs and ride some pretty rocky stuff, I know the my stock suspension is not adaquete for me but I don't have the money to upgrade so I wanted to get the most out of what I have.

I have bought some bel-ray 15w Fork oil that I was going to put in my forks and I also have some Yamaha SAE20 Fork and Shock Oil, I was going to try this in my shock. I want some higher weight oil in my suspension to stiffen up my dampening for my fat ass....right? Will these oils work?

I have read a ton of of articles and been to all the suspension pages out there, but this suspension tuning crap still seems so confusing to me, I can't make heads or tails of it. Any advice would be welcome.

  • Jon-D

Posted June 03, 2006 - 07:06 AM


I am not sure if you will get good results by using a higher weight oil in your shock. if you do this, then you may not get a plush feeling although I would expect it to help with bottoming. try adding a couple of shims on the compression stack and use shock specific oil (light). also, since you are in there replace the seal even though it may be good you don’t want to go back in when you are done.

another consideration, when you have the funds consider replacing the spring with a stiffer one and that will improve your ride as well.

  • cleonard

Posted June 03, 2006 - 07:09 AM


I've done part of what you are planning on doing. My problem is even worse as I weigh in at about 260. The best thing to do is get new springs. The setup will never be optimum without them. However, new springs are not in my bike budget at the moment, so here is what I have done.

My fork seals started leaking, so I had to re do them. When I had the forks apart, I saw that the bushings were worn out. New bushings and seals ran about $90. When I put the forks back together, I used Dextron ATF. It's about 15 weight. The stock fluid is 5 weight. I also put the maximum amount of fluid in the forks. I think that I now have the compression damping at 2 out from max. I only have a few rides on them after doing this, and it seems to be working great. I was worried that the non adjustable rebound damping would be too much with the higher weight of ATF, but so far I haven't had any packing issues yet. If I do, I'll go back to a lighter fluid.

On the rear, I cranked the preload on the spring to max level. I've also added some more compression damping. The clicker has 20 steps. I have my compression 7 from full hard. This is with the stock fluid. The overall result is great. I've been taking the bike off of some small jumps and it handles them way better than it used to.

I worry that 20 weight shock fluid may be too thick. While both compression and rebound are adjustable, they might not be adjustable enough to handle that thick of a fluid. I mostly worry about rebound. If the rebound damping is too high then the shock can't extend fast enough. In whoops, this results in packing. Each whoop compresses the shock. If the rebound damping is too much the shock can't extend for the next whoop. After a few whoops, the shock is mostly compressed and you end up with no more travel.

Why did you take the shock apart? Was it leaking? Did you do it just to change the fluid?

  • bigtooth

Posted June 03, 2006 - 07:58 AM


Thanks for all the great insite. So if the ATF fluid is about 15 wt then I think i'll try the 15w bel-ray in my forks. As far as the shock fluid what is the stock weight? Also can you tell me what the minimum spring height is when maxing out the pre-load? I took the shock apart because it had no nitrogen pressure when I got it off ebay and I figured I may as-well give changing the oil and checking out the parts a shot.

I read an article out there that talked about cutting part of a coil off the end of the spring and this will increase the weight limit of the spring. Has anyone tried this?

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  • creeky

Posted June 03, 2006 - 09:02 AM


I read an article out there that talked about cutting part of a coil off the end of the spring and this will increase the weight limit of the spring. Has anyone tried this?

Cutting off a short section of the spring ( I would start with not more than 1 or 1 1/2 inches) and replacing the cut off section with an equivalent length of PVC pipe will effectively bring up the spring rate. I have had several XR600s and would not reccommend putting oil thicker than 10wt in the forks. Most ATFs are 7.5 wt.

  • Strick

Posted June 03, 2006 - 09:43 PM


ATF is 7.5 wt. Heavier oil will aid in damping (a couple of clicks), but bottoming is controlled by oil volume (height).

I have heard about he same shortened spring trick. Actually I have a street bike friend that does this on bikes. I will have a talk with him. I have never done this though.

  • qadsan

Posted June 03, 2006 - 10:31 PM


You guys may want to recheck your viscosity numbers. Go to any oil manufacturers sight and look at the viscosity of their ATF's. For instance, Mobil's synthetic ATF has a viscosity of 7.2 cSt (Centistokes) at 100C and 35 cSt at 40C. Centistokes are completely different than SAE weights. 7.2 cSt at 100C (212F) equates to about a middle range SAE 20 weight oil. The oil in our forks probably won't be seeing this temp too often, so the 40C (104F) scale would be more appropriate. A 35 cSt fluid at 40C is kind of on the border between a SAE 15 & 20 weight oil. So depending on the temperature of a common ATF, it will likely be working in the 15 to 20 weight range for you and not in the 7.5 weight range that you've probably read from numerous other people who see the 7.2 cSt rating and assume that it means 7.2 weight. Here's the link to Mobil's Synthetic ATF for your reference...

And as you get into the racing ATF's or ATF's made for higher temperatures, you'll find that some of them are even more vicious, especially at lower temperatures. There's nothing wrong with using ATF in your forks. Don't let the weight scare you and do give it a try if you've never used it because you just may like it. If not, it will be a good learning experience. ATF's do have some very good properties that make them a great choice for some motorcycle suspensions, but its good that we have plenty of choices because there isn't a single fluid that's best for all riders, all bikes, all applications, etc.

  • bigtooth

Posted June 04, 2006 - 03:00 AM


man thats what I love about tt everytime I read a forum I learn somthing new. Well I got the shock back together with 5w honda fluid. The shock seems to be working great, no leaks and nice and smooth action. I'm going to go have it charged this morning, what pressure should I ask for 170 psi, 225psi? Also I would love to know how to adjust the pre-load to its maximum setting, how is this measured? Is it the spring height, if so does anyone know what that is? TIA1

  • cleonard

Posted June 04, 2006 - 09:09 AM


My Honda manual says this for the spring length (90-2000)

STD 200mm 7.87 in
MAX 195mm 7.68 in
MIN 205mm 8.07 in

I have mine at about 7.7.

Pressure is 15kg/cm or 213 psi

One more thing about the suspension. Play with those clickers. Start with them in the stock position and try changing the settings. Never change it more and two clicks at a time. One click is better unless it is way off. On mine one click on the compression settings makes a big difference. The changes that I have made to my XR600 have really helped the bottoming. It's come at a price however, the suspension is more harsh over rocks and smaller bumps. I'll take that for the bottoming improvement.

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