forks oil / air gap

[yozter]

Hi, Sorry to revive an old topic, but there haven't been any recent posts for a while.

I am about to attempt to do my front forks on my newly acquired GS500 2006 model with 46000 on speedo, its a bit run down and needs some TLC.

I have read quite a number of post and am now not sure exactly how to measure the oil/air gap. So just to be specific am I correct in assuming you add the recommended qty of oil and then use a tool (mine will be a large syringe with a pipe connected) to set the air gap at 91mm? tube compressed and spring out, damper rod in?

I am just replacing the oil and dust seals and oil with stock springs. Also is there anything else I need to know other than the excellent info that already abounds on this forum?

Also any tips on removing rust spots from the top of the chrome rod? between the steering

Thanks Yoz

[Endopotential]

Just in case you didn't come across this in your prior research...

FYI, for 2004+ model forks you'll need a slightly different tool to get the damper out.  A shaved down broomstick did the trick for me.

http://gstwins.com/gsboard/index.php?topic=70830.msg851134#msg851134

And keep really good track of the order in which all the washers and spacers come out.  If you get that order incorrect, the damper just won't screw back in.

Best of luck!

[yozter]

Thanks for the info,  I had not seen that and it's not really what I wanted to hear I was planning to make the tool) and had already read of some broom handle attempts not working but I will give it a go.

Thanks again

[crackin]

I have read quite a number of post and am now not sure exactly how to measure the oil/air gap. So just to be specific am I correct in assuming you add the recommended qty of oil and then use a tool (mine will be a large syringe with a pipe connected) to set the air gap at 91mm? tube compressed and spring out, damper rod in?

Yes that is correct.
I'll try to give some insight on how the oil level and viscosity affect fork performance, but i'm no expert.
Firstly, The total fork travel with the springs removed from the fork tubes is 110mm on a GS. So thats where it bottoms out.
  Secondly, the higher the oil level (=less air gap) the less fork travel you will have. And the opposite applies.
Thirdly, the heavier the oil the more dampening you have. And the opposite applies.
  Here is a trick i use to set up my front ends.
I put a zip tie around the fork tube right above the legs. Then i go for a hard ride, and measure how far up the tube the zip tie is. This shows how much fork travel i am are using. We only have 110mm of travel so if the zip tie is
only 70mm above the leg we have 40mm of travel to play with. If its 110mm up, its obviously bottomed out. You can add or subtract oil (increase of decrease air gap) to better use the fork travel.
  Oil viscosity affects the dampening affect. If you want a softer(faster) feel use light oil. If you want it a bit stiffer(slower) feel use heavier oil.
Either way you go, still need to adjust oil level (air gap) To get the most out of the forks.
  The GS has very old school forks, but they can work ok if you tune them correctly.
Then you have spring pre load on top of all this 
  Johno

[mr72]

the higher the oil level (=less air gap) the less fork travel you will have.

While this is technically true, it implies a very bad idea.

You never should have less than 99mm (factory) to 140mm (aftermarket) oil height with the springs removed and fork compressed. Under no circumstances do you want to get even close to bottoming on the oil. You will blow out the fork seals and likely bend and break things if there is not sufficient air space.

  Here is a trick i use to set up my front ends.

Please don't do this. It's completely misguided. This is a good way to evaluate the spring rate but not at all related to fork oil height. If you are bottoming during normal riding you may need heavier springs and vice versa. The zip tie trick will help you know that. But you need to set fork oil level according to the manufacturer instructions, either the Suzuki method for stock springs or the spring manufacturer for aftermarket.

The thing is the springs, washers and spacers all have some volume that needs to be accounted for. That's why there is a minimum extra space required, it's to allow the displacement of this stuff along with a minimum amount of air space. When the fork compresses, the air in the top is compressed as well like an air spring. So let's say there is 50mm air space with everything assembled and compressed, and therefore 160mm with it uncompressed. That air must compress 3.2:1 as the fork compresses. This adds extra spring rate to the fork in a progressive manner. Higher fork oil (the minimum) means less overall air space and therefore a higher extra progressive spring rate, and lower (more space) is less. So higher oil height will result in a firmer ride at the end of the travel, but should never be used to actually restrict travel as the previous post suggested. So there is a little bit of tuning available as oil height affects spring rate as the fork compresses, but you don't ever want to violate the minimum since it will break things.

[crackin]

I didnt mean to imply that going "over" the recommended oil level is the way to go here,its not. What i was trying to say is that you can go lower than the recommended level to improve the action of the forks. Going too high will cause a hydraulic lock and we definitely dont want that. And we dont want to bottom out either. It's a balancing act.
It was just a basic guide to fork tuning and i didnt mean to "misguide" anyone.
Johno