Just changed the fork seals in my 'stock '68 T100C. Prior to changing them, my forks were occasionally "bottoming out" when I hit a rough patch of road so while I had my forks apart, I decided to fit a new set of fork springs, too.
I ordered a set of "heavy duty" springs (as specified for the T100C in the parts catalogue) and it seems to have fixed my the problem of bottoming out, but NOW I have a different problem. Instead of bottoming out, the forks now occasionally "top out." That is, they "clunk" on the rebound.
The topping out isn't happening on every little bump, but the roads can be pretty rough around here -- open pot holes, lumps of asphalt, cobblestones, etc. -- and it's happening more than I expect it should. (it's topping out more often that it was previously bottoming out.)
I've been using ATF for fork oil. Is this too light? Will simply using a heavier weight oil solve the problem or is something else going on? I have some 20w fork oil on order (can't seem to find anything heavier than 15w at the local shops) so I guess I'll find out soon enough if this is a simple fix. In the mean time, any other ideas?
You could effectively soften the spring rate and lower the ride height slightly. Loosen the cap nuts a few threads,or take them off. Fully compress the forks.Use ropes or other implements of destruction to winch the forks to fully compressed.If you only loosened the cap nuts,allow 30 minutes for the air pressure to bleed out around the threads.If you removed them,just screw them up tight while the forks are compressed.
I religiously bleed the air out of forks and run on spring-rate,not compressed air.
Sounds like it'll take a bit of experimenting. 20w is "by the book" although I see 20w or 30w is recommended for the earlier forks. I've got the 20w on order so I'll try that first and see. I've never been able to find anything heavier than 15w fork oil in the local shops and since I have the ATF for my primary, that's what I've always used. Might've been part of the problem with the forks bottoming out in the first place. (Although, aside from them topping out, the forks feel a lot better with the new springs.) If the 20w doesn't fix it, I'll try going heavier.
Pete - I've never heard of bleeding the air out of the forks, but do you have to do that regularly? I can't imagine the forks are so air-tight that air wouldn't bleed back into the forks during and/or between rides -- Am I wrong?
There's too much air in the forks if they're assembled "normally",without bleeding them.You want no internal air pressure when they are fully compressed.I've used 50 grade engine oil in these early forks,but not on a "C" model with competition damping.
Some really smart forks are self-evacuating with one-way valves.Rickman frame is like this.
What you're saying makes sense. I suppose the air pressure could add enough force to make the forks clunk on the rebound. Maybe, while I'm waiting for the fork oil, I'll try to devise a suitable "implement of destruction" to try it out.
Hi Jamie, I think you put the damper sleeves in upside down. These are what look like spacers that fit between the fork bushes. The manual is not clear on which way up they go but as far as I can remenber the smaller diameter hole goes down. These sleeves stop the topping out by restricting oil flow when the bushes get closer together, ie when the forks are fully extended, but the sleeves only work if they are the correct way up. chances are you did not spot the difference in the hole size and put them in upside down. I am speaking from experience here hope this help Steve.
Anything is possible, but I never removed the stanchions from the lower fork legs so if the sleeves are upside down, they've been that way for a long time. It was dumb of me not to do a complete teardown and inspect the entire fork while I had them all apart. (Though, I'm always amazed at how much easier these tasks get with each successive try.)