Upside down fork question...

[08rangerdan]

I had a beauty of a bike. Was a 91 cr250r.. Restored. Was a monster to handle with the horrible forks up front.. Just no adjustability!! Except rebound.

All i can say is that any motocross suspension should be tested at higher speeds. Oil weights are a big factor, and make small adjustments! At low speeds adjustments wont be hardley noticeable. You gotta make that valving work.

If you are just booting around slow speed, id be running a heavy weight oil and firm setting all round. You dont want your wheel moving much, just limits its bite at slower speeds. And remember the rear shock setup directly affects the front suspension predictability. Make sure the rear is set for your weight.

 

[tkent02]

I had a beauty of a bike. Was a 91 cr250r.. Restored. Was a monster to handle with the horrible forks up front.. Just no adjustability!! Except rebound.

All i can say is that any motocross suspension should be tested at higher speeds. Oil weights are a big factor, and make small adjustments! At low speeds adjustments wont be hardley noticeable. You gotta make that valving work.

If you are just booting around slow speed, id be running a heavy weight oil and firm setting all round. You dont want your wheel moving much, just limits its bite at slower speeds. And remember the rear shock setup directly affects the front suspension predictability. Make sure the rear is set for your weight.

No it's different here. Nothing but rocks. Big rocks, little rocks, wet rocks and dry rocks, loose rocks and tight rocks. We have round rocks and sharp rocks too. There are some flat rocks, but usually they are standing on end. Bite isn't a problem, pain is. I ride as fast as the rocks allow, and no faster as falling onto rocks at speed hurts. Especially the big sharp ones. Doing that ten miles from a road or another person would be a bad thing. I want my wheel moving as much as it needs to for the big rocks, and it does. It actually feels pretty good on the big rocks, going fast it's really good. My problem is the little rocks. They hurt if the suspension doesn't move. It's not a damping thing, it a stiction thing. Damping does nothing if the forks don't move.
This isn't a motocross suspension, maybe it used to be but no more. It's a rock suspension.
However the bike works at higher speed and smoother places I can deal with, but it must have a smooth ride on rocks or I can't ride it.

Oh, we have some roots, and logs, but not as many as rocks.
Also we have holes, where a tight rock used to be, before it turned into a loose rock.

 

[Guest]

For rocky and woodsy type terrain you need to have the high speed shim stack opened up. And you need to have the proper spring rates as well. For motocross, we usually go up in spring rate to keep the forks riding high and control the movement via the shim stacks. For woods however, if I'm looking for a plush ride, there are two schools of thought. One is to again use stiffer springs and control the movement via the stacks. The other, more old school method, is to drop the spring rates and allow the forks to settle under the bikes weight about 1/2". I prefer the old school method if I know I will not be doing any jumping and will be hitting a lot of rocks and square edged type obstacles.

Stiction is going to be higher on the UPD forks than any RSU forks by design. Not much you can do about it.

It takes a lot of trial and error to dial in a set of forks but it does pay off. Good luck!

 

[tkent02]

For rocky and woodsy type terrain you need to have the high speed shim stack opened up. And you need to have the proper spring rates as well. For motocross, we usually go up in spring rate to keep the forks riding high and control the movement via the shim stacks. For woods however, if I'm looking for a plush ride, there are two schools of thought. One is to again use stiffer springs and control the movement via the stacks. The other, more old school method, is to drop the spring rates and allow the forks to settle under the bikes weight about 1/2". I prefer the old school method if I know I will not be doing any jumping and will be hitting a lot of rocks and square edged type obstacles.

Stiction is going to be higher on the UPD forks than any RSU forks by design. Not much you can do about it.

It takes a lot of trial and error to dial in a set of forks but it does pay off. Good luck!

Springs are right, sag is right.
Well my other bike with upside down forks rides smooth as can be in rocks. So do other bikes identical to mine that I have ridden. Shim stacks don't do much if the damned things aren't moving. Just rode it an hour on some dirt roads, nothing too rocky. This thing sucks. Worse than all the bikes in the garage, worse than the '84 XT 250 with the rusted original shock, far worse than my wife's DR 200, worse than the 1978 TS 250 with original crap suspension, worse than a stock 1979 GS 750 on a dirt road.

I think I'll part it out, just because I'm sick of looking at it.

 

[tkent02]

Actually I won't prt it out, I like everything else about it. It's light, powerful, great brakes, everything but the fork works perfectly.

Maybe I should keep an eye out for those '98 RM forks. I wonder if the triples would fit right in...

 

[Guest]

I know lots of folks swap out the WP for showa's or kyb units. I doubt the triples would fit. kTM uses odd ball sizes for just about everything...

 

[tkent02]

Ya know, there's still something wrong. I don't have to swap anything, just need to fix these. I have ridden other bikes just like mine, identical bikes, with the same forks and shocks. Also ridden quite a few other KTMs, different sizes and year models. These bikes have all given a very smooth nice ride. Comfortable, even in rocks. Plush. They were not all set up for my weight, but they were pretty darned nice anyway. And mine is a piece of crap.
Just need to find out what is making these forks bind up...

What else can it be?

The tubes are straight, all four of them. The axle is straight.

WTF?

 

[Guest]

Well, you said that they are ok on the big bumps but stiff on the squared edged, smaller bumps and rocks correct? If that's the case I doubt they are bent. So I would bet that either the damper rod is clogged with something or the compression adjuster is clogged. Did you disassemble and clean everything out good with compressed air? The compression adjuster will require some special tools to disassemble. Again, an expert will be required.

Someone may have also had them modified for SX or MX in which case they would exhibit this behavior. They could also have had the stock springs replaced for some very stiff ones for a heavier rider. If you can't compress the fork by hand an inch or so, then I would suspect the springs have been changed out for someone much heavier than you.

Good luck!

 

[tkent02]

Well, you said that they are ok on the big bumps but stiff on the squared edged, smaller bumps and rocks correct? If that's the case I doubt they are bent. So I would bet that either the damper rod is clogged with something or the compression adjuster is clogged. Did you disassemble and clean everything out good with compressed air? The compression adjuster will require some special tools to disassemble. Again, an expert will be required.

Someone may have also had them modified for SX or MX in which case they would exhibit this behavior. They could also have had the stock springs replaced for some very stiff ones for a heavier rider. If you can't compress the fork by hand an inch or so, then I would suspect the springs have been changed out for someone much heavier than you.

Good luck!

Yes, big rocks and landing off little jumps and such, very nice. Just sticks to the ground. Also with the front wheel in the air, very nice. The rear wheel just sticks to the ground and digs in. But unfortunately, I can't wheelie forever.
For every big rock there are a hundred little ones, they are bone jarring.

I put in the correct springs for my weight. I can compress them by hand once the fork moves. It is stuck. It's not a damping thing, it's a stuck fork thing.

If I hold the front brake and push hard, I can compress the forks about five to six inches, just about like my DR 350, which is also sprung for my weight. The difference is the DR will move a little if I push about an ounce, the EXC I have to lean into it to get it to move at all. Stuck. The other difference is the DR rides over rocks very well, smooth and comfy. Nothing is bent, all four of the tubes are straight. Measured with a dial indicator while rotating in a truing stand.

The suspension expert took the compression adjusters apart, made some valving changes, didn't mention anything wrong in there. I didn't take them apart, just took the whole cartridge out and brought it to him, but everything I saw looked clean as a whistle inside. The oil was clean, not perfectly clear green but pretty darned good.
I have never seen stuck forks before that didn't have something bent or extremely worn out.

I wish someone with your experience were here to look at this with me.

 

[Guest]

Yeah, me too tkent02. Something is certainly f'd up. I'm thinking something is not assembled properly or possibly an incorrect ID seal?

If you take the fork out of the triple clamp, can you move it by hand then? If so, remove the spring, put the cap back on, and see how easy it is to compress by hand.

Do these forks have preload adjusters?

 

[Guest]

I also wonder if the bushing is moving on the slider once it's assembled and jamming until there is enough pressure to get the fork sliding. It's time to disassemble and carefully check everything with a fine tooth comb. If you don't have a manual, it's time to get one I think.

 

[tkent02]

Yeah, me too tkent02. Something is certainly f'd up. I'm thinking something is not assembled properly or possibly an incorrect ID seal?

If you take the fork out of the triple clamp, can you move it by hand then? If so, remove the spring, put the cap back on, and see how easy it is to compress by hand.

Do these forks have preload adjusters?

The seal is fine, with the spring out it slides easily, a little seal drag but about what you would expect for new seals. The suspension guy measured it at two pounds pull which he said is normal. They slide smoothly the whole way, and both sides had the same two pounds pull. They have no preload adjusters, you add spacers on top of the spring. I have tried with no preload, and again with slightly less than was recomended by the spring guy. (ran out of spacers) Near as I can tell it sags the right amount, but with all the friction it's hard to tell. Then the suspension guy put a little more preload spacer in, still no change in the friction.

When you bolt the triples together with the forks and the axle with no springs and torque everything down it gets a little tighter, but still not too bad. When you put it all together on the bike and put weight on it is when it really gets sticky.

I don't get it.

 

[tkent02]

I also wonder if the bushing is moving on the slider once it's assembled and jamming until there is enough pressure to get the fork sliding. It's time to disassemble and carefully check everything with a fine tooth comb. If you don't have a manual, it's time to get one I think.

The suspension guy put in different bushings although the new ones I put in were fine. He shimmed them into the fork tubes to get rid of a little slop, but it made no difference in the friction.

 

[salty_monk]

If the sag is right I would look at the compression damping circuit....

Seems like you've tried everything!

 

[tkent02]

It's not the damping, it's just stuck. Friction or stiction, whatever you want to call it. The fork doesn't move at all until you really lean into it. But once you do force it to move, it moves about what you would expect. Then it sticks there. Same thing while riding, it won't move until you hit a big bump, or hit the brake or throttle. Then when it does move it sticks in the new position.

Tried a lot but not everything yet. I can replace the triple clamps, maybe they are misaligned just enough to screw it up. Actually I have a buddy here with the same bike, I can swap my forks into his bike, or maybe swap his forks into my bike. This would prove if it's the triple clamp or not. The only other part in the equation is the axle, but it is straight as close as I can measure it.

OK, thanks for helping me think of the fork swapping idea.

 

[tkent02]

OK, I found the problem. A buddy let me borrow his bike for testing purposes, a 520EXC of the same year. Put my forks on his bike using his triple clamps, torqued everything down, the forks move up and down as they should. Nice and smooth. Apparently my clamps are FUBAR.
Off to Ebay.