[ROSKO]

I gots me some Fork questions....

the set-up I'm running seems to have good compression but the rebound is SLOW. So i added some spacers for preload with very little result.

Anyone want to share their fork set-up techniques?

BORN TO LURK, FORCED TO WORK.
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[jbranson]

Give us some details.
Fork type, oil weight, new oil? old oil? disassembled and cleaned? etc.
JohnnyB

 

[FR]

Yeah.

FR

BTW, when you add spacers, do you just put them above the springs? And how many and what thickness? I've never frigged around with that, just changed the oil and tried to adapt to the forks, or frig around with the height in the triple clamp, or frig around with the rear shocks, since those are adjustable pre-load.

And then I just ride the thing. As Randy Mamola opined a few years ago when he was racing the BMW Boxer Cup, the thing that's adjustable is the rider. Not that I can even begin to suggest that I have any of his riding/adapting skilz, but I also don't have much chassis tuning skilz, so I try to learn how to deal with the vintage stuff by just riding it.

But having said all that, I'm guessing that the fork oil is too heavy, or maybe the springs are just shot?


FR

 

[jbranson]

FR,
Yep you just add spacers of various lengths above the springs under the top caps. Just has an old timer give me a hint...use valve springs of various types to "soften" the effect of spacers and make them somewhat tunable. But heck if you have 35mm forks...just get some real progressive modern springs.

My guess as to Rosko's fork question would be about the same as yours. Although if they haven't been disassembled and cleaned there could be some dirt clogging holes that is slowing the rebound.
JohnnyB

 

[ROSKO]

35mm forks from a cb550(?)
cleaned and re assembled,
10wt (which I used because I tried 20wt in my street bike and it turned into a pogo stick)
1/2" spacer (which was hard to add)

My thinking was to try different stock springs, I have a few more forks kicking around from 650, 750, 450. The thing is that they are all different lengths.

I have heard of people cutting down the springs and adding spacers...



BORN TO LURK, FORCED TO WORK.
www.NYCvinMoto.com
www.VinMoto.org

 

[jbranson]

Same forks I have, I run 20wt in them. I did run 1" spacers but just took them out as it was a bit too stiff.
It would be hard to diagnose without trying them out. Usually slow rebound is not an issue unless the forks are packing down on you in a race....meaning they progressively lose travel because they don't have time to rebound between compressions. It's an easy thing to spot...you come into t3 hard on the brakes in bumps and you crash because the forks are bottomed out and you have no suspension.

How slow is slow? I mean can you sit there and watch them slowly extend back to full length? Or does it just "seem" slow. Keep in mind when you are compressing the forks while sitting on the bike you've got the weight of the bike and you pressing down...on rebound you just have the springs to bring them back up again. When you are just sitting on the bike pushing down on the forks rebound will always seem about twice as slow as compression.

Usually it's better to diagnose a suspected suspension problem by looking for a problem on the track, it behaves a lot differently in motion.
JohnnyB

 

[FR]

When I first started racing flattrack, I put 20w fork oil in the stock forks. I didn't really notice a problem, except on some of the rougher dirttracks going into the turns over some of the "stutter bumps" which are probably similar to T3 at Louden, when I'd get some serious vibes/looseness. I didn't know what to do about it until I watched some video of me racing over those conditions, and the front end was waaay too stiff. It was actually scarier watching the vid than doing the racing, the forks weren't compressing very much 'cause the oil was too heavy, and I had them set up as high as possible in the tree. Just lowering the front end an inch made a big difference, and also going down to 15 or 10 weight helped smooth things out.
My point is, it helped me a lot to see some video to determine the solution to the problem.
I also learned to put a ziptie on my fork leg to find out how much fork travel I was actually getting, put it on just above the slider before you go out for practice. When you come back in note the position to see if you're getting full or close to full compression.


FR

 

[KeninIowa]

Here's some random thoughts;

Have you ever put a zip ty on the fork tube to see how much compression you're getting? We do that on dirt bikes sometimes if we're not sure exactly whats's going on.

Here's a link to a good article about springs http://dirtbike.off-road.com/dirtbike/article/articleDetail.jsp?id=332656 A lot of people don't understand preload vs. spring rate. The article kinda explains that. On a fork spring you can pretty much rate them with a bathroom scale and a tape measure if you want to experiment with the other springs you have. The formula for rate is in there too if you need to cut some down.

Normally if you increase spring rate you have to slow rebound down because like JB said the springs are what returns the forks. All else being equal stiffer spring = faster rebound.

I'd shy away from using valve springs or any other spring as a preload spacer unless you know exactly what you're doing. On a dirt late model thats called a double stack. The other night the driver of the car I work on did this without consulting me first. He put a 400 on a 250 to stiffen it just a little(he thought). Well the formula for this is (a*b)/(a+b)=spring rate. In my example he made a 153 lb spring. What you typically end up with is a soft spring with a ton of stored energy that unloads like a MF given the chance. Might be useful if you want to wheely easier.

04 KDX 200
05 KX 250f- SOLD!
01 GSXR 750
75 GT 550
68 Chevelle
Wanted: 86 FZ600 w/OEM paint

 

[kenessex]

Rosko,
Is it possible that you have some binding in the set up. I usually set the fork up by having all the associated bolts and nuts loose and then working the front wheel back and forth then progressively tightening all the pinch bolts and fender bolts and such so thet there is no stresses on them. If you elevate the front end and remove the fork caps and springs does the fork move smoothly up and down? If not then you might have a problem.

Ken

AHRMA 412
Vintage racing - old guys on old bikes

 

[ROSKO]

Thanks for all the replies! Basically my feeling is that there is too much rebound damping. Compression is fine she just springs back SLOW.

JB, of course you are right, who's to know what it will do unless in a track situation. Honestly the ONLY thing I noticed about the fork AT ALL was that it came back slowly out of the bowl...

FR, good idea. I'll check for stiction and/ or binding as you said

Ratty, thanks for the link! My next step will be to measure the other springs on a scale and compare.

Thanks guys, now that I'm armed with more info I should be able to sort it out.

BORN TO LURK, FORCED TO WORK.
www.NYCvinMoto.com
www.VinMoto.org

 

[ttilghman]

rosko-

shut up.

"the fork AT ALL was that it came back slowly out of the bowl..."

i'm afraid i'm more like FR, i don't think i'm digesting that kind of subtle feedback when i'm out there on the track. mine's more of an overall better or worse impression after a change. like, "wow, my bike gave me an overall worse impression when i ran the FCB race on one cylinder". and keep in mind, as soon as you're done braking/slowing for the bowl, you're turned in hard on the banking so the front may not act quite like it would otherwise.

earlywank-
tt

 

[ROSKO]

hey there buddy, if I keep my bike in the best shape possible I might be able to keep your single cylinder in sight after t2. I can't get much track time in here in NYC, so I'll have to work on the one thing I CAN.

Seriously? I can't help it. I have to do it.



BORN TO LURK, FORCED TO WORK.
www.NYCvinMoto.com
www.VinMoto.org

 

[texmawby]

rosko,

i don't know if mine has any oil in them.

they are attached to the wheel and frame.....good enough for me.

texy

 

[joe c]

you could always try oil in one side. that seemed to work great for zach. won him a championship.



jc

"tex, if your bikes a cheater, its not a very good one"

 

[jbranson]

Oil in one side is really no big deal. Marzocchis (sp) as supplied stock on early 90's Ducs like the 907ie only had dampening on one side.
JohnnyB

 

[racerchris46]

I've run those same forks with 20wt and couldn't get enough rebound damping-had to weld the damper rod holes shut a little to help. I don't think its a weight issue. I'd pull the lower legs off and make sure the holes are all clear and that the aluminum cone thingy in the bottom isn't f-d.

If I read your earlier post, it says you went with 10W because when you put 20W in your same street bike, it pogoed all over the place? If my street bike pogoes with 20w, going lighter is the opposite way of where I would go.

Try 20W50...

 

[ROSKO]

By 'pogoing' I meant it wouldn't compress at all. Might as well have had a rigid fork on there. terrible.


BORN TO LURK, FORCED TO WORK.
www.NYCvinMoto.com
www.VinMoto.org

 

[jbranson]

Rosko,
That type of rigidity....short of a mechanical issue...is almost always caused by way too much fork oil. It won't compress well...or at all. Doesn't take but a few ounces too much to do it.
JohnnyB

 

[rdrce698]

Hey Rosko check out www.strappe.com he details some interesting fork/spring information if I remember correctly
Pete

 

[KeninIowa]

Hey that Strappe site's got a ton of good info. Probably most important and the thing most people don't do is document what you do. His suspension chart is a good idea. I can't remember what I did this morning let alone last week.

04 KDX 200
05 KX 250f- SOLD!
01 GSXR 750
75 GT 550
68 Chevelle
Wanted: 86 FZ600 w/OEM paint