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Fork tuning

Old 06-19-2005, 08:23 PM
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Fork tuning

I finally got my former race bike street legal and on the road. (Pics on the way.) I covered about 200 miles on some wicked back roads with a buddy on his Ducati 900 Monster today. We swapped bikes a couple times, and each time he got off my bike he just shook his head at how "spooky fast" it was compared to his Duc. The hard riding pointed out some suspension tweaks that needed to be made.

I got the rear shock dialed in, but the forks are being difficult. My VTR came with CBR600F3 forks with Race Tech springs (I have no idea what weight the springs are rated), and they're adjustable for preload and compression damping only. The problem is that while rolling down the road they continuously "pogo" or bounce slightly. I just had new tires installed and balanced, and it was doing this with the old tires also. I've increased preload and damping, and set them back to factory specs, but it's the same.

Anybody had this problem? Any suggestions?

These forks will soon be replaced with inverted units (GSXR1000 or CBR 929/954 RR). This just irks me because everything else is performing flawlessly.
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Old 06-20-2005, 01:33 AM
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Re: Fork tuning

Are you sure its Compression damping and not Rebound you have ?
It does sound like your rebound is not set properly, or your valves are just not working.
Dont wind up the preload cause this will not be helping your cause, preload should be set to adjust the correct sag level and thats all... IMO

First thing I would do is change the fork oil so you know whats in there.
If you really do have no rebound damping, then i would suggest going to a heavier weight fork oil.
The compression damping will slow the rate that the fork rises, but without correct or any rebound damping they just going to bounce back as fast as the fork oil will let it.
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Old 06-20-2005, 08:04 AM
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Re: Fork tuning

I'll open them up this weekend. I haven't gone through a full static setup yet...that's likely what I need to do just for baseline settings.
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Old 06-20-2005, 09:01 AM
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Originally Posted by rc996";p=&quot View Post
I found this on the another site.
good, practical info thta might be helpful.
I agree, it sounds like too little rebound damping. It can really make the front end wallow. Check out the damper needle mod.. May be applicable to the 600 forks.

http://www.ablett.jp/bikes/vtr/vtr_sus.htm
Excellent link, thank you! I'll have to take one of the forks apart to see if this applies to the CBR forks. There are a handful of GSXR front ends on ebay right now. If I win the bid on one of them I may just live with it for now and do the fork swap ASAP. This bike was set up for the racetrack, and both ends were VERY firm. For the street I had to bring the preload down a lot on the rear shock. I weigh 195, and there was less than 1/2" sag at the rear with me on it. It was beating me up on the choppy back roads I rode yesterday, and occasionally stepping out over bumpy curves. The front end also chattered under heavy braking, which got a little hairy running hot into one particular switchback.

I gotta change out those race-compound brake pads, too. In stop and go traffic they're like wood.
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Old 07-06-2005, 08:37 AM
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Welp, after changing out the fork oil and doing a proper static setup, I finally got around to pulling off the front wheel and checking it for runout. Sure enough, the front wheel is bent like an egg. Someone must have hit a wall or slammed a botched wheelie pretty hard to do this, since it's not detectable with the eye. Thankfully I just bought a complete GSXR front end, including the wheel. As much as I hate to tear it down during the riding season, I may need to waste a good weekend with the front end transplant. Stay tuned... 8)
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Old 07-27-2005, 03:54 PM
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Re: Fork tuning

Quick Fork Damping Check

Compression of the forks is slowed mostly by the spring, so compression damping is doesn't need to be much, just enough to slow things down a bit.
On the other hand, rebound damping is working against spring tension and needs to be much greater.
That's why if any external damping adjustment is offered it will be rebound.
Some of the cheap fixes we do to lessen excessive High Speed compression damping on the shawk and other bikes effect these Slow Speed circuits as well, and sometimes to a greater degree than what we're trying to fix.

For example changing to a lighter fork oil, say 5w, will lessen the HSCD, but will also pretty much disable LSCD, and make LSRD so light that even with the adjustment screw seated (full hard) you can't select enough damping.
You can taper the needle to get more rebound adjustment (LSRD) with the lighter oil, but that won't bring back LSCD (much)*.
Another way is to drill holes in the cartridge body above the compression valve, this will lessen the HSCD as well, but also disable the LSCD that's pretty light to begin with.
Since it's difficult to feel low speed compression on the bike with nothing to compare it to,(external adjustment) you may not know it's missing.
The bike will feel plush but may not give you a controlled feel carving though turns, the suspension will be moving more and will take longer to settle when your in a turn.
Here's a quick way to feel the relationship between compression and rebound damping and the effect of your external rebound adjustment has.
What your mainly feeling here is Low speed damping.
The rebound you will feel pretty readily by pulling up on the damper rod.
The compression is much lighter, and you may have you push a little faster to feel it.
Compression will increase quickly with rise in speed.
If you push very fast you'll be feeling the high speed compression damping (HSCD) before the valve stack opens.
(I think you'd need more stroke and have to push pretty hard to feel the valve stack open)

What should you feel?
You should feel rebound for sure (resistance to movement) there'll be a lot, and it will change with screw adjustment.
You should feel 'some' compression when moving the rod at the same speed as you did with rebound, though it will feel much lighter than rebound.
If it's not there at all, zero LSCD, then you won't have the controlled feel in a turn that you could have.

By taking the spring out of the equation you can isolate to two damping circuits to help troubleshoot your front suspension.

http://pg.photos.yahoo.com/ph/caffei...=/1b23&.src=ph

(much)* external rebound adjustment effects compression to a degree, because it allows a bit more backwards flow though the rebound circuit, as does the rebound 'refill check-valve'.

HSCD / High Speed compression damping
LSCD / Low Speed compression damping

LSRD / Low Speed rebound damping

~Jeffers
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