Can you help me understand rebound damping as it applies to launching at track?

Kevin Jones

Kevin Jones

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Conditions given, a wheelie that keeps rising off the line until you have to let off the gas/clutch it/back brace/whatevertosaveyourass. Would increased rebound damping help bring the back wheel up and change the fulcrum to bring the front wheel down or do you need to reduce rebound to help that? Is rebound damping capable of any of those things? Do you have a particular setting that you think is a medium or set to maximum or minimum rebound damping? I have tried to read and understand these things and I am not grasping suspension tuning and getting frustrated. Most of my problems experienced, stock gen 1 shock and a 4" extension on a 230hp bike and now that I have extended the swing arm, I find I really dont know what I am doing. It will take forever to try to figure these things out with experimentation if I dont understand the basic concepts. I have been told what to set it at basically with sag and compression but that really will only go so far with me. I have to understand concepts. It is even more frustrating since I understood how to make a 4 wheel hook up, well, actually I never had that much hp to have to really really tune suspension other than buy add ons.

My current set up:
trac dynamics 465 HD shock (I couldnt afford a penske)
10" setting on swing arm (bringing in a little, spins violently, prolly to 8")
Stage 1 turbo (mild tune @230, HD clutch springs, 17/44t combo)
I weigh 170 suited.
I do not and will not strap down the front end at the track.
Launching at 3k since 4 spins me to the tach (on road, not track) all through 2nd, even with a babied clutch, pinned throttle right before tach before shift to 2nd.
I have yet to track test the 10" stretch and have been advised to bring it in to 8.
 
There are many articles on suspension tuning for twisties but not seemingly so many on drag applications. I do not corner. I had a 600 that I overconfidently laid on my knee 20 years ago and I will never ever go into that road racing mindset again. I could care less how it is set as long as I can still ride it on the road safely.
 
Rebound is the speed at witch the shock returns to full height ( expands upwards ) Compression / spring rate has a greater effect on wheelie control .
Two much of those two items = wheelies . Also to short of gearing can affect usable 1st gear . What gear are you using to go through the traps @ ?
If you say 6th gear this explains wheelies more . Less shifts in the 1/4 mile = lower E.T. on my drag bike I go through the traps in 4th ( 5 speed trans )
100 rpms from red line . a 4" extension on a 230 hp will wheelie on demand , even a 10 set @ 8 " with a sticky track can still manage it easily.
Are you still hand clutch ?

Last front end strap . Required if you have not built your front end to be low/set for drag racing . No strap on my bike you can forget having a decent 60' foot time . It will wheelie so bad you can not use wide open throttle till 3rd gear ( non turbo , 57" wheel base , 400 pounds wet , 145 hp ) with the strap
it carry's a low 1st gear wheelie , wide open throttle through all gears .
 
My understanding is more rebound dampening if you are on a slippery track. As the track gets better, you can let the bike rebound quicker and be less wheelie prone. Lots of stuff to consider and I also am not an expert in it. I get differing answers on some things, so I try to go with what at least makes sense. I've been told if the swingarm pivot is too far below the rear axle it can induce wheelies. Sort of makes sense to me ... the chain is pulling on the top of the rear sprocket so once that pivot goes down it would cause the arm to want to come on up in the back and make the rotation about the pivot point worse. Having the sag set properly helps combat that issue.

What is your aversion to a strap? I like mine as it gives me adjustability. I was at a track this summer where I had to run it completely loose due to poor traction but I certainly had more wheelies to deal with.
 
I just didnt want to have to mess with straps. Maybe it is not a big deal as I am making it. I do ride my bike an hour and 15 to the track one way and same back and anything that I can leave as is, I would rather. But if it cant be stopped, I will have to strap. Yes, still hand clutching. I cannot trailer it there, I do not have room for a trailer at my home. I am the only guy who I see does not but its not my fault.
 
My street Busas have had straps on. Just make sure the loose end is secure and there is enough slack for normal suspension action. They do help prevent wheelies on bikes less than 70".
 
Straps on the way will be here tmrw.

I shortened my swing arm to 8", took it out and the same thing happened. Shinko 003 at 30 psi. Did a burnout and I launched at 3k, slooowwwly let out the clutch and at about 8k went WOT. It was spinning bad, actually crossed lanes with the back tire and when shift light came on I hit 2nd and it spun so bad, it tach'd out instantly, shifted to 3rd, began to hook up and go hard. THat is the 3rd time in a row, same situation at 10" so bike is consistent! Of course this was road, not track. I believe I went too far with the 44t sprocket. Going back to stock if I cant get hookup. Setting sag on shock tonight as per Rob's recommendations. Will take some more air out of tire and try again tonight. AFR logs from the WEGO looked fairly good.

Question: How many of you cannot hit WOT in 1st with similar situations?
 
30 PSI is a lot. You can get by at that psi leaving softly with a stock wheelbase bike but not anything with power or length. I run 12-14 psi at 67". I could run 18-20 at 63-1/2". Not sure your tire is the best. I run a Shinko Hookup on my stuff. I can be at full throttle and clutch out by the tree on a 63-1/2" bike with 17/46 gearing stock motor. It is easy to spin on the street or poorly prepped track.
 
Yes, maybe I am judging this stuff and not being realistic as to totally different conditions on the track. I will take what I have now (17/44), strap it down, lower the PSI and try some runs Wednesday at Orlando. IF it doesn't rain out. Kills me when the only day I have to do that and it happens. If I cant hook up there, I can at least try different rpms, lowering them, 80% in 1st, floor it in 2nd or whatever. Maybe have someone take a vid of my launch technique. Dennis, are you easing the clutch slightly enough to have to hold yourself on the line with the front brake? Ive been doing that enough to make the back rise just a little bit or so that I can feel it.
 
No, I don't use the brake on the line. I roll in to light the pre-stage bulb, set my rpms and get the sound in my head (I like 5k-6k). Then I roll in to light the stage bulb. At that point I let the clutch out to where it feels like it wants to start pulling but the bike doesn't move, then squeeze the lever just a hair so it stops pulling. When it is go-time I try to hold the rpm's constant by opening the throttle and letting the clutch out together so that rpm's don't drop a bunch or flash too high. Do this as quickly as possible without a wheelie. If you do it right you should have the clutch out and full throttle by or shortly after the tree. This is for a sportsman's tree. My process is different for a pro tree.

 
Last edited by a moderator:
Set the sag and hopefully have it a little more under control. Going to try to track it tonight if no rain. I wish I had spent a little more on my shock and had some more compression adjustment. I keep saying that with everything I buy for this bike. "I wish I had spent a little more on that". Its like a mantra. I get the stage 1 of everything and wish I had waited and got the next step up.
 
I don't have adjustable compression dampening. I did make a spring adjustment this week to soften it up so the bike would get more sag and more motion during the launch. The goal was to reduce chance of spinning without increasing likelihood of a wheelie. I wonder if I should get a lighter spring and let the bike sit a little higher in the resting state. I even am questioning the 3/4" sag rule based on the success of a buddy who completely disregards that.
 
My 465 doesn't have the adj compression either but the rebound is. I did the sag test with my dad, came out to 1". I tightened the spring a bit and it looked like it brought it down to almost 3/4" sag, as best as I could tell? I lowered the rebound to lowest setting and took it for a launch on street with 36psi (holy crap did that feel loose going around a turn. I wouldn't drive it on the street at that rebound setting, no way). It didn't spin as bad as it normally does and it actually let me go through 2nd gear without spinning out instantly and I didn't see a new stain on my tiger tail. I will try again tonight, road test, let some air out of the tire. I think the track is going to rain out.
 
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