Suspension Q...Will adding oil to your forks affect rebound dampening?

[tstruyk]

Um its a little known fact JoJo eats lead and shits bullets... true story

[GSXR RACER MIKE]

My fork springs are a hair too stiff, the fork oil is too thick for my rebound valving, and the oil level was way too low.

The oil issues explain the damping problems, but too heavy of springs should have helped to prevent the suspension from bottoming - going to lighter springs seems wrong for avoiding the suspension from bottoming. The main thing I can think of that explains the suspension bottoming with stronger springs would be if your oil was so low that the compression valving was possibly cavitating and no longer able to do it's job (or air was trapped in the cartridge from never having been bled properly in the 1st place). Low oil may also not allow the anti-bottom out feature of your forks to work either.

Hopefully the current changes are going to fix the problem. 

[duckracer996]

I'm betting that too low of an oil level increesed the "air spring" so much that the springs were doing all the work, and like GSXR Racer said it allowed air to get into the compression valving throughing that way off.
   Oh and JoJo is Evil!!!!!!!!!!!!!!...He eats rabbid racoons for breakfast!!!!!!

[Speedballer347]

Oh and JoJo is Evil!!!!!!!!!!!!!!...He eats rabbid racoons for breakfast!!!!!!

JoJo is in deep depression since wednesday night when he f-i-n-a-l-l-y caught a 20lb raccoon and got the poop beat out of him
He is sad.
We all have to be super nice to him at the Gateway tracksetup and get his spirits up 

[banzai1]

SUPER Dave!!!!  8)  I was hoping you woould have advice in this thread


I can't get the forks shipped til the following Monday after the MCRA trackday. The forks were already rebuilt and the right springs/valving were supposed to be in there.  I always just figured they were...I knew nothing of suspension.

GSXR1000 2005. 
Rider sag at 30mm front. 4 lines of preload showing.  Rebound all of the way out.  Compression 6-out (19 or 20 click range)
The forks bottom so hard the zip-tie leaves indentions in the fork's dust-seal.
Don't know if it is bottoming on brakes or bumps, or both.
Bike feels great on the brakes, rear-end is a little light, moves around a little under super hard braking.
Bike packs up on the first bump (feels like it has run out of travel, and the rebound isn't letting it back out).  Just hits, bottoms, and then skips across the bumps.
With the rebound adjusters all the way out, the forks visibly return too slowly, just manually pumping the forks.

I honestly never really thought anything was wrong.
I had the forks/valving done by one of the best, believed it to be dead nutz correct.  Had it adjusted and tweaked by a knowledgable racer to were it felt really good everywhere except on the bumps. Geometry wise the bike is dynamite!
I figured the problems over the bumps was me tightening up and clenching the bars, inducing it.  Always felt like if I was more relaxed over the bumps the bike would be fine.

I finally tried that at Putnam a couple weeks ago, stayed light/relaxed on the bars and carried my speed over the bumps.  Instantly packed up and chattered.  I stayed relaxed and kept doing it, and the same result.  Went faster, same result.
I asked guys that were going the same pace as me if they were chattering in T2 and T9, and they said they couldn't feel any bumps.
Right then I knew something was wrong.  My zip tie was also at the bottom.

I went to Jay Q and he showed me correct rebound action on his forks, and mine is way slower in comparison, and I don't have any adjustment left. It takes about .5-.7 second for my forks to re-extend.
Then I checked the rider sag and I was at 30mm.  I believe I am supposed to be at 35.  I am sure my tuner friend cranked them down due to seeing his ziptie getting crunched when I would pit. I didn't know anything about suspension, all I knew was he made the bike a lot better, even in the bumps.  I figured my ass-clinching in the bumps was mental...I just foound out it's not 


I figured if I could keep the sag at 30mm, add some oil to keep it from bottoming, it would help some.  Also thought I could remove some of the Ohlins 15wt and replace it (and add 5cc) with 7 weight, it would dilute the oil to a lighter vicosity and reduce the excessive rebound.

Was also thinking of adding 1-2 pounds of air in my front Mich to aid in faster rebounding (of the tire at least).

I know this is all a bandaid, but no time to ship out forks till after GIR.  I blame this on me for not learning about suspension sooner so I could have caught the problem...NOT the guy who tuned them.

Now crush my GIR dreams 
.

Any advice you can offer Dave, is super appreciated!!! 

You have a combination of things going on. You fork spring rate is too soft allowing the bottoming.
The too stiif of rebound with minimum damping at the screw is because of a damaged fork rod. I had a problem with my rebound rods where the preload cap actually broke the rebound pieces in the rebound tube causing the exact same problem you describe. You'll need good rebound tubes to replace the broken ones (usually means purchasing good used forks). And replacing the rebound tubes in the forks you now have.

[Speedballer347]

You might be right.
With the rebound screws all the way out the bike has too much dampening.  With the screws all the way in, hardly anything changes, doesn't increase rebound dampening much worse than it is with them all the way out.  Something surely could be broken.  We will check for broken parts.  Thanks! 

[GSXR RACER MIKE]

The majority of damping is controlled by the shim stack on the damping valve, the external adjuster on your suspension ultimately controls what is more or less nothing more than a bypass around the shim stack. What your actually controlling with the damping screw is nothing more than fine tuning the rate at which a small percentage of the suspension oil travels from one side of the damping valve to the other.

The small bypass port (hole) around the shim stack is designed to work with a certain oil viscocity, as the oil gets thicker that bypass port will become less and less effective in changing the damping characteristics of your suspension. When you turn the damping screw all the way in (till it lightly bottoms out and blocks off the hole completely) the bypass hole is not being used at all and the shim stack on the valve is controlling all the oil flow (there may be a very small bleed hole in the actual damping valve as well, but it's not externally adjustable). As you turn the screw out it uncovers the bypass port and starts to allow some of the oil to travel around the damping shim stack. The greatest change in damping (by the damping adjuster screw) is in the very first turn of that screw because it's starting to unblock the bypass port, the further away from the bypass port the screw gets the less effect each turn of that screw has on damping. If the damping screw is turned all the way in till it bottoms, and then overtightened, it can potentially damage the end of the damping screw and/or the bypass port that it controls. If the tip of the screw was damaged it would most likely effect the first turn of the adjuster the most (not allowing good fine adjustment) and shouldn't really effect the later turns. If the bypass port itself was damaged (somehow crushed down smaller) then it could potentially effect the entire range of damping on the adjuster, but that's unlikely to happen.

I still believe it was the oil thickness that caused the damping problem.

[Speedballer347]

Jerry at WFO fixed my forks  8)

He measured the oil level and it was 5mm shy of max.  He said the rebound valving was a bit much.  Also said they were assembled to where 5 clicks of rebound range was missing.
He completely dissasembled and cleaned them.  Put in fresh oil and added 5mm of fluid to the level.  He also assembled them that now I have 5 more clicks of adjustment.

Now with the rebound adjusters all the way out, the forks manually feel perfect.  Rebounds noticably faster. He says he will watch my zip-tie at the track and will add more oil if necassary.
I am happy!  8)