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

[Speedballer347]

My fork rebound-valving is wrong, have too much rebound w/ the adjusters all the way out....packs up on bumps.
They also have too soft springs for my weight....bottoming out to the stops under braking.
Bike bottoms out and packs/locks up and (skips) on bumps.

Dont have time to ship them to suspension guy yet.

If I add 5cc of Ohlins fork oil, increasing my oil volume and thus decreasing the tendancy to bottom out....will my rebound be affected by it, meaning will it get even worse?  The last thing I want is more rebound!!!! 

I know compression will be affected, but what about rebound??? 
I dont think it would, but I want to make sure B4 I add the oil.
Thanks!!!!!

[Super Dave]

Well, if you know you have too soft springs, replace them. 

What's your sag at?

Adding oil isn't going to reduce your rebound dampening unless you add so much that the fork doesn't function. 

Dampening shouldn't overcome the spring.  Sounds like the forks have issues, and you've known about it for a while.  When are you planning on fixing the rest of the bike after you crash?  Seriously, if you've got that much rebound, you're just looking to throw it all on the ground, Eric.  Get it fixed so that you're not wasting your time and money on track.

[Speedballer347]

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

Well, if you know you have too soft springs, replace them. 

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.

What's your sag at?

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!!! 

[duckracer996]

Speedy, This is Randy. You could try a 5 or 7 weight oil as a band aid. to help the rebound return a bit quicker (thinner oil flows easier through the valve ports),and the dive on the brakes will be a product of the compression valving not set high enough which the lighter oil may help a bit. Adding fork oil will help slightly with the dive by reducing the "air spring" space, and not affect the dampening as long as you don't add so much as to hydrolic lock the forks.  but it is all just a bandaid, Like dave said you need to get the forks fixed asap! Oh and Long live Jojo bitts!!!!!

[Speedballer347]

Speedy, This is Randy. You could try a 5 or 7 weight oil as a band aid. to help the rebound return a bit quicker (thinner oil flows easier through the valve ports),and the dive on the brakes will be a product of the compression valving not set high enough which the lighter oil may help a bit. Adding fork oil will help slightly with the dive by reducing the "air spring" space, and not affect the dampening as long as you don't add so much as to hydrolic lock the forks.  but it is all just a bandaid, Like dave said you need to get the forks fixed asap!

Hi Randy, That is exactly what I thought and was hoping to get confirmed.
Do you think 5cc would be safe?  sufficient??
Forks are going back Monday after GIR :)

[Speedballer347]

JoJo Bits Rocks!!! ....stickin' it to da Man! 

[GSXR RACER MIKE]

Some food for thought....

Bottomed out suspension will definately cause the bike to skip over bumps, the damping at that point is not able to do its job very well (if at all). If the springs are too weak for your situation then damping changes may only help minimally.

Comparing 'free' and 'rider' SAG numbers basically identifies if you have the correct springs for your application, then if you do have the correct springs it allows you to make some tweeks to your set-up by adjusting pre-load. If the springs are wrong for your weight then you can more or less throw preferred SAG numbers for your application out the window and go to 'damage control' with what you have. Adding more preload to your springs (if their too soft) will mess up your SAG numbers, but it will also help to slow down your springs compression rate. In my opinion you need to get more spring into your situation to help stop the bottoming out situation in the 1st place, with the springs you have currently the easiest way to get that is more pre-load. Of course that's a compromise and may cause other handling issues because your suspension will have more initial spring pressure, but it may be better than what your dealing with currently.

I also wonder if you need a little more compression damping to help stop the suspension from bottoming in this situation, if that helps the suspension from bottoming out it should be better than what you have now.

Good luck! 

[MELK-MAN]

only had a minute and there already is a good deal of info posted already. May already be indicated, but as Dave indicated a bit more fork oil won't affect damping. Since fork oil LEVEL only affects the lower portion of travel, that is how you keep forks from bottoming provided your sag #'s are correct. In other words, if you have the right springs but are bottoming, adding a bit more fork oil will keep from bottoming. Goes without saying though, too much oil will hydro lock the forks and they won't work right.
I normally run 190mm from the top, but at VIR due to the long front st-8 braking area, i run another 10mm of oil. No other track i do has a braking area that is so long from such a high speed.

[Gixxerblade]

How does the rear feel?

[Burt Munro]

How does the rear feel?

Speedy's not that kind of guy!

[GSXR RACER MIKE]

Speedy's not that kind of guy!

I don't even want to know how you acquired that information!   

[Speedballer347]

Speedy's not that kind of guy!

LOL!!!!!!!!!

Rear is same as front, perfectly balanced front and rear.  Rear has infinite amount of adjustment so no issues getting it correct, or balanced to a changed front-end.