Rebound setting on 8100

[jstoezel]

Hi,

I would like to check the terminology used in the Penske manual for the rebound setting.
The manual alternatively uses terms such as soft/hard rebound or more/less dampening. I am getting confused now.

What is your understanding of soft rebound? Is this when the shock goes back to it's original position slowly (takes long to bleed the shock, rebound screw closed) or fast? Is more dampening equivalent to soft rebound?

Also, I run a D valve on the rebound. After the shock was rebuilt last Spring I could tell the difference in shock action when adjusting the rebound screw. The shock would either take a couple of seconds to come back to its original position (with the screw all the way in, after compression), or would come back right away.
After the first race this week-end I can barely see any difference when I adjust the rebound screw to either full closed or full open. I haven't changed the spring rate since I rebuilt the shock.
I don't see any oil leak, I don't exactly know at what pressure the shock is right now, I don't have the proper tool to check just yet.

The only thing I noticed and which may be worth mentioning is that both the shock and remote canisters got very hot after each session. The rebound setting doesn't seem effective even when the shock is cold.

Any idea why the rebound doesn't seem to "work" anymore?

Jean

[FV80]

My first question would be "where is it 'stuck'?" Does it have BUMP damping? Do you feel any notches in the bump (canister) adjustment?
These things are all clues, but about the only thing I can think of that would kill the rebound adjustment is NO oil in the shock ... unless it is stuck at full closed (and takes 2+ seconds to rebound).

I'm not sure what Penske had in mind, but in MY mind, a 'soft rebound' would be "not much damping" - a hard rebound would be "a lot of damping". But I'm not aware of any "rule" either way.

I know this doesn't help much, but answer the first questions and we'll see where it goes from there.

Steve, FV80

[jstoezel]

Hi Steve:

My first question would be "where is it 'stuck'?"

I am not sure I understand, but the shock seems to be moving freely both in compression and rebound.

Do you feel any notches in the bump (canister) adjustment?

I feel notches in the canister setting, from 1 to 6.

Does it have BUMP damping?

In all honesty I don't know. I have an A- valve installed in compression. Penske's documentation says that the softer the valving the less likely you would notice any changes made at the canister. Even after the shock was rebuilt I never really noticed any difference with the canister setting. I used to be able to see a difference when adjusting the screw for the rebound setting tough. How does the canister influence rebound though, isn't it just for compression?

These things are all clues, but about the only thing I can think of that would kill the rebound adjustment is NO oil in the shock ... unless it is stuck at full closed (and takes 2+ seconds to rebound).

It definitely does not take 2s to come back to its original position. I was monitoring the shock all week-end and I can't see any trace of oil. I guess I should have weight the spring before installing it, I would have been able to compare now.

I'm not sure what Penske had in mind, but in MY mind, a 'soft rebound' would be "not much damping" - a hard rebound would be "a lot of damping". But I'm not aware of any "rule" either way.

I know this doesn't help much, but answer the first questions and we'll see where it goes from there.

Steve, FV80

[FV80]

No - the bump adjustment doesn't affect rebound - but feeling the notches is a quick ck to see if there is a reasonable amount of pressure in the shock (you said you don't have the check tool).

By 'stuck' - I meant does the adjustment appear to be stuck either fully open or fully closed? Does the shock seem to 'perform a shocking function' when you bounce on the rear or is it totally non functional?

The bump doesn't change much with the canister adjustment, but if you have NO bump, it should be fairly obvious when you jump on the back.

Again ... just clues as to possible related problems.

I still can't think of any reason why the adjustment would not cause some change in rebound unless there is no oil. The adjustment is just a needle valve that controls the movement of shock oil during rebound. If you have the Penske manual (downloadable from Penske), you can follow the path of the oil in their diagram and maybe get a clue. You could also call James at Stimola Racing Products (google it for their number) and see if he has any ideas. .... or maybe Jim or someone else that rebuilds shocks a lot will have an idea.

Steve, FV80
edit - 1 more thing - how many 'flats' do you have of rebound adjustment? Should be about 25 IIRC. 1 flat = 1 hole.

[brian]

Assuming that you are comparing rebound behavior with the spring installed, the only other issue may be the shims. I have seen shims crack and bend reducing the dampening considerably. A hard landing will bend the shims quite easily. If your shock and canister is getting hot in the car, the components will need to be cooled. While a shock will get warm from activity, exhaust system radiant heat will greatly effect it's performance. I run heat wrapping around my shock and the canister is far from the exhaust system. Gas pressure will effect rebound as well. In very general terms, the higher the gas the quicker the rebound.

[jstoezel]

Hi Brian:

Thank you for the reply. I haven't checked the gas pressure just yet. I will rebuild this shock after the next race, I don't think I have enough time now.

Jean

Assuming that you are comparing rebound behavior with the spring installed, the only other issue may be the shims. I have seen shims crack and bend reducing the dampening considerably. A hard landing will bend the shims quite easily. If your shock and canister is getting hot in the car, the components will need to be cooled. While a shock will get warm from activity, exhaust system radiant heat will greatly effect it's performance. I run heat wrapping around my shock and the canister is far from the exhaust system. Gas pressure will effect rebound as well. In very general terms, the higher the gas the quicker the rebound.