Dynamic Damper Can Pressure

[Lyngengr]

Although I have an "owners" manual for these shocks, it doesn't say anything about what can pressure to use. I had mine converted to use a Schrader valve fill rather than the needle, so I've been checking the can pressure before and after each race weekend. The pressure drops around 10 psi after each weekend. Does anyone have some general or specific guidelines on what range of pressures should be used?

At my last event, we dropped the front can pressure by about 30% and that made a huge difference with understeer. I'm reluctant to take it any lower because the car was really good and I'm worried about cavitating the damper fluid.

[Steve Demeter]

While I can make no specific recommendation, Chris Billings at the shock Shop ( he has passed away since then) always recommended the lowest pressure you could get away with.

He felt that hgher than necessary pressures would increase "stiction" . On my Fox Shocks he recommended around 2120 PSI..

[Garey Guzman]

I was told that with the very small gas volume, checking would drop the PSI by a measurable amount. I would not be surprised to learn that every check dropped the internal pressure 5 PSI (2 checks per weekend dropping it 10PSI).

Fluid cavitation is a temporary condition so even if you go to far, you're not ruining anything, just pump it back up. I assumed you have a N2 bottle to refill it since you put a Schrader valve on.

[David Ferguson]

Lynn, You might consider contacting Bill Margraf in your area. I think he has a shock dyno, and spending some time with that could be very useful to learn about canister pressure and it's effect on damper forces.

[Lotus7]

I was told that with the very small gas volume, checking would drop the PSI by a measurable amount. I would not be surprised to learn that every check dropped the internal pressure 5 PSI (2 checks per weekend dropping it 10PSI).

exactly this !
those expensive, awkward-to-use needle gauges had purposely small internal volumes :-)

[Lyngengr]

I was told that with the very small gas volume, checking would drop the PSI by a measurable amount. I would not be surprised to learn that every check dropped the internal pressure 5 PSI (2 checks per weekend dropping it 10PSI).

Fluid cavitation is a temporary condition so even if you go to far, you're not ruining anything, just pump it back up. I assumed you have a N2 bottle to refill it since you put a Schrader valve on.

You are correct, Garey, every time you check the pressure it does go down a few psi. We now check it every day at the track, and we do have a N2 bottle to add more pressure if necessary.

[Lyngengr]

Lynn, You might consider contacting Bill Margraf in your area. I think he has a shock dyno, and spending some time with that could be very useful to learn about canister pressure and it's effect on damper forces.

Bill is 5 minutes away and said to not go below 75 psi or it will cavitate. I will also ask Dan Campbell, he does the dyno checks on my shocks.

[John Green]

How does the shock react if it bleeds down to 0psi during a race due to a leak?

[DanW]

How does the shock react if it bleeds down to 0psi during a race due to a leak?

The shock will quickly lose its damping due to cavitation and the grip will go away.

[Lee Johnson]

While I can make no specific recommendation, Chris Billings at the shock Shop ( he has passed away since then) always recommended the lowest pressure you could get away with.

He felt that hgher than necessary pressures would increase "stiction" . On my Fox Shocks he recommended around 2120 PSI..

Steve -- I imagine the "2120 PSI" (144 atmospheres, by my calculation ) in the above reply is a typo, isn't it? That type of pressure is in the range of pressure in a fully charged N2 tank from a welding supply house. Hard to believe a race car shocks remote canister & hoses would hold up.

Perhaps the value should be "120 PSI".

Lee

[tstrong]

Bill is 5 minutes away and said to not go below 75 psi or it will cavitate. I will also ask Dan Campbell, he does the dyno checks on my shocks.

If Dan is doing you shocks just leave them where he tells you to. You can’t get any better advice than he will provide. As mentioned above 75 psi is a kind of very generic lower limit but Dan can give you a much better idea. Todd