Penske 8100 presure check

[Dave SanF 50]

I have read the Penske Manual and could not find a reference to the proper technique for checking the nitrogen pressure. It mentions setting the shocks to 200 psi after reassembly and checking the shock pressure as part of a pre race maintenance program. My gauge documents states the shock pressure should be checked with no load on the wheels ie. car jacked up. But I have seen a lot a competitors check the pressure with the wheels on the ground (shocks loaded). It would seem that if the shock is compressed, the volume of the reservoir is greatly reduced resulting in a much higher pressure reading, you know, the old PV=nRT thingy.
Anybody know the correct way to check the pressure. Should we take the shocks out of the car and check them in the fully extended condition?
thanks
dave jalen

[problemchild]

This procedure is much like measuring ride height. Once you measure it once and determine reference points, you can find alternate (more convenient) ways to measure.

[DaveW]

It doesn't matter which way you do it as long as you do it the same way every time. Just don't exceed the maximum that the damper manufacturer recommends. Canister pressure has a small effect on spring rate, ride height, and low speed damping (more canister pressure adds more of each), and can be used as an additional suspension-tuning tool.

See this link for more info:

http://www.neohio-scca.org/comp_clin...print_menu.htm

[Dave SanF 50]

Thanks guys for the info. The link to the NeoOhio region site with the manual is great. If anybody has not seen it yet, it is a must read. Thanks for sharing your knowledge with us wangers, DaveW.
dave jalen

[T.A. Treat]

I agree, that's a great resource. I had never thought about the pressure thing until you brought it up. I usually do all pressures with the car on the ground. I'll have to do it both ways now to see if there is a difference.

In case you haven't found the official technical manual for Penskes, here is the link:
www.penskeshocks.com

Some drivers mix up which adjuster is bump and which is rebound. I still have to think about which way to adjust so I'm thinking about putting a label on each shock with a direction arrow and the text "more bump" and "more rebound". Does anyone have a better choice for the labels?

[DaveW]

Here's a diagram I made for the Penske's.

[T.A. Treat]

That saved me some work, thanks. Do you happen to have the same thing for the 8100 series for us poor guys?

[DaveW]

No, sorry. These are the only Penskes I've had. Do the 8100's just have the shaft adjusters?

[Ted and Nancy]

the pressure change from full extend to 2 inch compressed is very small, about 3 psi. Calculate the area of the piston rod and then how much volume is inserted into the shock at one inch and two inch compression.

[Ken Rozeboom]

It seems to me that ambiant air temperature has lot more effect on shock pressures than a bit of weight. What is the effect of 60 vs 100 degrees? In other words, set them in the garage but the track day temp a few days later is a lot different. And on-track temps add another variable, especially with the rear shocks on a db1. I don't plan on setting shock pressures at the 5 minute warning on pre-grid.

[Ian McKechnie]

A long while back, Joe Stimola told me that damper pressures should be check with them unloaded. That is the way I have been doing it ever since with no problem.

[Dave SanF 50]

I tested my 8100's with the tires on the ground and up in the air. As Ted and Nancy stated, very little change. Within the 3 psi window. More of a change with the pssssst of gas when installing the gauge then the loaded vs unloaded state. I think that the very small volume of nitrogen would not change much with ambient temperature either.
dave jalen

[R. Pare]

The presssure will change linearly with temperature - ie - double the temperature and the pressure will double.

[Scott Hanba]

Richard,
Isn't that relationship true only for Kelvin scale (I forget the English absolute temp scale)

30C to 60C change in temp would really be 303K to 333K or 10% change in pressure.

[Paul Derov]

Just to clarify, please note that temperature is in degrees R (or K in metric). A +50 degrees F (27.8 C)ambient temperature change will increase the pressure approx 10%.

Sorry, my post is now redundant. Thanks Scott you beat me to it.

[R. Pare]

Yep - you are correct! Kelvin it is - I keep forgetting that!

Give the man a cigar.

[Ted and Nancy]

Dave
You mentioned the pssssssssst when disconnecting the service line. That is why you need part number 556 from http://www.aircraft-tool.com for about $35. This tool allows you to pressurize the service line and with a built in T handle open and close the schrader valve without loosing pressure in the shock. Never check the pressure with a device like a tire pressure gauge as the volume is very small and when you remove the air chuck the pressure in the chuck is changed thus the pssssssst.

[Ted and Nancy]

Richard
The next part of the question is what effect does changing the can pressure have. Some people say that is changing the support!

[DaveW]

From one of my previous posts in this thread:

Canister pressure has a small effect on spring rate, ride height, and low speed damping (more canister pressure adds more of each), and can be used as an additional suspension-tuning tool.

See this link for more info:

http://www.neohio-scca.org/comp_cli...eprint_menu.htm

[R. Pare]

While the tool Ted described is absolutely necessary, you will still lose some pressure just doing a pressure check - about 10 -15 psi, depending on the volume you lose filling and pressurising the tool and the initial pressure of the canister. However, you won't lose any pressure after that initial filling, so the shock will stay at whatever the gauge says.

[Chris More]

Can someone clarify something for me?

I have Penske 8100's and I need to check the canister pressure.

I see that the schrader has a T-handle on it for cracking opening the pressure after the gauge is mounted to the male side.

Do you recommend mounting a pressure gauge right onto the schrader valve? Seems like using a hose to go up to a gauge would loose a lot more pressure then having a pressure gauge mounted directly to the valve.

So all I need is this:

http://www.aircraft-tool.com/shop/de...PRODUCT_ID=556

and a good screw on pressure gauge, right?

Thanks,
Chris

[R. Pare]

Every time you check the pressure,you will loose a little, no matter what setup you have. What you read will actually be below what WAS in there before you checked. I don't remember the amount for the typical Penske-supplied checker, but I seem to remember that it was in the neighborhood of 10 lbs.

The way to find out what your particular checker does is do a series of checks - you will loose the same amount each time and should be able to note the change.

More canister pressure adds bump damping and decreases rebound by the same amount - it effectively shifts the whole curve upwards. It doesn't actually change spring rate per se, but it does increase the amount of static force that the shocks imparts to the support of the car.

[Chris More]

Richard,

I just wanted to know if i should mount a pressure gauge to the side of the schrader valve or hook a small hose to the side of the schrader valve with a guage at the other end?

I guess like you said and others it shouldn't matter as long as if I can figure out how much it loses each time I check it and factor that in.

Though the first thing on my list is to get a nitrogen tank!

Chris

[R. Pare]

Chris: The shorter you make the connections, the less is air let out to fill the gauge, etc, and the less pressure drop you will see when checking.

[DaveW]

More canister pressure adds bump damping and decreases rebound by the same amount - it effectively shifts the whole curve upwards. It doesn't actually change spring rate per se, but it does increase the amount of static force that the shocks imparts to the support of the car.

Richard and I have agreed to disagree on this one!

IMO, adding canister pressure DOES NOT ADD BUMP DAMPING AND/OR DIMINISH REBOUND DAMPING! It DOES shift the raw (gas load not subtracted) damping-force curve upward as explained in the next sentence. The real effect is to add to the load that the damper carries statically (and offsets the damping curve - that is why the pressure load needs to be subtracted from the damping curve, so you can see the actual DAMPING force values), similar to cranking up the spring perches and compressing the spring. The result is that you need to set canister pressures before setting ride height. If you set the pressures after setting ride height, and make a big pressure change, the ride height will also change. In addition, there is a minor increase in spring rate and seal friction when canister pressure is increased.

Again, see this link for what adjustments do what:

http://www.neohio-scca.org/comp_cli...eprint_menu.htm

On checking damper pressures - when I check with the Penske unit (pressure source not attached), the unit's volume makes the damper pressures decrease by about 7 to 8%, based on absolute pressure (gauge pressure + 14.7 psi).

[Scott Hanba]

What is the max pressure typically used in these shocks (if anyone is willing to share)? There is a big price difference between a 250 psi regulator and a 1000 psi regulator for the nitrogen tank. I thought 300 psi range was not uncommon.

[DaveW]

What is the max pressure typically used in these shocks (if anyone is willing to share)? There is a big price difference between a 250 psi regulator and a 1000 psi regulator for the nitrogen tank. I thought 300 psi range was not uncommon.

From a several year old Penske manual - they suggest 150-300 psi. I have also heard that going higher increases the risk of blowing out the seal. I personally have not used more than 200 psi cold.

[Scott Hanba]

Thanks Dave, sounds like the 250 psi regulator is the ticket!

[Rick Iverson]

[size=1]Fellas;[/size]

[size=1]Any suggestions where to buy a good quality regulator with both supply (3,000 - 4,000 psi +/-) and delivery (250 - 300 psi +/-) gauges?[/size]

[size=1]I have found the Harris Model 25-500C-580 with delivery pressure range of 0-500 psi, delivery pressure gauge of 1000 psi and supply pressure gauge of 4,000 for $79.50.[/size]

[size=1]Also, Model 25-200C-580 with delivery pressure range of 0-200 psi, delivery pressure gauge of 400 psi and supply pressure gauge of 4,000 is also available for $67.85, but that might be too light of duty. [/size][size=1]Any recommendation on a good value?[/size]

[size=1]Iverson[/size]

[Ian McKechnie]

You can get a suitable regulator from your neighborhood welding supply store. That is what we did a number of years. We bought a Uniweld regulator. The gage on the supply side goes from 0 - 4,000 psi and the delivery side gage goes from 0 - 600 psi. The supply side, of course, gives the pressure left in the tank and the output of the delivery side is adjusted via a screw with a "T" handle. We did not need the supply side to go that high as any Nitrogen tank I have ever gotten has 2,000 psi. We also have not used any more than 225 psi on the delivery gage. We use a large tank for home use and a medium size tank to take to the track, using the same regulator for both. No matter what regulator you get, put a quick release on the output end and a matching one on the hose. You will find that it is very convenient that way. We use the large tank to check damper pressures and to do the major tire changes at home as part of our race preparation.