Refilling shock

hardingfv32-1
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Joined: December 1st, 2006, 8:01 pm

Re: Refilling shock

Post by hardingfv32-1 »

Using the "100 psi and moved the shock 1", the pressure would be 133 lbs, a difference of 33 lbs and about 6.5 lbs additioanl force on the shaft" as an example, the spring rate would be 6.5 lb/in?

How do you combine two spring rates to get one value?

Brian
kps
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Re: Refilling shock

Post by kps »

The spring rate of an extended gas shock is created by the difference in area between the bottom of the piston and the top where the shaft connection takes up some of the area. The three factors that impact the end result are piston diameter, shaft diameter and gas pressure. Assuming that the first two are constant that leaves gas pressure as the variable. You can try this for yourself by taking a shock off the car and letting out all the gas. You can compress the shock to whatever position you wish and it will stay there. Add 60 lbs of gas and you will observe that the shock will extend to its full length by itself. If you apply pressure you can feel the added force required to start it compressing. Compress it to the same distance as in the first case and let go. It will extend. Now add 140 psi of gas. You will be able to feel the difference if you repeat the compression exercise. Depending on you car and set up loss of gas pressure can cause changes in camber and ride height. This is a little hard to visualize but consider that the overall volume of the shock does not change only the piston position changes. Oil is forced from under the piston through valves to the top of the piston so total volume is unchanged and the gas is not compressed beyond what you put in. This is almost true except there is some lag time for the oil to move to either side of the piston when the shock is compressed and extended and therefore some pressure change will occur. As Brian pointed out there is also some volume decrease as the shock compresses due to the volume of the piston shaft. Without a shock dyno and a lot of math it would be very difficult to calculate contribution to spring rate for a given situation but at steady state there is spring rate. John
jpetillo
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Joined: August 26th, 2006, 2:54 pm

Re: Refilling shock

Post by jpetillo »

KPS (John) is right. Since when there is pressure in the shock it extends, means that it acts like a spring. Yes,the internal volume changes by the shaft volume as Michael said. Brian H. you can calculate the effective spring rate for two springs in parallel (the shock is parallel to the regular spring) by simply adding their rates. But, air volume displacement "springs" have nonlinear rates. John
cendiv37
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Joined: June 25th, 2006, 7:29 pm

Re: Refilling shock

Post by cendiv37 »

jpetillo wrote: But, air volume displacement "springs" have nonlinear rates. John
Maybe if the air is in a flexible bladder, but in a rigid cylinder I don't think so.

For a given starting shock pressure P and gas (nitrogen or air) volume V. Assuming the shock fluid is incompressible...

PV=nRT... assume the right side is constant ... P = Constant/V

It's been a while, but unless I'm missing something, it looks pretty linear to me: pressure inversely proportional to volume, pressure proportional to effective spring force

Therefore an air spring has a nice constant K.
The pressure change with shock movement would depend on and act over the shaft cross sectional area.

I think...

Good night...
Bruce
cendiv37
jpetillo
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Re: Refilling shock

Post by jpetillo »

cendiv37 wrote:Maybe if the air is in a flexible bladder, but in a rigid cylinder I don't think so.

For a given starting shock pressure P and gas (nitrogen or air) volume V. Assuming the shock fluid is incompressible...

PV=nRT... assume the right side is constant ... P = Constant/V

It's been a while, but unless I'm missing something, it looks pretty linear to me: pressure inversely proportional to volume, pressure proportional to effective spring force

Therefore an air spring has a nice constant K.
The pressure change with shock movement would depend on and act over the shaft cross sectional area.

I think...

Good night...
Perhaps it has been a long time. The effective spring rate is from the force along the shock shaft, not the pressure - that's the difference. If you rewrite your equation as PV=P*Area*L=Constant, and define force F as P*Area, and using the definition of spring constant K = (change in force)/(change in length), you come up with K = NewConstant/L^2. So, it's not constant, but highly dependent on shaft location. It's because of this that the internal gas volume needs to be large compared with what the shaft can displace - so that the effective L never gets small. This is a notorious problem with front forks in motorcycles - too high an oil level and you have too little volume left for the air.

I believe that bladders work better as springs because they can change volume as they pressurize.

John
jpetillo
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Re: Refilling shock

Post by jpetillo »

With regard to the torsion springs, the rate as seen by the tire is not linear as well. When the torsion arm is less than horizontal, it's a decreasing rate, and when it goes above horizontal, it's a rising rate. John
Mystique Racing
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Re: Refilling shock

Post by Mystique Racing »

hardingfv32-1 wrote:Using the "100 psi and moved the shock 1", the pressure would be 133 lbs, a difference of 33 lbs and about 6.5 lbs additioanl force on the shaft" as an example, the spring rate would be 6.5 lb/in?

How do you combine two spring rates to get one value?

Brian

Don't forget to factor in the motion ratio of the shock when applying this added spring rate.
Scott

Diamond Formula Cars

http://www.diamondformulacars.com
hardingfv32-1
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Re: Refilling shock

Post by hardingfv32-1 »

Would that not make the wheel rate half of the spring rate in the case of most FV suspensions making the shock pressure even less relevant?

Brian
Mystique Racing
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Re: Refilling shock

Post by Mystique Racing »

hardingfv32-1 wrote:Would that not make the wheel rate half of the spring rate in the case of most FV suspensions making the shock pressure even less relevant?

Brian
Absolutely, but I would use the term less effective, not less relevant.

All of the top runners in any class understand that it is the small things that create separation from the competition. This thread is about one of those small things. Every part of a race car is relevant, some because they perform better, some because they are stronger and wont fail. Many people say things like "that will never work", or, "that makes no difference", etc, etc, Well, most of those people have never tried the specific thing that they are saying won't have any effect. Or maybe they have tried it and just could not feel the difference, that doesn't mean that there wasn't a difference. I think we all have made a change to the car, driven it, and thought "this feels great" then found out that the lap times were slower. Racers are the most guilty group of this that I have ever seen. I can't believe some of the absolutely baseless, unfounded thoughts and opinions that come out of some of these "highly respected" people in our industry. There are a lot of very smart people that contribute to this forum and have very creative ideas and thoughts, however, my advice would be try it before you condemn it.

BTW, I am not taking shots at anyone in particular, just a overview of 30 plus years of doing this.
Scott

Diamond Formula Cars

http://www.diamondformulacars.com
hardingfv32-1
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Re: Refilling shock

Post by hardingfv32-1 »

Until this thread, I was under the impression that the gas pressure had nothing to do with spring rate. Now I know it does, but only by a very small amount. I personally use shocks designed for about 15-20 psi gas pressure, so the spring rate gain is even smaller. If you then apply this 2-3 lb/in spring rate to my 400-500 lb/in spring, I'm fairly certain that I'm not going to notice it.

Brian
brian
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Re: Refilling shock

Post by brian »

Scott, I completely agree with what you're writing. The interesting thing about "conventional wisdom", is that when I test many old standing beliefs, I rarely can I objectively confirm any of them. Some of it has to do with the limits of my flow bench, dyno and onboard data, but many conclusions are tainted by opptimism. It reminds me of the joke about aggregating all the 1/2 hp tricks we've heard about into a 70 hp vee engine. I have to admit that when something on a car is out of tolerance, or not working properly, great gains can be made. But drawing conclusions about the new component is open to challenge. If I install a new hickenlooper on an engine that had a weak hickenlooper, and the engine gains hp, does that make my new hickenlooper a trick? The real test is to find the best hickenlooper around and compare it to the new one. The first thing Joe Mondello tells his students at his cylinder school is not to trust one test on the flow bench. I can't remember how many times I came back the next day to retest something on the flow bench and not get the same results. I think that's why overly technical stuff gives me a headache. Mother nature never does anything the same way twice.
The above post is for reference only and your results may vary. This post is not intended to reflect the views or opinions of SCCA and should not be considered an analysis or opinion of the rules written in the GCR.
cendiv37
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Joined: June 25th, 2006, 7:29 pm

Re: Refilling shock

Post by cendiv37 »

jpetillo wrote:
cendiv37 wrote:pressure inversely proportional to volume
Perhaps it has been a long time.
John
Indeed, too long. Inversely proportional is not linear. Silly me :oops:

As you say, the non-linearity would be greater when the volume of the pressurizing gas is small relative to the volume displaced by the shaft in suspension travel. The smaller the gas chamber, the more change in spring rate with suspension travel.

Black Penske fronts would be more sensitive to this than shocks with a separate, larger pressure canisters. Whether this a good or bad thing might not be so obvious. The effective spring rate of a Black Penske with 100 psi may still be too small to matter. And as you say, there are possible advantages to rising rate springs.
Bruce
cendiv37
jstoezel
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Re: Refilling shock

Post by jstoezel »

Hi,

Could someone walk me through the process of refilling a Penske shock with remote canister? More precisely, I bought the 300psi Penske gauge that comes with a valve, I was hoping it would come with instructions... How do you use it?
Also based on what was said in this thread, it looks like straight air will highly vary in pressure with temperature, and will deteriorate the oil.
I don't think I will have access to high pressure nitrogen by the next race. Would it be ok for me to use straight air if I plan on re-valving after the race and if check the pressure along the day?

Jean
Jean-Sebastien Stoezel
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FV80
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Re: Refilling shock

Post by FV80 »

Jean,
The Penske gauge is simply a 'valve' with a pressure indicator. You screw the gauge onto the bottom of the canister first - tighten it up (the gauge should NOT have changed and should still read zero). THEN, you screw in the 'valve' (T handle) to depress the schrader valve - allowing gas pressure to reach the gauge to show the shock pressure reading. At this point, you can either add pressure (using the 'tire fitting' at the end) or just UNscrew the T handle to close the schrader valve. Then remove (unscrew) the device from the canister.

There is such a small amount of volume inside the canister that attempting to read the pressure directly will dump most of the pressure that is available. The 'super duper valve device' is a method to allow reading the pressure without loosing too much of the charge in the process. Despite that, you will still lose 5 lbs or more each time you read the shock pressure.

If this isn't clear, maybe you could call me (or someone else closer to you) some time and have them explain it "as you go" - it might be more clear.

In answer to your other question. You really should NOT try to pressurize your shock with simple "shop" AIR. It will introduce contaminants, cause foaming, and generally muck things up somewhat. HOWEVER, if you are in a pinch, it would be better than nothing...(say you had a DEAD shock and HAD to repair it). Of course, you should dump everything and re-pressurize with Nitro at the earliest opportunity - and probably change the oil again then. Using shop air, the pressure change you can expect will be WITHIN a given session - so, unless you plan to come in every 3 or 4 laps to check/change it, it won't be a good thing. As the heat builds up in the shock (rather rapidly unless you are on a billiard table track), the water vapor in the shop air will expand and change the pressure rather markedly.

Steve, FV80
The Racer's Wedge and now a Vortech, FV80
SR Racing
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Re: Refilling shock

Post by SR Racing »

jstoezel wrote:Also based on what was said in this thread, it looks like straight air will highly vary in pressure with temperature, and will deteriorate the oil.
It has nothing to do with air verses Nitrogen, Gas law is the same for both gases. Temperature will effect air and Nitrogen exactly the same way contrary to what others have said.

The Ideal Gas Law equation is P*V = n*R*T where: P = pressure, T = temperature, V = volume, R = the ideal gas constant and n = the amount of gas.

This assumes dry air. Which you should always use if you use it. whether it is for tires, tools or shocks

NASCAR etc. use N for tires etc. because it is dry, cheap and safe. The temperature expansion factor is the same.

Nirogen is inert, thus is will not degrade the oil, contribute to any oxidation or chemically combine to anything in the shock. That is the only reason N (N2) is used. It is also cheap and safe in in pressurized containers and dry when you buy it. That being said,... dry ambent air will probably work fine especially when you change the oli periodically. (5W shock oil is cheap and one bottle will fill several shocks.)

As pointed out the N or air is only used to keep foaming from occuring. The true shock dampening will remain the same with higher or lower pressures. However your overall effective spring rate will go up with the pressure. So you could do some "tuning" with gas pressure. For proper dampening the minimum recommended pressure will keep the shock from foaming. More than that and you are just adding some spring rate.
jstoezel
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Re: Refilling shock

Post by jstoezel »

Hi Steve:

Thanks for yet another good reply related to shock setup. I think I'm good with the way you described how to use the gauge.

Following your previous advice I called a few welder's shops. While the nitrogen refills are somehow affordable (about $35 for a small tank), the smallest tank available is about 5 gallon and priced at $170. I asked for a tank rated for up to 250-300psi since Penske's manual recommends around 200psi of pressure. This is why they are more expensive (the tanks are rated up to 2200psi).
Other stores send more affordable tanks, Canadian Tire and Princess Auto have 5 to 11 gallon tanks from $49 to $69. They're only rated up to 125psi though.

What do you guys use for nitrogen tanks? There must be cheaper solutions than those $170 ones.

I need to do something though, playing with the canister I've lost all pressure, now I know what you mean when you were asking if I could feel the notches in the compression setting on the canister. I can't anymore.

Jean


FV80 wrote:Jean,
The Penske gauge is simply a 'valve' with a pressure indicator. You screw the gauge onto the bottom of the canister first - tighten it up (the gauge should NOT have changed and should still read zero). THEN, you screw in the 'valve' (T handle) to depress the schrader valve - allowing gas pressure to reach the gauge to show the shock pressure reading. At this point, you can either add pressure (using the 'tire fitting' at the end) or just UNscrew the T handle to close the schrader valve. Then remove (unscrew) the device from the canister.

There is such a small amount of volume inside the canister that attempting to read the pressure directly will dump most of the pressure that is available. The 'super duper valve device' is a method to allow reading the pressure without loosing too much of the charge in the process. Despite that, you will still lose 5 lbs or more each time you read the shock pressure.

If this isn't clear, maybe you could call me (or someone else closer to you) some time and have them explain it "as you go" - it might be more clear.

In answer to your other question. You really should NOT try to pressurize your shock with simple "shop" AIR. It will introduce contaminants, cause foaming, and generally muck things up somewhat. HOWEVER, if you are in a pinch, it would be better than nothing...(say you had a DEAD shock and HAD to repair it). Of course, you should dump everything and re-pressurize with Nitro at the earliest opportunity - and probably change the oil again then. Using shop air, the pressure change you can expect will be WITHIN a given session - so, unless you plan to come in every 3 or 4 laps to check/change it, it won't be a good thing. As the heat builds up in the shock (rather rapidly unless you are on a billiard table track), the water vapor in the shop air will expand and change the pressure rather markedly.

Steve, FV80
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Re: Refilling shock

Post by SR Racing »

Looks like we posted at the same time. Go back and read my last post.. You will not want a small bottle (or big one) with only 200 or 300 psi. You will be out of gas in a few minutes. You can get a small ~2500psi bottle and that will last you for a few years. But you will need a regulator (plus your shock filler tool)

Remember that any testing of your shock is destructive. ie. If you test it you must refill it. The volume of N in the shock is quite small and the test alone will drop the pressure dozens of lbs psi.

Call me at the shop and I can walk you through testing, refilling and options for N tanks.

Now seeing Steve's post, it looks like he covered most of it.

Jim
jstoezel
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Re: Refilling shock

Post by jstoezel »

Hi Jim,

Thanks for the reply. Hum yes now I realize I need much more pressure in the tank than what I need in the shock. There's one more shop I need to call today.

Thanks Steve and Jim for offering to assist by phone, I might just do this if I can't get things sorted out.

Jean

SR Racing wrote:Looks like we posted at the same time. Go back and read my last post.. You will not want a small bottle (or big one) with only 200 or 300 psi. You will be out of gas in a few minutes. You can get a small ~2500psi bottle and that will last you for a few years. But you will need a regulator (plus your shock filler tool)

Remember that any testing of your shock is destructive. ie. If you test it you must refill it. The volume of N in the shock is quite small and the test alone will drop the pressure dozens of lbs psi.

Call me at the shop and I can walk you through testing, refilling and options for N tanks.

Now seeing Steve's post, it looks like he covered most of it.

Jim
Jean-Sebastien Stoezel
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jstoezel
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Re: Refilling shock

Post by jstoezel »

A couple more questions.

I may have tried to fill my shock with nitrogen and more while I was playing with it yesterday.

I don't have the shock with me but looking at the assembly diagram I'm guessing I should be able to (completely?) purge it by opening the filling valve (at the canister) and pushing the shaft all the way in.

Is this the proper way to purge a shock?

When pressurizing it, does it really make a difference if the shock is mounted and under compression (some I guess, the shaft would be in a bit), as opposed to fully extended? My understanding based on previous posts is that the pressure difference (compressed and extended) would be negligible.

Jean
Jean-Sebastien Stoezel
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jstoezel
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Re: Refilling shock

Post by jstoezel »

Thank you Steve "Greg" Davis for taking the time to go over this with me over the phone. Your help is greatly appreciated.

I went to Canadian Tire to use their nitrogen plug. I was very lucky, they did have a straight long reach tire inflator, which helped fix the canister lid malfunction. I asked and that straight tire adaptor is not available in their store, so I'm not sure where they got it. But it exists.

I also got to use the Penske gauge, and now I understand what you mean by isolating the shock. Initially I though there was a valve in the penske gauge, but instead that tap on the side drives a pin in the shock valve, to control when you want to open it (preferably when the gauge is fully mated with the shock). Anyhow, I filled the shock, it seems to hold the pressure, so I don't think there's any damage.

They only had 135psi available though. I tried another Canadian Tire and that one only had 60psi, apparently at some stores they limit the output to prevent technicians from blowing tires. I plan on racing at that pressure, unless I get to find a place that has nitrogen at a higher pressure.

Thanks again Steve, you have been of great help.

Jean
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Re: Refilling shock

Post by FV80 »

Jean,
135 PSI should work just fine in your shock. Penske is now recommending about 125 PSI in the shocks as nominal. You can adjust it up to as high as 250 or so and down to as low as 60 (so I'm told) to 'fine tune' things at the track. The on track effect is minimal, but it does change the spring rate a bit and some may have REALLY sensitive butts :mrgreen: .

Steve, FV80
The Racer's Wedge and now a Vortech, FV80
jstoezel
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Joined: September 19th, 2010, 6:21 pm

Re: Refilling shock

Post by jstoezel »

Thanks Steve, this is good to know as the user manual from the Penske website specifies 200psi.

I found that the compression knob is not as sticky at 135psi as it used to be when the shock was pressurized at 200psi. But it could be just an impression.

FV80 wrote:Jean,
135 PSI should work just fine in your shock. Penske is now recommending about 125 PSI in the shocks as nominal. You can adjust it up to as high as 250 or so and down to as low as 60 (so I'm told) to 'fine tune' things at the track. The on track effect is minimal, but it does change the spring rate a bit and some may have REALLY sensitive butts :mrgreen: .

Steve, FV80
Jean-Sebastien Stoezel
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