Gas-charged shocks and temperature...

[AlexL]

I noticed today when doing some work on my car that I had tons of clearance (an inch or more) between the jack at its lowest height, and the bottom of the car. This struck me as strange, because there have been times earlier in the year where I've had very little clearance, and at mowog 1 I actually had to lift up on the car to make room for the jack to go under. Everything about the car and the jack has remained the same, and the only real difference I can think of is temperature. It was in the 30s when I was trying to change tires at Valleyfair, and in the 90s today.

The only thing I can think of is that the gas in my shocks expands when it gets hot and increases my ride height. This could also explain why it's a lot easier to get a jack under my car after an event than it is before. I know it has to happen to some extent, but could it really cause a varitation of over an inch in ride height?

[GodSquadMandrake]

Alex your probably onto something there, but it could also have to do with the springs an tires. I have found that my suspension feels pretty loose when it's hot out, like 90 degrees, and tightest at around 60 degrees, it gets weak when it goes below freezing though. I daily drive my car so I get to know it pretty well!

I still have that processor Alex but I haven't plugged it in yet, i'll get around to it soon sorry about the delay.

[MNbiker]

Tire pressure?

[AlexL]

Tire pressure?
That could exlpain the difference from the start of the event to the end, but I check the pressures on my street tires almost daily and always keep them at 30 PSI.

[SmokingTires]

That could exlpain the difference from the start of the event to the end, but I check the pressures on my street tires almost daily and always keep them at 30 PSI.

We're you driving it at all prior to working on it that day? A quick trip on the highway could make a good deal of difference.

[AlexL]

We're you driving it at all prior to working on it that day? A quick trip on the highway could make a good deal of difference.
Nope, and I can assure you that it wouldn't account for an entire inch of change.

I did a little more testing this evening, and the results are interesting. I took the car for an 80 mile drive and about 15 minutes after I got home, I measured the distance from the center of the hub to the fender lip on both rear wheels (this makes tires irrelevant). It was 13.5" left and right. About an hour and a half after that, I made another check, and found that the car was 1/8" lower on both sides. About three or four hours after that (a few minutes ago), I made one final check, and the car was another 1/8" lower. The difference in ambient temperature from the first reading to the last was approximately 10° F, but since I can't tell you how hot the shocks themselves were for the first couple measurments, I can't give an accurate guess as to how much change there is in ride height with a given change in temperature. I can, however, say that I think there is a clear link between temperature and ride height, and that I believe it is caused by the expansion and contraction of the gas in my shocks with temperature.

[washburn]

Neat observation...I've never noticed anything like that before. One solution: de-gas those shocks.

[GodSquadMandrake]

De-gas the shocks? :?

Good runs today Alex, you were right on my tail at the end there. If we did two laps you probably would've passed me!

[Shrep]

Beacuse of the valving within the strut, isn't the only physical limitation the shaft length? And then even so, wouldn't it have to be at one extreme or the other to cause a change? Instead, the car is suspended by the spring, rather than the strut.

I've noticed this before on several of my cars...and have decided to blame it on temperature, as you have. However I think it works like this...

The spring is nothimg more than a torsion rod, which twists as the spring compresses and expands. This twisting causes friction/heat within the metal, the effects greatly impacted by ambient temperature. With heat, metal expands. This expansion can cause a diameter change within the spring itself. Just like a sway bar, the diameter of the bar increases rigitiy to the 4th power, so even minor changes can make a huge difference.

What's likely occurring is a spring rate change due to the heat expansion.

The changes measured and felt as the day goes on, are likely a result of the heat not dissipating well, as ambient temps increased. Note that you also paddocked your car almost immediately after pulling off track...

In the winter the same can be said about the feel...as the metal contracts, as you're just starting out, the spring rate is weakened, and will feel soft'er until the metal has had a chance to heat up, which is difficult considering the icy temps we have here, constantly working to cool the metal as you drive.


I've got no proof....but it makes sence to me. :D

[Chin]

Gas will expand ~33% (given a sealed system) when the temp goes from 0C - 100C, maybe you could do the math....

Don't forget your chemistry... PV=nRT

:P

Christian Banks
#202 DSP
'00 Impreza 2.5RS

[washburn]

De-gas the shocks? :?

Yup. Ran de-gassed Koni's on my Neon "back in the day". Nitrogen charging is there to reduce oil cavitation, which can happen under high temp/stress conditions, almost never reached in autocrossing. All "gas" shock are still hydraulic in operation. Even though the springs supports the cars weight, the nitro charge actually provides an additional spring rate, thus lifting the car up according to the amount of charge that's in the shock. By de-gassing, you take away this additional rate, and the cars lowers. We lowered the Neons quite a bit by doing this. Lower CG = better autocrosser. This does not affect the damping rates.

[Chin]

Beacuse of the valving within the strut, isn't the only physical limitation the shaft length? And then even so, wouldn't it have to be at one extreme or the other to cause a change? Instead, the car is suspended by the spring, rather than the strut.

I've noticed this before on several of my cars...and have decided to blame it on temperature, as you have. However I think it works like this...

The spring is nothimg more than a torsion rod, which twists as the spring compresses and expands. This twisting causes friction/heat within the metal, the effects greatly impacted by ambient temperature. With heat, metal expands. This expansion can cause a diameter change within the spring itself. Just like a sway bar, the diameter of the bar increases rigitiy to the 4th power, so even minor changes can make a huge difference.

What's likely occurring is a spring rate change due to the heat expansion.

The changes measured and felt as the day goes on, are likely a result of the heat not dissipating well, as ambient temps increased. Note that you also paddocked your car almost immediately after pulling off track...

In the winter the same can be said about the feel...as the metal contracts, as you're just starting out, the spring rate is weakened, and will feel soft'er until the metal has had a chance to heat up, which is difficult considering the icy temps we have here, constantly working to cool the metal as you drive.


I've got no proof....but it makes sence to me. :D

I would tend to believe it is the gas expansion, not metal. Short of finding the equation for expansion of metal, I would doubt the rates change considerably, but I am not a material science expert. In a perfect world (physics land, as my old HS teacher called it), the shock would have no purpose other than damping the system, but in reality, the gas and oil serve a dual purpose, adding a certain amount of "spring rate" to the system.

Christian Banks
#202 DSP
'00 Impreza 2.5RS

[Shrep]

I would tend to believe it is the gas expansion, not metal. Short of finding the equation for expansion of metal, I would doubt the rates change considerably, but I am not a material science expert. In a perfect world (physics land, as my old HS teacher called it), the shock would have no purpose other than damping the system, but in reality, the gas and oil serve a dual purpose, adding a certain amount of "spring rate" to the system.

Christian Banks
#202 DSP
'00 Impreza 2.5RS
I don't argue that, and there is some inherant "spring rate" to the system, however being a system designed to control the speed of movement, rather than movement itself, it would affect handling, indeed, but rideheight?

[washburn]

I don't argue that, and there is some inherant "spring rate" to the system, however being a system designed to control the speed of movement, rather than movement itself, it would affect handling, indeed, but rideheight?

Absolutely. Read my post above. The gas charge is what pushes the shaft of a shock out. Like the gas strut on a hatchback or hood. No gas charge = no force to push the shaft out. Get enough force pushing the shaft out and you raise the car. De-gas and the car drops. Simple.

[phile]

I don't argue that, and there is some inherant "spring rate" to the system, however being a system designed to control the speed of movement, rather than movement itself, it would affect handling, indeed, but rideheight?
Nomally, shocks do not affect ride height. Gas shocks are an exception. The gas pressure is there to control the oil, but a side effect is that the gas shock adds support to the car and raises it. I have personally seen the difference when changing shocks.

If you don't believe this, you have never actually handled gas shocks. Gas shocks come packaged with keeper strings. When you cut the string, the shock extends all by itself. This force does raise the car and add to the effective spring rate.

[Shrep]

Nomally, shocks do not affect ride height. Gas shocks are an exception. The gas pressure is there to control the oil, but a side effect is that the gas shock adds support to the car and raises it. I have personally seen the difference when changing shocks.

If you don't believe this, you have never actually handled gas shocks. Gas shocks come packaged with keeper strings. When you cut the string, the shock extends all by itself. This force does raise the car and add to the effective spring rate.Ahh...yes. Gotcha.

[phile]

Ahh...yes. Gotcha.
And the official scorekeeper will please note that I had not seen Pat's final posting when I composed mine. I was not piling on. :)

[Shrep]

And the official scorekeeper will please note that I had not seen Pat's final posting when I composed mine. I was not piling on. :)
I've been working on spring/strut combo's for so long that for whatever reason I could not remember having worked on anything else. It wasn't until you mentioned the strap that the "old" stuff came rushing back. :lol:

[73GT]

Justin
You never remembered having to turn the shaft on a strut type cartidge to get it to come out when installing? Same thing only different ;)


73GT

[Shrep]

Justin
You never remembered having to turn the shaft on a strut type cartidge to get it to come out when installing? Same thing only different ;)


73GT
Well, yeah...there's that too...:lol: Good thing I only work on my cars eh? ;)