F1technical.net

Thanks for the info Belatti.

Sorry,
I think we had a slight miscommunication, I did mean to only warm up the gas canister, so that you could see the change from the gas pressure, without the effect of the change in oil viscosity.

But, it´s no big deal. I think, you understand what a change in gaspressure does to your damper graph.

My little "model" calculated the increase of gas pressure to 237 psi, using a starting pressure of 205 psi, but I had to make a guess about your oil volume.
Nevertheless, it shows, that my numbers are not totally "on the moon".
I will post the last part of my "gas spring story" a little bit later.

I agree with Dave about the "valve chuckle", one possible reason could be the piston shape you use.

If it is a standard Penske lin-deg piston, then the linear side has a very large sealing area.



if you compare it with the dig. side or with an lin.-lin.- high flow piston



if permitted by the rules, you could try a modification

going from this



to something like this



reducing the sealing area and thereby the possible cappilar effects of the oil.
The modified piston is also less likely to "stuck-open" when you have condamination inside your oil. As a particle traveling with the oil has less possibility to be traped between the shim and the sealing surface of the piston.

one another note, you could try to turn the second shim (the one on top of the sealing shim) around, or deburring the outer edges of the shim a bit.
Sometimes, this helps

747heavy wrote: Sorry we had a slight miscommunication, I did mean to only warm up the gas canister, so that you could see the change fom the gas pressure, without the effect of the change in oil viscosity.

NP

the warm up / pressure check was one event, the dyno test with different temps was another...

Thanks for your inputs both 747 & Dave!

I find it lucky that that particular damper had that chuckle problem, I learned something new there.

I remember about the force that can be built between two very smooth surfaces from a practice in the metrology lab at Uni (standard gauges used to calibrate).

check this out!

here, a zoom of the "chuckle" zone with 4 different runs...

the green one is the softer one (15 clicks) in expansion and the effect disapears...



blue, brown and pink are 5,7&11 clicks repectively

I have just dismantled the damper.

It has this piston:


Next test is to try to balance pressure according to the suggetion 747 gave me from the penske paper.

o.k. let´s see, if we can wrap up our small excurse into the gas spring effects in a conventional monotube damper, by looking at the influence of temperature.
We have seen, that the ratio between the volume which get´s displaced by the shaft and the gas volume in our damper, are important to define the progression rate of our gas spring.

As larger our gas volume compared to the displaced oil volume, as less the spring rate will be affected over the stroke of the damper.

For most racing dampers this ratio is sufficient to not have an large change in gas spring rate over the stroke of the damper, so the effect will be small.

But there are maybe cases, where larger shaft diameters are used and where, due to packaging, weigth or cost constrains, the gas volume is a bit marginal, then the gas spring progression comes into play and adds spring rate to our main spring rate.

This effect will be less perceptable in a stiffly sprung car, but maybe leads to an perceptable change in a lightly sprung car, which uses larger strokes.
(such as Sprint Cars on Dirt, or-off road racing buggies etc.)

When we talk about temperature, we have to consider three effects.

The first one, is the change in oil viscosity and it´s effect on flow rates and forces generated by the valves.
This can/will have a effect on the overall damping values of the damper, as seen in Belatti´s experiment.

Depending on the damper in question, it´s design, it´s valve technique and the oil used, this can have major effects on overall damper performance and overshadow the effects from the gas spring.
But this is a different topic, and not directly related to the gas spring effect.

For our assessment on the gas spring effect we need to consider the follwing two effects, the temperature has on our damper.

The first one is the increase in pressure inside the gas chamber in the damper - canister.

This is again governed by the "ideal gas law".



But to make matters worse, we have to consider, that our oil will expand, due to the increase in temperature, as well.
By doing so, it will take away volume from our gas chamber, by moving the seperation piston further inwards (towards the gas).
This change alone would increase the gas pressure slightly.

How much does our damper oil expand by temperature?
Well, the precise value will depend on the oil used, it´s density and CTE, but for most practical applications an estimate of 0,01%*oil volume*delta T can be used, and will give sufficient accurate values - IMO

Applying this to our example JRZ damper, this means.

Let´s say our damper warms up during use from 25°C to 65°C, which is a delta T of 40°C.
Based on our dimension, we estimate the oil volume with ~152 cm^3 and we have the same value as our gas volume.
When our damper heats up , the oil volume increases to ~158 cm^3 and our gas volume decreases to ~146 cm^3.

If our damper had a gas pressure of 150 psi when it was @ 25°C it will now have a pressure of:

Phot= (339K*150psi*152cm^3)/(299K*146cm^3) = ~177 psi

when @ 65°C (note that the calculation uses absolute temperature in K)

This means our preload increases from 393N to 465N, a increase of ~72N or 0,1 lbs
If our car would be fitted with 70N/mm springs and would have a motion ratio of 1:1
the car would be ~ 1mm higher now.

The average spring rate increases by 0,27N/mm or 1,52lb/in, so this is a secondary effect, which is neglectable in this case.

some graphical representation of the values:




note: the preload, was taken off the data in both cases, to show only the difference in force @ the shaft vs. stroke.
The pressure/force of the "hot" damper whould have an offset of 72N to the "cold" damper value.

Belatti wrote: I heated up the whole (damper and canister) in the most homogeneous way I could. I tried to maintain a stable carcass temp for a while to make sure the same was true inside the thing...

The drifting graph was from 150 to 100psi, however the temp test was performed at 205psi... here, the gas pressures:
23°c ~ 205psi
39°c ~ 220psi
55°c ~ 235psi

my "estimate" for your damper, but as I said, I needed to take some guesses for your oil and gas volume, but I would like to think, that it is resonable close.

so finally, what about the comment of this Roehrig fellow, who claimed that 20 laps into the race, when your damper get´s hot, it would be the equvivalent of throwing a 25lb/in (~4.4N/mm) spring change onto your car.
Is this possible?
Or is Speedsense correct, when he says that this has never happened to him?

Well, I guess, as with most things in life, there are different ways to skin a cat, and you can prove allmost everything, if you try hard enough.
As long as the damper has a sufficient large gas volume, in relation to the shaft volume, it is very unlikely that you see a meaningful increase in spring rate.
If your gas volume is a bit marginal, and you are dealing with large strokes (soft springs), the progression of the gas spring can add to your car spring rate.
The value of 25 lb/in is maybe a bit high, but 12 lb/in are doable under realistic condition.
Here are two examples where such a thing could happen, at least theoretical.

a Sprintcar / Dirt model damper with 8 " stroke




and the Porsche 996 Cup front damper, as used in the earlier example




Due to the highly progressive characteristic of the gas spring, it is maybe not 100% correct to think about it, as adding a constant spring rate.
The spring rate increases progressivly with stroke.
A better comparsion would probably be to say, it´s like adding a bump stop to your car.

While the extra spring rate is neglectable, the increase in damper preload/gas pressure due to temperature is a very "real" effect.
Even on relative heavly sprung cars (such as an F3) this can lead to an ride height change of ~1mm on the rear.
This effect is more pronouced on mid engined race cars, which generate a lot of there downforce by underbody aerodynamics.
While the front dampers operate in "relative" cool conditions inside/or on top of the tub, the rear dampers are subjected to exessive heat from the engine/gearbox which is in very close proximity in some cases.
So the resulting ride height change will be larger at the rear of the car, leading to a change in the aerodynamicly important "Rake" of the car.
This effect and the resulting inconsitencies where one of the considerations for using through rod/through shaft dampers in these type of cars.
Because a TRD/TSD damper has the same surface area on both sides of the piston, there is no resulting force for any pressure in static conditions, therefore a change in gas pressure has no "lifting" effect on the car.

O.K. that´s the end in my long winded explaination about the gas spring and it´s effects in an conventional damper.
I hope, some of the things make sense, and that some of you may found something interesting while reading this.

Great post 747!

747heavy wrote:Even on relative heavly sprung cars (such as an F3) this can lead to an ride height change of ~1mm on the rear.
This effect is more pronouced on mid engined race cars, which generate a lot of there downforce by underbody aerodynamics.
While the front dampers operate in "relative" cool conditions inside/or on top of the tub, the rear dampers are subjected to exessive heat from the engine/gearbox which is in very close proximity in some cases.
So the resulting ride height change will be larger at the rear of the car, leading to a change in the aerodynamicly important "Rake" of the car.

When I read about this temp effect I wonder if it would be legal to put F1 dampers into thermal insulation and have a system that can cover or uncover an cooling air intake.

Remember the fuz about RBR ride height change? Something to post in the 2011 innovation thread

Much to consider, 747. I like especially your plotting software....

I have to question just one statement:

747heavy wrote:Even on relative heavly sprung cars (such as an F3) this can lead to an ride height change of ~1mm on the rear.

It's not hugely important, & your calcs support the statement. However, at the risk of being accused of nit-picking, my recollection was that the change in ride height could be larger than the 1mm you suggested. I did my own calcs based on an actual F3 rig test (with no comments on the numbers). 16mm shaft dampers, pressurised to 27 bar were replaced by 16mm shaft dampers pressurised to 10 bar. The length of the 700 springs fitted actually reduced by 2.6mm (computed to be 2.8mm, so in reasonable agreement). Motion ratio was approximately 1.3, so the computed change in ride height was approximately 3.7 mm. Through rod replacement dampers would have reduced static ride height by over 5mm. in this case....

you are not nit-picking Dave, and I´m allways open to discussion with you, and there is allways the very real possibility that I´m wrong or made a mistake somewhere.

The statement you quoted was in regards to the ride height change due to the increase in temperature. (difference in running temp front vs. rear)
I used the 900 lb/in rear spring which was quoted in the Dallara manual.
I based this estimate on a KONI damper, where I made some guesses in regards to the gas and oil volume.
I assumed a delta T of 70°C and a 14 mm shaft.



I did not wanted to "break the bank" with some extreme values, so I opted for ~1 mm.
But clearly this value is open to discussion, as I have no first hand data/expirience with an F3 car, and it will depend on shaft size, base gas pressure etc.

Running the data you provided, I come up with these numbers:



Therefore I think we are on the same page with our data, and the way we look at it.
Now it would be of interest to know, what the actual temperature difference between front and rear dampers in an F3 (or F1) car is, so that we can make an assessment/guess about the true ride height change.

Thanks for your feedback and input Dave it´s much appriciated.
P.S.: the "ploting software" is just Excel - nothing special

Drat.. Sad case of sleeping on the job, I'm afraid.

I'm no expert on operating temperatures under the engine covers of open wheelers, but I believe "high temperature" seal materials are used by damper designers in rear dampers destined for such vehicles, & I think high temperature means > 150C. One well-known designer seems to like to produce vehicles that self-immolate occasionally, especially after a safety car has been deployed.

Finally!!!!

I got the pressure balancing "thing" from the penske paper right.

Check this out:



It is the same damper, tested at the same conditions. The black is the "rebalanced" one. To get similar peak forces I had to change clicks here and there.

Taking into account it was the first damper I assemble I guess it was a pretty good job. It took me a little while to purge the system to get all the air out.

looks like a job well done to me !!!

and maybe also shows that the flexing hose was not the main culprit in this case.
As I assume, you still use it (the flexible hose), you just made his life a bit easier I guess.

How much adjustment range, did you loose in bump direction (if any)?
But seeing, that it is just a 2-way adjustable damper, where I assume, the adjustments are low speed (bleed), this may does not show up on your damper.

BTW where can we send the invoice for our consulting to?

Yep, the same thing I thought about the flexing hose. Less pressure inside of it and less hysteresis.

The red plot was in 50% of the click scale. Now the black one is in the 80% so, yes, some adjustment range was lost.

Err... Invoice? How about in kind payment? For holidays in Argentina, you already know where you can stay...