Author Topic: Truck Arms & Air  (Read 2550 times)

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Offline Peter C

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Truck Arms & Air
« on: May 07, 2010, 09:00:42 AM »
An old thread had some good info on truck arms and panhard bars. Using air bags with the truck arms like Doug Odum uses sounds like a great idea.
A couple of questions I have.
First question - How do you work out rear down force from the airbag pressure. I mean what would a 20psi increase work out to in lbs downforce. Is it something you work out in the shed by sitting a 200lb buddy on the rear and checking pressure.
Another question - could you raise the rear as you are making your run to increase downforce.

Offline Peter Jack

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Re: Truck Arms & Air
« Reply #1 on: May 07, 2010, 09:31:49 AM »
You don't raise the downforce at all unless it comes from changing the attitude of the truck. The only way to change the downforce is to change the aerodynamic attitude, add ballast or move weight from one location to a location further back. Adding air pressure will simply raise the back of the truck.

Pete

Offline Peter C

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Re: Truck Arms & Air
« Reply #2 on: May 07, 2010, 09:56:44 AM »
Thanks Peter.
Thats what I ment. Raising the back of the vehicle to change aerodynamic attitude. I guess on a car with a wing or a spoiler it would change the angle a fraction. Probably a dangerous thing to mess with on a run. Could get into all sorts of trouble if you went down instead of up!!!!

Offline bbarn

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Re: Truck Arms & Air
« Reply #3 on: May 07, 2010, 10:03:46 AM »
You don't raise the downforce at all unless it comes from changing the attitude of the truck. The only way to change the downforce is to change the aerodynamic attitude, add ballast or move weight from one location to a location further back. Adding air pressure will simply raise the back of the truck.

Pete

Pete, I think his question was more along the lines of using the airbags as a gauge of sorts. If I read it correctly, he was looking to take a static reference pressure, then compare that to the pressure during a run to see if he is making down force or not.

Your approach of adding weight while static and measuring the pressure difference at a given weight should give you a "reasonable" method of measurement. Instead of having just a 200# friend jump on, add known weights and make a chart. I don't know if the PSI increase on the bag will be linear, you can work that out by varying the weight and measuring the PSI.

I would do a couple of measurements to see, 100,150,200,250# and check the PSI at each weight level. If it is linear, you can easily calculate the amount of down force being generated by writing a simple formula and plopping in the PSI reading at the bag.

For fun, get a bathroom scale and put it on your jack. Lift the car up 50,100,150,200# and take a reading to calculate lift. Might be interesting to see if there are speeds at which down force goes away and by how much.
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Offline doug odom

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Re: Truck Arms & Air
« Reply #4 on: May 07, 2010, 10:23:38 AM »
I use air bags for many reasons. First the ride is very smooth and your eyeballs can focus. The pressure tells me if I have down force or lift. I adjust the ride height with 1/2 inch shims if I want to keep the same bag pressure but change the height.
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