Chassis Dyno question ?

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tod85
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Chassis Dyno question ?

Post by tod85 » Sat Jun 25, 2011 6:23 pm

Does anybody know if there are any consequences to a "super" tight tiedown on the rear of the car while on the dyno ?

The Dyno shop on a recent pull had strapped down the rear with 4 straps down so hard it brought the rear end half way down the shocks.... I am not sure, but it would make sense to me this is added resistance considering the artificial weight literally driving those tires into the dyno wheel. If I thought about turning that dyno wheel with the car strapped down like that manually, it would make sense that it would be harder to turn..... but not sure.

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91stang
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Re: Chassis Dyno question ?

Post by 91stang » Sun Jun 26, 2011 12:34 am

Depending on power and the roller size and the tire on the car it is usually necessary due to tire spin on the dyno give you a faulty reading.
1991 mustang 3040 with driver all motor all the time 9.70 @ 138.75 1.31 60' street driven mufflers and real tires, mt 275/60/15 radials with a 950hp holley.

RednGold86Z
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Re: Chassis Dyno question ?

Post by RednGold86Z » Tue Jul 12, 2011 11:40 am

tod85 wrote:Does anybody know if there are any consequences to a "super" tight tiedown on the rear of the car while on the dyno ?

The Dyno shop on a recent pull had strapped down the rear with 4 straps down so hard it brought the rear end half way down the shocks.... I am not sure, but it would make sense to me this is added resistance considering the artificial weight literally driving those tires into the dyno wheel. If I thought about turning that dyno wheel with the car strapped down like that manually, it would make sense that it would be harder to turn..... but not sure.

Thanks
Yes, tire pressure and deformation affect the measured power (but slippage must be avoided - best way is as high a gear as possible without blowing up the tire or overspeeding the dyno). A better way is with the hub mounted dynos (no tire). Even better is an engine dyno with all of the vehicle exhaust and intake at the normal operating temperature.

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MadBill
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Re: Chassis Dyno question ?

Post by MadBill » Sun Aug 21, 2011 10:43 pm

If your tires and power level don't push the traction limit, you could just substantially back off the tension and see what happens power-wise.
Felix, qui potuit rerum cognscere causas.

Happy is he who can discover the cause of things.

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Re: Chassis Dyno question ?

Post by rskrause » Fri Sep 16, 2011 7:34 pm

At least on a Dynojet it doesn't seem to make a big difference. Even so, if you are using the chassis dyno correctly the thing you need to do is strap it down the same way each time and use the same tire pressures. Think of it as another variable that needs to be controlled/minimized. The point is repeatable results, not an "accurate" number. It's a tuning tool, not necessarily a way to "accurately" measure hp.

Richard

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