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Re: DYNO RESULTS -- STOCK vs AFTERMARKET WHEELS
>From: 16v Jason <jason@scirocco.org>
>
>Hey all,
> There was a local dyno day sponsored by the RX-7 club in my area..
>snip<
>THE RESULTS:
> There was *no* appreciable power loss anywhere in the rev range using the
>heavier wheels.
Great job on getting some numbers put to an old cunundrum.
Question: Did this Dyno automatically recalculate the figures into HP at the
crank?
I don't know much about Dyno's but it would seem to me that if the dyno took
some readings while everything was spinning down, then it could calculate
the effective crank HP figures.
If done right this would throw out the losses/gains due to wheel weight
(theoretically). The result would show no HP loss due to the heavier wheel.
It would be great to get some 0-60 and 1/4 mile times to compare the
acceleration with the two setups. 60-0 braking distance would also be
interesting.
But none of this would mean anything unless the rotational inertia of the
two wheel/tire combos is known. This can be measured, a crude test would be
to let the wheels roll down an incline and measure the time it took. A more
detailed and scientific test would be to set the wheels on a free spinning
rig. Then put a heavy weight (of known mass) on the end of a string and wind
the string around the outside of the tire. Let the weight drop and use a
stopwatch to time how long it took to reach the ground. Immediately count
the RPM's of the wheel. Repeat for other wheel. You now know the torque put
on the wheel, and for how long. You also know the acceleration of the wheel
(change in RPM in the time it took the weight to drop). Using F=ma you can
calculate rotational mass.
but I digress...
My point is this. Heavier wheels will not reduce the HP output of your
motor. Heavier wheels (actually wheels with greater rotational inertia) will
hurt performance, no way around it. Torque output from the motor will be
used to turn the wheels rather than accelerate the car. More rotating mass
will reduce the deceleration caused by your brakes. More unsprung weight
will alter your suspension's behavior.
A dyno would only be able to detect the change in torque output to the
ground. We (of all people) should know that there is much more to the
performance of a car than it's HP output.
food for thought
Jonas
85 Scirocco 2.0
For direct reply: jonas@metabolex.com
http://www.geocities.com/scirocco2l8v
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