Rwhp vs tire size on a dyno - Page 2 - Great Lakes 4x4. The largest offroad forum in the Midwest

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Old June 7th, 2012, 12:32 PM   #21
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No nothing about the tires changes HP except the ability to maintain enough traction on the rollers
It sure does effect usable hp you get to the ground.
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Old June 7th, 2012, 12:33 PM   #22
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Here's am interesting read about dyno's and how easily the results can be fudged.
http://www.duncanracing.com/TechCent..._March2011.pdf
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Old June 7th, 2012, 12:35 PM   #23
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It sure does effect usable hp you get to the ground.
I definitely agree. I did some reading on a diesel forum. Those guys love power numbers... There are countless threads about increasing tire size and lower dyno numbers.
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Old June 7th, 2012, 12:37 PM   #24
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it sure does effect usable hp you get to the ground.
omg!!!
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Old June 7th, 2012, 01:00 PM   #25
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tire size affects the usable torque. You cant multiply or divide HP only torque. when you change gears you are multiplying torque, the hp is unaffected
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Old June 7th, 2012, 01:11 PM   #26
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tire size affects the usable torque. You cant multiply or divide HP only torque. when you change gears you are multiplying torque, the hp is unaffected
But big tires weigh a lot more than a cars street tires, and take more effort to spin.
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Old June 7th, 2012, 01:43 PM   #27
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Again, there are different types of dynos.

Inernia dynos, eddy current ones, etc.

An inertia dyno 'calculates' HP based on how fast you accelerate the rollers. It does not consider your tire weight.

So if you swapped heavy tires/wheels for lighter ones of the same rolling radius, you would 'gain' horsepower with these tires.

This is not really accurate, in a numbers thing, as both sets of tires (unless one has a lower rolling resistance also) sap the same power when moving at a steady speed. So something like a Tow rig with slow acceleration and lots of steady speed loaded pulling will not really see a difference.

But swapping heavy for light will show up in real world 0-60 and 1/4 mile numbers when you are trying to accelerate fast.


So you'll see some camps say that makes the inertia based ones(most dynojets) not real accurate (especially if you are the type to calculate backwards to determine what you think you have at the flywheel).

But as hoser said, while the number might not mean much, the results reflect what happens on the street. Add weight and you will slow acceleration. You see that as 'HP loss' on the dynojet. If you added 10 HP from better exhaust or 'added' 10 HP by swapping to lighter wheels, you'd see similar effects.
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Old June 7th, 2012, 02:06 PM   #28
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The HP value displayed will be different. I've heard lots of issues getting accurate numbers with offroad rigs....everything affects it. Big tires is a pretty major thing but you can even get variations in the numbers with changes in tire PSI, different rubber compounds even with tires of the same size, a couple extra clicks out of the hold-down straps, you name it. It all plays a role in the numbers. On an offroad rig a chassis dyno is more of a tuning tool than a tool for gauging actual horsepower since there are so many variables. Torque is a reading and HP is a calculation, and all those variables come into play on a chassis dyno.
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