Big rotors questions >
ROFTLMAO but true!
12-15-2004, 01:56 PM
#22
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on measured wheel hp.
Rotors won't change the output of the engine obviously. But what a dyno measures may change slightly due to the change in inertia of the wheel/brake system.
12-15-2004, 02:17 PM
#24
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12 HP measured difference is very unreasonable...(long, with math and physics)
(edit: ACK! Edited to correct and simplify)
the actual amount could be calculated with enough info.
Here's a quick and dirty look at what 12 HP would mean.
The kinetic energy of a rotating object is 1/2mr^2w^2, where m is the mass, r is the radius and w is the angular velocity. If we compare the amount of kinetic energy at any given speed for two brake disc, the only difference being one disc moves the effective mass out 1", we can get an idea of the difference.
From the formula, everything stays the same except the r term, which is squared. So if we compare a disk with a 7" radius with one with an 8" radius, the difference in energy is (64/49)-1 = 30%. If at full acceleration that difference is the claimed 12 HP, then 12 x 100/30 = 40 HP is going just to turn the brake discs up to speed. Wheels and tires easily weight double a disc, and are bigger, so well over 80 HP must be used to bring those up to speed. If we're talking about a TT225, the remaining 105 HP must be going to spin the rest of the rotating parts (engine, driveshafts, etc.) AND accelerate 3000+ lbs linearly. NOT.
the actual amount could be calculated with enough info.
Here's a quick and dirty look at what 12 HP would mean.
The kinetic energy of a rotating object is 1/2mr^2w^2, where m is the mass, r is the radius and w is the angular velocity. If we compare the amount of kinetic energy at any given speed for two brake disc, the only difference being one disc moves the effective mass out 1", we can get an idea of the difference.
From the formula, everything stays the same except the r term, which is squared. So if we compare a disk with a 7" radius with one with an 8" radius, the difference in energy is (64/49)-1 = 30%. If at full acceleration that difference is the claimed 12 HP, then 12 x 100/30 = 40 HP is going just to turn the brake discs up to speed. Wheels and tires easily weight double a disc, and are bigger, so well over 80 HP must be used to bring those up to speed. If we're talking about a TT225, the remaining 105 HP must be going to spin the rest of the rotating parts (engine, driveshafts, etc.) AND accelerate 3000+ lbs linearly. NOT.
12-15-2004, 02:32 PM
#25
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Right
Article was provided only to provide more info on dyno's and tuning. Not to support any arguments on rotational inertia affecting HP.
Also to note that a non roller dyno like those used in the article could be used to eliminate any wheel/brake rotor inertia arguments, since the wheels (and brakes if you want) can be completely removed for the dyno.
Also to note that a non roller dyno like those used in the article could be used to eliminate any wheel/brake rotor inertia arguments, since the wheels (and brakes if you want) can be completely removed for the dyno.
12-15-2004, 02:42 PM
#26
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Good effort! >
Thanks much. I'm an architect, so I do have physics background enough to grok, and appreciate your taking the time to go through the basic math. Too, as I mentioned in a thread above, the delta between a stock (something) rotor and a 'big' rotor is still a relatively small number, an inch at most.
Again, gracias.
Again, gracias.
12-15-2004, 05:02 PM
#29
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Let me Clarify>>> Everything you read is not FACT>>> Dyno Results Do Not =
how the car behaves in on the road... this is a Suspension / road performance issue - NOT a drag race calculation - Engine / Torque / HP / Gearing / Suspension / Unsprung weight PLUS DRIVER SKILL is what makes cars to FAST - don't fret - do what's right for Your Style and Goals !