Wheel weight effect on acceleration
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Originally Posted by Hormazd' date='Dec 6 2004, 05:46 PM
Wolverine:
I think your exercise has answered its own questions.? It looks to me like we have two issues here:
1) Wheel and tire weight.
2) Wheel Diameter.
The weight should make a very small difference,? probaly not noticeable at all.? 40lbs is nothing.? Like having a suitcase in the car.
Now the diameter... the extra inch is giving you an extra 3 inches in circumferance. I would think 3 inches on each revolution of the wheel would make a big difference. This would not only get you going faster,? but might also provide inaccurate speedometer readings.? Does this make sense?? I am not the engineer!
Hormazd
I think your exercise has answered its own questions.? It looks to me like we have two issues here:
1) Wheel and tire weight.
2) Wheel Diameter.
The weight should make a very small difference,? probaly not noticeable at all.? 40lbs is nothing.? Like having a suitcase in the car.
Now the diameter... the extra inch is giving you an extra 3 inches in circumferance. I would think 3 inches on each revolution of the wheel would make a big difference. This would not only get you going faster,? but might also provide inaccurate speedometer readings.? Does this make sense?? I am not the engineer!
Hormazd
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What I'm finding out is wheel/tire weight has two contributions to acceleration. First, you use HP to accellerate the rotational speed of the wheel. Imagine the car is on a lift, it takes a certain amount of HP to accelerate all four wheels at a given rate even though you're doing nothing to move the car. Then you also have the actual weight of the wheels, that you carry as if they were a suitcase just riding along with the car. The two combined make wheel weight pretty important in acceleration. If you ask the folks on the racing forums, the general consensus is that 1 lb weight saved on the wheels equals 3-5 lbs saved on the car.
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Originally Posted by wolverine' date='Dec 6 2004, 08:56 PM
The diameters of the two setups are virtually identical.? The OEM 18 inch is 25.579 in, and the BBS 19 inch is 25.496 inches, a difference of less than .3 percent.? I'm using 245/35 19's vs 245/40 18's in front and 275/30 19's vs 275/35 18's in the rear.? So gearing is virtually identical.
It is pretty simple to understand if you consider that a merry go round accelerates slower if you add extra weight to its outer edge.
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Rotational inertia is the issue where the two wheel/tires in question have different net mass and different weight distribution radially. The wheel/tire with all the mass in the center will accelerate and brake better than the same diameter wheel/tire with the same mass but placed on the outer radius. Now, add the difference in mass and the placement of that mass, the two setups can differ considerably. The change in rotation inertia goes as the 4th power of radius. That is why, in general, larger wheels place more mass to the outer radius and raise inertia considerably even if no overall mass was changed. This applies to brake rotors too. Bigger rotors may stop better but also require more horsepower to get them up to speed.
One can calculate the difference in rotational inertia but requires a map of the wheel/tire cross section to allow an integration of mass with radius. I haven't seen anyone do that except the designers with finite element algorithms.
I wonder who reads these messages to the end?
One can calculate the difference in rotational inertia but requires a map of the wheel/tire cross section to allow an integration of mass with radius. I haven't seen anyone do that except the designers with finite element algorithms.
I wonder who reads these messages to the end?
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Well, I actually did all the calculations, and I won't bore you with them. Of course, I used some simplifying assumptions - wheel weight distributed 2/3 on the rim, constant acceleration over a 1/4 mile, 1/4 mile of 13.8 at 103mph.
I used actual wheel and tire weights for the OEM runflats and my BBS setup. The BBS setup is 54 lbs lighter than the OEM. According to the calculations, this equates to a 140 lb weight reduction, or if you put it another way, about about a 14 horsepower increase. I'm suprised at how much it affects the performance.
I used actual wheel and tire weights for the OEM runflats and my BBS setup. The BBS setup is 54 lbs lighter than the OEM. According to the calculations, this equates to a 140 lb weight reduction, or if you put it another way, about about a 14 horsepower increase. I'm suprised at how much it affects the performance.
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Wonderful wolverine!
Can you give a little more details on the calculations and assumptions - especially about the wheel structure?
I won't be bored... promise!
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Originally Posted by sg530' date='Dec 9 2004, 01:01 AM
Wheel weight affects acceleration not gas mileage...unless you litteraly always are accelerating and braking and never cruising. ![Roll Eyes (Sarcastic)](https://5series.net/forums/images/smilies/imported/rolleyes.gif)
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