tire size versus acceleration
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tire size versus acceleration
it's easy to find speedometer differences between tire sizes. how about trying to get that extra diameter and mass moving? i'm trying to decide if moving from a 23575r15 up to a 265 (31) will negate all the torque advantages gained with my present mods (i spend a lot of time pulling the pulling the passes in colorado during the winter).
probably be purely subjective, but i'd appreciate what you've learned.
probably be purely subjective, but i'd appreciate what you've learned.
#5
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Originally Posted by javadoody
it's easy to find speedometer differences between tire sizes. how about trying to get that extra diameter and mass moving? i'm trying to decide if moving from a 23575r15 up to a 265 (31) will negate all the torque advantages gained with my present mods (i spend a lot of time pulling the pulling the passes in colorado during the winter).
probably be purely subjective, but i'd appreciate what you've learned.
probably be purely subjective, but i'd appreciate what you've learned.
I'm trying to think of a way to calculate this without actually having to different wheels set up but am not doing a very well.
Here's one thing, engine mods almost aways increase torque higher up the rpm band. Larger tires make the engine run at lower rpms. As a result, the mods are usually ineffective in compensating for larger tires. This is the reason we regear as opposed to build up engines when we go up in tire size.
Frank
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Originally Posted by elripster
This is the reason we regear as opposed to build up engines when we go up in tire size.
Frank
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I moved from a 235/75R15 (or was it /65?) to a 31 and it did make it a bit slower, but nothing really major. Kinda hard to keep it at 75 though, but I think that is more due to the 3.0 than the tires. I was expecting it to be worse than it really was. I'm running a 3.90 rear for now too. What's your gear ration now?
Have fun all.
Daniel
Have fun all.
Daniel
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Originally Posted by DH6twinotter
I moved from a 235/75R15 (or was it /65?) to a 31 and it did make it a bit slower, but nothing really major. Kinda hard to keep it at 75 though, but I think that is more due to the 3.0 than the tires. I was expecting it to be worse than it really was. I'm running a 3.90 rear for now too. What's your gear ration now?
Have fun all.
Daniel
Have fun all.
Daniel
#10
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Originally Posted by elripster
I'm trying to think of a way to calculate this without actually having to different wheels set up but am not doing a very well.
For Thick Cylindrical Shell:
So it can be seen that increasing the outer diameter of the "tire" will have a squared effect on the moment of inertia. So changing a tire's outer diameter by 6% (31" to 33" for example) will change the tire's moment of inertia by 10% (and this doesn't take into account the added weight.)
Then, say the bigger tire weighs 6% more (could be more or less, depends on the brand of course...) that means the moment of inertia would change by 17%!
This is a big deal when it comes to acceleration and braking, not to mention the brakes have to work harder because of the larger overall diameter.
Granted this approximates the tire as a solid when it is not, but the approximation can give you an idea of what's going on.
Ok, lecture's over
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Originally Posted by elripster
This is the reason we regear as opposed to build up engines when we go up in tire size.
Frank
Frank
Not to mention we can regear 35% for $600. Try to find 100 more horsepower for that....
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Originally Posted by mastacox
For a quick idea of the nature of the problem, just look at the mass moment of inertia for a thick cylindrical shell (approximation of the tire) and cylidrical disc (wheel, assume doesn't change)-
For Thick Cylindrical Shell:
So it can be seen that increasing the outer diameter of the "tire" will have a squared effect on the moment of inertia. So changing a tire's outer diameter by 6% (31" to 33" for example) will change the tire's moment of inertia by 10% (and this doesn't take into account the added weight.)
Then, say the bigger tire weighs 6% more (could be more or less, depends on the brand of course...) that means the moment of inertia would change by 17%!
This is a big deal when it comes to acceleration and braking, not to mention the brakes have to work harder because of the larger overall diameter.
Granted this approximates the tire as a solid when it is not, but the approximation can give you an idea of what's going on.
Ok, lecture's over
For Thick Cylindrical Shell:
So it can be seen that increasing the outer diameter of the "tire" will have a squared effect on the moment of inertia. So changing a tire's outer diameter by 6% (31" to 33" for example) will change the tire's moment of inertia by 10% (and this doesn't take into account the added weight.)
Then, say the bigger tire weighs 6% more (could be more or less, depends on the brand of course...) that means the moment of inertia would change by 17%!
This is a big deal when it comes to acceleration and braking, not to mention the brakes have to work harder because of the larger overall diameter.
Granted this approximates the tire as a solid when it is not, but the approximation can give you an idea of what's going on.
Ok, lecture's over
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IN simplest terms - changing the tire diameter changes the torque multiplication directly, so a 15% increase in tire diameter will result in a 15% loss of power.
BUT, if you change the final gearing far enough to drop down the engine torque curve to an rpm where less horsepower and torque are available, that simple 15% might actually net a much bigger loss. You have to re-figure the entire drivetrain math again to find the real effect.
Here's one of the neatest dissertations I've seen yet:
http://www.mustangsandmore.com/ubb/D...orqueVsHP.html
BUT, if you change the final gearing far enough to drop down the engine torque curve to an rpm where less horsepower and torque are available, that simple 15% might actually net a much bigger loss. You have to re-figure the entire drivetrain math again to find the real effect.
Here's one of the neatest dissertations I've seen yet:
http://www.mustangsandmore.com/ubb/D...orqueVsHP.html
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Originally Posted by Flamedx4
IN simplest terms - changing the tire diameter changes the torque multiplication directly, so a 15% increase in tire diameter will result in a 15% loss of power.
https://www.yotatech.com/forums/f116/few-questions-about-my-90-4wd-3-0l-69025/
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Originally Posted by mike_d
just being nit-picky, but i think it's important: 15% increase in tire diameter will result in 15% less force applied to the road, but the power output will stay the same. see the proof in this thread:
https://www.yotatech.com/forums/showthread.php?t=69025
https://www.yotatech.com/forums/showthread.php?t=69025
"you will not necessarily reach a lower top speed. You will be in a different torque requrement with different tires at speed. I lost 20% in mpg going from stock to 31's on an '84 pu. You won't be able to redline in high gear in either case."
now THERE'S another issue!
#18
285s are nothing to turn...especially on a 3.4L. If you want a race car street mobile, then stay w/ a street stock tire size. I turn 37s on my 4banger 5 speed and run up and down the interstate 80 mph+ w/ NO problem. It is all about your gears to maintain the proper relative speedo reading...mine is off by 5 mph...which ain't bad considering I'm like 4.2 million sizes over the stock tire size.
Last edited by waskillywabbit; 11-04-2005 at 04:57 PM.
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Originally Posted by mike_d
just being nit-picky, but i think it's important: 15% increase in tire diameter will result in 15% less force applied to the road, but the power output will stay the same. see the proof in this thread:
https://www.yotatech.com/forums/showthread.php?t=69025
https://www.yotatech.com/forums/showthread.php?t=69025
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