performance loss with bigger rim?

so i searched around and found some weights of other rims in comparison with other rims..nothing that i was looking for though

i currently have factory 5 spoke 16x7 alloy ranger rims with 245/75r16
and im looking to go to
replica 20x8.5 cobra R rims with 265/50r20, im not that worried about the tires, tire rack had those size tires weighing plus or minus 2 lbs of the ones i have now..

im worried about the rims, is that gonna be a killer? and how much performance should i expect to suffer?

 

[lifted4.0]

i tried to go in and edit the thread, but somehow i ended up creating 2, if someone could delete the other please?

 

[kert0307]

Weight isn't the as much of an issue with the larger rims, it more about the increased moment of inertia with the larger wheel.

 

[lifted4.0]

im trying to make sense of that post?

are you saying bigger overall tires (going from 31 to 35 inch tall tires) is what will kill the power? versus the weight of the 31 inch tall tire?

i emailed the place that im gonna get the rims from..and they said the rims are approximately 30lbs

 

[Kriegen]

Where'd you come up with 35" for your tires?
245/75r16 = 30.4685"
265/50r20 = 30.4330"

 

[little x]

What he was saying about the moment of inertia is that as the weight distribution is moved farther from the center of rotation, it takes more energy to turn it at a given speed. Larger wheels will move the weight further from the hub, and even if the larger wheels weighed the exact same as the smaller ones, it would still require more energy to turn them. Watch this video for a great demonstration of the effects:
http://www.youtube.com/watch?v=AaCLbJqT_k8

 

[lifted4.0]

darn physics...i got 35 inches, from the post above mine, i couldnt understand it, so i thought he was talking about something about a larger tire or something.

so even if the 2 combos (16inch rim with tire, and 20 inch rim with tire) even if they weigh the same amount, the 20 inch is gonna require more force to turn because it pushes the weight distribution out from the center more..?

how much are we talking though? is there a way of finding that out?

 

[ahodges]

a larger diameter tire will decrease your performance hands down.. I know from experience i only went 1.5 inches larger diameter and have a noticable performance loss.. if you have the 4.0 OHV you will definately notice it, the 4.0 SOHC has more HP and the difference wont be as bad, depending on how much bigger than stock you're talking.

 

[lifted4.0]

im not trying to go with a bigger tire, im staying with a 31 inch tall tire (or ~30.4 inches) im wondering about going bigger in the rim department

i have 281rwhp/314wtq to work with, its not stock, but i dont wanna lose like 20percent of that just cause i have a 20 inch rim on there

 

[ahodges]

oh.. you'll be fine and probably won't notice much of any loss.. rim size itself doesn't matter, just the over all diameter of the tire

 

[little x]

With that much power I wouldn't worry about it. There will be a difference, despite what ahodges said (he obviously didn't rear his physics homework) but it won't be massive. If your taking it to the track, your times will slow down, but for street use you'll still have plenty of umph.

 

[lifted4.0]

Okk ya I don't really plan on taking it to the track. Maybe once who knows.

 

[ahodges]

With that much power I wouldn't worry about it. There will be a difference, despite what ahodges said (he obviously didn't rear his physics homework) but it won't be massive. If your taking it to the track, your times will slow down, but for street use you'll still have plenty of umph.

First off what does "rear my physics homework" mean?

Secondly.. so i was simplifying for a car stand point and the diffierence will be sooo small that it's pointless discussing it here..

I am a third year engineering student and have had plenty of physics to understand the concepts behind this theory. The rotational inertia difference between the two different sized rims is minuscule. Just to clear that up.

 

[Gadget X]

Maybe I'm missing something here, but if the overall outer diameter of his new wheel/tire combo is smaller ( I know, only by 0.03 inches, but still) then he will actually see a miniscule INCREASE in his power to the ground.

 

[ahodges]

Thank you MayJan.

 

[Elessar65]

just remember that since the moment of inertia is greater, not only will it take more force to get going, but also to stop. Easiest wat to understand rotational inertia, or moment of inertia (same thing) is to think of baseball. Like choking up on a bat.

 

[ahodges]

"choking up on a bat" is equivalent to changing the tire diameter

 

[Gadget X]

just remember that since the moment of inertia is greater, not only will it take more force to get going, but also to stop. Easiest wat to understand rotational inertia, or moment of inertia (same thing) is to think of baseball. Like choking up on a bat.

Ok, this is where I am confused. Why will there be a greater moment of inertia if the outer diameter is actually smaller with his new tire/wheel combo? Wouldn't the moment of inertia be less at that point?

 

[ahodges]

I know what little x is saying because the mass of the rims would be focused farther from the center of the rim but it wouldn't really make a difference in performance, especially if the tire diameter is slightly less or the same.

 

[Elessar65]

"choking up on a bat" is equivalent to changing the tire diameter

I just said it would be an easy way to explain moment of inertia, not his particular case.

Ok, this is where I am confused. Why will there be a greater moment of inertia if the outer diameter is actually smaller with his new tire/wheel combo? Wouldn't the moment of inertia be less at that point?

Not necessarily, because the wheel is larger, the weight of the tire (we'll forget about the weight of the wheel for simplicity's sake here) is further from the center of rotation, or axis. There probably won't be much of a difference at all actually, because the overall diameter is smaller, and the tire will weigh less cause its thinner. Might offset the larger diameter wheel completely. You'll have to ask the engineering student to be sure. I'm sure he'd love to do the calculations for you.

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