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?

 

[ahodges]

correct

 

[KPSquared]

I don't know much about the physics of the whole thing, but my friend who owns a body shop says that all the guys rollin on dubs have been great for his business. Even without an increase in rubber size, the larger rims make it WAY harder to stop. He said he's had more SUV's on 20's, 22's etc in than he's ever seen that have rear ended people cause they couldn't stop.

Best I can figure is, if it's harder to stop, it's probably harder to go...

Upgrade your breaks while your at it!

 

[techboj]

The effect of rotating mass can be calculated using Moment of Inertia (MOI). MoI is related to not only the mass of the rotating object, but the distribution of that mass around the rotational center. The further from the center, the higher the MoI. The higher the MoI, the more torque required to accelerate the object. The higher the acceleration, the higher the torque required.

A bigger tire and a smaller wheel will have a different MoI than a smaller tire and bigger wheel. The distribution of weight is different in relation to the rotational center - regardless if the total final diameter of either combination is the same.

Tire inches is generally lighter than wheel inches (to a certain degree). So moving up from a 16 to a 20 will actually result in a heavier wheel. This change alone is enough to alter the MoI since now you have a heavier rotational mass towards the center.

This is why the general rule of thumb is to increase or improve braking when you change to bigger wheels even if you keep the overall final size the same. In some applications the combination of the new tire and wheel may have minimal impact on rotational inertia but it's always better to err on the safe side

 

[Gadget X]

Ok, thank you. That's what I was confused about. The increased weight of the larger wheel will make it harder to start the rotational process that is acceleration! I was thinking that the issue was similar to the "loss of power" (so to speak) that is incurred when one goes from, say, a 30" O.D. tire to a 35" O.D. tire without changing the gear ratios at the same time. That, I understand! I wasn't aware of the similar forces involved in just increasing the wheel/tire ratio without increasing the overall O.D.

 

[lifted4.0]

yaaa soo i just read up on all of this thread, i didnt know this was gonna turn into a huge science lesson, ahah but its all good...my brakes will probably need to be replaced soon anyway..soo..probably get that done..

soo whats the overall consensus? these 20s gonna kill me? should i still be able to tear through first/second gear?

haha

 

[Gadget X]

My $.02 is that you won't notice any real difference at all, other than the obvious lack of shock absorption! Oh, and you will have more precise handling than with the stock size.

 

[Elessar65]

If your motor is putting out the numbers you said before then you'll be fine. It will ride harsher though, and definitely upgrade the brakes.

 

[techboj]

Your original sidewall is approximately 7.2 inches (245/75-16)
The new proposed sidewall is approximately 5.2 inches (265/50-20)

The loss of sidewall represents not only the missing 2 inches of inflated rubber but also a noticeable stiffness in the tire. To make up for the size decrease, the smaller tire will have harder and stiffer sidewalls + a significant decrease in load carrying capacity. Most SUV tires in this size range also require a higher tire pressure (some as high as 51psi) this would also affect the ride equation

As mentioned by other people - this will translate to a harder / harsher ride

 

[little x]

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.

Hey dude, not trying to start a flame war with you. My comment was based on the fact that even though Moment of Inertia had already been covered in the thread, your post said that it didn't matter and wouldn't have any effect, which isn't true. BUT I do agree with you that with that much HP he will hardly notice the difference, unless he takes his truck to a drag strip and actually times it.

 

[turboexplorer]

Interesting, learned a lot reading this thread. And i agree though you wont see much difference, the most of which will be in ride quality!