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Discussion Starter #1
I just calculated the moment of inertia for a 17" vs 18" stock rim (with tires) and I am very surprised at the results. Basically, to accelerate the 17" rim (enkei rpf1 17x8), it takes about 60% of the torque it takes to accelerate our stock rims.
This is a huge upgrade! I am definitely going to be buying these rims asap. At 15.85 lbs each, vs our stock rims weight of 25 lbs each, these would be the equivalent of removing about 150-200 lbs at constant speed, and upwards of 400 lbs during acceleration and deceleration. Absolutely worth every. Single. Cent.

The only downside is the inability to upgrade to larger brakes, but, thus far, i have found that larger diameter brakes would actually not be even close to as beneficial as these wheel upgrades would be. (Especially when reducing rim size to 17")

Tell me what you all think and whether I just found some sites that gave random numbers to me for the weight equivalencies lol

377861
 

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Don't you think there might be a mistake in your calculation?
I might as well be wrong with my assumption, but the inertia of the tire has to be larger with a smaller rim and the (assumed to be) same outer diameter. Because a 235/45 R17 needs more rubber than a 235/40 R18 and thus, more inertia - or am I thinking wrong?
 

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I don't know about the numbers but there was a huge diff in the way my car drove and accelerated when I put on 18" RPF1's.
 

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Discussion Starter #4
Don't you think there might be a mistake in your calculation?
I might as well be wrong with my assumption, but the inertia of the tire has to be larger with a smaller rim and the (assumed to be) same outer diameter. Because a 235/45 R17 needs more rubber than a 235/40 R18 and thus, more inertia - or am I thinking wrong?
Well yeah if I used the same outer diameter tires. I'm planning on reducing the weight by a large amount though, not adding it. So I will be going with either a 235/40 17, 245/40 17, or 245/35 17. Depends on if I can find the correct tire that I want.

These calculations were done with a 245/40 17 michelin pilot sport 4S and a 235/40 18 michelin pilot sport 4s
 

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Discussion Starter #7
Hey guys I'm not sure how but I did the math wrong lol. The 17" end up with a moment of inertia of 0.95514 kg.m2. Still a huge decrease, but not anywhere near 40% dectease lmfao

I'll still be doing the upgrade asap.

Can we use forscan how to adjust 4 tire size. If not your Speedo is going to be way off.
Yeah I'll need to get my speedo adjusted for sure. I'll be getting my tires done and having a shop fix the speedo at the same time, probably the same shop for both.

Theres also all kinds of useful calculators here: Fitment Industries
 

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Hey guys I'm not sure how but I did the math wrong lol. The 17" end up with a moment of inertia of 0.95514 kg.m2. Still a huge decrease, but not anywhere near 40% dectease lmfao

I'll still be doing the upgrade asap.



Yeah I'll need to get my speedo adjusted for sure. I'll be getting my tires done and having a shop fix the speedo at the same time, probably the same shop for both.

Theres also all kinds of useful calculators here: Fitment Industries
I was told removing one pound of rotating Mass was like removing 20 lb of sprung weight from the vehicle
 

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Hey guys I'm not sure how but I did the math wrong lol. The 17" end up with a moment of inertia of 0.95514 kg.m2. Still a huge decrease, but not anywhere near 40% dectease lmfao
Now that sounds somewhat more realistic.

I was told removing one pound of rotating Mass was like removing 20 lb of sprung weight from the vehicle
I was told it's times 7. So 1 pound rotating mass = 7 lb sprung weight. But yeah - there are many different numbers out there on this specific matter.
 

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Now that sounds somewhat more realistic.



I was told it's times 7. So 1 pound rotating mass = 7 lb sprung weight. But yeah - there are many different numbers out there on this specific matter.
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Either way my wheel and tire combo took 30 lbs off of the wheels that's pretty significant.
 

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Yes lighter wheels, help a lot in acceleration as well as a more responsive suspension. Its a great mod and easy to achieve for anyone.

as does a lighter flywheel, lighter brakes (AP Racing) lighter underdrive pulleys and more.

when my ACT HD clutch wears out I’ll ditch the factory flywheel most likely for a single mass flywheel.

while i love my WEDS wheels that are nice and light, you forget about the gains in about 2 days.
 

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Discussion Starter #12
I was told removing one pound of rotating Mass was like removing 20 lb of sprung weight from the vehicle
I've heard so many different numbers, anywhere from 4 up to 20. However, I've never found an actual reliable source for the info lol

I've also heard unsprung weight is worth 3x static weight and that every 10 lbs lost is ~1 hp.... never from a reliable source and there is some truth to it and some untruth to it.

Under acceleration and deceleration, the difference in mass will affect everything. However, at constant speed itll be approximately the same as static weight. It also has weird effects when gravity isnt directed as a normal force to the ground (up and down hill), or so I've heard lol

I have found very little information on any of these things and if someone could post a link or something that would be awesome!
 
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