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Old 05-11-2004, 04:02 PM   #1
1DOWNCLOWN
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Default the effects of larger wheels...

maybe this should be in the wheels and tire forum, sorry if in the wrong place. Ok i know we have discussed that larger OD wheels change the rotational mass of the wheel, and this effects the speed in which the wheels spins and the effects the power need to spin that wheel. My question is going from a 16" wheel to an 18" wheel will effect someone's 1/4 time by how much? I know your answers will be aprox. but are we talking tenths of a second, or more like a half of a second? Thanks in advance

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Old 05-11-2004, 04:38 PM   #2
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if you do it right, going from a 16 to an 18 won't change your overall wheel/tire diameter. I put my 18s on, and they almost exactly the same height as the stockers.
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Old 05-11-2004, 04:52 PM   #3
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The question was about lost speed however and the difference is much more than you would like to believe. There is a formula out there hopefully someone may post about X amount of weight in the car is equal to X amount on the wheel/tire assembly. The point is I know we all think large wheels look great but if you care about speed and efficiency (you will drop MPG as well) you will not put 18 wheels on the car unless they are very light (read: very expensive and possibly fragile). I cannot tell you exact numbers in the 1/4 but lets just say I would LOVE to see my competition slap on some 18s before a race.
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Old 05-11-2004, 06:05 PM   #4
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The difference should not be that huge in daily driving situations. But if you are talking about a tenth of a second, then it will matter.

There is no way to give you an apprx number because there are so many factors involved. But roughly speaking 1 lb wheel/tire mass counts as 1.5 ~ 2.0 lb chassis mass, so think of the total weight difference between the old and new tire/wheel combo and use that factor. Divide that by the total weight of your car and you'll have some idea how much difference it will make.

Search the tire/wheel forum with "rotational inertia" and you will find a few very good thread. Most of them will have some boring physics/math to it, but unless you understand what's really going on you'll always be quoting someone else's misguided guestimations.

Good luck.
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Old 05-11-2004, 06:18 PM   #5
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your 1/4 miles times wont be affected by much. even though the difference will probably be negligible, you will feel a difference in weight... or some people are really sensitive to that. i know i am. when i went from my stock 16s to some 17" sti wheels with stock sti tires, i definitely noticed the sti wheels being heavier, but only at low end. its quite annoying. the engine bogs down easier and takes a little more to get it going. but when you get it up to speed its fine, which is why your 1/4 mile time wont hurt much. i now have Toyo T1-S tires on and the weight went back down pretty much to stock, at least it feels that way. one thing you do have to worry about with heavier rims is that your brakes will wear out much faster. trust me on this one.
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Old 05-11-2004, 06:27 PM   #6
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a lot of this depends on the weight of the wheels you put on... some people go from the 16.5lb stockers to 22 or 23 lb 18's.. this can make a few tenths difference in the 1/4 and will make your handling feel a little more sluggish at low speeds.
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Old 05-12-2004, 12:11 AM   #7
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Quote:
Originally posted by MustGoFast
some people go from the 16.5lb stockers to 22 or 23 lb 18's.. this can make a few tenths difference in the 1/4 and will make your handling feel a little more sluggish at low speeds.
Gross understatement of the month, here...
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Old 05-12-2004, 02:03 AM   #8
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same weight and larger diameter can make a significant difference. On one of the tuner shows they installed some upgraded disk brakes and lost 13 AWHP even though the weight was the same. The new disks were larger diameter and that increased the rotational inertia enough to cost them some HP under acceleration.

At our typical power to weight ratio it costs about 8 - 10 hp to gain 1 mph in the 1/4 mile, so larger wheels can easily cost you you 1 - 2 mph, and maybe a tenth at the drag strip.


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Old 05-12-2004, 07:18 AM   #9
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Quote:
Originally posted by hotrod
On one of the tuner shows they installed some upgraded disk brakes and lost 13 AWHP even though the weight was the same. The new disks were larger diameter and that increased the rotational inertia enough to cost them some HP under acceleration.

Larry
Wouldn't some of this loss be due to the increased drag of the bigger disks?
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Old 05-12-2004, 08:04 AM   #10
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thanks for the info guys....
I ran at the track last night and was getting half way consistant times, so maybe next week i will put the stock wheels and tires back on and run it again to see the diffrence. I feel that my car handles 10 times better with the aftermarket wheels. If im only going to loose 1 tenth of a second then that i can live with but if its more like a half second or something major like that then i will probably sale them.. Kind of sucks because i just got them less than a month ago
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Old 05-12-2004, 11:00 AM   #11
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Quote:
Originally posted by hotrod
same weight and larger diameter can make a significant difference. On one of the tuner shows they installed some upgraded disk brakes and lost 13 AWHP even though the weight was the same. The new disks were larger diameter and that increased the rotational inertia enough to cost them some HP under acceleration.

I've heard of that story before. But with all due respect, that is totally bogus. Those guys might know their cars, but they certainly don't know physics. There is no frigging way that upgraded disk can make anywhere near that much difference in power. There is nothing mystical about rotational mass.

If I remember correctly, what happened was they did powertrain modification where they expected horsepwer gain, but in the end they actually lost some ponies. So they blaimed the new disk. But I think they screwed up something else and just came up with some voodoo physics to justify that. I wish 13 WHP was that easy.
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Old 05-12-2004, 12:02 PM   #12
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i've seen some 17" wheels on tirerack that are in the 13-14 lb range for about $200. I don't know if these wheels are very good, but you CAN go up to a 17" wheel without putting on extra weight...and actually losing some. The other thing to consider is this: although you may not be changing the MASS of your rotating wheel by much, you ARE changing the DISTRIBUTION of that mass. moving it farther away from the hub makes it harder to spin. that's why I don't think lager discs can make ou lose that much power....unless they're made of lead or something....they're so close to the hub they matter less than wheels that are farther from the hub.

I think the wheels I saw were Kosei K1 TS

any good?
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Old 05-12-2004, 12:12 PM   #13
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The physics of what Larry said is very much true. Now, I dont know about a whole 13 whp loss but, it is very correct. If you get a pipe that is 6 inches long and weighs 10 pounds you will have a pipe with a center of mass at 3 inches. That will give you a torque at the junction of the pipe and the holder of 30 lb/in. Now, if you go get a piping that is 60 inches long but still weighs that some 10 pound it will have a center of mass of 30 inches and have a torque at the junction of the pipe and the holder of 300 lb/in. That would be with both pipes held parallel to the ground. So, as you can see... you if you have the same weight but and larger diameter you still have to counteract the extra leverage that moving the center of mass out a few inches would done. That is compounded when you try to get that mass to begin spinning very quickly.

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Old 05-12-2004, 01:19 PM   #14
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Quote:
Originally posted by eightballrj
The physics of what Larry said is very much true. Now, I dont know about a whole 13 whp loss but, it is very correct. If you get a pipe that is 6 inches long and weighs 10 pounds you will have a pipe with a center of mass at 3 inches. That will give you a torque at the junction of the pipe and the holder of 30 lb/in. Now, if you go get a piping that is 60 inches long but still weighs that some 10 pound it will have a center of mass of 30 inches and have a torque at the junction of the pipe and the holder of 300 lb/in. That would be with both pipes held parallel to the ground. So, as you can see... you if you have the same weight but and larger diameter you still have to counteract the extra leverage that moving the center of mass out a few inches would done. That is compounded when you try to get that mass to begin spinning very quickly.

Richard
First of all I don't get your method of calculating the torque. If you apply 10 pounds of force at the end of the pipe, then yes, you can multiply the length by the force to get the torque. But if I understand correctly that doesn't seem to be the case, unless the mass of the pipe is 100% concentrated at the tips.

Important thing is the rotational inertia. You can talk about the torque or mass distribution or anything related to the rotation, but they are all incorporated into the concept of inertia. (as you may already know) If you use the standard formula, you will find that a "thin ring" behaves like it has the twice the actual mass, while a "uniform disk" behaves like it has 1.5 times the actual mass. The real wheels/tires have to fall inbetween these two extremes. This means if you add 1 lb rotational mass it will be like 1.5 ~ 2 lbs chassis mass.

I said the break disk thing is bogus not because the fundamental physics behind it is wrong, but because the actual values are totally ridiculous. If you were to drop 13 WHP on a stock WRX, the new disks have to be roughly 40 lbs heavier than the old ones PER CORNER. Even if the new ones are a bit larger, the number will still be around 30 lbs. If they "upgraded" the disk to ones that weigh 30 lbs more EACH, then they are total idiots. Plus as I mentioned it was as far as you can get from a properly controlled experiment.

The bottom line is that heavier wheels will affect the performance, but often times the effect is exaggerated by wheel manufacturers and owners having placebo effect. I trust people saying how much difference lighter wheels made on track, but I don't trust people saying their butt can tell 3 lbs per wheel difference. That's less than 1% of the total effective mass of the car.
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Old 05-12-2004, 01:54 PM   #15
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Quote:
Originally posted by Len
...
The bottom line is that heavier wheels will affect the performance, but often times the effect is exaggerated by wheel manufacturers and owners having placebo effect. I trust people saying how much difference lighter wheels made on track, but I don't trust people saying their butt can tell 3 lbs per wheel difference. That's less than 1% of the total effective mass of the car.
I have observed just the opposite from wheel advertising. It seems wheel manufacturers and sellers don't want to mention the effect weight and diameter will have on performance. Look how reluctant they usually are to post wheel weights on every wheel. They don't want the customer to even think of that, because most wheels on the market are heavy. Because most wheels are made for the mass-market at affordable prices, they use low-pressure casting which is cheaper to produce, but weaker and heavy. The huge-diameter chrome wheels are very stylish, but they are also very heavy, and have massive inertia from their large diameter.

The wheels that are advertised as light, are the ones that are made to appeal to the performance and racing market, because these are the markets that know that the weight has a significant effect on performance, and value a light weight wheel for that reason.

Check any serious competetive autocrosser, track racer, or drag-racer. They will not be running 19 inch chrome wheels if they are trying to win. The performance effect is real, and the racers and engineers know this.
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Old 05-12-2004, 02:48 PM   #16
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ok so you all have good points.... but what about the effects in tenths of senconds that a 2 inch larger wheels would have? I believe that my rims and tires are now lighter than stock. The rims are in fact larger though. Are we talking one tenth of a second diffrence or many more than that???
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Old 05-12-2004, 04:58 PM   #17
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Quote:
Originally posted by blarg
i've seen some 17" wheels on tirerack that are in the 13-14 lb range for about $200. I don't know if these wheels are very good, but you CAN go up to a 17" wheel without putting on extra weight...and actually losing some. The other thing to consider is this: although you may not be changing the MASS of your rotating wheel by much, you ARE changing the DISTRIBUTION of that mass. moving it farther away from the hub makes it harder to spin. that's why I don't think lager discs can make ou lose that much power....unless they're made of lead or something....they're so close to the hub they matter less than wheels that are farther from the hub.

I think the wheels I saw were Kosei K1 TS

any good?


I saw this show. It was a acura RSX with mugen parts. It wasnt BS.

Edit: though the car didnt lose AWHP because its a FWD
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Old 05-12-2004, 11:28 PM   #18
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Quote:
Originally posted by 1DOWNCLOWN
ok so you all have good points.... but what about the effects in tenths of senconds that a 2 inch larger wheels would have? I believe that my rims and tires are now lighter than stock. The rims are in fact larger though. Are we talking one tenth of a second diffrence or many more than that???
You would need a VERY consistent racer to drive the car back to back with both sets and adequete time for cool down etc etc to get hard numbers. It depends on the exact weight of the new combo vs the old. Please see the section called "basics of unsprung weight" in this link.

http://www.nissanperformancemag.com/july01/axis.shtml
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Old 05-12-2004, 11:32 PM   #19
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Old 05-13-2004, 03:33 AM   #20
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Not to step on any toes or anything, but the post above talking about the car that lossed about 13WHP.... If it's the same show I thnk he's talking about, I think it was an Acura. I think they put on an exhaust (&maybe other related mods), bigger brakes, & bigger wheels. They loss the HP w/ the big brakes & wheels, & I think they said they broke about even after the prformance mods where done. They also explained how the bigger brakes & wheels could help performance in track & mountain like driving but wasn't as ideal for street or drag use.

Also I have 18's (about 18 pounds) on my car & it drives great! One other thing to consider when upgrading to larger wheels is to keep in mind the correct tire size to retain factory spec diameter or circumfrance, That way you will not do damage to your drive train if you ever have to drive on you spare tire. (so they're the same size). Hope any of this helps.... if not, then oh well.
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