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It depends.... [IMG]i/expressions/face-icon-small-smile.gif[/IMG]

This is discussion for rim size and effect on dyno readings and performance.

"Power changes due to wheel/tire changes on roller dynos depend upon the moment of inertia. This is directly related to the mass of the wheel/tire combo and how that mass is distributed around the hub axis. If the 18" rim weighs significantly more than the 15" rim, it will probably read lower hp on the dyno. If the 18" wheel weighs close to the 15", it may actually read higher. This is because the 18" tire may weigh a bit less because it has less sidewall. Since the tire mass is furthest from the axis of rotation, it has the largest effect on MoI.

Realistically, if you keep the same outer diameter for the wheel/tire combo, the heavier the combo, the less power you'll read on a roller dyno."
--quoted from Ultimate Lurker on s2ki.com

So just because you get larger wheels doesn't mean that your car will accelerate slower. The overall wheel diameter and weight of the tire is what makes the difference. When you go to a 2" larger rim, ie. a 15" to a 17" rim, you generally get lower profile tires. This means that you have a lower MoI because the tires are the furthest weight from the center of the wheels. Also since you have lower profile tires, your overall diameter may be similar. For example... your rims are 15", and your tires are maybe 4". Overall diameter is 19". You get 17" rims, but lower profile tires that are only 3". This gives you an overall diameter of 20". However, the new tires will probably be lighter, giving you less rotational mass further from the center, and the overall setup may be a similar weight..... meaning you may actually see your dyno HP go up and an actual INCREASE in performance/acceleration.

Cliff Notes:
The weight with the wheel on, matters more than overall diameter, however, diameter does play a role. So if you had 15" rims on your car that weighed 34lbs a piece, and you put on lower profile 17" rims which had a similar diameter with the tire on, and didn't gain much weight, you could actually see a GAIN in hp on the dynojet, as well as acceleration increase.

 
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gearing wise, a smaller then stock wheel will allow you to accelerate quicker, but give you a lower top speed. and a Larger then standard wheel will make you accelerate slower and give you a higher top speed. This is disregarding weight issues.

to clairify a little. This is when dealing with larger and smaller total size, not just a larger/smaller wheel and puting tires on them that would make it match the stock size. like if you ran 15's but instead of putting a 185/65 15 (Stock Size) you put on say a 185/30 15. In this fashion, a stock tire rotates 823.59 times per mile, while the smaller tire rotates 1036.98 times per mile.

this translates to roughly 1.259 to 1. meaning the smaller tire will acellerated .25x quicker then the stock one. dosent sound like mutch, but in autocross where times are mesured to the .001 th of a second, being 25% quicker can make a difference.

I'm not shure of this but acording to the math,
If your car did a 16 sec 1/4 mile with stock size wheels then youre car will now do a 12 sec 1/4 mile.
16*.25 = 4 and 16-4 = 12 that is of coures ignoring shift times (because you will probably be going through all 5 gears), wheel spin ect. but a "Perfect" 1/4 mile with one gear. which Dosen't exist except on drag bikes, but it would be nice.

of course that also means that you have a 25% decreace in top speed, so for an ex, with a suposed top speed of 127, youre new top speed is 95.25MPH not very fast, but you'll get there quick.



here's my math
ratio on tire rotation.
1036.98/823.59 = 1.259097366......... aprox 1.25

with a 1.25 to 1 ratio since 1 = 1 then the added bonus is .25 or 25%

the top speed 127*.25 = 31.75
127-31.75 = 95.25

I'm pretty shure this is all right.

The same holds true if you uped the size of the wheel except in the oposite fasion, Making you slower to accelarate, yet have a higher top speed.

please note that none of this would show up on a Dyno. that is recording engin hP and tourque not acceleration.
though you could calculate acelleration if you Knew the gear ratios.[IMG]i/expressions/face-icon-small-smile.gif[/IMG]

 
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I disagree with the weight of lower-profile tires. Even low-profile 17" tires weigh ~20lbs while the stock 15" tires weigh around 16lbs. It's most likely that the tires will be heavier as the rim size increases, not lighter. A smaller sidewall giving way to less weight is a common misconception.

Plus, no matter the size of the rim, if the overall diameter is the same or very close, it's the wheels inertia that ultimately matters. A heavier wheel has more inertia which means it is harder for the engine to spin than one that has less inertia.

 
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Gearing is the biggest thing I enjoy with my 15's, weight and economics were also factors, since a pair of 12lb. 17"s will set you back a few thousand while 12lb 15"s are more reasonable. Once again though, I enjoy the lower gearing [IMG]i/expressions/face-icon-small-smile.gif[/IMG] (205/50/15's are 1.5" smaller in diameter than stock (roughly 10% gearing increase)

 
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Yes, all correct, disregarding weight factor that is. [IMG]i/expressions/face-icon-small-smile.gif[/IMG]

 
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So if you want to accelarate quicker, get a verry small verry light wheel and tire combo.[IMG]i/expressions/face-icon-small-tongue.gif[/IMG]




Physics are your friend[IMG]i/expressions/beer_yum.gif[/IMG]

 
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You can find tires that have a similar weight as original tires when only increasing 2" in tire size. Also, less weight means less MoI (moment of inertia) on the outside of the circle. If you are getting lower profile tires with less sidewall, you should be able to find lighter tires if you are only increasing size by 2". Do you know where you can get tire weight specs from? We could try to look into this more.
[IMG]i/expressions/face-icon-small-wink.gif[/IMG]

 
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Yes, go to most any tire manufacturer's website. I visited several when shopping for wheels quite some time ago. All the 17" tires that I could find weights for weighed more than a 15" tire by 4-5lbs on avg. (keeping the same overall diameter as stock) Edit 2

Edit: Are we operating under the assumption that the 15" and 17" rim weigh the same?

 
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Cool, I'll have to go look up some tire weights and see if it's easy enough to find lighter tires or not. Should be interesting since it's not something I've ever actually looked for. Does tire rack give specific weight of a tire? In order to maintain stock overall diameter, does the new tire have to be 30mm (is that 2"?) less in height than the stock tire? (when changing from say 15's to 17's)

Rim weight can be whatever you want. Even if you have lighter rims, the tire weight plays more of a role since it's furthest from the center, but lighter rims can help too.... it's just that tires are the bigger factor. Of course heavier rims don't help. lol

 
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yea it depend on the weight on the wheel also, here the thing bigger the wheels the more force to take to spin the wheels, that why u never seen any drag on 17 or 18". smaller the wheels less force to spins the wheels

 
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Mbow..i agree with you that lighter tires are quite hard to find. I went thru it with my rims too...almost every tire weigh more like how you explained it. Anyways, the way i see it is, even if the 17in does have lighter tires than the 15in. The 17in should spin slower off the line still. For example...tie a 1 1/2pound hammer to a 5inch rope and spin it around. Then tie a 1pound to a 7inch rope and spin it with the same force(i know its impossible..=P). From my understanding, the hammer with the 7inch rope will be harder to spin. You lose power the more distance it travel. This is the only explaination i can come up with on why my friend had a lower HP reading with his 17s.

 
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I just remebered this Quote from Mickey Thompson Performance Tires and Wheels web site, and although they mainly deal with off road perfomance I think this would apply to street as well

Quote:
4. Weight & performance
In some cases it is acceptable to run a racing tire without a tube. When a car competes in a heads up class, unsprung weight is an important factor. Tires and tubes are all unsprung weight. One pound of "unsprung" weight is equal to 8 pounds of "sprung" weight. The average tube weighs about 6 pounds. Therefore, if you eliminate the tubes you have eliminated the equivalent of 96 pounds from the car. For every 10 pounds of unsprung weight removed you may pick up as much as .01 seconds in quarter mile ET.


They are talking about Removing innertubes, but Decreasing the Tire/Wheel weight would have the same affect.

This can also be applyed as Every 10lbs of weight added to your wheels, you will lose .01 sec of your 1/4 mile ET.


Mickey Thompson Performance Tires and Wheels

 
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Good post.