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      12-29-2009, 02:44 PM   #1
wa128
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Wider tires=more weight; acceleration effects?

Can't seem to find any discussion about this topic? Trying to decide if it's worth it to go wider in the rears.

Majority of the members seem to be concerned with fitting the widest tire possible without rubbing issues. Yes I know, wider tires look good. What about the effects on acceleration of added weight as tire size increases? For example, if the rears with Michelin PS2s are switched from a 225/40/18 to a 255/35/18, that's an added weight of 3 lbs per tire plus another pound or two for the proper rim width size to fit the wider tire. It doesn't seem to be much but I know a lot of people are concerned about decreasing weight as well. Does anyone know if the performance factor of a wider tire will always offset the negative aspects of added unsprung weight in terms of acceleration and of handling?

Thanks for your feedback!
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      12-29-2009, 03:46 PM   #2
larryn
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That's why we are also getting rid of the anchor weight stock wheels too. The wheels I'm waiting to be shipped to me are 10lbs lighter each corner (going from 28lbs stock to 18lbs aftermarket). Remember that runflats have lots of extra rubber on the sidewalls, and a 245 stock runflat weighs just about the same as a 275 non-runflat.

I was running a square 245 Star Spec setup on my 135i for the 2008 season. The wheels were 16lbs each, making the unsprung weight reduction be in the neighborhood of 15lbs a corner. It was incredibly noticeable on acceleration.

To be honest, it would be pretty hard to find aftermarket wheels that weigh more than the stock wheels. So, even if your tires did weigh a couple pounds more, you probably have lighter wheels, making it a wash.
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      12-29-2009, 08:36 PM   #3
wa128
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Quote:
Originally Posted by larryn View Post
...To be honest, it would be pretty hard to find aftermarket wheels that weigh more than the stock wheels. So, even if your tires did weigh a couple pounds more, you probably have lighter wheels, making it a wash.
That's true. However, I am already on aftermarket wheels and non-RFT tires. What I'm trying to find out is...if I stick with the same tire brand and model and go wider in the rear say from a PS2 225 to a PS2 255 size, will the larger contact patch translate into better grip and acceleration than the 225 and possibly more than offset the weight gain which normally translates to slower acceleration considering all other factors being equal?

Or, is the net effect so miniscule that it's not even worth fretting about?

Thanks.
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      12-29-2009, 09:01 PM   #4
larryn
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You'll definitely get more grip from a 255 PS2 versus a 225 PS2, given the same wheel/offset/camber, if the wheel is 8.5" at a minimum, so as to not crown the contact patch.

If you are talking about a pound or two for wider rubber, you're likely gaining more grip, and the additional grip would outweigh any meager detractors from the additional weight, IMO.
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      12-30-2009, 10:30 AM   #5
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It's been discussed in the track forum and Im pretty sure everyone agrees that the extra traction you get from wider driven tires outweighs the weight gain (no pun intended). That's assuming you actually need that traction in this first place. The truth is you don't want 100% grip all the time. If that were the case cars could never turn. You always need a little slip for performance driving, especially for standing starts (IE: drag racing). This is part of the reason alot of track cars that are RWD use square or even under-square setups. Sometimes you need less grip.

Having said that, the 135i seems to respond well to a few more millimeters on the rears.
Especially if you are going to be replacing your tires with some higher performance compounds, anyway. Might as well stick a bit more rubber under there while you're at it.

The reason you don't see a lot of square setups on the 135 is that we simply cant get enough width in the front without replacing the fenders while is illegal in a lot of amateur race classes. Also the n54 responds so well to simple mods that you simply can't sacrifice rear width and still hope to get the power down once you start throwing software and methanol at it.
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      12-30-2009, 07:54 PM   #6
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This can be a very complex discussion in some cases. However, with our limited offset options and high horsepower I'd bet you'll pretty much always be best off going as wide as possible.

I'm a big fan of weight reduction. With my wheel setup I pretty much don't have any choice on tire size. But I've recently been heavily researching tire weights vs traction ratings trying to make my next tire choice. My rears are gone and I've settled on Hankook V12's, my second choice though will be the lightest tire I've seen which are the General Exclaims. The Hankooks are a little heavier but still relatively light and reported to have great traction. I need all the traction I can get. I also hate tire noise and don't mind soft sidewalls since I'm already running 19's and a stiff suspension.
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      12-30-2009, 08:48 PM   #7
wa128
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I've been researching articles online regarding the effects of rotating mass on acceleration to try and validate at least mathematically what everyone has been saying. Not trying to make this a technical nor theoretical discussion but I came across a spreadsheet that's kind of interesting. You plug in car specs plus specs for two different wheel/tire combos. The end result basically tells you how much more or less torque is used in comparing the two sets. Less torque used to get the wheels and tires moving should mean quicker acceleration. It also calculates a chassis weight equivalent given the weight difference between the two wheel/tire combos. Bottom line...it appears to be saying that you can go as wide as you want as long as you achieve a weight savings with a new setup versus your current one. The magnitude of the gains given a few pounds saved is surprising. How this translates to ticks on a stopwatch, I don't know. That's another story.

Here's the link to that article and its spreadsheet if you're curious.
http://the-welters.com/racing/rotational.html
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      12-30-2009, 09:40 PM   #8
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Quote:
Originally Posted by Eyeman View Post
My rears are gone and I've settled on Hankook V12's, my second choice though will be the lightest tire I've seen which are the General Exclaims. The Hankooks are a little heavier but still relatively light and reported to have great traction. I need all the traction I can get. I also hate tire noise and don't mind soft sidewalls since I'm already running 19's and a stiff suspension.
I have the General Exclaim UHP's on my 332ti, only because I bought the car with them. I have only put 6k miles on them, and I hate them with a passion. They are loud running and make as bad a racket on bumps as the runflats do. I'll be throwing them away in the spring, even though they are near new looking. They also have little traction, and I have to be very careful to not spin when taking off. That car has less HP and TQ than our 1ers. What do you want for a mud & snow rated tire though, which they are. The Dunlop 3D snow tires I have on that car right now handle better and a quieter than the Generals.

If you are looking for a summer tire, get a decent one, and those Generals should be run away from. They are rotating garbage.
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      12-30-2009, 11:45 PM   #9
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Thanks for the info Larryn. That's got to be the worst rating from a reliable source that I've ever heard. But the Generals are light..lol.

So I'm looking at V12 with my second choice as Toyo T1R's then. There's less choices in 19's btw. I hear Toyos run narrow and I'm planning to try to get my wheels machined to a slightly higher offset. So if I do try 255's I'll probably try the Toyos.

wa128, for sure lighter will help. You can even find reports of tuners using Dyno specific wheels to up there numbers. Shiv has had some interesting comments about how his drag radial setup gets him better quarter mile times but decrease his trap speeds.
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