Why use larger wheels? 20" vs 18" etc.

Nothing IMO (for a street car, it's all bling)

 

Basically you got it. Pretty much a exercise in style ( or lack of). It seems that the 18" wheel has the best comprimise of sidewall stiffness and bump absorption. Thats why most sports cars in racing still run them. For the street many choose larger sizes for the look. Tends to fill out the wheel wells better when you go to the larger size.

 

Style is the reason, unless you drive a purple '96 Caprice with metallic flakes the size of a bass boat's, with 26 inch chrome wheels (spinners, of course) that raise the car 4 inches higher than stock and makes it look like some weird bastardized off-roader. Of course you have to have the gold trim package installed as well. It contrasts nicely with the Dimetap-purple paint. Then the bigger wheels are an issue of BAD styling.

I guess the rule is your wheels have to be at least 6 inches bigger than your largest woofer.

 

I'm changing my 360 wheels to the BBS Challenge wheels. I'm keeping them at 18's with a slide adjustment to ride height. I did look at different wheels, but didn't feel comfortable going outside the Ferrari stable when it came to recommended wheels and specialist experience. Stock wheels on the 360 are not too impressive, but stock on a Boxer look fantastic. Its a tough one, but take a 355, stock wheels look great, but for some reason Ferrari didn't spend too much time getting the wheels right with the 360 IMO

 

Apart of any style or estetical consideration, more technical reasons for big wheels ( large diameter ) are :
1- More room to accommodate larger brake rotors and/or to cool them.
2- Use of advanced low profile tyres.

Above needs were originally requirements on racing cars but progressive adopted on street performance cars.

 

You aren't missing anything. The larger diameter wheel, generally puts more weight away from the hub increasing the difficulty to get the wheel spinning, and like you say, increases the likelihood of damage from pot holes etc... From my limited understanding, It is just to make the car look more contemporary and aggressive. The most high performance cars have a substantial sidewall with fairly stiff suspension to allow the tire to take up some of the differences in road surfaces. The F1 cars don't sport DUBs.

BT

 

Nada, zilch, zippy, zero, nothing... In 1997, with the introduction of the C5 Corvette, GM two different size wheels on the vehicle - 17" up front, and 18" out back. In terms of performance, you would want a stiffer sidewall on the tires that "turn" (i.e., the fronts) as opposed to the drive tires (rear), so for optimal performance, Chevy should've put 18" wheels up front, and at least 17" or 18" in back. Instead, they chose to favor aesthetics and put the 18's in back and 17's in front for a nice, staggered, high-performance look.

Those individuals who put 19's or 20's on a sportscar on par with the levels of Ferrari, Porsche, Lambo, and Corvette are doing so *SOLELY* for aesthetic reasons. Nobody in their right mind could think that chrome "Dubs" will help them get around the track any faster - as if they actually have ever SEEN a track.

And as for the 26"ers on Hummer H2s, Suburbans, and Navis, puh-lease. Performance?!?! No way, Jose. It's all about the bling.

Aaron

 

That is not the full story. I have a 1979 308 that came with 14" wheels. The wheels are now 16"which greatly improves handling and choice of more modern tires.
Bob

 

A larger diameter wheel/tyre package will have a larger contact patch for the same width tyre AND with less rolling resistance and road noise than a smaller diameter combination with the same size (area) contact patch achieved through a wider tyre.

 

I read somewhere that there is a speedo issue when upgrading to a larger set of wheels. Any of you guys care to enlighten me on this one? thx

 

Rolling circumference.
If you increase the circumference of your wheel/tyre package then you will change the "gearing" accordingly, e.g., larger circumference = further travelled in one revolution of the wheel and hence your speedometer will not read the same as it did before the change.

 

Herewith is a handy little spreadsheet so you can check the effect of rolling circumference:
Image Unavailable, Please Login

 

Sure....think about this:

Take the circumference of a tire, and "unroll" it. That is to say, if a tire is 24" inches around (completely random, arbitrary number I just made up), you move 24" in a straight line for every revoultion of the tire. Assume this is stock. Now, imagine a larger tire that the engine and drivetrain are moving.

For the sake of my example, make it 1ft larger in diameter (again, completely arbitrary). The circumference of the tire, "unrolled" if possible, is a foot longer than the stock tire. Your speedo does not know this - it thinks it's moving the same ol' stock tires. Instead, you're getting an extra 12" per revolution over the stockers... At highway speeds, you will be moving CONSIDERABLY faster than your speedometer indicates on account of those extra 12".

The key is, when upgrading to larger wheels, to take that difference in wheel diameter into consideration and absorb that difference (in the case of going from 17" wheels to 18" wheels, a 1" increase) with the sidewall of the tire.

If you want to keep the same ratio, or as close as possible, and run larger wheels, bigger wheels = narrower sidewall. A typical upgrade would look something like this (again, completely arbitrary numbers...just for arguements' sake)

(Stock) 245/45/ZR17 -- 17" wheel

(Aftermarket) 245/35/ZR18 -- 18" wheel

The difference is in the second number - 45 vs. 35. The first number is the width of the tire in millimeters - 245mm...or 24.5cm...or just under 10" wide. The second number, which refers to the sidewall thickness, is a PERCENTAGE of the WIDTH, so a 245/45 has a sidewall that is 45% the width of the tire. If you increase the diameter of the wheel by 1", you want to decrease the sidewall by approximately the same amount, so while the with of the tire's contact patch remains the same (245), the second number (45) goes down (35) to keep the speedo reading as close to "factory" as possible.

Hope that helps...

Aaron

 

That's what I said.

 

...and the spreadsheet makes it easy to see.

 

And Zertec, you obviously have missed many prime time TV shows doing this sheet!

Wow! Fantastic.

Great work without question!

Didn't you at any point "tire" of doing this?

I have found that using this site : http://www.miata.net/garage/tirecalc.html : has been a big help in seeing real time what the numbers actually do to the speedo readings.

This helped me find out that the rear tires on the 360 I just bought were incorrectly sized throwing off the speedometer.

 

Speedometers are normally optimistic (i.e., they read high) to avoid people suing the car manufacturer (if the speedo was reading low) when they are "done" for speeding.