Hey There, Big Wheels!

[spitz7985]

A good, surface article from MotorTrend's May '04 on the effects of different size wheels (summarized).

"We're not saying "Don't" to oversized alloys and skinny rubber-but you need to consider what you're messing with....we wondered how the substitution of mega-diameter wheels affects performance and whether they're safe on that 5800 lb truck...Although swapping on big wheels with short-sidewalled tires is as simple as loosening and tightening lug nuts, it's nevertheless an act of significant reenineering. Those boring, standard wheels and tires you're so restless to replace are, in actuality, highly developed components, painstakingly integrated into the physics of your vehicle's suspension. By tossing them aside, you've just spun the roulette wheel of ride, handling, and braking performance outcomes. Feel lucky?We did a few simple measurements comparing two wheel-and-tire packages: a pedestrian 18 inch wheel and P275/65/18 tire on a stock Lincoln Navigator and an Oasis brand 23 inch alloy shod with 305/40/23 rubber unbolted from an aftermarket modified Nav. On the scale, the standard wheel/tire set weighed 76 lbs. The [dubs] creaked the scale's needle to 84- that's a bit over 10 percent heavier. It's not a lot, but if these things were the latest 26- or 28 inch models, the differential would be greater- and even more so with spinners. The increase is all unsprung weight, the worst kind.Acceleration requires turning the rolling stock, too, so we must also include their rotational inertias. Here, the 23 inch wheel's big rim- spaced a cruicial 2.5 inches farther from the wheel center- takes a second bite out of acceleration...added reluctance to spin is equivalent to four times the increase in wheel and tire mass. Last, this bigger wheel-and-tire package exacts a third performance price by adding 0.75 inches to the rolling radius as mounted on our truck, making the final-drive ratio effectively five percent taller.Heavier [dubs] also guarantee an erosion of ride quality. Less obviously, this extra weight means they trace the road's profile less accurately, too, degrading cornering and braking. Racing wheel designer and builder Bill Jongbloed of Jongbloed Racing Wheels (currently developing wheels for the 2005 Indy 500) belives that braking is most affected: "we did a test where we intentionally added weights to the wheels, and the consequence on braking distance due to the tires losing contact with the road was eye-opening...Jongbloed notes "As you go up in diameter, you increase the loads at the hub, and some of the hubs I've seen on the street have only about five-eigths of an inch of material under the lug. That would worry me." Bill adds, "Nobody really ever thinks about wheel maintenance, but in Indy-type racing, wheels are checked for cracks after every 10 races. on the street, wheels are never checked." An army of vehicle-development engineers worked hard to give that H2 or G500 its world-class off-road prowess. By strapping on wheels and tires with little to no sidewall flex, you've tossed that rock-crawling ability out the door....[(information about testing standards of the aftermarket wheel industry as specified by the SFI Foundation and the Society of Automotive Engineers)]...Even J2530 [(Soceity of Automotive Engineers' badge of standards compliance)] won't soften that auto manufacturers's assertion that only they can properly engineer [dubs] into a vehicle's suspension system. In some cases, aftermarket fitment will void your warranty. The natural concern is that even if a wheel and tire survive their reasonable lifetime of pothole pounding, the suspension itself was never designed for this sort of expanded hammering. Bearings, spindles, and more are exposed to considerable, unanticipated stresses. We've heard of a manufacterer discovering to its horror, cracks in suspension components after rigorous fatigue testing with large-diameter aftermarket wheels....don't go overboard on size choice. A well-chosen +1 or +2 wheel/tire combo can yield improvements in handling performance to go along with The Look. Replace your 18s with 26s, and you're long past diminishing returns..."

Also, there are a couple things that the article did not touch on:A wider track width often a result of larger, wider wheels and tires will reduce the roll angle. But, throwing on wider wheels will increase the scrub radius (most of the time, depends on the offset). Bumps and cornering forces exibitied on a tire exerts a twisting force on the steering that is proportional to the length of the scrub radius. So, the best scrub radius is zero, which would result in zero twisting force. Lessening the scrub radius results in improved handling and control. Wider tires also increase the swing arm length, which lessens camber gain with suspension travel. The worst effect of larger wheels, IMO is the increase in unsprung weight. A lot of unsprung weight hurts the tires ability to follow the road's irregularities and reduces the tire's contact patch. Not to mention the additional force needed to accelerate and decelerate heavier wheels or larger wheels with more rotational inertia.

-Brandon

[aither]

I think this only really applies to bling bling set ups. I don't think throwing 19" forged wheels on my s13 with super low profile tires would change performance that much.

[spitz7985]

You don't think so? Try it. I bet it will slow you're car down more then you think. Mathematically it will. Like I said before, the biggest hurt on the performance will be the increase in unsprung weight.

[aither]

my point is that if you are not changing the total weight by more than a couple of pounds for the wheel and tire, and the overal diameter is close to stock, there will be little performance loss to a bling bling set up.

[Nismo_Freak]

my point is that if you are not changing the total weight by more than a couple of pounds for the wheel and tire, and the overal diameter is close to stock, there will be little performance loss to a bling bling set up.

Hmm... let's put that to some application.

17x7.5 Volk Racing TE37 - 15.2 lbs. x 4 = 60.8 lbs. 215/45/17 Bridgestone S03 - 24 lbs. x 4 = 96 lbs.

Rolling diameter difference of 2%

All together weight: 156.8 lbs.

Now an equal 19" setup

19x8.5 Volk Racing TE37 - 19 lbs. x 4 = 76 lbs. 245/35/19 Bridgestone S03 - 28 lbs. x 4 = 112 lbs.

Rolling diamter difference of 3.2%

All together weight: 188 lbs.

Difference in weight: 31.2 lbs.

Rotational/Unsprung effective difference: 124.8 lbs.

125lbs. of effective dead weight will make a noticeable amount of difference.

[Exar-Kun]

thank you alan!...not to mention, yet again, that theres the serious rate of dimishing returns on lower-sidewall height tires, and "slip angle" is more of a factor of inflation and load index once you cross into the typical 45-or-lower aspect ratio tires.

plus, and it bears repeating, contact patch rear is not determined by thw width of the tire....that only changes the shape.-chet