Please explain wheel size differences.

There have been several threads lately about wheel sizes, but I'm a bit in the dark as to what difference wheel diameter makes. I seem to remember hearing that changing wheel diameter doesn't necessarily change spedometer accuracy. If that's the case, then the tire thickness (rim to road) must be less with larger diameter wheels. Then it would seem to follow that there would be less tire distortion when cornering, all other things being equal. Am I on the right track? Are there other advantages/disadvantages? Should I just go for a ride and not think about it?

Reply to
Dave Smith
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"Should I just go for a ride and not think about it?" Yes! Life is too short to worry about the minutiae!

Short sidewalls must be very stiff in comparison to a normal height sidewall. Remember that the sidewall has a part in improving the ride as well as effecting handling. To me, the current crop of large rim diameters and narrow sidewalls are more for appearance than any other useful reason. We watches a poor fool try to autocross a car with the narrow sidewall tires and large rims. The rims ground in sharp corners (sparks and all that) If the tire does not absorb shock, the suspension and shocks/struts are the next line of defense.

Reply to
chuckk

Correct. All the common Miata sizes have about the same overall tire diameter: 185/60-14, 195/50-15, etc.--only the size of the hole is different See the tire calculator in the miata.net Garage, which lets you compare revs/mile for any two tire sizes.

That's one theory. In reality, the inflation pressure, brand, and model of tire make much more difference to grip than the profile (aspect ratio). Extremely low ratios (less than 40 or 50) do not protect the wheels as well from pothole impact damage. In general, larger-diameter wheels also weigh more, and so do their tires (despite the bigger hole), which is detrimental to both ride and handling. The sweet spot seems to be 195/50-15, which also offers a greater choice of tire options than

14" sizes.
Reply to
Lanny Chambers

I suggest you pay attention to the aspect ratios typical of racing cars when not limited by regulation...

Reply to
Alan Baker

I bought my '99 with 205-40-17s, and suffered with them for three years-- terrible ride, acceptable but unremarkable handling and the embarrassment of being seen driving a car with high "bling"/low function wheels. I just now came into some money and could afford to swap to 195/50-15 Heliums by Koenig, and it's like I got a new car. I'm guessing I've got about a 20lb saving in unsprung weight at each corner. As long as you have a driver's mentality (as opposed to whatever thought process makes someone buy spinning rims or an Escalade to put them on),there is no advantage to big rims. The marginally quicker turn in is offset by the weight slowing the suspension response and harsher ride.

Maybe it's because I was starting so far behind the 8-Ball, but MAN I like these Heliums!

Cheers

Dave Smith wrote in news: snipped-for-privacy@4ax.com:

Reply to
Mack

You got it. Bigger rims are useless unless the plus sizing is kept down to about an inch. The only benefit of anything bigger than that is "the look." Shows you the mentality of anyone driving with rims like that on their car: they know nothing about REAL performance driving. They'll all drive 90 MPH on the freeway and way too fast elsewhere, but as soon as anything bad happens like a sudden case of understeer or oversteer they'll be screwed. I'm currently searching for an NA Miata, and every time I see some idiot who's put 17" rims (about the biggest that can fit) on the poor thing, I usually look elsewhere. If the person is ignorant about rims and tires, they've probably been ignorant about other things as well. I would consider 15" on an NA Miata, where the stock rims were 14," but no bigger.

There are rare cases where super big rims are a good idea: when they weigh less than the stock rims. Then the thin rubber doesn't flex as much providing more instant turn-in and steering response. However, these are usually true racing rims and cost enormous amounts of money.

~Anthony

Reply to
Anthony

I am not disagreeing, but why do corvettes and vipers run such large tires? To keep the Hp down on the ground? The miata's I have seen at the SCCA races usually run 14's, but with really wide rims and low profiles.

embarrassment

Reply to
Dustin

Same reason as Ferrari: the factory thinks the owners expect bling-bling wheels. And they're right--few buyers of these cars know or care about handling. They bought image, not performance.

FWIW, the first thing a savvy Corvette owner does is lose the heavy run-flat tires.

Reply to
Lanny Chambers

There are a couple of legit reasons for bigger wheels: Allows bigger brakes (which is sometimes, though extremely rarely the case). Allows shorter sidewalls for a given wheel OD which, up to a point, will give a more responsive ride. Though likely with increased harshness.

Now almost everyone mentions the increased weight. Yes, the wheel weight will increase, but the tire weight will decrease! [Given we stay at the same tread width and OD]. And tires weigh as much as wheels. So, the overall weight doesn't really have to change much.

But I agree most of it is just about style. Big wheels are in. My Miata has the stock 16", I'd be as, or more, happy with 15's.

I'm with you on that! Damn this move to run flats and no spares! What does Porsche do now, give you a can of fix a flat?

Reply to
Dave

Not to mention the cost of tires when the time comes. Stock 14 and 15" tires are inexpensive, relatively speaking. My 4Runner came stock with

17" tires and although they look fine on an SUV, the tire sizing leaves very few choices in replacement tires and the pricing reflects it! I'm finding out every day that my 92 Miata just gets cheaper and cheaper to maintain, repair, etc.. It may end up being my forever car.

Tom

92 Red
Reply to
Tom Howlin

I thought that was to change the gearing.

Reply to
Leon van Dommelen

A long time ago, Richard Decker posted data showing that this is typically not true. I don't have them anymore, but if I look at the web site of our favorite T1s's, it lists

205/55R15 88V weight: 18.3 outer diameter: 23.9 205/40ZR17 84WRD weight: 18.5 outer diameter: 23.5

Moreover, it is somewhat misleading, since the 18.5 lb R17 tire is the lightest available. With a 14" wheel, you have the option of going down to a 15 lb 195/45R14 77V. In addition to having a much lighter wheel. If you are in a situation where keeping your tires on the road is a consideration, you are out of luck with a 17" wheel.

Leon

Reply to
Leon van Dommelen

Yet the Corvette continues to outperform the Miata's handling despite our lovely car's nimbleness. Could we possibly be over-exaggerating the wheel issue here?

Reply to
Bryan

I am not disagreeing, but why do corvettes and vipers run such large tires?

I would suspect they were designed to use them from the start.

Reply to
JEN LURA

Damn you for bringing data to the table! 'Course it is only one data point ... exactly one more than I had :-)

Seems strange though. I'd expect a shorter sidewall would require less thickness for a given spring rate and strength, thus lighter. Maybe something else is happening. Anyway, most go to wider wheels and tires which certainly does add more weight. But even with all that, I think the rotating mass arguments generally used on the net are exaggerated. And maybe the unsprung mass issues?

Be that as it may, I'm still not a fan of big wheels & tires. There simply isn't much benefit. There are some drawbacks (whether minor or not). And they cost a ton more.

Reply to
Dave

I would guess it may have to do with design trade-offs making the sidewall conform to a flat contact patch in a short distance. Fatigue jumps to mind.

The ones I have seen are. The rotation adds a bit to the effective mass, but nowhere as dramatic as some claim.

That is a real issue to me. While your wheel is off the ground, its traction performance is quite poor. It also reduces driving comfort.

Leon

Reply to
Leon van Dommelen

By what measures does it outperform the Miata's handling? "Handling" does not have an objective definition as far as I know, so I wonder what this means.

Leon

Reply to
Leon van Dommelen

I would think so too. To accelerate fast in a straight line with an oversize engine, a major concern is to keep the tires from slipping. The first way to squeeze more traction out of rubber is to increase the contact patch size. That is mainly determined by reducing tire pressure. I would think that the larger patch would tend to push you towards larger tires. Something else you can do is change the patch shape. Making it wider makes the wheel locally look flatter, reducing curvature effects, and thus making the contact patch more effective. How do you make the patch wider? Obviously by making the tire wider.

Ergo, big, heavy tires. Are those heavy monsters with big, wide-patch, moment arms going to be nimble? I would doubt it.

And what is the fun in going fast in a straight line? Unless you want to go to jail for it, of course. :)

Leon

Reply to
Leon van Dommelen

I'd expect the sidewall to be much thicker (and thus heavier), to reduce the odds of bending wheels in potholes. Folks with 35- and 40-series tires report plenty of bent rims anyway, but I guess it could be even worse.

They also must be stiff enough not to roll under at all. I recently heard of a kid with some cheap, flimsy bling tire rolling it under in an autocross and actually catching the rim on the pavement. Cars have flipped from that!

Reply to
Lanny Chambers

Man, you are so biased it's crazy. I love my Miata to death also, but in no way has it distorted my view of reality. There is absolutely no way you can realistically downplay the Corvette's handling abilities to this extent. While it is probably true a Miata is much more nimble with it's shorter wheelbase and smaller size, this performance rarely pays off when compared with the Vette other than in the tightest autocross formations. Not to mention with triple the torque, it's going to take a little more concentration to take advantage of the Vette's abilities. Corvettes average between .89 - .98 lateral g's on the skidpad (Convertible - Z06) while Miatas register .86 - .92. While many of us have upgraded our suspension package, the highest (Miata) ever (officially) recorded was a racing beat Miata with 1.00 g's which set a record in Car and Driver with a 73.6mph slalom. Now I doubt our cars are not quite to this level, a typical miata pulls a 66.7 mph slalom which is only marginally better than the previous model Vette's 66.4 mph. Yeah, yeah what do numbers mean? Well a Corvette and Miata are in two different leagues and it's completely unfair to test them head on, but lets just settle that the Corvette is a fine handling car, especially for American manufacturers.

Reply to
Bryan

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