Why are 17 and 18 inch Wheels Considered the best for Performance?

[tyoneal]

To All:

I know this has been brought up offhandedly from time to time, so I thought I would ask it out right.

Why are 17 and 18 inch wheels considered the, "Best" choice for overall performance, (Road Race or Performance Street driving) while larger wheels (> 19 inches, and, < 17 inches) and smaller wheels are generally considered second rate when ONLY handling is considered?

The new Vette and Camaro etc. all have moved to 19 and 20 inch wheels, and at least the Vette is considered one of the finest handling production cars in the world?

Is this just "Old" reasoning, that is now considered out of date or is there some factual reasoning for this that I have overlooked?

Does Staggering the size have any effect one way or the other on the handing of the car, (i.e.. 19 inch in the rear, and 18 inch in the front), or is it just strictly for looks?

I have noticed that many great performing racing cars have a taller, wider tire on the rear and a slightly shorter and thinner tire in the front. (IRL, F1)

Is there any good reason for having the rear of the car higher in the air than the front for a road racing application?

Is it a function of aerodynamics, while under heavy acceleration and "At Speed" running, that the down force will adjust the downward pitch of the car, so while running normally it will move into a nice level stable attitude?

I had heard that depending on the front to back weight ratio of your car, that in some cases this weight differential helps dictate the width of your tires. (i.e.. A car that is heavier in the front than the rear will handle better with a wider tire up front as to help keep understeer to a minimum. While a car with more weight in the back will benefit from a wider rear tire to help reduce oversteer. Is this correct?

Before answering, the drive wheel/wheels should also favor a wide wheel as well.

For instance. A 68 Camaro with 55% of it's weight to the front/ 45% of it's weight to the rear, AND being a Rear wheel drive car. Would benefit from having wide tires in the rear, say 315's (Due to rear wheel drive) and would handle better with front tires being somewhat similar in size as the rear tires, say 295's because the front wheels are having to work harder at changing the direction of the car due to the front being heavier than the rear. (in other words a natural tendency to try and understeer)

This would be one of the reasons, other than weight and aerodynamics, why Drag racers run skinnys up front and as wide as possible in the rear. (Given of course it is rear wheel drive) Changing direction is not an issue so understeer is also a non issue.

Whereas, a rear wheel drive, rear engine car while having 315's in the rear, AND having a weight bias also in the rear could get away with wide tires in the rear and substantially thinner tires up front say 215's without having an undue challenge of dealing with car induced oversteer and understeer.

Is this a correct thought all other influences being static?

Do larger diameter tires as a rule run cooler than smaller diameter tires because the friction of the tread against the pavement is spread out over a larger surface?

I'm sure this is blatantly obvious to most people, but I wanted to clear this up in my mind while giving some thoughts to some project issues.

Thanks for your patience.

Best Regards,

Ty O'Neal

[ironworks]

I would say that as cars get faster and faster you need bigger and bigger brakes to stop the car and 13" used to be the top of the food chain and now you can get 13's all day long. Plus as they are able to improve the design and strength and weigh of the bigger wheels they can run the bigger wheels and still keep the unsprung weigh down of the wheel and the rotor. I think bigger wheels comes with the territory of running bigger brakes.

I would also say there is a benefit with the overall diameter of the tire which you are able to be run which helps with top speed and gear ratio. I think a lower profile tire will not grow at speed as much. You see this alot more in drag racing.

As far as tire widths I would say most of the cars you listed are mid engine, which changes everything from the toys we play with. The Lemans cars we have now racing are so far out there technology wise I don't think you could even compare them to the toys we have. That stuff could kill any street car on the track running on half of it's cylinder and the brakes locked up in the front. As much as we want to think our junk handles it is not even in the same time zone. We just think it handles good. One of my employees buddies is on one of those formula SAE racing teams in the colleges and they have there car running a sustained 2.4 lateral g in the turns. Try that with any street car.

Just my thoughts, whatever they are worth.

Rodger

[byndbad914]

Originally Posted by tyoneal

Why are 17 and 18 inch wheels considered the, "Best" choice for overall performance, (Road Race or Performance Street driving) while larger wheels (> 19 inches, and, < 17 inches) and smaller wheels are generally considered second rate when ONLY handling is considered?
unsprung weight and I would argue smaller than 17" is better in cases where the car is light enough to run smaller brakes

The new Vette and Camaro etc. all have moved to 19 and 20 inch wheels, and at least the Vette is considered one of the finest handling production cars in the world?

Is this just "Old" reasoning, that is now considered out of date or is there some factual reasoning for this that I have overlooked?

Does Staggering the size have any effect one way or the other on the handing of the car, (i.e.. 19 inch in the rear, and 18 inch in the front), or is it just strictly for looks?
you answered all three with the final - it is design IMO. You don't need 14" rotors for a streeter or a typical Joe like us at the race track. It is the typical bigger is better and posers want what real racers are using tho' they never will. You need the big wheels to fit over the brakes (brakes are driving the size v. actually wheel/tire performance). Only argument could be if you wanted an exact tire diam for some reason, say 26", then it is better for performance to have a shorter sidewall, driving a larger wheel choice. But designing the car to the chosen wheel diam instead means a smaller diam tire and different shaped wheelwells.

I have noticed that many great performing racing cars have a taller, wider tire on the rear and a slightly shorter and thinner tire in the front. (IRL, F1)
they have more rear weight and power is applied there is a reason for the big rears. The open wheel cars also started narrowing up the fronts years ago as they create drag, so a skinnier tire had less wind drag at high mph.

Is there any good reason for having the rear of the car higher in the air than the front for a road racing application?
slight downforce can be created on the car, but that is very shape dependent and how air generally flows over the car. Cars with heavy slant to the nose and so forth like it (Porsches for instance)

Is it a function of aerodynamics, while under heavy acceleration and "At Speed" running, that the down force will adjust the downward pitch of the car, so while running normally it will move into a nice level stable attitude?
No, it won't necessarily level out as you actually tend to push on the front more than the rear with rake (the roof tends to block the rear of the car from the flow), tho' again based on shape, the aero can change how it flows over the car with rake.

I had heard that depending on the front to back weight ratio of your car, that in some cases this weight differential helps dictate the width of your tires. (i.e.. A car that is heavier in the front than the rear will handle better with a wider tire up front as to help keep understeer to a minimum. While a car with more weight in the back will benefit from a wider rear tire to help reduce oversteer. Is this correct?

Before answering, the drive wheel/wheels should also favor a wide wheel as well.

For instance. A 68 Camaro with 55% of it's weight to the front/ 45% of it's weight to the rear, AND being a Rear wheel drive car. Would benefit from having wide tires in the rear, say 315's (Due to rear wheel drive) and would handle better with front tires being somewhat similar in size as the rear tires, say 295's because the front wheels are having to work harder at changing the direction of the car due to the front being heavier than the rear. (in other words a natural tendency to try and understeer)
so this is sorta yes and no. If you are front heavy, in theory you want more front tire. But you also don't want to spin the wheels out of the corner, so you run as wide or wider rears (unless you are so underpowered the rear wheels won't spin with narrower, "weight matched" tires. So one option is to run the same size all around with some sort of front downforce device to create more grip there when you would plow in the corners, and have the rear tire to reduce power wheelspin. Key to a good car can really come from tuning aero devices - even the race Vettes have a rear wing and splitter they can tune to balance the car v. the street cars.

Rear heavy, you will want wider in the rear period (assuming your suspension setup isn't all f'd up) i.e. look at every race 911-variant Porsche and you will see massive rears v. front widths.
This would be one of the reasons, other than weight and aerodynamics, why Drag racers run skinnys up front and as wide as possible in the rear. (Given of course it is rear wheel drive) Changing direction is not an issue so understeer is also a non issue.
unsprung weight and less friction with the ground that come to mind. Unsprung weight doesn't transfer well to the rear (unless you get the wheels completely off the ground) and the skinnier the tire, the less input it takes to roll it. Also, those cars use heavy rear brake bias or chutes and little front brakes strictly for the waterbox essentially.

Whereas, a rear wheel drive, rear engine car while having 315's in the rear, AND having a weight bias also in the rear could get away with wide tires in the rear and substantially thinner tires up front say 215's without having an undue challenge of dealing with car induced oversteer and understeer.
possibly - again all about how the suspension was set up. Bad suspension design and wrong spring choice, etc can induce understeer even with similar width tires. But, assuming all's good with the suspension, it is more or less true to say that. Again, look at Porsche racecars.

Is this a correct thought all other influences being static?

Do larger diameter tires as a rule run cooler than smaller diameter tires because the friction of the tread against the pavement is spread out over a larger surface?
I don't buy into that one completely, tho' I have no exact real world example to say yes or no. I know this much, the difference in contact patch between a 16"x10" tire and an 18"x10" tire is virtually nil according to the folks at Hoosier who test this stuff. Width is the key to contact patch, not diameter. So equal width wheels have nearly the same contact patch regardless of diam, so heat will get in or get out somewhat evenly I would think, tho' the concept of thermal mass would certainly say it is harder to heat the 18" than the 16". I suspect the difference is small tho'.

I'm sure this is blatantly obvious to most people, but I wanted to clear this up in my mind while giving some thoughts to some project issues.

Thanks for your patience.

Best Regards,

Ty O'Neal

That is all FWIW and my opinion based on a lot of similar questions over the years involved with racing and tire pros. Put it this way, on my race car I have 12" rotors, good pads (which make all the diff in the world), and 16" wheels to run the smallest diam tire possible on my car to get the least unsprung weight possible. I can drag that car from 140mph+ down to 80mph damn fast. I also built a mid-engine car because I road race, and have a 55% rear bias as I knew I wanted to run slightly wider rears (cuz I have room for it). I run a 25.5" diam rear right now as it works with the track I frequent (cuts down on my shifting on a couple stretches), but will switch back to 23.5" diam due to proposed track changes that make them longer so I have to shift anyway (hit rev limit right now thru the esses for instance).

A porsche site had similar threads (they do all the time) and I just spoke with the guys at Hoosier last week again about this and he assured me he will always run the smallest diam, widest tire he can on a race car.

Now if you are talking running with the big boys in serious racing like 24hr races, then you need large brakes to keep the heat out of them over a duration like that, and funky materials and so forth, so I understand why they run huge carbon ceramic rotors and therefore large wheels, because the unsprung weight tradeoff doesn't outweigh the braking benefit.

But for the street or a weekend track warrior, you are a fool to run anything more than a 17" wheel IMO unless the car is 3200lbs+ (because a 12" rotor with good pads and good 4 piston calipers is PLENTY of brake if you have any clue), then maybe 18" up to 4000lbs due to more weight to haul down and subsequent heat, etc. Anything beyond those based on car weight, you are just slowing yourself down. And with the popularity of 6spds especially, tire diam to help with street gear ratio isn't that important. I would run in the higher gears on the track with the smaller tires/wheels. Won't even get into mass moments of inertia...

And trust me, this discussion is far more in depth that I typed above, I just tried to hit some key points but there are many more. Suffice it to say, brake the car with only the maximum brake you will need for what you do and wrap the smallest tire and wheel package around it. Smaller on every component means less overall weight and less unsprung weight (key).

[byndbad914]

pic of my car just for kicks - 525HP destroked 400 (353 cubes with a 327 crank) on pump gas, solid tappet cam for simplicity, 5100-7300 rpm band tho' the car is light enough to pull like a freight train from 3000rpm. The car is 2650lbs almost on the money with full tank, 55% rear weight bias, 4spd Porsche trans so lllllooooonnnnngggg pulls in mph thru the gears as the car is good for 60mph in 1st gear, 105mph second, 141mph third and then whatever my pucker factor can handle in 4th hahah with the 25.5" diam tire That is heavy for a Porsche racer but it is full tube chassis with steel body and doors, V8, etc etc and I run Porsche 930 Brembo brakes right off the stock vehicle in 1977-1978. I had 15" wheels before but went to lightweight Kodiak 16s as the tire selections were 3"+ smaller in daimeter in 10" to 12" widths and the slicks are lighter than DOT-R tires!

[RECOVERY ROOM]

Can i beat on it,sorry i mean drive it.Looks like a blast to drive.

[byndbad914]

it is fun for sure. The radials require about 3deg neg camber in the rear so I can blow them off pretty easy cuz the contact patch in a straight line is less than in a corner, but if I could hook it up, power to weight ratio put me in the 0-60mph in 3.0-3.2 sec range. All in 1st gear so no gear change to slow me down. But is just lays rubber instead.

Here is some track footage for an idea of the car and how I just run it in essentially one gear. I had a bracket flexing on the shift linkage so getting into 3rd on the front stretch was painful (you will hear it). That will be fixed when it warms up here

https://www.youtube.com/watch?v=3ICOD1MFYLc

sorry for the off-topic with this BTW, but sorta proud as I built the car completely myself, minus the chassis (my design but had a fab shop bend and weld - I can't weld for crap). I am a momentum driver so I don't run deep and hammer the brakes like an amateur but use the compression to slow down. In a competitive environment I will double foot and ride the brake and stay on power, but that is rough on brakes and my budget doesn't call for new rotors and pads twice per year at $400 per end. I have well over 12 sessions on the current pads and rotors. It's funny cuz the in car seems slow sounding, but there are 30-40mph drops in mph on decel with the long gears! The external view gives you an idea of speeds tho'. Search out my username and there are other track day vids and you see me chew up some subies for a relative acceleration comparison.

[XcYZ]

Looks like a great time, Tim.

[TravisB]

thats all I have to say about this topic.......

[chicane]

Answers, thoughts and ideas in bold...

Originally Posted by tyoneal

To All:

I know this has been brought up offhandedly from time to time, so I thought I would ask it out right.

Why are 17 and 18 inch wheels considered the, "Best" choice for overall performance, (Road Race or Performance Street driving) while larger wheels (> 19 inches, and, < 17 inches) and smaller wheels are generally considered second rate when ONLY handling is considered?

Compliance. If its driven on the street, that is the biggest aspect that shouldn't be overlooked. That... and suspension design (geometry and damper technology) has come ten fold in the past 15 years... so you don't need to run 850lb spring anymore to get the chassis to work at speed. Not to mention that there is suspension in the sidewall of the tire itself.

The new Vette and Camaro etc. all have moved to 19 and 20 inch wheels, and at least the Vette is considered one of the finest handling production cars in the world?

Yes... it is. Without question.

Is this just "Old" reasoning, that is now considered out of date or is there some factual reasoning for this that I have overlooked?

Old theory... yes. Newer technology is that good.

Does Staggering the size have any effect one way or the other on the handing of the car, (i.e.. 19 inch in the rear, and 18 inch in the front), or is it just strictly for looks?

One idea is that the smaller front tire and wheel are effected less by gyroscopic effects from the movement of the package weight. This would effect the transitional steering input less. Interesting thought...

I have noticed that many great performing racing cars have a taller, wider tire on the rear and a slightly shorter and thinner tire in the front. (IRL, F1)

Is there any good reason for having the rear of the car higher in the air than the front for a road racing application?

No. Other than an aero package that would utilize the possible down-force created... I don't really believe that there is anything viable.

Is it a function of aerodynamics, while under heavy acceleration and "At Speed" running, that the down force will adjust the downward pitch of the car, so while running normally it will move into a nice level stable attitude?

Uh... no, I don't believe so.

I had heard that depending on the front to back weight ratio of your car, that in some cases this weight differential helps dictate the width of your tires. (i.e.. A car that is heavier in the front than the rear will handle better with a wider tire up front as to help keep under-steer to a minimum. While a car with more weight in the back will benefit from a wider rear tire to help reduce over-steer. Is this correct?

Before answering, the drive wheel/wheels should also favor a wide wheel as well.

For instance. A 68 Camaro with 55% of it's weight to the front/ 45% of it's weight to the rear, AND being a Rear wheel drive car. Would benefit from having wide tires in the rear, say 315's (Due to rear wheel drive) and would handle better with front tires being somewhat similar in size as the rear tires, say 295's because the front wheels are having to work harder at changing the direction of the car due to the front being heavier than the rear. (in other words a natural tendency to try and under-steer)

This one, really needs to be laid to rest. A wider tire doesn't do as much for forward traction as it is speculated. Which is why I generally laugh when people stuff 315's to 345's on the back of a first gen with and 235 to 255 on the front. All that does is induce handling problems... mainly, as you stated, under-steer.

A wider tire helps with lateral traction... and a taller tire helps with longitudinal traction. The tires used in per-say... Top Fuel classes... isnt used because its wide, its used because the rotational inertia of the tire itself makes it grow in diameter... which means a longer foot print and an increase in forward traction.

This would be one of the reasons, other than weight and aerodynamics, why Drag racers run skinnys up front and as wide as possible in the rear. (Given of course it is rear wheel drive) Changing direction is not an issue so under-steer is also a non issue.

Whereas, a rear wheel drive, rear engine car while having 315's in the rear, AND having a weight bias also in the rear could get away with wide tires in the rear and substantially thinner tires up front say 215's without having an undue challenge of dealing with car induced over-steer and under-steer.

Is this a correct thought all other influences being static?

Yes... all we are trying to do is control weight and the transfer of it. The hard part is being able to handle it in all directions.

Do larger diameter tires as a rule run cooler than smaller diameter tires because the friction of the tread against the pavement is spread out over a larger surface?

Like compounds and construction ?? Kinda of. If the tire is larger in any aspect, there is more mass to absorb and retain heat. But on the flip side... the taller tire will have timeoff of the surface and will have more time to cool (in theory).

This is still open for debate. I did some tire testing for Goodyear some time ago at Talladega where we ran a Corvette around the track for a 24 hours (to an endurance record... that still stands to this day). Anyway... we mounted three infared cameras in the leading and three on trailing... and also had a camera focused on the sidewall. It was an interesting study to say the least. Although we were looking specifically at 275, 295 and 315 section widths in the 17" diameter... I am sure the data would cover other diameters as well.

I just wish I could have done the same kind of testing on their drag race brothern. It is somewhat more difficult... to say the very least.

I'm sure this is blatantly obvious to most people, but I wanted to clear this up in my mind while giving some thoughts to some project issues.

Thanks for your patience.

Best Regards,

Ty O'Neal

[byndbad914]

Originally Posted by chicane

Is this just "Old" reasoning, that is now considered out of date or is there some factual reasoning for this that I have overlooked?

Old theory... yes. Newer technology is that good.

That is too vague to arbitrarily be true IMO. Take a magnesium center and carbon fiber barrel and even so, a 16" wheel is less unsprung weight and easier to both accel and brake from an inertial standpoint than an 18". Or use a 14" diam if the brakes can be small enough to work and fit. Only reason for big wheels is to fit bigger brakes from a performance standpoint that I am aware of. A true race car can be geared to work for any given track and any tire size including small diams.

So, again, if you don't need 14" rotors you don't need big wheels, and 98% of the people that would buy a Vette will never really need the big rotors even in a racing situation. You mention below working with a record setting Vette, so maybe you (or the driver if you aren't the driver) can actually put so much heat into the brakes you need the large diam for thermal issues. Even then, I would ask if you have ever taken that car and tried a smaller, cryogenic'ed rotor (is that a word? hahaha) and good pad and found you overcooked 'em? If so, then you need to run the big brakes and therefore big wheels.

Originally Posted by chicane

A wider tire helps with lateral traction... and a taller tire helps with longitudinal traction. The tires used in per-say... Top Fuel classes... isnt used because its wide, its used because the rotational inertia of the tire itself makes it grow in diameter... which means a longer foot print and an increase in forward traction.

Not disagreeing from a drag standpoint other than a taller tire doesn't "help" with traction as much as the inertia hinders acceleration. In fact, contact patch is what creates traction period, so wider is better. Watch a TF car launch, the launch is at full width and the growth as rotational velocity increases is a byproduct of the sidewall design that is necessary to reduce launch shock by wrinkling. The car is also so overpowered the transmissions have stacked clutches that slip the entire length of the track and are replaced each round, but if they could keep the tire from growing they could put more power down with the wider contact patch. They have all sorts of clutch designs of course to work with that byproduct. Also note they still use 15" diam rims because the taller the sidewall, the more wrinkle and shock absorption. Of course tire size becomes class limited too. But anyway, I used 28" diam drag slicks on a 68 Mustang because they would fit inside the wheelwells and it had a 2spd powerglide, so the tall tire helped the big end at 9000rpm thru the traps. I would have ran 26" but then to get out the backdoor, the rpm drop was too great between the gears and the car was slightly faster with the large tire and optimal rpm drop in the shift.

From a Lateral g standpoint, you want smaller sidewalls and less wrinkle for overall performance. Smaller diams are easier to accelerate and brake. 'Course, use a 23" diam overall tire on a 69 Camaro and it will look like butt and there is something to be said about liking the way your performance car looks. But, I say get the small tires for the track and the larger ones for the street. I don't care what it looks like as long as it is fast

Originally Posted by chicane

This is still open for debate. I did some tire testing for Goodyear some time ago at Talladega where we ran a Corvette around the track for a 24 hours (to an endurance record... that still stands to this day). Anyway... we mounted three infared cameras in the leading and three on trailing... and also had a camera focused on the sidewall. It was an interesting study to say the least. Although we were looking specifically at 275, 295 and 315 section widths in the 17" diameter... I am sure the data would cover other diameters as well.

Very cool, what was the overall findings of the test??

edit - the above TF stuff is what I understand to be the issues, not necessarily gospel so I am open to facts that change that. Also, the growing tire adds extra "gear" that is beneficial.

Also, thanks XcYZ for the kind words, forgot to say that! Kinda shows why I am more of a lurker than poster since it ain't a Lateral G, but my favorite car to this day is Big Red and love the car culture it started, so here I am