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Is this just "Old" reasoning, that is now considered out of date or is there some factual reasoning for this that I have overlooked?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

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

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.

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.

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.

Originally Posted by byndbad914

hopefully it isn't considered lame here to post a thread from another forum, but a lot of talk on one of the Porsche forums about this:

http://forums.pelicanparts.com/showthread.php?t=386520

That is a racing forum discussing experiences with this stuff and tire types as well. THis thread starts with a link from another thread, so there are two there to read thru. THe linked thread has a few really competitive AX guys weighing in but keep in mind you need even less thermal mass in your braking system, so they run the smallest diam tires they can get in a good width for the quickest acceleration between turns. Anyway, fair amount of info there.

I freely admit all my opinions are based on what I understand things to be and am involved in these threads to hopefully learn along with the next guy... but here you will see for a typical racer large wheels have not proven out better and other threads have pointed to needing bigger brakes (sometimes because the driver is a bad driver ;) ) so therefore bigger wheels to cover. The late models are getting heavy, so 17" wheels start becoming the small size due to need for larger rotors at a fast track.

Admins - if it is rude to post a link elsewhere, I am cool with that and can delete this post if you want me to. I am here on your terms, not mine.....

Originally Posted by 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

Originally Posted by byndbad914

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).

Originally Posted by 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 :P 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!

Originally Posted by chicane

Answers, thoughts and ideas in bold...

Originally Posted by tyoneal

===================================
Rodger:

Thanks for your thoughts. I know you are right about our, "Toys", not being comparable to the, "Real Deal", however, the physics of what is going on is still the same and while I know on our cars we may never feel the difference, I really do enjoy learning what is going on, and try to separate the Facts of True Physics/Racing, from the Fiction of Fads/Bling.

Thanks again,

Ty

Originally Posted by tyoneal

===============================
Tim:

That's a killer 914. Did you stay with a Porsche V8? (It sounds like your running a Chevy, however the 928 (5.0) would be really trick in that car)

Ty

Originally Posted by tyoneal

=========================================
Tim:

Thanks for the post. Very informative and the personal experience is always the best type to get. There are a couple of things I wanted to bounce off you.

The current Car I'm messing with will weigh 3300 to 3800 lbs. realistically when finished unless I can go crazy with some CF or something. I have 17's on the car w/ 13 inch rotors and was thinking of putting 18's possibly later with a 14 inch rotor and 6 pot Caliper. I guess given the weight of the car this isn't to out of line. (I'm sure this is over kill anyway)

Originally Posted by ironworks

I bought this book a few months ago and it is will answer all your questions. And create even more.

http://www.amazon.com/Race-Car-Vehic...0411881&sr=8-1

Rodger

Originally Posted by byndbad914

This all really depends on what you are doing with the car. If you are going to mostly street drive it and go to the track to have some fun at a DE (Driver's Ed) or an AX, then it really doesn't matter what you do with the brakes. If you want to be able to say you have 14" rotors and shiny red 6 piston calipers showing thru the wheels, trust me, I have no problem with that and it looks damn good. Just understand, that 19/20 wheel thread is proof that G-machines are more about looks than performance with 98% of the cars out there in the category, and that is totally cool with me. Got no problem with that. THere are some high-end ones with tube chassis and so forth I know will never be actually "used" and that is fine.

So if you want your car to look like a G-machine v. a rattle can painted racer like mine :unibrow: I have no issues. My next project after I burn out on that will be the car I have also wanted to build for years now - based loosely on Big Red, which I fell in love with the first time I saw it in Hot Rod in high school. Closer to performance than street car, but it will probably have 17-18" wheels as that would fit the look, but full of over-the-top performance ideas not seen in a 1970 Mustang fastback yet. Like a detuned 500 cube Pro Stock engine around 850HP that is totally street legal.

On the other hand, if you are strictly performance oriented, then you should stick with what you have IMO and buy quality pads and run good, high temp brake fluid and flush regularly if you are going to track the car a lot. A 13" rotor with good calipers, quality pads and fresh fluid will haul down a 3800lb car all weekend long running four or five half-hour sessions each day, which is in road racing a damn good event to get that much time. And you won't need to replace the pads either assuming your "skilled" as a driver. Lower skilled drivers are hell on brakes period. High skilled drivers are tough on brakes as the more and more competitive you get, you will find there is a point where momentum can cost you a little time v. sometimes riding the brake a bit while staying on power a little (with a neutral to slight oversteer car of course) as you go thru a corner and then shoot out of it. Dumping the throttle is okay with an understeer car to get weight transfer, but a truly fast car is typically the ones with very slight oversteer characteristics (that is well documented BTW in various handling and setup books) so you can hold power on.

Just remember, 12" are all over NASCAR circuits getting the crap beat outta them every weekend. Even down to a SouthWest Tour (Late Models now) team I worked with running 100-150 lap races and that is more extreme than a weekend warrior at a road race day will get.


Here are my guesses at answers... first, it is cheaper to make one slightly larger rotor than two smaller ones. Second, pretty much any idea listed above may remove unsprung mass in one item, but you pick it back up elsewhere or even gain it. Example, if you ran a rotor inside of the wheel, you would need structure to get the rotor inside of the wheel and that is weight. Or you have a myriad of engineering issues associated with that, such as shear bending in fasteners if you offset with long bolts, or stresses in whatever "structure" you used between what was once a direct shear load path between the rotor and wheel studs, or just an extra caliper.

Mass moments of inertia and unsprung weight are not the end all be all by any means. So you're smarter to weigh benefits, and in hardcore LeMans racing for instance, the larger diam rotor provides benefits that outweigh the cost in unsprung weight and mass moment. So the Corvette race car has big wheels. Then the average Joe wants to buy a Vette, but one like the actual race car cuz it is a "performance" car. But the hot ticket in hot rods is going back to the hot rodder run 1" bigger rears, so design and sales team says you gotta have a 1" larger diam wheel and tire. So now the basic design has it and therefore the race car runs it (I have yet to hear any viable reason from any forum or internet research I have done for running a larger diam rear wheel as the rear braking is always less than the front so caliper clearance CAN'T be an issue).

I mentioned in one of the other posts that documents show how design teams argued with Duntov and "the other guy" that was a real engineer (I don't remember his name now, but Duntov's "competitor" in the company's early Vette design phases thru the early 60s). Duntov was more forgiving, but the other dude argued like hell with the designers as what they wanted based on market was counter-intuitive to what he wanted engineering-wise in the car. Design had power and Duntov bent better, so Duntov designs moved forward because he went with what design wanted v. a real engineer would want.

Fact is that on paper, bigger brakes "always" work better. But in the real world, if you don't NEED bigger brakes, then what seems obvious is you don't NEED bigger brakes hahah ;) So you don't need bigger wheels, bigger calipers, etc that add unsprung weight. Issue with unsprung weight is controlling it so the lighter you can get, the easier the suspension can control it and the less of everything you need to do so. Car gets slightly lighter yet and the suspension can focus more on handling v. controlling wheel motion.

Colin Chapman is a name I would become familiar with

http://en.wikipedia.org/wiki/Colin_Chapman

I dunno if Wiki mentions one of his adages, but when he worked with Lotus racing, one of his quotes was "when something breaks, make something else lighter". Brilliant way to look at it.

Also look into getting some Carroll Smith books (I borrowed about 5 from a friend that worked to design and built Indy cars while we went to college together - works for Honda now - and they were great primers). A lot of his info is "old" in that it is back in the 70s IIRC, but the tenants of what he discusses are still relevant. Even many of the newer books will cite him as a reference, but they just update with newer car pictures and examples.

edit - oh yeah, the Milliken and Milliken book Rodger mentions above was also one of the books my buddy had I borrowed a few years ago, so +1 on that as a good book.

Originally Posted by tyoneal

===================
Roger:

Thanks for the tip. I feel some interesting reading in my future.

How dry a read was it?

Ty

Originally Posted by ironworks

Lets just say you will not get through to quick. It is almost 900 pages and tells you more then you want to know. If it was about any other subject i probably would not read it, No scratch thatI would not even pick it up. I have been slowly working my way through it for a while. It is worth every penny. It is a great book. I heard about it from a builder on the east coast.

Rodger

Originally Posted by tyoneal

========================
Do you know if people who aren't professional racers with huge budgets and the like, really deal with true, "Ground Effects", engineering on their cars?