Lateral-g Forums - Why are 17 and 18 inch Wheels Considered the best for Performance?

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Quote:

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

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Roger:

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

How dry a read was it?

Ty

Quote:

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.

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

I checked out the link you provided about Colin Chapman. It was very interesting and the associated links were excellent on chassis design, aerodynamics etc.

That was several hours of well spent time.:thumbsup:

Thanks a bunch for the tips.

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?

While I was in college I hung out with a bunch of aerospace engineers. For fun We built and flew a bunch of R/C planes. It is remarkable how much of the same technology is used in F1 Racing to stay on the ground, as it's used by the Air Force to get off the ground.

Couple of things I thought I'd bounce off you before signing off.

1) Since the New Corvettes are using the Carbon/Carbon Braking Technology, wouldn't you think the wheel diameter would get smaller, as the room needed for appropriate brakes would be shrinking, and thus taking advantage of the dreaded unsprung weight issues everyone tries to avoid.

2) Do you remember the blue, six wheeled, "ELF", cars of a few years back? They had 4 small front wheels. What was the purpose of having that configuration? The only things I could think of is that:

a) More steering wheels=better turning ability
b) More front wheels=Better Braking Ability
c) Smaller front wheels=Smaller frontal area (Less Drag)

For awhile at least I thought they were successful with that configuration. Was it outlawed because of success, dropped because of cost, or dropped because of performance issues?

Thanks again for your post.

Ty

Quote:

Originally Posted by tyoneal

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

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

How dry a read was it?

Ty

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

Quote:

Originally Posted by ironworks

The M&M book is - no lie - a text book. So it is about as dry as any engineering text is. With that said, dry is subjective - I am such a dork and so into cars that it was the most interesting book I have ever read, and ON MY OWN TIME, not part of school. As I mentioned, it was one of the books my friend that worked building Indy cars had and it is really the sh!t to get a formal understanding of vehicle dynamics.

We are both engineers, so the math makes sense to US. That might be hard to follow if not an engineer, but reality of engineering v. having a basic concept is that for what you will want to do with your car or understand is high level. You don't need to solve integrals ;) You can read the chapters, breeze past the math and still get very good conceptual knowledge.

Quote:

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?

Sure, some do. There are some crazy cars out at some of the road races I have been to in the past with money poured into under car skins, diffusers front and rear, wings of different shapes and so forth testing various config and taking data with $5K+ data logger setups. I personally haven't worked with many privateers that do that, mostly helping out other guys with similar cars and budgets to my own. But I have met a few rich dudes with plenty of money to spend messing around.

Quote:

While I was in college I hung out with a bunch of aerospace engineers. For fun We built and flew a bunch of R/C planes. It is remarkable how much of the same technology is used in F1 Racing to stay on the ground, as it's used by the Air Force to get off the ground.

Yep, you see more and more aircraft stuff hitting cars. "Wings" (airfoils, which on a car are inverted airfoils) are of course common, but now you see vortex generators on the rear roofline of Mitsus and so forth - next time you are on a plane look across the top rear of the wing and you will see the same shaped vortex generators to improve air separation. I am going to be putting a different shaped vortex generator across the rear of my roof this season to better bend the air down around the underside of the wing.

Quote:

Since the New Corvettes are using the Carbon/Carbon Braking Technology, wouldn't you think the wheel diameter would get smaller, as the room needed for appropriate brakes would be shrinking, and thus taking advantage of the dreaded unsprung weight issues everyone tries to avoid.

Not necessarily. Thing is a LeMans car will use all the brake it can get because you can gain speed with trail braking and so forth, that generates a ton of heat, so the more pad area you have to burn, and the more rotor area you have to dissipate the heat instead of transfer it into the caliper and fluid, the better.

The carbon tech is used for various reasons, but unsprung weight is actually a driver... since they need such a big rotor, the carbon is used to get the weight back down. So trust me, they are worried about unsprung weight in the racing world and are running bigger wheels to get around the brakes out of necessity. My major point is that if you aren't pro racing in ALMS or similar, you will never need monster brakes and big wheels, and in fact, will be slowing yourself down by carrying unnecessary weight around.

But the average Joe spending $100K for a Vette wants big wheels cuz they look cool and big brakes cuz they are cool, regardless of whether there is a benefit. I would bet at least 90% of the people that buy the ZR1 will never actually experience that car anywhere near its potential. A few hardcore guys might scrape together the bucks to do so... maybe.

Quote:

Do you remember the blue, six wheeled, "ELF", cars of a few years back? They had 4 small front wheels. What was the purpose of having that configuration? The only things I could think of is that:

a) More steering wheels=better turning ability
b) More front wheels=Better Braking Ability
c) Smaller front wheels=Smaller frontal area (Less Drag)

For awhile at least I thought they were successful with that configuration. Was it outlawed because of success, dropped because of cost, or dropped because of performance issues?

Thanks again for your post.

Ty

I do in fact remember those when I was a kid... Wikipedia is your friend :D

http://en.wikipedia.org/wiki/Tyrrell_P34

Easier to post a link than type out my own opinion :lol:

Tim:

"I do in fact remember those when I was a kid... Wikipedia is your friend

http://en.wikipedia.org/wiki/Tyrrell_P34"

Easier to post a link than type out my own opinion
__________________

You were doing just great until you said, "I do in fact remember those when I was a kid..."

...but then thinking back, I was a kid then too.

Your forgiven.

God time goes by quickly.

Ty

hahaha, yeah where did the time go.

My dad was 36 when I was born... I turned 36 last summer, looked in the mirror on my birthday and realized I was the exact SAME height, SAME weight, though a little behind but starting the balding process and essentially can take a baby picture and swear to God I look exactly like him... and thought WTF happened?????

I guess I just didn't realize how good looking my dad was :P :rofl: