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.....

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Tim:

FWIW: I posted this thread Because I want to learn. If you think it would be helpful for posting an additional link, I'm grateful you took the time to do it.

That said, It's not my forum and like you, I wouldn't want to do anything to put someone's panties in a bunch, (I promise this is said without disrespect to the owners or Moderators. It is after all your forum and I respect that. It's just a figure of speech for the sake of communicating. No, "Visual", is intended or implied):unibrow:

Thanks Tim again for adding to this thread.

Regards,

Ty

Now to the rest of the posts.

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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

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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)

Anyway, regarding the unsprung mass issue. As mentioned earlier smaller and lighter Wheel-Tire Combinations are certainly the best for acceleration and deceleration. That said, instead of running larger brake rotors thus larger wheels in an effort to dealing with the braking issues mentioned above, why hasn't braking technology taken the route of keeping the smaller wheel tire combinations that are more advantages, and either running the rotors outside of the wheels, (So a larger rotor could be used with a smaller wheel), or running a couple of smaller rotors and calipers per wheel? I realize that some important design changes would have to be made to the current steering and suspension designs, but clutch Technology has gone to multiple smaller disc. as well to handle greater amounts or power. In this way both are dealing with similar rotational forces.

I would think that the heat buildup that is a constant battle to maintaining good brakes, might be able to be handled in a more efficient manner dealing with smaller lighter rotors. (Perhaps?)

Anyway, thanks again for chiming in and helping to "Learn" me.

Best Regards,

Ty

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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

Chicane:

Could you elaborate on some of the findings with regards and observations to the studies on the tires?

Thanks,

Ty

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

Busy at work today so I will give my thoughts are the more elaborate question above later tonight at home. That said, the car has a Chevy. 525HP out of essentially 350cubes pump gas, 11:1 CR long rod motor to better deal with the constant high rpms.

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.

just throwing a little more real world findings out there from one of the racing forums I am also on. This guy has been AXing and road racing his 4-cyl car and, in the above thread link there are a couple AX guys I mentioned that also chimed in about smaller wheels. He picked up what some of us have been dropping and here is his response:

So he went down in wheel and tire diam and went 5mph faster in top speed. He mentions a low torque motor, and the little 4-bangers are, but that said, this is just additional proof that larger wheels take more to accelerate, so regardless of the power, it takes more of it to get bigger wheels going.

Funny that I am seeing a fair amount of reconsideration in the racing circles for going back to a smaller wheel and tire combo and picking up speed and overall faster lap times.