Okay now I have a serious chassis and tire Question.

[syborg tt]

Okay now I have a serious chassis and tire Question.

If you look at all the new late model super cars they have much smaller wheel & tire combo then the older muscle cars that are being built on this
site and other site. I know the philosphy that bigger is better but at what point is it overkill. If you have that much unsprung weight on a car you have
to increase the power of the engine and increase the brake size to over come said weight. I am just trying to figure out when a 12" Tire became small.

BTW I am not a suspension guy so I really looking for wisdon on this.

Here is the only example I can give you.

Joe has a car with 500hp an John has the same "Exact" car with 1200hp. Both of them have the same exact problem - Traction.

If the reason for traction is acceleration then it makes sense. But if you look at a Lotus Exige that car has tires 1/3 the size of what people here are putting on there cars.

[arue333]

great question, anybody have an answer besides production costs?

[hectore3]

So much goes into this equation that simply tire size is one factor. The tread wear rating,alignment,chassis stiffness etc play all into this along with ego. The Nissan GT-R has only 285MM and 255MM tires respectively and touches near or exceeds 1G in cornering force. But like I said just to take tire size in consideration doesn't tell the whole picture. Others more knowledgeable than myself will hopefully chime in on this subject.

[parsonsj]

Marty, I'm not sure I follow. My Z06 has a 335 / 295 combination. II Much ran a 315 / 265 combination.

[syborg tt]

Hi John,

I agree the vette has large tires. But more specifically look at the imports. They are doing the same with much smaller tires. AS stated above the Nissan GTR, Mazda RX?, Supra, Lotus. It seems that American Cars are into the big is better theory.

ps - I know the Lambo has huge wheels in the rear however the fronts are not nearly as big.

I quess I get the bigger front tire to gain traction in the corners. But how big do you really need and where is the point of overkill. Could the exact same be done with a 275 front & 295 rear. If the car was actually set up correctly or is the fact that the older muscle cars flex so much that the tires need to overcompensate for the car weakness.

On the 2010 Camaro that I own I am going to switch to 275 / 295 mostly for looks.

[SLO_Z28]

Marty, I'm not sure I follow. My Z06 has a 335 / 295 combination. II Much ran a 315 / 265 combination.

And on the flip side to that, a lot of guys gain lap time with smaller diameter and skinnier tires in corvettes. The less the tire weights the more horsepower you have, and at a certain point the tire width is detrimental to handling.

[LowFast]

Which "sueprcars" are you refering to, as mentioned above most supercars or semi-supercars run very wide tires. The exception are those that are AWD as they do not suffer in the traction department as much. I would say that all RWD supercars try to cram as much rubber under the fenders as possible. Porsche runs a 315/325 size out back of its high HP models, Corvette, Lambo (also AWD), Bugatti (AWD), Viper, etc. Don't see a narrow tire in the bunch. A Lotus is a terrible comparison with old American Iron, it is basically the exact opposite of our cars, super light, relatively low HP, tiny. Tire compound has much more to do with grip than width. Some on this forum need the width to put down the power, others, like me, do it even if we do not "need" it because it looks killer to have big fatties out back.

[parsonsj]

Marty,

I'm with you now. There are lots of things to consider, but the three main factors are vehicle weight, front/rear balance, and driving wheels. For example, the GT-R is AWD, so it can share some of its forward acceleration traction needs with the front tires. The RX8 is much lighter (less than 3000 lbs), so it needs less tire than our typical 3500 lb vehicles. Same with Lotus vehicles, etc. One thing to notice is that all of them have wider tires in their motorsports editions.

In our front steer / rear drive combination, the limiting factor is the front tire. For lateral movements, the bigger the better. The practical limit for most of our cars is about 265 mm, with larger sizes possible with aftermarket sub-frames, fender flares, reduced turning radius, and the like. Something else to remember is that even though it's pretty easy to fit a much larger rear tire (e.g., 335 mm) with mini-tubs, that much rear tire actually causes handling problems. When the rear tires have a lot more traction than the front, the car will push and have sluggish turn-in.



Hope that helps.

[parsonsj]

a lot of guys gain lap time with smaller diameter and skinnier tires in corvettes.

Smaller diameter yes, but I've not heard anyone ever claim that less tire width (especially on the front) was helpful on the race track.

[syborg tt]

I agree big tires look good and are pretty darn close to the old pro-street look. Heck the 335 BFG is only a 1/2" narrower then the Pro-Street tires of the 80's. Some of you young guys won't remember that era of muscle cars.

AS for the cars in reference- see my above post about the imports ( post 5 ).

I quess in the end what is the main purpose of the car. I've ridden in a mini-tubbed leaf sprung 335 2nd gen car and to say the same quality as my old Toyota pick up with 44's on it would be an understatement. My goal is to build a car that rides and drives nice, but don't want the "walking" that goes with the big tires when your on crappy pavement.

I've compared a bunch of tires and found the the actual numbers on the side of the tire are worthless. I have put the 295 BFG next to 4 different 315 and it's the same width if not slightly wider. I even put the bfg 335 next to the mich pilot 345 and it the bfg was about an 1" wider. So the really question is there a magic width or contact patch. Say 10" front and 12" rear ???

[syborg tt]

Marty,

I'm with you now. There are lots of things to consider, but the three main factors are vehicle weight, front/rear balance, and driving wheels. For example, the GT-R is AWD, so it can share some of its forward acceleration traction needs with the front tires. The RX8 is much lighter (less than 3000 lbs), so it needs less tire than our typical 3500 lb vehicles. Same with Lotus vehicles, etc. One thing to notice is that all of them have wider tires in their motorsports editions.

In our front steer / rear drive combination, the limiting factor is the front tire. For lateral movements, the bigger the better. The practical limit for most of our cars is about 265 mm, with larger sizes possible with aftermarket sub-frames, fender flares, reduced turning radius, and the like. Something else to remember is that even though it's pretty easy to fit a much larger rear tire (e.g., 335 mm) with mini-tubs, that much rear tire actually causes handling problems. When the rear tires have a lot more traction than the front, the car will push and have sluggish turn-in.

Hope that helps.

yep that actually helps a bunch.

My goal is to have a great driver and still have fun when I need it.

[ace_xp2]

Yeah, an exige doesn't have the kind of torque output that can really take advantage of a 335 in the rear.
SLO, do you have any links to people that found speed on a corvette with smaller tires? The only time I would think that would happen is when the car has a power deficit such that the rolling mass slows it down. That, or the smaller tires have a way better compound, which given the crap the stock corvette rides on, isn't very hard to find.

Oh, maybe heat input can become a problem with lots of tire? I'd suppose that with little enough car and large enough tire it could take some work to get the tires up to temp. That could be viewed as detrimental on the street, but I would think that it would be easy enough to get the heat in on the track. Don't forget the GTR is made to be "all weather" and larger tires can make a very skittish rain car, and a very stuck snow car .

Wow this thread is moving fast! The only thing I would mention is that this is one of the advantages of a mid engine car, having less weight on the font tire means you can use less tire, which usually makes tram lining less of a problem and can let you get away with less power assist.

[Bryce]

Your question relates to the theory of diminishing returns. When does the tire/wheel combo become to heavy? When does the rolling resitance slow the car down at the topend without adding any more lateral grip?

So are you building a race car that needs to get the last 10% out of the package or a street car that needs to handle good or somewhere in between?

[SLO_Z28]

Yeah, an exige doesn't have the kind of torque output that can really take advantage of a 335 in the rear.
SLO, do you have any links to people that found speed on a corvette with smaller tires? The only time I would think that would happen is when the car has a power deficit such that the rolling mass slows it down. That, or the smaller tires have a way better compound, which given the crap the stock corvette rides on, isn't very hard to find.

Oh, maybe heat input can become a problem with lots of tire? I'd suppose that with little enough car and large enough tire it could take some work to get the tires up to temp. That could be viewed as detrimental on the street, but I would think that it would be easy enough to get the heat in on the track. Don't forget the GTR is made to be "all weather" and larger tires can make a very skittish rain car, and a very stuck snow car .

Wow this thread is moving fast! The only thing I would mention is that this is one of the advantages of a mid engine car, having less weight on the font tire means you can use less tire, which usually makes tram lining less of a problem and can let you get away with less power assist.

Ill have to ask my chassis building friend for the hard numbers, but he gained time at VIR going from the 315/17 to the 275/17 Hoosier A6, and both were massively faster than his 18's. This is only a consideration if youre building a 10/10ths race car though, and would me a moot point on the street.

[Roadbuster]

Something to consider is that the tires only come in certain sizes. Most of our cars have wheel wells that require a large diameter tire to fill them out. Its a trade off. Trying to get the right diameter and width with a wheel size that gives the look that you want and fits. So it comes down to the look you want, for some that is steamrollers in the rear.

It is also about proportion. I have 305/35R18 tires on the rear of my GTO and they look wide. I have 305 tires on my Yukon and they don't look as wide. Why? They are way taller since they are 305/40R22.

[syborg tt]

The reason I started this post is I was talking with a Race Car Chassis builder. Most of the cars he build are for long tracks not short tracks like autocross at Good Guys Events.

Long story short he is building of all things an s10 pick for a gentleman that has a 250,000 porsche race car. The tire size he is putting on the truck is 275 front and 295 rear. Engine is a 383, forgot to look at the trans. He is confident that with the suspension set up and scratch built chassis it will handle every bit as good as the owners Porsche race car if not better.

They are building it for fun to beat other high end cars. Nothing like getting spanked by a little S10 pick up. When I asked him why not go bigger he kinda laughed and said ,"Son bigger isn't alway better"

By the time he was done explaining everything to me it made more sense. I figure since all this guy does is actually build track cars he must know what he's talking about. I remember a VW van that was built with the same philosphy and it surely didn't have big tires

http://www.youtube.com/watch?v=0MLCuaPdPew

[Matt@BOS]

Great discussion here. I know with our cars, (as everyone before me has said) the big front tires are a bit of a compromise to help even out the big rear tires, which create a ton of understeer on our typically front heavy cars. People have mentioned Porsches having wide rears, but they're a completely different animal since they have more weight over the rear, and don't need that much tire up front.

I also saw an interesting clip of an interview with SVTs Jamal Hameedi (don't quote me on the spelling) discussing why the new 650 horse GT500 still had 285 tires on the back. He said that their primary concern was to create as balanced of a car as possible that had decent turn in, and road feel. He stated that they decided to revise all of the gearing in the car, to get as much usable torque at the wheels on a 265,285 combo as possible, rather than go with a 285, 305, or 295, 315 combo. He tried to explain the benefits of the smaller tire with regard to crisp turn in feel, feedback to the driver, and also everyday driveability i.e. tracking on the highway. It was really apparent in the interview that he was doing his best to simplify his explanation so the average person could understand. I'd like to know more as well, but I'm afraid without a really detailed explanation most of it would be lost on me.

Matt

[rfrankb4]

Not going to say that is impossible but it is going to be very hard for a solo builder to match the engineering hours and expertise that goes into any of the non-budget porches. I have no doubt he could probably build a car that was better in one given area. But to make one all around better than a modern high end sports car is a tall challenge.

There was a thread here a while back on this point and the consensus was that even the most BA builds on here would be hard pressed to compete with those 250k production cars. That said the builds on here have a lot more cool factor but that does not get you around the track any faster.

[syborg tt]

Matt - seems the video you saw and the conversation I had with the chassis builder was one in the same.

In the end I agree this has been a great discusion and i'm still learning.

Heck last week Troy at Rad Rides was explaining the new rear suspension on there latest salt flat car. Very cool information.

I also saw an interesting clip of an interview with SVTs Jamal Hameedi (don't quote me on the spelling) discussing why the new 650 horse GT500 still had 285 tires on the back. He said that their primary concern was to create as balanced of a car as possible that had decent turn in, and road feel. He stated that they decided to revise all of the gearing in the car, to get as much usable torque at the wheels on a 265,285 combo as possible, rather than go with a 285, 305, or 295, 315 combo. He tried to explain the benefits of the smaller tire with regard to crisp turn in feel, feedback to the driver, and also everyday driveability i.e. tracking on the highway. It was really apparent in the interview that he was doing his best to simplify his explanation so the average person could understand. I'd like to know more as well, but I'm afraid without a really detailed explanation most of it would be lost on me.

Matt