What size tire to pick or when is the tire overkill..

[matty b]

Hey I guess this is more of a theoretical question that I already know relies on multiple variables but what is the best way to determine how much tire you actually need? I guess Im asking this cause there seems that tire size options fall into three distinct categories being: TOO SMALL, TOO LARGE and THE GREY AREA. I know you cant really expect a 225 width tire to handle on a 3500 lbs car and for all but the extreme a 335 is probably really pushing it as far as when the tire is making up for a lack of something other than traction. Where Im puzzled is seeing some builds and what would appear to be a rather small tire for a very high skid pad number pops up and Im left scratching my head. Art Morrisons Bel Air is what made me wonder since it has a 285 tire on it and came real close to hitting 1 g on the skid pad. For the size of that car I just cant get over thinking that it would need a lot more tire under there. This affects me cause Im in the very early stages of my build and my rear tire choice could potentially cost alot of money and time, both I would rather save if I can help it. So the question lies in being what do I pick? Is this a situation where you kinda guess and hope for the best or is there any kind of formula I can use to figure out a ball park as far as what tire width I should pick? This is fairly important cause I more or less get one shot at buying rims and building a rear end considering my budget isnt really allowing for major do overs. Any help on the subject would be awesome and Im looking for more of general rules of thumb than a specific tire for my car. I say that cause I have no idea how much power my engine is gonna make nor do I know what the final weight or other things like that affect the decision. I do know im going to run a 3 link rear end and coilover front and rear, beyond that nothing else has been spec'd. Let the discussion begin!!

[CurtiSS 69]

There is more to it that just tire size, as you suspect. Tire compound and how the suspension "uses" of the tires makes a huge difference.

Tire Compounds: R-compound -softer- tires such as BF Goodrich R1's will deliver the grip and you may not need to go very wide -for instance 235/60-15-to get very good skid pad numbers. On the other hand if you got 335/35-20's that had a treadwear rating of 700 they could not match the performance of the narrower R1's. A few things on R-compound tires: they wear out very very fast, they need to be at operating temperature for maximum grip (you won't get there in normal driving), and the suspension needs to be up to the task to take advantage of them.

Suspension: How well does the car use the tires. Does the tire stay planted using its entire tread during hard cornering? Tire temperature profile after track use will tell you this. If your setup is correct you will see no more than a 10 degree variation from the outside to the inside of the tread. Then you will know your suspension system is using the tires well. Conversely, if the temperature is 50 degrees hotter on the outside than the inside you are not exploiting the available grip from the tires, and there is work to do. Ask a race guy about this.

If you are on the show circuit. Max it out! Fill the wheel wells as full as you can with tire, and avoid any unsightly gaps! Beware of maxing out the size for track use unless no scraping of any type is encountered in extreme maneuvers. It is very easy to cut down a tire in a track situation due to contact with the body, and the result may not be good for you or your car.

It all depends on your goals for the car Matt. This forum has a matrix on tire fit for different cars that is excellent. Use the info, as it is there for you.

Regards

CurtiSS 69

[matty b]

my project is involving new rear frame rails so the tire size FITTING isnt an issue. What Im trying to get my head around is whats the best way to come up with the correct tire width. I can assume that for my application a 315 will probably be the right size for me but then I was thinking about benefits of a smaller rim and tire such as reduced rolling resistance, unsprung weight, inertia of the wheel and tire slowing down acceleration and affecting braking. I usually try to copy factory cars that closely match my goals and do that. Im shooting to make 550-600 whp and want to be able to use it but I also dont want a pro street car with the shocks a foot apart because I chose such a wide tire. I know its a very open ended kind of question but I guess what Im asking is how do vehicle designers go about figuring what size tire they should use? is it trial and error or do they have a system for figure this out?

[fast Ed]

Look at some of the high-end German cars ... many of the 3,500+ lb. BMWs and Mercedes with 400+ hp are running on 235 to 255 section width tires, and doing just fine. Putting on the largest that fits the hole won't necessarily give you the best handling, but it will look cool anyway!!


cheers
Ed N.

[CurtiSS 69]

Matty tell us what you are going to do with the car. If you are going to track it, you want the most you can get from it, and you are building a custom car. Go with the stickiest, widest tires on the lightest forged wheels and flare the fenders out to clear it all. If you can't put flares on then you need to comprimise somewhere. To get the most out of your setup you need to tune, tune, and tune more to get tires, suspension, power delivery, and braking working well together. This is not a bolt it on and go scenario, and it not determined by a simple formula. If you are doing it for show or the street put the widest tires that will fit inside the body for the look and be happy with it because you car will look great.

Auto design engineers will generally get handed a specification from marketing on how much passenger and cargo space, what price, what look, what platform, and what performance level the car is to achieve. They will develop the car as a many separate teams and comprimise all over the place. For instance a 255 tire may be all they can fit comfortably and stay with the original cost effective platform. It's pretty complicated, and there is an enormous amount of testing and tuning to get to the final result.

You seem like you are ready for a book called "Tune To Win" by Carroll Smith which talks about race car development and tuning. Remember a lot of what he discusses applies to Formula and Cam Am cars. The book will give you an idea of what to do if you have a clean slate.

Good luck with your project.

Regards

CurtiSS 69

[matty b]

That makes sence as far as them just compromising on those aspects of the car. My plan is for the chevelle to be totally streetable and run it at Thunderhill, Infineon and Laguna Seca for fun not competetively. As far as looks, yes I would like a good looking car but Im definitely not building a full on 6 figure show car. Im tubbing the rear end so I can go as wide as I want in the back but with that much freedom I dont want to get greedy and put a tire on that will F..K up handling such as a car that understeers ect. I figured a 315 was a good choice cause it is whats on a porsche so the tires are readily available unlike some other wider sizes and they arent ungodly expensive (although they arent the cheapest either) plus they sell drag radials in that size if I want to go to Sac raceway for a little dragracing action. Ill take a look at that book, Ive heard it been talked about quite a bit and Im sure there are a few things I can learn from it. Im just in a new area of car building since all my earlier projects were drag cars where you just want to get as much tire under the car as possible. This is a balancing act which is a bit more complex to me (or maybe Im just making it that way) so I find myself asking questions such as this. So yeah on my car itll be a 315/35r18 so hopefully Ill be ok ;)

[CurtiSS 69]

Matty,

It sounds like you are in the Bay Area or by Sacramento. How long will be before finish your car? I'd like too see that car on the track. It sounds like a good one.

As far as the tire issue goes no matter what you put on you will have to tune it at the track.

Regards

CurtiSS 69

[streetk14]

I think there are a few factors that could effect your tire choices. A 315 is a good sized tire, and you really can't go wrong with that. I think you will want a 255-275mm in the front to help balance the car out.

Looking at German cars is a good idea since they are extremely well balanced and great handling cars. What you have to realize is that most of these cars (BMW comes to mind) have a perfect 50/50 weight distribution and less power than you are looking at. They don't need as much front tire because they are not nose-heavy.

The last generation M3 had a 225 in front and a 255 in back. It weights in close to some of our cars (3300 lbs I think). But this car only has 333 hp out of a 3.2 liter motor that didn't make much in the way of torque and has much less weight over the front tires than a Chevelle of the same weight.

I want as much rear tire as I can get on the street with close to 600 ft/lbs of torque. Wide tires also look great, IMO, as long as they are well matched. I have a 315/35/17 right now, but will be going to a 335/30/18 next. I will have a 275/35/18 in the front, but I have an aftermarket subframe that helps me fit larger tires.

Look at the Dodge Viper; that is a car that is along the same lines as what you are planning. Hope this helps a little.

Andy

[matty b]

that helps a ton and definitely makes alot of sense. I was looking at the M5 as far as tires go but never really thought about the viper but the more I think about it the more it makes sense that Id look at the viper instead. I was just trying to figure out when to call it good so I can move on with planning everything else out. Sounds like I will stick with the 315 and Ill consider the 335, problem with the 335 is that there are not that many tires to choose from in that size.