The reason why someone was using 17" on the rear and 18" on the front was because you could get a wider tire out back without rubbing (or tubbing). With more sidewall in the rear, you'd be able to put the power down better. With the 18's in the front, you would have better turn in because of the lower profile. I just thought that this was a strange combination though.
I am going with stock Z06 wheels and Toyo T1-S tires in stock sizes for street and for when it rains at the track.
I have a set of CCW 18x11 and 18x12 wheels for 305 front and 335 rear Hoosiers for the track when it's dry. If I want I can also try many other brands on these wheels including Goodyear slicks, Michelin Pilot Sport Cups, Kumhos, etc. Lots of options.
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Brian A. Marks
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With the 18's in the front, you would have better turn in because of the lower profile
That combination also allows use of larger calipers on the front, ie: StopTech which only makes a front caliper for the C5/Z06. In addition the 17" rears permit using a wider variety of tire sizes, including those with taller sidewalls; this aids traction when going to throttle, as in corner exit, because of the more flexible sidewall.
In addition, the overall diameters, both front and rear, remain very close to stock given the right tires.
That combination also allows use of larger calipers on the front, ie: StopTech which only makes a front caliper for the C5/Z06. In addition the 17" rears permit using a wider variety of tire sizes, including those with taller sidewalls; this aids traction when going to throttle, as in corner exit, because of the more flexible sidewall.
I know.. It makes a lot of sense when you think about it. I just thought that it was odd at first having 18" up front and 17" in the rear.
I like Redshift's option too. That's more what I had in mind for track wheels and tires. I also considered 17's all around, but you'd run into problems if you went with a bigger brake package, as you noted No Doubt.
What kind of rubber is most plentiful if you're looking to buy used race rubber?
It basically depends on what the rules will allow you to do.
For example T1 is limited to stock size rums and tires or 315's on all 4 with 11" rims in the rear and 10" in the front.
However if you are not limited by regs you will first try to correct the factory design faults and enhance the Corvette's natural strengths.
Chevy designs a car for the street with understeer to be safer for the average driver. The car exhibits both static understeer and mid corner understeer.
The Corvette's best attribute is acceleration and straight line speed.
A larger tire in the rear will contribute to understeer.
There are a number of things you can do to decrease understeer, the easiest being to enlarge the front contact patch area relative to the rear. Use grippier tires ie softer compound. More negative camber, Neutral to a touch front Toe Out and skid pad testing to dial in tire inflation pressures. You also want to increase front steering response and modulation and increase turn in.
The old adages: larger is better, wider is better and lower is better hold true.
Running 18" wheels in the front is not odd, in fact the C6 will have them and the C5R's run them and most ASP & SM2 Solo guys run them.
Here is the current DOT applicable availability in tires from Hoosier for example in both A & R compounds in the S04's (S03's are close). (I didn't have a Kumho chart in front of me, sorry).
To me a perfect combination on a non-tubbed regular body would be:
305/30x18 front with a 315/35x17 rear.
On a tubbed and/or wide bodied car with higher HP I would run:
335/30x18 front and 335/35x17 rears.
Notice that all the tires are the same overall diameter, only the width changes.
The larger than stock tire in front equals the rear contact patch area and neutralizes the cars handling by reducing understeer, both static and mid-corner and helping turn in. The lower aspect ratio sidewall reduces flex and helps turn in, steering response and steering modulation. This allows later braking, higher corner entrance speed and overall cornering speed.
The rear higher aspect ratio helps the car to squat and aids in acceleration, traction and corner exit speed and allows greater driver use of oversteer for correction.
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Williams GT Engineering @ www.WilliamsGTEng.us
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AS DJ said, in SCCA T1, I'm limited to either stock or 17x10 front and 17x11 rear with 315/35-17 all around as my tire combo of choice.
Car is actually fairly well balanced with this combo. Lots of tire choices in these sizes as several manufacturers make these for SCCA T1 competition, namely Goodyear, Hoosier, Kumho.
A larger tire in the rear will contribute to understeer.
I thought that a larger tire out back contribute's to oversteer, not understeer?
I'm not limited to any kind of rules because I don't race the car. I just want the best combination of tires/wheels to go have fun with at DE's and make the most of it. Something that's going to give me the most options. I also want a balanced car. Everyone strives for neutrality, don't they?
I thought that a larger tire out back contribute's to oversteer, not understeer?
Generally, a wider tire will grip better, so a bigger tire in back will contribute to less oversteer (less loose)... or more understeer (more push) depending on how the car is set up.
Neutral handling may be an ideal, but it also may not be fastest around a particular track. An inexperienced DE driver may want a car that understeers slightly as it is generally safer. An experienced racer may want a car that is neutral to slightly loose. Further, how the track is laid out makes a difference as well. In a track with lots of high speed sweepers, I think I would prefer a neutral car. But in a track with slow speed corners leading to long important straights, I may want a car setup that allows me to get the power down better so I have a better exit speed. Trying to figure out which one is fastest is tough. Requires lots of testing and a good, consistent driver. I can't afford the former, and am not sure if I'm the latter.
In my opinion, like most street cars, the Z06 inherently understeers. But with 405 HP, power on oversteer is never far away.
Although the C6 has 18" fronts they also have 19" rears and the C5R's run 18" wheels all around. The important thing is the size of the tires themselves with width being only one of the factors. Specifically the relationship between front and rear widths as DJ alluded to.
When you increase rear grip in relation to the front you will increase understeer. If you decrease rear grip in relation to the front you increase oversteer. When talking about tires this relationship is achieved not only with tire widths but the tire compounds themselves as well. A stickier compound (typically softer) will have more grip than a harder compound. Hence a sticky front compound (in relation to the rears), as in most race tires, will have more front grip thereby lessening understeer. A harder rear tire compound (in relation to the front tires) will increase oversteer. For this reason it's usually a good idea to have both front and rear compounds the same or close to it. There are times when a slightly harder rear compound can be beneficial though. If the car has a lot of power and wheelspin is prevelant then a slightly harder rear compound can help balance things out. As it gets hotter it gets softer without becoming too soft like a soft compound might.
There are other factors involved in choosing tire compounds as well, such as tire wear. Generally speaking a harder compound will last longer, especially when the ambient temperature is high. In addition, tires get softer as they get hot. For this reason a tire that is too soft for the conditions will lose grip as it gets hot, or become "greasy".
Besides tires, suspension setups obviously influence a cars handling. For instance, if you want to decrease understeer you can soften the front spring rate, soften the front shock compression setting/s if the shocks have that capability or use a smaller swaybar, amoung other things. Likewise, if you want to decrease oversteer you can do the same to the rear suspension. Doing just the opposite, that is making things firmer, will increase understeer if on the front and oversteer if on the rear. A good rule of thumb is to work on the end of the car that is experiencing a problem. In other words don't stiffen up the rear in order to decrease understeer by inducing more oversteer.
Doh! I do realize that a wider tire out back would control oversteer by inducing understeer. That's why you see the huge back tires on the earlier 911's, because they were tail happy. That's also why you don't see wider rear tires on front wheel drive cars.
All of your explanations are great! I really appreciate it! It makes a lot more sense to me than the camshaft tutorial that No Doubt linked for me. That's one is still sinking in..
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