Is there such a thing as "too much tire"? With the widebody setup and even some people going BIGGER than factory widebody, is there a point where the tires get too big and defeat their purpose for handling, cornering and acceleration?

 

There's a 'sweet spot' somewhere depending upon your car's setup and what you do with the car. For me its more about performance but some Corvette owners seem to choose wheel/tire sizes based on appearance. For street driving I think the stock Z06 wheel/tire sizes are a good mix of qualities. Serious Corvette road racers (including the Corvette Racing C6.R's) use 18's all-around...not 19's...not 20s. Not only do 18" wheels give you a better selection of tires to choose from but IMO 18's help you attain a optimal (low, 'level') setup for the track. Many racers I know use 345's on the rear. On the front I've been using 305 tires on 11" rims but some friends have suggested I might want to try slighter narrower front tires. Personally, I'd just have to try smaller front tires to see how the car feels and what effect they have on my lap times.

I can't recall which race car it was but not so long ago there was one car running identical size wheels/tires on all 4 corners. I'm sure the car was extremely well balanced and the theory was they'd sacrifice a bit in a straight line but more than make it up in the corners. If you're a drag racer you want to run wider rear tires and 'narrow' front tires.

I'm no expert but IMO if the car is primarily used on the street I think the stock Z06 wheel/tire sizes offer a good mix of performance and 'roadability.' There is no one perfect wheel/tire size...it depends on what you do with the car, the car itself, your own driving style and what you want. Adjust accordingly.

 

Thanks for the input!b

I've been reading some car testing and I found it interesting that even with the widebody setup of the GS (which incorporates the wide Z06 stance), in some tests it could only pull off .96g -.98g.

Then in other tests the base coupe with Z51 has pulled .98g and even 1.00g. I know driver variances along with weather conditions all play a role in those numbers.

It made me think. Why can the Z06 pull off a constant 1.00g+ and the GS which has the same tire size cannot pull that number?

My conclusion is that weight (the GS weighs a couple hundred pounds more) and the HP/Torque of the GS is substantially less, therefore it cannot pull through the added loss it has due to rolling mass. The larger the tire, the more HP it takes to turn it and the more rolling resistance one gets.

Therefore, like you reiterated, there is a "sweet spot" where just enough tire, combined with suspension engineering, weight, HP, give the car its perfect cornering capability. Slapping huge tires on the car can actually slow it down and give it poorer handling capabilities.

 

Tire width isnt the only part of the equation. Alignment, shocks, springs, tire side wall, and sway bars contribute to the whole. In fact, a car running coil-overs with an aggressive wheel alignment, can run less tire and far out handle a car that has a more potent tire with stock shocks and stock alignment.

Straight line acceleration for cars running 9 sec passes almost never run a 345 tire out back. Most of them run a 275 wide tire to do it it because tire side wall rather than width is the primary traction component.

The C5R and C6R cars run 18" rims rather than 19" for optimum handling too.

Tire width is not as important to handling or straight line traction as is the comination of other factors.

 

optimal tire width can only be selected tracking the car and taking tire temps. If you aren't going to track the car, buy whatever looks good to you. If you are looking to optimize the car on the track, and without going into all the camber discussion, etc, suffice it to say that once you have that all the other variables at best settings, you will have an average distribution of temperatures across the tire, measured near the core (actually probing deep into the tire down near the chords). The tire mfrs have their own ranges, but for the sake of discussion say around 200F is an optimal tire temp for a race tire. If your average temp is lower than that, the tire is too wide. If it is hotter, you need more tire.

Disclaimer is that is super simplified, but the fundamental test of whether you need more or less tire is in the temps v. mfr recommended temp range.

 

Racecars have to follow rules, so the C6R probably uses 18's because they have too.

The ZR1 uses a 19"f - 20"r combo

The Acura LMP car had front & rear matching tires

 

All drivers/conditions identical, can a base C6 coupe with Z51 and good tires, run with a C6 Grand Sport? Or will the GS's wider stance and tires make a difference?

 

ALMS mandates 18" wheels for GT2

 

I would point to the the fact that the reason competition tires dont regularly come in 20's is 'probably' because they dont perform well. Try searching for competition tires in a 20" model.

The McClaren F1 uses 17's all around.
Ferarri Enzo uses 19's all around
Lamborghini Gallardo: 19

Since sidewall is so much of the equation for acceleration, there has to be a balance for traction vs handling. While it can be argued that sustained high speed may be better with lower profiles, racing that requires a wide range of speeds and thus acceleration would then need a compromise for average performance. Since the corvette's main issue is traction limited perfrmance when you mod, the median of 18 is the best compromise. I have driven cam only cars that cant hook in 1st or 2nd on a 305/20 100 compound (nitto) tire while a 305/18 100 compound nitto hooks to 500rwtq. 20" then goes the wrong way. I would imagine the 20" thing is for bling and would be the first thing I would get rid of if I obtained a ZR1.

I doubt the C6r would use a lower profile out back regardless of rules. Your assumption is just that though. Perhaps Lou Gigiotti will chime in to clarify as he builds a real race car and would know what it takes for all around performance on rim sizes.

In any event please dont say that because GM used 20's on a the ZR1 that it validates that they are the best for performance. I dont think I can handle "GM does it best" today.

 

Talk of rules limiting tire combos negates the whole conversation since you don't get to pick now, you have to set the car up to work with the stated tire combo, so not gonna bother with that.

But I am long winded

Without spinning this way out, what any real racer wants to do is minimize unsprung weight. With the uprights on the new Vettes, you can NOT go any smaller than 18" diam wheels, so that is the lower limit. Going with bigger wheels = more unsprung weight, so to me the smallest wheel that fits over the brakes (or for the Vette brakes and uprights) is the best wheel diam. Width then is selected by tire temps.

The C5s and C6s (and all sorts of "performance" cars - and yes that is in quotes) have bigger diam rears than fronts because that appeals to the average Joe for some reason. Bigger somehow now = better, so the ZR1 adds some more weight to each corner because guys want to say they have 19/20 combos - absolutely beyond me but GM is out to sell cars that perform great but mostly sell based on appeal, not necessarily performing to the maximum. That is why the Vette looks like it does and the aftermarket exists for those that want to maximize what they bought based on appeal. In guy terms, you got a pretty hot chick, but she could use a boob lift and drop a couple more pounds to be stunning

Looking at the Lambo stuff, the Gai and Murcs are for guys with big pockets, not necessarily track, but the Lambo Murc LP670-4SV uses 18s front and rear that would just barely fit over 15" rotors - so their top model they claim is oriented to racing uses the smallest wheel that fits their brake package.

I agree that on most cars, even old muscle cars, 18s all around look good. I do like 17s with a bit more sidewall on muscle cars, but 18s are maximum for any car IMHO. The only thing I don't like about my Vette is the way the tires look like they belong on a forklift, everything else just looks awesome. And does the typical owner ever need a 14-15" brake setup... NO 12" rotors with 4 piston calipers and good pads are more brake than 90% of drivers would ever need frankly.

I have 12" diam rotors and 4 piston Brembos on my race car as that is all it needs for my purposes - I used 16" wheels ONLY because I could get smaller diam tires (so shorter sidewall) and overall lower weight at wide widths v. the 15" even. Lower mass moment to accelerate and brake is a further bonus. Compared to an 18" combo for my car I save ~20-25 lbs PER CORNER in unsprung weight with that combo if you consider the larger tire weight, the larger wheel weight, the larger rotor weight, and the larger caliper weight. If you don't need it, that is the answer, don't add it.

Hence why selecting components for tracking isn't a simple consideration, and surprisingly, there is a lot that goes into selecting tire sizes when you are allowed to do so

Take this is coming from a guy that is a bit of a 1%-er, I don't DD the car and only make a change to maximize performance. If I put race wheels on my car down the road, I will consider 18x11 fronts and 18x13 rears with 315/345s. My tires would be Hoosiers given I drive the car periodically around town or to work and never in the rain, or on the track here and there. If I take a road trip, the stockers would go back on.

 

So GM sometimes takes the route of "bling" over functionality and performance. Eye candy.

I wonder if the Grand Sport widebody stance is more about bling than performance. I would be curious to see a back to back test with the same driver, same tire design, one on a 2008-2009 Z51 coupe and then with a 2010 GS. Which would truly handle better if driver, conditions and tire design were all the same.

I've read reviews where the GS only pulls .96g - .98g and then the Z51 base coupe is pulling .98g - 1.00g.

It would be interesting to see the results of said test. Is the GS widebody just more rolling resistance, on a heavier car?

 

My guess, and it is just that, a guess, is that since the GS is a heavier car, the slight gains in added with the wider tires probably doesn't make up for it, or at best breaks even. New Vettes have pretty wide tires regardless of model, so unless you prefer the looks of the wider body car like I do, then go with the narrower car IMHO and just run slicks on the track.

Since the two have the same engine, the lighter car will be better on the track since lap time is a collection of averages, so if the lighter car can corner 0.1mph faster in every turn, and slightly better HP/wt ratio means juuuust a bit higher speeds in the straights, the lighter car will win. So to answer your question, based on your scenario the lighter car should have a slight advantage in my guesstimation.

Now, if the two cars are wheel to wheel racing, and the GS can brake later due to better grip and larger brakes, then things get more interesting. HP being equal, even if the GS is sliiiiiightly slower thru the corner and on the straights, on a typical road course it won't matter because the GS will dive in deeper and pass into the turns, then simply block the other car that can actually carry a bit more speed thru the turn. In the straights, the GS beats it out of the turn, and even if the lighter Vette with that little bit more power/wt ratio can get up beside the GS, he just brakes deeper and maintains the lead at the next corner. Often, both cars run slower lap times than they would in an independent timed lap test because battling for position won't equate to perfect lines. But in my best guesstimation, in this scenario the GS has the advantage.

To end a bit tongue in cheek, the GS will win no matter what because the dry sump will keep the engine oiled and in one piece

It's all bench racing at this point, but a fun convo nonetheless

 

Not once did I say "GM does it best" Wayne mentioned 19's & 20"s - I was merely pointing to the fact that GM chose 19's & 20's for their flagship, not that it was actually better, you misinterperated. Production streetcars are one big compromise to make the most profit possible, by your post I think we agree?

A while back (many years ago) Tirerack did almost this exact test, but with their BMW 3 test cars. They used the same tire manufacture & widths, but used different wheel sizes. They started with 16's & a 215 IIRC & recorded lap times. They then put 17's, 18's & 19's on the car still with the same 215 width. The results were: 16's were slowest, 17's were faster, 18's were the fastest & the 19's were slower than the 18's.

There conclusion was, at least on that BMW 325, was that anything bigger than an 18" wheel was an exercise in diminishing returns. So basically what others above have said, bigger than 18 = show not go

BUT, in reality, most people buy wheels for show not go anyway.

 

Keep in mind that the results need to be taken with a grain of salt for flavor - using a different diam wheel doesn't say anything about the tire used... my point is this:

1. If they were all the same diameter, then the sidewall becomes a second variable (wheel size is one) and was taller on the 16" than the 18" and that will hurt performance. As noted before, shorter sidewall is better for the track (to a point, all things have limits). The tall sidewall tire would roll over and run on the sidewall v. optimal contact patch of the tire and howl and slip. This was very evident on my F150 Lightning pickup I used to have - I would track it for grins periodically and I would scrub down almost an inch on the sidewall

2. If the tires all had the same sidewall height, then the overall diam becomes the second variable and the overall final gear ratio is changing between the tires. For my race car, on one of the local short tracks I use the 23.5" diam rear tire as it nearly optimizes my gearing for the course without opening the trans. On the other track locally I run the 25.3" diam (note the 3 and 5 swap). The ~2" larger diam tire gets me just a few more mph at top rpm, which works better in the main straight (don't need to shift to go only 300ft more and downshift again) and I can get thru the esses in one gear as well and go deep into the next turn.

So, for #2 we could deduce my car would be slower with the 16" wheel and 23.5 tire v. an 18" wheel with 25.5" diam The unsprung weight wouldn't make enough difference v. having optimized overall gearing. I am certainly happy to have those two options and partially why I chose the combo I did for my race car.

True tests only vary one variable at a time, unfortunately that is WAY easier said than done, and magazines like easy articles and sales, not science, so they draw conclusions at will To only vary one of the variables (wheel diam) they would have had to hold sidewall height constant and re-gear the transmission each time they swapped overall diam.

 

Tirerack's test was an attempt to eliminate all variables except wheel size while keeping the proper BMW specified tire diameter for a 3 series. Sidewall flex/tire rollover, as you said in your truck reference, was the weak link, except in the 19's.

I wanted to reiterate their test here for discussion purposes, 325's & Z51/GS's aren’t both apples.

 

I hear you, it's all good, bench racing fun

 

Good post...I never thought of these other variables

 

That is the "manual based" GS has the dry sump. The autos do not have the dry sump.

What's your take on just normal street driving with occassional high-speed maneuvering. Is a dry sump needed?

 

You have a valid point. To me the GS is mainly an "appearance package", which is fine and dandy. It comes with the old Z51 option but is offset with the wider & heavier tires, along with the car itself being heavier.

Here is that same thought being expressed by a car reviewer:

"On hilly, twisty Kentucky roads, the Z06 couldn't hide its substantial, lane-filling width, which hindered the hustling cause and held back its lane-change speed, too"

I've been in cars that had narrower tires and the car felt more nimble and "zippy". With the GS, it felt big. If the driver felt this way in a Z06, how much more with a GS which weighs a couple hundred pounds more than a Z06 and doesn't have the same power to weight ratio.

 

Occasional high-speed maneuvering probably isn't going to tax the oiling system enough for the need for a dry sump. Even tracking the car as a beginner might not require it. It will depend on the track of course and what kind of Gs your pulling in the turns. A stock tired C6 probably wont be pulling enough Gs to starve the oil pick up. (GM recommends adding 1 extra quart of oil if you will be racing) It's more likely the stock tires will give before you need more oil pumping. Tracks with high banking turns, specifically left handers I believe, can possibly put you in the danger zone though. But again, this is dependant on tires and how fast you're entering.