tim414

I have recently purchased a set of 360 Forged Wheels (Concave Mesh) in ZR1 sizes. They are supposed to arrive tomorrow with new tires already mounted.

I just read a thread which indicated the larger wheel size is NOT good for HPDE's/track, which IS something I plan to become more involved in.

Can someone enlighten me on the bennys of larger wheel size -vs- drawbacks of the larger size? I hate to believe I have made a multi-thou$and dollar mistake....in buying the 19x10/20x13 wheel size.

(My mods to start with were to be wheels/tires/suspension (got in on GB DRM Blitsens w/poly bushings ) followed by exhaust and cabin electronics.)

Someone please share your knowledge/insight on wheel sizes PLEASE....I hope I have not made a expensive mistake.....(I still have the factory 'speedline' wheels though!)

thanx

jschindler

Tim, I'm not sure what some of the track junkies will say, but I think the only real downsides would be that bigger wheels/tires add additional unsprung weight and rotating mass. In laymans terms, that simply means that handling, acceleration and braking could all suffer slightly. But in reality, I cannot imagine that you have anything to be concerned with.

I road raced motorcycles for a few years "back in the day". What I learned is that at an amateur level, way too many people spend too much time worrying about their vehicle having all the trick stuff to maximize lap times. Then someone goes blowing by you who has nowhere near the sophisticated equipment.

Most amateurs would be far better just making sure the vehicle is safe. Make sure everything is working, bolted down and safety wired, tires are inflated properly and the brakes are sound. Then worry about driving the car the way it is.

tim414

Thanks 'js'....I agree with your philosophy....that's why I chose the Invo tire rather than the Pilot Sport to start out with.....all the info I had read is that the money difference did not equal enough gain difference unless a highly skilled driver is in driver seat (which I am not).

Also the reason I chose to start with the particular mods wheel/tire/suspension parts.....

add: My thinking is that a wider 'foot print' in the rear would not be a bad thing. That's what lead me to look for better shocks too. These DRM Blitsens are supposed to allow the suspension to 'travel' over bumps rather than 'crash into' the bumps-allowing suspension to stay more stable and planted plus to adjust for the wider tire in rear.

GMuffley

The added weight/rotating mass is the only disadvantage as jschindler points out. Certainly not a major problem.

peterpiper

Performance tire selection is a bit smaller in those sizes. -pp

tim414

I think I had a choice of Pilot Sport, Invo, P-Zero.....

MitchAlsup

There are 3 effects: Rotational inertia, weight, and tire performance.

20" wheels have the metal farther from the hub than 19" wheels, in addition there are 20" of that metal for evey 19% of metal in the smaller rims. THus more rotational inertia--and mostly more actual weight than the smaller wheels.

When it come to tire performance you really do want some side wall height. The ALMS prototypes get away wiht really scrawny tire side walls because they need more room for the brakes and are willing to give up a triffling of tire performance to get the brakes to last the distance.

5 years ago we were arguing whether the best drag tires were of 17" or 18" sizes. A few years ago wer were arguing whether the circuit tires were better in 17" or 18" sizes. The tires have advanced since then and the argument is now whether the best comprimize is between 18" and 19". 20" in my opinon, is on the downward side of tire performance. {You will never notice the difference on the streets between 19" adn 20" unless you have Mario's butt.}

MOSPORT1

just had a track day last week at Mosport,,, one of the other Z06s running
had switched to the 19/20 invo combo,,, he wasn't very happy with the performance,,,,

I'm sure other guys with track experience will chime in,, you should have just invested that $$ in track time with an instructor,,,, the Z is more than capable in it's stock form. I'm not sure what your experience with the Z is but any novice is going to take a long time to find out what it can do. So keep it stock and learn to drive it first..

if anything,, upgrade your brakes,,, high temp fluid and stainless lines to start. Then upgrade your pads. So use your new set up for the street and keep the stock ones for the track. The GYs aren't as bad as some say,, they might not be great for street but the do pretty good on the track..

160KTS

The larger the wheels the more rotating mass you're incurring which does slow you down. but I think more than the heavyness of the wheels the issue is the rubber band of a tire you have to run (345/25/20?) which leaves very little room for flex in the sidewall and if you want to drag race you will never catch traction. Also your tire choices are so limited that you're spending a fortune for tires you probably won't be too impressed with for track use (INVOs)

tim414

Thanks. I have already decided that the factory speedline wheel will be 'off road' use, and as one guy suggested to keep these (360 Forged) for street use....I was thinking a wider foot print in the rear would not be a bad thing, but maybe it is.

AzDave47

The DRM Bilstein shocks are the best improvement you did for both street and HPDE's.

18" front/rear wheels give the widest choice of track tires, but I was able to pick up a set of GM Speedlines 18/19 at a good price from a forum vendor so I went that way. If you have limited track experience, you will be best off using high traction street tires and spending $ on driver instruction. On a 90 second lap, good instruction can reduce your times by 5-10 seconds and improve your driving safety. Moving to track tires should occur when you are going thru cheaper street tires faster than a set of track tires. As others have mentioned, the 19/20 combo drawback is primarily higher unsprung weight and rotational inertia. An expensive lightweight 19/20 forged combo will probably be lighter than a cheap set of 18/19's

Everett Ogilvie

I agree with pretty much everything you said and wrote a reply to this thread earlier today but did not post it because this question comes up often and sometimes causes a pi$$ing match - BUT, the technical reasons are indisputable, and if one wants to OPTIMIZE their track performance then 18s all the way around are still the way to go, because of the track tire selection and for the technical reasons you already stated.

ALMS Corvette race cars use 18s front and rear (the prior GT1 car and the current GT2 car). Grand AM DPs use 18s front and rear, and so do the GT cars. Even LMP1 and P2s still use 18s (I just re-checked the rules).

The new Michelin slicks, especially the 31/71/18 on the rear, are moving back towards increased sidewall height - for compliance and forward bite exiting corners - this is well documented in the Michelin catalog.

Most tires for the larger diameter wheels have very short sidewalls which hurts performance (no compliance), and if they DON'T have short sidewalls, they raise the height of the car - again bad for performance.

My comments above are about optimizing track performance via one's wheel/tire choices, but plenty of Z06 guys do very well on track with the stock 18/19 sizes, especially for occasional track days. It is when they become truly addicted and the car starts to become less of a street car and more of a track car, that they buy a set of 18" wheels (and Hoosier A6s or Michelin slicks).

TAILWAG

The stock OEM wheels are plenty...only some manufactures make wheels forth track...bbs, ccw, etc...you will carry more weight, rotational weight, wear your brakes a lot quicker and could probably risk bending those wheels too during track use. Keep them for the street and take your oem's for the track.

parsonsj

A very rough rule of thumb (analysis from Mazda Racing) is that each 1" increase in wheel diameter is equivalent to adding 50 lbs of weight to the vehicle. That's per wheel. Weight is the right way to think of the performance penalty since larger wheels affect acceleration and braking.

My Z sports 19/20 around town, but runs on the track with 18s on all four corners.

That's roughly equivalent to shedding 300 lb. for track use.

Admiral Ballsy

All else equal (and when is it, really?) the more rim/tire width and/or the shorter the sidewalls you run on the front, the 'twitchier' the steering, and the more tramlining you'll experience.

I've read a couple reviews that indicated that the ZR1 tramlines a lot more than the Z06/GS. I've never driven one, so I don't know personally.

ferravette

Interesting thread.

I bought my Z w 18s on the rear and was thinking to put 19s on to get back the performance i thought i lost. But after reading this thread, i think i will stay w 18s and just get some good tires. Any recommendations as to size and grip/brand. I am running about 600 RWHP. The rear rim is 12" wide. was thinking 345 30 r 18 (that should keep it close to stock height).
Thanks

Admiral Ballsy

Unloaded diameter of the stock 325/30s is about 26.7 inches. 345/30s on 18s would be smaller, about 26.2, and the effective diameter would be slightly smaller still since more sidewall generally means more static deflection under load.

I think the closest match would be 325/35s, but I have no idea if such a thing exists. That'd be just under 27.0, but would deflect a little more.

315/35 would be perfect but you'd be getting rather narrow for a 12-inch wheel.

ferravette

Thanks for the Reply,

I have a MT et Street on there now 345/35R 18 and when i put a ruler to it i am almost at 28" under the car's weight. That's a bit higher than the actual calculations (4.8+4.8+18=27.6). Checked w tire rack no325/35 size. I'll do a little more checking, but if you have not heard of the size, it probably does not exist. Looks like i don't have too many choices.

TapOut

I believe everyone has accurately answered your questions and concerns with the larger wheel sizing.....but no one has mentioned anything about the brand of wheel.

360's are big heavy street/show rims. All of their wheels are made for looks and not performance. That being said, most people keep their stockers as track wheels and throw on 360's etc for the street. The only wheels I would multipurpose would be CCW, HRE etc. And if you really wanted to track a 19/20 combo, or upgraded to the carbon brakes and required this setup....I would always go with the stock ZR1 wheels (Lighter than other 19/20 combos and tested by GM race engineers).

Rathmullan

RIGHT ON!!!! Unless the only thing you are interestd in is beating that CTS-V or Camaro ZL-1 etc. in the 1/8 mile off a stop light, take the money you put into mods to increase horsepower and torque and go take a course at Bob Bondurant's or Ron Fellow's driving school. Both use C6 Grand Sports. Learn to properly use the horsepower and incredible handling capabilities you have in your C6! GM doesn't give away driving school instruction with each ZR1 as a marketing gimmick. They do it to enhance the driver ownership experience and, most importantly, personal safety. Because 638hp will definately get you in trouble and 430 hp can as well.