I've been studying threads on wheels & tires and think I've got a handle on the whole offset issue and my options. Most threads seem to focus on "will it fit", "how will it look", or "this is my favorite tire/wheel" but I'm not getting a good sense of how the various options will translate into DD street performance.

Background:

stock '87 with automatic. (see newbie post in intro section)

Tires are old:
GY Eagle ZR50 P255/50ZR16 on front with a 1995 date code
GY Eagle GT+4 P255/50R16 on rear probably older (date code facing in)

Issues so far:

Rear end is scary loose in wet weather. I'm used to this having driven small pickups since the early '80s but wasn't expecting it in the Vette. I just drive my s10 crew cab on rainy days now---I can practice banjo in the back of the cab at lunch and it doesn't break loose as easily. (Not sure it's even legal to pack a banjo in a Vette. )

Rear traction may be less than ideal even in dry conditions. It seems to corner fine but I stuck my foot into it for grins recently and was surprised by quite a fishtail at the 2nd or 3rd gear shift.

I'm not looking to race or increase power in the near future. I pretty much stick to the speed limits, but do like to put my foot down occasionally when it seems safe to do so.

Options as I see 'em:

• Stick with the 16" wheels and hope to find some BF tires or decent alternative.
  
• Upgrade to 17" 9.5 wide rims and go for 275 tire widths on all 4 corners.
  
• Upgrade to 17" with 9.5/275 up front and 11/315 in the back.

So, wadya say?

Is my loose rear end an artifact of the old tires, resolvable by new rubber on my existing rims?

Is a switch to 9.5/275 going to do the trick?

Is 11/315 in the rear overkill?

I want to be able to launch this puppy safely.

 

First thing I'd do is get new tires. If those are even close to being 15 yrs old, then you run a high risk of tire failure.

Tires get old and can become brittle as they age. Once you get a new tire, they should hook up much better.

As far as diameters and tire sizes, that'd be your choice. I like the tire to be the same all around for rotation purposes, but the wide tires look better.

 

Old tires are, well, scary at best. Downright dangerous at worst. With new 275's in the back with the manual in my 88, yes I can make it get stupid, but you have to try... a little. Add in the fact that my alignment has positive rear camber with toe in, and a ton of negative camber up front with so much toe out it's halfway around a turn before you use the wheel.

I think you would be very happy with 17x9.5's and 275's all around with new rubber.

 

Tires are the single most effective thing you can install on any car to affect handling characteristics. If your tires have 1995 dates on them, you should not be driving on them as they are dangerous. They could experience a catastrophic failure or simply fail to provide the level of traction you are expecting in a given situation causing you to crash.

I road raced at many road courses in the SCCA up and down the east coast in the early 1990's in open wheel formula cars and Nissan's and learned a lot about tires/suspension. Just the number of heat cycles (heating up and cooling off) a racing tire experiences over a given period of time greatly changes the amount of adhesion.

Get a set of new tires asap!

 

Thanks for your advice. New tires and wheels on order from America's Tire Company
:

4 (38597) BF Goodrich
275/40ZR-17 98W BSW
g-Force Super Sport A/S

4 (23133) Replica Wheel
17X9.5 5-120.65 38S
ZR1 Replica

Got the no-sales-tax promo (real money in CA). Installation Friday.

 

You will be so much happier with the way your car acts.

 

Man are you going to notice a difference. In both looks and driveability.

 

Looks like you have the tire and wheel issue solved with a good selection for a tire and ZR-1 style wheels. The dimensions of the 17" tire will be nearly identical to the original 16" 255/50 tire in terms of diameter and tread width. Getting the 38mm offset will insure that the wheel will sit "correctly" in the wheel well and there will not be an issue with rubbing.

But you might also think about the condition of the shocks. How old are they and what brand? Worn shock will affect ride and will also have a negative effect on handling especially with "spirited" driving. Same with alignment; it would definitely be a good idea to have the car properly aligned when the tires are installed.

Find a good shop that knows how to do a 4-wheel alignment properly on Corvettes. Not that there are any "tricks" but making sure the alignment specs are correct for the car. Most newer alignment equipment have printers that will provide the before and after settings.

 

Good points c4cruiser, thanks.

I'd thought about a 285 wide tire, which would have been an exact diameter match according to the tire calculators I've seen. But 9.5 is the minimum width for those tires and I worried that would translate to handling or tire wear issues. (sorry for repeating something I posted yesterday in another thread). I'd love feedback on this point.

After buying it I gave myself a 1 month ignorance-is-bliss period then took it in to my mechanic for a thorough check up---he wouldn't let me have it back for almost a week! I specifically asked about the running gear---tie rod ends, shocks, anything rubber---& he assured me all was fine. It came with the FG3 Delco/Bilstein shocks (I have the orignal purchase paperwork) but I haven't peeked to see what's there now.

As for alignment, well...I was mulling over what cost adders might come up when I actually go in Friday and I remembered...alignment! Not the time to scrimp though. I'll ask if they can provide a before/after printout.

 

Beginning in '93, 285/40 tires were installed on 17x9.5 wheels for the rear of the C4. So a 285 tire would fit that wheel but going with different sizes would keep you from rotating the tires for longer tread life. Later C4's with the Z07 performance suspension got 17x9.5 wheels and 275/40 tires all around.

The 87 had 3 different shock types based on the suspension package. The base FE1 suspension got an AC/Delco shock that I think was black. The Z52 Sport suspension got a Bilstein "sport" shock and the Z51 performance suspension got a specific HD shock for that package only. Z52 was available for both the coupe and convertible while the Z51 was coupe only.

Both Z51 and Z52 cars got 16x9.5" wheels.

For alignment, if you go to this page and select "Alignment Specs" you can see some good alignment specs. I use the "Advanced Street" setting for my 87 for autocross and track use but it's still easy to drive on the street.

Also check out this page for some information on the suspension for C4's.

 

Thanks again.

I'll study those links & try to post some pics next weekend.

 

So far so good. Seems to be more sure footed. Definitely stuck on one curve I almost always slip on when it's wet. I'll have to live with them a while to be sure---I'm not going to go out of my way to break 'em loose.

I'll post Pics this weekend.

 

I contacted the people at www.factoryreproductions.com and got a set of chrome zr1 wheels for my 87 Vette, 17x9 on the front and 17x11 on the rear. The off sets that they offer allow the wheel and tire combo to fit perfectly under the cars fenders. Wheels, tires, lug nuts and locking lugs ran me about $1400 two years ago. They shipped them to my door and i put them on and been enjoying it ever since.

thanks
David

 

No fiddle lesson today so, after sending the AX7000 back from whence it came, I drove down to the bay & took some pics: