Hi ya,

I'm taking possession of my vette tomorrow but started the task of desk engineering wheel choices now.

I'm thinking of running 17x8 19mm offset wheel and 245/45-17 tires on a 94 Convert. Now I know that goes against conventional wisdom, but I'm looking to reduce unsprung weight and rolling resistance while running a better tire (falken rt-615's) in the interest of improving fuel economy and acceleration other than off the line.

Doing the math on the new tire wheel combo vs. the oem tire/wheel combo, I come up with:

tire size - offset - rim size - distance from hub
255 - 56mm - 8.5"(216mm) - 71.5mm (oem front)
285 - 56mm - 9.5" (242mm) - 86.5mm (oem rear)
245 - 19mm - 8.0" (203mm) - 103.5mm (new setup)

So.. here's my two questions:

1. This change means the tire will stick out 1 1/4" further in the front and 5/8" further in the rear.
Is the new distance from the rotor face to the tire's outer edge going to be flush with or exceed the fender lip?

2. Will running a 19mm offset instead of the 56mm of the oem adversely affect steering geometry to the point of making the car more twitchy or cause the tires to scrub more on turns?

I know, pretty heady stuff, but since I'm the newbie here and working off theories alone, I wonder if anyone else has experience in playing with a similar setup.

Rob

 

OK I just saw on another recent post that people are running 315's on 17x11" rims, front and rear without issue.

Now If I recall there is two offsets for the 17x11 rim... 36mm and 50mm. Taking those into account, 121.5mm and 107.5mm are the rotor face to outer tire specs so I think I'll be safe not having the rims stick out of the wheel wells considering I'll be less than either of those setups.

I just wonder if they'll be flush...

 

With a 19mm front wheel offset with an 8 inch rim width, the outer edge of the wheel will stick out 18mm more or 0.7087 of an inch.

It'll drive fine but, will look bad.

Also, the 17x11 wheel with a 50mm offset will stick out 0.9843 of an inch and the 17x11 wheel with the 36mm offset will extend 1.535 of an inch. That's why the ZR1 rear quarter panels are extra wide and the GS uses wheel well flares.

 

Thanks for the info - I came to the conclusion that rim width plays a smaller role in the equation than the tire width does, so rim width as long as it's the ideal size for the tire being used is not needed for the comparison.

so, with calculator and my formula at hand, here's what I have for information.

formula:
tire width/2 - offset = tire distance from rotor face.

255/2 - 56 = 71.5mm (2 13/16") 1994 base front
275/2 - 56 = 81.5mm (3 3/16") 1994 z51 front/rear
285/2 - 56 = 86.5mm (3 7/16") 1994 base rear
315/2 - 36 = 121.5mm (4 3/4") 1994 zr1 rear
315/2 - 50 = 107.5mm (4 1/4") 1996 GS Cpe

my proposed tire/wheel combo:
245/2 - 19 = 103.5mm (4 1/16")

1 1/4" farther out than 1994 base 255's
7/8" farther out than 1994 z51 275's
11/16" farther out than 1994 base 285's
11/16" further in than 1994 ZR1 rear (wide body)
3/16" further in than 1996 GS Cpe. (flares)

now I've seen pics of newer non GS/ZR1 C4's with 315's on front and back here on this forum board and the fronts seem to fit under the wheelwell and the backs are barely there... perhaps flush to sticking out an 1/8" or so.

this leads me to believe that my proposed combo, with 3/16" less protrusion than a GS wheel, and a taller tire aspect ratio (tread part widens as the ratio numbers drop). A set of 315/35-17's have 12.4" of sidewall width and 11.5" of tread width where the same brand tire in 245/45-17 has 9.6" of sidewall width and 8.9" of tread width, meaning I'll be further in at the tread. I think I'm going to be ok.

Typically high positive offset wheels were a front wheel drive domain and rear drive cars ran little to no offset so I was wondering if the Corvette suspension was prone to bad steering issues if one reduced wheel offset.

Rob

 

not true...'section width' should be considered to prevent tire sidewall rub...'specified' section widths are as measured on a 'stated' rim width, and for all practical purposes, section width will change 1/2 the rim variation from that 'stated' rim.

btw, 'scrub' is not necessarily a bad thing...what may become a problem is added 'overhung load' on your wheel bearings, bearing failure (esp rears) is a burden for many c4 racers with tire centerlines/loading in stock location.

 

Are you confusing wheel offset with backspacing? Wheel offset is the distance from the hub mounting flange face to the centerline of the wheel. Backspacing is the distance from the flange face to the inside edge of the wheel.

While the two measurements have a relationship to each other, one can change more than the other as the width of the wheel increases. For example, if the center section is more towards the outside of the wheel rim, both values change but change the thickness of the mounting flange and it's still possible to have a wider wheel with little change in the backspace or offset values.

As far as the two versions of 17x11 wheels and the 315 tire, you can fit the "real" ZR-1 wheel and a 315 tire in the front of an 84-87 C4 The offset of the OEM 84-87 wheel and the ZR-1 wheel is all but the same (36 vs 38mm). The GS 11" wheel with a 50mm offset will fit but there could be some interference with rear suspension components and there could be some tire rubbing on the wheelwells.

A mounted wheel and tire should be checked for fit when you begin to experiment with sizes and dimensions that are way outside of known combinations.

The other thing to consider with running smaller tires than stock is that the handling characteristics of the car were designed with tires of a certain height and tread width. For example, the 84-87 cars used a 255/50-16 tire; the 88 and later cars had 275/40-17 tires. There is very little difference in tire diameter and tread width between these two tires.

When the Corvette engineers changed the front tire to a narrower 255/45 tire, it was done in part to solve complaints of the wider 275 tire "tracking" in the wear grooves in pavement caused by heavy usage. The narrower tire solved that problem along with slight changes in alignment values.

 

Yes, backspacing is a measurement regarding what the wheel is doing from the rear lip of the rim inward as opposed to offset which refers to the measurement of the wheel centerline to the mounting surface.

I do not like to use backspacing as a measurement because it leaves an open variable when doing calculations, which is the thickness of the rim to the tire mounting bead.

You can use backspacing to determine offset, but only if you know what the true width of the rim is.

wheel absolute width/2 - backspacing = offset

I think before I buy the rims, I'll be removing the wheels of the car and will measure the rotor face to wheel lip on both front and back wheel wells to determine for sure.

Rob

 

There's a chart at the Corvette Action Center for all C4's...http://www.corvetteactioncenter.com/...usp_chart.html

If you have an '87 or earlier, you can run the ZR1 rears and they fit PERFECT!

You can call John @ CCW...he can tell you what you need for the year model you have. When we did my race rims, he was dead on...running 12" x 18" Eagles on the rear and 11.5" x 18" on the fronts and they sit right where they should. I wish I remembered what the offsets were...been almost 10 years ago since I bought them, and I've slept once or twice since then :o

 

I meant for this comparison of checking tire overhang... thats why I put the mention of "ideal size". I realize that stretching a smaller tire on a larger rim is going to affect the sidewall measurement.

Thanks for the info on scrub not being a bad thing, I am concerned about making the car an understeering pig from my wheel/tire change.

As for the offset and bearing failure, I think by reducing the unsprung weight of the wheel and tire, and still running some sort of positive offset on the new wheel, I hopefully will not increase stress loads on the rear bearings.

 

There's a Corvette convertible in my driveway! Woo Hoo! Just brought the beast home so the armchair work subsides and the real work ensues.

The car currently has the 255/45-17 fronts and 285/40-17 rears on 8.5 and 9.5 chrome oem wheels. I took a plumb and hung it from the wheel well to see what fender clearance I currently have to the sidewall of the tires and found the front to be 1/2" and the rear to be 1/16".

A little quick math says I have 84mm (3 5/16") in the front and 88mm (3 1/2") in the back. Hmmm. Looks like I'll need to rethink my wheel selection.

Thank you though for all your help - I'll have to get the 45mm offset rim in 9" in order to make it look presentable.

 

285/40/17 is not the stock rear tire size for a C4 Vette.

The stock OEM Rear tire and wheel size for your 94 Vette is 275/40/17 on a 9.5" rear wheel.

 

I believe you'll find the 285/40 a "fit" and an OE tire for the '94 C4 on the 17 X 9.5 wheel.

 

Explain your math!

Your comment regarding "actual width" when calculating offset is correct BUT using +1" will get you very/very close and is the "accepted" rule. All of my wheels (several sets) are between + 15/16" and +1".

 

I have the original window sticker to the car and it says 285/40-17 on it, and that's whats presently on the car. It is the oem size for a 9.5" wheel when equipped with the 8.5" wheel on the front (base models).

 

front:
255mm/2 - 56mm = 71.5mm
71.5mm + 12.7mm = 84.2mm (3 5/16")
(12.7mm or 1/2" distance from tire to wheel well lip)

rear:
285mm/2 - 56mm = 86.5mm
86.5mm + 1.6mm = 88.1mm (3 1/2")
(1.6mm or 1/16" distance from tire to wheel well lip)

As for the 45mm offset comment, that's the alternative size available in the torque thrust wheels I want. If I use a 255 or 275 tire it will work out to 82.5mm (255/2 - 45 = 82.5mm) or 92.5mm (275/2 - 45 = 92.5mm).

Rob

 

My bad, the 285/40/17 tire change started on the 93 Vettes.

Since your goal is to reduce unsprung weight, installing Heavier 17x11 and Heavier 315 rear tires which you were considering in your posts above, will have a negative result.

 

both f/r wheels experience significant 'camber gain' during jounce, you gain clearance at the fender opening (and lose inside).

check recent f/s threads by member 'kubs', IIRC he had 45mm offset 10.5'' TTM's with 285/40's all around and only had slight rub in ugly places.

 

Also, some C4 Vettes are not symmetrical side to side when assembled from the factory.

I had an 89 Vette and the front drivers side wheel and tire stuck out 1/8" from the wheel well lip and the passenger side front wheel and tire fit in perfectly.

My current 91 Vette's front and rear wheel and tire measure almost evenly side to side.

 

the 315 tire question wasn't me, i just hijacked this thread a bit to understand what will fit in the wheel well.

what started this inquiry for me was i found a nice set of AR torq thrust replica wheels sold by summit racing that were 17x8, 19mm offset, and 5.25" backspacing. i love the look of those wheels and on top of it i found some BFG KDW's in 245/45-17 for $88 each.

this setup would serve my purpose beautifully except for the nagging q that even though i'm running a narrower tire, would that offset have them sticking out too far.

as it turns out, on paper, the 245 tires with the 19mm offset wheel would protrude less than 315's on 50mm GS wheels so they would fit and work, but they might look a tad goofy if they stuck out of the wheel well, so i was trying to nail down how much, if any, would they pass the top of the wheel arch.

Rob

 

I found a real neat little backspace to offset converter at ccw wheel... http://www.ccwheel.com/files/technical.php