Tires & Wheels in the New Millennium
by Mark Reed
Thinking about putting new tires and/or wheels on your Corvair? Considering changing their size or style? Even if not, do you understand the latest nomenclature? Because we now face more options than ever before, we have to stay on top of what's available even when we want our cars to remain stock. Having recently purchased new tires and wheels that are larger, differently styled, and suited to a different purpose than the stock ones they replaced, I can share what I researched about tires and wheels with respect to the Corvair so that other Corvair owners can be as happy with their tire/wheel decisions as I am with mine. Here, then, is that info. (Note: Though some of the following details don't apply to Forward Control vehicles, the general information does.)
A combination of numerous variables comprise the description of any given wheel. Factory Corvair rims (rim = wheel) are steel. Aftermarket rims are either steel or alloy. Most but not all alloy rims of the same size are lighter and all are stiffer than steel ones. Almost all alloy rims and some steel rims have openings that both reduce weight and help vent heat from the brakes. "Early" Corvairs use 4 x 4 1/2" rims, meaning that each wheel has 4 lugs on a 4 1/2" dia. circle. "Late" Corvairs use 5 x 4 3/4" rims. All Corvair rims have a 2 7/8" center hole diameter. Factory wheels have a 13" wheel diameter and a 5 1/2" wheel width, both of which are measured where the tire's bead seats. A wheel's backspace is the distance between the wheel's inside mounting surface (that interfaces with the brake drum surface) and the plane of the inside (car side) edge of the rim. Corvair wheel backspace is about 4 1/4". Offset is the distance that the wheel's inside mounting surface is from the midpoint between the two outer planes of the wheel. The mounting surface on a Corvair wheel is offset 1" towards the outer side of the wheel. Because experts debate whether this 1" offset is negative or positive, it's best to avoid using those terms and to, instead, understand what they are intended to mean. Factory lug nuts are 7/16"-20 RH thread (there is no metric equivalent nut) with a conical taper on the wheel side of the nut head.
Tires describe themselves on their sidewalls. However, the nomenclature has changed several times over the years and reflects tire development. For brevity's sake, I will describe only the current nomenclature. Radial replacements for stock tires are 185/80R13 for all '66 - '69 cars and '61 and '62 station wagons and 175/80R13 for all other cars. Bias ply tires won't display the "R" but may display a "B", "C", or a "D". The first number (e.g., the 175) is the section width in millimeters. The section width is the tire's width measured at its widest point which is almost always at the sidewalls. The second number (e.g., the 80) denotes the ratio between the section width and the section height. The section height is the distance between the tread surface and the tire bead (where the tire's bead meets the rim). Thus, a 175/80R13 tire's section height is 80% of the tire's 175 mm section width. The section height is, therefore, calculated to be 140 mm. The ratio denotes what series a tire is (e.g., an 80 series tire).The lower the series number, the lower the tire profile. For example, a 45 series tire, is a very low profile tire. The number following the "R" is the tire's "inside" diameter (at the bead) in inches. Yes, the nomenclature will likely change again in order to eliminate the use of both imperial and metric measurements in the same specification. One important dimension not indicated on a tire is its overall diameter. Stock Corvair tire diameters are within a 1/4" either side of 24" depending upon the year and model. Increasingly, tires also display a speed rating and a load index. An example of such a tire is 205/60R15 89H. Speed ratings that are relevant to Corvairs range from Q (99 mph or 160 km/h) to ZR (over 150 mph or 240 km/h). For some reason, H refers to 130 mph or 210 km/h. The speed indicated is the top speed for which the tire is certified. Load indexes relevant to Corvairs range from 75 (852 lbs) to 95 (1521 lbs).
Obviously, tires and wheels have to physically fit each other. But this means more than simply matching the tire's inside diameter with the rim size. The tread and rim widths must be "reasonably" close in measurement, as well. A "reasonable" rule of thumb is that the variance not exceed 1" either way. A tire that is too wide or too narrow, relative to the rim width, distorts its design geometry such that the tire's optimum lateral movement is compromised. Further, a too narrow tire does not seat its beads as firmly in the rim as it ought to and risks separating from it. The rims must fit the car, too. Ideally, the rim's center hole should fit snugly onto the brake hub. This way, the lugs don"t carry the full burden of holding the wheel in position, though, in practice, they do. Still, lugs break. Larger than stock tire/wheel assemblies are limited by steering and suspension components on the inside and by the fenders on the outside, especially at the car's front. Also, early model wheel wells have less room than do late model Corvairs. Nonetheless, both early and late models can fit some pretty large tire/wheel combinations before clearance limits are reached. Larger than 13" rims that fit the early models come from Mazda, Datsun/Nissan, and Toyota as well as from early Nova station wagons, some '61 - '63 Olds F-85s, and VWs. These can be up to 15" in diameter and 6" wide. For late models, any 14", 15", or 16" rim that is up to 7" wide that fits all but the most recent generation of Camaro will bolt up. Typically, front tires having a tread width up to about 205 mm and mounted on rims with near stock offset can clear the fenders even in hard turns. Modifications to the inside lips of the fender buys significantly more room, especially for late models which can then fit 17" x 8.5" rims at the rear and almost as large rims at the front, all with 50 series tires. Flaring the fender wells really opens things up, of course. It seems that Corvair owners are fitting larger and larger tires and wheels on their cars, though more in the interests of aesthetics than practicality (which is not to value one over the other, but to raise the issue).
The Effects of Deviations from Stock
I will just list these without discussion.
|raise car; lower engine rpm and car speed; speedo error; poorer acceleration; poorer lateral handling; heavier
|usually require wider wheels; better lateral handling; more prone to aquaplaning; use more gas; heavier
|poorer steering response; rob horsepower, use more gas
|most are lighter; most improve brake cooling; can break (brittle)
|allow wider tires while retaining stock tire diameter (if using lower profile tires); stiffer setup; heavier; create room for disk brakes
|allow (usually require) wider tires; heavier
|big, alloy rims/tires
|can be as light as stock setup; better handling and braking
|offset towards car
|premature bearing wear; twitchy steering; better lateral handling
Other things to consider
Alloy rims are stiffer than steel ones. Lower profile tires are stiffer than higher profile ones. Stiffer rim/tire combinations improve steering response but give a rougher ride. Also, low profile tires can be large and small. With everything else being equal, the wider the tire, the higher the lateral handling limits. However, the larger the tire, the poorer the acceleration. Tire/wheel size selection is always a compromise. As well, having the rear tires larger than the fronts helps compensate for the rear weight bias; it improves handling and raises the lateral limit before going into oversteer. Look at any rear-engined Porsche. Finally, all rim designs reflect the fashion of the era that they were designed in. Stock rims always match the car's era. Non-stock rims reflect only their own era. Having hip wheels can be an ongoing and expensive process.
To put all of this together by way of example, I offer my own recent tire/wheel purchase experience. I own a '68 coupe with a slightly tricked up 110 hp 4 spd. I want to do some amateur 'street-legal' autocross, drive the car daily, and go on the occasional inter-province trip. I decided to change to 15" x 7" rims with good 50 series tires. Because I am a full time student, everything had to be inexpensive. Alloy rims are too expensive for me. So, I bought 4 white, 8-spoke, steel, Chevy rims (as usually found on pickups) for $60 each. Local stock car racers seem to have a direct connection to all the used ones. The dealer threw in the chrome lug nuts. Wanting some pizzazz, I had a local body shop paint the rims green. The car is yellow. The tires are Kumho Ecsta Supra 712s: 205/50R Vs on the rear and 195/50R Vs on the front for $125 and $110 each, respectively. An article in Grassroots Motorsports, Aug. 2001 edition, described them as one of the best "ultra-higher performance" street tire for the money. The best (BF Goodrich, Michelin) are only marginally better at the limit and cost 50% more. I weighed both my stock tires/rims and the new ones. The stock ones weigh 32-34 lbs each and the new ones weigh 43-44 lbs. Not good but not unexpected. The holes in the rims make them lighter than otherwise and help to vent brake heat. The much more expensive alloy rims are significantly lighter. The overall diameters are 22 3/4" (F) and 22 7/8" (R). The rim offset is "0". This 1" deviation from stock is not large but is the maximum recommended by Seth Emerson, famous Corvair racer. I noticed some twitchiness immediately. I also noticed the slightly slower acceleration due to the extra weight, despite the smaller overall diameter. On the other hand, I don"t have to slow down for corners. The handling is awesome. Still, when I can afford alloy rims, I will try to retain the stock offset.
Other Info Sources
All the foregoing is just the basics. More detailed information is available from many sources. I recommend checking out at least some of the sources listed below:
Corvair Technical Guide (available from CORSA)
Grassroots Motorsports magazine (esp. August 2001 edition)
How to Keep Your Corvair Alive 8th ed., by Richard Finch
<http://www.corvaircorsa.com/mixed02.html> (early model with 17" rims)
<http://www.geocities.com/MotorCity/9164/lugcircle.txt> (wheel lug information)
<http://www.geocities.com/MotorCity/9164/tires.html> (tire and wheel information)
<http://www.tireguides.com/tip2.html> (tire speed rating and load indexing info)
<http://www.wheelvintiques.com> (makes custom wheels)
If you are particular about the kind of ride you want to get from your Corvair, a lot of thought should go into the kind and size of tires and wheels you put on it. Any old set of tires and/or inappropriate wheels just doesn't do the Corvair justice. The car was ahead of its time. A smart choice from today's crop of tire and wheel products will help it perform and drive like the best cars this millennium has to offer.