Tyre Pressure

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gjb49
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Tyre Pressure

Post by gjb49 »

When the car was made, it came with bias ply tires. The pressure had to be 10 psi different between front (18 psi) and rear (28 psi).

The tires currently on my car are radials and my mechanic is of opinion that the tire pressure should be higher.

I will confess that when I was driving my '63 in the 60's and my '66 in the '70's I put 28 front/32 rear and did not have any problems. And, yes, I read 'Unsafe at Any Speed' and had done everything Nader said not to do and survived.

So, what is the current opinion/wisdom on tire pressures.
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terribleted
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Re: Tyre Pressure

Post by terribleted »

Most Corvairs will feel loose at speed if the front and rear pressures are even. Factory spec on most was 28 rear and 14 front. I have found over many miles of Corvair driving that a 10 LB pressure differential works well. I generally run around 32 rear and 22 front on radials, but will play with slightly different pressures around these numbers to see if the particular car has a sweet spot it likes.
Last edited by terribleted on Fri Jul 28, 2017 11:46 am, edited 1 time in total.
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bbodie52
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Re: Tyre Pressure

Post by bbodie52 »

Sizing Corvair Wheels and Tires wrote:Tire pressure.

The factory recommended tire pressure settings were selected to dial in understeer and provide a comfortable ride on bias ply tires being the important conditions. Although radial tires are much more forgiving, you should still maintain a pressure differential of 8 to 10 pounds front to rear, but you can improve the responsiveness and handling by raising the tire pressure up to about 27 front, 35 psi rear for street use (don't exceed the maximum tire inflation on the sidewall). Autocross use is a bit trickier, as the construction of the tire has an effect on the amount of pressure in the tire. In short, there are now three types of "Street" tire:

Real Street Tires

The first are tires to drive around on all the time, these are designed to give a reasonable service life (~50k miles), good all weather performance, and perhaps a sporting amount of grip. The BF Goodrich Radial T/A, Goodyear Eagle Aquatread, etc. fall into this category. These tires have a relatively soft sidewall which gives them a nice ride, but they unfortunately allow the tire under hard cornering to try to roll under the rim, meaning you're now cornering on the sidewall instead of the tread. Bad plan.

The proper solution is to get a set of competition tires (see below) which don't do this. But if you're just trying out the sport, there is a band aid fix - more air. The additional pressure will stiffen the sidewall, which will help the handling, up to the point where the contact patch is so much smaller that the tire slips anyway. Generally, a 5 to 10 psi increase works well, *but* you may need to increase the front and rear tires different amounts to make the car well-balanced.

DOT race tires

The second group are "R" tires, that is tires which meet the letter of the DOT regulations for street tires, but are really meant for competition use. Goodyear's Eagle VR-S, Yoko AD08-R, and Hoosier's R-6 are examples of this breed. These tires have the sidewall stiffened to avoid the tire rolling over on its shoulder during hard use, so using the 27/35 tire pressure is still a good baseline. These tires will respond to pressure changes differently, you may need to lower pressure to get more traction. I suggest using a pyrometer with these tires, but chalking will work to start.

Street Touring tires

With the advent of the new SCCA "Street" and "Street Touring" classes, there are now tires which come very close to the concept of a tire you can drive on the street that will also work at an autocross. The Dunlop ZII and BFG Rival are examples of this type, with a minimum tread wear rating of 200. As with the DOT race tires, start at 27/35 and go from there.

Chalking

Here's how to "chalk" tires to find a good pressure, from a post by Mike Lukacs to the autocross list (Team.Net):

The classic method of determining correct tire pressures for autox is as follows:

  • A) raise your tire pressures to ~10 lbs above mfg recommended pressures to start.
  • B) mark the outside tread/sidewall corners of each tire with 3 or 4 patches of white shoe polish (sneaker polish) spaced around the tire.
  • C) after your first run, check the shoe polish patches to see if you are using the whole tread width, but not rolling over onto the sidewall too much.
  • D) if the shoe polish is getting rubbed off of the sidewall area, raise that tire's pressure by 1 or two pounds. If the shoe polish is NOT getting rubbed off of the outer tread area, lower that tire's pressure by a pound or two.
  • E) repeat steps B, C, D, until you are happy.

Caveat: this method is only approximate, a tire pyrometer to measure the evenness of temperature across the tread is better.

Note: Don't forget to go back to your normal street pressures before you leave the event! [BB]
:link: http://autoxer.skiblack.com/tires.html
Brad Bodie
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Re: Tyre Pressure

Post by 66vairguy »

An understeering car is more predictable and tends to correct automatically by just lifting off the accelerator.

Most of the Corvair books note that GM deleted the front sway bar on the original Corvair and relied on lowered front tire pressure to reduce front tire adhesion when the car was at it's limits as in a fast or abrupt turn. Rear heavy cars tend to loose rear tire adhesion first at the limits in a turn prompting a spin at the hands of the basic driver (most of use). A substantial front sway bar (64 and later) helps unload the inner front tire in a turn to reduce front adhesion vs. the rear in a turn. If you look at the old 911 Porsche racing you'll see the inside front tire OFF the pavement in a turn, yes cornering on just one front tire. This provided a balanced front to rear adhesion in a rear heavy car at the corning adhesion limits.

So the sway bar size, type of tire, or running a wider tire on the rear all affect the best front and rear tire pressure.

Those with skills go to a track and carefully test tire pressures for the best handling response.
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