<VV>Still mo' tirez

Padgett pp2 at 6007.us
Sun Jul 30 11:36:25 EDT 2006

> > What is the correct tire size for my 65 Corsa coupe?  By correct I mean 
> the size that would cause my speedometer to read correctly =P.

Corvair is different from most RWD cars in that the speedometer is driven 
by the front wheel. This means that no matter how big you go in the rear, 
the speedo will not change.

Now a 6.50x13 was 24.3" in diameter and a 7.00x13 was 25.3" (1) but in the 
front that is not all you need to be concerned about. For clearance (a near 
thing on the front of my 66 with 195x70x14s on 14x6" Snowflakes) you also 
have to consider the pivot radius, the distance between the center of the 
steering arc of the wheel and the outer edge of the tread.

In my case this was a problem. The tires were slightly smaller diameter 
than the stock 7.00x13 at 24.8" but the wheels have about 1/4" less 
backspacing and are 1/2" wider than stock which moves the outer rim out 
about 3/4". Also the tread is about an inch wider than the stock skinnies. 
Put all of this together and the steering arc the edge of the tire makes is 
about an inch and a quarter more than stock (this also makes the steering 
slightly heavier). At one turn of the steering wheel from center in either 
direction the lip of the wheel opening in front was so close to the tire 
that you needed a feeler gauge to measure the clearance, and the pointy 
edge a bit further in was worse. Application of a speed wrench increased 
this clearance to a knuckle which seems to be enough at least there are no 
shiny spots.

So it is all of the changes added up that determine if a tire will fit and 
wheel width/tread width/backspacing can have even more influence than the 
diameter. I have been told that there are some suspension adjustments 
(including moving the whole crossmember back slightly) that can improve the 
situation but I have not needed to go to that extreme.

Shouldn't this be in a FAQ somewhere ?


(1) page 222 "The Classic Corvair" by Bob Helt 

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