A tire question...but wanted to hit as many eyeballs as possible so posting here

[Al N.]

Hey all....about to get a little freshening for SB1341 (aka "Bessie").

 

What pressures does everyone run? IIRC, most people were running significantly lower than spec'd on the tire...like in the teens.

 

This is for a 175-185/70-13 tire on a vintage car.

 

TIA for your input.

[Kitcat]

Probably depends on the tire. At this point, for track days, I start at 12 front, 15 rear. And then as they heat up (last track day, 10 lbs per tire!), I bleed them down to 18 rear, 15 front. Since tires don't heat up as much in normal driving, maybe start at 18/15? Or, since your tires are "taller", perhaps up that 2-4 lbs to 22/19?

[Al N.]

Thanks Kitcat.

 

Speaking of "taller"...I'm considering trying a vintage-specific repro tire (Vredestein Sprint Classic) that comes in a 6.40 x 13 size. The only thing is that these tires had something like a 90 aspect ratio. So the difference in diameter from what I'm running now is about 4 inches.

 

I know I can get the speedo recalibrated for this new rolling diameter....but there have to be other downsides to going with taller tire...right?

[Nick OTeen]

With the taller tire (taller sidewall) you can typically expect to experience some loss in steering response and some loss in overall handling. Additionally, the 4 inch taller tire means you'll raise the car 2 inches from the ground if you don't adjust the ride height to compensate. That would raise the center of gravity which would further effect handling.

[Kitcat]

It would be like adding over drive:). Engine speed would drop (as would acceleration), rpm's would drop, ditto noise, gas mileage would improve. But your car has pretty minimal power to start with and my guess is it would turn it into a slow Prius, esp with skinny tires' effect on cornering. Plus all the stuff Nick O just noted. But, this is all theory, maybe it would be great?

[Al N.]

@Kitcat: I had the engine redone last year and it (1500 pre-crossflow) dyno'd 128hp at the crank...so it's got more pep than it had originally.

 

Hmm. No one here runs Avon cr6zzs that you know of, do they? It's a pricey tire, but I've had my eye on them for a while. It is the one that comes in 175 or 185...the 6.40 is the Vredestein Sprint Classic.

[James A]

My answer is that it completely depends on the tire!

[m wirth]

I have been lucky at several track days that the promoter has dedicated personnel to take tire temps and pressures immediately after a session and have found that at ORP and The Ridge they recommended 26 psi front and rear, this pressure yielded only a 2 degree difference across the tires, I did notice a difference especially in the off camber corners at ORP. I run the same pressure on the street . Obviously cross section makes a big difference. They also recommended always setting a base line on cold temp pressure and using a needle type pyrometer not a laser. More likely than not the hot temps will vary from track to track and cw to cc travel and from tire position but it will get all but the top scca racer in the ballpark.

[Al N.]

In case anyone is interested in what Avon CR6zz tires look like in narrower sizes (they are apparently very popular for Cobra and GT40 folks in the wide sizes as well). Not cheap, but I think they look great in 175/70x13 size currently fitted to my car.