Bias 2 ply tire pressure?

[James B.]

A couple of weeks ago I drove my recently restored '67 coupe to Bloomington Gold (225 miles one way) on my new Coker bias ply 7.75 tires. Coker's recommendation on tire pressure was OEM recommendation (24 psi). However, a couple of friends recommended 32/33 psi which I went with. The trip down wasn't great. Car would 'dart' with the least touch of the steering wheel (power steering, steering box rebuilt, new ball joints, just aligned). Keeping the car centered took a lot of concentration.

On the way back I reduced tire pressures to 28 and the car drove much better. Still had to concentrate- car didn't center as well as I would like but it was almost a joy to drive and cruised well at 70-75mph.

Now I wonder if further reducing pressure to 26 or recommended of 24 would make further improvement. I would be curious re the experience of others with tire pressures on bias plys.

I will be driving it to chapter judging meet in August and that will be about 150 miles one way so a good opportunity to experiment.

Once its show life is over the radials are going back on!!

Jim

[Michael J.]

Re: Bias 2 ply tire pressure?

I run my bias ply driving tires (7.75 X 15 Goodyear Blue Streaks) at 25 psi on my '64. The max psi is shown on the sidewall as 32 psi. They are not great, but are adequate, as long as there are not too many grooves in the road that they tend to follow.

[William F.]

Re: Bias 2 ply tire pressure?

Bias tires in today's world?? I know judging standard is "as delivered" back when. But it's unrealistic and unsafe to require driving or even being judged on bias tires. This probably contributes to trailer/garage queens rather than being able to enjoy safely driving vintage Corvetes.

[Timothy B.]

Re: Bias 2 ply tire pressure?

I don't believe bias tires are unsafe but no way I would be driving 70-75 mph with them.

I experienced the same thing with the bias tires on my 67, at 24lbs they seemed so much better, I think they gain pressure faster than radials.

After I tightened the steering box on my 67 many issues with the bias tires went away.

[Michael J.]

Re: Bias 2 ply tire pressure?

Don't get me wrong, I'm not taking long road trips on these tires, usually less than 100 miles to cars and coffee and car show events. It just seems at some places, people appreciate seeing the cars as delivered, skinny tires and all. I have radials on Torq-Thrust Ds for longer trips where originality is not a factor.

[Michael H.]

Re: Bias 2 ply tire pressure?

I did a 900 mile trip last May with my 67, I had redline Goodyears on the car with 30 psi and it drove great at 70 mph. You might want to check your alignment.

Mike

[Jim D.]

Re: Bias 2 ply tire pressure?

Once its show life is over the radials are going back on!!
Jim

Until that day, you'll just have to put up with a car that handles like crap. I put Coker bias ply tires on mine and I quit driving it because of the tires. I put radials back on it and all is well. Sold the POS Coker's for $100 for the set with less than 200 miles on them.

[Jeff B.]

Re: Bias 2 ply tire pressure?

During the 60s, most all passenger tires were 4 ply (bias ply, some nylon cord and some rayon). However, there were also some tires that were actually 2 ply, 4 ply rated. I worked for a Gulf dealer in the mid 60s that had some of the 2 ply tires, and think they were called the Gulf Pacer, and the next better tire was called Crown, and the best was called Deluxe Crown. Seems to me that all 4 ply bias ply (nylon) tires called for 24psi (cold), but I think the 2ply (4ply rated) specified 26psi.

[Timothy B.]

Re: Bias 2 ply tire pressure?

I will say this, IMO, the look of the bias tires on the car is so much better than the radial tires.

[James B.]

Re: Bias 2 ply tire pressure?

'I don't believe bias tires are unsafe but no way I would be driving 70-75 mph with them'. Timothy, interesting, I never gave it a second thought (after about 300 miles; testing everything at lower speeds). We certainly didn't question 75mph w/ bias plys in the 60s. Maybe I will give Coker a call, I've never looked into design max speed (I wonder if DOT would even approve a highway tire at <100mph??).

Another interesting question is, with 50 years of advancement in tire technology, materials, mfgring tech etc how do current bias ply tires compare to the originals. I don't have a clue but it would be a fun inquiry.

'I experienced the same thing with the bias tires on my 67, at 24lbs they seemed so much better, I think they gain pressure faster than radials' Yes, I think you are right about gaining pressure faster than radials. I'll try 24 on my next trip.

[James B.]

Re: Bias 2 ply tire pressure?

Michael,
Re your comment about checking alignment. The shop that did it is well regarded and does a lot of C2/3 Corvettes. I've checked the machine printout and it matches OEM specs for the '67. However, good idea and I will call them and see if they have any opinion about the 'twitchiness' and if a slight change could make it center better.

[James B.]

Re: Bias 2 ply tire pressure?

Jeff,

I worked for a Standard Oil dealer in the 60s and yes, remember the '4 ply rating 2ply vs true 4 ply' debates. And, the 7.75 repro tires for the 67 are Firestone Super Sports and are 4 ply rated 2 ply tires. I don't know what that means in terms of drivability, sidewall stiffness etc.

[Bob R.]

Re: Bias 2 ply tire pressure?

I have a set of Coker tires for my 63 for judging. A friend of mine is in the tire wholesale business and purchased and mounted them for me. He told me they were almost impossible to properly balance even using the latest equipment. His thought is that Coker is making these tires in small batches and they have poor quality control. Remember they are not made by the original manufacturer as the originals were Coker is just using the original name. It is best to drive only at reasonable speeds with these tires on the car. For NCRS judging you can only have weights on the chassis side of the rim so I had to glue weights on the inside of the rim to balance close enough to be able to drive on the highway. As soon as the judging process finished for me last year a new set of radial tires went on the car. Safety is more important then the looks of the original tires.

[James B.]

Re: Bias 2 ply tire pressure?

Bob,

Re your friend's assumptions about Coker, I don't know enough about Coker to comment. I would guess that 90% of Coker's classic tires go on 'trailer queens' so their tires are rarely exposed to prolonged highway speeds. I will say mine balanced very easily & the shop guys said I was 'good for 100mph'. I don't intend to test that remembering Duntoff's comment that above 100 mph the C2 becomes a really bad airplane... I did chassis side weights only (due to judging) and the car is very smooth at 75mph.

I will say that on 5 mid-years over the past 25 years I have run Coker RADIALS, yellow stripe and red stripe to get a correct look. I have driven those cars at highway speeds without worrying and have never had a problem so I am not sure your friends fears are well founded.

As you said, as soon as judging is over the bias ply tires are coming off. Radials will go on but they may well be Coker to keep the red stripe (matching the tank sticker).