Tire air pressure question?

[olddog]

My Cobra has BFG T/A radials P295 50R 15 rear and P245 60R 15 front.

I noticed the rear tires are wearing in the centers. The driver's more so than the passenger's side. Tire pressure was 19 psi rear and 20 psi front. I have only had the car a couple months. I'm thinking of dropping the rear to 16 psi, but thought I would ask those with more experiance. What say you?

[ROUSHAC]

16 psi is getting a bit low IMHO.

How wide are your rims? If they are narrow for the width of the tires, that will also cause wear in the middle of your tires.

[olddog]

Originally Posted by ROUSHAC

16 psi is getting a bit low IMHO.

How wide are your rims? If they are narrow for the width of the tires, that will also cause wear in the middle of your tires.

Rims measure 11" across outside. They look plenty wide.

[ROUSHAC]

That is a good width for that tire to get the entire tread to the pavement. So that eliminates that issue.

The pressure that you were using should have been fine for the rear tires.

Not to be a wise guy, but how accurate is your tire gage? Have you tried more than one gage? Maybe you have more psi in the tires than you think.

[olddog]

I double checked with a second gauge and got the same reading.

Mind you I haven't had the car long, so I do not know where the pressure has always been. It's possible that the previous owner had carried more pressure, seen the problem, and already lowered the pressure to correct for it. That is why I thought I would ask the question. The tires set well at 19 where they were at.

My thought was that since the fronts are P245 at 20 psi and wear looks good, and weight is about the same front to rear, then the rear tires being wider should be less pressure. Theoretically 245/295*20= 16.7, so 17 psi in the rears should hold about the same weight as 20 in the front. I'm sure there are more complicated factors, but thought that might be a good rule of thumb. Again since I do not know the history, thought I would seek advise.

Thanks

[pgermond]

I have the same size tires as you - 24 psi all around. 5.4k/mi and no problems.

[Got the Bug]

I'm running the Yokos and they seem to handle well at 26 front and 24 rear. Probably can't go wrong in the 24 to 26 range for street use.

[Rwillia4]

Wearing out in the middle of the tire? Are you applying too much pressure on the right pedal? Could it be a burn out issue?

[olddog]

Originally Posted by Rwillia4

Wearing out in the middle of the tire? Are you applying too much pressure on the right pedal? Could it be a burn out issue?

I've never had the tires loose. So far, I haven't felt the need.

[JBo]

At 14-16 pounds for street tires you should be underinflated. Never heard of an underinflated tire wearing out in the middle unless the rim width was narrow in relation to the width. jack the rear end up and put tire shine on a foot or two of the tire, slide butcher paper under the tires, lower the car and gently roll the car fore and aft. jack car back up and pull the paper out and "read" it, color or dampness should be uniform across tread. if not, accepted technology says to inflate to put more in center, delate to move to edges of tire. Less inflation allows tires to cup as the side walls expand over the sidewalls. The only kicker here is side wall construction(stiffness) could effect it as well.

[MaSnaka]

Olddog, same issue with me. Same tire size, same wear pattern. When I took possesion of my car it had 15 lbs in the rear. I thought that was to compensate for the wide tread and center tread wear. I noticed the rear end seemed to shimmy at certain times. I have since raised the pressure to 25lbs, much more stable handling and 6k miles later no noticable change in wear pattern. I run 26 in the front.
John

[1985 CCX]

Here is what I run for psi:
Rear: 23psi cold
Front 25psi cold
Happy driving.......................Jeff

[ROUSHAC]

Originally Posted by olddog

I double checked with a second gauge and got the same reading.

Mind you I haven't had the car long, so I do not know where the pressure has always been. It's possible that the previous owner had carried more pressure, seen the problem, and already lowered the pressure to correct for it. That is why I thought I would ask the question. The tires set well at 19 where they were at.

My thought was that since the fronts are P245 at 20 psi and wear looks good, and weight is about the same front to rear, then the rear tires being wider should be less pressure. Theoretically 245/295*20= 16.7, so 17 psi in the rears should hold about the same weight as 20 in the front. I'm sure there are more complicated factors, but thought that might be a good rule of thumb. Again since I do not know the history, thought I would seek advise.

Thanks

I think you may have answered the question yourself. Sounds like the most logical explanation for the wear pattern.

Sounds like you are pretty good where you are for letting the tires wear evenly from this point on.

Just remember to put a few more pounds in the tires if you plan on doing any "spirited driving" to keep the tires from rolling over on the rims or worse.

[olddog]

Thanks everyone.

Sounds like I should put a little more in the fronts.

[olddog]

Originally Posted by JBo

At 14-16 pounds for street tires you should be underinflated. Never heard of an underinflated tire wearing out in the middle unless the rim width was narrow in relation to the width. jack the rear end up and put tire shine on a foot or two of the tire, slide butcher paper under the tires, lower the car and gently roll the car fore and aft. jack car back up and pull the paper out and "read" it, color or dampness should be uniform across tread. if not, accepted technology says to inflate to put more in center, delate to move to edges of tire. Less inflation allows tires to cup as the side walls expand over the sidewalls. The only kicker here is side wall construction(stiffness) could effect it as well.

Now that sounds like a technical way to know you have it right.

We use carbon paper on white paper at work on rubber nip rollers. It works well. So why didn't I think of this?

They also sell a pressure sensitive paper that changes different colors for different pressures. More expensive, but very interesting. I have seen adds with pictures where they used it on an engine head.