Tire pressure?

[MadRiver]

Howdy all. I'm running Coker 185R15 tires on my TR-250 -- which is the standard size -- and I had a question about pressure. The recommended tire pressure listed on the plate inside the glovebox lid seems very low to me: 20psi front, 24 psi rear. The tire itself indicates that 40psi is the max, and I of course wouldn't even dream of approachig 40, but still, 20 and 24 seems low. I would think 30 would be more appropriate. Thoughts, comments?

Thanks as always!

B.

 

[roofman]

Following the tire pressure rating on the tire is usually the best advice.

 

[PeterK]

I run 32F/30R in the TR3. 20/24 is too low for good gas mileage and responsive handling. I run less in the rear to help it stick better on fast corners.

 

[Geo Hahn]

I run the max in the rear and a couple pf pounds less up front... e.g. 34F & 36R on Michelins, Dunlops & Coopers.

The ride may be a bit harsh for some -- but it feels right to me.

 

[Alan_Myers]

Hi all,

The original tire pressures given in the manuals or imprinted on the glovebox lid for TR2/3/4/etc. are totally meaningless today, since these are almost all for bias ply tires and not applicable to today's radials. Today's tires need to use higher pressure.

It's also not a good idea to use the "max pressure" shown on the side wall of any particular tire. Just as it says, that's the max, not a recommended pressure to use. Some tires have a range imprinted on them, but even that is only an approximation.

You really, really have to experiment with your particular car. Too hard will let the tires slip more in corners, and so will too soft. In the case of too hard, it's because only the center portion of the tread is in good contact with the road. In the case of too soft, it's because the tire deforms during cornering and only the outer portion of the tread is in good contact with the road.

It's also a falicy that softer settings mean better "stick-tion" in fast corners. Just the opposite, because softer settings allowing the tire to deform/roll more under hard cornering stresses.

Road racers often use 40 psi and higher, and the smartest ones never bother to ask what someone else is running in their tires because they know it has no bearing on what will work best in their particular car. Due to differences in weight distribution, tires/tire age, suspension setup, power to weight ratio, the ambient temperature, track surface and other factors they each work it out by trying different settings and seeing what works. Basically, they raise the pressure as high as possible until the tire starts to break loose (oversteer if it's a rear tire, understeer if it's a front tire breaking loose), then back off a little looking for a good, controllable balance between front and rear that allows the shortest possible lap times thanks to the quickest possible corners.

Heck, watch a NASCAR race sometime and you'll see them tweaking by a one pound or less, from front to rear and side to side, to really dial it in and get the best possible performance.

Of course, these racers are also the guys who say. "If you ain't scared, you aren't going fast enough." That's the extreme of tuning, and we're talking about street cars, right? After all, you're probably not looking to shave 2 seconds off your commute, right?!

Most tire stores set pressure about 32 to 36 psi on newly mounted tires. This is just an initial setting and likely needs to be reduced, especially for a lighter car like our TRs.

I'd suggest you try 30 and drive it "spiritedly", see what happens. You will probably end up a little higher or lower, and probably different front and rear. This can change, too, Summer to Winter and with different tires or as tires wear. For example, my LR used 28 front, 32 rear when it had Michelins on it, but now I use 32 front and 38 rear with a set of Pirelli. The very best test is in the wet, when it's raining. Here a too-low or too-high tire will show up much more obviously! Just be a little careful about the testing, to be safe.

/ubbthreads/images/graemlins/cheers.gif

 

[PeterK]

<< It's also a falicy that softer settings mean better "stick-tion" in fast corners. Just the opposite, because softer settings allowing the tire to deform/roll more under hard cornering stresses. >>

A too high rear pressure in a RWD car will allow the rear to kick out more easily (Nascar term is fightin' a loose condition.)
Reducing pressure in the rear, even with tire roll, keeps the car pointed in the corner because the rear doesn't slide out until the limit of the tire roll is reached. And you're right in that it all depends on conditions and your driving habit. I grew up on VWs and 911s so I like to kick out the rear around corners but for my street driving, lower in the rear works best.

 

[angelfj1]

This presents an interesting question. Like Bill I often wondered about the tire pressure indicated on this glovebox plate. When we finished the restoration in 1985, we mounted Michelins, 185 x SR15. We inflated them per the glovebox plate and they looked kind of flat. Like most of you I always run them about 10 psi higher. Surely Triumph was aware of this perception, so why would they specify the low pressure. Could it be that they were sacrificing tire tread life forperformance? If this was an unsafe practice, wouldn't Triumph and Michelin have been open to litigation? Of course our society wasn't as litigious then as it is now.
Just a thought.

 

[Andrew Mace]

[ QUOTE ]
...We inflated them per the glovebox plate and they looked kind of flat. Like most of you I always run them about 10 psi higher. Surely Triumph was aware of this perception, so why would they specify the low pressure....

[/ QUOTE ]Likely as not, manufacturers probably did (maybe still do?) specified tire pressures on the lower side to AVOID litigation. The lower pressures in front would tend to promote a bit of understeer, which is much easier for the average (or below-average) driver to deal with than is oversteer.

As others have noted or hinted at, playing with tire pressure is THE CHEAPEST modification one can make to "improve" handling. And, as one might suspect from reading various replies here, it's somewhat personal and subjective amongst drivers.

Case in point: Many years ago I visited a friend of mine in IL. While there, she and I went to her local Triumph club's autocross, and we both drove her TR6. She was very used to the car and was very particular about tire pressures. Granted, I was much less used to the car (having autocrossed mostly Spitfires and GT6s), but I absolutely could not deal with the car the way she set up tire pressures. It worked for her; for me, it was more like terminal understeer. I ended up boosting the front tire pressures before my run and bleeding them back down each time before her run. Worked outt well for both of us! I was just the opposite, though, when I autocrossed my own GT6+; I always ran the rear tires about 4 lb higher than the fronts, while other GT6 drivers did much the opposite.

So many other factors as well: type of tire (bias v. radial), aspect ratio, wheel width, etc., etc.! When I autocrossed the GT6+ on stock 4.5" steel wheels and 175/70 x 13 radials, my pressures often were at or higher than the sidewall markings...upwards of 40 psi. On my Spitfire 4 autocrosser, we usually had 5.5" or 6" alloy wheels and castoff FF racing slicks...and about 22-24 psi.

So basically whatever works for you...and doesn't cause outrageously uneven wear...is cool. /ubbthreads/images/graemlins/cool.gif

 

[PeterK]

When we autocrossed our F500, we would run pressures between 9-14lbs depending on conditions, temperature, etc. and ofter would change a pressure by as little as 1/2lb between runs. If allowed us to dial in the balance between front & rear. On tight courses, I would crank the brake bias to the back so I could brake steer with power. I guess it worked because I spun around the courses alot, but that ate time so I really never did well. Had lots of fun with that car though!