Tire Inflation

[padgett]

Something is decidedly strange here.

1) Measured the section width on both the 2001 Michelins and the 2009. Both are the same even though the specs indicate the 35 psi tire should be 1/2" wider.

2) Contacted Michelin (now Michelin/BF Goodrich/Uniroyal ?) and asked about the tire inflation pressure. They answered (quite quickly) "As far as the recommended air pressure to run we suggest that you go by the pressure

recommendation as noted on the door placard."

This makes no sense, if it needs more pressure to achieve the rated load then the placard pressure should need to be scaled up accordingly. Comments ?

[reattadudes]

no question I would agree with Michelin on this one.

I always have to correct the oil change guys when they try to put any more pressure that the 30 pounds that is specified by GM. anything more than that, and the car rides terribly. I live on a bumpy dirt road, and my car bounces all over the place with anything more than 30PSI.

this is doubly true for convertibles, as increased pressure really exacerbates the cowl shake problem. it is critical for convertible owners to buy the softest available tire for the best ride. the Goodyear Assurance Triple Tread is a great choice here.

auto manufacturers spend millions tuning suspensions to cars, and tire pressure is a very important part of that. when auto manufacturers went to recommending 35PSI in tires, they made changes in the suspension to compensate for that. the Reatta does not have those changes.

I should also mention here that many times the tires you buy at warehouse clubs or Wal-Mart are not the same tires you buy from a regular tire store.

for example, a Michelin Symmetry, size 225/60/16 from Wal-Mart/Sam's Club/Costco has a treadwear rating of 460. from a tire store, the same tire has a treadwear rating of 600. there was a difference of $4 per tire.

Mike

buickreattaparts.com

[padgett]

No question they are specially made for the warehouse stores, it even says so on the Michelin web site.

http://www.michelinman.com/tires/passenger-car-minivan/x-radial-dt/

Do not see any difference in wear (740) though. What I do know is that they work well on my cars.

The intersting thing is that even though the specs are different the tires seem to measure the same so the question is why 44 psi vs 35 for the same load ? Or is it just marketing ?

[Richard S]

I had opened a simlar discussion a couple of weeks ago that was spurred by the same concern about 44 psi tires on a car made when the max was 32. I posted an inflation formula from an article I found that made some assumptions that may or may not be correct. For our cars curb weighs and weight distribution, the formula yielded around 35 psi. Mike is correct, however, that this makes the ride pretty harsh. I wonder if I were to run about 32 if the ride would improve at no real cost in terms of wear on the outside of the tire?

[HRP]

My 2 cents worth. Agreed that automobile manufacturers design the suspension and tire pressures to provide for a compromise between milage, ride, handling, etc, but the tire manufacturers are legally liable if they advise tire pressures at variance with the vehicle manufacturer. Many of us prefer a firmer ride which can be accomplished by increasing the tire pressure. I base my pressures on these criteria: the ride and handling I like as well as the tire wear pattern. This results in 36-38 lbs in my '91 Reatta front tires and 33-35 lbs in the rear. Add a couple of lbs per tire in my '02 Lexus LS430. However, my '77 Excalibur is essentially a center mount engine and weighs 4500 lbs without me in it, so she gets 40 lbs all around. With frequent alignment and rotation and balencing, my Michelin Sport Pilot tires wear evenly and I get the ride and handling I like. FYI, many states spell out the tire pressure and maintainance for their highway patrol vehicles are recommend 40 lbs for handling, milage and wear. Many of these are available on-line.

[padgett]

So even if the tire construction materially changes (as in the same max load but at a higher pressure), the lawyers prevent the engineers from advising the customer of the change.

I wonder should a tire should fail due to underinflation when the placard pressures are used and the tire company advised the same if the tire company would be liable ?

This is doubtful to be a Reatta concern because even the stock tires have a more than sufficient load capacity (my change to 225x60x16s was more for appearance, availability, and ability to use on different cars than any engineering need.

Add in the fact that thutty yar ago I found that increasing the diameter of the driven tires helped MPG (but 26.5" seems to be the limit with any clearance for a Reatta) and there you are. I do find it amusing to see the carefully worded non-responses from Michelin. They do respond though.

[HRP]

It would seem unreasonable for tire &/or automobile manufacturers to update specs for anything more than 10 years old or the lenght of time they are legally required to maintain parts for a specific model. However, there are new, used, and rebuilt parts sources available for those of us who own even the cars of a "rara avis" variety. As well as people who are knowledgeable to assist us with problems or questions like the one under discussion here. Especially in light of the other aftermarket non-OEM parts we've all used. ie: Who hasn't replaced their shocks or other suspension parts with non-OEM ones.

From this site:

http://www.4crawler.com/Diesel/Tires.shtml

albeit 8 years old, is a plethora of information about tires, their construction, pressure, etc including a rather neat and complex formula for figuring tire pressure based on a specific tire and load. They also suggest the following tests for those of us (like me)who aren't savvy enought (or too lazy) to figure and plug in all the numbers.

1. The "Business Card Test": On a smooth, hard surface, try inserting a business card between the tire and the pavement. If it goes in less than about 3mm-1/8", the the tire may be under-inflated, if it goes in more than about 6mm-1/4", it may be overinflated.

2. The "Chalk Line Test": Draw a heavy chalk line across al the trear faces. drive slowly forward in a straight line for a few revolutions of the tire. Get out and observe the wear pattern of the chalk. If it has worn away evenly, then the inflation is correct. If either the edge or center of the line is worn first, then the tire is under or over inflated, respectively.

3. The "Water Puddle Test": Similar to test #2, but drive through a puddle of water in a straight line, then get out and observe the wet tire tracks and see if the wet imprint is even, especially as the track starts to dry out after a few revolutions.

4. Heat is the #1 enemy of high-speed tires. The flexing of the tire's sidewalls as the tire rolls under load is the source of the heat. Higher inflation pressures mean less flexing of the sidewall and therefore less heat. Another test for proper inflation pressure is to measure the tire pressure when cold then again after 15 minutes at highway speed. If the pressure rise due to the temperature rise is more than about 3 psi, then the tire may be under-inflated.

Hope this adds to the information offered for proper tire pressure.

It will be very interesting to see what the new Michelin "Tweel" will offer when introduced (scheduled for) later this year(???).

[Richard S]

The roads in Greensboro must be a helluva lot better than they are here in SD. Sure, we squandered all our City's road money went to pay pension benefits, and all the State's road money went to bad investments, but I tried those high pressures in my 235/55/17s and even the bots dots were pretty intrusive. Could be the low-end struts I installed [i'm gonna try some Koni's next time just to see if there really is a difference]. And HRP is running that in a convert to boot. Wow!

[padgett]

Interesting article and matches the way I used to inflate the A-60x13s (even the factory said B's were too big) on the front of my V-8 Sunbird for autocrossing. Of course it came out to about 50 psi but the first time I tried it, I hit inside pylons.

Note that the formula L = 0.21*mL + (0.79*mL/mI)*inflation includes that mL/mI factor I have been talking about. Clearly if mL stays the same and mI goes from 35 to 44 psi, for the same load, "inflation" must increase.

Rearranging things we get Inflation=(L-.21mL)*mI/(.79*mL).

Figuring 60% of 3400lbs on the front tires and we get 2040 lbs on the front or 1020 on each front tire.

Now my 225x60x16s on the blue car are 1609 lbs at 35 psi = 18.78 psi. For 44 psi, 23.6. Obviously something is rong.

I suspect that the problem is in the load - while correct staticly, it is incorrect dynamically. On hard corners & braking, I used to figure that the entire weight of the front end was on the outside front tire (have some photos of the Sunbird with air under the inside front tire). This doubles the necessary pressure to 59 psi front (44) and 47 psi (35). This is what I would use if I wanted the maximum in cornering. For street driving and comfort, 50% is about right and gives 32 psi front for the 35 psi max tires and 41 psi for the 44s. Real world, 30 and 32 is probably good for comfort.

However this assumes the tire MFR is telling the truth. Since both tires seem to have identical outside dimensions, at the higher pressure the newer tire should have a higher load rating. I suspect some creative accounting is going on.

Real world 30 and 35 seem about right for the front. By all accounting the rears should be much less but 28-30 seems about right (I get somewhat better mpg at 30 psi). As for over/under inflation, radials do not seem to care much from a wear standpoint but rolling resistance goes up if low and has to come out as heat.

BTW I agree that the car manufacturer should not be responsible for updating the placard but the tire manufactures should be responsible for specifying the right pressure for the application. There is something wrong here.

[HRP]

Most of our roads in N.C. are kept up pretty well and we rarely have significant weather problems which can mess them up. I've driven through S.D. around 3 years ago and they are pretty worn. But we've got higher fuel taxes than our neighbouring states.

Having mostly driven sporty cars with a stiff suspension, I prefer the higher tire pressures. And my tires do wear evenly at the higher pressures and do not increast the pressure more than a couple of lbs at a high speed for >15 min.

All technical points aside, as long as you do not underinflate or overinflate below or above the tire nufacturer's

recommended pressures, keep the tires rotated and balanced and the car aligned, and are driving in reasonable weather, almost all of use can achieve the ride and handling we desire.

[padgett]

Keep in mind that many of the mythconceptions about tires come from the bias ply days. Then heat build up on an underinflated tire has a significant effect. OTOH modern radials are much more forgiving and change little with driving.

My only concern is tire growth at speed but have found nothing on the subject (other than tires seem to be measured at 45 mph. This might explain why when I measure tires at rest they seem very slightly shorter than the spec).