maximum tyre pressure?

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maximum tyre pressure?

Postby Ducked » 22 Apr 2012, 13:39

Anyone know / able easily to find out the max pressure for Bridgstone 175/70R13 SF-226 radials?

As is my wont, I've been running pretty high pressures, in the interests of fuel economy and reducing tyre wall flexing. I've noticed the tread depth is now a couple of mm down in the centre, compared to the tyre edge. (around 4 v 6 mm)

"Mining" tread depth doesn't really matter here, since the tyres are at or near the end of their life anyway. Its probably bad for handling and braking, though, and it would be a good idea not to exceed the maximum pressure, especially on new replacement tyres.

However (and I think unusually) this is not apparently moulded on the sidewall.

IIRC max pressures on the sidewall of tyres in the UK were typically around 50 psi. My Cheng shin spare, OTOH, says 36 psi, suggesting Taiwanese tyres may be made of less stern stuff.

I was recently running 38/40, and though I've reduced that to 34/36 for the "wet season", its still pretty high.

I'll probably run Redwagon's "tire tuning" procedure, but it would still be good to know what the official max pressure was.

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Re: maximum tyre pressure?

Postby Super Hans » 22 Apr 2012, 14:55

I have no idea about the max pressure of the tyres you have specified, but sometimes tyre brands supply the information on the wall of the tyre.
In Taiwan, I wouldn't go anywhere near maximum pressures due to heat expansion in the summer (and therefore the risk of catastrophic failure) and less resistance to punctures. Also, and you already know all this of course, less of the tread is in contact with the road. They also wear down more quickly as they take longer to warm up if they are inflated too much.
I think any money you save on fuel consumption would be spent on increased tyre wear and increased suspension and steering wear.
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Re: maximum tyre pressure?

Postby Ducked » 22 Apr 2012, 18:51

Super Hans wrote:I have no idea about the max pressure of the tyres you have specified, but sometimes tyre brands supply the information on the wall of the tyre.
In Taiwan, I wouldn't go anywhere near maximum pressures due to heat expansion in the summer (and therefore the risk of catastrophic failure) and less resistance to punctures. Also, and you already know all this of course, less of the tread is in contact with the road. They also wear down more quickly as they take longer to warm up if they are inflated too much.
I think any money you save on fuel consumption would be spent on increased tyre wear and increased suspension and steering wear.


Hi Super Hans. Thanks for your reply.

I think (provided one is within the maximum specified by the manufacturer, which I don't know, which is not moulded on the sidewall, and which a zero-zhongwen web search has so far not revealed) that internal heating, and consequent catastrophic failure is much more likely with underinflated tyres.

From a fair bit of reading now, this is by far the commonest failure mechanism.

As you point out yourself, "they take longer to warm up if they are inflated too much." I think your implied statement that cold tyres wear more rapidly is questionable, but its likely that over-inflated tyres will wear more unevenly, as mine apparently have.

I think you're right that they're likely to be more vulnerable to punctures. I can live with that, partly because I havn't had any, and also because puncture repair is dirt cheap here.

I suppose there may also be more suspension and steering wear due to more vibration transfer from harder tyres.

As I said, wear on these tyres doesn't matter much, since they are at or approaching end-of-life.

With new tyres, balancing the (unknown) increase in tyre costs against the (unknown) decrease in fuel costs sounds like a "my guess is as good as yours" exercise, except that my guess is better, because I have the advantage of knowing my tyres are likely to die of old age before they wear out. (With me, the distinction is less clear and its likely to be a closer call.)

The Redwagon link above gives a method of optimising the tyre pressure/footprint which should more or less address these issues, so I'll try that, but it would be nice to know what the safe upper limit was.

I sent an email via Bridgstones US website (telling a few automated lies filling in inapplicable obligatory fields) but they'll probably ignore it.
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Re: maximum tyre pressure?

Postby jaame » 22 Apr 2012, 21:27

I'm no expert but 50 psi sounds miles too high for car tyres to me. I have always believed that they should be between 28 and 33, maybe 35 if the boot if full of something heavy. Every car I've ever had has had the tyre pressures in the owner's manual. I've always stuck to that myself.
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Re: maximum tyre pressure?

Postby Ducked » 22 Apr 2012, 23:20

jaame wrote:I'm no expert but 50 psi sounds miles too high for car tyres to me. I have always believed that they should be between 28 and 33, maybe 35 if the boot if full of something heavy. Every car I've ever had has had the tyre pressures in the owner's manual. I've always stuck to that myself.


I'd guess most people do. I think I did the first couple of cars I owned, but I've been running higher pressures (though of course well below the sidewall max) for many years now.

I might have been overdoing it a bit lately though and I should probably get around to properly "tuning" the pressures.

I don't have a handbook but I'm sure the pressures recommended in it will be lower than I'm running, or would run. They are generally biased toward ride comfort.

These are the first tyres I've encountered which don't show the max pressure on the sidewall, and I don't much like it. Seems evasive to me, and if I can't get a number for them I won't be buying the same one's as replacements.
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Re: maximum tyre pressure?

Postby Super Hans » 23 Apr 2012, 18:35

Hi Super Hans. Thanks for your reply.


You're welcome.

I think (provided one is within the maximum specified by the manufacturer, which I don't know, which is not moulded on the sidewall, and which a zero-zhongwen web search has so far not revealed) that internal heating, and consequent catastrophic failure is much more likely with underinflated tyres.


I think so too, but I think it all depends on the degree to which they are over inflated. Grossly under-inflated tyres would obviously get chewed up and also get very hot, and creasing would result from cornering. My point was that if you inflate to the manufacturer's maximum recommended pressure, I'm not sure whether this would account for further expansion due to environmental factors such as outside air temperature, outside air pressure, road surface temperature, heat from breaking and heat from friction, or whether the maximum pressure includes all of these possibilities. What I mean is would 50psi be a safe pressure to inflate to, or would 50psi be the maximum pressure the tyre has been rated for?




As you point out yourself, "they take longer to warm up if they are inflated too much." I think your implied statement that cold tyres wear more rapidly is questionable, but its likely that over-inflated tyres will wear more unevenly, as mine apparently have.


I had cause for various reasons to take an advanced driving course whilst in the UK. It was always drummed into us that we had to keep our fleet in good "nick," if you get my play on words here. I think if cars are used constantly and for demanding driving techniques, this factor is far more important than for a family car used for less demanding journeys.

I think you're right that they're likely to be more vulnerable to punctures. I can live with that, partly because I havn't had any, and also because puncture repair is dirt cheap here.


You're incredibly lucky. I've just had to fix a slow. Last year, or the year before, I had a stick enter through the side of the wall.

As I said, wear on these tyres doesn't matter much, since they are at or approaching end-of-life.


Ah - I see what you mean now. They're actually deteriorating, not wearing down.



The Redwagon link above gives a method of optimising the tyre pressure/footprint which should more or less address these issues, so I'll try that, but it would be nice to know what the safe upper limit was.


Yeah, I haven't looked at that yet, but it'll be interesting to take a look, so I'll save it for my bed-time reading later.

I sent an email via Bridgstones US website (telling a few automated lies filling in inapplicable obligatory fields) but they'll probably ignore it


Ha. Good luck with that, but if you get any feedback, I'd like to know what they say. I've just forked out on a couple of Bridgestones for the rear, and will be purchasing the front lot next week.
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Re: maximum tyre pressure?

Postby Ducked » 24 Apr 2012, 21:37

Super Hans wrote:
My point was that if you inflate to the manufacturer's maximum recommended pressure, I'm not sure whether this would account for further expansion due to environmental factors such as outside air temperature, outside air pressure, road surface temperature, heat from breaking and heat from friction, or whether the maximum pressure includes all of these possibilities. What I mean is would 50psi be a safe pressure to inflate to, or would 50psi be the maximum pressure the tyre has been rated for?



Err...Good question. Recommended tyre pressures are usually quoted cold, and I've been assuming the maximum recommended tyre pressures are quoted on the same basis, but I don't know that for a fact.

I've heard a 10%-below-maximum "rule-of-thumb", but I don't know how soundly based it is.

In the past, with my UK tyres, which, for whatever reason, seemed to have relatively high maximum pressures, I'd have been below that. Here, I don't know.

This Wikipedia article is somewhat more categorical in its endorsement of high tyre pressures than I would be.

http://en.wikipedia.org/wiki/Tire

For example : "Modern tire designs allow for minimal tire contact surface deformity during high pressures, and as a result the traditional wear on the center of the tire due to reasonably high pressures is only known to very old or poorly designed tires."
"reasonably high pressures" is undefined, but this is rather more specific (my underlining):-

"It may be, that very high tire pressures have only two downsides: The sacrifice in comfort; and the increased chance of obtaining a puncture when driving over sharp objects, such as on a newly scraped gravel road. Many individuals have maintained their tire pressures at the maximum side wall printed value (inflated when cold) for the entire lifetime of the tire, with perfect wear until the end. This may be of negative economic value to the rubber and tire companies, as high tire pressures decrease wear, and minimize side wall blow outs."

This matches my prejudices, so of course I'd like to believe it, but I don't know how soundly based it is, and I don't find the implication that tyre companies would deliberately encourage (or at least not discourage) sidewall blow-outs very plausible. (Edit: though in the buck-passing context of my last point below, perhaps it makes sense.)

I havn't had time to check all the articles source refs, but this one:-

http://www.nhtsa.gov/Cars/rules/rulings/TirePresFinal/FEA/TPMS3.htmlTIRE PRESSURE SURVEY AND TEST RESULTS
is a survey of the prevalence of low tyre pressures in the US vehicle fleet (VERY HIGH) and a test of tyre pressure effects on braking. The highest pressure tested is "only" 36psi, though, so it doesn't really address this issue.

Super Hans wrote:Ah - I see what you mean now. They're actually deteriorating, not wearing down.


Well, they have plenty of tread, but they're about 7 years old, and they have sidewall cracks, which could have been caused or aggravated by my high pressures. I think these are mostly cosmetic, but some of them are big enough to give me mild concern and suggest early replacement. (or they're "death traps", of course, depending on one's attitude) .

I havn't been able to find anything specific on what constitutes unnacceptable cracking, and I suppose it would be difficult to define or measure. The UK MOT requires that you can't see the belting, which is pretty liberal. A couple of US RV (motorhome) sites suggested "no deeper than 2/32".

Less quaintly, In the rest of the world, that's about 1.6 mm, which is the same as the UK (and most US states) minimum legal tread depth. This might be a coincidence, or it might be to save them the pain of pulling another arbitary number out of their ass. Anyway, I don't drive a US RV, (many of which are big enough to be comparable to a British Heavy Goods Vehicle) and I havn't seen any description of how to measure crack depth. I can't generally get a 0.15 mm feeler guage 1mm into the tyre, but I can in places on the worst looking one, at the highest pressure I've run (40psi), so that one, at least, might deserve the "death trap" description.

I think I have good reason to believe that sunflower oil treatment could significantly slow (though of course not reverse) crack development, but since I have another, silent thread on that I shouldn't discuss it here.


Super Hans wrote:
You're incredibly lucky. I've just had to fix a slow. Last year, or the year before, I had a stick enter through the side of the wall.



Well, you do drive a Jeep, and presumably, in "the sticks". I don't drive much at all, and very seldom off road (beach tracks a couple of times, but with lowered pressure).

My previous car (Sierra) seemed to pick up an awful lot of nails, though it had wider tyres which I suppose increases the probability if they are randomly distributed. OTOH, it could be my apparent good fortune is down to increasing student apathy, though I don't really think so. :ponder:

Super Hans wrote:Ha. Good luck with that, but if you get any feedback, I'd like to know what they say. I've just forked out on a couple of Bridgestones for the rear, and will be purchasing the front lot next week.


As you probably know, Bridgestone/Firestone had a damaging legal dispute with Ford over Explorer rollover incidents, which seem to have been due a combination of relatively poor tyres, a relatively poor vehicle, and absolutely awful advice from the vehicle manufacturer.

There seems to have been a subsequent trend among tyre manufactures to pass the buck to the vehicle manufacturer on as many issues as possible, and it may be that the lack of a recommended maximum pressure on the tyre reflects this kind of corporate-lawyer caution.

If so, I'd expect max pressures on tyre sidewalls to be an endangered species.
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Re: maximum tyre pressure?

Postby thirdstring » 28 Apr 2012, 23:36

I worked at a repair shop before, and we had people coming in over 80 psi! That was the highest my gauge went. Definitely don't recommend that.

Actually, at least in the States, each vehicle determines proper tire pressure. On US cars, the recommended pressure for correct performance (handling, braking, fuel economy) is posted on the door pillar with a lot of other info. I don't know if Taiwan requires those labels or not, but you can find the recommended pressure in the owners manual I'm sure.

As for actual max pressure for typical tires themselves, 35 psi is most common, while some are 44 psi. "Cold" doesn't need to be cold weather, just don't come off the highway & set it at that pressure. Don't worry about them over-pressurizing due to various factors if you set it at 35 psi. I'm sure the burst pressure is at least 4 times the rated amount, and you're not going to heat it hot enough to get close to that.
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Re: maximum tyre pressure?

Postby Ducked » 30 Apr 2012, 18:13

thirdstring wrote:I worked at a repair shop before, and we had people coming in over 80 psi! That was the highest my gauge went. Definitely don't recommend that.

Actually, at least in the States, each vehicle determines proper tire pressure. On US cars, the recommended pressure for correct performance (handling, braking, fuel economy) is posted on the door pillar with a lot of other info. I don't know if Taiwan requires those labels or not, but you can find the recommended pressure in the owners manual I'm sure.

As for actual max pressure for typical tires themselves, 35 psi is most common, while some are 44 psi. "Cold" doesn't need to be cold weather, just don't come off the highway & set it at that pressure. Don't worry about them over-pressurizing due to various factors if you set it at 35 psi. I'm sure the burst pressure is at least 4 times the rated amount, and you're not going to heat it hot enough to get close to that.


Thanks. Reassuring.

I'd taken them down to 36 psi ( the max pressure on my spare and the lowest I can remember seeing, though I only recently started paying much attention), partly because of the impending rains, partly "playing safe" with the max-pressure-unspecified Bridgestones.
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