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Old 04-30-2020, 03:19 PM   #1
RidingOnRailz
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Cool Another Tire Pressure Question! Front-Rear Considerations

If, especially in front-wheel drive family sedans such as Accords, Altimas Camrys, Elantras, and Malibus, there is a 55/45 or greater percentage difference in front-rear weight distro, why do car makers recommend the same cold TP all around on the Tire Placards in their vehicles?

IE: In a typical sedan, the placard would read 32psi for all four tires, where 1-2psi more might benefit the front wheels, and 1-2psi less might do, in the rears. IE: Front: 34psi, Rear: 31psi.

Rare is the BMW that doesn't recommend different front and rear cold pressures, and not a few Subaru models: Typically it's the opposite than in only two wheel drivers, with a late model BMW or Subaru recommending 30front and 34rear.

In the former case, is equal pressure specified just to make it easier for the average driver to remember? I know my wife's numbers(30psi all around) and mine(32psi) by heart. But if I think a slight redistribution of the pressures mkght improove the handling, it couldn't hurt to try, right?
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Old 05-01-2020, 08:26 AM   #2
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Re: Another Tire Pressure Question! Front-Rear Considerations

Different vehicle manufacturers take different approaches.

Some do as you suggest - portion the pressures based on the load.

Some think it is better to have a single pressure on all 4 corners, so that tire rotation doesn't involve resetting pressures - and those guys adjust the springs and sway bar rates to compensate.

And since evenness of wear isn't strongly affected by inflation pressure, wear isn't greatly affected.
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Old 05-01-2020, 11:28 AM   #3
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Cool Re: Another Tire Pressure Question! Front-Rear Considerations

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Originally Posted by CapriRacer View Post

And since evenness of wear isn't strongly affected
by inflation pressure, wear isn't greatly affected.

^This^ flys squarely in the face of everything I've ever been told, by automotive technicians, mechanics, and have read on-line, about tire pressure and tires in general.

Inflation pressures are secondary only to where, and how, one drives, and how much one hauls regularly.
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Old 05-02-2020, 09:37 AM   #4
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Re: Another Tire Pressure Question! Front-Rear Considerations

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^This^ flys squarely in the face of everything I've ever been told, by automotive technicians, mechanics, and have read on-line, about tire pressure and tires in general.

Inflation pressures are secondary only to where, and how, one drives, and how much one hauls regularly.
Indeed this does fly in the face of what is generally accepted. On the other hand, what you have cited as experts aren't tire engineers - and even then there is disagreement. Allow me to tell you where I developed this.

For about 10 years in my career I was a development tire engineer - I designed them! I had access to a lot of data - and one of those was detailed tread measurements of wear tests being performed. Because of all the data available, I was able to see that position on the vehicle had a major effect on how even the tires wore. Because these tests were done at the GAWR for each axle, loading was even for the vehicles in question.

As it happened, I chose some test data that involved a full sized RWD sedan. The data from that test showed that the front tires slowly developed shoulder wear, while the rears slowly developed center wear. (Mind you, the loading and the pressures on the tires was the same.) That meant that there was a wear effect independent of the load/pressure.

We also used to perform a city driving test - where a vehicle was driven around a simulated downtown area. The tires wore extremely fast. In some cases, we had tires wear out in 5K miles!! Here the test called for empty vehicles (curb weight) and placard pressures, and since we were using SUV's, one would expect the tires to be worn in the centers due to the lack of loading. That was not the case. Just like the other tests, the front tires wore in the shoulders and the rears wore in the center.

Later in my career, I was working in field testing (regular vehicles in service), I noted the same sorts of things - EXCEPT, many of the vehicles were small FWD cars, and there, the front tires wore fairly evenly as did the rears. This flies in the face of the conventional wisdom you cited above.

So why? Because the fronts on a FWD do both the drive (torque) and the cornering, where these functions were separate on a RWD. In other words, the position effect (drive vs cornering) cancelled each other on a FWD.

But that does not explain why the rear tires on a FWD wear evenly - and that's because the affect load/pressure has on tires is small compared to the effect torque and cornering have on wear.

But all of what I said above applies to steel belted radial tires. Bias ply tires don't behave that way because they don't have that reinforcing belt under the tread. That belt stiffens the tire which results in much longer wear life (among other things.)

And here's where it starts to make sense. The conventional wisdom was based on bias ply tires. For some reason that I don't quite understand, these sorts of "wisdom" don't seem to change over time, even when they should. For steel belted radial tires, this "wisdom" is hard to refute because there are enough examples where it is true, and even more where it is hard to sort out all the effects going on.

Take for example the rears on an empty pickup wearing in the center. Conventional wisdom would say that the uneven wear would be caused by the light load/ high pressure thing - and since very few people know about the drive tire/steer tire thing, they would conclude that the wear is 100% the result of load/pressure mismatch. Ergo, it reinforces the faulty convectional wisdom.

So I hope that explains things. I have spent the last 20 years trying to get people to realize that not everything one hears about tires on the internet is true - and this is just one of many.
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Old 05-02-2020, 11:32 AM   #5
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Re: Another Tire Pressure Question! Front-Rear Considerations

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Originally Posted by CapriRacer View Post
Indeed this does fly in the face of what is generally accepted. On the other hand, what you have cited as experts aren't tire engineers - and even then there is disagreement. Allow me to tell you where I developed this.

For about 10 years in my career I was a development tire engineer - I designed them! I had access to a lot of data - and one of those was detailed tread measurements of wear tests being performed. Because of all the data available, I was able to see that position on the vehicle had a major effect on how even the tires wore. Because these tests were done at the GAWR for each axle, loading was even for the vehicles in question.

As it happened, I chose some test data that involved a full sized RWD sedan. The data from that test showed that the front tires slowly developed shoulder wear, while the rears slowly developed center wear. (Mind you, the loading and the pressures on the tires was the same.) That meant that there was a wear effect independent of the load/pressure.

We also used to perform a city driving test - where a vehicle was driven around a simulated downtown area. The tires wore extremely fast. In some cases, we had tires wear out in 5K miles!! Here the test called for empty vehicles (curb weight) and placard pressures, and since we were using SUV's, one would expect the tires to be worn in the centers due to the lack of loading. That was not the case. Just like the other tests, the front tires wore in the shoulders and the rears wore in the center.

Later in my career, I was working in field testing (regular vehicles in service), I noted the same sorts of things - EXCEPT, many of the vehicles were small FWD cars, and there, the front tires wore fairly evenly as did the rears. This flies in the face of the conventional wisdom you cited above.

So why? Because the fronts on a FWD do both the drive (torque) and the cornering, where these functions were separate on a RWD. In other words, the position effect (drive vs cornering) cancelled each other on a FWD.

But that does not explain why the rear tires on a FWD wear evenly - and that's because the affect load/pressure has on tires is small compared to the effect torque and cornering have on wear.

But all of what I said above applies to steel belted radial tires. Bias ply tires don't behave that way because they don't have that reinforcing belt under the tread. That belt stiffens the tire which results in much longer wear life (among other things.)

And here's where it starts to make sense. The conventional wisdom was based on bias ply tires. For some reason that I don't quite understand, these sorts of "wisdom" don't seem to change over time, even when they should. For steel belted radial tires, this "wisdom" is hard to refute because there are enough examples where it is true, and even more where it is hard to sort out all the effects going on.

Take for example the rears on an empty pickup wearing in the center. Conventional wisdom would say that the uneven wear would be caused by the light load/ high pressure thing - and since very few people know about the drive tire/steer tire thing, they would conclude that the wear is 100% the result of load/pressure mismatch. Ergo, it reinforces the faulty convectional wisdom.

So I hope that explains things. I have spent the last 20 years trying to get people to realize that not everything one hears about tires on the internet is true - and this is just one of many.
Well, my evidence to the contrary is more empirical, more hands on. In my former 2015 Hyundai Elantra, the handling felt more planted and confident with the rear pressure slightly lowered and the front slightly raised. Instead of 33psi all around, I ran 32 rear and 35 front. Less jittery, less step-out from the rear end, more car-like! Same for my wife's Corolla, which already had an advantage over the Elantra: independent rear end.

So my question to you is: Is what BMW and Subaru are doing wrong? Should drivers ignore the recommended F/R pressure offset, and split down the middle? IE, instead of F29, R33, just run 31psi all around? I kind of tend to trust the information vehicle manufacturers put on the tire placards on their vehicles, so it seems reasonable to accept that those two mfgs knew what they were doing when they did that.

I'm concerned less with wear than with how the thing feels when I'm behind the wheel. And you'll be happy to note that I do not follow the conventional wisdom of running lower tire pressures during winter - au contraire: I run them 2-3psi higher in winter.
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Old 05-03-2020, 08:11 AM   #6
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Re: Another Tire Pressure Question! Front-Rear Considerations

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Well, my evidence to the contrary is more empirical, more hands on. In my former 2015 Hyundai Elantra, the handling felt more planted and confident with the rear pressure slightly lowered and the front slightly raised. Instead of 33psi all around, I ran 32 rear and 35 front. Less jittery, less step-out from the rear end, more car-like! Same for my wife's Corolla, which already had an advantage over the Elantra: independent rear end. .
What you did there was to reduce the amount of understeer in the car. That, of course, is going to make the car feel more nimble.

Quote:
Originally Posted by RidingOnRailz View Post
..So my question to you is: Is what BMW and Subaru are doing wrong? Should drivers ignore the recommended F/R pressure offset, and split down the middle? IE, instead of F29, R33, just run 31psi all around? I kind of tend to trust the information vehicle manufacturers put on the tire placards on their vehicles, so it seems reasonable to accept that those two mfgs knew what they were doing when they did that.
No, you should not ignore what pressures are on the vehicle tire placard. They are there for a reason.

At the same time, you should consider them as a starting point if you want to cheaply change the way the car feels. The problem, here, is that it is easy to over do and you could get yourself in a great deal of trouble if you don't thoroughly think this through and do the testing necessary to validate that sort of change.

Quote:
Originally Posted by RidingOnRailz View Post
..I'm concerned less with wear than with how the thing feels when I'm behind the wheel. And you'll be happy to note that I do not follow the conventional wisdom of running lower tire pressures during winter - au contraire: I run them 2-3psi higher in winter.
That's another one of those bits of "wisdom" I am trying to change.

The way I see it is that the higher pressure tends to penetrate the snow and allows the tire to contact the road surface, instead of riding on the cushion of snow. The road surface will always have better traction than the snow, so it's the safer proposition.
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Old 05-03-2020, 07:54 PM   #7
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Cool Re: Another Tire Pressure Question! Front-Rear Considerations

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What you did there was to reduce the amount of understeer in the car. That, of course, is going to make the car feel more nimble.



No, you should not ignore what pressures are on the vehicle tire placard. They are there for a reason.

At the same time, you should consider them as a starting point if you want to cheaply change the way the car feels. The problem, here, is that it is easy to over do and you could get yourself in a great deal of trouble if you don't thoroughly think this through and do the testing necessary to validate that sort of change.



That's another one of those bits of "wisdom" I am trying to change.

The way I see it is that the higher pressure tends to penetrate the snow and allows the tire to contact the road surface, instead of riding on the cushion of snow. The road surface will always have better traction than the snow, so it's the safer proposition.
1. Understeer. I love understeer! So instead of increasing the front psi in that Elantra, it would have been better only to drop the rears to the 2011 spec(32psi), and leave the front tires at 33psi(2016 door placard). By the way the 2011 generation Elantra carried until 2016. Hyundai just upped the tire pressures on them starting in m.y. 2013 or 14.

2. I asked about ignoring BMW's F/R pressure difference because it sounds to me like you are more in favor of the same cold tire pressure all around, based on your reasoning in post #4.

3. Snow - a good song by the Chili Peppers. Also, something we do agree on - slightly higher pressure in the colder months.
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Old 05-04-2020, 08:08 AM   #8
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Re: Another Tire Pressure Question! Front-Rear Considerations

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1. Understeer. I love understeer! So instead of increasing the front psi in that Elantra, it would have been better only to drop the rears to the 2011 spec(32psi), and leave the front tires at 33psi(2016 door placard). By the way the 2011 generation Elantra carried until 2016. Hyundai just upped the tire pressures on them starting in m.y. 2013 or 14...…
I'm getting a mixed message here. On the one side, you said you liked what the change in tire pressures did, and on the other you said you like understeer. Those of mutually incompatible positions.

And I think it would be better to raise the fronts and not lower the rears. Lowering the rears makes the rear end looser, while raising the fronts makes the front end tighter.

Quote:
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…….
2. I asked about ignoring BMW's F/R pressure difference because it sounds to me like you are more in favor of the same cold tire pressure all around, based on your reasoning in post #4. ……
I am not in favor of deviating from the placard specs without careful consideration.

And I certainly wouldn't change the front/rear split without that same sort of consideration.

What I enjoy is a crisp turn-in, which I get by adding a few psi to both the front and rear. I think this principle operates regardless of the way the car was set up.

So, No! I am not in favor of setting the pressures the same all around, unless they were specified that way.

Quote:
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…….
3. Snow - a good song by the Chili Peppers. Also, something we do agree on - slightly higher pressure in the colder months.
Yes, we agree. But I like a few psi year round.

And one last point:
Quote:
Originally Posted by RidingOnRailz View Post
…….Hyundai just upped the tire pressures on them starting in m.y. 2013 or 14...…
Not according to Tire Guides. They list all Hyundai Elantras with a 32 psi spec in the 2013 and 2014 model year.

Last edited by CapriRacer; 05-04-2020 at 08:47 AM.
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Old 05-04-2020, 10:50 AM   #9
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Cool Re: Another Tire Pressure Question! Front-Rear Considerations

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I'm getting a mixed message here. On the one side, you said you
liked what the change in tire pressures did, and on the other you said you
like understeer. Those of mutually incompatible positions.

And I think it would be better to raise the fronts and not lower the rears.
Lowering the rears makes the rear end looser, while raising the fronts makes
the front end tighter.



I am not in favor of deviating from the placard specs without careful
consideration.

And I certainly wouldn't change the front/rear split without that same sort of
consideration.

What I enjoy is a crisp turn-in, which I get by adding a few psi to both the front
and rear. I think this principle operates regardless of the way the car was set up.

So, No! I am not in favor of setting the pressures the same all around, unless
they were specified that way.



Yes, we agree. But I like a few psi year round.

And one last point:


Not according to Tire Guides. They list all Hyundai Elantras with a 32 psi spec
in the 2013 and 2014 model year.
1) Looser Rear Ends: Lowering rear TP might have that effect in most cars, particularly those with indie in back, but not with that particular Hyundai Camar-COUGH COUGH!! -Elantra(!). With that generations revert to a torsion beam rear end, my lowering the rear cold pressures actually calmed the da__ed thing down. I was taking turns even more aggressively with it set up that way than before. The Elantra rear tendency to kick out in turns on rough pavement, or when traversing highway deck joints, was pleasantly reduced.

2) Elantras 32 spec: Then my M.Y. Elantra must have been the first of the 'MD/UD' generation to increase cold pressure to 33psi. Likely primarily in the interest of boosting CAFE compliance.

I bought the car in 2016 as a wreck replacement. I drove it for one week on 32psi cold all four tires, in the spring, not too hot or cold seasonally. It felt like a CAR again, instead of the basketball-on-wheels I had purchased, at 33psi. By 2018, I was doing the Rear32, Front34 offset thing, didn't lighten steering up too much, and helped my gas mileage a bit.

Capri:
I still maintain that the main(not only) reason car makers specify a single cold pressure in cars with a greater GAWR split(60/40) than in cars with a lesser GAWR split((55-45) that do specify a pressure difference) simply because it is easier for the demographic that buys that particular car to remember where to set it. That demographic typically checks their tire pressures at most 2x per year, and any more than that only when something looks or feels 'off'. As long as it gets them to work, that's all they care about, not fine-tuning the handling.

Applying a pressure split in my Elantra and 2005 Corolla worked for us. And that's all that matters. I might not have to apply much of a split in my present Accord's pressure, what with a wishbone up front and indie rear, and a recent good alignment, I'm already lovin it.

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Old 05-04-2020, 04:17 PM   #10
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Re: Another Tire Pressure Question! Front-Rear Considerations

Semi-related:

What, personally, would you consider enough 'resolution'(steps of measurement - not accuracy) in a digital tire gauge for your personal use?

1.0psi? 0.5? 0.1?

Examples of all can be had for under US$50, just look for sturdy build quality.
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Old 05-05-2020, 08:56 AM   #11
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Re: Another Tire Pressure Question! Front-Rear Considerations

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Semi-related:

What, personally, would you consider enough 'resolution'(steps of measurement - not accuracy) in a digital tire gauge for your personal use?

1.0psi? 0.5? 0.1?

Examples of all can be had for under US$50, just look for sturdy build quality.
For general purposes a digital tire gauge needs to only display single digit psi. Example: 35. psi. The fact that most digital tire pressure gauges show to the nearest 0.1 psi, I think, is just to give the user a degree of confidence.
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Old 05-05-2020, 10:57 AM   #12
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Cool Re: Another Tire Pressure Question! Front-Rear Considerations

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For general purposes a digital tire gauge needs
to only display single digit psi. Example: 35. psi. The
fact that most digital tire pressure gauges show to the
nearest 0.1 psi, I think, is just to give the user a degree
of confidence.
I keep my Honda Accord pressures at 33psi.
The difference I felt when driving was when going from:

(Using old mechanical clock-face gauge)
Front L: 32.7psi, cold
Front R: 33.0
Rear L: 33.2
Rear R: 32.9

to:

(Digital resolving to nearest tenth)
All four tires: 33.0psi cold, exactly.

Whether you choose to believe I felt a difference, or not, is up to you. I was in the car in question. I felt it. I was satisfied with the results.

To me, a digital tire gauge displaying, as you mentioned, to the nearest 1psi, is little more than a clockface or pencil gauge with a battery: useless. No more than just an 'emergency' gauge tossed in one's glovebox, or neath the saddle of their bike. Not to dipsarage analog gauges - there's some good ones out there, but often one must spend a little more for a really precise one than has to spend on a competitive digital model. And you drop one onto the pavement from more than knee height, is like flushing $50 for a good one down the toilet. Analogs can run up to $200.

Both a nearby tire installer and at least one garage I frequent are equipped with X.1psi resolution gauges. Why, because the exact psi, or kPa or whatever, matters to a lot of people. The tire place manager has all his gauges calibrated twice as often as required by state regulations.

It.

Matters.
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