Kooky Tire Pressure Question

On a lot of cars, I'm guessing 1 out of three typical sedans, a different tire pressure is specified for the front axle vs the rear. Typically, it will read something like my old Ford Contour: Front: 31PSI - Rear: 34PSI. On other cars with different front/rear pressures I've seen "F: 29 - R:31" and even "Front: 33/Rear 36psi".

Now, folks, Smooth here lives by his GUT. And my gut instinct tells me, that in modern front-wheel drive cars, at lest 55-60% of their weight will be borne by the Front(read: LEADING) axle. Ergo, I would want more pressure in the tires under the heavy end of the car, and less pressure at the lighter end - so as to increase contact patch area and keep it planted back there in sudden turns, etc.

In my 2008 Kia Optima, Kia recommends on the door jamb cold F/R 30psi. I run Front-33, Rear-31. Just that small offset has not only firmed up my ride but had eliminated some under-steer. My wife's 2005 Corolla - same thing. F:32psi, Rear:31psi. She maneuvers that thing through traffic on the Merritt Parkway - one-handed - 70 to 80mph!

Why do mfgs with front-rear tire pressure offset seem to have it backwards according to my own gut & experience, putting more psi in the already potentially fishtailing rear axles of these already tail-light front-wheel drive cars?

Because manufacturers are stupid! That's why we have companies like Edlebrock and Summit Racing.. Because manufacturers don't know jack about the way cars SHOULD be run! :)
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Because manufacturers are stupid! That's why we have companies like Edlebrock and Summit Racing.. Because manufacturers don't know jack about the way cars SHOULD be run! :)
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And this has WHAT to do with tire pressures? :loser:

Manufacturers engineer cars to exhibit certain specific handling characteristics. Typically, benign or safe, predictable characteristics are chosen over 'performance' characteristics. For example, most cars are designed to understeer as one does through fast corners. The faster you go, the more understeer you get.

This is intended as a built-in warning device, so that the faster you drive through corners, the less responsive the car gets, so drivers know they are approaching the car's cornering limits. As drivers, we see this so much that we think this is natural. But often, it isn't, it is simply an engineered-in characteristic for safety. One could just as easily engineer a car to be 'neutral' that is, have no understeer, or in fact, oversteer as one goes through corners.

So, having a higher tire pressure in the rear is simply a way of 'fine-tuning' such safety-related handling characteristics. A slightly lower pressure in front would increase understeer a bit, so the front end would 'push' more in fast cornering.

But you do not actually have to go with this. I agree with you..... tire pressures should be set to achieve the same-size contact patch and sidewall shape on the front and back------ which usually means higher pressures in front for front-engine cars....... unless the car is fully-loaded.

BTW, such tire pressure differences can get pretty extreme. The first-generation Chevy Corvair (1960-64) has a rear-weight bias and swing axles at the back (like an old VW Beetle or Porsche). This made for very odd, unpredictable handling at the back. So, GM specified 25 psi for the rear tires (normal for low pressure bias-plies) but only 15 psi at the front. Yes, just 15 psi.
Now, the problem here was that unless one was aware of the specifics, people often put the same pressure at both ends (25 psi or so) which emphasised the squirrirly handling. Lots of Corvairs ended up rolling over, partly due to improper tire pressure, as reported in Ralph Nader's book, Unsafe at any Speed.

Now, the problem here was that unless one was aware of the specifics, people often put the same pressure at both ends (25 psi or so) which emphasised the squirrirly handling. Lots of Corvairs ended up rolling over, partly due to improper tire pressure, as reported in Ralph Nader's book, Unsafe at any Speed.

Hmmm, sounds a bit like what was happening with Explorers, mixed with driver's over-reacting when they had a tire blow out. Considering how easy and quick it is to check tire inflation, it's amazing how few people actually make it part of their normal routine.

-Rod

For all the cars/trucks I have owned, I have never seen the correlation between those labels and what I find to be the balance point between front & rear. The labels usually just state "35F/35R", presumably to just eke the most mileage out at the expense of the tires wear pattern.

I have typically run 31-33 psi in the fronts and 29-32 in the rears (generalizing for all my vehicles). Never had issues with uneven wear unless I was lax about checking them and the temps changed on me.

To the best of my knowledge, psi varies according to the brand of tire. I always inflate to within 2psi listed on the tires sidewalls. i.e. my Goodyears list Max. psi 44. So I go with 44 in winter & 42 Summer. (Outside temp. causes tire pressure to fluctuate slightly.

To the best of my knowledge, psi varies according to the brand of tire. I always inflate to within 2psi listed on the tires sidewalls. i.e. my Goodyears list Max. psi 44. So I go with 44 in winter & 42 Summer. (Outside temp. causes tire pressure to fluctuate slightly.

The tire pressure on the sidewall is maximum, and as I understand it...it only applies if you are running the maximum allowable load on the tire. Typically, it is not applicable and will just result in an over inflated tire (good gas mileage but poor wear and possibly poor grip).

The vehicle specified tire pressures are based on tire size (OEM tires) and vehicle weight/distribution...also as stated above it sounds like there is some safety factors thrown in for good measure. So I would vary tire pressure based from the recommend vehicle pressure not the maximum tire pressure.

To give a few more points of reference here are the recommended tire pressures for my vehicles...

Full Size Van - 35/44 psi [Front/Rear]
Old Toyota- 27/27 psi [Front/Rear]
New Civic - 32/32 psi [Front/Rear]

Just adding my 2cents as a mechanic: tire pressures are dangerous at 2 pressures : Too high and too low. Set em between 32-36 and don't think too hard about it. The sidewall is for MAX LOAD not normal driving but, know what it says and don't go over it. On newer cars with TPMS the door sticker may say set em at 29 but, I can tell ya, when that tire hits 26 the light will come on and you will get tired of it :(

BTW, such tire pressure differences can get pretty extreme. The first-generation Chevy Corvair (1960-64) has a rear-weight bias and swing axles at the back (like an old VW Beetle or Porsche). This made for very odd, unpredictable handling at the back. So, GM specified 25 psi for the rear tires (normal for low pressure bias-plies) but only 15 psi at the front. Yes, just 15 psi.
Now, the problem here was that unless one was aware of the specifics, people often put the same pressure at both ends (25 psi or so) which emphasised the squirrirly handling. Lots of Corvairs ended up rolling over, partly due to improper tire pressure, as reported in Ralph Nader's book, Unsafe at any Speed.

#1. I'd venture to guess that at least half to 70% of drivers in this country do not know(or care!) where the correct tire pressure for their car comes from - yet in most cases they swing themselves right over this source of information, the sticker adhered to the lower b-pillar below the door strike, everyday - as they get into the drivers seat of their cars!

Now, to tie this into the Corvair case. Would strict adherance to F:15/R:25psi have drastically cut down on the number of loss-control cases, flipping over cases associated with the Corvair?

Also - it is general knowledge that the 1st Gen Corvair rear axles comprised of a primitive "swing-arm"

http://www.corvaircorsa.com/tech/earlyaxle.jpg - sorry! can't get the damn image to display in my reply!

suspension in which under extreme movement the rear camber would drastically alternate between negative & positive, contributing to that squirrely handling.

Would the above setup have contributed to far less of those incidents if the correct tire pressures(+ - 2lbs) were present?

#1. I'd venture to guess that at least half to 70% of drivers in this country do not know(or care!) where the correct tire pressure for their car comes from - yet in most cases they swing themselves right over this source of information, the sticker adhered to the lower b-pillar below the door strike, everyday - as they get into the drivers seat of their cars!

Now, to tie this into the Corvair case. Would strict adherance to F:15/R:25psi have drastically cut down on the number of loss-control cases, flipping over cases associated with the Corvair?

Also - it is general knowledge that the 1st Gen Corvair rear axles comprised of a primitive "swing-arm"

http://www.corvaircorsa.com/tech/earlyaxle.jpg - sorry! can't get the damn image to display in my reply!

suspension in which under extreme movement the rear camber would drastically alternate between negative & positive, contributing to that squirrely handling.

Would the above setup have contributed to far less of those incidents if the correct tire pressures(+ - 2lbs) were present?

There were several factors that contributed to the Corvair's rollover problems which is why I said tire pressure was partially responsible.

The recommended tire pressure made the car understeer more, so it was less likely to slide the rear end when cornering. But once one got it sideways enough, the swing axle would lift the rear, allowing the wheel to go off-camber.

When this happened, another Corvair deficiency came apparent.... the lack of "safety" rims. The old-style rims allowed the rear tire to peel off the rim, when the wheel was off-camber allowing the steel rim to dig into the pavement..... and over it went.

So, correct tire pressure made the car less likely to get into trouble, but the car still rolled far too easily, and thus was considered a hazard. BTW, GM fixed this problem, with a redesigned rear suspension and "safety " rims in late -1963, 18 months before Ralph Nader's book was published.
The old VW Beetle had worse safety problems than the Corvair, and Nader never went after VW. Why? Imo because Nader was somewhat self-serving, GM was a bigger target and better for launching his career.

This video has a great analysis of the suspension problem and is an interesting analysis of the book. It makes the common claim that, if Nader had not targeted the Corvair, GM would have been more progressive and innovative in their car design in the '60's through '80's, and not let the imports get so far ahead. I am inclined to believe this..... GM never again built a truly innovative high-volume car.
http://www.youtube.com/watch?v=IF-8a6B8Ljk&feature=related

There were several factors that contributed to the Corvair's rollover problems which is why I said tire pressure was partially responsible.

The recommended tire pressure made the car understeer more, so it was less likely to slide the rear end when cornering. But once one got it sideways enough, the swing axle would lift the rear, allowing the wheel to go off-camber.

When this happened, another Corvair deficiency came apparent.... the lack of "safety" rims. The old-style rims allowed the rear tire to peel off the rim, when the wheel was off-camber allowing the steel rim to dig into the pavement..... and over it went.

So, correct tire pressure made the car less likely to get into trouble, but the car still rolled far too easily, and thus was considered a hazard. BTW, GM fixed this problem, with a redesigned rear suspension and "safety " rims in late -1963, 18 months before Ralph Nader's book was published.
The old VW Beetle had worse safety problems than the Corvair, and Nader never went after VW. Why? Imo because Nader was somewhat self-serving, GM was a bigger target and better for launching his career.

This video has a great analysis of the suspension problem and is an interesting analysis of the book. It makes the common claim that, if Nader had not targeted the Corvair, GM would have been more progressive and innovative in their car design in the '60's through '80's, and not let the imports get so far ahead. I am inclined to believe this..... GM never again built a truly innovative high-volume car.
http://www.youtube.com/watch?v=IF-8a6B8Ljk&feature=related


Yeah, the 'Vair's not the only thing Nader could've wrecked. He helped Bush get elected in 2004(by winning the hearts & minds of Democrats!), and could have - but mysteriously didn't - helped McCain out, just by throwing his independent hat in the ring.

I believe in my diagram the lower illustration reflects the changes you referred to, regarding the Corvair rear end.

I believe in my diagram the lower illustration reflects the changes you referred to, regarding the Corvair rear end.

You are correct. Also that video link I posted uses a simple diagram to demonstrate the Corevair rear end problem.

Also, you brought back some memories. I recall Democrats publicly pleading with Nader not to run for the reason you stated. Of course, his running was a pointless, self-serving exercise.

But this video demonstrates Nader's attitude, he is in favor of taking action to make a point or to draw attention to a cause, regardless of the consequences or futility of the exercise.

http://www.youtube.com/watch?v=Lmd3I3Q33zE&feature=related

I've always adjusted my tire pressure according to how the tires were wearing. Starting in the neighborhood of the manufacturer's recommendations, check your wear pattern after 5 or 10k miles. If the tire is wearing in the center, it's overinflated. Edges wearing? Underinflated.

Agreed that max inflation pressure should match up with max weight on the tire. However with all the manufacturers and different tire designs and suggested pressures, you can't just go with one number, especially if you are putting a different tire than original on the vehicle.