AWD Tire Pressure Question

Wheel and tire information and upgrade discussions
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MrWhoopee
Posts: 35
Joined: Mon Dec 14, 2015 6:45 am

AWD Tire Pressure Question

Post by MrWhoopee »

From the owner's manual, I find that Toyota recommends 32 psi tire pressure front and rear for FWD vehicles, but 35 front and 32 rear for AWD. WHY???
The recommendations for tires on the AWD state that all tires must be the same brand and have the same tread pattern, presumably to ensure equal diameter and handling characteristics. It would seem that higher pressure in the front would create a slightly larger effective diameter of the tire due to a smaller deflection from vehicle weight. This would create a continuous load on the viscous coupler and a small amount of torque transfer to the rear wheels. Is this the intent of the higher pressure spec, or is there some other reason?

Facts, opinions and speculations welcome.

Enquiring minds want to know.
Craig
'06 Matrix Base AWD
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kumquat
Posts: 379
Joined: Wed Sep 18, 2013 9:01 am

Re: AWD Tire Pressure Question

Post by kumquat »

Probably because there's more weight over the front wheels than the rear wheels. In between the front wheels is the entire engine and transmission. In between the rear wheels is, usually, an empty metal box.
MrWhoopee
Posts: 35
Joined: Mon Dec 14, 2015 6:45 am

Re: AWD Tire Pressure Question

Post by MrWhoopee »

kumquat wrote:Probably because there's more weight over the front wheels than the rear wheels. In between the front wheels is the entire engine and transmission. In between the rear wheels is, usually, an empty metal box.
That would explain it generally (both FWD and AWD), but why only for the AWD? The weight of the differential offsets the transfer-case and the drive-line is more or less balanced. Why.........?
Craig
'06 Matrix Base AWD
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kumquat
Posts: 379
Joined: Wed Sep 18, 2013 9:01 am

Re: AWD Tire Pressure Question

Post by kumquat »

MrWhoopee wrote:
kumquat wrote:Probably because there's more weight over the front wheels than the rear wheels. In between the front wheels is the entire engine and transmission. In between the rear wheels is, usually, an empty metal box.
That would explain it generally (both FWD and AWD), but why only for the AWD? The weight of the differential offsets the transfer-case and the drive-line is more or less balanced. Why.........?
Matters less for FWD because there's no drive power to the rear wheels. In slippery conditions where you're going to be transferring power to the rear wheels with some frequency, you want to make sure they have as much traction as the fronts. If they're overinflated and have significantly less contact patch and traction, then they're just going to spin as well and not help you out. The driveline is not even remotely balanced. The rear differential weighs, what, 50-100 lbs max? How much do you think the engine and transmission weigh?

Also, besides traction, as you mention is diameter. At the same pressure, the front tires will have a smaller effective diameter. The computer could become confused and have less opportunity to provide optimal traction if it sees the rear wheels turning more slowly than the front.

It's all about the weight distribution, and there's a lot more weight in the front than back.
MrWhoopee
Posts: 35
Joined: Mon Dec 14, 2015 6:45 am

Re: AWD Tire Pressure Question

Post by MrWhoopee »

kumquat wrote:
MrWhoopee wrote:
kumquat wrote:Probably because there's more weight over the front wheels than the rear wheels. In between the front wheels is the entire engine and transmission. In between the rear wheels is, usually, an empty metal box.
That would explain it generally (both FWD and AWD), but why only for the AWD? The weight of the differential offsets the transfer-case and the drive-line is more or less balanced. Why.........?
Matters less for FWD because there's no drive power to the rear wheels. In slippery conditions where you're going to be transferring power to the rear wheels with some frequency, you want to make sure they have as much traction as the fronts. If they're overinflated and have significantly less contact patch and traction, then they're just going to spin as well and not help you out. The driveline is not even remotely balanced. The rear differential weighs, what, 50-100 lbs max? How much do you think the engine and transmission weigh?

Also, besides traction, as you mention is diameter. At the same pressure, the front tires will have a smaller effective diameter. The computer could become confused and have less opportunity to provide optimal traction if it sees the rear wheels turning more slowly than the front.

It's all about the weight distribution, and there's a lot more weight in the front than back.
When I said that the driveline was more or less balanced, I meant that it's weight was equally distributed front to rear.

The computer does not factor into this, the AWD system is strictly mechanical and I don't have traction control.

You have given me a likely explanation. The extra pressure in the front compensates for the extra weight, giving the front and rear wheels an equal effective diameter, keeping rotational speeds equal front to rear and eliminating any preload on the VC. It's not necessary to do this with FWD, so the tire pressures can be equal, giving a little softer ride.

I've been driving with equal pressure front to rear (having only just discovered the AWD specs) and have noticed a tendency to wander a little on slick roads at higher speeds (with studded snow tires). I will adjust the pressure today and see if it makes a difference.

Thanks for playing.
Craig
'06 Matrix Base AWD
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