Tire pressure Forester

[Bobbie Mayer]

I am at my mechanics with my 2000 Subaru Forester and he says the door sticker says 24 lbs in front and 36 in back. I have always run 32 or 36 everywhere. He says it says for towing 41 in back, which I think exceeds the tire specs. What do you do, and why would tire pressure be lower in front when not towing, other than comfort which costs gas mileage?

Bobbie

[Kevin K]

The simple answer is under steer. Lower front tires pressures generally induce under steer.
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Reccomended tire pressure - fronts lower? does that make sense? [Archive] - Toyota Nation Forum : Toyota Car and Truck Forums

[Greg H]

Follow the tire manufacturers recommended air pressure, obviously those are not the original tires.

[honda03842]

Originally Posted by Bobbie Mayer

I am at my mechanics with my 2000 Subaru Forester and he says the door sticker says 24 lbs in front and 36 in back. I have always run 32 or 36 everywhere. He says it says for towing 41 in back, which I think exceeds the tire specs. What do you do, and why would tire pressure be lower in front when not towing, other than comfort which costs gas mileage?

Bobbie

I'm not familiar with your tires but I do know that the maximum tire pressure is listed on the tire. Mine is 44psi. We increase our rear tire pressure to 39#s when towing to increase their load carring ability and stiffness.

We also increase our front tire pressure to 32 psi. I don't have any science for the numbers.

Norm

[Bobbie Mayer]

What is understeer? Obviously if I've always driven with all four the same I drive fine that way. I can see bumping up for towing but not if the tires won't support it.

I remember when the Ford's were rolling over; one issue was under-inflation of tires as the manufacturer recommended lower pressures to make the ride better. At that time they said go with the tire's recommended rating, not the vehicles.

[Brian Van Snell]

Originally Posted by Bobbie Mayer

I am at my mechanics with my 2000 Subaru Forester and he says the door sticker says 24 lbs in front and 36 in back. I have always run 32 or 36 everywhere. He says it says for towing 41 in back, which I think exceeds the tire specs. What do you do, and why would tire pressure be lower in front when not towing, other than comfort which costs gas mileage?

Bobbie

Bobbie:

We drive a 2003 Forester. The door sticker says: Light load 29 psi front, 28 psi rear, Full load 29 psi front, 36 psi rear, Towing 41 psi rear.

We use Michelin Energy MXV4 plus tires which have maximum rated pressure of 44 psi. We have a small 12 volt air compressor which I use to adjust the pressure. When we are not towing, ride is better using the rated Light load pressures. When we are towing I put 38 psi in the rear tires which seems to work okay with our light trailer.

Brian

[Bobbie Mayer]

I need to check- I think he misread 29 as 24. 41 rear is what mine said for towing but I think that exceeds the tire rating; I'll check that, too. But I don't see how they can rate the car rather than the actual tire unless you keep putting the same tires back on (and mine were not available by the first time I needed tires.)

[Greg H]

That's pretty much what I said earlier, follow the tire makers instructions not the car makers. the car maker put those pressures on there for the original tires.

[HikeSticks]

Originally Posted by Greg H

That's pretty much what I said earlier, follow the tire makers instructions not the car makers. the car maker put those pressures on there for the original tires.

Tires come in a wide range of sizes and specifications. When purchasing tires consider how they will be used. The car owner's manual will specify the size range. If the car owner's manual specifies 41 psi for the rear tires while towing it is likely there is a tire available that will accommodate that but they may not be 'in stock' at every tire store. If your tire store doesn't have a tire in stock that will meets your needs you may have to wait for them to order them in or go to another store.
Good luck,
Barrie

[Greg H]

Yes Barrie your right, but car makers put tires on the car at the factory, lets say Detroit, Mi. This week they have a warehouse full of Goodyears, so the cars built this week get Goodyears with the proper size and weight capabilities of the car, next week they have a warehouse full of Generals, those cars are going to get the Generals, the Goodyears and the Generals will both be capable but that doesn't mean they will have the same inflation recommendations. The door tag is good for tire size and weight carrying capabilities but I will always go by the tire makers recommendations not the car makers.

[Bobbie Mayer]

I'm getting a new car soon so what new tires I put on is moot but I may try varying the pressure and see what pulls best if I use the trailer before getting the new Forester. I can see there is little agreement on this issue, though! That's what I find online, too, little agreement.

[Steve L.]

Originally Posted by Bobbie Mayer

What is understeer?

(Geek hat on!) In broad terms, an understeering vehicle leaves the road front first. An oversteering vehicle leaves the road bass-ackwards. In both cases I'm assuming the driver has lost control in a corner and something bad is about to happen.

Tires develop lateral force as a function of slip angle, tire pressure and load. Increasing any of them increases the cornering ability of the tire. Both the front and rear tires must develop lateral force (a turning force) in a turn. All a tire can do on its own is to increase the slip angle (the direction the tire is "facing" relative to where it's going.) It's en-tire-ly (pun intended) possible for the front tires to get the vehicle turning more quickly than the rear tires can keep up. .

Generally, it's thought that it's better for a customer's front tires to "wash out" before the rear tires. Therefore they specify less pressure in the front tires so that front tires run out of traction sooner than the rears. So as the road turns, the driver turns the steering wheel (and tire) more and more until the limits of the front tire are exceeded. The vehicle is understeering with respect to the curve.

There's actually a bazillion things going on from the tire's perspective during a corner but this is sort of the big picture…

[Ed Harris]

Sure Steve you took the words right out of my mouth!

Oh and don't mention what you do during the day?

Ed

[Cam A]

Originally Posted by Bobbie Mayer

I am at my mechanics with my 2000 Subaru Forester and he says the door sticker says 24 lbs in front and 36 in back. I have always run 32 or 36 everywhere. He says it says for towing 41 in back, which I think exceeds the tire specs. What do you do, and why would tire pressure be lower in front when not towing, other than comfort which costs gas mileage?

Bobbie

I towed aprox. 6000 Km with my 98 Forester/Surfside combo at the recommended 42 psi. rear tire pressure, with the fronts at 36. No issues, nor abnormal tire wear.

[Bobbie Mayer]

I checked the sticker: it is 29-26 for low load, 29-36 for loaded, and 29-41 for towing. I can see the idea of more in front for unloaded (the weight is in front) and more in back for loaded (weight shifts toward back) but my major problem towing if anything is bouncing in back on rough roads; wouldn't higher tire pressure make that even worse? I forgot to look at the tires and will do that soon.

[Roger C H]

Oversteer and understeer are just what they say.

Oversteer: the car turns (steers) more than is expected.

Understeer: the car turns (steers) less than is expected.

To make things more complicated, front wheel drive and rear wheel drive have some opposite handling characteristics, however higher tire pressure adds more steering grip to the axle it is on. That is front axle = oversteer, rear axle = understeer.

YouTube - Car Handling - Grip, Oversteer & Understeer explained by Tiff Needell

Everyone gets used to the handling of the car they drive. If you change the ratio of tire pressure front axle to rear axle, you can affect how the car handles. Front wheel drive and rear wheel drive are discussed in the video. AWD, or All Wheel Drive avoids the power over/understeer of the single axle powered car.

Apparently Subaru desires an understeer condition when towing because of the danger of the TV sliding out and the trailer consequently jackknifing.

I recall hearing a screaming sound and then observing a large white van sliding driver side first sideways down the highway with a utility trailer actually pushing the van down the road. The van had just negotiated a left curve, so I assume he had over compensated for an oversteer condition to the left and caused an oversteer to the right and once the jackknife occurred and the trailer took over, he was helpless to do anything but ride it out.

[mcbrew]

Originally Posted by Greg H

Yes Barrie your right, but car makers put tires on the car at the factory, lets say Detroit, Mi. This week they have a warehouse full of Goodyears, so the cars built this week get Goodyears with the proper size and weight capabilities of the car, next week they have a warehouse full of Generals, those cars are going to get the Generals, the Goodyears and the Generals will both be capable but that doesn't mean they will have the same inflation recommendations. The door tag is good for tire size and weight carrying capabilities but I will always go by the tire makers recommendations not the car makers.

Where do you find the tire manufacturer's recommendation on the inflation for your vehicle? You mean the MAX inflation on the sidewall? Filling to the max inflation (or some given percentage of it) is like always taking an elevator to the top floor. The MAX is the MAX, but not necessarily the proper pressure for the size, weight, and handling characteristics for a given vehicle.

[Bobbie Mayer]

I think Subaru is just basing it on where the weight is likely to be. Low load, put more pressure in front where the engine and passenger(s) are. Bigger load, put more in back. Towing, put still more in back.

[Greg H]

mcbrew, If you look at the door jam on your vehicle you will see the recommended pressure the car maker wants you to use, but remember that was with the original tires. I don't run at the max I run a little below max and at a pressure that rides good. Also remember the pressure stated is at cold tire temp.

[mcbrew]

Originally Posted by Greg H

mcbrew, If you look at the door jam on your vehicle you will see the recommended pressure the car maker wants you to use, but remember that was with the original tires. I don't run at the max I run a little below max and at a pressure that rides good. Also remember the pressure stated is at cold tire temp.

Not what I was asking. Some people mistakenly inflate all tires to the MAX sidewall pressure and disregard the pressure recommendations from the car or trailer manufacturer or the tire manufacturer's weight chart (for trailer tires).