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LuckyGuy's avatar

How much does tire inflation affect wheel alignment on an All Wheel Drive car?

Asked by LuckyGuy (38674points) 2 months ago

I had new tires put on my 2015 Subaru with All Wheel Drive. I also paid for a 4 wheel alignment.
The specification for tire air inflation is, Front: 30 psi, Rear: 29 psi. I happily drove home with my new tires and aligned wheels and noticed how differently the car handled. (A little oversteer)
Three days later, I checked the tire pressure and found: the front tires were 31 and 34 psi, and the rears were 37 and 37 psi! The outside temperature was 40F, the car was in the shade and had not been driven all day. I let the air out to set them correctly.
But now I am worried about the wheel alignment.
I was taught that the first thing you are supposed to do is set the tire pressure correctly before you do an alignment. I know the alignment is based upon the wheel itself, not the tire, but doesn’t the incorrect pressure affect how the car sits on the road? Caster? Camber?
How much of an effect is it? Is my alignment off now?
Is there any data that quantifies the effect of tire pressure?

I called the shop and they will recheck alignment but my confidence is shaken. What else did they miss? Lug nuts?

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16 Answers

JLeslie's avatar

My husband is no expert, but his reaction was, “they should be checking the pressure before doing the alignment.” Lol.

He says he thinks it will have some minor impact, but it depends on how strong the side walls are and also if they are low profile or not.

The alignment is toe and camber, and the pressure affects height probably so it wouldn’t have a lot of impact. He said it’s all computerized.

kritiper's avatar

Wheel alignment is not affected by tire pressure.
To others who may want to know; the pressure in a tire depends a great deal on the temperature of the tire. A warm tire will read a higher pressure than a cold tire. Tire pressure is to be checked when the tire is cold, whatever temperature that is! I check my tires in the morning, before being driven, when the outside temp is about 50 degrees. (A tire is considered warm after being driven for 1 mile.) The correct pressure of the tire is what the vehicle manufacturer recommends, not the maximum pressure shown on the tire.

Tropical_Willie's avatar

Tire pressure does not affect alignment, it does impact tire traction (sidewall flex and tire contact area).
Sounds like you need to take Subbie back for another alignment (don’t think they did it right) tire pressure wrong and a rookie doing the alignment ! (Toe-out in the rear and not slight toe-in can cause the over-steer and sloppy handling). Have the boss do the alignment or at least check current alignment!!

JLeslie's avatar

@kritiper The OP measured the pressure when his tires were cold.

kritiper's avatar

@JLeslie And yet the OP questioned the difference in pressure when he rechecked them.
The tires are warm in the store before being purchased, and they are mounted, basically, at room temperature. Then they sit outside overnight before being rechecked. It figures that the pressure would be different. Also, tire pressure gauges could differ some. Normally, people who sell tires always inflate tires to 35 PSI no matter what the vehicle sticker says, so I always recheck my tires after coming back from the tire store. (Most tires say maximum pressure is 35 PSI but some have a maximum pressure of 44 PSI.)

stanleybmanly's avatar

Are the pressure specs from the tires themselves or from the owners manual?

LuckyGuy's avatar

The pressure specs were from the car itself. On the door jamb. Front 30, Rear 29.

@kritiper I checked with a good gauge and read: Front Left 31, Front Right 34 , Rear 37, 37 when the tires were stone cold in the shade. The rears were grossly overinflated and the fronts were mismatched.

stanleybmanly's avatar

I wouldn’t worry about throwing your alignment off as long as the tire pressure is within the range clearly embossed on the sidewall of the tire itself.

LuckyGuy's avatar

The tire pressures were not even close. I will see next week when the car is realigned.

kritiper's avatar

The pressure listed on a tire itself may say something like this: “Max load 670 KG (1477 lbs.) at 300 KPA (44 PSI) cold max pressure”
So at maximum pressure, for the tire to sit properly on the ground for maximum traction and wear, each tire would be inflated to 44 PSI and have 1477 lbs. of weight on it, so the car would have to weigh 5908 lbs., or just under three tons. I doubt the car weighs that much. Also, with that much air in the tires, and the car not weighing so much, the tires would wear out prematurely due to over inflation (a center wear pattern), and the actual amount of rubber in contact with the road would be miniscule, which would result in less traction and a rougher ride. Best to go with what it says on the door post or glove box sticker.

My 91 Nissan pick-up calls for 35 psi front and 26 psi rear. (max psi on the tire is 44 psi) The front tire wear would normally be a center wear pattern with 35 psi, but is off set by being a turning/steering wheel which evens out the wear.

kritiper's avatar

@LuckyGuy It’s possible that two different people with two different gauges mounted and filled the front tires.

LuckyGuy's avatar

@kritiper Oh man. Two different people inflating the front tires and checking with 2 different gauges? If that were true that does not build my confidence in the place. it is a tire shop! They should have the best pressure gauges possible. The tires should never have been inflated to those levels, period.
These are not cheap tires. They are total performance Michelon CrossClimate + 225/R60–17. Winter is coming and it was definitely time to replace the old ones. Bald or low tread.

I have been looking for info about this situation and found an interesting comment:
“Absolutely your tire pressures will affect alignment readings! This should be the FIRST thing the tech does when he pulls the vehicle on the rack. Quite possibly your tire pressures may not affect your toe settings, but definitely will affect other things like camber settings, which WILL wear your tires prematurely. If you found uneven tire pressures after a shop performed an alignment, I would personally be looking for a more reputable shop to be performing your alignments from now on. Hope this helps, and good luck!”

It is too cold outside right now to run the experiment but when it warms up I will put a dial between the bumper and the ground and see how much the car body is raised when the pressure is increased.
In any case I will know the alignment result on Thursday when they do the alignment and supply before and after numbers. I’ll update you.

I was hoping there was a technical paper somewhere that addressed this.

kritiper's avatar

@LuckyGuy Some people may inflate tires to the exact pressure, others may get it close.
Yes the tire pressure will affect alignment readings but only when they are not correct, before the alignment is to be performed, and while the technician is doing the alignment. Low tire pressure would affect height adjustment measurement. One tire being lower on one side of the car will affect the camber measurement. Low tire pressure will not affect toe-in or caster measurement. BUT it won’t change the alignment after the alignment has been completed.
Ask your tire pro if he/she sets the tire pressure at the vehicle recommended pressure. (Good luck finding one that does!) Every tire shop I have ever seen or worked at sets the pressure at 35 lbs. They don’t go by what it says on the car!
I have mounted and balanced thousands of tires. I trained as a automotive technician at Denver Automotive and Diesel College for over 1200 hours, and this training included front end alignment procedures.

LuckyGuy's avatar

@kritiper I usually do most everything myself. For example, my oil changes take twice as long because I try to squeeze every last bit of the old oil out before putting in the new. (I also take a sample and send to Blackstone Labs for analysis every other time.) But for tires and wheel alignments I must rely upon someone else.

I think I can safely assume the tire pressures I measured (Frt: 31, 34 R 37, 37) were close what they were set to when the tech did the alignment. He showed me the report which indicated the alignment was out Now I am wondering if it was the difference in tires pressures that made the alignment appear off. (Right or wrong, my confidence is shaken.)
We will see.
Thanks for your input. I really appreciate hearing from the guy on the other side of the machine. I will send you the report.

kritiper's avatar

Two or three pounds of pressure won’t make a difference. Once the adjustments are made on the alignment machine, with the tires aired to the same pressure in all tires, the adjustment won’t change due to some tire being low while being driven under normal circumstances. One tire, say, the right front, being at 15 lbs and the left front being at 35, will affect the camber measurement ON THE ALIGNMENT MACHINE. Camber is how the tire leans in or out at the top. If it leans in past 0 degrees, it’s negative camber. If it leans out past 0 degrees, it’s positive camber. So you can see how one tire being REALLY low could affect the measurement, because the car is leaning to one side.

LuckyGuy's avatar

Tomorrow I will have the alignment checked and we will see how much of an effect the different front pressures and incorrect rear tires had.
BUT I have an idea or the magnitude already. (You knew I would do this!)
I used a digital caliper to measure the height of the car body above the ground and the height of the lowest spot of the wheel above ground as I changed the pressure from 39 psi down to 30 and back up again. (My reading to reading repeatability was well within 0.05%) A averaged the two readings for each pressure and plotted the curve. It was close to a straight line.

For my car, a 2015 Subaru Forester with Michelon CrossClimate + 225R60–17 tires, over that pressure range I found the car height changed 0.3mm per PSI. A 10 PSI difference will make a 3.2mm difference in height between wheels. I then related that height to body tilt assuming rigid body and calculated a tilt of 0.12 degrees. Small – but should be zero.
We will see tomorrow.

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