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While changing my tires for the winter, I checked my tire pressure and it was low (30psi) compared to the car recommended pressure (35psi).

This made me curious as to what the minimum tire pressure is for my tires and I could not find it. I know that both under and over inflating tires is bad, but I was curious what would be considered the minimum is.

Nokian (My tire manufacturer) says the following:

The correct inflation pressure will be provided by your tire retailer; if not, refer to the vehicle decal. These inflation pressures must be maintained as a minimum. However, do not exceed the maximum pressure rating indicated on the tire sidewall.

To my understanding, that means I should always stay between 35psi and the maximum rating. However, I also heard that this is the recommended pressure for the way the manufacturer wants the car to handle. (Can't remember where I read this, sorry)

Just how low can I go before I damage my tires from low pressure?

Edit:

It seems like the number in the door panel is the "minimum" according to current answers. Why is that the case, how does the weight of the car affect the minimum pressure for "all brand/models of tires"?

  • I will add a comment that the only exception to lowering pressures below recommended values that I know of is if your stuck in sand or soft dirt. I have seen tires lowered to 5 PSI to get out of the soft material. But then they must be inflated to proper values before travel. – spicetraders Oct 24 '16 at 18:06
  • The recommended value is just that, a value, not a range. So if the recommendation is 35 psi, 34 is too low and 36 is too high. Realistically, one or two psi in either direction isn't a big deal. – cory Oct 25 '16 at 15:50
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Year round you want to maintain your tires at the recommended tire pressure (COLD) for the stated vehicle load. For the most part it is a fixed value to maintain (not a range from there to maximum) with the exception it can be increased (not to exceed maximum) if the load is increased.
This value is normally on a tag found inside the driver side door frame or in the owners manual.
OEM values provided are set to a value to provide the most comfortable ride under normal conditions with optimized handeling. The MAX PSI on the tires side wall is the PSI that the tire will carry the most weight.

Under-inflated tires can damage the tires, lose traction, lower fuel economy, and in wet weather increase chance of hydroplaning.
It has been shown in testing that a higher number of blow outs occur in under-inflated tires then over-inflated.

Based on these factors. Lowering the pressure below the automobiles manufactures designed value can not be approached with out CALCULATIONS based on tire design, vehicle weights, atmospheric and road conditions and expected use.
Doing so will present possible unsafe conditions. Again it is safer to go higher then to reduce pressure.

Tire foot print, and side wall pressure needs to be set at a optimum profile for the tire. A tire that becomes under inflated will roll to much into cornering wearing heavy on the sidewall. Under inflated will cause cupping of the tire leading to outer edge wear shorting the tires life. It presents a smaller surface foot print on the ground reducing traction.

  • I do understand that, my question however is what defines an under-inflated tire. Is it simply being under the number in your door panel? I've seen a lot of discussion of people changing their values to get what they believe is a better driving feel. Such as lowering it for more grip (which spawned my question) and increasing the pressure for various reasons. – Yousend Oct 24 '16 at 18:13
  • @akadian I can understand people like a "softer" ride and by lowering the pressure stating a better driving feeling. But that does not mean the tire is not being pushed harder. I would really like to lower pressure on my truck to take out some of the harshness. But looking at the rolling pattern with even 5 pounds out 65 down to 60, I can see a distinct reduction in the footprint and cupping starting. Add in as I stated lower tire pressure has more chance of side wall blow out is my life worth it. Going a few pound higher has a plus on fuel use but tire wear also increases. – spicetraders Oct 24 '16 at 20:03
  • @akadian it would be nice if we could roll our cars on a thick piece of glass then raise and lower the pressure to find a sweet spot where max rubber meets the road producing the best foot print. – spicetraders Oct 24 '16 at 20:06
  • Thanks, needing to do calculations based on the various conditions satisfies me as an answer. Having switched my summers from 215/45R17 to 225/40R17 also had me wondering how accurate that would be for that. I always go 1PSI over recommended in case I lose some in the process and "recommended" implies that you can go higher or lower and be somewhat fine, hence my question. – Yousend Oct 25 '16 at 14:23
  • I was just conversing about this this morning, trying to decide on adding a post. IMHO Basically the minimum could be as low as 2 or 3 PSI if your car had no weight. The lowest pressure is what is needed to keep the bead locked when the most extreme lateral pressure is exerted (cornering). So as weight goes to zero, pressure goes down, but realistic unless your stripping cars weight no change there. If the two tire sizes your swapping semiannually show a max pressure difference (still having the same load range) then possible one could translate that difference to the car PSI values. – spicetraders Oct 25 '16 at 15:52
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You can go as low as the minimum recommended pressure. Lower than that will start to damage the tire and increase wear.

Mac Demere of Popular Mechanics states;

Without proper air pressure, the internal components of the tire—fabric, steel, rubber, and composites—flex beyond their designed limits.

Tire pressure is important to maintain the shape of the tire under load. With improper pressure, the tire can not only wear unevenly, it can cause loss of traction.

enter image description here
Why Tire Pressure Matters

  • I understand that pressure matters, my question is where do I get to the point where it is "under inflated". It just seems counter intuitive that the car manufacturer would be able to say the recommended is the minimum for all types of tires. Can you clarify on that part? – Yousend Oct 24 '16 at 20:35
  • You should keep your tires inflated within recommended pressure and the max pressure. As stated in my answer, if the pressure is below the recommended, then it is under-inflated. Keep in mind, tire manufacturers don't know what vehicle you own. That is why you should use the recommended tire size and pressures listed by the vehicle manufacture because they now the weight and loading specs for the vehicle. It is really that simple. – CharlieRB Oct 24 '16 at 23:26
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    @akadian the manufacturer specification is not for "all types of tires" it is for the specific size tires that came on the car. My car actually has three pressures listed based on which wheel package (including smaller snow tires) you configured it with. If you install aftermarket wheels or different sized tires then the car manufacturer's recommendations no longer apply. – Ukko Oct 27 '16 at 15:14
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Here's my take on this. As @alephzero mentioned to support vehicle weight properly and not damage tires you need to maintain certain contact area which will guarantee correct tire profile. Let's consider Honda Civic which has a curb weight of 2800 lbs. Obviously manufacturer doesn't specify pressure as a function of load, so we can safely assume that recommended 30 PSI means full load, which for this car is 850 lbs, i.e. full weight is 3650 lbs. So if there are only two people in Civic, its weight would be around 2800 lbs + 400 lbs = 3200 lbs. Thus to maintain the same contact area we only need 30 * 3200 / 3650 = 26 PSI. Obviously weight may not be distributed 50/50, so 26 PSI is a bit optimistic, but in general if you only drive alone or with one passenger 26 PSI is enough (not that I recommend it though).

On the other hand if a tire is underinflated it will heat up fast because or extra flexing and pressure will rise, providing some negative feedback.

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Useful pressure info should be printed on the side of the tire.enter image description here

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    That's not the minimum tire pressure for the tire, though. That's the maximum pressure the tire can handle. Is the minimum tire pressure listed anywhere? – Ellesedil Oct 24 '16 at 21:43
  • @Ellesedil The minimum pressure depends more on the weight of the vehicle than on the tire itself. See my answer. – alephzero Oct 25 '16 at 3:07
  • @alephzero I actually understand that. I'm challenging the person who wrote this answer to improve it. – Ellesedil Oct 25 '16 at 3:09
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The sticker on the drivers' door will give you the proper tire pressures. It's not a matter of minimum versus maximum, but a matter of contact patch.

For instance, you may think that lowering the tire pressure when it rains would help, but it's the opposite that happens an you

The tire pressure that is indicated on the door is based on the stock tire size, so if you replace your tires with an identical size, just follow the recommended pressures (to be measured when the tires are cold)

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The basic "mechanical engineering principle" behind tire pressures is fairly simple.

Ignoring the stiffness of the side walls, the air pressure in the tires has to support the full weight of the vehicle. The weight of the vehicle has to be balanced by the tire pressure, multiplied by the contact area between the tire and the ground.

If the pressure is lower than recommended, the contact area will be larger (that becomes very obvious when you have a completely deflated tire!). The increased area will affect the sideways friction force on the tires when the vehicle is cornering, increase the amount of flexing of the tire tread as the wheels revolve (because of the "flat spot" in contact with the ground), and tend to increase the load through the side walls, which causes more flexing of the walls and potentially more damage to them.

On the other hand if the pressure is too high, the contact area with the road will be reduced, which makes it more likely that the tires will start to slide when cornering or braking.

The "minimum pressure" will probably correspond to an the design conditions for an unladen vehicle, and the pressures should be increased as the amount of load increases - that factor is obviously is more significant for a truck than for a compact passenger car.

Higher pressures increase the tension around the circumference of the tire, which resists the air pressure inside. The maximum pressure will correspond to the safe limit for that tension, before the structure of the tire is damaged - this is the same basic reason as the maximum pressure limit for a metal compressed air tank.

The tire pressure also affects the stiffness of the tire in resisting changes in vertical loads caused by bumps in the road. The stiffness will affect the behaviour of the vehicle suspension, and therefore affect the handling.

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