Just wanted to conduct a quick informal poll about how many people reduce their tire pressure when they install wider tires. It seems like an obvious move to me, but I read/hear so many people complain about harsher rides with wider tires. I discovered that they were all still inflating their tires to original factory specs. They failed to realize that they are now spreading the same amount of weight over a larger area; hence pressure should be lowered to preserve the ride quality.

 

when I went to the 235 width I upped my tire pressure to 36F/34r, then again I dont care to much about ride quality since I have coilovers....

 

Pressures should be based more on sidewall profile rather than overall width. Going wider with lower profile while dropping pressures is asking for rim/tire damage.

 

I didn't say anything about going lower, just wider. But the principle still applies: you will still get a degredation of ride quality if you use stock tire pressures on wider tires that have stock sidewall height. The ride quality just gets even worse if you use the same tire pressure in tires that are also lower profile.

Pressure is a physical quantity. It is the amount of force per unit area, hence why tire pressure is measured in PSI (Pounds per Square Inch). Take my car for example. It weighs about 3200 pounds (force of gravity on the mass of the car). The factory tire pressure is 32 psi. From that, we can calculate that the total contact patch area created by all four tires is 100 square inches (3200 lbs / 100 square inches = 32 psi). Now if I put on tires that are 10% wider (say 245mm vs. 225mm), my total contact patch area will also be 10% wider, or 110 square inches. But my car is still 3200 lbs. So to get the same ride quality, I would need to lower my tire pressure to 29 psi (3200 lbs / 110 sq. in. = 29 psi). If you inflate them to 32psi, your tires may (depending on design/construction) start to round up in order to reduce your contact patch area back to 100 sq. in. That defeats the entire purpose of getting wider tires in the first place.

 

Higher tire pressure may adveresely affect ride, but it will increase tread life and fuel economy.

 

yeah i have 245/35/20 and i was running low pressure and my side walls cracked

 

bigEL'S math makes sense but there's got to be real world/usable limits. That would mean 28lbs on 245's and 34lbs on 205 snow tires. Upping pressure still increases performance by supporting sidewalls. And changing the tire profile actually changes the shape of the contact patch rather than just increasing it. I'm biased since I NEVER run below stock pressures except for drag slicks.

 

On agricultural tractors (and monster trucks that use agricultural tires), the tires are so wide that they only need 2-16psi to support 5+ tons. That's why than can drive on soggy mud that would gobble up anything with thinner tires. It's basic application of Newton's 3rd law (action = reaction). It's the same principle as spreading your weight out on thin ice or quick sand or a bed of nails. In WWII, the military used tires so wide on beach-landing personnel carriers that they used less than 1psi. Those tires could roll over people without even bruising them.

Yes, this is true. My example was very simplified. Tire construction/design must be considered too. Plus tire width is not measured at the contact patch, so going from 225mm to 245mm may not increase your contact patch by 10%. Calculating your actual contact patch area is as simple as raising/lowering your tire onto a piece of paper. This creates a "stamp" of your contact patch (use some ink or something on your tire to help visualize the patch if you want). Again, this is only for people who care about their ride quality or are just curious. I'm not saying everyone has to do this or something horrible will happen to them. Go with what you are comfortable with. Cheers.

 

I hadn't thought of that idea. I was simply sliding paper up to the front, back, and sides of the contact patch to get a measurement. Your idea is so much easier. Last year I needed to get accurate patch measurements when comparing tires - both for cornering grip and braking compares.

 

Jeep guys check for under/over inflation by drawing a chalk line across the tread and driving to see how the chalk is transferred onto the pavement.

 

You have to also take into account that when Nissan came up with the PSI standard it was for the tires that came on the car, so they could get the best ride comfort, fuel economy and handling. Different tires react differently to PSI. I could never run my 235/45/17 Conti Extreme Contacts at 32/32 let alone 29/29. The handling would be awful, yeah I might not feel as many bumps, but with the thinner sidewall I would most likely bend a rim. with the lower tire pressure they get mushey... it really depends on the tire.

You cannot just throw some simple math down and call it a day. you have to take many things into consideration. and using the cars weight at 3200lbs is also unrealistic, that does not account for you or anthing in your car. If you and a friend go out are you going to adjust your tire pressure accordingly? I doubt it, they use 32/32 because you have to guess someone might load up a car everyonce in a while to go on a trip or go the club with friends and your math is out the window.... You WILL get worse gas milage as well and you are going to wearout the edges of your tires.

But it is your car, if you are that confident in your highschool math, take the chance. See what happens when you and a friend are out and you throw your car into a turn and really LOAD up that tire....

 

I respect everyone's opinion, so there's no reason to get bent out of shape. I openly stated that my example was very simplistic and that there were other factors to consider. I'm not trying to hide anything or deceive anybody. I also said that I just mentioned it as an informal poll and something for people to consider and that I'm not trying to change anyone's mind or say that you will/won't damage your wheels if you do/don't follow my suggestion. It is just a suggestion. I further stated that everyone should feel free to do whatever they are comfortable with. Besides, you should realize that +/- 3psi is a noticeable, but not catastrophic difference. Your tires probably vary by that much normally due to temperature changes and/or friction. So the guy who cracked his sidewalls probably wasn't simply due to reduced tire pressure. Now if you'll excuse me, I have to finish my arithmetic homework before my mommy will let me ride bikes with Timmy.