18" Tire Pressure?

[JustinVGT]

Hey Everyone, I'm wondering those of you with 18" tires, what PSI are you inflating them to? I thought I've read that they are inflated more than the stock 16s. The tires I purchased are 225/40WR-18 Kumho Ecsta ASX SL. You may be curious what they are being wrapped around, you'll have to wait and see! We're going to be installing mine on Saturday, can't wait! I'll definitely post pics!

[ragingfish]

As far as I know, 32 psi is the pressure, regardless of tire size.But I could be wrong.

[DavidPIL]

Hiya...I've read that upsizing will allow you to use a bit more pressure in the tires (I think I read it on Ronal's site). I went with a wider (not larger in diameter) and the tire place filled em up to 44psi. I thought it was just to make sure it sealed to the rim but I went back a week later (to get an alignment done) and they said... no, these tires shouldn't be set any lower than 40psi. I was surprised since the max pressure was only 44. I had to set it down to 34-36 because I kept the stock wheel 17x7 and the tires need a 7.5" minimum wheel. If I had a wider rim width (which I should have and will eventually, I can increase the pressure to help maintain even tread pressure to the road. Now with the 7" width rim, apparently too much pressure causes the centers to round which would cause premature center tread wear. I dunno if this is all accurate but it sounds theoretically right to me.Anywho... I would check with where you get the tires to see what they recommend but I would think, especially if you go a little wider as well as larger rims, a little more psi (2-4 lbs) would be appropriate.Dave

[NovaResource2]

Quote, originally posted by silverawd26 »Bigger truck tires require more air depending on what the manufacture of the tire states.Volume and pressure are two different things. Yes, a larger tire will require more volume of air because the tire is physically larger however, the air pressure shouldn't need to be changed.Pressure is in PSI (pounds per square inch). That means that there is so much weight of the car carried by the square inch area of the tire contact patch. Unless you change the weight of the car you shouldn't have to change the tire pressure.For example of a car is 4000 pounds and the balance is a perfect 50/50. Then each tire holds 1000 pounds. With 32 psi in each tire that means there is 31.25 square inches of tire contact with the road at each tire (1000 / 32). That's about a 6" by 5.5" area per tire. Total, that equals approx 125 sq-in of traction. You want to maintain that area for better traction. If you raise the pressure to say 35 psi, then each square inch of tire contact carries more weight. Since the weight didn't change, the area decreases. Each tire still holds 1000 pounds since the weight of the car didn't change. With 35 psi in each tire that means there is 25.57 square inches of tire contact with the road at each tire (1000 / 35). That's about a 5" by 5" area per tire. Total, that equals approx 102 sq-in of traction. You've just lost 23 square inches of traction.The reverse is true for lower pressures, you get more tire contact with the road but the tradeoff is more rolling resistance and more heat buildup.

[joatmon]

seems like the tires for 18" wheels have short sidewalls. Do you need higher pressure in the tire to keep stuff like potholes from damaging the wheel rim? I know it would take a really serious pothole or a head on into a curb or something, just wondering if that was a consideration in tire pressure.

[Triton]

I was told to go with 35 lbs. in my 18's. I may back them down to the 32 lbs. recommended though after Scott's post.

[NovaResource2]

The 50/50 balance was an EXAMPLE to make it easier to understand. I know 99.9% of cars are no balanced that way. In reality, most cars are balanced left to right either. Cars as far back as the 50's Chevy Corvairs have used different tire pressured for front an rear tires.Trucks have higher pressures because they weigh more. Yes, sometimes cars require higher or lower pressures when you change tire sizes but that depends on the load rating of the vehicle. The factory pressure should be used unless the tire manufacturer suggests otherwise.You blanket statement that all larger tires get more pressure is not correct. A good example is that my 12-speed bike uses 100-psi for the tires. Since the tires on your Vibe are larger should you put more than 100-psi in your Vibe? No. My bike has a tire contact area of about 1 square inch pre tire. That's 2 square inches total. If I'm 200 pounds that means that those 2 square inches are supporting 200 pounds. That comes out to 100 pounds per square inch. Amazing, isn't it?

[Mavrik]

I'd go with the tire pressure indicated on the wheel. Mine suggest 40psi so thats whats in it and I check them once a week.

[syclnjr]

The 18's that I ahve on my show truck I run 40 psi to keep the sidewall flex down when hitting a bump. I have many buddies that have had there rims bent from running lower pressures and I didn't want this to happen. The fronts have 35 series and the rear is 40. I have yet to have a problem in this air pressure. I wouldn't go below 35 though. Food for thought. Hope it helps. Later

[JustinVGT]

Thanks for the input everyone. I installed mine yesterday and Tirerack had them inflated to 35.5 psi. I'm going to call and see what they recommend, but for now it seems as though that may be good.

[joatmon]

Mavrik, 40 psi seems high for stock tires. all the vibe documentation recommends 32. You may want to drop it some.syclnjr, that what I was thinking would be an issue. my son hit a curb in a differrent vehicle and took out a chunk of the alloy rim. the rubber on those 18's looks really short.Justin, glad you got some kind of answer, too bad nobody else who has 18's on had any advice. Kumho's site doesn't offer much help, all it says is if you replace with the same size tires as stock, follow the vehicle recommendation. You didn't, so their advice doesn't apply. Try to not hit any curbs if you do get an actual recommended number, let us know.

[joatmon]

Mavriks sig says "16" steel wheels/bolt on wheel covers.AM/FM 6 disc CD player with digital equalizer-6 speaker 200watt amped premium sound system.17" ICON Gun metal rims/chrome lip."I only saw the 16" steelies. Sorry Mavrick, you should pick one type of wheels instead of mixing and matching them like that Sorry Rob, but I think Scott's traction patch physics is more applicable to Justin's question than your abstract ideal gas law chemistry and large truck tire examples.

[scherry2]

now now boys no arguing

[goodvibe]

I would keep the pressures high so that your sidewalls and rims safe in case you you hit something. Tires do deflect and regardless of formulas, low pressures will allow them to do so more easily at a given point even though the greater width increases the load carring spec. which would indicate that you could run a lower pressure if you wanted. By the way, I think your tires are run flats so lowering the pressures will have less of an effect on ride due to the stiffened sidewalls. I went with 7.5" 16's so that I could play with pressures if I wanted a better ride and the rim width would help support the 225/50 sidewalls laterally when the pressures are lower.

[NovaResource2]

Quote, originally posted by silverawd26 »You have no idea on the Formula and the Link I posted above do you?You have no idea, PERIOD.Quote »I am not even talking about contact patches of rubber.That's your problem because it's the contact patch of the tire to the road that's important. Quote »But you said air pressure has nothing to do with volume, which it does, and so does temperature if it is not held at a constant.Incorrect. I said volume isn't important when it comes to larger tire sizes. Pressue and load are what are important.Quote »And Scott, lets keep this out of the thread, IM me for further discussion. No. This is what the thread is about. It's on topic. That's what you're always complaining about. Plus, I blocked you from my IM because I tired of all your insults and attacks through IM.

[rasermon]

I found a good link here to lower the pressure in this thread. Finding the Best Street Air Pressure http://www.film.queensu.ca/CJ3....html

[goodvibe]

It's an informative link, but there is a wide range of pressure that he can use and still have a good contact patch on a stiff sidewall low profile tire on a wide wheel. It's a very different situation than a high sidewall offroad tire on a 4wd Jeep though at extremes the info still holds true.

[NovaResource2]

Quote, originally posted by silverawd26 »Btw, load is what causes the pressure and that is what should make the PSI go up, Correct, load causes the pressure. When you add larger tires/wheels, you aren't changing the load.