Tire load range / optimal street psi?

[cshontz]

I'm used to running a Load Range C tire at around 32 psi on the street. This is the first time I've ever run a Load Range E tire which has a much higher maximum psi.

http://www.bfgoodrichtires.com/assets/pdf/all_terrain_ta_kd.pdf

My instinct is to continue to fill my new E tires to 32-35 psi, but I'm wondering if this is somehow less than optimal for a tire that may have been designed to run at a higher pressure.

Can anyone enlighten me on this subject?

 

[OldSven]

I had a set of 235/85/16 BFG's on my Tacoma and they were a load "E" rating. The max said 80psi and I ran them at 60 and they were way to stiff so set them at 35, and they ran fine.

 

[paulj]

Is the weight of your vehicle close to the maximum load for the tire? Those E range BFG tires are listed at some thing like 3043 lb at 80psi. Presumably that means that if your truck has a loaded weight of 6T, you must run your tires at that maximum rated pressure. At 2-3T you should be fine with much lower pressures.

That BFG table also has a footnote: 'To assure correct air pressure and vehicle load, refer to vehicle owner's manual or tire information placard on the vehicle.'

Tirerack has some good tech notes on tire pressure, such as
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=1

Car manufacturers put a recommended pressure in the manual or on a sticker by the door. They may even specify different values front and back. When people ask about tire pressures on Element forums, I (and other experienced owners) say, stick with this recommended pressure, or step up from there a few psi at at time till you get the handling you want.

I have not seen a good discussion about whether you should use a different pressure if you change tire size. It is possible that with a larger tire (i.e. larger air volume) that you can get by with a lower pressure, but I hesitate to recommend that.

Of course these sticker pressures apply to highway driving, where heat buildup do to tire flex is an important consideration.

paulj

 

[calamaridog]

I frequently reference this post. I thought it covers all the basics.

Air Pressure for your new tires:

Originally posted by KWRANGLN on Pirate BBS

<So you've got new tires, and they're bigger than the ones you took off so the factory recommended pressure printed on the door jam wont work. Neither will the max pressure stamped on the sidewall of the tire. Why not you ask? The max pressure is followed by a weight, that is the amount of weight that the tire can support safely at a given pressure. In most cases the entire weight of your jeep could be supported by one or two tires if inflated to max pressure, you don’t need that much. So, what to do?

I know, I'll post the question and see what everyone else is running and use that...

Wrong answer, thank you for playing, please have a seat and listen up.

Everyone’s rig is going to have a slightly different "optimal" pressure to get the best ride and tire wear. Even similar setups will have different weight distribution and options that affect how the tires wear. Joe TJ who has added nothing but a spacer lift and tires is going to need different pressures than John YJ who has added a V8 engine and one ton axles, even tho they have the same sized tires and rims.

So how do you find out what’s right for you and your rig?

Well, it’s fairly easy, and only takes a little time. Allot about an hour to figure it out and make sure you have access to a compressed air source. Make sure you have everything in the jeep that you normally have in there, if you carry 100 lbs of tools to work, make sure to include them, etc. Do this with about 1/2 tank of gas.

You will need:

Chalk or paint pen, something that will make an easy to see line across the tread of your tire.

A road to drive on, preferably mostly strait, and dry.

A tire pressure gauge.

A source of compressed air to inflate tires.

Here’s how to figure it all out.

Draw a line across the tread of all 4 tires, which means sideways, from left to right.

Drive a bit, and stop to look at the lines you drew.

If the lines are wearing off in the center more than the edges, you have too much air. If the lines are wearing off on the edges more than the center, you don’t have enough air. 99 times out of 100, you'll have way too much air to begin with.

Let air out or add air depending on what the wear pattern of the lines told you, and repeat the process. The goal here is to have the lines wear completely evenly, while having as much pressure in the tires as possible.

Once you get even wear of the lines, check the pressure and record your readings. Don’t be surprised if the numbers are lower than you expected, and don’t be surprised if they are different front to back.

Just as an example of how low the numbers can be, I ran one setup that took 21psi front, and 19psi rear to get even tire wear. Ended up getting close to 50K miles out of those tires before I got rid of them to get a bigger set, they still had usable tread on them.

Now on to trail pressure. A good rule of thumb to follow is to use half of your normal road pressure, round down. If your running 19psi, air down to 9psi for the trail. Don’t be surprised if you loose a couple psi over the course of a weekend, as the tire will "burp" a bit from time to time as it pulls partially away from the rim when twisted, bound, or pulled by a rock or root. Adjust as necessary for the next wheelin trip, but don’t get carried away unless you or someone in your group has an air source. Different trails will require different pressures, so its personal preference once you leave the pavement.>

 

[paulj]

A long term variant on the line-on-the-tire method is to keep an eye on tread wear. Excess wear on the shoulders (compared to the center) indicates too soft, and conversely, excess center wear points to too hard.

Race car pressures are tuned by checking tire temperatures after a few laps. The goal is to have even temperatures across the tread. This, though requires a means of checking tire temperature, preferably a short distance into the tread. I suspect tire flex is as important as tire wear in determining the temperatures.

Something that these methods don't address directly is the efficacy of the water evacuation grooves. Tirerack has a technote illustrating how an under inflated tire is more prone to hydroplaning.

paulj