Effectiveness of airing down for beach driving???

[nevada]

I probably should have posted this in the first place. [MENTION=78273]Jeremy556[/MENTION] and all guys who go in the sand off road will hate me because I will give you more useless knowledge to depress your friends and family lol.

I worked on a project awhile back that was for a new vehicle that will be used in sand mining all over the world. Suffice to say it is an off highway vehicle and rubber tired to boot. It's top speed is only around 30 mph and one of the requirements was flotation over all types of sand. When hearing the initial presentation and specs I thought to myself "Alright, different types of sand?" "Sand is sand right?" Sadly I was mistaken as I was about to find out! I will not bore you with the esoteric details of types of sand but suffice to say there are a LOT of different sand types and especially SHAPES of sand grains that absolutely influence the "pack-ability" of sand. We have all heard of sugar sand and glass sand (Hawaii) but that is just a few types. My part of the project was torque distribution and ways to limit said torque. Sort of a electro-hydraulic "Crawl Control" (way different than the TE though) that would give the operator different settings depending on where in the world he (and the machine) is located. Of course tires and ground pressure were extremely important but I did not work on that portion. There are several different tire shapes, types, compounds and sizes depending on what type of sand this machine will see. Anyway, I learned way too much about sand so now I will share some of that pain with you. Here are two elementary links that will briefly describe the type and shapes of sand. Have fun!

Oh, shape of sand grains is a bit more important than types of sand.

Sand Types - Sandatlas

World Atlas of Sands » Blog Archive » Shape of sand grains

No wonder nobody ever wants to talk to me at a party. This is the type of crap I babble on about...

this is true about snow also.

I wont bore anyone with my stories..

but anyone who has done much snow wheeling knows that the type of snow makes a big difference on how you drive in it. also, that it can change throughout the day, so driving styles and equipment much change to match. this includes, but not limited to tire pressure.

also, once you get down into single digits(below5 psi usually), even 1/2lbs make a difference.

 

[Jetboy]

Stock size 265/70/17 LT tires need 44psi to have the same load rating as the stock size P metric tires.

That's only because the safety margin is calculated differently. They dont actually hold less weight at 35 than a p rated tire. Thats a common myth because of the euro safety margin formula and the division between thetypes of tires.

 

[1engineer]

That's only because the safety margin is calculated differently. They dont actually hold less weight at 35 than a p rated tire. Thats a common myth because of the euro safety margin formula and the division between thetypes of tires.

I'm sure you have documentation or a link you can post on this "myth?"

http://www.toyota-4runner.org/engin...s/110299-lt-tires-need-more-air-pressure.html
http://www.toyota-4runner.org/general-discussions/110300-ask-tire-engineer.html

 

[Jetboy]

I'm sure you have documentation or a link you can post on this "myth?"

http://www.toyota-4runner.org/engin...s/110299-lt-tires-need-more-air-pressure.html
http://www.toyota-4runner.org/general-discussions/110300-ask-tire-engineer.html

Logic should be enough. Do u run 500 psi in an XZL?

Im busy but yes. Ill exlexplain how safety factors in tires are calculated when im at a keyboard.

 

[Jetboy]

Im struggling to find the engineering safety factors used in the US in a public website. I'll keep looking later today.

Heat is an issue with thicker sidewalls, however my moving to 44psi you're also significantly reducing contact patch area and traction/safety on the highway. It's always a balance of different safety issues - traction vs tire heat vs handling, etc.

Static factor of safety at 20*C is usually 10 for P-rated tires and IIRC about 15 for LT (this is what I'm trying to find - it's been almost 10 years since I did anything tire related for work). Intuitively it would seem like LT would be closer to 20. Anyway by 135*C the safety factor drops to about 3.5 for P rated and something like 4.5 for LT tires.

If we use a p265/70/17 at 35psi and a load rating of 2425 and a LT 265/70/17 35psi with a commercial rating of 1890 - or a passenger vehicle rating of 1770 - or an EU rating at 36psi of 1808, and compare the safety factors 2425 * 3.5/4.5 = 1886 lbs. It ends up a lot closer than a 25-30% increase in psi for the same load and safety-factor.

If I can find the safety factor publications I'll either link or copy and paste the relevant parts- they might also be somewhere back in the volumes of Tire Science and Tech. I don't have time today to search though.