NJTramcar

How to calculate air down pressure

39 posts in this topic

Posted (edited) · Report post

I have a 6 cyl, 2012 Toyota 4Runner.

 

How do I figure out what pressure to air down to?  Is there a formula?

Edited by NJTramcar

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Trial and error. Every beach is going to be different. Start and 20psi and work your way down from there. I've never had to drop below 16 in my FJ.

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Want a formula: with the recommended door jam inflation, measure the height of the bottom of the rim above hard ground.  Say it's 4".  Air down to 75% or that, 3".  Measure the pressure and use that.

 

Or:

Air down to 15 psig, never been stuck over 22 years (Ford Expedition, Toyota Sequoia, 4Runner).  If you bog down, go lower to 12 or 10 psig.  Yes, I've run 18 or 20 on wet sand but many will say ~15 is a good starting point that doesn't strain the trans or running gear.  Take you foot off the gas, does the truck float to a stop or dig in immediately?  You want it to float. 

 

Watch this: https://www.njbba.org/beach-video.html

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7 hours ago, Darth Baiter said:

Want a formula: with the recommended door jam inflation, measure the height of the bottom of the rim above hard ground.  Say it's 4".  Air down to 75% or that, 3".  Measure the pressure and use that.

 

Or:

Air down to 15 psig, never been stuck over 22 years (Ford Expedition, Toyota Sequoia, 4Runner).  If you bog down, go lower to 12 or 10 psig.  Yes, I've run 18 or 20 on wet sand but many will say ~15 is a good starting point that doesn't strain the trans or running gear.  Take you foot off the gas, does the truck float to a stop or dig in immediately?  You want it to float. 

 

Watch this: https://www.njbba.org/beach-video.html

There is no formula, as all beaches are different, and all sidewall heights are different.

6 hours ago, SoCalStripers said:

15-18 psi. Air up as soon as you can when you get back on the road or you’ll end up with a ton of cracks in your tires. 

This!!

50 mins ago, LouC said:

the 75% of full inflation sidewall height works well. We use 12-15 lbs on the Jeeps.

Can't go by that, as all trucks and wheel/tire sizes are different. Newer vehicles have larger rims, that equate to smaller sidewall tires. Not too mention weight, width and wheel bases of vehicles are different.

 

 Best advice is to air down to 20, then deflate from there depending on tire size and beach conditions. Optimum, is 16" wheels with larger sidewall tire. You want wider traction when deflated, that only happens with smaller diameter rims.

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33 mins ago, Fishtale7 said:

There is no formula, as all beaches are different, and all sidewall heights are different.

This!!

Can't go by that, as all trucks and wheel/tire sizes are different. Newer vehicles have larger rims, that equate to smaller sidewall tires. Not too mention weight, width and wheel bases of vehicles are different.

 

 Best advice is to air down to 20, then deflate from there depending on tire size and beach conditions. Optimum, is 16" wheels with larger sidewall tire. You want wider traction when deflated, that only happens with smaller diameter rims.

75% of starting height takes into account starting height differences amongst old/new rims.  75% of x is a variable based on what x is.  X varies which each vehicle and may change with weight but width and wheel base are not factors.  Embrace the science!  Airing down elongates the tire contact patch, it does not widen the patch as much as it elongates.  Go fact check and let us know the results.     

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Posted (edited) · Report post

No it doesn't. 75% of a 5" sidewall tire does not match the 75% of an 8" sidewall. I only dispel science from Democrats, but math does not lie. Footprint of the tire is the deciding factor of traction, not the percentage of the sidewall or pressure of the tire.

Edited by Fishtale7

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10 hours ago, Fishtale7 said:

No it doesn't. 75% of a 5" sidewall tire does not match the 75% of an 8" sidewall. I only dispel science from Democrats, but math does not lie. Footprint of the tire is the deciding factor of traction, not the percentage of the sidewall or pressure of the tire.

I never said the numbers will be the same.  You air down to 75% of the starting height and record the pressure.  Of course 75% of 5 is not the same as 75% of 8, I'm not a republican.  You believe the big lie?  Just use 15 psig and put the tape measure away. 

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Posted (edited) · Report post

On 5/14/2021 at 7:19 AM, Darth Baiter said:

I never said the numbers will be the same.  You air down to 75% of the starting height and record the pressure.  Of course 75% of 5 is not the same as 75% of 8, I'm not a republican.  You believe the big lie?  Just use 15 psig and put the tape measure away. 

Obviously, you do not follow the science. There is NO formula. It all depends on rim size, tire size, weight of vehicle, beach conditions, etc... 75% of crap, is still 75% of crap. To the OP... start at 20psi, and work from there. Don't listen to the mathematicians here. Beach conditions & vehicle will dictate tire pressure, not any formula. Your truck will let you know if you need to drop more air, not a calculator.

Edited by Fishtale7

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There’s no need for any of these crazy calculations. I own a 4Runner and it has been on the beach. Just air down to 18 unless the sand is super soft then go lower. Most beaches at least up here in MA tell you what maximum PSI you should be aired down to. 

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8 hours ago, Darth Baiter said:

Found this on the interwebz, disclaimer: it's from Oz.  Remember, as previously stated:  "there is no formula"

 

 

 

4WD Tyre Pressure for Sand.pdf

this is exactly how we do it starting 10 years ago, and have never been stuck. It turns out that the 75% rule gives us a PSI of 12-15 lbs. We have 3 different Jeeps with 3 different tire sizes but it works the same. Weights, are about the same, 4200-4500 lbs.

'17 Wrangler 245/75-16 "C" rated all terrains (30.5" height)

'07 Grand Cherokee 245/70-17 "P" rated all terrains (30.5" height)

'98 Grand Cherokee 30"x9.5-15 "C" rated all terrrains (30" height)

 3 different vehicles, 3 different tires sizes, same rule, works the same, 10 years of experience, never stuck anywhere on the South Shore of Long Island. 

You can make it complicated, but its really quite simple. 

Another quick test, when you take your foot off the gas, if the vehicle coasts, rather than stopping right away, you are probably aired down enough. 

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