Clearance loss from airing down

[Scarsman]

I did a little experiment today in the shop. I wanted to see just how much ground clearance I lose when I air down at different pressures. Enjoy!


Test subject:
2005 Hummer H2 with Cooper STT 37 x 12.50/17 tires aired to 50 PSI

Loss:

-0" @ 50PSI

-1/2" @25 PSI

-5/8" @20 PSI

-3/4" @18 PSI

-1" @ 15 PSI

 

[alrock]

Interesting. Now someone needs to try it with 35s and 33s too

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[Scarsman]

I was surprised, as I thought it would be more than that. When you are airing down it just looks like it is going way down! I imagine with a stiffer tire, like the Toyo, it would be even less. These Coopers are pretty soft.


I also found that it takes about 15 minutes to air a tire up from 15 to 35 PSI with my Costco jumper/compressor. :giggle: Someday I'll have a Power Tank!

 

[Hunner]

It has always been my belief airing down a 315 to 15 to 18 will put you back to a 285 aired up, EVEN if the circumference of the bulk carcass is still larger. Which I think is what he has just shown. I showed about 3/4 inch more clearance with 315's over 285's with about 35 psi.
I still want to check on that in all my spare time.

 

[Steve #1]

It should be prety easy to determine how much clerance you lose when you air down as that is what you should be focusing on rather than the PSI you air down to. You typicly want to reduce your sidewall by 25%.

Math was never my subject but with a 37 on a 17" wheel your sidewall should be 10". Let the presure out until you have reduced the sidewall by 25% (7.5") which reduces your clerance by 2.5".

 

[Scarsman]

Hmmm. I don't know about that. PSI is important for retaining your beads. At least if you don't have bead locks. I think if I let mine down until I droped 2.5" I would be below 10 psi. And it would also be VERY squishy. I am sure traction would be awesome, but I think there is a balance between traction, clearance, and not losing a bead. If I was just playing in snow or sand on flat ground, it might work, but I am sure if went that low on a trail I would lose it on a sidehill or corner.

When I get my truck back I will try going to 2.5" and see what the psi is.

 

[Steve #1]

Those rocks in your sig look sharp. You may not want to push it, but at the same time, I don't air down unless I'm going to make it worth it. I see a lot of people "air down" but don't go far enough to get anything out of it. 25% is a "rule of thumb" different tires and different situations may require different pressures. I'm also a bit old school and that was from the days when 16" was a big wheel.

I'm betting you can still safely get more out of those tires. How wide are the H2 wheels?

I'm probably at about 12PSI here.

 

[Ron B]

With my 39.5 Iroks I lose about 3/4" going from 40psi to 20psi

 

[Scarsman]

H2 wheels are 8.5" wide. I usually air down to 15 psi. At 15 they are very "squished" and deform very easily to wrap around roots, rocks, stumps, etc. H3slate can attest that in the rock field in my sig pic, they ate those rocks up.

I did air down to 10 in sand, and they were quite noticably more "squished" than 15 psi.

I agre though, a lot of people don't air down enough to get a traction benefit from it. They do get a smoother ride, but I would bet not a great increase in traction.

 

[LagunaH1]

Maybe I'm misunderstanding this but I always thought having soft aired down tires was a good thing when wheeling on sharp rocks?

When I had my H3, I aired my 325/17/70 Nitto Terra Grappler's down to 13 pounds and never had a problem with the bead on regular rims.

 

[HUMMER INVESTMENTS]

H2 wheels are 8.5" wide. I usually air down to 15 psi. At 15 they are very "squished" and deform very easily to wrap around roots, rocks, stumps, etc. H3slate can attest that in the rock field in my sig pic, they ate those rocks up.

I did air down to 10 in sand, and they were quite noticably more "squished" than 15 psi.

I agre though, a lot of people don't air down enough to get a traction benefit from it. They do get a smoother ride, but I would bet not a great increase in traction.

Thats what i am usually at, 15-25 PSI when i am wheeling

 

[Scarsman]

Maybe I'm misunderstanding this but I always thought having soft aired down tires was a good thing when wheeling on sharp rocks?

When I had my H3, I aired my 325/17/70 Nitto Terra Grappler's down to 13 pounds and never had a problem with the bead on regular rims.

No, that is correct, the softer they are the less chance of a puncture. My worry with the beads is where we go up here there are places where there are some really off camber stuff, and the sideways force is what will pop the bead off. Especially when you are off camber, down hill with a corner, I have seen the tires be rolled under pretty good that way.

Of course the only way to know for sure how far I can go is to keep airing down until it does lose a bead, but I really don't want to know that bad!

 

[LagunaH1]

I ran my truck at 18 PSI in Moab this past week and it worked really well

 

[Steve #1]

Maybe I'm misunderstanding this but I always thought having soft aired down tires was a good thing when wheeling on sharp rocks?

When I had my H3, I aired my 325/17/70 Nitto Terra Grappler's down to 13 pounds and never had a problem with the bead on regular rims.

Yes, it is a good thing for the tread, but at the same time you are exposing the sidewall which isn't as strong as well as the rim. If you pinch the sidewall between a sharp rock and the rim, things may not go well which is why I say you may not want to puch it.

 

[Steve #1]

No, that is correct, the softer they are the less chance of a puncture. My worry with the beads is where we go up here there are places where there are some really off camber stuff, and the sideways force is what will pop the bead off. Especially when you are off camber, down hill with a corner, I have seen the tires be rolled under pretty good that way.

Of course the only way to know for sure how far I can go is to keep airing down until it does lose a bead, but I really don't want to know that bad!

That's why I ask how wide the wheels are. A narrower tire on a wider rim will be more likly to lose the bead.