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Unread 11-02-2013, 08:42 PM   #1
Charley3
Web Wheeler
1999 XJ Cherokee 
 
Join Date: Mar 2013
Posts: 3,584
Have you ever wondered how much a taller tire increases floation?

I wondered how much increasing tire height 1.2" would affect flotation compared to increasing width.

I'm looking at Cooper AT3 here.

My 30 x 9.5 R15 are 29.3" tall. If I increased to a 245/75R16 it would be 30.5" x 9.5".

I wondered how increasing height affected floation compared to increasing width.

According to my calcs 30.5/29.3 = 1.04

1.04 * 9.5" = 9.89"

9.89- 9.5 = 0.39" which I will round to 0.4".

So in effect, increasing from 30 x 9.5 to 31 x 9.5 (245/75R16) has an equivalent effect on floation as adding 0.4" to the width of a 30" tire.

i.e. - a 245/75R16 tire has same flotation that a 30 x 10" tire would have.

The 245 has less debri to plow ahead of it (in mud, sand, or snow) and has a better angle of attack due to larger diameter (compared to a 30 x 10).

A 31" x 10.5" would have equal floation to a 30 x 11" tire.

So there are some interesting thoughts, IMO.

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Unread 11-02-2013, 08:43 PM   #2
Charley3
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1999 XJ Cherokee 
 
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Based on figures in my earlier post:

Increasing from a 30 x 9.5 to a 31 x 9.5 (245/75R16) would increase my floation 4%.

Increasing from a 30 x 9.5 to a 30 x 10 would increase my floation 4%.

Increasing from a 30 x 9.5 to a 31 x 10.5 would increase my floation 14.5%.

I know from prior years experience with various Jeeps, tire sizes, and amounts of gear that I like enough floation to get me by on sand (when aired down), but little enough floation to let me have good grip on Winter roads without having to air down.

I got that ^ beautifully from a stock 96 XJ with 30 x 9.5 tires.

But my 99 XJ recently weighs 4% more due to gear added to it (skid plates, light winch mount and light winch with syn rope). So now 30 x 9.5 tires no longer give me enough floation for sand, even aired down. I'd sink to much. It'd be great on Winter roads, but poor on sand.

I want to add 4% floation back to it with larger tires so I'm back at the same contact pressure I had with a stock XJ on 30 x 9.5 tires. I know from experience that is low enough contact pressure to work on sand, and high enough to work good on Winter roads.

So I was trying to decided whether to upgrade tires to 245/75R16C (31 x 9.5) or 31x10.5R15 tires. I want to add as close as possible to 4% to my flotation, or to say that another way, I want to reduce my contact pressure 4%.

So after doing the math, I know that 31 x 10.5 would decrease contact pressure (increase flotation) to much, while 245/75R16 tires would be perfect (for what I want). I already suspected this intuitively, but now I've confirmed it mathematically.
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Unread 11-03-2013, 03:53 PM   #3
jimk403
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2009 JK Wrangler 
 
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I have suspected that taller tires would result increased contact, just never did the math. If we had snow enough to worry about, I would have to find a 35x10.50 for the wifes JK.
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Unread 11-03-2013, 05:52 PM   #4
Charley3
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Quote:
Originally Posted by jimk403 View Post
I have suspected that taller tires would result increased contact, just never did the math. If we had snow enough to worry about, I would have to find a 35x10.50 for the wifes JK.
There are no 35 x 10.5 that I know of, but there are one or two 35 x 11.5, such as the Trxus MT (lots of sipes for Winter) 35 x 12.5, which is really a 35 x 11.5 in real life.
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