Skinnies

[BuildBreakRepeat]1. Contact area only increase in the front to back direction (done so by lowering pressure) so a wider tire only increase contact area by a very small amount.

that must be why every competition vehicle on the planet uses as wide a tire as regulations permit

[BuildBreakRepeat]2. Pressure per square inch significantly increases while using a narrow tire.

I guess that depends on your definition of significantly.
 
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[BuildBreakRepeat]
That statement is wrong.

How so?

[BuildBreakRepeat]But it’s obvious you aren’t asking questions to learn.

I "leaned" that using a slightly wider tire only increases traction slightly but using a slightly narrower tire increases pressure significantly.
 
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freedom_in_4low said:
what tire would that be? If I could find such a tire in load range C and with good reviews, I'd be up for trying it.
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It’s the kenda klever RT - luckily one of the few D range 17” tires available. Not as good as a C range but better than all the E range tires out there for 17”s. Pretty cheap tire and good reviews online.
 
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1. In talking about on-road behavior between tires of different widths, it's easy to conflate the impact of the wider tire with the impact of the change in steering geometry (scrub radius) caused by the smaller-backspaced wheel needed to run that wider tire. I run 32x11.5 and will be next using 33x10.5 but I don't run the common 3.75-4" BS wheel - I run 5.25, which is only barely reduced from stock.

2. While a lot of the principles cited in that blog post hold water, I'm not sure I follow his separation of contact patch, contact pressure, and inflation pressure. To me, it seems that as you lower a tire onto a surface, the point at which it stops deforming is when the pressure on both interior and exterior surfaces of the contact patch are equal - so if you have 1000lb load on a tire inflated to 20 psi, it will deform until you have a 50in^2 contact patch, and both the air pressure pushing the tire down and the road surface pushing it up will have 20 pounds on each of those 50 square inches. Following this logic, the contact pressure would be completely unaffected by the width of the tire, as either tire will deform until the contact patch is sized to equalize the pressure - the wider tire will just have a wider patch, and the narrower tire will have a longer one.
 
freedom_in_4low said:
1. In talking about on-road behavior between tires of different widths, it's easy to conflate the impact of the wider tire with the impact of the change in steering geometry (scrub radius) caused by the smaller-backspaced wheel needed to run that wider tire. I run 32x11.5 and will be next using 33x10.5 but I don't run the common 3.75-4" BS wheel - I run 5.25, which is only barely reduced from stock.

2. While a lot of the principles cited in that blog post hold water, I'm not sure I follow his separation of contact patch, contact pressure, and inflation pressure. To me, it seems that as you lower a tire onto a surface, the point at which it stops deforming is when the pressure on both interior and exterior surfaces of the contact patch are equal - so if you have 1000lb load on a tire inflated to 20 psi, it will deform until you have a 50in^2 contact patch, and both the air pressure pushing the tire down and the road surface pushing it up will have 20 pounds on each of those 50 square inches. Following this logic, the contact pressure would be completely unaffected by the width of the tire, as either tire will deform until the contact patch is sized to equalize the pressure - the wider tire will just have a wider patch, and the narrower tire will have a longer one.
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I agree. What most struggle to understand is that as pressure is lowered for off road use, the tire’s contact patch does not get wider, but rather longer. This theory was proven in an experiment show below:

778E66AB-C372-4CDC-83C2-556842D343DA.jpeg


As I mentioned before, a wider tire only provides benefit in dry conditions, and when extreme flotation is needed. The reason the military runs skinny tires is due to the need for them to work in any environment.
 
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BuildBreakRepeat said:
I agree. What most struggle to understand is that as pressure is lowered for off road use, the tire’s contact patch does not get wider, but rather longer. This theory was proven in an experiment show below:

View attachment 168432

As I mentioned before, a wider tire only provides benefit in dry conditions, and when extreme flotation is needed. The reason the military runs skinny tires is due to the need for them to work in any environment.
Click to expand...

Ok, I'm gonna nerd out a little bit here...I LOVE data!

That table does show that my "model" was missing something when it comes to correlating it all (I had completely neglected forces from the tire carcass as if it had the rigidity of a party balloon) but that I'm generally on the right track - once those pressures are converted from gauge pressure to absolute pressure, the contact pressure and inflation pressure trend together fairly well, and the contact area not quite doubles as the pressure is halved (I used regression equations to extrapolate for 12.65 psig inflation pressure, which in absolute pressure would be half of 40psig).

1591650872618.png


I still stand by my previous position that on a dry surface where digging vs flotation is not a factor, there should be approximately zero difference in traction between narrow and wide section tires of the same tread design, sidewall rigidity, rubber formulation, and inflation pressure.
 
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freedom_in_4low said:
Ok, I'm gonna nerd out a little bit here...I LOVE data!

That table does show that my "model" was missing something when it comes to correlating it all (I had completely neglected forces from the tire carcass as if it had the rigidity of a party balloon) but that I'm generally on the right track - once those pressures are converted from gauge pressure to absolute pressure, the contact pressure and inflation pressure trend together fairly well, and the contact area not quite doubles as the pressure is halved (I used regression equations to extrapolate for 12.65 psig inflation pressure, which in absolute pressure would be half of 40psig).

View attachment 168475

I still stand by my previous position that on a dry surface where digging vs flotation is not a factor, there should be approximately zero difference in traction between narrow and wide section tires of the same tread design, sidewall rigidity, rubber formulation, and inflation pressure.
Click to expand...
Ahh I love seeing a fellow engineer break it down. So what I’m drawing from your tables is that a wider tire would only be more beneficial when flotation is critical (sand, deep snow not on road)? I’ve always found narrow tires to perform well all around, it’s nice to see some data.
 
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BuildBreakRepeat said:
Ahh I love seeing a fellow engineer break it down. So what I’m drawing from your tables is that a wider tire would only be more beneficial when flotation is critical (sand, deep snow not on road)? I’ve always found narrow tires to perform well all around, it’s nice to see some data.
Click to expand...

exactly. A wider tire will be more beneficial when you need to float, and a narrower would be more beneficial when you need to dig, and that's all there is to it.

The one thing I still haven't worked out is why in my force balance the contact pressure (tire load distributed over the contact area) doesn't fully counteract the inflation pressure, and the missing chunk is way too big to be explained by the rigidity of the tire carcass. I'm a little outside my wheelhouse here being a thermal-fluids guy. I don't have to do pressure calcs for a vessel or tubing/pipe often, but when I do, I don't have to consider that it might be significantly deformed sitting on the ground on its own weight. :ROFLMAO:
 
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