[expeditionswest]

I have received a few PM's and emails about this tire, and how they fit, so I spent some time on an installation review. It shows required trimming, pictures, ramp results, etc. Thought it might help a few of you

Installing the 255/85 R16

 

[TomTRD]

great information! thanks

 

[AK98Taco]

I have a few questions for you:

Firstly, the tire height in inches is not the one that BFGoodrich provides, nor does the math work out.

BFGoodrich says the tire has an overall diameter of 33.3" on a 7.0" rim: http://www.bfgoodrichtires.com/assets/pdf/mud_terrain_ta_km.pdf

Logically, using a wider wheel will reduce the overall height given the same tire pressure.
Mathematically, the tire is calculated to have a diameter of 33.1".

This isn't really a big deal, just a discrepancy.

Secondly, when you say the 255/85 offers improved traction over a 33x12.5 or 285/75 size tire, I must ask where you get the support for this claim, other than experience basing your assumption.

In physics, friction, and therefore traction, is not influenced by surface area. The only factors are the materials and the normal force, or force resisting gravity in a direction perpendicular to the surface.

The 255/85 has a higher load rating, indicating that it is probably of a sturdier tread compound than that of the 33x12.5.

Just wondering...

I like your website very much, and it's incredibly well put together.

 

[expeditionswest]

I have a few questions for you:
BFGoodrich says the tire has an overall diameter of 33.3" on a 7.0" rim: http://www.bfgoodrichtires.com/assets/pdf/mud_terrain_ta_km.pdf.

Thomas,

Thanks for the kind words on the site . Regarding your height question, that was the measurement I made when first bringing them home. You are right, the BFGoodrich site does say 33.3. I was just listing my measurement, and will change the document to reflect that. (I always take my own measurements )

Secondly, when you say the 255/85 offers improved traction over a 33x12.5 or 285/75 size tire, I must ask where you get the support for this claim, other than experience basing your assumption.

In physics, friction, and therefore traction, is not influenced by surface area. The only factors are the materials and the normal force, or force resisting gravity in a direction perpendicular to the surface.

The coefficient of friction (Ff = Cf x Fv) as you describe requires a level surface, and discounts the traction elements of deformation and mechanical keying, which are both influenced by vertical load (contact pressure). Most surfaces encountered off-highway are irregular, which provide a benefit to a narrow tire (with the exception of deep sand, which requires flotation for the low shear surface).

I have done considerable research on the subject and my findings can be reviewed here (not claiming to be an expert): Tire selection for expedition travel

Are you located in Alaska? What a beautiful state I have been on two treks there. One to Boulder Creek and the other to Ruby Lake. Some of my best trips!

 

[themariachi]

nice writeup, thanks

 

[03trdblack]

Awesome, thanks a lot for that. I know I was one of the folks asking you about going to that size tire. Glad everyone can share this now.

 

[expeditionswest]

Your welcome

This is a great forum. I am glad to be able to contribute a little bit...

 

[TQM3 (Last IFS in AZ)]

I have received a few PM's and emails about this tire, and how they fit, so I spent some time on an installation review. It shows required trimming, pictures, ramp results, etc. Thought it might help a few of you

Installing the 255/85 R16

Nice write up. I may go to this size next time!

 

[AK98Taco]

That is an awesome write-up. Definitely worth adding to the favorites.

There are only a couple of things I would like to comment on:

1. Tires designed for racing are not only wide for the stability and heat dissipation, but also due to the softer tread compound that makes them adhere to the road surface. Due to the low sidewall height, the tire must make up in width what it loses in height so that the weight of the vehicle is supported.

2. One would think that the softer tire, i.e. wider and lower load rating, would have increased deformation when compared to a narrower tire of the same load rating - the 33x12.50 over the 33x10.50.

3. Rim size: the 15" wheel versus the 16" wheel. The 33" tire on the 15" wheel will have more available sidewall to flex and provide additional protection for the wheel.

Those are some suggestions.

Yeah, I'm from AK. Going back up for the summer. I been on the Ruby Lake trail once, riding along. It gets worse by the year (worse meaning deeper ruts and bigger mud holes) but the local Anchorage clubs have done a lot of maintenance to keep the trails intact and divert water flow under the trail. I'm definitely going to run some trails this summer, as I am yet to actually wheel in Alaska.

If you plan on coming through Anchorage or the Kenai Peninsula give me a hollar and we can meet up.

 

[expeditionswest]

1. Tires designed for racing are not only wide for the stability and heat dissipation, but also due to the softer tread compound that makes them adhere to the road surface. Due to the low sidewall height, the tire must make up in width what it loses in height so that the weight of the vehicle is supported..

You are correct, great point. All of the books I read on tire performance sure was an education

2. One would think that the softer tire, i.e. wider and lower load rating, would have increased deformation when compared to a narrower tire of the same load rating - the 33x12.50 over the 33x10.50.

The softer the durometer of the rubber and more flexible the carcass, the better the deformation and mechanical keying. The downside comes from reliability and wear(tearing). The more flexible carcass (lower load rating like a P rated tire) would fail from trail hazards. So for the 12.50 to perform the same as the 10.5, it would have to be a lower load rated tire to exhibit the same deformation as the vertical load is spread over a larger surface.

3. Rim size: the 15" wheel versus the 16" wheel. The 33" tire on the 15" wheel will have more available sidewall to flex and provide additional protection for the wheel.

I agree. I run the 33x10.5 15 on my Jeep, which is what I use for the rock crawling trails. The 16" wheel with the higher load rating does have less performance on the rocks than a C rated 15" equivelant. But for me, I like having the stronger tire and reserve capacity the D rated provide (for my application).

You are a lucky man to be spending the summer in AK. I love that place!

Have you been to the Double Musky (near Alyeska)? Oh man :xcrazy:

 

[AK98Taco]

Good stuff. I'm glad someone finally did some quality research on tire performance offroad.

I've been to the Double Muskie once - had a kickass English burger!

Until this winter, I'd been living in AK fulltime since '96. Missing the AK winter was depressing, though I guess it wasn't a great winter. I am counting the days until I get back across the Canadian border and roll into Anchortown.