Mike Romain wrote:
>
> That is a good link. Your version crashes my Netscape, here is a clean
> link:
>
> http://www.film.queensu.ca/cj3b/tech/tires.html
>
> The skinny tire has more psi on the ground. The shape is also a long
> grab vs a square one. This means it doesn't spin as easy as a wide tire
> in most conditions. Rock crawlers used to go for wide for more area
> grip, but I hear they are even going back skinny.
>
> When I run through mud or up sand pit walls or on snow I keep the rpm
> steady and low and I just leave tracks up to the stop point. If I then
> want to spin them up, I punch it and they look for bottom 'fast' and
> 'usually' hit it so I keep on crawling through or up or have to back
> down for more momentum. Wide tires spin and dig easier.
>
> It makes going and 'playing' in the mud pits almost no fun any more. I
> mean if I have to clean up all the under stuff I want the top to be
> covered too! That doesn't happen any more unless someone else gets
> me....
>
> I have had a winch on for 4 years and 99% of it's use is recovery of
> someone else. Had to use it twice last week though. I was twisted up
> in a bog bad enough to snap my rad side support so the bottom tank was
> left hanging on the rad cooling tubes and leaking. I got photos of that
> and the twist is wicked. Gotta get the film developed and I will post
> them on alt.binaries.pictures.autos.4x4.
>
> Same for snow. No spin. That is the big trick, no spin for going and
> stopping and a good tread for turning.
>
> On the highway I had 10.5" muds that measured a real 10.5" and couldn't
> go much over 40-45 mph without them floating up and losing steering
> control. It is 'very' white knuckle when a semi tractor passes a Jeep
> CJ7 in a snow storm doing 55 mph and you are doing 40.....
>
> With the 9.5's that actually measure 7.5" on the tread because they are
> taller, I have full control to keep up to the trucks in a snow storm
> running in 4" to 6" of road snow cover, they are great!
>
> Every vehicle is going to have a different footprint that has the best
> traction and float trade off. My CJ7's is what the military use or
> under 8" of actual tread width like my 7.5". That is it's 'sweet
> spot'. I can run on cold snowmobile trails without cutting in so I have
> good floatation still.
>
> It only weighs in at 3000 lb though. On a heavier vehicle, that skinny
> might dig too much but I doubt it. The loggers use skinnier than mine.
>
> Mike
You also have to figure in the force required to push the tire through
the snow, since any reasonable sized tire is going to sink in to some
depth. It takes more force to push a wider tire through the snow, more
if its dense spring snow. I've had my truck buried up to the axles in
spring snow and I stalled out a 9000 lb. winch trying to pull it
forward. It wasn't until I put the transmission in gear and let the
tires turn slowly, that it came out. Point is it can take a lot of
force to push something through snow.
I've run various 33" tires from 9.5" to 15.5" wide on my '85 4Runner and
they all seem to work about as well. The very wide tires at 2 psi would
let me float up on deep snow better, but it was a lot of work to get
them back on top of the snow if/when you broke through. You had to keep
making runs at the unpacked snow to get enough momentum to get up and
out of the ruts, then it went good until you hit another softer area or
whatever and sank again. I find the narrower tires (I currently run the
10.5 ATs) might not float as high, but are much easier to get going and
keep going. All my current snow driving is up in the northern CA Sierra
Nevada and the snow is affectionately called "Sierra Cement". May fall
as soft powder, but a few days in the warm weather and sun, it gets
pretty heavy. It also tends to be deep, no way to dig down to a hard
surface.
What I have found to be important for snow traction is letting the tire
treads pack down the snow under the tire as it slowly rolls forward,
then with the snow locked into the treads, you can use that bonded snow
like a cogged railroad. A skinny tire, as mentioned, has a longer
footprint for a given inflation pressure that a wider tire, so you have
a longer patch of compressed snow to push off of. A wider tire won't
pack the snow as hard and the contact patch is not as long and it tends
to be easier to overpower the snow and spin the tire. On one of our
last club snow runs, there were many vehicles with 33x12.50 and
35x12.50s and on one hill several were getting stuck left and right.
Took a lot of work to get one of the trucks ahead to manuever around
enough to pull them out. Getting bored waiting, I pulled out to the
side and easily drove up past them made a 180 turn and drove back down
the other side and then back up again.
Unless you are talking about something like an 18.5/44 Cepek Fun Country
tire, you are not going to be floating over bottomless, soft snow in a
normal vehicle. In the 33" tire diameter I've run, there is perhaps a
2:1 ratio of actual tire widths and in terms of footprint even less
difference and and unless you are in conditions that are just on the
borderline of firmness where a 15" wide Swamper would stay on top and a
narrower tire wouldn't, it won't make all that much difference.
--
Roger
