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Posted: Mon Jul 13, 2009 2:42 pm
by T_Diesel
love_mud wrote: Like most decent tech threads (this on actually has some very good tech in it)
Depends on your definintion of very good.

Posted: Mon Jul 13, 2009 3:53 pm
by frp88
mike_nofx wrote:
frp88 wrote:I have 33/12.5/15 muddies on my Cruiser and have never needed to aired down on the beach . I put it down to clearence if you are high enough you don't get stuck.
aaah... suspension clearance, or diff clearance?

33s dont put you that high in a cruiser...
I 'll do some measuring I am not sure on the numbers myself. All I remember when I bought it it came with 237/75/15 and the springs were flat I got stuck on the beach all the time. I only have a 2 inch lift and 33 and have not needed to my stauns again.

Posted: Mon Jul 13, 2009 4:20 pm
by mmaaxx
I refer you guys to take the time and have a read of the technical writeup on my earlier mentioned link on how and why a tall narrow tyre is better than a wider one here -
http://www.expeditionswest.com/research ... _rev1.html

But for those that dont want to take the time to read it here is a summary of sorts.
While the coefficient of friction (Ff = Cf x Fv) is linear and not affected by width (on a perfectly smooth surface, traction is consistent despite width), it is the variability of the road surface conditions off-highway that improves traction for a narrow tire. The greater the contact pressure, the greater of effectiveness of the friction elements of Deformation and Mechanical Keying. A narrow tire also presents less rotating resistance on a soft surface, like shallow mud, snow and sand. Additional performance is gained by the assumption that most vehicles can fit a taller tire if it is narrower, which provides greater axle clearance. Final arguments are made for the benefits related to reduced rotating mass and unsprung weight
The benefits of a narrow tire:

The Argument: A tall, narrow tire is a better choice for all off-highway surface conditions with the exception of soft sand, snow and soft mud that's depth exceeds 110% of the vehicles minimum ground clearance. Here is the explanation.

Contact Pressure: Contact pressure is expressed as the vehicles curb weight distributed over the contact surface of four tires. The contract pressure is not equal to all four tire road surface contact points as the vehicles weight is not perfectly distributed. To ease the description, let's assume that the test vehicle weights 5,000 lbs and has a perfect weight distribution. Each of the vehicles four tires would be creating 1,250 lbs. of vertical pressure on the terrain. Let's assume for the sake of this example that the vehicles tires are 10” wide, where the load and tire pressure results in a total surface area of 30 sq. inches. The total pressure per square inch (without equating the secant) would equal 40 lbs.

Off-highway effects of contact pressure:

Deformation- On a smooth surface (like concrete), a tire gains most of its traction by adhesion. On an irregular surface like granite and boulders, a tires contact patch will deform as a result of vertical pressure. The wider the tire, the less the rubber will deform to the surface irregularity given the same vertical pressure. The greater the deformation, the greater the tires resistance to shearing forces (spinning). This is the strongest argument to using a narrower tire.
Real world example: When climbing a ledge with a jagged surface, the narrower tire will wrap the protrusions with more contact due to the increased deformation depth. The wider tire will rest on the surface of the protrusions and will have a greater chance of spinning (shearing).
Image
Mechanical Keying: This is the second critical benefit of a narrower tire. As the vertical load increases, so does the compression and flexing of the tires tread and rubber to the surface protrusion. A narrower tire generates greater vertical load on the rubber and the tread, increasing tread compression in conformance to the surface irregularity. A wider tire in contact with more surface conforms less, and will shear sooner than a narrow tire.

Adhesion- On a flat surface, the adhesion rate of a narrow tire and wide tire are the same, as the wider tire makes more contact (friction area), but the narrow tire generates more pressure (vertical load force). On a highly irregular surface, the higher vertical load force of the narrow tire becomes an advantage, increasing molecular bonding between the tires rubber and trail surface. That bonding becomes so great that either the vehicle moves forward, the tire tears leaving rubber on the surface, or the trail surface breaks away.
There is alot more there to read, but thats the main summary of it...hope that helps some of you understand the difference in performance of both.

Posted: Mon Jul 13, 2009 6:03 pm
by -Nemesis-
Not sure a heap of that makes sense IMO.


The picture for starters, tyre tread won't deform in that direction like pictured unless you're running flat HT's maybe. Even so, the minute bulge in between the ridges isn't going to offer traction. Touching an extra 4 ridges might though.



Then parts like adhesion:
"Adhesion- On a flat surface, the adhesion rate of a narrow tire and wide tire are the same, as the wider tire makes more contact (friction area), but the narrow tire generates more pressure (vertical load force)."

If that was true to any extent, then powerful cars wouldn't be running wide tyres hey. A drag car can get as much traction on cheese cutters because there's more downwards pressure and the same adhesion rate


Traction is purely friction. A bigger footprint on hard surfaces provides more surface area for friction to occur. Hence why we air down to give a longer footprint. Otherwise with the above logic you wouldn't air down for rocks, because at 40psi you'd have heaps more downards pressure on the tread surface that touches.



Anyways, I'm not saying wider is better, I just doubt alot of the background behind the above 'tech' stuff. :?

Posted: Mon Jul 13, 2009 6:16 pm
by mmaaxx
yes Nem', a bigger footprint on a flat even surface does give you more traction, that info above agrees with that....what the above 'tech' does say is it doesnt apply on uneven terrain offroad......a wide tyre will not deform the same as a narrow tyre will.....

a wider tyre will only make traction with the top edges of an obstacle even when aired down, thus not giving you as much traction as you need, whereas a narrower tyre having more pressure per cm squared applied to it will have more force pushing the tyre deeper into the spaces between uneven terrain giving you more traction....

hope that makes sense.....no it doesnt apply in 100% of offroad situation but most in general....

it is why you see army trucks running tall and narrow offroad tyres even today, not just on ww2 jeeps as previously mentioned when tyre selection was few and far between...theres a reason the army run tall and skinny's otherwise you'd see then all on 12,13" wide rubber if there was benefit in it!

Posted: Mon Jul 13, 2009 6:41 pm
by -Nemesis-
It's definitely horses for courses and terrain based, but I'm still not convinced it's as simple in real world situations as he implies. There's rarely a surface that will create lateral tread deformation, and when you have less tread width there's less to deform anyway lol....


Anyways, how many comp trucks, in what off road environments, run skinny tyres? If the real world was the same, everyone would be running 10" wide tyres, but I can't remember the last truck I saw with them (apart from the odd simex jobbie)


Toyota vs Nissan anyone lol!

Posted: Tue Jul 14, 2009 9:32 am
by coat
Well didnt i open a can off worms here.. Ive had 12.5 for the last 5 years now and they were great but i thought i would try10.5 for a change.. The ute does more road km these days so i mite get a bit more fuel eccom out of it aswell..

Posted: Tue Jul 14, 2009 10:24 am
by Patroler
Something that hasn't been mentioned yet is punctures, mates old man - whos done a lot of outback travel, swears by 7.50x16's on the gq when going outback, not only are they easier to change on splits, but his theory on punctures was that the 7.5 inch wide tyre being narrower than the 10.5 would mean you have less chance of running over stakes (7.5 is about 70% of 10.5 - therefore you should get 70% of the punctures that guys with 10.5's get)
Sieems a pretty simple thought, but can't really see why it wouldn't be true - probably as likely to get sidewall damage as there is still 2 sidewall per tyre.
Ain't gonna stop me running 265/75's just yet, but is i was offered a cheap set of splits i consider them for that purpose.