4x4 / Off-Road TechThis section contains all discussion related to taking the FJ Cruiser in Off-Road situations, 4x4 applications and any armor modifications.
As I stated above, the document is flawed. They assume same PSI air down levels. I counter that if you air down two similar height tires to the same side wall height, the contact patch will be better on the larger tire. They also failed to discuss rotational friction.
It's not a scientific paper nor site. It's an outdoor automotive site, written by someone who may or may not be a physics major. I guess on the "not" side. A search of "wikipedia" or "howthingswork" will give a different explanation. And yet, their own calculations suggest what we already know -- a 9.0 to 11.5 width tire is ideal for our rigs.
Airing down isn't done to acheive a sidewall height and I'm not suggesting that you're saying it is. I just want to make a point about pressure in tires and surface contact.
In fact, loss of height is the critical DOWNFALL of airing down because it directly affects ground clearance. The advantage of lower and lower tire pressures, on the other hand, is having the ability to MUSH the tire surface into the contours of obstacles in order to obtain a "mechanical key-in" much like when you sand the surface of a piece of metal before you put primer on.
On the other hand, because of the CONVEX shape of the tire's surface, a low enough pressure will cause it to POP INWARD and "cup" so that the pressure applied across the center of the tread is decreased and along the edges it's increased.
You can play games on flat ground with chalk on your tires and a pressure guage to see where it bows and where it cups while airing up and down... but in the rocks, all bets are off. The ground ain't flat and your weight ain't evenly distributed.
My issue with the above statement is about the line that says:
"I counter that if you air down two similar height tires to the same side wall height, the contact patch will be better on the larger tire."
Let's think about this for a moment and create a mental physics experiment.
Consider a solitary inflated tire with a weight load on it. Ignore rubber stiffness or tread design issues for a moment. Just think about the weight and pressure.
Since we assume that this tire is "at rest", meaning that it is not moving, we can conclude that either there are NO forces acting on it, or whatever forces are acting on it are perfectly balanced by opposing forces so that the net force is zero.
Gravity is acting on this system so the "weight" of the system is a force downward.
Therefore the support of the ground is "pressing upward" with equal magnitutde but oppositely directed force.
Acting on the flexible wall of the tire where it touches the ground, then, are two balanced forces. One is the upward force of the ground supporting the system, the other is the air pressure in the tire.
We KNOW what the amount of the upward force is. It's exactly equal to the weight of this one-tire system. Therefore it is exactly balancing a force on the inside wall of the tire being exerted by the air pressure.
If the weight of the system is "W" in pounds,
and the pressure is "P" in pounds per square inch,
then W/P is equal to the square inches of tire rubber in this system over which W pounds of force are exerted!
This will be the size of the "footprint" contact area.
Notice that it is irrespective of width or length. This tells me that if you use a narrower tire, you'll get a longer contact area. If you use a wider tire, you'll get a shorter contact area at the same pressure.
Since we typically air down to a pressure and not a height, we'd end up with the same result either way in terms of surface contact area.
What if you DID air down to a height? Then the skinny tire would be limited to a SHORTER contact area to acheive that, and it would still have more pressure in it. This would limit deformation useful to squish the tire into the shapes of the spaces between rocks, if thats the type of terrain you're in.
Quote:
Mtbcoach previously said:
Good points, yet, with all due respect, I don't know of a "scientific paper/study" which has been published on such a topic. If you know of one, please share. I can see the validity of doing such a project yet doubt anyone would invest the time or money to do something more than a "Mythbusters" type experiment...
And, as to width size, there are many who are running 12.5" tires and larger in width where my "pizza cutters" are in the middle at 10.5, which, in my mind is the perfect width tire for the FJ Cruiser. Yet, to argue against myself...when I was in Moab this summer, going through "Wedgy", I saw the advantage of the wider tires there. All the guys with wider tires than mine had a bit more margin of error than I did. Nothing that couldn't be dealt with but it was clear that a couple inches here and there can be helpful...
One of the advantages of wider tires is nothing more than having bulging rubber bumpers stick out sideways from the wheel wells!
...This can be very useful when going through a V-notch so that the width of the tire also supports it's sidewall against lateral forces.
Okay, I'm not going to get into a "SeanK" with you... :-) Just try to clarify what I was saying. Yes, back "in the day" (I'm guessing you're not a 20 something - either am I), the only way to try a new tread design was to hand cut it. Freddie Spencer had to hand cut his tread when he first started motorcycle GP because the Americans were used to races being called for rain, not in GP, so after the pit crew stood around staring at each other, Freddie cut his own tread pattern. My point was I don't think DC sat in his garage cutting tires for 17 years. As I posted, Gates Rubber made many different "brands" of offroad tires for other companies in the '70s, don't know about the '60s, and DC-FC's were very popular in the '70s.
As far as the Side Biters, um, yes I understand. I personally run load range E BFG's, and you're more than welcome to look at the scoofs/scrapes/gashes in my sidewalls anytime (I could have just said "DUH" here, but that would have been rude). My point was look at the biters on a BFG, whether AT/MT or Krawler, very basic, and it works. Look at the "biters" on a Nitto Mud Grappler, they look "cool". Just by looking at them it appears that an argument could be made that they are actually angled improperly for maximal functionality, but, hey, they look COOL. Does the cool texturing on the sides between the biters do anything? Look at the homepage for BFG and for Nitto, marketing to two different crowds I might think.
Hope that makes sense.
Oh, not familiar with the Atlas z pattern stuff you mentioned, if you have the time could you please PM me some info/links on it? - thanks!
Ok, I will get the link for you, I found it late one night, but you have to admit it was that z pattern that everyone emulates.
I did not mean every tire DC produced was hand cut, no infact he had his manufacturer do his patterns, but remember, DC is a small, and at one time regional manufacturer. he, like MT was a huge influence on tire design for off road tires. But that is what it was all about, It took a major company to design what we now call a off road tire that had street creds.
Look at the picture of those old jeeps, they look like the side of the tire has mud cuts and the middle is bald. Man would you like to ride that on the street? I bet that thing drifted right and left in the rain. And so went Major tire tech for many years. The offroad market was not big enough for guys like Good Tear and B F Goodrich to jump in full force with a AT or MT, they were the realm of the specialty market. Once Atlas (you know it may have been Armstrong now that I am looking up all these tire companies with A's) broke loose on the national market, things got easier for us to find wide Off road tread tires. Before that, wide tires were what we were running on our muscle cars, and those things would get ripped up in seconds off road.
Now Yes there were small makers of tires in the 70s that were making tires for specialty markets, I in 79 had a set of DCs ( i think they were FCs) shipped to Louisiana for my Jeep. I some times have to remember a general statement can be totally misconscrewed here. As small guys experimenting, and regional manufacturers are solid evidence, I was refering to the national market.
It is like the 2 different ways to look at history
factually or in a sociology method. One wants hard facts, one wants significant developments, I was looking at the statement in a sociology way of the movement getting major creds. You, my friend, were looking at the roots.
and yes, 17 years in a garage is funny, I think he was in it for less than a year, I think he actually went into contract with something to do with a Hair styling thing, weather it was who held the rent for the first store, or the store itself, I cannot remember. But 2 yeqars down the road he quit his job and was full time till he died in 1983, his son ran the business till it went under, and MT bought out the tire line and name.
PS, I still consider the Fun Country II one of the best AT tires out there. I have wore several sets out down in Baja. I currently run MT MTZs on the DD and Baja Claws on the Jeep
Here is an extremely well written paper on tire choice, not something you see in off-road rags that bolt on the latest Big Bling and place the vendor's advertisement on the same page.
What is important is not their final choice but the actual thought process used to reach that choice.
Oh... and I read this article a while back when I was asked to by 1legLance. To say that this guy isn't a physicist is an understatement.
Frankly, I found it infuriating. It's a classic example of a little bit of knowledge being worse than none at all. I got my hackles up after the first couple of paragraphs where he describes his approach as "empirical" which is a word that he's obviously not familiar with using. Empirical data is what you get from actually doing real-world experiments, not just sitting around thinking about it. The rest of his paper is just some thinking and it's based on SEVERELY 2 dimensional thoughts.
Another problem was that he doesn't understand the difference between static and dynamic friction. Dynamic friction is what happens when surfaces slide against eachother. Although this HAPPENS to tires, this isn't their standard mode. They roll, and the surface of the tire touches the ground in ONE PLACE and then lifts off. This is why you can actually SEE the print of someone's tire tread in the dirt. If it were SLIDING then you'd just see a gouge in the dirt the width of the tire.
Nascar tire design might have to take dynamic friction into account because of the 4 wheel drift when they go around the turns. I don't know for sure... not my area of expertise...
Salt-flat speedsters probably have to take this into account because the tire is "burning out" for huge distances while the actual speed creeps up to the tire speed. My friend's dad used to do this and he described using the tires like paddle wheels in water!
Most of the time, when we 4 wheel, we're under STATIC conditions with the tire biting. When it loses traction YOU KNOW IT!!! This is when the tires bark, the lockers get engaged or you need ATRAC if thats the way you run... The whole idea is to get BACK to static conditions as best as possible.
When wheelers go DYNAMIC, they're typically burning rubber with wide open throttle to get over or through an obstacle that's kicking their butt. It's typically a technique of last resort and poses significant risk of mechanical misadventure!
Notice that it is irrespective of width or length. This tells me that if you use a narrower tire, you'll get a longer contact area. If you use a wider tire, you'll get a shorter contact area at the same pressure.
Edited above post a little for brevity, but not to cut out a good explanation.
With a wider tire it would be a shorter but wider contact area to achieve a footprint similar to the narrow tire's longer and narrow contact area, right?
Also the loss of ground clearance is pretty important. Some of the really large tires on rigs can be aired down to incredibly low pressures and the percent loss of ground clearance is not as critical.
__________________ Jim '05 4Runner V6 4x4
Hannah sliders, Downey skid plate, Downey coilovers, Deaver rear springs 30%/2.5", Donahoe rear shocks, rear diff breather mod, Garmin 276C GPS, LT265/75/R16 Bridgestone Revo's, Dual Batteries, Engel fridge/Two-Zone, All-Pro Ultimate LCAs, All-Pro LCA skids, Total Chaos A-Arm skids, cyclone air pre-filter, All-Phase rear diff skid, and ARB Compressor. Next --> ARB rear locker!
When wheelers go DYNAMIC, they're typically burning rubber with wide open throttle to get over or through an obstacle that's kicking their butt. It's typically a technique of last resort and poses significant risk of mechanical misadventure!
I think that is why so many guys prefer smaller engines with more torque, hard to spin the tires and snap the axles. I watch the built up rigs run hemi engines with insanely large axles muscle over stuff, and next a samuri creeps up it.
sorry for the hi-jack
I think that is why so many guys prefer smaller engines with more torque, hard to spin the tires and snap the axles. I watch the built up rigs run hemi engines with insanely large axles muscle over stuff, and next a samuri creeps up it.
sorry for the hi-jack
i see four bangers break dana 44's. its all technique when it comes to breakage.
Also the loss of ground clearance is pretty important. Some of the really large tires on rigs can be aired down to incredibly low pressures and the percent loss of ground clearance is not as critical.
in the book, "The 4 Wheelers Bible" there is a good chapter on airing down with pictures that explains quite a bit, worth checking out.
i see four bangers break dana 44's. its all technique when it comes to breakage.
I have seen 4 bangers break d 60s, If you want to go tit for tat we could argue for hours. what I was saying is it can be humbling to watch different rigs, and what is popular in the magizines can get a inexperienced guy in trouble fast.
Dorans entrance last year, watch 2 guys in rigs, almost brand new, with every bell and whistle on them, come up to the entrance and comense to destroy the rigs, and these rigs looked like they had just gotten out of the 4 wheel drive hardware garage with a unlimited budget. Carnage done, well last I saw they were on cell phones calling down to San Diego looking for a trailer to tow them each as both would not move. Dorans 2, built rigs with no clue 0. And talking to the guys they had full confidence they had built the rigs to conquer the entrance. Both were new to wheeling. Funny thing, a YJ with 2 inches of lift and a 4 popper went right on up before them. No Carnage. Having been doing this since high school in the 70s, I get told all the time my equiptment is to small, axles to weak,tires to small. But somehow I seem to be asked to lead a lot of runs, and asked what lines to take. I cannot figure it out, these guys have bigger badder rigs than mine.
Another problem was that he doesn't understand the difference between static and dynamic friction. Dynamic friction is what happens when surfaces slide against eachother. Although this HAPPENS to tires, this isn't their standard mode. They roll, and the surface of the tire touches the ground in ONE PLACE and then lifts off. This is why you can actually SEE the print of someone's tire tread in the dirt. If it were SLIDING then you'd just see a gouge in the dirt the width of the tire.
Belly, going back MANY years, I think '79-80, one of the offroad mags (much better then than now, IMO), had the only semi real article on tire traction that I think I've ever seen. I wish my mushy brain could remember which mag. Essentially they did a sort of draw bar test. Using a homogenous, I think dirt surface, and I don't remember the exact tire brand/model. Obviously we do not offroad on homogenous surfaces, etc. but most physical modeling starts with a very simplistic view, then refinements are made. Enough of that... the one thing I remember, is that they found the tires tested had maximum traction at 104% wheel spin. Maximum traction under Dynamic friction --> 4% slippage. This always stuck in my head, just find it interesting.
Oh... and I read this article a while back when I was asked to by 1legLance. To say that this guy isn't a physicist is an understatement.
Frankly, I found it infuriating. It's a classic example of a little bit of knowledge being worse than none at all. I got my hackles up after the first couple of paragraphs where he describes his approach as "empirical" which is a word that he's obviously not familiar with using. Empirical data is what you get from actually doing real-world experiments, not just sitting around thinking about it. The rest of his paper is just some thinking and it's based on SEVERELY 2 dimensional thoughts.
Another problem was that he doesn't understand the difference between static and dynamic friction. Dynamic friction is what happens when surfaces slide against eachother. Although this HAPPENS to tires, this isn't their standard mode. They roll, and the surface of the tire touches the ground in ONE PLACE and then lifts off. This is why you can actually SEE the print of someone's tire tread in the dirt. If it were SLIDING then you'd just see a gouge in the dirt the width of the tire.
Nascar tire design might have to take dynamic friction into account because of the 4 wheel drift when they go around the turns. I don't know for sure... not my area of expertise...
Salt-flat speedsters probably have to take this into account because the tire is "burning out" for huge distances while the actual speed creeps up to the tire speed. My friend's dad used to do this and he described using the tires like paddle wheels in water!
Most of the time, when we 4 wheel, we're under STATIC conditions with the tire biting. When it loses traction YOU KNOW IT!!! This is when the tires bark, the lockers get engaged or you need ATRAC if thats the way you run... The whole idea is to get BACK to static conditions as best as possible.
When wheelers go DYNAMIC, they're typically burning rubber with wide open throttle to get over or through an obstacle that's kicking their butt. It's typically a technique of last resort and poses significant risk of mechanical misadventure!
What is interesting to me is that it is all in one paper to be read and discussed, refuted or embraced in a sort of peer review process. Other than magazine articles, the act of documenting the thought process is rare in the 4Wheel world. This forum has seen plenty of "I know better and if you don't beleive me I am taking my toys and leaving!!!".
It is no problem that flaws are found with the paper that I referenced. If the flaws are pointed out and well thought out counterpoint is offered I am all for it.
__________________ Jim '05 4Runner V6 4x4
Hannah sliders, Downey skid plate, Downey coilovers, Deaver rear springs 30%/2.5", Donahoe rear shocks, rear diff breather mod, Garmin 276C GPS, LT265/75/R16 Bridgestone Revo's, Dual Batteries, Engel fridge/Two-Zone, All-Pro Ultimate LCAs, All-Pro LCA skids, Total Chaos A-Arm skids, cyclone air pre-filter, All-Phase rear diff skid, and ARB Compressor. Next --> ARB rear locker!
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