Common 4X4 myths

[BusaDave9]

Now I would like to retract what I said earlier. I now think that using inner tubes can help maintain air in a tire at low pressures without ripping out the valve stem.

In a normal tire there is about 32 pounds per square inch of pressure all around the bead pressing it against the rim. (of course the pressure is everywhere else in the tire too) Or if you air down there may be 8 pounds per square inch pushing the bead against the rim. At that lower pressure the tire may turn and/or break the bead.
Now if you are using an inner tube then the inner tube is pressing outward all over the tire AND rim. Even at 5 psi I don't think the tire and inner tube will rotate or even move. The inside of the inner tube is rubbing the circumference of the rim. It's not going anywhere.

 

[BusaDave9]

Myth #6

With winter approaching people often put extra weigh in the back of their pickup or other rear wheel drive vehicle for traction. If you put 200 pounds in your vehicle I hear people say it is best to put the weight over the rear wheels or axle. That way all the weight is on the back tires.

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That much is true but if you put the weight further back it will actually add more weight to the rear tires. If you put 200 pounds back by the tailgate of a pickup it will add MORE than 200 pounds of weight to the rear tires. How is that possible? Leverage. It will actually make the front lighter. Yes, it will take weight from the front and add it to the rear in addition to the weight you added.
If you have a pickup with a wheelbase of 100 inches and add 200 pounds 33 inches behind the rear axle it will add 266 pounds of weight to the rear tires and front tires will now be 66 pounds lighter. Sports car manufacturers know a 50/50 weight distribution is best and a pickup starts off with the front end very heavy. (not good for snowy driving)

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Of course you wouldn't want to put the weight out on the open tailgate. My example is for illustration purposes. It goes without saying the upward force of 66 pounds on the front of the pickup won't lift the front of the pickup but it will make it lighter.

The above picture is can be compared to a lever and fulcrum.

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In the above picture the leverage of pushing down on one end with 200 pounds of force will push upwards on the other end with 66 pounds of force. It is important it realize that if there is a bolder on the left side then you are not supplying enough force to lift it but you will still lighten the load of the boulder by 66 pounds. The fulcrum will hold up the 200 pound weight plus the 66 pound force.

A clown wheelie car takes this idea to an extreme.
They put a lot of weight behind the rear axle. Most of the weight is on the rear tires. The front still has a lot of weight such as the engine but it is countered by the weight behind the axle. The front can now come off the ground with only a little acceleration.

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References:
First Class Lever
The Lever

 

[tarry99]

Now I would like to retract what I said earlier. I now think that using inner tubes can help maintain air in a tire at low pressures without ripping out the valve stem.

In a normal tire there is about 32 pounds per square inch of pressure all around the bead pressing it against the rim. (of course the pressure is everywhere else in the tire too) Or if you air down there may be 8 pounds per square inch pushing the bead against the rim. At that lower pressure the tire may turn and/or break the bead.
Now if you are using an inner tube then the inner tube is pressing outward all over the tire AND rim. Even at 5 psi I don't think the tire and inner tube will rotate or even move. The inside of the inner tube is rubbing the circumference of the rim. It's not going anywhere.

We have used Liners in Drag racing for years...........25 PSI pushing against the inner beads and 5.5 lbs at the traction surface.........the Liner has an ingenious stem that you thread in and out to fill both from one location. Threaded in your adding air to the liner and out adding air to the Drag slick. Both ends are O-ringed and need to be locked with a small wrench........Of course when we used these we also screwed the outside tire beads to the Rims...........We no longer use this method as tire technology has advanced to the point that the side walls do a much better job of supporting the car & tire under acceleration notwithstanding getting rid of the extra weight. We still use bead lock wheels that effectively clamp the tire to the rim.


Would they work off road? I'm sure they would..........Goodyear use to make them but I believe Hoosier may be the only player left.

 

[BusaDave9]

Myth #1 explained

I realize I did a poor job explaining some of these myths.
I hope this clears things up.

The Myth:
I often hear it said that an open differential will, at times, send all the torque to the one tire with the worst traction. And that a locking differential (or spool) is needed to send torque equally to both tires on an axle.

The Truth:
The truth behind this myth is almost completely opposite from the above paragraph.
1. An open differential will ALWAYS send torque EQUALLY to both tires. The total amount of torque that can be applied to an open differential depends on the tire with the worst traction. If one tire is on poor traction and very little torque can be applied to it, then the other tire will get the same torque.
2. A locking differential can send all the torque to the one tire with the best traction. This will happen when the other tire has zero traction such as when it is off the ground.
3. ANY tire that is off the ground has zero torque. This fact is true for a locking differential, an open differential or a limited slip.

It is also important to note that I am disregarding the inertia of the tire and wheel. In other words lets say the wheels mounted on the jeep don’t weigh anything. This inertia is neglectable. Especially when you consider the force needed to propel a jeep over rough terrain.

Let me first talk about point #3 above.
You can not apply torque to any tire that is off the ground.
We all learned this when we were 10 years old helping our dad replace a tire.
You turn the lug wrench and the tire turns. Then you try holding the tire with one hand as you turn the wrench with the other. You find it's just as hard to hold the tire as it is to turn the wrench. (To every action there must be an equal and opposite reaction) You try to suddenly turn the wrench and you realize you can apply a little torque as you are accelerating the tire. Once it it turning a uniform speed you can't apply torque to it.
Then Dad says “We'll lower the car enough that the tire presses against the ground. Then you can apply all the torque you want.”

It's easy to rotate a tire that is off the ground but don't get speed confused with torque.

To understand the physics behind this myth people need to understand torque. TORQUE IS ROTATIONAL FORCE. I can apply 50 pounds of force to a torque wrench or 50 pounds of force to a lever.

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Resources:
Physics Help: Torque
Physics tutorial: Torque Sensors, Domain Theory

see the bottom of page 50 of this Physics Manual:
http://physics.wm.edu/~labs/107_manual/ch8.pdf
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The piston in an engine pushes with downward force. This force gets converted to rotational torque by the crankshaft. The torque gets converted back to a force as the tires push against the ground to propel the jeep.
Keep in mind that the torque applied to a tire becomes the force that propels the car (or jeep). If the tire is off the ground it can't be applying any torque and can't be pushing the car forward.

References:
How to Convert Torque to Force | eHow.com

As Newton said “To every action there must be an equal and opposite reaction”.

If you are trying to apply a force to something, it must be able to resist. If you go into a gym and bench press an empty barbell and tell everyone around “I am pressing up with 500 pounds of force on this barbell”. Everyone would yell “BS!”. Now if someone put 500 pounds of weight on the barbell above you then you better be pressing up with 500 pounds of force because the weight is pushing down with 500 pounds of force. If you can't push up with 500 pounds of force then the barbell will come smashing down onto your chest.

You can't work up a sweat bench pressing an empty barbell.
Likewise you can't make a torque wrench read anything other than 0 before the nut gets tight. It doesn't matter how strong you are. The torque wrench needs something to push against to come up with a reading. Likewise the tire needs something to push up against to have any torque applied to it.

Resources:
Newton's Third Law

People get confused by torque but, for some reason. not by force. To keep things simple I would like to talk about force first.

Lets say I am driving my jeep and run across a Bronco with engine trouble. I volunteer to push him up a hill with my jeep. I am curious how much force it takes to push this Bronco so I mount 2 scales (similar to heavy duty bathroom scales) to the front bumper of my jeep. I put one scale on each side of my bumper. Now when I go to push the Bronco I don't line up my jeep properly with the rear of the Bronco. Only one side of my bumper is pressing against his rear bumper.

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The right side of my bumper is doing all the pushing. The scale on that side says it's pushing with 600 pounds of force. The other scale says 0 pounds because it's not pushing against anything but air. Since the left side of the bumper isn't exerting any force it doesn't mean you can stand in front of that side of the bumper. If you do, you will get plowed down. The scale will show how much force it takes to run you over.

The above bumper can be compared to a locking differential (or spool). All the force can be exerted on one side even when there is zero pounds of force exerted on the other side.

As stated before: A LOCKING DIFFERENTIAL CAN APPLY ALL THE TORQUE TO THE ONE TIRE WITH THE BEST TRACTION. This will happen when there is one tire that has zero traction such as when it's in the air or on glaze ice.

Next lets say instead of a regular bumper that is solidly mounted to the front of my jeep, lets say there is a bar extending strait out the front of my jeep. There is a bumper mounted on the tip of this bar. My bumper can now pivot in the middle.

Lets say I try again to push the Bronco. Once again I do not line up properly with the Bronco. The bumper can pivot and I drive past the Bronco without putting any force on it.

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Now this bumper is comparable to an open differential. There MUST be equal forces on both sides of the bumper. Since one side of the bumper is now pushing against nothing but air it is exerting 0 pounds of force. The other side MUST also exert no more than 0 pounds of force. Since both sides of this pivoting bumper must exert the same amount of force IT IS NOT POSSIBLE TO PUSH THE BRONCO UNLESS BOTH SIDES OF THE PIVOTING BUMPER HAVE THE ABILITY TO PUSH. I must have my Jeep lined up with the Bronco. This is comparable to an open differential. If one tire of an open differential has nothing but air to push against then the tire with traction won't be able to do any pushing either.

As stated before: AN OPEN DIFFERENTIAL WILL ALWAYS APPLY TORQUE EQUALLY TO BOTH TIRES.

If you have an open differential and the right tire is in snow covered ice, it will spin. It will be able to provide very little torque (we'll say 20 foot/pounds of torque). The left tire is now on dry pavement but MUST also apply the same amount of torque (20 pounds). This adds up to only 40 foot pounds of torque for both tires and is not enough to move the vehicle. You are stuck!

Now if the car has a locking differential. The tires will turn in unison. More importantly the tire on the dry pavement will be able to apply all the torque that the engine can produce. The tire on the ice will just be along for the ride. In this case the tire on the dry pavement will be experiencing all the torque. If you break an axle shaft, it will be the one on the pavement.
Parts break when they experience a force greater than what they can withstand.

Kinetic Energy: Let me take a minute to explain kinetic energy. This is another area that confuses people into thinking a spinning tire must have torque.
Kinetic energy is energy by virtue of motion. It pertains to any moving body or spinning tire. Kinetic energy depends on the weight and speed of the object. It does not depend on the power or force it took to get the object moving. You can not easily stop a heavy tire spinning in mid air and you can't easily stop a rolling jeep. If my jeep is rolling down the trail and hits a tree the amount of kinetic energy that is involved depends on the weight and speed of the jeep. It doesn't matter if I am still giving gas or putting on the brakes. Of course I would want to be slamming on the brakes before impact. By decreasing the speed of my jeep I would decrease the kinetic energy and the force of impact. A torque wrench or scale can not measure kinetic energy. If I had a scale mounted to the front of my jeep it would read 0 pounds of force as I am moving. If I were to run into a tree the scale would instantly max out over 1000 pounds of force even at a very low speed. My bumper can not withstand that much force and my jeep would be damaged.
If I were to crash my jeep into a tree at 25 mph it doesn't matter if I have a big block V8 that got me up to 25 mph or if I am coasting downhill with my engine off. Either way the kinetic energy (and resulting impact) only depend on my speed and the weight of my jeep. This also applies to a rotating tire.
If I relied on kinetic energy to get over rocks then I could just coast fast enough to slam over the rock. It wouldn't matter if my engine is running or if I had an open or locking differential.
Do not confuse kinetic energy with torque or force.

Further explanation:
I think I explained torque and differentials but if anyone doesn't agree with my analogy comparing the solid and pivoting bumpers to a locking and an open differential then read on. This section is for extra credit and will not be on the test.
View this video for an excellent explanation of how the differential works.

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This video starts off using a fixed piece at the center of the differential. Here is the view at 2:55 into the video.

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In my screen-shot above the part I colored red does not rotate. This setup is comparable to a spool or locking differential. The 2 axle shafts rotate in unison. If you really want to be exact it is most like a welded differential. The red part is like a spider gear welded in a differential.
The red part above can be compared to the solid bumper when I was pushing the Bronco. You could mount a small scale on each end of the red part to read how hard the red part is pushing the blue parts. In the above picture the man is holding one tire adding extra resistance. This could be compared to trying to roll over a big rock. A scale would show that there is extra force where that side of the red part is pushing against the blue part. The other wheel is in mid-air. A scale on that end would show that there is no torque going to that tire.
This “locking differential” would now have all the torque going to one tire and zero torque going to the tire off the ground.
If the wooden sticks in the above picture are weak then the blue stick on the side that the guy is holding the tire would break. Or that end of the red stick would break. That's because that's the side experiencing all the torque.


Later in the video this red part is replaced with one that rotates.

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This is an open differential. It is like the pivoting bumper from earlier in my post.
They guy is holding one tire adding tension to the one side. You can imagine that tension is a rock the vehicle is trying to get over. Now the middle piece can pivot. For that reason it is very easy to hold the tire. Once again, you could use small scales to help measure how much torque is going to each wheel. And just like always it doesn't take any torque to turn the tire off the ground. Since this is an open differential both tires will get the same torque. As the guy is holding the tire he doesn't hardly notice any force trying to turn the tire. Now if BOTH tires were on the ground he then both tires could be pushed. Now the problem (as noted in the video) is that since the center part can pivot it will end up slipping slipping off the side piece (comparable to an axle gear).
Next they add more spokes to these “gears”.

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Now in the above picture the axle can work more like an open differential.

People think that if an axle has a locking differential than it should be applying torque to both wheels just because the axle shafts are locked together. If you are sitting at one end of a park bench all the force of your weight is on the bench legs under you. If the opposite legs are off the ground then you are even more sure there is no force on those legs.

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Private conversations let me know I did a very poor job of explaining myself originally. I hope this is a better explanation of torque and differentials. If I am not completely clear or if you have any questions at all please let me know. Either reply to this thread or PM me.

 

[PaRenegade]

MYTH #7

"JEEP" is the name given to a Liberty, Wrangler, Cherokee, Commander, or any other vehicle the Chrysler / Jeep company produces. -FALSE

The term JEEP was given to the 1/4 ton GP vehicle Willy's produced for WWII, which was later converted to civilian use and still named the Jeep with a CJ model designation. After a 42 year run it's production was ended in early 1986. It was not changed to be called a Wrangler or anything else.

The Jeep trademark did not exist till 1950 and the Jeep company did not exist till 1970. Therefore, the only vehicle that can be legitimately called a "Jeep" as it's given name, is the military lineage of GP/MB/CJ. All others have a name and different model designation. They are a vehicle produced under the brand called Jeep, not an actual Jeep.