Highway 4wd low

[pjtoledo]

got a piece of chalk and a straight line to test it in? my vote is the squished tire will rotate more, and generate more heat.

 

[Ranger850]

Wouldn't the soft sidewalls of the tire allow enough flex for this "Problem". I could see if it was a stiff side walled tire like a low profile, but a 33 inch tire minus 15 inches of rim leave 9inches if side wall to flex on the bottom side, allowing for the difference in rotations. I see what @ericbphoto is saying, If the wheels were not rubber, say wood, or something that does not flex or give, what would the outcome be?

 

[alwaysFlOoReD]

(BTW, circumference = pi x diameter)

OOPS...Good catch. the formula I posted should be pi x Radius x 2.

Doesn't change the fact that the radius of the center of the axle to road surface changes with load. Tire pressure or temperature also changes the effective radius.

If the tire wasn't flexible then the radius wouldn't change.

Ever hear a tire "chirp" as the vehicle is turning in a parking lot? That happens with a locked differential. The rubber can't compensate for the difference in tire travel so it "chirps".

 

[don4331]

Sorry. I’m not convinced. Where did the extra rubber go? Why don’t we recalibrate our speedometers for heavy loads? The axle is closer to the ground. But the distance around the tread has not changed. We just Chang the shape from round to “D”-shaped. That measurement from pavement to axle is not a true radius because we’re not talking about a circle. If you measure from the axle to the top of the tire, you will find that the radius has not changed.

(BTW, circumference = pi x diameter)

Where does the extra rubber in the sidewall bulge come from? Answer - from creation of the 'flat spot' due to weight of load. And if mathematicians were creating the speedometer they would want load compensation, but engineers are fine with "close enough for practical purposes." They design the speedometer to read within 1% at the average load with the stock tires. So, yes the axle center to pavement is the radius you should be using and circumference = pi * loaded radius * 2.

Aside the flexing the tire to create the "flat spot/sidewall bulge" is what creates heat in tire.

Finding 4 identical tires on the rack is nearly impossible. Given a 30" tire rotates ~675 times/mile, a .05" difference in radius results in dragging a tire the length of the truck every mile if you don't have differentials (either in axles and/or transfer case).

Back to the original problem: The planets in the low range gear train, turn at more/less the speed of the reduction. I.e. 2.48 * input speed. If input speed is: Engine 4,500 rpm / .75 for OD, the poor little planets are spinning ~15,000 rpm!

It has historically been a issue with Jeep Rubicons: Their transfer case has 4.0:1 reduction, so when people forget to shift out of 4lo, and attempt to drive highway speeds, their planets spin up to 30k rpm for a few moments, then there are all sorts of transfer case pieces on the pavement.

I would check the oil in the transfer case to see how badly heated it was. If it smells burnt; you need to investigate further.

And who reads manuals??

 

[Curious Hound]

Guys. I’m not trying to get anyone upset or worked up. I’m willing to learn something new. My brain is hung up on the fact that for a 30” tire, the manufacturer lays a strip of tread rubber about 90.25” long around that tire. So it seems to me that that tire must travel 90.25” for every revolution unless we cut some of that out and glue the ends back together for a smaller tire. That’s just my hang-up. Y’all just ignore me and go on with life. I’ll be ok. ?

 

[rusty ol ranger]

This is why we should all run 10 plys.

 

[don4331]

This is why we should all run 10 plys.

Or 45 or lower aspect ratio tires which have no sidewalls to compress.

ericbphoto:

I have a similar issue around melting ice: I know philosophically, the temperature of freezing water/melting ice is 0C/32F, no matter how much energy is being removed/introduced, but I always want it to be slightly cooler/warmer.

 

[Snoranger]

Guys. I’m not trying to get anyone upset or worked up. I’m willing to learn something new. My brain is hung up on the fact that for a 30” tire, the manufacturer lays a strip of tread rubber about 90.25” long around that tire. So it seems to me that that tire must travel 90.25” for every revolution unless we cut some of that out and glue the ends back together for a smaller tire. That’s just my hang-up. Y’all just ignore me and go on with life. I’ll be ok. ?

Quite a few years ago NHRA was trying, once again, to slow the top fuel cars down. They imposed restrictions on tire diameter, gear ratios, max engine rpm. I think they were trying to limit them to 320 or 325mph. By all the calculations, it should have worked. The next weekend Tony Schumacher ran 330+ mph. Long story short, they were running a 14" wide rim instead of the normal 16" wide rim. That allowed the tire to "grow" a little more. Which, in turn, gave them a higher top speed. No one cut the tire and added anything too it, the rubber just did what rubber does... it shrinks, it stretches, it conforms.

 

[rusty ol ranger]

Or 45 or lower aspect ratio tires which have no sidewalls to compress.

Thats all well and good till ya smoke a pothole and bend a rim.

 

[sgtsandman]

Everyone is focusing on the wrong tire rotation difference issue and 4X4. The issue is the difference in tire rotation of the tire on one side of the axle and the tire on the other. If a truck is traveling in a relatively straight line, there is not problem.

The problem is in a turn where the outside tire needs to spin more than the inside tire to cover a longer distance. With the front and rear axles locked together, the system will bind up and something needs to give to relieve the binding.

On dirt and gravel, this isn’t a big problem since the dirt or gravel will provide the needed slip to relieve the binding. Pavement doesn’t provide that slip. Add in an engaged locker and the problem is compounded even more.

That is the issue of concern with the problem the OP may or may not have.

 

[fastpakr]

Everyone is focusing on the wrong tire rotation difference issue and 4X4. The issue is the difference in tire rotation of the tire on one side of the axle and the tire on the other. If a truck is traveling in a relatively straight line, there is not problem.

The problem is in a turn where the outside tire needs to spin more than the inside tire to cover a longer distance. With the front and rear axles locked together, the system will bind up and something needs to give to relieve the binding.

On dirt and gravel, this isn’t a big problem since the dirt or gravel will provide the needed slip to relieve the binding. Pavement doesn’t provide that slip. Add in an engaged locker and the problem is compounded even more.

That is the issue of concern with the problem the OP may or may not have.

We're not missing that, it was explicitly covered earlier. The current drift of the conversation was based on a comment about straight driving being an additional issue based on drivetrain weight.

 

[sgtsandman]

We're not missing that, it was explicitly covered earlier. The current drift of the conversation was based on a comment about straight driving being an additional issue based on drivetrain weight.

I either missed or forgot about that part. Thanks.

 

[brokemechanic3000]

Through a convoluted series of events that I don't fully understand, my brother drove my ranger about 6 miles on the highway (dry pavement) in 4wd low. I'm assuming at regular highway speed (55'ish miles per hour) but details are sketchy.

Me: Didn't you notice it was in 4wd low?!?!
Him: Well, I did have to give it a lot of gas....

I can hear an exhaust leak now; maybe from getting hot and shook to hell; maybe unrelated. Anything else I should look for? Years off the life of my tx case? I took it out on a dirt road, and 4wd seems to work fine. But I rely on this vehicle for work, so I'm willing to preemptively swap a part if it prevents an on the job breakdown.

It's a 98 3.0 automagic. I ditched the PVH and went with live axles, so yes, the hubs were definitely engaged :/

Thanks,
Cahman

My wife did the same to my truck years ago and it has been fine

 

[4x4junkie]

Not sure if this helps, or just makes it all more confusing, but my theory is that a greater weight load on a tire (or less air pressure in the tire would also create the same circumstance) does NOT change the circumference of the tire (the circumference is held firm by the steel belts in the tread, the belts cannot flex in a manner that would allow a change in circumference). What happens is the greater deformation of the tire's shape causes more squirming of the tread blocks right where the tread initially contacts the road, and then again where the tread leaves the road with each rotation. This causes more friction (and thus heat) and also has the effect of causing the tire to rotate at a speed something in between that of a non-deformed tire, and a non-deformed tire of the same radius as the deformed tire.

On the other hand, there will always be a tiny bit of give due to the flexibility of the tread blocks, even on dry pavement. It's just enough give so that things should not destroy themselves (not right away, anyway) if the truck is driven in a straight line on dry pavement in 4WD. It is not enough to allow for turns on pavement however, and this is generally where things will start to grenade.

 

[19Walt93]

Lock yours in 4x4 and head down the road, you'll feel it starting to bind very quickly and if you go too far something will break and then you'll be sure. After 42 years in the service department I've seen running in 4x4 on pavement break many parts. It's not hypothetical, it's fact.