Transfer Case Clunking

[James Morse]

I understand most of that, except, if you are making a turn in a perfect circle, seems to me the front and rear axles are travelling exactly the same distances. The lefts and rights are different distances, but front/rear on one side and front/rear on the other side would be the same distance. That's what I don't get.

That's why I wondered is it something that happens only when you begin the turn. Because if you are going in a perfect circle, your lefts and rights are travelling in different circles, true, but the fronts/rears are in the same circle. That's where the explanation doesn't seem to hold up.

In mine I have lsd in back but that doesn't prevent it from acting open when it has to, and having open or lsd doesn't seem to come into this at all.

Edit: If the front steering causes the distances front/back to change as a result of the steering geometry that could impact it.... ?

Thanks.

 

[00t444e]

The front and rear axles are not traveling the same distance when you turn in a circle, only way that would be possible is if you had a vehicle with front and rear steering.

 

[James Morse]

but you'd agree if the truck is going straight they are travelling the same, surely
so that means the steering is changing the distance between the front/rear contact patches, right?
I mean there are all kinds of considerations in steering ass'y, camber, caster, all that. but if you say, when you turn, it changes the front/rear distances on one or both sides, then, that would explain binding because if you have 2 differentials that act the same, then there's no way (I think) that any kind of differential action will take up the differences front/rear on -both-sides. If one side is in synch, the other side is being asked to do something it simply cannot do

am I on track here?
in other words if I'm reading this correctly, when you're going straight, the tires (contact area) make a rectangle, but when you steer, it creates a trapezoid.

and I suspect the side that drags/skips/slides will be the non-driven side, is that right?
the driven side will be the inside of the turn?
I might be wrong there but it has to be one or the other

still not clear why you see it at the front and not rear when front has more weight on it, seems like it should cause the rear to drag/slip

thanks

I guess I could answer the geometry question by cutting the wheels all the way and measuring the two sides. seems to me like, why can't you build steering where when you turn the wheels they maintain the same distances to the rears
then you wouldn't have this issue, if that's what causes it. the pivot point would have to be precisely in the center of the wheel, maybe you can't build something like that.

it's not that the lefts and rights are going different distances in a circle, that's true, but differentials take care of that
it's that for the two sides, the fronts and rears are different distances from each other and that creates a situation asking the differentials to do something they don't do
in other words diff's that are built the same will act the same and they are being asked to behave differently, and they can't

hope I'm on track.... gonna keep hammering away at it until my brain can wrap it

 

[00t444e]

but you'd agree if the truck is going straight they are travelling the same, surely
so that means the steering is changing the distance between the front/rear contact patches, right?
I mean there are all kinds of considerations in steering ass'y, camber, caster, all that. but if you say, when you turn, it changes the front/rear distances on one or both sides, then, that would explain binding because if you have 2 differentials that act the same, then there's no way (I think) that any kind of differential action will take up the differences front/rear on -both-sides. If one side is in synch, the other side is being asked to do something it simply cannot do

am I on track here?
in other words if I'm reading this correctly, when you're going straight, the tires (contact area) make a rectangle, but when you steer, it creates a trapezoid.

and I suspect the side that drags/skips/slides will be the non-driven side, is that right?
the driven side will be the inside of the turn?
I might be wrong there but it has to be one or the other

still not clear why you see it at the front and not rear when front has more weight on it, seems like it should cause the rear to drag/slip

thanks

I guess I could answer the geometry question by cutting the wheels all the way and measuring the two sides. seems to me like, why can't you build steering where when you turn the wheels they maintain the same distances to the rears
then you wouldn't have this issue, if that's what causes it. the pivot point would have to be precisely in the center of the wheel, maybe you can't build something like that.

it's not that the lefts and rights are going different distances in a circle, that's true, but differentials take care of that
it's that for the two sides, the fronts and rears are different distances from each other and that creates a situation asking the differentials to do something they don't do
in other words diff's that are built the same will act the same and they are being asked to behave differently, and they can't

hope I'm on track.... gonna keep hammering away at it until my brain can wrap it

Again you can't turn in a perfect circle because only the front wheels turn and the rears are fixed. Ever made a turn too sharp and hit a curb with a rear tire? That happens because your rear axle doesn't take the same track as you front when you turn, go into a snow covered parking lot and turn a circle and you will see how the front and rear track differently. Also there is no driven or non driven side in an axle, both sides are always being driven even in a turn. The only exception to that is if you have an automatic locker and in that case in a turn the outside tire uncouples and spins faster while the inside tire is being driven, then it locks back up when the speeds equalize.

 

[pjtoledo]

maybe this will help.

ackerman angle explained - Search (bing.com)

 

[James Morse]

why don't I have auto locker? seems like that would have been something to include. I don't suppose it's something I can retrofit?

you hit on something (!) with the tracking because I do recall that from years past and I remember my puzzlement at the time.
Ackerman angle is what I'm talking about we know the left/right front turn at different angles, if it's set up right it will be exactly the Ackerman angle.
note that if your right front was turned all the way to the right (at a right angle to the right rear) the left front would still be at an angle. the truck would pivot around the right rear making three tracks.
normally of course the angle is less severe and the truck makes four tracks. why? because if you look at that picture you'll see the distances from the center are different for all four tires.
the wheels are fixed points on the truck so it has to be that way. in the pic it looks like the left rear will have the inside track, then probably the left front, then right rear, then right front.
you'll get 4 tracks. you'll never be able to drive so that it makes two tracks (unlike a lot of examples for steering).
the tires are always tangent to the circle they must travel in.

this pic below helps a little to visualize it. they only drew the circles for the right front and left rear, but if you took a compass and drew the other 2 circles, then it'd be very clear, there have to be 4 tracks.
as the steering angle gets less and less the tracks will get closer and closer together... but only at 0 degrees steering (straight) will you have two tracks
it was a surprise to me

all four circles obviously have different diameters meaning all four wheels travel different distances

if it is the situation that two differentials on the same truck must act exactly the same (I'm guessing that's true) then in order for there to be no fighting/binding between them,
the -difference- travelled between the two fronts, and the difference travelled between the two rears, has to be the same. I suspect this basically never happens.
in other words if the difference were the same, then both diff's could act exactly the same and, no binding. you could figure mathematically when does this happen and I bet you'd find out, if it does ever happen it would be a rare situation.

so that solves that part of it - wheels do -not- travel in the same circles.

now the rest of the question is, why do we see the slipping/dragging at the fronts? wouldn't it be just as logical for it to be at the rears? It seems like the rears take precedence somehow....
one of the differentials is going to be working to take up the difference, and it seems to be the rear, you don't see slipping at the rear, right? it evidences at the front, I think.
so if the rear is "working properly" (accounting for the difference left/right), it is forcing the front to behave the same say, but, the difference of the fronts left/right isn't the same, so it's being asked to do something it simply cannot.

I'm going to draw this out to show all the circles, if you know minimum turning radius spec and wheelbase and track you have all the info needed to find out what are the respective differences, that might answer some of it.
what I'm saying is the proportions of the circles of the rears, and the proportions of the circles front, aren't the same, and one of the differentials will work to take up the difference but then it forces the other to do the same

if the proportions of the diameters of the rear tracks were, say, 20' : 16', then the proportions of the front tracks would likewise have to be that same proportion, like, 25:20 and my guess is they won't be.
one differential is going to act to take up the difference where the proportion is biggest... or smallest.... and force the other to do the same, which will be wrong for that other axle thus the binding

the problem isn't that the left/rights are different it's the front l/r and rear l/r aren't different in the same proportions

we'll run it to ground yet

 

[James Morse]

I may have omitted that there is no match of front/rear that's the same besides no match of l/r. So even if the proportions were equal as I was talking about above, you still have a problem of no matchup.

However, let's say in the pic that the left front and right rear were the same tracks. Then they would be in synch as to rotational speed. You could construct a steering angle where the left front and right rear were tracking the same circle, I believe. So no binding there between those 2 wheels.

Then you have the issue of what happens to the other wheels. They have independent differentials that can compensate their opposite wheels. Again no binding.

But it has to be that situation only. Otherwise no front/rear combination is equal.

In a turn probably your inside front and outside rear, w/ respect to the turn direction, are the most equal.

So in a real-life experiment there should be a turn angle, that is somewhat less than the sharpest you can turn, where you will have no binding. Everywhere else you will have binding, but you notice it the most on a very sharp turn because that's when the differences are the greatest. In a shallow turn it will be there just not as much. Obviously at the shallowest turn of zero degrees there's no binding.

It means there is, I think, a scenario where you get only 3 tracks (front inside and rear outside are in the same circle). And no binding. Unless I'm missing something about how diff's operate.

 

[00t444e]

Yes you can add auto lockers in the axles, people do it all the time I have one in both axles of my Jeep and F250 and I am going to put one in the rear of my Ranger, but that still doesn't allow you to turn on dry pavement in 4x4. You need a center differential in the transfer case for that. You keep saying only the front wheels slip when turning in 4x4 but that's not true, the front or rear tires could slip.

 

[James Morse]

OK good to know about the rears can slip too because that would make a lot more sense. I have almost no experience with it. Thanks.

I need to read about the auto lockers this is totally new to me. Thanks.

 

[pjtoledo]

add 2 more circles to your diagram.
first one is exactly centered between the tracks created by the front tires.
second is centered between the rear tires.

those new circles represent where each differential would be at equilibrium, nothing sliding/skipping/hopping, PER AXLE.

but the front axle circle is bigger than the rear, therefore the 2 driveshafts should be turning at different speeds.
but they can't because the transfer case has them locked together.

why does the front left slip???

different traction at each wheel for openers.
then there is a littler matter of physics and something called, well I don't know what it's called. (it's related to vectors)
an example is a socket with ratchet applied directly to a nut, 10 lbs of force on the ratchet puts 10 lbs on the nut.
add a 3 inch extension, 10 lbs applied is now less than 10 lbs applied to the nut.
add a 12 inch extension and a lot less is applied to the nut.
add a 24 inch extension and that nut ain't gonna move because there is now very little rotational force.
this assumes the head of the ratchet is not supported.

guess what, the front left combination of shafts is the shortest extension so it gets the most torque.
chassis & suspension flex is the equivalent of not supporting the the ratchets head., sort of.

auto lockers, study up on how sprague clutches work then we can apply that to an auto locker,,,not-GM weight flinging thingies

 

[don4331]

an example is a socket with ratchet applied directly to a nut, 10 lbs of force on the ratchet puts 10 lbs on the nut.
add a 3 inch extension, 10 lbs applied is now less than 10 lbs applied to the nut.
add a 12 inch extension and a lot less is applied to the nut.
add a 24 inch extension and that nut ain't gonna move because there is now very little rotational force.

In words of Sir Isaac Newton - for every action there is an equal and opposite reaction - 1st law of physics.

So, whether you have a 1" or 24" extension, if you put 10 ft lb of rotation on the ratchet, you put 10 ft lb on the nut.

Now, because the extension isn't made of un-obtainium, it twists. We'll say the 1" twists 1 degree - you hardly notice that... But the 24" extension twists 24 degree to achieve the same rotational force, and you notice that.​

​

With open differentials, all shaft (and therefore wheels) get the same torque.

As was noted in another post, as noted in other post, car don't steer like Conestoga wagons, but rather the front wheels are turned with "Ackermann" geometry. And Ackerman geometry only has the front 2 wheels turning the correct angle at two (2) diameters of circle - straight ahead (infinitely large circle) and some other point determined by the suspension engineer (usually about 3/4 tightest circle as this minimizes differences at other positions/being at minimum circle for any amount of time is unusual). At any other angle, both front wheels are at a slight wrong angle (one too small diameter, the other too large). The result is the tire is already slipping slightly. Add in a little weight transfer to outside wheel. A little suspension geometry with TIB/TTB, helped by body roll, that puts the whole outside tread flat again road, while lifting inside of inner wheel.​

​

The result is the inside front tire loses traction and slips.​

​

As noted by @00t444e, as the rear wheels have less weight on them in unloaded truck, it might be the inside rear wheel that is easiest to slip.​

​

Auto-lockers, e.g. Detroit Locker, need some minimum torque threshold to operate. So, in really slippery conditions, act like a spool and drive both wheels same amount and the truck goes straight. The old F-100 needed 4wd much more (Dana 60 in back had Detroit Locker), as the front wheels needed to pull the truck around corners.

 

[00t444e]

My truck with a Detroit locker in the rear doesn't need the front wheels to pull it around corners, it turns fine in slick or dry conditions.

 

[pjtoledo]

In words of Sir Isaac Newton - for every action there is an equal and opposite reaction - 1st law of physics.

So, whether you have a 1" or 24" extension, if you put 10 ft lb of rotation on the ratchet, you put 10 ft lb on the nut.

Now, because the extension isn't made of un-obtainium, it twists. We'll say the 1" twists 1 degree - you hardly notice that... But the 24" extension twists 24 degree to achieve the same rotational force, and you notice that.​

​

With open differentials, all shaft (and therefore wheels) get the same torque.

As was noted in another post, as noted in other post, car don't steer like Conestoga wagons, but rather the front wheels are turned with "Ackermann" geometry. And Ackerman geometry only has the front 2 wheels turning the correct angle at two (2) diameters of circle - straight ahead (infinitely large circle) and some other point determined by the suspension engineer (usually about 3/4 tightest circle as this minimizes differences at other positions/being at minimum circle for any amount of time is unusual). At any other angle, both front wheels are at a slight wrong angle (one too small diameter, the other too large). The result is the tire is already slipping slightly. Add in a little weight transfer to outside wheel. A little suspension geometry with TIB/TTB, helped by body roll, that puts the whole outside tread flat again road, while lifting inside of inner wheel.​

​

The result is the inside front tire loses traction and slips.​

​

As noted by @00t444e, as the rear wheels have less weight on them in unloaded truck, it might be the inside rear wheel that is easiest to slip.​

​

Auto-lockers, e.g. Detroit Locker, need some minimum torque threshold to operate. So, in really slippery conditions, act like a spool and drive both wheels same amount and the truck goes straight. The old F-100 needed 4wd much more (Dana 60 in back had Detroit Locker), as the front wheels needed to pull the truck around corners.

I specifically mentioned the head of the ratchet not being supported. that puts my example in a whole different ball game.