Author Topic: tandem axle?  (Read 6864 times)

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Offline Little_Girl_Big_Truck

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tandem axle?
« on: March 4, 2009, 11:45:30 AM »
ok what is it how does it work? Google didn't tell me enough but form what I got from it semi trucks and trash trucks have them or could... so does it do what I think it does? power all the wheels in the back of the truck.

Offline Psycoticredneck

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Re: tandem axle?
« Reply #1 on: March 4, 2009, 02:12:07 PM »
yes most tandem medium and heavy duty trucks do have both axles powered.  main shaft runs into the pinion of the front diff.  there is a output yoke on the rear of the the front diff housing which links to the rear diff input with a short driveshaft called a thrushaft.  Older trucks were direct drive back in the day.  Most newer ones have an air shifted gear box built into the front diff called a power divider that essentially is a differential for the two axles to allow slippage between the two and save tires.  This is much like the open differential action on a car or light truck axle to keep the tires from barking everytime you go thru a tight turn.  An air switch on the dash can be flipped which locks the two axles together for low traction situations.................now you know 8)
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Offline magnumRC

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Re: tandem axle?
« Reply #2 on: March 4, 2009, 02:27:50 PM »
Psycoticredneck covered it good while I was typing.  Here's my post anyway.
 
Heavy truck tandem drive axles are fairly simple in operation.  If they are fully open, it is possible to get stuck if only one wheel looses traction.  This means that if one of the four (or eight) wheels lose traction, one wheel will spin and you will not move.  Think of it as a fulltime 4x4 truck with open axle diffs and an open transfer case diff.

If the heavy truck has a Power Divider Lock, it locks the two drive axles together.  Traction has to be lost on two wheels (one on each axle) in order to be stuck.  It's like locking the transfer case diff in a fulltime 4x4 truck with open axle diffs.

If the truck has a differential lock, it will lock the axle solid so both wheels on the locked axle will spin together.

Some trucks have just a PDL.  Some have just diff locks.  A common setup is to have a diff lock on only one axle and a PDL between the two.  Some trucks are 6x6 with diff locks in all three axles and a PDL.  It just depends on what options the original owner ordered.

The problem is that when the PDL is thrown in, it gets difficult to steer.  It's easy to turn the wheel, but the front tires on the truck will plow.  If the PDL is in and the Diff Lock is in, good steering is next to impossible.  In slick conditions, with everything locked up, steering is impossible and the truck will go the way it is pointing.

PDLs and Diff Locks shouldn't be used on surfaces with good traction unless you really know what you are doing.  It is very easy to break stuff under those conditions, and the stuff in those axles is not even close to being cheap.  Generally speaking, the PDL and diff locks can be engaged at any speed as long as the wheels are turning together.  Hope this helps.

Erich
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Offline Elwenil

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Re: tandem axle?
« Reply #3 on: March 4, 2009, 03:30:38 PM »
And before you ask, yes you can convert a pickup to 6WD if you really want to go through the headache.  The easiest thing is to add a dummy axle in the back that is not driven.  A lot of medium and heavy trucks have axles that are off the ground and are either lowered in place by air or forced down by the load to help support the weight.  A truck with a dummy axle on the ground all the time can find itself in situations where there is not enough weight on the driving axle to keep it from spinning.

A driving axle is much harder since very few light truck axles have a "through drive" to allow and axle behind it to receive power.  I have seen a few 9" Fords fabbed up to use a pinion in the rear and a couple other axles but I think all were custom projects.  The other options are swapping to Rockwell axles from a M44 series 2.5 ton military truck.  The input and output on these axles is on the top, so they require a lot of lift to clear the chassis and especially the engine in the front.  But since the axles are all the same configuration, you can drive any of them from the front or back.  The M135 series military trucks do also have driven tandems but use a more conventional design axle with the yokes in the front of the rear axles and have a pillow block on the forward rear and a shaft to transfer power to the rearmost axle.  Th transfer case has dual rear outputs for this setup so you are forced into using this very large and heavy transfer case with this setup.  Another note on the M135, the front axle is backward, meaning the drive shaft rotates backward from all common front axles.  So again, you are pretty much forced to use the transfer case with that setup.

Another setup is to use a second transfer case or transfer box to transfer power to the rearmost axle but you have to run the transfer case at near vertical and this can cause a lot of lubrication problems that will need to be worked around.  A transfer box is pretty much a custom deal and very expensive.  The now (rightfully) out-of-business USA6X6 used transfer cases and a custom transfer box as well as the Rockwells with mixed results.  Unless they farmed out the work for their transfer box to a quality shop, I would not trust it.

Now for some points on 6WD.  The main problem with 6WD on a smaller vehicle is ground pressure.  When you add more points of contact to the ground that support the weight of the vehicle, you spread the load over all those points.  So a truck that weighs 6000 lbs and has 1500 lbs supported by each tire on level ground would only have 1000 lbs on each tire in a 6x6 configuration.  Naturally in this simple example I am not taking the weight balance of a real truck into consideration.  In most vehicles you would actually end up with more weight on the front and less on the rear axles.  Now you take that truck and put it in some off road obstacles and transfer the weight around and you can easily end up in situations where you can't climb because there is not enough weight on the tires to get traction.  In a lot of cases a 4x4 truck would preform much better than the same truck converted to 6x6.  Now if you are adding a lot of weight to the truck and are exceeding the load rating of the two axles under the truck, you could convert to three axles to support the load and still have decent ground pressure.  This is going to be sort of rare in a light truck since the frames will only support so much and you begin exceeding the design parameters of the original vehicle.  To say the least, a 6x6 conversion requires quite a bit of math and thinking to decide if it's necessary or is going to function as well or better than a 4WD.  Most are done for show or they quickly find that their truck is only good in light mud or sand where low ground pressure is advantageous.

There are other points to consider with 6WD conversions such as tire scrub.  The rear axles do not turn so you will slide or drag most of the rear tires in any tight turn.  This can cause a loss of traction on slick surfaces and will easily decrease the life of the tires.  Another point is when you do have a working 6WD vehicle that does get good traction, you can overpower the front axle and push it out of a turn in certain situations.  Your turning radius will easily suffer and in some situations you will not be able to turn at all.

There are other points to consider and think about, but that is the core of the matter.  Now, does anyone want to cover 8x8 conversions with dual steering axles?  I've thought my way all over that one also, lol.
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Offline SuperBurban

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Re: tandem axle?
« Reply #4 on: March 4, 2009, 05:00:37 PM »
There are other points to consider and think about, but that is the core of the matter.  Now, does anyone want to cover 8x8 conversions with dual steering axles?  I've thought my way all over that one also, lol.
Sure go for it, I won't cover the problems with your 6 wheel explanation, overall it was close.
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Offline KThaxton

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Re: tandem axle?
« Reply #5 on: March 4, 2009, 05:31:25 PM »
ok what is it how does it work? Google didn't tell me enough but form what I got from it semi trucks and trash trucks have them or could... so does it do what I think it does? power all the wheels in the back of the truck.

If I understand your question, I think you are looking for a much simpler answer than what has provided so far. A tandem axle is well, a second axle in tandem with the first.
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Offline Elwenil

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Re: tandem axle?
« Reply #6 on: March 4, 2009, 05:52:22 PM »
Sure go for it, I won't cover the problems with your 6 wheel explanation, overall it was close.

Feel free to counter, I'm not an expert, but have looked at doing these conversions for many years and have been researching various military 6x6 and 8x8 trucks for a long time.  If there are points where you think I'm wrong, or a little off, I'd like to hear them since you may be considering something I haven't thought of.
L.Clemons

1988 Ramcharger-Mil-Spec AW450 Project-318EFI-NP435 4 speed-NP205 Transfer Case-Front & Rear Dana 60s-Braden Wormdrive Front Winch
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Offline Little_Girl_Big_Truck

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Re: tandem axle?
« Reply #7 on: March 4, 2009, 06:02:18 PM »
wow my head hurts now.... but I think the point is its a bad idea to try to use them on a pick-up. I guess I'll just have to face it if I want a truck with 3 axles 6X6 or not I'm going to have to buy one that already is one or a school bus bolted up on/over a military 6x6.... thanks I now know some thing new.

Offline Elwenil

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Re: tandem axle?
« Reply #8 on: March 4, 2009, 06:16:36 PM »
Why not just get a military M35?  They are pretty cheap for their size and can still be bought at auction.  Research them a lot before you make the leap to such a large truck as there are a lot of points to consider and working on them is just about all heavy work.  Check out www.steelsoldiers.com if you are interested in military trucks.
L.Clemons

1988 Ramcharger-Mil-Spec AW450 Project-318EFI-NP435 4 speed-NP205 Transfer Case-Front & Rear Dana 60s-Braden Wormdrive Front Winch
I do not like shortcuts.  Any job worth doing is worth doing right.

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Offline Little_Girl_Big_Truck

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Re: tandem axle?
« Reply #9 on: March 4, 2009, 06:26:32 PM »
Why not just get a military M35?  They are pretty cheap for their size and can still be bought at auction.  Research them a lot before you make the leap to such a large truck as there are a lot of points to consider and working on them is just about all heavy work.  Check out www.steelsoldiers.com if you are interested in military trucks.

theres only a cab with enough room to drive. though I'm really tiny so it may be pretty big to me.. but I like the idea that I could live in my Ramcharger or another Suv of that size or bigger I was thinking on useing a crew cab pickup not so big but it would have to do.

Offline Elwenil

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Re: tandem axle?
« Reply #10 on: March 4, 2009, 08:17:23 PM »
With a 12' bed, an M35 can carry a lot, plus the cab seats 3 but you other two passengers are going to wish they had better seating arrangements, lol.  You could get an M109, which is a M35 2.5 ton truck with a command box on the back:



But it's more than likely overkill for what you want, lol.
L.Clemons

1988 Ramcharger-Mil-Spec AW450 Project-318EFI-NP435 4 speed-NP205 Transfer Case-Front & Rear Dana 60s-Braden Wormdrive Front Winch
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Offline magnumRC

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Re: tandem axle?
« Reply #11 on: March 5, 2009, 03:08:58 AM »
There are other points to consider and think about, but that is the core of the matter.  Now, does anyone want to cover 8x8 conversions with dual steering axles?  I've thought my way all over that one also, lol.

LOL!!  My brother drives the M1074 PLS in the Army.  They are full time all wheel drive 10x10s.  2 straight axles, 3 steering axles (axles 1,2 and 5). All axles are drivers, have lockers, and have planetary hubs.  It has 500hp from a 8v92TA Detroit, a CTIS for all 10 tires, can ford 4 feet of water, and has 24" of ground clearance.  They even have their own crane.

I WANT TO OFFROAD ONE....but not bad enough to join the Army to do it.

Erich
83 Ramcharger w/92 Cummins, NV4500, NP208, D44/ 9.25 axles, 3\" lift, turned up VE pump (mild).

Offline Elwenil

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Re: tandem axle?
« Reply #12 on: March 5, 2009, 03:26:10 AM »
Lol, yeah there is a guy on SteelSoldiers who owns a PLS and several other Pallet trucks, some of which are experimental versions.  He uses them out in the midwest to fight wildfires.  There are also a few HEMTT owners on there as well.  Some of the coolest 8X8s are the old Ford trucks from the '60s that supported the Pershing missile system.  I think one guy still owns one of the experimental amphibious 8x8s that were built by Chrysler in the late '50s that were never given the nod for a contract.
L.Clemons

1988 Ramcharger-Mil-Spec AW450 Project-318EFI-NP435 4 speed-NP205 Transfer Case-Front & Rear Dana 60s-Braden Wormdrive Front Winch
I do not like shortcuts.  Any job worth doing is worth doing right.

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Offline Little_Girl_Big_Truck

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Re: tandem axle?
« Reply #13 on: March 5, 2009, 09:19:43 AM »
With a 12' bed, an M35 can carry a lot, plus the cab seats 3 but you other two passengers are going to wish they had better seating arrangements, lol.  You could get an M109, which is a M35 2.5 ton truck with a command box on the back:



But it's more than likely overkill for what you want, lol.

yeah I think the box would be to big. how long is it from the back of the cab to the end of the frame? I was thinking maybe I could take like a van or a mini school bus or if needed use a part of a full size to make it like a HUGE SUV! I think it would be a pretty mean looking truck. 2 of my friends say I should just get a full size school bus already and be done with the I want a big truck thing... maybe I should I was told a few years ago by my school bus driver I could get one for like 1500 cause there pretty much worthless when they can't be used for schools anymore and they just want get rid of them. I'm thinking if I do that on adding like Zombie Apocalypse survivor armor like in dawn of the dead to it would be just as mean looking.

Offline Elwenil

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Re: tandem axle?
« Reply #14 on: March 5, 2009, 01:59:11 PM »
Most of the M44 series 2.5 ton trucks have a 12' bed.  The box on the one above is also 12'.  There are a few models like the M36 and M292 have a longer chassis and bed.  5 tons also have a longer bed and chassis on most models.  And yes, you can get a school bus after they are retired but they are pretty worn out by then and can be a money pit in some cases.  Prices can vary but generally you can get one for about half of what a 2.5 ton Deuce would cost.  Most are a bit under powered with a full load and can be pretty slow.  I can tell you that you probably won't get much respect from the "truck crowd" driving an old school bus but they can be a great deal if they fit your uses.
L.Clemons

1988 Ramcharger-Mil-Spec AW450 Project-318EFI-NP435 4 speed-NP205 Transfer Case-Front & Rear Dana 60s-Braden Wormdrive Front Winch
I do not like shortcuts.  Any job worth doing is worth doing right.

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Offline Blasphemous

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Re: tandem axle?
« Reply #15 on: March 7, 2009, 12:17:09 AM »
I'm a member over on SS and I'm a Deuce owner as well.  That calms my needs for 6x6, heavy hauling and a HUGE diesel engine that runs on anything.  It's only slightly inconveniencing when driving to the store or something like that.  It's FUN as HELL though ;D
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