Swapping in a front axle into the rear

[Hawssie]

I dont have years of experience dealing with the rockwell axles found in the 5 tons like some on this forum, but by looking at the dual front and rear input/output flanges it seems like it would be a natural to swap in a front axle to the rear for rear steering. I cant be anywhere near the first to think of this as it seems so obvious. Right off I can see the allowable turning angle would be less in the rear spot due to the outboard springs but that wouldnt be a problem as you wouldnt need that much angle to make a big difference in maneuverability and tuning radius. You would lose some weight rating though. I know its not for everybody and would still require a lot of fab/hydraulic work but what am I missing. If you did just the last axle it would be kinda like a mini HET.

 

[Recovry4x4]

The geometry would have to be spot on for sure.

 

[wheelspinner]

Isn't that pretty much how the monster trucks and such do it?

 

[doghead]

I think he means like a tag axle, with all three axles still in place, WS.

 

[Dead Reckoning]

Isnt the front axle about 4" narrower than the rears?

 

[Naterik]

Assuming you're talking about a bobbed truck, to have 4 wheel steering it really would be easier than using a rear straight axle. It would all bolt up. Only complications are the hydraulics and centering pins for on road use.

 

[tim292stro]

If you're doing 4-wheel steering and streeting it, you will find that if you make a turn over 25MPH with rear counter-steer - the rear will try to pass you .

If you're doing 3-axle (front rear steer, middle static) similar to the LVSR and MTVR, you can get away with more rear axle steering input due to the static axle(s). The trick, if you are only using one steering wheel, is to figure out your linkage lengths. You will want to keep the axle facing the same direction (front-to-front) so that you get the benefit of caster at speed. Spin the drop pitman 180* from the steering box and now the same direction input rotation will have the opposite movement. Change the pitman length and you can have a single non-gear-reduced input shaft which has more or less effect on the rear steering. To get the Ackerman angles correct for on-road speed manuvering, draw a ling from the center of the static axle's left-to-right center-line through the king-pin bearings on the knuckles.

Your knuckle-to-knuckle tie rod should mount at the intersection of the two lines and the distance the tie-rod is mounted from the bearing (as long as possible for best leverage, but no longer than necessary to avoid being an impact target, or running into the tires.

If you want to get really complicated, you can make everything variable and do like what the Quadra-steer trucks do and invert at >25MPH (so both the front and rear turn the same way) for better stability in highway speed lane changes. Also, you could "crab" into a parallel parking spot (this would mean that all three of a three axle truck would need to be steerable).

 

[tim292stro]

One think you might consider is the insurance aspect of that - any design changes you make will affect your liability post-accident.

 

[bigford]

I'm on my second truck with rear steer, the first was street legal, and the second one will be when I'm done. It is a fun upgrade. Right now I have only a single rear axle, but have worked out the details for adding another rear. It would require the middle axle to be able to alter the ackerman angle depending on whether you are steering or crabbing.
As for the single rear, I use a 4-way snowplow electric / hydraulic unit with a left-right toggle switch mounted on my shifter. It is not linked to the steering box at all. I have a small pneumatic cylinder that drives a 1" pin in a drilled out steering arm to lock it straight. It is mechanically self-centering so you don't have to worry about lining up the locking hole. Somewhere on here I posted some pics a long time ago.
You might turn your truck over if you use rear-steer only, even at low speeds. It's the same as backing up and cutting the wheel. On my first truck, while checking it out I brought it up on 2 wheels (not far but just enough to cause some seat discoloration). I only use the rear steer at low speeds and off-road.
I did have to move my rear springs inboard under the frame rails to be able to go lock-to-lock.
The monsters use some electronics to self-center. I came to the conclusion I would have to do the same if I added another rear axle. It would take several sensors as the middle axle travel would be set as a percentage of the rear axle travel in steer mode and copy it in crab mode. I have had to put this part of my project on the back burner for now.
By the way all of this is under an F-700 but bobbing a mil truck will give the same results. By the way the 5 ton front springs and mounts are nice for moving the rear springs inboard on a bob job.
Hope this helps... and if you decide to try it I'll be glad to send you some more pics or share what I've learned.

 

[Hawssie]

My idea was to just do the last axle so you would have the middle static. Yes I would put it in the same way as the front so caster should hopefully want to keep it straight. The front may be shorter but I was planning on just running the rim flipped the same way as the front so the track would be the same. As far as steering goes my idea ( and this may change or not work) was to add a hydraulic steering valve like tractors have, run it off the steering shaft with a chain drive so it wouldn't effect the front steering at all. Then I would have a lock pin that would lock the rear steer in the straight position for over the road usage. Then I would add a double pump in place of the single steering gear pump to allow for two separate hydraulic circuits. I would also have a light that tells you when the rear steering axle is centered just for safety. So when you wanted to use it you would first start with the front axle straight, then unlock the pin to allow the axle to steer (either with pneumatic or hydraulic maybe electric actuation for lock unlock) Then you would be starting with the front and rear steer axles straight so they would synchronized. after your done you would turn the wheel to center the rear, lock it out and then you would be ready fro normal (safe) steering. You could also have a selector valve to change the rear to crab if you wanted. You would have to fool around with the gear ratio for the steering chain drive gears, cylinder size length etc, and pressure settings to get the steering where you wanted it react according the the mechanical front, but adding mechanical linkage sounds like a nightmare to me. But then again I was reading the OP manual last night for the m1070 and it appears to use a mechanical set up. Hopefully I will be able to see how that works in person for myself soon. You would also need to re direct the hydraulic flow away from the steering valve when not in use.

 

[tim292stro]

Oshkosh uses a KISS principal on their military trucks - they did offer a more flexible steering system under their Pierce brand of Fire trucks (same company different product line) called "All-Steer", but they have since discontinued that due to reliability and training difficulties. Simple shafts, fixed ratio gear boxes, and calibrated length connecting rods and pitmans are the way to go for safety - Oshkosh and Pontiac found this out the hard way. In the video below, this is a 3-axle All-Steer truck - but the biggest complaint I have is the location of the axles - the rear of the truck has a 14-FOOT overhang past the last rear axle!! As you will see, the truck has a tendancy to throw the rear of the truck outside the turn, this is coupled with driver's inexperience displayed as over-input for the first turn. I can understand just by looking at this why it may have been the cause for several accidents.

If you are keeping the mid axle fixed, you will notice a stability improvement at speed with both rear axles locked straight. You only gain the rear counter-steering benefit at low speeds with large steering input (less scrub). I can think of a mechanical linkage with a very large effort spring return, tied into an axle drive speed mechanism - under 25MPH it allows connection to the front steering only when aligned to the front (i.e. you might have to center your steering wheel to "pick up" your rear steering). Over 25MPH it disconnects the rear steering, and the high force spring returns the shaft to center where a keyed index locks it down to dead center (and no connection to the front steering at all.

This would serve three purposes: no electronics, mechanical simplicity, and a noticeable change in the feel of the steering (which subconsciously you would detect and train yourself to recognize when you have which mode). The high force spring would act against the front steering when the rear steering is picked up (i.e. it would become a bit harder to steer), and when the rear steering drops out there would be no force acting on the steering which would allow for smaller steering input required for higher speeds without the fatigue the high force spring would induce.

 

[Hawssie]

I see what you mean. My idea(s) stem from the fact that all my experience with rear steer is from the heavy equipment or Agriculture realm, and that means hydraulic. I knew that this limitation in knowledge would have to be addressed prior to advancing from the the theoretical.

 

[tim292stro]

Even with hydraulics, you would have to have a good relable way to disconnect when centered and pick up in-sync with the front steering. A similar mechanism would be required if you went the purely mechanical route.

If you go electronic it's not all sunshine and flowers either, drive-by wire software and life critical systems is an interesting industry unto itself. Something like steering would require redundancy and incredibly complex and thorough design review and implementation validation by a third-party. You would get the benefit of being able to operate your steering based on a mathematical model, or in actuality several models, so in theory you could have incredible flexibility.

Agricultural vehicles are not normally designed for road use, so they have more leeway as to failure modes - if you lose you're steering, you're only likely to run over some crops, in traffic if you lose your steering there are potentially much more significant different consequences. Lets not forget the driver in these two different scenarios: In agricultural, the driver is likely to be more passionate about their vehicle and understanding it since it is directly associated with their livelihood - a car is most often a commute vessel - and these days (just look at the numerous threads here complaining about cell phone use), many drivers just don't care about their vehicle. Just different level of involvement and mental commitment to the operation of the vehicle, and different levels of "giving a $#!+" about the function of the vehicle.


All that asside, hydraulics would be good for power amplification, but would make the total system design more complicated. Not trying to deter, just stating it.