# Differential hydraulic action

If I had a 4X4 car and wished to raise it up to increase ground clearance by 10 feet, (looking something like a travel lift) keep the diffs and axels in their original position and attach 4 separate hydraulic pumps to the end of the axels to drive 4 hydraulic wheel motors at ground level – Would I achieve all the same driving characteristics? i.e. differential action, braking, forward and reverse and gears.

Danger
Gold Member
Welcome to PF, Ducatiman.
If you have hydraulic motors, there is no need for any differentials or even driveshafts. You just run flexible hoses to the motors from your pump, which should be driven directly from your gas or diesel engine.
There's almost no way that any mechanical driveline or steering system using existing automotive technology can survive a 10' lift. If they can, then you don't need the hydraulics. I'll think on it some, but I think that your project is somewhat impractical.
Anyhow, it most assuredly would not have the same driving characteristics. To start with, and most importantly, your centre of gravity will be so high that you'll probably tip over if you hit a rabbit.

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In this exercise i need to obtain differential and not skid steering for the vehicle and just thought it may be possible to use a combination of mechanical and hydraulic. thanks

Danger
Gold Member
This is just theoretical, then? You're not actually trying to build one? That's a bit different.
In that case, it is possible to do it. No way will you avoid massive changes in the driving characteristics, though.

edit: Okay, I'm back. (Bailed out for a while to make sure that I didn't miss the last 10 minutes of 'Apollo 13'.) The first problem that you have to overcome is that you will need multiply-telescoping driveshafts and steering shaft, and more importantly either a series of U-joints or some way to rotate your axles longitudinally in order to maintain some workable geometry between the driveshafts and the differentials.

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Theoretical at the moment but i need to use hydraulic wheel motors - the height is unimportant. really just wanting to know if i can go down the path of using hydraulics as in the example above to replace your suggestion of multiply-telescoping drive-shafts and U joints without getting a degree in hydraulic engineering. Just finishing watching BSG 2/4 blue-ray.

Danger
Gold Member
You can't eliminate the telescoping bits if you plan to run your pumps from the axles. In essence, all that you are planning to do is introduce an hydraulic pump/motor linkage between the axle and the wheel. You still have to drive the axles from the engine. That's not only unnecessary; it's also downright inefficient. The only reason for going to hydraulic motors would be to eliminate the need for extremely flexible driveline geometry. And unless you plan to use differential speed of those motors for steering control, you're still going to need a telescoping and flexible steering shaft.
I rented the first season of BSG a few years ago (if you mean Battle Star). The doctor guy was irritating, but I pretty much liked it. I plan to check out the rest when I can afford it.

Your right about the doctor and about the steering control. Steering is handled by hydraulic like outboards on a boat. Its the introduction of an hydraulic pump/motor linkage between the axle and the wheel that i was concerned about. Do you know if this introduction can act in the same way as a physical link? would you still get differential action braking etc?

Danger
Gold Member
This is getting very confusing for me. Your braking should be unaffected, as long as your lines are long enough and flexible enough to handle the suspension adjustments. After all, your calipers will still be able to grab the rotors (or shoes, and drums, if using that type). Additionally, hydraulic motors can be braked by simply valving the flow. That should theoretically increase your braking potential, since you would have two systems working in tandem.
I agree that a purely hydraulic steering system would work; I never thought of it.

edit: Damn you! Now you've got me all interested in this stuff. I'm going to bed as soon as 'Hellboy' is over, but I know damned well that I'm going to be working on this for days. :grumpy:

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Could it be done by varying flow to the motors based upon the steering angle?

Danger
Gold Member
Sure could, which would again eliminate the need for differentials.