forcing a differential

Thread Starter

strantor

Joined Oct 3, 2010
6,782
I think you are going to have problems if you don't at least have pivoting wheels. A rigidly mounted wheel wants to track straight ahead. Turning two wheels at different speeds on a common axis is not too much of a problem and the object can turn about the vertical axis that passes through the common axis pretty freely. But if you add another wheel that does not share that common axis, then it will fight you. This is true even in aircraft that use differential braking on the main gear to steer -- the nose/tail wheel still needs to pivot.
I've thought about this, and touched on it in the thread I linked to in post #1. I think that degree of difficulty of turning will be dependent on the ratio of distance between the 2 front wheels and the distance from the center point of the 2 front wheels to the rear wheel. Plus rolling resistance of the rear wheel, and air resistance along the sides, increasing with speed. I think that a traditional "aerodynamic" body style (long, slender, tapered toward the rear) would make this vehicle very hard to turn, especially at high speed. The easiest body style to turn I think would be a hemisphere, like an upside down bowl. I don't know how efficient that would be though. Definitely an area which deserves more thought.
In a conventional four wheeled vehicled, the two front wheels not only need to pivot, but they need to pivot by different amounts. because you want the axes of all four wheels to pass through a common point at the center of the turn. The steering linkages, such as the Ackerman steering arm, are pretty amazing when you understand the subtleties of what they do.
I guess it wouldn't hurt to familiarize myself with how a regular car works before reinventing the wheel.
 

WBahn

Joined Mar 31, 2012
29,979
I've thought about this, and touched on it in the thread I linked to in post #1. I think that degree of difficulty of turning will be dependent on the ratio of distance between the 2 front wheels and the distance from the center point of the 2 front wheels to the rear wheel.
Have you ever had a shopping cart that had a front wheel that wouldn't swivel? Not very pretty to handle, is it.

Also, think about trying to park (i.e., turn the vehicle at very low speed). Two of the four wheels (at least) are going to have to be dragged sideways.
 

Thread Starter

strantor

Joined Oct 3, 2010
6,782
Have you ever had a shopping cart that had a front wheel that wouldn't swivel? Not very pretty to handle, is it.

Also, think about trying to park (i.e., turn the vehicle at very low speed). Two of the four wheels (at least) are going to have to be dragged sideways.
This was also brought up in the first thread. I don't think I addressed it in that thread, but what I've been thinking is some kind of mechanical dampener to eliminate oscillations of the caster wheel. For sure you wouldn't want something like the wobbly walmart shopping cart wheel on the back of your car as you cruise down the highway. Some sort of mechanism that would allow full rotation of the rear wheel for the scenario you mentioned, parking, or turning (this automobile should be able to turn 180 degrees in one spot, dragging the rear wheel in an arc of course), but maintain a steady direction at speed.
I think it can be done passively but I'm not sure how. If not, the rear caster could be positioned by a servo or stepper. A bit of math in a microcontroller should be able to tell the rear wheel exactly what angle it should be at for a desired turn.

...but then, if it were done that way, it would negate the need for the differential controlled steering all together. Steering could be accomplished by the back wheel. This would also allow for aerodynamic body design...

mumble mumble mumble.... walks off shaking head...
 

gerty

Joined Aug 30, 2007
1,305
How about a hydraulic motor on each rear wheel, each powered by a proportional valve. That's how they do Bobcats and small tracked backhoes.
 

Thread Starter

strantor

Joined Oct 3, 2010
6,782
How about a hydraulic motor on each rear wheel, each powered by a proportional valve. That's how they do Bobcats and small tracked backhoes.
Yes most examples of vehicles I'm aware of that employ this type of steering do employ the hydraulics, except for military tanks, and I believe ZTR lawnmowers use belt clutches. The thing that all the hydraulic machines have in common is that they are high torque, low speed. I think flow friction becomes a problem when you want high speed. But then there's gearing to overcome it, but I don't know all the particulars.

In any case, it goes back to what I was saying in post #2:
This is why I'm on the trail of a differential solution, because if the wheels are mechanically linked, then should something happen, it would happen to both sides.
If there were 2 seperate drive mechanisms and one failed at high speed, the vehicle could lose control catastrophically.
 

WBahn

Joined Mar 31, 2012
29,979
Yes most examples of vehicles I'm aware of that employ this type of steering do employ the hydraulics, except for military tanks, and I believe ZTR lawnmowers use belt clutches. The thing that all the hydraulic machines have in common is that they are high torque, low speed. I think flow friction becomes a problem when you want high speed. But then there's gearing to overcome it, but I don't know all the particulars.

In any case, it goes back to what I was saying in post #2:

If there were 2 seperate drive mechanisms and one failed at high speed, the vehicle could lose control catastrophically.
Not necessarily. Cars losing a wheels (as in lug nuts come loose and wheel goes flying) at highway speeds is not too uncommon an occurrence nationwide and it seldom results in a catastrophic event. When I was young I was in a pickup that threw a rear wheel and, as the driver, I've had two front tire blowouts (on a front-wheel drive care) at 70+mph. All three events were remarkably sedate and well controlled. Now, I realize that I still had steering, but I didn't need to do anything except guide it off the road and so the wheels stayed pointing straight ahead and that's how it tracked. Now, if you have a castoring wheel, you might have bigger problems. But, then again, even with your single drive mechanism the most likely failure is probably going to be a blow-out or loss of a wheel on one side. So you'll have to deal with that possibility one way or the other.
 

Thread Starter

strantor

Joined Oct 3, 2010
6,782
Not necessarily. Cars losing a wheels (as in lug nuts come loose and wheel goes flying) at highway speeds is not too uncommon an occurrence nationwide and it seldom results in a catastrophic event. When I was young I was in a pickup that threw a rear wheel and, as the driver, I've had two front tire blowouts (on a front-wheel drive care) at 70+mph. All three events were remarkably sedate and well controlled. Now, I realize that I still had steering, but I didn't need to do anything except guide it off the road and so the wheels stayed pointing straight ahead and that's how it tracked. Now, if you have a castoring wheel, you might have bigger problems. But, then again, even with your single drive mechanism the most likely failure is probably going to be a blow-out or loss of a wheel on one side. So you'll have to deal with that possibility one way or the other.
I think a regular car is much more stable in failure mode than this 3 wheeled vehicle would be. If there were 2 independent drive mechanisms for the 2 front wheels and (worse case) one of them locked up, i imagine it would be the 'traditional car equivalent' of all your steering linkages falling out on the road while simultaneously one front brake locking up.
 
Top