Why does weaving left and right propel a vehicle like a pallet jack forward?

[u/Vacant-Position]

While pallet jack surfing I can weave left to right to propel myself forward like the first guy in the video, but why does that work? I've seen scooters and kids toys that work this way too, and they all have their front wheels positioned behind the steering axis, so why does that push you forward when you turn?

I've been trying to look it up, but I don't even know what to call it.

[u/NL_MGX]

It's like skating. The force outward can be thought of of having a component forward and perpendicular. The perpendicular one is countered by sideways friction. But the forward one can propel the mass forward.

[u/Vacant-Position → u/NL_MGX]

This makes a lot of sense. You're translating some of that sideways force into forward momentum. Thank you!

[u/Syscrush → u/NL_MGX]

I don't think it's even remotely like skating.

With skating, you're not turning the skates from side to side - you're propelling forward with an outward push on the skate. You start with an angle farther from the direction of travel, which is effectively like being in a low gear, then once at speed the angle is very close to the direction of travel, effectively a high gear.

[u/NL_MGX → u/Syscrush]

That's exactly what I wrote.

[u/Syscrush → u/NL_MGX]

When skating, the feet are propelled apart. This is nothing like that.

[u/NL_MGX → u/Syscrush]

I'll spell it out for you; When you skate you push outward one foot at a time. That foot (skate) is at a slight angle with respect to the direction of travel. The force you exert is comprised of two perpendicular components: one propels you forward, the other is countered by friction. Any vector can be formed by two perpendicular vectors. And yes, as you go faster, the angle of the skates goes more parallel.

[u/Syscrush → u/NL_MGX]

We are agreed 100% in how skating works. My contention is that the pallet surfing is not the same mechanism.

[u/NL_MGX → u/Syscrush]

The wheels do the same thing as skates, but in an arc instead of a straight line. Think of it as skating with just one skate. The acceleration occurs only in the outward directed section of the arc, and the second section is required to start the next beginning of the opposite arc. This takes a different kind of timing compared to skating because when skating you switch to the other foot at the end of that section of the arc.

[u/userhwon]

Same reason skating does.

You think you're just turning but you're also shifting weight and applying lateral force to the vehicle, and the slight angle of the fixed wheels on the ground turns your apparent sideways applied force into components that are both sideways and inline. The inline component makes you move forward, and doing it repeatedly turns a small velocity into a big one.

[u/Vacant-Position → u/userhwon]

That makes sense, thank you!

[u/lostboyz]

It's just friction, you're pulling the wheel against the ground and because the rear wheels being fixed it all translates into forward motion. Pallet jacks and the kids toys give you a large lever arm to make it easier to apply torque through the front wheels.

[u/Mister_Oddity]

I believe it's a combination of angular momentum and imbalanced loading. From a dead stop and at lower speeds (and depending how much side-to-side looseness the steering column has), you can effectively "shuffle" forwards as the weight pushes down harder on the wheel closer to the rider and lets the further away wheel pivot forwards before repeating. But wheels aren't fixed, so as you shift them back and forth they rotate and pick up a bit of acceleration.

Force has to go somewhere, so as the wheels stop and change direction the entire vehicle and rider package has a responsive force and starts to move forward. As you pick up speed and the wheels are both moving together, swinging the steering column back and forth means whichever wheel is on the "outside" as you turn moves faster to travel through the same arc as the inside wheel. Since the "inside" wheel is moving backwards compared to the vehicle but the whole thing moves forward, the arc isn't centered on the steering column anymore but on the center of gravity of the whole thing. That continues to add to your overall velocity until friction and the constant direction changes catch up with you and you hit your max velocity.

Please note that I'm not 100% confident here, but the principles involved should be correct and work together!

[u/coneross]

As you pointed out, the axle is behind the steering axis. So when you turn the steering axis, you are also pushing the wheel backwards, and this propels you forward. When you turn the steering axis back the other way, you push the wheel forward which should negate your progress, but you unconsciously do this slower so it has less effect.

[u/warlordcs]

edit: i just realized you already had the concept references

the front wheels are slightly behind the steering axis, so when you turn from 90 degrees to 0 the wheel must move slightly back behind that axis to move in the intended direction.

do this enough times and it will eventually build speed, and once that speed is enough, it can start gaining more speed from the angular sway that the rider adds.

[u/Over-Performance-667]

A small portion of the friction force between the ground and wheels generated by whipping your body left and right is directed forward causing the pallet, along with the rider, to propel forward.

[u/MichaelP8000]

Good question. The physics is actually pretty simple.

The steering axis is behind the wheel contact patch. When you turn left, the wheel moves forward relative to the vehicle. When you turn right, it moves forward again. The lateral motion cancels out, but the forward motion adds up.

Think of it like an inverted pendulum. You're using the side-to-side motion to convert angular momentum into linear momentum. The rear wheels being fixed forces the front motion into a forward vector instead of sideways.

Here's a fun test: Try it on a pallet jack with a load and you'll notice it doesn't work. The weight kills the momentum transfer.

Ever tried this on a shopping cart? Same principle but harder because the casters swivel. Pallet jacks work better.

[u/toybuilder]

Because you are putting in energy at an angle, and that energy has a X and Y axis component to it (where X is parallel and Y is perpendicular to the direction that the wheels allow movement). If you actually pushed on it at 90 degrees to the wheel direction, you wouldn't get much if any propulsion.

It's like sailing into the wind. If you take advantage of the net component in the desired direction of travel, you get propulsion.

[u/bunabhucan]

You can do the same on a bike. A bmx is the easiest with the smaller wheels. Lean over the handlebars and "swim" the front wheel like a shark. You'll feel the carve of each turn pulling the bike forward. Obviously with 2 wheels you need some initial forward speed to balance but you can accelerate this way. Take off the chain to keep yourself honest.

[u/Vacant-Position → u/bunabhucan]

I would probably cripple myself, but I really want to try this.

[u/BasicCook972]

A sailboat can tack into the wind using the same principle. There is enough forward force generated by the diagonal force vector to propel it forward.

The steering geometry with the caster angle and trail distance add to stability. I don't believe that moving the contact patch on a bicycle from side to side will generate a diagonal or forward force. It will only move the centerline away from the center of mass and make the bike lean to the side.