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Comment by woriuweflksdncn | Hacker News Reader

I wonder how those rear wheels handle corners. I'm a little surprised they didn't use two independent drive motors, versus a single motor and solid axle, which could be extended to implement an electronic differential.

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Indeed, this thing will likely eat tires if used on-road a lot. Without a differential, it'll also be harder to steer and will want to "push" through the turns. You can see this at 2:31:

https://youtu.be/vuMg0QwKAGI?t=151

The turning radius isn't that great, and the lack of a differential helps explain why. Definitely an interesting design choice.

Version 1.0 will always be a functional proof-of-concept. They have the basic platform, now they can start iterating.

If they've gotten this far, they've already been iterating. In fact he says so at the beginning. "A year ago I welded two bicycles together..."

Having built two recumbent vehicles (a bike and a trike) I can say that the options for a rear wheel drive are either:

- Single wheel drive

- Dual wheel drive with freewheeling action

   - The inside wheel is driven in turns

- A differential

   - The outside wheel is driven in turns

- No differential at all

   - Both wheels are driven equally all the time

He's chosen the latter. It's simple, cheap, and provides the best traction in almost every situation except a turn. It's a good compromise for where this thing is intended: Dirt. If it were optimized for street use, he'd just drive one wheel, which is what I'm doing with my own trike.

For reference, this is my current trike build. It's not done yet. Needs finishing, and paint:

https://i.imgur.com/pnrDQMS.jpg https://miscdotgeek.com/wp-content/uploads/2019/11/IMG_20191...

Dual wheel drive with freewheels needs a way to block the freewheel to be able to reverse.

Indeed. This is a unique feature to his machine because he's not concerned with pedal power at all, and that opens up a lot of possibilities.

Almost think the ultimate solution here would be 2 motors, one per side, and skid steer action. Can then pivot in place, or turn at any radius desired. Would map quite naturally to the two joystick control system too.

You can drive all 4 wheels then which will do even better off-road, and the suspension actually gets a lot simpler since you can totally do away with streering and all the assocaited geometry.

The problem with skid-steer is that it tears up surfaces with high-traction tires.

Probably just fine. It seems they intend this mostly for use on soft surfaces, where the lack of a differential isn't really an issue, even on large vehicles, because the surface itself is compliant enough to make up for the lack of differential action.

But even on a hard surface it might work just fine, because the turning radius looks pretty shallow.

The advantage is that a solid axle is simple, cheap, reliable, and performs well in low-traction situations.

I mostly agree about the advantages, but two motors would be arguably simpler from a mechanical standpoint.

Yes, I might have gone for two smaller hub motors, rather than a solid rear axle. But maybe they tried that too.

Then you have two motors instead of one, and either need an ESC that handles two motors or an additional ESC, and then electronics/software to make sure they work in tandem.

Yes, I agree. I was careful to say from a mechanical standpoint.

But far more expensive. Likely the motor - possibly the batteries - is the most expensive part of the design.

Perhaps, but I don't think it would be impractical for something that's nearly $5,000 already. You would likely spec two smaller, cheaper motors; altogether it would cost more, but not as much as two of the motors it has now.

One drawback that I haven't seen mentioned yet, is single-wheel torque. If you look at the video, there is a pretty sizeable motor that's even geared down with a sprocket an chain. So there is a lot of torque and power available, and due to the solid axle, you can send it to a single wheel. This might be important in off-roading situations.

With direct drive motors, you have to make a pretty big compromise somewhere:

* Two smaller motors (same total power): drastically less torque per wheel

* Two larger motors (>2x total power): somewhat less torque per wheel (still don't have gearing!), cost, weight

If you're adamant on avoiding the solid axle, I think a good-ish solution might be hub motors on all four corners. That way you're far less likely to depend on a single wheel to get you moving.

Or alternatively, a locking diff in the rear (forget about LSDs). They usually provide some gear reduction, so you can skip the chain and drive it directly with the electric motor.

In short, the solid axle is a really good choice that you would be hard pressed to replace, without making a sizeable compromise somewhere.

I don't doubt there's a good reason for this design, but from my armchair engineer perspective, I'm surprised he didn't go with dual hub motors on the front wheels.

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