When I turn, the car hops up and down.

PrudensOptimus

Hello,


I have a 3 wheel autonomous small car... built out of legos.

And umm... the problem is when I turn the back two wheels in opposite directions... to produce left or right turns... the autonomous vehicle tends to "up and down"... ... what causes that?

faust9

Why are you turning the back wheels in opposite directions?

hypatia

is it a solid axel? if it is both wheels are turning at the same speed, this makes the inner wheel hop. Because the inner is trying to turn as fast as the outer wheel.
you need a limited slip axel.

DaveC426913

One question: your front wheel, is it on an axle, or can it freely rotate like a caster (like on a shopping cart)?

If your front wheel has an axle, then here's the problem:

By turning the rear wheels in opposite directions, you are rotating the vehicle about a vertical axis that is centred on the rear axle - the front wheel gets dragged sideways, causing the hopping.

Instead of trying to turn "on a dime", try turning while moving forward slightly. Rotate only one wheel - keep the other one wheel fixed or even moving forward slightly while turning the other forward. This will allow the front wheel to move forward as it pivots.

minger

Yes, his front wheel is on an axle.

PrudensOptimus

Yea on an axle...

Also... do i even need the front wheel?

What is the front wheel there for anyways? lol... i'm kinda confused myself lol.

Any benefits if i keep it there?

brewnog

Have I missed something with your posts about 3 wheeled vehicles? Surely the purpose of having more than two wheels is so that your vehicle doesn't fall over?

PrudensOptimus

What if my vehicle is balanced without the front wheel? Will there be any downside to remove the front wheel?

physicsCU

No vehicle is balanced on two wheels. Even a bike, your body is constantly adjusting to maintain balance.

You need the third wheel for balance and stability.

PrudensOptimus

What if the "third" wheel causes turning difficulties? What can i do about that.

faust9

You could use a smooth plastic "nub". A little nylon sphere on a stick would work well(I've seen a lot of sumo-bots with these). Or you could go crazy and install a handfull of 3-axis motion sensors and a complicated algorythm to continually balance your bot(like the Segway).

brewnog

Or you could have a caster (or did we discount that in another thread?)

DaveC426913

This is not true.

A vehicle whose centre of gravity is below the axle balances just fine thank you.

(Of course, in practice, you'll want to accelerate slowly, lest the wheels remain motionless and the vehicle does the rotating...)

physicsCU

Yes, that is true, but I think in practice, it is very difficult to get the CG below the axle, unless you know what you are doing.

It is a hard mix of balance and math and design to get it though. Most people who do it end up there not on purpose

brewnog

I wouldn't say getting the centre of gravity below the axle would be an accident, nor would I say it's difficult. The difficulty is controlling such a two-wheeled vehicle.

PrudensOptimus

Is controlling the movements of a humanoid difficult? LIke Asimo, is it hard to control something like that?

minger

I think you should stick to your three wheeled car.

The movements of a humanoid, I would think, would be very difficult to reproduce.