Making a vehicle go straight on carpet

[Kiblur]

Homework Statement

The "statement" is a 5 pages long pdf describing a project we have to do. The tl;dr is that we have to build a gravity-powered vehicle that has to do a round trip on narrow strip of carpet. The carpet is 5 meters long with a shag that's about 0.2 in. or 5 mm thick.

Relevant Equations

There is no equation

There is no solution per se. I am currently in the brainstorming stage and I am looking for advice.

I have to choose between a free axle or a fixed axle for the freewheeling wheels. Either the wheels or the axle will be mounted on bearings. In either case, there will be a way to align this axle to make it parallel with the driving axle.

I am however worried that the shag of the carpet will cause the vehicle to deviate. Am I right to worry about this? It's not a lumpy carpet, but it's carpet. I would like to know which, between a free axle or a fixed axle, would best help the vehicle stay on a straight path in these conditions. I would also like to know about any other factor that might help the vehicle's ability to stay on a straight path besides wheel alignment and making sure that the wheels are the same size, such as wheel width. I'm thinking that perhaps, if the wheels are wider, the vehicle won't sink in the carpet as much, so it might be less affected by the shag of the carpet.

Thank you for your input.

 

[DaveC426913]

Homework Statement: The "statement" is a 5 pages long pdf describing a project we have to do. The tl;dr is that we have to build a gravity-powered vehicle that has to do a round trip on narrow strip of carpet. The carpet is 5 meters long with a shag that's about .2 in. or 5 mm thick.
Relevant Equations: There is no equation

There is no solution per say.

Sorry. "per se".

Pet peeve.

I am currently in the brainstorming stage and I am looking for advice.
I have to choose between a free axle or a fixed axle for the freewheeling wheels. Either the wheels or the axle will be mounted on bearings. In either case, there will be a way to align this axle to make it parallel with the driving axle. I am however worried that the shag of the carpet will cause the vehicle to deviate. Am I right to worry about this? It's not a lumpy carpet, but it's carpet. I would like to know which, between a free axle or a fixed axle, would best help the vehicle stay on a straight path in these conditions. I would also like to know about any other factor that might help the vehicle's ability to stay on a straight path besides wheel alignment and making sure that the wheels are the same size, such as wheel width. I'm thinking that perhaps, if the wheels are wider, the vehicle won't sink in the carpet as much, so it might be less affected by the shag of the carpet.

Thank you for your input.

I have not done this experiment but I am pretty sure that larger wheels (in either/both dimensions) will definitely improve tracking by minimizing the randomizing effect of the carpet pile.

I am not actually sure what you mean by a free axle versus a fixed axle.

 

[Kiblur]

I am not actually sure what you mean by a free axle versus a fixed axle.

Free axle : The wheels are fixed on the axle and the axle rotates
Fixed axle : The axle is fixed on the frame and the wheels rotate independently on the axle.

 

[haruspex]

Sounds like it has to do quite a tight turn. That suggests a differential on the driving wheels and either toe-in on the steering wheels or maybe a single steering wheel.
How do you plan to make it do the turn at the right point, and through 180°?
The shag has the effect of making it an uphill journey. I think a larger wheel makes the "slope" gentler.

 

[scottdave]

I'm not sure which type of axle would be better, but could you have one of each? One type on front and one on back. Just thinking. Try some free body diagrams of how you think the forces would act on each style of axle. Think about how a disturbance or imbalance would affect each style. If you can, make a smaller scale model to try out.

I don't understand how is it gravity powered in both directions?

 

[Kiblur]

Sounds like it has to do quite a tight turn. That suggests a differential on the driving wheels and either toe-in on the steering wheels or maybe a single steering wheel.
How do you plan to make it do the turn at the right point, and through 180°?
The shag has the effect of making it an uphill journey. I think a larger wheel makes the "slope" gentler.

It does not need to turn. It will go forward one way and backward the other way. I will simply insert a peg into the driving shaft and wrap the cable with the weight around it in one direction, then wrap it around the peg, then wrap the cable the other direction.

 

[Kiblur]

I'm not sure which type of axle would be better, but could you have one of each? One type on front and one on back. Just thinking. Try some free body diagrams of how you think the forces would act on each style of axle. Think about how a disturbance or imbalance would affect each style. If you can, make a smaller scale model to try out.

I don't understand how is it gravity powered in both directions?

There will be a vertical shaft at the end of which there will be a pulley which will hold a weight on a cable. The cable will be wrapped around the driving axle and the weight will pull on the axle, causing it to rotate. This axle will have to be a fixed axle. As for how it will drive in both direction, see my previous reply.

 

[DaveC426913]

Free axle : The wheels are fixed on the axle and the axle rotates
Fixed axle : The axle is fixed on the frame and the wheels rotate independently on the axle.

Ah. It's the wheels that are free, not the axle. Got it.

 

[berkeman]

I am however worried that the shag of the carpet will cause the vehicle to deviate. Am I right to worry about this?

You might think about using wheels that dig into the carpet a bit to avoid getting deviated very much. Something like sharp gear-shaped wheels or similar...

From a Google Images search on sharp gears: