# Homework Help: How much power is needed to steer tires?

1. Jan 22, 2013

### qpham26

1. The problem statement, all variables and given/known data

I am trying to design a car with simple front rack-and-pinion steering system.
Exactly like this one

I am having trouble find the right motor for the job, assuming typical tire of regular car and the car weight 1ton (included everything)
The shape will be simple rectangular
Assume everything is given like μs, R of wheel
2. Relevant equations

3. The attempt at a solution
I haven't deal with this subject for a very long time and especially this particular problem isn't similar to the things I have dealt with.
I was able to find out the drive power needed for the rear axle but for the steering I couldn't go very far and I have tried asking my professor but he said he hadn't teach this stuff for decade so he couldn't help me as well.

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

Last edited by a moderator: Sep 25, 2014
2. Jan 23, 2013

### Simon Bridge

Back of envelope:

taking power as work over time...
$P=\tau_{app}\theta/\Delta t$

$\Delta t$ is the amount of time you want the wheel to rotate in... you'd think of it as a steering response.

The minimum applied torque at the tires would be just that needed to overcome friction ... $\tau_{min}=\mu Mg$

If you are using BTC steering like the video - you'll need to account for the resistance of the return mechanism.

There will be some wrinkles depending on the exact setup - and I'm sure the engineers here will have some advise too - but you should be able to figure it from there.

3. Jan 23, 2013

### qpham26

I just want it to turn, returning isn't necessary.
and I dont get the way you expressed power. since I dont have any idea about the amount of time i want it to rotate. I just want it to be able to turn and stay turned is fine to me.

would it be possible to find the force to over come the static friction and use it to find out the torque or power from the motor?
and how would I do something like that?

thanks.

4. Jan 23, 2013

### haruspex

Are you trying to turn the wheels while stationary or only when moving?

5. Jan 23, 2013

### CWatters

If you are actually building it then I'd measure the torque required, then knowing the speed at which you want the wheels to move you can calculate the power.

If not then you would need to calculate/estimate the frictional forces. Will be tricky but not impossible. You could probably work out the frictional forces properly but I would cheat...

Basically you have a contact patch under type. Lets say it's a rectangular patch of length L. I would calculate the force required to drag the whole wheel sideways (that's independant of contact area) then assume the force acts at say L/2 from the pivot when the wheels are turned. That should give a conservative answer for the torque at the wheel.

Then use whatever gearing you have to work out the torque and speed required at the motor. Perhaps assume lock to lock neds to be a few seconds?

Once you have the rpm at the motor convert it to angular velocity (radians/second).

Then multiple torque & angular velocity to give the power required.

Perhaps apply a safety factor of say 2?

Last edited: Jan 23, 2013
6. Jan 23, 2013

### Simon Bridge

The wheel won't turn instantly - it takes some time to change angles. You need to know how fast you want this to be.

If you choose a slow time, then you save power, but the steering will feel sluggish.
Of course - you could see how auto manufacturers do power steering.

The others have covered the rest so I won't repeat it here.

7. Jan 23, 2013

### CWatters

PS. If the professor can't help he won't know if the answer is wrong :-)

8. Jan 23, 2013

### qpham26

would the power needed to turn the wheels while stationary be greater than while moving?
of so than using the motor with that much power would be fine.

9. Jan 23, 2013

### qpham26

that is what I was thinking. That is why I am trying to do this as simple as possible xD

10. Jan 24, 2013

### CWatters

Yes. It's much easier to steer when moving. This was very obvious before full size cars had power steering.