Starting torque to move vehicle

[Ole Forsell]

I am currently working on designing a drive system for a vehicle, and I am calulating the necessary torque to accelerate a vehicle. I am using the guidelines of this source:
http://www2.mae.ufl.edu/designlab/motors/EML2322L Drive Wheel Motor Torque Calculations.pdf

I still feel that there is something missing. Isn't there a required force to even start the vehicle to move from rest? Some kind of friction force which need to be taken into account?

 

[Nidum]

There is but it is seldom taken into account because there is no need . Any practical vehicle has more than enough power available to overcome it under normal operating conditions .

Starting resistance tends to be more of a problem in harsh environments and where a mechanism has been dormant for a long time .

It can be a particular problem in aerospace systems .

 

[Ole Forsell]

There is but it is seldom taken into account because there is no need . Any practical vehicle has more than enough power available to overcome it under normal operating conditions .

Starting resistance tends to be more of a problem in harsh environments and where a mechanism has been dormant for a long time .

It can be a particular problem in aerospace systems .

So neither the static friction nor the kinetic friction have an impact that works as a counterveiling force? This means that only the rolling resistance is working against the acceleration on the wheels? In my case the rolling friction is very small and almost neglitiable. This means that only the air drag, and gradient force is working against the driving force. Is this right?

 

[Nidum]

All shaft and bearing combinations and things like wheels on roads require a brief but relatively large torque to unstick them and take them from a motionless condition into a normal running condition .

Not a significant problem in normal vehicle design because available driving torques are almost always of overwhelming magnitude .

The only instance that I know of in ground vehicle design where any of this matters is in big trucks for use in oil exploration in the ice fields . Additional engine power has to be specified to ensure that they will start moving reliably when lubricant in bearings has almost set solid .

 

[Clonemonkey]

In the caes of a bus, does it matter that 2 wheels are applying force and the other 4 are providing resistance?

 

[jbriggs444]

In the caes of a bus, does it matter that 2 wheels are applying force and the other 4 are providing resistance?

By design, the resistance of the unpowered wheels is minimal. Similar to the un-powered wheels in a trailer.

 

[sophiecentaur]

I am calulating the necessary torque to accelerate a vehicle.

The specifics of the motor are relevant here. An internal combustion engine has zero torque until it has started rotating and so a clutch mechanism is needed to allow slipping between engine and drive shaft. Assuming an electric motor of a steam locomotive then the maximum torque can well be at zero RPM. The relevance of friction to getting going is that the wheels must not slip. Loss of energy is seldom relevant at near-zero speeds.

Not a significant problem in normal vehicle design because available driving torques are almost always of overwhelming magnitude .

Yes. The only exception is when a vehicle is 'bogged down' in mud, when the drive wheels slip before the rolling resistance is overcome.

The basic answer to the OP will depend on what acceleration you want. Drag racing and a quick getaway at traffic lights are both limited by traction and not by resistive friction.