# Friction Drive Forces

Discussion in 'Friction Drive' started by cycledude, Nov 16, 2009.

1. ### cycledudeMember

I have been trying to figure out some of the forces involved in this friction drive concept, there are several, and just too complex for me.
Does anybody know a physics dude that will be willing to sketch the thing?
:detective:

2. ### loquinActive Member

Did you see the sketch here? It indicates the torque transfer.

3. ### cycledudeMember

I did, and it was super informative, I did several searches through the g-engine and even found books on the gear ratios.
Got to tell you I am still iffy on it.

Here is what I mean by forces, is the roller and back wheel propelling the bike or is the carriage-whell roller what is driving the bike forward.

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4. ### loquinActive Member

The diagram you posted is incomplete. Although the drive roller does have a force acting on it that pushes it towards the front, it pushes equally briskly on the tire in the other direction. Essentially all the non-rotational forces on the bike, generated directly by the roller, balance out. (They are transferred to the seat stay and then through the chain stay to the rear axle. Essentially, the bike frame acts as a rigid object, and all forces acting internally on the frame/axle balance out)

You could also think of the tire as a lever, and the axle, as the fulcrum. The drive roller pushes forward on the top of the tire, it rotates, and pushes rearward against the road surface. The motor PULLS against the seat tube/seat stay, and they in turn push against the seat stay and chain stays, eventually pushing against the axle (the fulcrum.) Since, under normal operational procedure, none of the individual pieces of the bike fly away as you're using them, you can be sure that the net force on each piece sums up to zero. (for every positive acting force, there is a negative acting force to balance it.)

However, the torque is converted to a net linear force 'where the rubber meets the road.'

The Wheel is an idler roller, and IT is what is pushing the bike forward. Granted, the motive torque comes from the engine, via the drive roller.

The forces on the rubber of the tire cause it to deflect (as a type of spring, if you will) and act on the rest o the tire, less any small internal frictional losses due to the distortion of the rubber surface.

There are also generally one or two drop rods, used to apply the tension to the drive roller, keeping it against the tire. The two triangles formed by the bike/drive system as a whole (ref the attached pic) keep the assembly rigid, and transfer the forces so that no net linear force is generated anywhere, except at the tire-road surface interface. Think about it - if there was a NET linear vector being generated at the motor, it would fly off. All the linear forces everywhere on the bike, they all balance out. except at the road.

And, if the net linear force weren't being applied at the rear tire/road interface, if the bike were sitting on a perfect frictionless surface (super ice) , it would move forward when you goose it. But, on a perfect frictionless surface, that wouldn't happen - the rear wheel would spin, and you would sit there blowing smoke out the exhaust...

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Last edited: Nov 17, 2009
5. ### cycledudeMember

I am going to make something to attach the U bracket to the bolt holding the seat pipe in place, like a rack.
I don't know if it will make any difference, just though I try it.
The interesting part for me is how to get more performance from the little 35.