How does it work?

The Basics

The drive usually sits totally disengaged from the tire, adding no resistance to the normal operation of your bike.

When the user applies the throttle the motor comes in contact with the tire with a variable amount of engagement based on the amount of assistance being applied. This maximises efficiency for both pure pedal and assisted riding.

When the throttle is released the motor slowly disengages returning to a totally disengaged position.

Motor Engagement:

The Commuter Booster uses a spring to counter-balance the motor weight such that it will sit just clear of the tire, with the lower deadstop adjusted to this position as well. This stops the motor from pivoting further away from the tire than necessary.

Once the throttle is engaged the reaction to the start up torque of the motor actually pivots the swing arm, and lifts the motor into contact with the tire. Once it makes contact with the tire it then relies on the motor gripping the tire to further engage.

How much it engages is then dictated by four things:

    1. Motor Torque
    2. The geometry: Specifically how much interference there is between the arc the motor swings through and the tire.
    3. Tire pressure & size: This sets how much force the tire reacts with, and over what area.
    4. The upper dead-stop: This is used to limit how much the pivot arm rotates, and how far the motor interferes with the tyre.

Friction:

Friction is how the torque of the motor is transferred to the tire, rather than using a belt or chain reduction. The main advantages of this system are:

    • It can totally disengage for zero additional drag when not in use
    • Good effect reduction ratio
    • Simplicity

But you are at the mercy of the friction coefficient between motor and tire.

The aim is to have a high coefficient of friction, as this reduces the force with which the drive needs to engage into the tire for a given torque. The real benefit here is in efficiency. The more you engage into the tire, the more energy you waste in deflecting the tire structure. Distorting the tire less will reduce that wasted energy, and most likely have a positive impact on tire life as well, as the tire surface in the contact region is not trying to move tangentially, or axially relative to the contacting friction roller (the motor can in my case) as much. Set-up right the friction roller engagement should not be significantly worse for tire wear than the usual tire to road surface wear.

The trick really is to make sure it doesn't slip, as you lose power transfer and start eating your tire up. The issue being things can change once set-up. Maybe your tyre pressure goes down, resulting in a lower reaction force, and reduced max torque before slipping occurs. Or it rains, and this reduces the coefficient of friction, again limiting the amount of power you can lay down. This is one of the reasons I went with an upper dead-stop that limits the amount of engagement into the tire, as it gives the user the potential to quickly adjust for more engagement for just these scenarios, without affecting the other set-up parameters.

Max Power:

These friction drives are not really suited for high power set-ups for a couple of reasons.

    1. You would need very aggressive engagement into the tire to transfer the torque, leading to more wasted energy. Making other gearing options attractive.
    2. The small motor sizes just can't handle a lot of waste heat. One of the reasons to only use them above a walking pace, as motor efficiency is pretty crappy below this.

Friction drives are definitely more suited to the light-weight assist type market, than the all out speed/torque beast user. Having said that they can work well for over 1kw, which is a very usable amount of power.

Variable Engagement:

Finally the geometry of this, and other friction drives, allow for automatically adjusted variable engagement. Essentially the drive will engage less when low torque is applied, improving efficiency. Then when you give it more throttle and demand more torque, the drive will engage more allowing for greater torque transfer.