gfulton wrote:Hi adrian_sm and others,
I've just been lurking here off and on the last couple of years. I have a question about the engagement of the drive to the wheel... is it always touching the rear wheel via spring force... but just barely? Once the throttle is engaged and power is put to the motor does it 'climb' the rear wheel until hitting a deadstop? From reading the previous posts I wasn't quite sure how you had it rigged.
The ESC, motor, servo-tester and throttle setup is all clear, I have everything going, even a thumb throttle... I just have to understand the mechanics of the mount. Thanks.
Wow. Long time lurker indeed. Glad you dropped by my little thread. How it Works:Motor Engagement:
My drive uses a spring to counter-balance the motor weight such that it will sit just clear of the tyre, with the lower deadstop adjusted to this position as well. This stops the motor from pivoting further away from the tyre than necessary.
Once the throttle is engaged the reaction to the start up torue of the motor actually pivots the swing arm, and lifts the motor into contact with the tyre. Once it makes contact with the tyre it then relies on the motor gripping the tyre to further engage.
How much it engages is then dictated by three things:
1) the geometry, specifically how much interference there is between the arc the motor swings through and the tyre.
2) tyre pressure & size, this sets how much force the tyre reacts with, and over what area.
3) 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 tyre, 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
But you are at the mercy of the friction coefficient between motor and tyre.
The aim is to have a high coefficient of friction, as this reduces the force with which the drive needs to engage into the tyre for a given torque. The real benefit here is in efficiency. The more you engage into the tyre, the more energy you waste in deflecting the tyre structure. Distorting the tyre less will reduce that wasted energy, and most likely have a positive impact on tyre life as well, as the tyre 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 tyre wear than the usual tyre to road surface wear.
The trick really is to make sure it doesn't slip, as you lose power transfer and start eating your tyre up. I don't quite have good guidelines for this process yet, as it is a balance between efficiency and robustness. 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 tyre, 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 tyre 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 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 on the forum, 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.
I haven't studied this properly for my geometry to verify it works enough to make a difference, but it sure sounds nice.
That probably covers a few things you already know, but hopefuly it answers your questions.
So have you started building something yet. Would love to see what you have come up with. I like the two speed set-up you mentioned in your first post
. It might help with overheating of motors when lugging up hills. But I guess I would just be tempted to go for a bigger motor with more thermal mass, or limit power.