Commuter Booster - <1kg Friction Drive

I've been a big proponent of variable pressure friction drive but honestly... I recently made a plain ol' friction drive with no movement for my beater mountain bike and it works great. I think maybe we're all over thinking this stuff.

Sure, the variable pressure systems do help with making the motor more efficient but I'm starting to doubt that it's by a huge margin. Now that I have a watt meter I'll have to do some testing on that theory.

My current opinion (and keep in mind that this changes day by day :lol: ) is that for lower power systems you simply don't need a mount that moves. It's really not that hard to get the correct pressure on the roller. Yes, you'll need a clutch bearing in the roller, but man, it's so much easier.

If I wasn't afraid of blowing up an esc I wouldn't even use a clutch bearing. A smaller rc motor doesn't put much drag on the tire at all.

Now if you want a high power system then yes, you need a roller that can move and really bite into the tire.

I guess I'm just surprised at how much time and money people are willing to put into friction drive when it can be so damn dirt cheap, easy, and work very well at the same time.

I think you have a fair point in relaton to over complication but I suppose it depends on what different people find complicated. I personally find the process of coupling up a roller to a motor just as complicated as making a swing arm for a direct drive setup. No doubt though, simpler usually is better.

adrian_sm said:

When I say via geometry, I don't mean locking the drive to a position, and hence fixed losses.
I am still talking about a free to pivot progressive contact pressure just like your design, with the motor torque dicatating the contact pressure. I just want to limit the maximum engagement via a deadstop so I can stop too much contact pressure. I think you acheive the same thing but by using a soft-stop, ie. your foam dampener. This applies more and more reaction force as the drive engages, until a balance is found. I was think of a more defined stop to limit excess contact pressure, and hence losses.

Click to expand...

Problem is without a damper, the motor will typically go to the stop when put under load. Because of the length of the pivot, you wont be able to control this no matter how much you play with the geometry, or at least I never could. It will either be on or off. The damper solved this issue.

Hi Todd, can we have a photo please

EVTodd said:

I've been a big proponent of variable pressure friction drive but honestly... I recently made a plain ol' friction drive with no movement for my beater mountain bike and it works great. I think maybe we're all over thinking this stuff.

Sure, the variable pressure systems do help with making the motor more efficient but I'm starting to doubt that it's by a huge margin. Now that I have a watt meter I'll have to do some testing on that theory.

My current opinion (and keep in mind that this changes day by day :lol: ) is that for lower power systems you simply don't need a mount that moves. It's really not that hard to get the correct pressure on the roller. Yes, you'll need a clutch bearing in the roller, but man, it's so much easier.

If I wasn't afraid of blowing up an esc I wouldn't even use a clutch bearing. A smaller rc motor doesn't put much drag on the tire at all.

Click to expand...

I would be really interested to see what sort of drag your "plain ol' friction drive" has. If it spins down faster than a hub, then personally I would probably just stick with a light weight hub instead. The fact that the variable pressure systems can totally disengage is the only reason I was keen to try it, and the only reason I would put up with some of the down sides of friction drives.

And, yeah, got any pics?

Kepler said:

Problem is without a damper, the motor will typically go to the stop when put under load. Because of the length of the pivot, you wont be able to control this no matter how much you play with the geometry, or at least I never could. It will either be on or off. The damper solved this issue.

Click to expand...

Starting to get my head around this now. Got to think about the partial load scenario more.

Thanks.

Kepler said:

I think you have a fair point in relaton to over complication but I suppose it depends on what different people find complicated. I personally find the process of coupling up a roller to a motor just as complicated as making a swing arm for a direct drive setup.

Click to expand...

Really? I just drill a hole in the motor shaft and put a roll pin in it. Here's a photo. Doesn't show the pin but in the bottom one you can see where it would go.



As for a picture. Yup, it's an old picture but same old drive I've been making. That top plate doesn't slide, it's to stiffen the small channel aluminum I was using.

adrian_sm said:

I would be really interested to see what sort of drag your "plain ol' friction drive" has. If it spins down faster than a hub, then personally I would probably just stick with a light weight hub instead. The fact that the variable pressure systems can totally disengage is the only reason I was keen to try it, and the only reason I would put up with some of the down sides of friction drive.

Click to expand...

You have a good point about being able to totally disengage the motor. I think you missed the part of my post that said I'm still using a clutch bearing though, so the "plain ol'" version freewheels anyway. I have tried it without and the drag really isn't bad but it would depend on which motor you're using. Never had a hub motor so I can't compare the two.

Don't get me wrong. I'm still using the sliding mount on my main bike. That roller would slip like crazy if I didn't. I'm just talking about low power "assist" type systems.

I think your drive is turning out great btw.

EVTodd said:

You have a good point about being able to totally disengage the motor. I think you missed the part of my post that said I'm still using a clutch bearing though, so the "plain ol'" version freewheels anyway. I have tried it without and the drag really isn't bad but it would depend on which motor you're using. Never had a hub motor so I can't compare the two.

Don't get me wrong. I'm still using the sliding mount on my main bike. That roller would slip like crazy if I didn't. I'm just talking about low power "assist" type systems.

I think your drive is turning out great btw.

Click to expand...

Thanks for the pics. I love how simple these friction drives can be.

Have you done the spin down test on the "plain ol" freewheeling version? So lift the rear wheel, go full throttle, then see how long it takes for the wheel to come to rest. This for me is a quick and dirty test for how much drag the system has.
For example my crystalyte 408 hub motor takes about 8s.
The friction dirve because it disengages takes for ever, 20+ seconds.
If I had the drive fully engaged like the test data above, it spins down really quick, like 2 seconds.

Oh, and thanks for the comments on my drive. Once I implement everything I have learnt on this one, it should be much better.
But I want to sort out my throttle, and get this one set-up right first.

- Adrian

adrian_sm said:

Thanks for the pics. I love how simple these friction drives can be.

Have you done the spin down test on the "plain ol" freewheeling version? So lift the rear wheel, go full throttle, then see how long it takes for the wheel to come to rest. This for me is a quick and dirty test for how much drag the system has.
For example my crystalyte 408 hub motor takes about 8s.

Click to expand...

I have tried that but I couldn't tell you how long it takes to stop spinning. I'll check when I get home later today.

Of course it all depends on how tight the roller is on the tire. If you have a mount that doesn't move you have to find a compromise between slipping and resistance. Again, not a great thing with very high power, not a big deal for a little assist.

I know that with the clutch bearing it takes a lot longer than 8 seconds to stop spinning. With my sliding system it spins like a normal wheel with pretty much no resistance.

I don't have it set up this way, but with no freewheeling roller it would stop pretty quick for sure. It wasn't that noticeable while riding but I can see why you wouldn't want that on a road bike.

EVTodd said:

I know that with the clutch bearing it takes a lot longer than 8 seconds to stop spinning.

Click to expand...

Great. That should mean a well set-up friciton drive can compete with a hub motor in terms of system loses. Great news.
I had it in my head that it would never get it this low for a freewheeling, constant contact friction drive. Now I can see why you were saying we can make it even simpler for a low power set-up.

Thanks again for sharing to the knowledge base.

Cheers,
Adrian

Update:

Batteries
Good news:
Got two 5s 5000mah LiPo packs.
No load speed on 5s is now ~54kph. That should be kinder on the whole drive system.

Bad news:
One pack had a dodgy cell. :x
And there is no return centre in Oz, even though they have a warehouse. Great.

Throttle
Good news:
I managed to find a easy way to mount a throttle button in a great ergonomic spot.
Using a $2.41 delivered rubber and velcro flash light mount from DealExtreme




Bad news:
Still no success in integrating the button and CA override in to the servo tester.
The button is a momentary switch. So I put it on the wiper output of the pot. Thinking that if the button isn't pressed, it breaks this line meaning zero throttle. Then when you press it down, you get what ever throttle the pot is set to.
But in practice when the button is held down eveything is fine, as soon as I let go of the button, it goes full throttle.


Well two steps forward, one step back....

adrian_sm said:

But in practice when the button is held down eveything is fine, as soon as I let go of the button, it goes full throttle.

Click to expand...

you need a pull down

Adrian, try this.

drifter said:

Adrian, try this.

Click to expand...

Adrian it will be even simpler if put the swith you have on +5V so it connected wene push the buttom wene realesed you have no power to the pot and it will set the outpot to zero.

umejopa, have you tried that. I know that with my ESC, that will not work. If you press the button the ESC will go to start routine and the throttle setting will be set to maximum depending on position of pot then pot has to be turned to low position to start motor.

Well I have test it with my own built servotester then I have swith to max ,center , low but you have to learn the ESC the "low" off setting and high.
The auto learning trottel will not work good soo you have to do that evry time you power up.

//Jonas

Well I worked out that previously when I made the mods to the servo tester, I cut a trace in the wrong spot. Fixed that up, played with trying to get the CA override working, but failed. I think I will have to remove the 1k Ohm resistor inside the CA to get it working. I was just trying to avoid this, because of how I use it on my other bike, but looks like there is no way around it, while keeping the upper end of the throttle range.

Jonas,

Thanks for the tip. Putting the momentary switch on the 5.5V input to the pot works perfectly.

Drifter,

What servo tester are you using? It worked for me. Have you got the switch on just the pot 5.5V, or the whole 5.5V rail.
BTW, my servo tester uses a 10k pot, and the switches I want to use are normally open.

- Adrian

I have not tried it with a CA.

Update:

Made a few mechanical adjustments, to fine tune the torsion spring.
I upgraded the spring to a heavier guage wire.
Added an adjustement screw, so I can tweak the angle the motor sits at without disassebmling the whole drive system to bend the spring.
And added an softer deadstop.

Here are a few pics.


View attachment 3
View attachment 2



- Adrian

Update:

Took the bike out for a quick shake down with the upgraded mechanicals, and 5s battery. Seems good so far.

The new throttle buttons works a treat. The location is great, could even change gears while holding the throttle button in.
5s battery is definitely more gentle on the motor, I will stick with it.

Cycle analyst is now hooked up. Here are the stats from some quick stop/start runs around the block, going up and down hills, and a flat speed run.
Max Speed: 49.2 kph
Avg Speed: 26.0 kph
Time: 8 mins
Watt-hrs: 22.8
Wh/km: 7.3

Motor pickup on the tyre was very good. Still need to tweak it, as managed to get it to slip.
No sync issues.

Biggest issue I have is the throttle. For a few reasons:

1) Button throttle is annoying when you don't want to use full power.
Let me explain. If I am not wanting to blast around at 1500+ watts all the time I might set the pot throttle at say 60%, so I have maybe 800-1000w.
This is great, helps me up the hills, nice lazy cruise on the flat. But then I start going down a hill, suddenly I start actually peddling quicker than the motor is set for. Then the motor lifts away from the tyre and is just spinning in free air. Then I start slowing down on the next flat, or up hill.
Now unless the drive is set such that it is always dragging on the tyre, it will not engage again, unless I let off the throttle and hit it again. Not exactly intuative for the user.

Solution 1a: variable trigger/thumb/twist throttle
Solution 1b: current limiting controller, so I can drop the power but not the top speed
Solution 1c: get the CA to limit current. But response will probably be too slow

2) Button throttle is annoying when you first engage it
Okay now I think well I'll just turn the power up. But these little motor/ESC combos spin up pretty darn quick and have a lot of power, so the pick up is pretty savage. Even when I have the ESC set to the softest start.

Solution 2a: somehow get a slower throttle ramp up, in the ESC, or a uC taking control of the throttle signal.
Solution 2b: some sort of spring/dampener like kepler to soften the engage
Solution 2c: tweak the geometry to use the tyre in similar manner to soften the engage
Solution 2d: have a soft dead stop, rather than hard and leave it at that.


At the moment I am leaning towards trying to get a variable throttle working somehow. So I would welcome any thoughts on how to achieve this on road bike handle bars. Extra points for links to suitable parts, and sketches. And I'll buy you a beer if I end up using it.

3) Slip
Well not exactly button throttle related, but if the drive does slip it is not obvious to the user. The problem with this is, to make the drive system as efficient as possible you actually want to have just enough contact force to avoid slip. As anymore is just deflecting the tyre, and the motor is just wasting power deflecting the tyre. But you don't always have control over the co-efficient of friction. So if it rains you actually want more contact pressure to overcome the lower friction coefficient, and still be able to lay down the power. It can also start to slip if you bounce over bumps, and the motor lifts of the tyre a bit.

Solution 3a: dry & wet setting for the drive engagement. This could be a simple adjustment of the dead-stop that sets the maximum tyre engagement
Solution 3b: traction control. Have a uC monitoring motor, and tyre speeds, and it can actually cut the power if the motor starts spinning quicker than the tyre.
Solution 3c: fix up the grip tape (that has ripped holes from hitting the seat tube before I fixed the dead-stops) so it doesn't slip

Finally on a more positive note to keep me motivated.
4) Weight
This bike is so light it is amazing. Riding it without power feels just the same as without the Commuter Booster on.
One of the things I have found annoying with my dual suspension ebike is it weighs 30kgs, and is a real pain to move around unless I am riding it. Like I had to go rescue my wife when her car broke down, and it is not the sort of bike I can quickly throw in the back of the car, or on roof mounted bike racks, even the tow ball mounted bike rack I have I feel reluctant to put it on since it weighs so much. Where as my road bike + commuter booster weighs 12 kgs. So easy to put on a roof rack, dismantle and put in the back of the car, or on a standard bike rack on the back. It is still a bike with all the light weight, easy to ride, easy to throw around, easy to transport, easy to store advantages, but also the awesome advantages of an 1000+ watt ebike.

I have got to fine tune this thing and overcome these little annoyances so I can realise all the awesome potential of a 12-13kg 1000+ watt 50kph 50km+ ebike.

- Adrian

I agree on the weight issue. Many people do not care about huge weight on a bike, but I sure do. I want a bike to still be a bike when the power is not used.

Looks good so far.

Matt

Excellent analysis. One thing I am familiar with is "Solution 1b: current limiting controller, so I can drop the power but not the top speed." Well said. My present homemade controller is this way and it's great. I have yet to get my friction drive completed and have only used this type of controller on a through-the-gears drive.

I have now make a controll loop in the wattmeter that take down speed if current go to high only thing can not test it on E-bike before the snow is gone
How fast have you the controll loop Solcar ?
Do you contoll a RC controller or have a complettly own ?
I now check current and adjust the speed evry 20mS

Solcar said:

Excellent analysis. One thing I am familiar with is "Solution 1b: current limiting controller, so I can drop the power but not the top speed." Well said. My present homemade controller is this way and it's great. I have yet to get my friction drive completed and have only used this type of controller on a through-the-gears drive.

Click to expand...

How about using the CA to do this. Modify the CA to have a variable current limit on a pot. Then have the CA control the ESC throttle to keep the power below the current limit.

Solcar said:

Excellent analysis. One thing I am familiar with is "Solution 1b: current limiting controller, so I can drop the power but not the top speed." Well said.

Click to expand...

I quite like this one, as it could be an off-the-shelf type solution. Which is in keeping with me wanting to make this an easy thing for people to copy.

umejopa said:

I have now make a controll loop in the wattmeter that take down speed if current go to high only thing can not test it on E-bike before the snow is gone ......... I now check current and adjust the speed evry 20mS

Click to expand...

That is fantastic news. Well done Jonas.

Need anyone to help you test it? I just happen to have a watt-meter and summer is just hitting us in Australia. :lol:

jbond said:

How about using the CA to do this. Modify the CA to have a variable current limit on a pot. Then have the CA control the ESC throttle to keep the power below the current limit.

Click to expand...

I only have v2.01 CA at the moment. So I can get it to limit the throttle based on current, speed, or voltage (at elast I could on my main ebike), but it doesn't do variable current via external throttle without a firmware upgrade.

I still haven't been able to wire it up properly yet either, and to be honest I would prefer a different solution as most people that would use this type of drive don't need a full ebike computer.

- Adrian