Commuter Booster - <1kg Friction Drive

[adrian_sm]

Update:

1) Torsion Spring
I have increased the spring stiffness. The last one was too light to take the torque of the motor weight.
This new one is better, still not right, and I might be able to make it adjustable with a screw rather than the current bend the spring to the right shape method.
I now have the drive tuned such that the motor just touchs the tyre. I might back it off a bit later, but it works for now.

Note: This was the method Kepler appears to use when setting up one of his eboostdrives.
- When the tyre spin forward it just drags on the motor but not enough to spin it.
- When you spin the tyre backwards the motor actually engages and spins.

3) My new back tyre is not round.


3) No Load Power Test
Now that I have the Cycle Analyst on the friction drive bike, I thought it was time to get some data.
So here is the no load power the drive requires just to spin the wheel.

Commuter Booster - No Load Power (2010-11-09).PNG (23.6 KiB) Viewed 5920 times

Note 1: The no load power consumption of the motor alone at full throttle is ~52W.
So only about 12W for the light engagement friction drive loses, and to keep the wheel spinning.

Note 2: There is no load on the tyre, so the drive is only lightly engaging with the tyre.
When fully engaged the power required is a lot more. ie. 160W total at 66kph.
So about 100W more than minimal engagement. That is like ten times the losses. Yikes.
I think I might need to spend some time optimising the engagement.

Note 3: The ambient temperature was 22 deg-C, and the motor temperature stablise to about 49 deg-C after 10mins a full thottle.
This means a 27 deg-C temperature rise for about 50-60W of waste heat.
Will be interesting to compare this figure to other motors, as the motors ability to dispense heat is key to survival for light drives like this.

That's it for now.

- Adrian

[Kepler]

Thats good data Adrian. The drive setup against the tire is how I setup my drive as it does need the lightest contact to pickup cleanly every time. Still has no impact in relation to drag on the bike with this setup though.

As you have proved, having the contact either off or full pressure contact is not the best for efficiency. I came to the same conclusion during my development and was the reason why I designed a damper arrangment. Progressive contact pressure is the way to go. I look forward to seeing what arrangment you come up with. Finding the right damper material is the key. You need to find a material that gives the correct amount of compression but also has good rebound properties. Rubber sheet from Clark rubber works OK but its rebound property isnt very good. I am still looking for the perfect material for this. I am thinking silicone rubber would work well but its hard to find in sheets. Spinningmagnets suggested the silicone rubber used in making the soles of shoes. There was also a suggeston to mold your own using a silicone based modeling material.

[adrian_sm]

Thats good data Adrian. The drive setup against the tire is how I setup my drive as it does need the lightest contact to pickup cleanly every time. Still has no impact in relation to drag on the bike with this setup though.

As you have proved, having the contact either off or full pressure contact is not the best for efficiency. I came to the same conclusion during my development and was the reason why I designed a damper arrangment. Progressive contact pressure is the way to go. I look forward to seeing what arrangment you come up with.

Why can't this be done purely by the geometry? I have it in my head that the angle between the pivot point, the contact patch, and the wheel axle sets the ratio of contact pressure to motor torque. So by adjusting dead-stops, and pivot point locations, you should be able to set the correct contact pressure ratio to ensure you don't get slip.

Your dampener material I believe is essentially just reducing the contact pressure for a given torque. If the drive started with a bigger angle (mentioned above) to start with, while still maintain your just touching scenario, you could reduce the force required by the dampener. But the system might become more sensitive the tyre pressure, or geometry imperfections such as out of round tyres. I know you mentioned something about the drive bouncing off the tyre, maybe that is another reason for your geometry & dampener set-up.

I feel like I am really missing something here. I need to play with it more to understand.

Thanks again for helping out.

- Adrian

[Kepler]

You can do it just with geometry if you are happy to accept a fixed loss due to the pressure exerted on the tire when the drive is activated. However, you can get away with less contact pressure under lighter loads and hence less loss. It makes sense to me to try and capitalize on this using progressive contact pressure strategies.

[adrian_sm]

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.

Here is a pic of where my motor sits.

Friction Drive Geometry - pic1.PNG (26.18 KiB) Viewed 5865 times

Here is a close up of the motor to tyre contact.

The motor torque grabs the tyre, resulting in the driving force in green (F1).
The geometry (specifically the angle shown with a double headed black arrow) sets what the resulting contact force with the tyre will be (F2)
If the coefficient of friction is high enough, then F3 = F2 x coefficient of friction > F1. And the drive will bite, and not slip.

Friction Drive Geometry - pic2.PNG (38.15 KiB) Viewed 5865 times

If the pivot point moved to the right, then you would get a lower contact force (F2)for an identiacal driving force F1, and more likely to slip, but less losses.
If the pivot point moved to the left, then you would get a higher contact force (F2) for an identiacal driving force F1, and less likely to slip, but more losses due to deflection of the tyre.



So for me the coefficient of friction dicates the maximum angle necessary to apply the contact pressure required to ensure the drive doesn't slip.
For a perfectly stiff tyre and motor, there would be no more angular travel after initial engagement.
Softer tyres will need more travel angle between pick-up point and fully engaged.
Stiffer tyres will require less travel angle, as the contact force will increase more rapidly.
Higher torque means higher contact pressure required, which means bigger travel angle.

If you get the geometry right, then F3 [EDIT - Ooops had written F1 before by mistake] is only slightly bigger than F1 to ensure engagement, and miminal losses. But in reality you will have a decent safety margin, so out-of round tyres, or tyre deflaion doesn't mean you start burning holes in your tyres.

Does that make it any clearer ? Or am I just confusing myself, and everyone.

- Adrian

[EVTodd]

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 ) 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.

[Kepler]

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.

[Kepler]

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.

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.

[drifter]

Hi Todd, can we have a photo please

[adrian_sm]

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 ) 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.

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?

[adrian_sm]

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.

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

Thanks.

[EVTodd]

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.

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.

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.

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.

[adrian_sm]

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.

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

[EVTodd]

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.

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.

[adrian_sm]

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

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

[adrian_sm]

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.
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

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IMG_1334.JPG (80.19 KiB) Viewed 6296 times

IMG_1326.JPG (96.29 KiB) Viewed 6302 times

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....

[full-throttle]

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.

you need a pull down

[drifter]

Adrian, try this.

[umejopa]

Adrian, try this.

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.

[drifter]

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.

[umejopa]

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

[adrian_sm]

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

[drifter]

I have not tried it with a CA.

[adrian_sm]

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.

Drive in bits, and random springs I will try to use.
Note the old torsion spring at the bottom centre of the image.

IMG_1365.JPG (43.31 KiB) Viewed 6101 times

New Spring ready to be installed. This is a thicker guage wire, and started life as an extension spring so each coil is in contact with it's neighbour.
Much better.

IMG_1366.JPG (44.77 KiB) Viewed 6101 times

Spring installed. One of the legs sits in the main clamp threaded hole.

IMG_1367.JPG (44.1 KiB) Viewed 6101 times

The new adjustement screw on the top right, acts on a leg of the spring which sits in the hole on the right.

IMG_1368.JPG (40.01 KiB) Viewed 6095 times

Here it is all together. Note the second leg tucks under the deadstop screw head.
Here you can just see that I now have 8 active coils, versus 4. Much better.

IMG_1369.JPG (39.42 KiB) Viewed 5936 times

- Adrian

[adrian_sm]

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