Commuter Booster - <1kg Friction Drive

Now you can test a low setting ex 30A and see how it works.
I ran out of ESC they may not like to shorted.
Was running for a time but hade synk problem then it stop but no smoke this time.
New are on way but it will take some time.
The code I hade is simulate at 8Mhz to get time right.
The messurment for Ah and Wh my be wrong now for you.
//Jonas
 
Progress has been a bit slow, as work got busy again.

Let's see what has happened
- Replacement motor arrived
- Modified the CB geometry to increase the rate at which the drive digs in to the tyre, which reduces the angle it needs to rotate through.
- This means it will fit more bikes.
- Modified the design back to a single swing arm, which requires more accurate machining of the pivot axle hole.
- This also means that we no longer need to modify the motor before assembly. Previously it was necessary to slide the motor shaft along, for the support bearing. But this also required removal of a circlip that prevented axial movement of the can. Which I did see when I had my motor failure.
- I have the modified watt meter now to be able to control the motor, but still need to get some good road time to stress test it.
- Started a little production run of parts. Have about 10 pivot blocks and 10 swing arms half made.
- Bought a cheap band saw to split the pivot blocks, but it was a POS. Blade guide adjustment was friggin' hopeless. So it went back to the shop.
- Might have to drop over to the brother in laws to use his, as hand cutting the acetal is a PITA.

That's it for now.

- Adrian
 
Update: Video

Just realised I have never posted a video of the drive, except for one of my early failures. So here is a video from a quick ride around the block. Quality is pretty average due to a dodgy mount and dirt cheap camera.
But it does nicely show how the drive engages, how it pulls itself into the tyre more as it needs to when applying more power, and how it totally disengages when not in use.

- Adrian

P.S. It is uploading now so if you might have to check back later.

[youtube]n_GBu4OkmJ4[/youtube]
 
Thanks everyone for the kind words. Very happy with the mechanical side of the design, just need to limit the power now. This was never designed as a high power setup, so when I am pushing 2500W + through it, I am make too many compromises.

I just swapped the remote shunt cable on the modified watt meter to a shielded on, so hopefully this will give me both good current readings and a clean throttle signal. Then maybe Jonas's code will have a chance of do it's job of limiting power.

I made the same trip that killed the last motor today, now that I have fixed the mechanical issue, and the motor windings were only 74 deg-C at the end of the trip. So I don't think lack of airflow/cooling was the issue. It was alignment, and physical clashes due to allowing the can to move axially. This is no longer an issue, as I no longer modify the motor. So the can can't move axially, and I have removed the secondary swing arm it clashed with. So no more unintending rubbing.


I should put up a video of installing one of these on a bike, to show how quick it is. I would love to show the whole process from a standard bike, to having the kit installed and riding off. The whole process only takes minutes, should make for a good video. Unless you take into account the time it takes to remove the grip from the handles bars so you can put a twist throttle on. :lol: That takes just a long as the rest of the process. :x

- Adrian
 
gtadmin said:
Hi Adrian, start the video with the grip off :idea:
Click to expand...
Yeah. Sort of cheating though.
gtadmin said:
Are you going to "snaffle" Kepler's throttle interface?
Click to expand...

Trying my hardest to get one. :) As I am really keen to try out Kepler's interface on my drive. John has said one of the next batch has my name on it. But I wanted a DIY solution forthe more adventurous out there hence the other two approaches I am working on.

I gave the interface spin on his drive and was really impressed. Can't wait to try the refined version on my drive.
 
adrian_sm said:
...Yeah. Sort of cheating though...
Click to expand...
Or .... cut it off and replace with a new one?? Which is what I would do.
... But I wanted a DIY solution forthe more adventurous out there hence the other two approaches I am working on...
Click to expand...
Those that can see you mean :evil: (not necessarily adventurous). Even if I can't use the ideas at the mo' this thread still makes unmissable reading 8)
 
I think I have found the next weakest link. The shear strength of the tyre rubber. :D Even with heaps of engagement, and high tyre pressure, I can rip up the tyre. So I did a simple test of the amount of torque I can apply before the motor starts ripping the rubber off the tyre. I measured 3kg @ 0.45m, so about 13Nm.

torque_test.jpg

Based on the torque constant of this motor........
adrian_sm said:
Just thought I would post a useful link to the conversion between motor Kv, and the torque constant. This tells you how much torque the motor will provide for each Amp of current going through it. Obviously the possible motor current drops as BEMF rises with motor speed, but it can tell me what sort of thrust the motor is providing
Jeremy Harris said:
Kt (N-m/amp) = 9.5478 / Kv
Click to expand...

So crunching the numbers for a few of the motors we are playing with:

6374-200kv, Kt = 0.0477 N.m/amp
Click to expand...

....... I can calculate the max current before I start shreading tyres.

13Nm / 0.0477Nm/amp = 272 Amps.... :shock:

Hmmm. Pretty sure I wasn't dumping that much current into.

Maybe the drive is not fully engaging to this depth. Which means I still have the geometry/force balance wrong.

Time to work out best way to setup the drive on the bike, rather than just by feel. :roll: At least that explains why I started shreading tyres, when I wasn't before. :D
 
Hmmm. Just redid the torque test but rotated the lever arm 180 degrees, such that I was pulling down rather than up.
Still get the motor climbing the tyre, and similar peak torque levels......

Geometry looks okay. :?

Maybe it is more of a dynamic effect. Or out of round tire. Or bumps causing disengagement...... I really wish I could tell when it is slipping/burning up the tires. So hard to tell when you are riding.

- Adrian
 
:lol:

Think I would prefer to buy a flight to NZ instead for myself, and bring the bike. I haven't been then since I was a teenager, but loved it.

I'll drop you line line if I can convince the wife and kids to let me go on a holiday myself. :lol:
 
Ive got a better idea. fly the family to tassie and we can go for a ride while they enjoy the sites!
 
Oh forgot to say.

Speaking of NZ torpedo7 have helmet cam HD 1080 cameras on sale for $199 or $299. That is probably cheaper than flying around the country side :)
 
Managing tire wear is a major consideration with friction drives. Although belt sander material grips well, I am not convinced this is the ideal material for the job. Todd reports that he has had minimal tire wear with a knurled roller and he is pushing plenty of power through his drive.

Current control makes a big difference to tire wear especially current limiting based on speed. Pumping low Watts at low speeds is much gentler on the tire but the trade off is reduced performance.

Measuring motor speed and comparing it to wheel speed may also be an answer. This can be done with my interface and may be added in the future, but for now I need to see if the issue can be solved without going to the extra electronic complication.

Tire pressure has a big effect also. Max pressure is not ideal as this tends to take the top off the tire. Spreading the contact patch with less tire pressure improves the overall wear. I set tire pressures mid way between max an min recommended on the tire.

I think we still need to do more work on coming up with the ideal friction material. Something that mimics a knurled roller is what I think we need to consider. I am trying to get my hands on some stainless steel wire mesh used on security doors at the moment. I think this is certainly worth experimenting with. Another option would be fit a knurled sleeve over the motor. Hard part with this option though is keeping the outside diameter as small as possible. Also this is relatively expensive to make.
 
Kepler said:
Current control makes a big difference to tire wear especially current limiting based on speed. Pumping low Watts at low speeds is much gentler on the tire but the trade off is reduced performance.
Click to expand...
I thought it would just be a current thing. Current should dictate torque. Which is what should be breaking traction. Why does a lower limit at lower speeds help?

Kepler said:
Measuring motor speed and comparing it to wheel speed may also be an answer. This can be done with my interface and may be added in the future, but for now I need to see if the issue can be solved without going to the extra electronic complication.
Click to expand...
Oooo. I would love something that simply told me if the motor was spinning faster than the tire. (ie. surface speed) If it could alarm whenever the speeds aren't matched (above some threshold) I could tell when I am breaking traction. Which would really help use identify the root cause.


Kepler said:
Tire pressure has a big effect also. Max pressure is not ideal as this tends to take the top off the tire. Spreading the contact patch with less tire pressure improves the overall wear. I set tire pressures mid way between max an min recommended on the tire.
Click to expand...
Interesting.

Kepler said:
I think we still need to do more work on coming up with the ideal friction material. Something that mimics a knurled roller is what I think we need to consider. I am trying to get my hands on some stainless steel wire mesh used on security doors at the moment. I think this is certainly worth experimenting with.
Click to expand...
Metal flyscreen material might work. But I think it is typically aluminium, and wire strength may be too low.

Kepler said:
Another option would be fit a knurled sleeve over the motor. Hard part with this option though is keeping the outside diameter as small as possible. Also this is relatively expensive to make.
Click to expand...
:idea: If you are serious about going commercial on a decent scale, you may be able to get a motor manufacturer to knurl the can before assembling magnets etc. Would be worth asking a few manufacturers how much this would cost, at various volume points.

I think my slip is probably during the initial engagement, when the geometries lever ratio is at the lowest, the motor is spinning up faster than the tire speed, hits the tire, burns the rubber off until the motor has slowed down to the tire speed, then climbs the tire and engages properly. This would explain why I only started seeing it recently, when I moved the pivot point, and hence contact lever ratio at initial pickup.

I should be able to tune this via a few methods
- geometry of the motor engagement with the tire, by sliding the drive up the seat post, and adjusting dead-stop and spring to suit.
- ramp rate for the throttle, which is easy.

If it is purely the motor developing more torque that the tire can handle, the only answers appears to be bigger contact patch, or less motor torque hence current limiting.

- Adrian
 
Thanks for the links guys, but I am actually really happy with the belt sander material. It has been showing no signs of wear, and is designed for high shear loads, and wear resistance. All the grip tapes I have tried do not have a very strong backing material, and can not handle the shear forces.

Update: Tire slippage
Okay the problem I had with slippage was that the drive was incorrectly installed. :? :roll:
I had it too low on the seat tube, so the motor would slip when initially engaging the tire.

The simple check for this is lightly lifting the swing arm until the motor engages with the tire, then turning the motor. In one direction it should lightly slip, in the other direction it should climb the tire. If it slips in both directions, then the drive needs to be raised on the seat tube.

So now I have some sort of installation procedure.
<Cue drum roll>

Installation Procedure v1
1) draw a line on the grip tape that aligns with the centre of the seat post hole on the main pivot block
2) back off both dead stops
3) install drive on the seat tube, and tighten the clamp screws such that the drive does not slide down under it's own weight, but loose enough to slide it by hand
4) lightly lifting the swing arm until the motor engages with the tire, then turning the motor. In one direction it should lightly slip, in the other direction it should climb the tire. If it slips in both directions, then the drive needs to be raised on the seat tube. Repeat until the drive climbs the tire.
5) twist the drive to align the mark on the grip tape with the centre of your tire.
6) tighten the clamp screws, then repeat steps 4&5 if neccesary
7) increase spring tension until motor is approximately 2-3mm clear of the tire. (spin the tire to ensure it is clear for the full revolution)
8) adjust lower dead stop to stop the drive swinging more than 4-5mm from the tire
9) adjust upper dead stop to ....... TBD


Step 9 is still a work in progress. Since I am not sure what the best maximum engagement is. It depends on a number of factors, like maximum motor torque, tire pressure, wet or dry riding conditions, etc. But from my tests so far I can get more than enough engagement for the torque my motor is providing. So it is more a matter of limiting the engagement to reduce the rolling resistance, and preserve the tire.

Feeling a lot better now that I think I have resolved that issue.

- Adrian
 
Sounds like a good set of procedures there Adrian. Setup is definitely a major part of keeping tire wear down. I still are not convinced that belt sander material is the best choice though. At the end of the day, I think the best results will come from a combination of strategies. Still more work to be done on this front I think.

I thought it would just be a current thing. Current should dictate torque. Which is what should be breaking traction. Why does a lower limit at lower speeds help?
Click to expand...

I could be wrong here but it seems to me that say a current limit of 40A is more likely to break traction on the tire at low RPM then 40A when the tire has good momentum and is spinning at a higher RPM. Kind of like how you will spin a tire in the dirt off the mark at low speed but the same amount of power wont break traction when traveling at a higher speed.
 
Kepler said:
Sounds like a good set of procedures there Adrian. Setup is definitely a major part of keeping tire wear down.
Click to expand...
Since I had never noticed slip before I just got lazy/cocky setting it up. Now I have learnt my lesson.

Kepler said:
I still are not convinced that belt sander material is the best choice though. At the end of the day, I think the best results will come from a combination of strategies. Still more work to be done on this front I think.
Click to expand...
Your in a much better position to comment on long term tire wear than me. I just don't have the kms under my belt.

Kepler said:
I thought it would just be a current thing. Current should dictate torque. Which is what should be breaking traction. Why does a lower limit at lower speeds help?
Click to expand...

I could be wrong here but it seems to me that say a current limit of 40A is more likely to break traction on the tire at low RPM then 40A when the tire has good momentum and is spinning at a higher RPM. Kind of like how you will spin a tire in the dirt off the mark at low speed but the same amount of power wont break traction when traveling at a higher speed.
Click to expand...

Did you mean to say "same amount of power" or "same amount of torque"? There is a big difference between power and torque. Remember, Power = torque x speed
If you hold power constant, and half the speed, torque would have to double. Conversely if you hold torque constant, and double speed, the power will double.

For the same torque the tire will see the same shear force, and we will set up the same force balance engaging the motor. You will still naturally have less power at lower speed, but I still can't see a reason justifying a lower current limits at lower speeds.

- Adrian
 
Update: Modified Watt Meter

Okay a combination of the remote shunt, and Jonas's code doesn't work. The throttle signal is somehow screwing up the current signal.

I get good readings of current when I don't have any of the throttle input or output connected. But when I have the throttle hooked up, I am lucky to see the current get off zero, even when I know I am pulling 100 amps.
But I did test it on the bench before I making the remote shunt mod, and it definitely had reasonable current readings, and throttle control.

Time to get the oscilloscope on the shunt and see what the signal looks like, with and without the throttle hooked up.
 
adrian_sm said:
Did you mean to say "same amount of power" or "same amount of torque"? There is a big difference between power and torque. Remember, Power = torque x speed
If you hold power constant, and half the speed, torque would have to double. Conversely if you hold torque constant, and double speed, the power will double.

For the same torque the tire will see the same shear force, and we will set up the same force balance engaging the motor. You will still naturally have less power at lower speed, but I still can't see a reason justifying a lower current limits at lower speeds.

- Adrian
Click to expand...
I do mean power.
Taking a sample current limit of 40A @ 25V for 1000W power.

For simplicity this is a raw numbers only example (power / speed = torque)
1000W / 5kph = 200 torque
1000W / 10kph = 100 torque

So by this example, if 100 Torque was the target figure to ensure slip did not occur, 500W would be the target watt limit for a speed of 5kph. (500W / 5kph = 100 torque)

This is one of the advantages of mapping speed against a given current limit. The other advantage is that the chance of sync loss is greatly reduced.
 
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