Messing with my e-bike

GrahamGo

Senior Member
I have not built anything yet. But want to!

My e-bike is a crank motor type (by Bosch) and provides various assist levels up to 25km/h, above which it just stops providing assistance. But I would really like to go a little faster at times. I believe that the crank mounted motor detects the pedal force (me), and derives the speed from a magnet fixed to a spoke which passes a sensor on the rear wheel support. Here is my concept.

1). I move the magnet along the spoke away from the Bosch sensor
2). Add a new magnet detector and hook it to a picaxe
3). Have the Picaxe sense the passing magnet and simply repeat the same magnetic pulse to to Bosch sensor.

So so far nothing changes as far as the Bosch system is concerned. But now the trick.

4). When the speed reaches say 24km/h I halve (or fix) the speed sent to the Bosch sensor. Now the bike should think that it is travelling at 24km/h and provide motor assistance beyond the preset limit.

I think the approach is sound, but what I cannot decide on is how to provide a magnetic pulse from the Picaxe to the Bosch magnetic sensor. I would appreciate any thoughts. My first thought is to rob a 5V relay of its coil.

Anyway, its just a bit of fun. Illegal, I know......
 

premelec

Senior Member
Not clear what the speed sense unit is - does it sense single position on wheel or multiple? In short one pulse per rev or multiple pulses?
 

GrahamGo

Senior Member
Yes, one spoke has a small magnet attached which passes a magnetic sensor once every revolution, they are about 5mm apart. At a speed of 15km/h that relates to about 2.0hz. It just provides a speed reading on the readout. What I dont know about is the sensor, I guess its a hall effect. I'm just wondering how sensitive it might be and what it takes to fool it into thinking that a magnet is passing by.

The illegal remark was a joke, emphasis is on fun :)
 
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Jeremy Harris

Senior Member
My understanding of the Bosch crank drive is that it works pretty much exactly as you say. The torque sensor in the pedal crank shaft sets the assist torque, and the wheel speed sensor only sets the maximum assist speed, nothing more. I remember reading a full tear-down of this unit, a few years ago, with a bit of reverse engineering to find out how it worked. IIRC, there may be a simpler option than just spoofing the speed limiter, as there's something in the back of my mind telling me that the control board can be set for different maximum speed limits, to allow the same unit to be sold in countries where the legislation on maximum assisted speed is different. It might be worth digging around in the depths of some of the ebike forums to see if others have spoofed the Bosch system. It's a popular system, and delimiting ebikes is extremely common (yes, I know it's illegal, but it's like speeding, loads of people do it).

I rode an ebike with the Bosch crank drive a few years ago, and was impressed with the way the system works, it's probably one of the best systems for hill assist (within the legal EU power limit) there is. At low speeds on hills it will out-perform my 1.5kW, 35 mph capable, rear wheel geared hub motor ebike (not that I would EVER think of riding such a powerful ebike on any publicly accessible area.................).
 

GrahamGo

Senior Member
Jeremy, thank you for the info. Today I bought a cheap bike speed/distance unit. So will use its hall effect sensor to see what the best form of magnet coil is needed to trigger it. Then it should work the same with the Bosch sensor. I don't know whats best yet, fewer turns of a heavy transformer wire, or lots of thin wire. Time to find out! But this little experiment is really about finding out what I can do with a Picaxe chip. I'm going to use an 08M2, should be a bit of fun and not costing much at all.

I'm impressed with your 1.5kw rig. Makes my 400w seem trivial. We live in Holland most of the Spring/Summer. So yes, obviously hill assist is a big asset. Actually not, its so flat the bike could do with a larger cog drive on the motor, which is something that i will do. I have just bought a new ebike for the wife. Its 500w, Bosch crank drive with a fully automatic linear gear change from NuVinci Harmony. Pretty weird to ride, you just set your preferred cadence and it holds it irrespective of your cycling speed.

I did search for Bosch conversions, and yes. Quite a few around at around £80. But this, as a little Picaxe project just seems more interesting ATM. I was hesitant about posting this subject because it didn't seem to fit in with what is normally discussed. I think it would be nice if they could put a little Miscellaneous section next to the Sandbox section. Where vaguely Picaxe stuff can be discussed. Just a thought....
 

The bear

Senior Member
@ Jeremy Harris,
What a brilliant website (Post #7). Until this O.P I hadn't even heard of an e bike! Must get out more.
Jeremy, is there anything you haven't done?

Regards, Bear..
 

Jeremy Harris

Senior Member
@ Jeremy Harris,
What a brilliant website (Post #7). Until this O.P I hadn't even heard of an e bike! Must get out more.
Jeremy, is there anything you haven't done?

Regards, Bear..
Yes, loads yet, I'm only 64, so I've time to have a lot more fun yet, I hope!

I like building things, so far I've built a couple of cars, this was my favourite:


I flew in the "toy" in the background, too, as a part of my day job back then. I also like flying, especially this floatplane:



and this microlight:



I've designed and built a couple of small boats, like this one, made from aircraft fabric over a light alloy frame:



and for the past four years I've been designing and building our new home (and to get back on theme, there are currently 14 Picaxes working away in the house, doing various interesting stuff) all the trials and tribulations of designing and building a "negative energy" home are in my blog, here:

http://www.mayfly.eu/
 

The bear

Senior Member
Thanks Jeremy,

That will do nicely, impressive stuff.
Will look at your blog later.

Regards, Bear..
 

GrahamGo

Senior Member
1.5kW on a very light alloy folding bike does tend to make you grin a bit! I had to fit a Schlumpf two speed bottom bracket high speed drive gear (some details here: http://www.schlumpf.ch/hp/schlumpf/faq.getriebe.engl.htm) so that I could get tall enough gearing to make it look as if I'm legal and peddling at 35 mph..................
Jeremy, I came across a Russian site offering ebike kits, I thought that you might be intrigued. You will see that one kit offers a 2kw 80km/h solution. Not for me, but interesting. No pedaling required! Or in a Picaxe context the battery pack would keep an 08M2 going for quite sometime :) here is the link :- http://e4bike.ru/?lang=en cheers.
 

Jeremy Harris

Senior Member
The bike is interesting, as it looks like a copy of one that a chap on the Endless Sphere forum was making, as a custom built job, around 5 or 6 years ago. IIRC, that bike used the biggest available Crystalite hub motor and ran at around 72V. I don't have the data to hand, but seem to remember that the big Crystalite motor was good for around 6 kW or so, the only issue being that it's heavy, and tends to be tough on the rather short spokes that have to be used.
 
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