Royal update: Why hub-motors are better
And why they may actually win longer term
We got a new King. The old Queen stepped down and let her son take over. So now we have two queens and a king. Rare but possible in chess and real life. The new king of Denmark is Mr. X. Royals do not really have last names, so he is just Frederik. The X is for number 10.
As part of getting a new king, all Danes are officially allowed to put the royal monograms on anything for a limited period. So here we go - this is now a royal update.
This monogram is for Frederik and his wife Mary.
What does this have to do with hub motors?
Nothing. Other than the fact that Frederik is known to be quite sporty (some years back when my kids were handing out bananas and drinks at a local MTB race they got to say hi to him - even though he would have his own bananas and such).
MTB is not the focus of this story. But when I keep seeing this marketing about how mid-drive motors on e-bikes are good because of the low center of gravity I get the feeling that it is all about MTB. Having a low center of gravity may matter for MTB. Let us take a look at that marketing claim with a quick calculation.
Starting point
This is where the center of gravity is on a bike before adding a motor etc. to it. This point (see the red cross) is where you could theoretically hang the bike in a string and it would be able to stay horizontal.
The two components on an e-bike that shifts this around are the heavy parts:
Downtube battery (let’s assume this is 4 kg)
Motor (let’s assume this is 3 kg for a mid-drive and 2.6 kg for a rear hub motor)
In comparing the two systems, you can look at how much they shift the center of gravity.
Let us assume the bike itself weighs 12 kg before we start adding stuff. Then if we add a battery and mid-drive motor to it, this is what happens:
The center of gravity (the red cross) moved a bit up and to the front (about 4-5 cm total movement).
If we add a battery and rear-hub motor to it, this is what happens:
Now the center of gravity moves a tiny bit up and a bit to the rear.
As you can see, the argument that mid-drive is superior because of the lower center of gravity is really far-fetched and a good example of typical bike marketing. If you add a person on top of the bike into the calculations, the difference becomes ridiculously small.
But…
Yes - there are other advantages to hub motors on e-bikes. The main one is that the chain only needs to carry the force from the human. Not from the motor.
If you design an e-bike with a mid-drive, the chain will need to transfer the combined power from both the human and the motor. So a chain and chainring for a mid-drive motor needs to be stronger (read heavier).
But what about the gears?
Well, that is potentially the biggest disadvantage of hub motors. Internal gear in the hub is difficult to do when that space is taken up by a motor (often motor + internal fixed gear). So you need an external gear system. Standard derailer type or a gear in the crank area (like Pinion). Or no gear. An urban e-bike really only needs one or two gears. Cowboy works well in most situations with a fixed gear - as an example.
I remember seeing a 2-speed crank-area gear solution also, but can’t seem to find the one I was thinking of. The only one I found was a (failed?) Kickstarter. Maybe you have pointers?
Also, the gear in a hub motor setup only applies to the human power, not to the motor power. As e-bike motors are not outputting much at low RPM, starting uphill from a standstill is a weak point for hub motors where mid-drive can work a lot better.
The conclusion still holds, however: Stop talking about a low center of gravity as an advantage of mid-drive over hub-motor. And don’t rule hub motor out as the best solution for many types of e-bikes.
(thanks to unsplash for the photo)
TECH STUFF
Non-rotating Torque/Force Sensor
How do you easily measure torque on something that is not rotating? Or not rotating more than a few turns - and then back again? Do you really need a full wireless torque sensor system for that?
That problem pops up in many projects, and there is a simple solution to it. These types of forces or torques can be measured using a strain gauge and with this little circuit called SG12, the measurements come streaming out on a serial port. Up to a rate of 1000 samples per second.
If you are building anything where sensing force, torque, strain, etc. can be done without having to use a wireless connection - this is for you. Get in touch and we can get you one of these tiny modules. And they are tiny - the active part of the electronics is less than 10x15mm. So if you want that integrated into some part of your system, we can also do that.
NERD STUFF
Hasan Yavuz Özderya writes software. He is the guy behind a great open-source project that we have used a lot over in Sensitivus. It is called “SerialPlot”.
This software is ideal if you have any type of data streaming out on a serial port and you want to plot that live. Like this:
We love it. And we love the fact that it is open source. Open source means we could take it and add our own stuff to it. That is exactly what Kristian just did.
Since the software is so great to use with a torque sensor, we have used it a lot. But we wanted to add one extra feature to make it next level. We wanted to be able to update the sensor firmware from within SerialPlot. That is now possible.
So here is the new “Sensitivus SerialPlot” open-source software. Ready for you to further add or modify as you like.
FUN STUFF
Build your own cargo bike. I mean, how hard can that be? This guy explains how it can actually be a significant challenge if you want to make something really nice.
That cable-steering mechanism is so well done. Wow.
FRIENDS
Miriam Mayer
She was sitting on a bench with her purple Yeti touring bike outside the hippest kiosk (so hip they have to spell it Kihoskh) in what was earlier the red-light district of Copenhagen when I first met her. The bike was loaded for touring. From Munich to Nordkapp. That is a long ride, and also sort of a standard touring ride for those going up north in Europe.
Part of the reason for the heavy load it turned out is also why I want to share this: Miri takes photos. Great photos. Obviously, she was carrying a lot of photo gear as well on that purple Yeti.
So - if you ever need a photographer for outdoor stuff/bike stuff - check out Miri here:
https://www.instagram.com/miri.outandabout/ and on her website.
And if you want an article to go with the photos, she is also a talented writer. #2for1.
ABOUT
I have been spending the last 10 years on sensors and technology for bikes. Both are things that I really enjoy. Technology for bikes can be many things - these are the types of products and technologies I can talk about from experience:
Power meters (to measure how hard you pedal for training purposes - but actually valuable for much more than just that). A product with a high retail price and a low BOM price. Mostly sold as after-market.
Case: Technology still on the market by THM - The Clavicula PM crankset.Brake sensors (for optimizing MTB downhill performance). Very niche product for elite riders, which may eventually slowly become more widely accepted. Pure after-market.
Case: Technology on the market from BrakeAce.
e-bike torque/cadence sensors (to make e-bikes feel more like “analog” bikes on a really “good leg day”). Super high volume product with a fairly low sales price - a pure B2B product for integration into a motor design or similar.
Case: Technology on the market from Sensitivus, but not quite out in stores yet.
Each of these examples has occupied a good chunk of my life over the last 10 years, but there is more. A lot more. This is a story for another day.
P.S.
Over on EE-Training I have some super nerdy courses coming up - check this out if you are into designing the very fastest and most efficient digital bords today with serious memory interfaces:
Open the Black Box of Memory, Munich, Live, Feb 26-28, 2024
Signal Integrity w/Hands-On Simulation, Online, Mar 4-8, 2024
Signal Integrity w/Hands-On Simulation, Online, Apr 15-19, 2024
Find it all on ee-training.dk
P.P.S.
Just to prove that the new king around here actually has been pretty active on the bike.
(thanks to @copenhagenbikefestival for the photos)
[[ LET’S KEEP UNF***KING THE PLANET - ONE EBIKE AT A TIME ]]