Sturmey Archer Dynohub magnet upgrade: early results

My idea for a GH6 magnet upgrade seems to be working.  I had an adapter machined out of mild steel to space out 20 0.25" x 0.75" x 0.125" neodymium magnets, which I attached with J-B Weld.  It took a while to fit it into the hub shell and I ultimately had to steal a spacer clip from another GH6 to make it fit without the armature rubbing.  Unfortunately, I didn't document any of this with pictures, so you'll just have to take my word for it for now!  The cogging effect is quite strong, but once mounted in the fork the wheel spins fairly easily. Not sure how this will translate into actual drag while riding.  


I connected it to my test circuit, which consists of an Arduino controlled buck converter driving two LEDs in series at 100% duty cycle.  The series forward voltage of the two LEDs is about 5V so you can infer power from the current but I didn't actually measure Vf, which goes up modestly as current rises. The data was acquired on my motorized testing jig. I'm using the Arduino to log the current at different speeds. Speed is calculated using zero-cross detection to measure the frequency of the pulses coming from the hub. My jig has crappy speed control so all I can do is ramp it up and slow it down and catch the current at each speed. At each speed I collect the average current of two complete wheel rotations (again, as measured using hub pulse frequency) four times and plot the average of that. Error bars are standard deviation of the mean.

The hub seems to saturate around 750 mA, although my multimeter is reading 900-1000 mA at higher speeds (update: confirmed with another multimeter that the hub is indeed saturating at 900 mA, so my high side current monitor is not linear :( grrrr!).  Thus far, it looks like the magnet upgrade is a success. I was worried, based on the accounts of others, that the armature would saturate at a much lower current, but that doesn't seem to be the case here.  I'm going to try to use 1/16" block magnets to see if I can get a similar power output with less drag.  There is a chance that the 1/8" magnets are overkill, causing unnecessary drag and reducing efficiency.


Decreasing the gap between the magnets and the claw poles of the armature might also help make the hub more efficient. At the moment they are about 0.050-0.060", whereas the original magnet is probably closer to 0.010-0.020".


Will post photos of the upgrade as soon as I have them!


Update:  I found a photo of the adapter sticking to my fridge.

GH6 magnet adapter upgrade with Neodymium magnets in place

Update 2:  here it is mounted in the hub shell with the armature (the magnet cover plate has been removed)

Specific aim 4: a Dynohub magnet upgrade

I think I have established that I have an unhealthy fascination with the Sturmey Archer GH6 dynohub.  The hub has an output of about 1.8W, although doing a little experimenting with my digital buck converter on my dynamo testing jig, it appears that the hub reaches its saturation current at considerably higher speeds than contemporary hub dynamos (at least in my test setup with a 1966 GH6).  If the point of the buck converter is to extract more power out of the hub, there's not much practical use if that power isn't available until you're going 40 km/h!

Speed versus current of a GH6 powering two series LEDs through a digital buck converter at 100% and 50% pulse width duty cycle.  Error bars are standard deviation from the mean of 4 readings taken at each speed.

 The GH6 is old technology and seems primarily limited by the fact that it has a rather weak 20 pole Alnico magnet, which is reported to lose a significant amount of flux over time.  The few GH6 experts out there maintain that if a Dynohub has been serviced there is a good chance that the mechanic damaged the magnet by removing the armature.  Amazingly, Jobst Brandt, of The Bicycle Wheel fame, built a re-magnetizing device (very bottom of page) to refurbish the magnet in the event of the hub being disassembled improperly.

GH6 20 pole manget (GL343) and armature (GL603)

I wondered if it would be possible to upgrade the magnet with modern Neodymium magnets.  After some more searching I discovered that I wasn't the only one that had thought of this.  Ben over at gotwind.org tried a couple of configurations and got something that worked, although not well enough for his application.  His results are promising for my application though as he reports getting higher voltage due to the increased flux of the new magnets and perhaps even a higher saturation current.

Ben@gotwind.org replaced the 20 pole Alnico ring magnet with 20 neodymium magnets in a GH6 hub

Based on Ben's description, a magnet upgrade to the GH6 might be able to bring it up to spec with modern hub dynamos (rated voltage at low speeds, saturation current around 0.5A), albeit with more drag and probably lower efficiency.  After some pondering and a bit more research, I think I'm going to have a go at making an adapter to fit the GH6 with 20 new Neodymium magnets. One planned improvement over Ben's efforts will be to keep the spacing between the stator and the magnets as close to the original specs as possible. The actually spacing is probably in the order of 0.010 to 0.020". I'm aiming for around 0.050", whereas Ben's design is reported to have 3mm (0.118") spacing.  The trick will be to pick magnets of appropriate strength that are strong enough to saturate the stator at low speeds, but not so strong that they add undesirable cogging effects and drag.  Thus far, I have a design I'm shopping around to machinists:

Proposed magnet adapter with armature

3D view of proposed GH6 magnet adapter

Not sure yet what material it should be made of. At first, I thought of using ferromagnetic steel so the magnets would stick to it.  This is more expensive and heavier than using 6061 aluminum, which might be a better solution as the longer I consider it the more I realize that whatever the material, I'll need to epoxy the magnets in place.

So, fingers crossed that these efforts will result in a GH6 with improved lower speed performance!

Update: magnets arrived today and, boy, are they strong! It's been said that Nd magnets are 10 times stronger than Alnico magnets. Now that I have some in hand I believe it.  As stated previously, I think the challenge will be to use a magnet that is just strong enough to saturate the stator but not so strong that it adds additional drag due to cogging torque.

Specific aim 3: a dynamo wheel testing jig

In order to test my buck converter with a dynamo I need to be able to spin the wheel at different constant speeds.  This is not done practically by spinning the wheel by hand, so some kind of motorized system is required.  Fortunately, (DIY dynamo lighting legend) Martin over at pilom has already built one and he gave me some helpful advice on how to build my own. An abandoned bicycle fork recovered serendipitously from under a bridge and a washing machine motor salvaged from my in-laws' basement are the main ingredients.  Rudimentary speed control comes from a router speed controller, which I believe is TRIAC-based.  It took about 3 hours in my Dad's wood shop to put it all together:

hub dynamo testing jig with 700C wheel

1/3 hp washing machine motor. Routed slots allow motor adjustment to accommodate different sized wheels

The 6 inch drive wheel is based on achieving a maximum speed of 50 kph, or close to 400 RPM, from the motor's full speed of 1750 RPM.  Speed control is not great; the sweet spot cruising speed (25-35 kph) is very difficult to maintain.  In this region the motor's relay cycles on and off, resulting in a sinusoidal speed with a period of a few seconds.  Constant speeds in the 5 kph - 20 kph are easily achieved, then it cycles and finally takes off to a maximum speed.

So far so good. The contraption remains stable with minimum vibration at high speeds. I ordered a super cheap bicycle computer from DealExtreme to measure the wheel speed, but while I waited for it to arrive I figured how to calculate the speed based on the pulses coming out of the hub using a zero cross detector.  The cycle computer has arrived, so I'll be able to see how close my zero cross dynamo speedometer works (more on that later...).

Specific aim 2: a dynamo digital buck converter

I am developing hardware and software for a dynamo peak power tracking system.  Maximum Power Point Tracking (MPPT) is something I learned about over on a thread at CPF and have subsequently become fascinated about.  Widely applied in the solar industry to extract the maximum available power from a photovoltaic array under all conditions of illumination, MPPT is, in essence, a way to tweak the load resistance to reach a peak power point on a current/voltage curve under dynamic conditions.  Bicycle dynamos operate primarily as constant current sources. They reach a saturation current at reasonably low speeds and the only extra power to be gained from a 'passive' system is through the increase in voltage that comes at higher speeds.  Passively extracting more power is done typically by adding more LEDs in series. If you have 1 LED with a Vf of 3.0V you can get a maximum of 3V x 0.5A = 1.5W.  If you add a second 3.0V LED in series you can get 6V x 0.5A = 3.0W, a third gives you 9V x 0.5A = 4.5W, etc.  The problem here is that for each LED added there is an increase in the minium speed at which the dynamo reaches Vf of the series chain. Below that you get no light, or, at best, very blinky lights.  However, there are some clever passive low speed boost circuits out there to get more power at lower speeds.

Today's power LEDs have maximum currents in the range of 1-3A, so a dynamo that saturates at 0.5 or 0.6A is not able to run these even close to their maximum theoretical outputs.  I want to a employ a single LED design for front and rear lamps, so running the LEDs at more than the dynamo saturation current would be useful.  In order to pull maximum power out at all times, a switched mode buck converter can be employed.  I'm working on a digitally controlled synchronous buck converter that uses LED current and bicycle speed as feedback.  Thus far I've developed a peak power tracking system that works with a bench top supply.  Essentially, under fixed current conditions (like a hub dynamo), it modifies the pulse width of the switching signal to maximize the power from the available voltage.

200 mA in, 370 mA out!

LED Current vs pulse width duty cycle at different input voltages where Vf of the series LEDs is 4.1V

Putting dynamo LEDs under microprocessor control may seem excessive, but I'm not the first to do it.  In theory, it should be possible to drive LEDs up to and beyond 1A at cruising speeds, with the obvious caveat that the extra power has to come from the cyclist's legs!

Buck converter hardware

Hardware is under Arduino µC control

The software still needs some significant tweaking to work with the fluctuating DC of the rectified dynamo output.  I'll post a full description of this buck converter soon, hopefully once I have the peak power tracking working with a dynamo.  I'm using the Arduino platform for development, which is great and easy to use, but I did need to employ a few under the hood tricks to get it working. I will ultimately have to switch to another, smaller AVR microcontroller for the final design.

Specific aim 1: making an LED light bulb

I need to mount a single LED with sufficient heat sinking into the base of an E10 Edison screw thread bulb.

I've collected some lovely French lamps. Historically, these would be powered by a bottle dynamo.  My intention is to power them using one of the... um... 10 or so hub dynamos that I own.  Old incandescent bulbs don't light up much so a LED upgrade is in order.  Experience has taught me that commercially available LED E10 Edison bulb upgrades are OK, but I think I can do better, especially with the selection of high power LEDs now available.  Power LEDs get hot, so whatever retrofit I come up with will need to be able to deal sufficiently with the heat.

Late night scrawlings

Thus far, I think I have an idea involving a turned copper pillar of some sort that can be pressed/epoxied into an E10 bulb base.  Something like this:

Copper pillar LED heat sink

Pressed into something like this:

MES/E10 lamp base

This way, any cycle lamp that takes an E10 threaded bulb can be retrofitted without modification.  Getting the heat out of such a small package will be a challenge, though; something that my previously proposed solution might not be up to.  The best solution would be to mount the LED directly to the copper heat sink, which eliminates any of the thermal resistance encountered in an FR-4 board perforated with plated vias or in a metal core PCB that is then attached to a heat sink with thermal epoxy.

There are several elaborate methods described for doing this. Lambda Lights seem to have mounting LEDs directly to a large copper heat sink down to a science, so I might contact them before attempting to reinvent the wheel.  From what I can tell, they turn a copper pillar and then machine a square nub on the top where a PCB is mounted with a slot milled in it so the nub can poke through and be soldered directly to the thermal pad of the LED.  Something like this:

While ideal, this solution may be cost prohibitive...

Sturmey Archer lamps: second attempt

Back in the summer of 2008 I was quite proud of myself when I managed to get a hold of a set of NOS Sturmey Archer lamps for my wife's Raleigh Superbe from a Canadian seller for about half of the going rate at the time ($67, to be exact).  Two years later my faith (what little was left) in humanity was shattered when a very bad man (I've called him worse) very craftily stole the rear light.  This faith was somewhat restored when a very nice reader of this blog offered to send me a replacement rear lamp at no charge (thanks Jeff!).  A last minute discovery of an auction with less than an hour to go has turned up another NOS Sturmey light set, also going for well below market price (and also, coincidentally, sold by a Canadian seller).  I bid, finished my dinner and was delighted (but not surprised) to find that I had won!

Sturmey Archer NOS light set

Just a week ago I won an auction for a NOS GH-6 Dynohub, a 1978, from the same seller:

1978 NOS Sturmey Archer GH-6 Dynohub. Not quite as elegant as earlier models.

Setting goals and things in the works

Being housebound taking care of two small children leaves the mind with enough excess capacity to come up with an increasingly complex and ambitious list of aims.  To try to keep track of all the stuff bumping around up there, I'm going to commit some of these goals to this here blog page.

Specific aims:


1.  Mount a single LED with sufficient heatsinking into the base of an E10 Edison screw thread bulb.


2. Develop hardware and software for a dynamo peak power tracking system. 


3. Build a speed-controlled dynamo wheel testing jig. 


4. Upgrade the Sturmey Archer GH6 magnet to modern Neodymium magnets.


A flurry of posts will address each of these aims.