This mid drive build was started way back in 2008, but…it was so ahead of it’s time, it is still awesome by todays standards. Deecanio is the member-name of an endless-sphere.com builder from the UK who created this historic milestone E-bike.
He decided he wanted something completely new and cutting edge. He had read about how the components for Radio-Controlled (RC) aircraft models had grown large enough that they were beginning to be used on electric bikes. RC components can be found that are very powerful, but the reason for their high price is that they are very small, and it is having high power in a small package that he was willing to pay extra for.
Deec likes riding off-road, so he was immediately drawn to the idea of a mid drive system. The RC motor. controller, and reduction might be light, but it would be extra sweet if the modest weight of the drive could be centrally located, and also placed down low for a great balance.
2008 was about the same time that Matt (ES member “recumpense”) was doing a lot of development work to make highly professional mounts and also drive-reductions for Astro Flight motors. His designs are works of machinist art, and he is a pioneer in this field.
Here is a link to his monster yellow trike from 2012.
Motor and Reduction
Early in the design stage, Deec knew he wanted to have a light mid drive. He had owned a powerful rear hub bike before, and although it was fun…when he was in slow uphill technical sections of off-road crawling, the hub would get hot from low RPMs. Also, when he took jumps, he noticed that the extra hub weight in the rear had a noticeable effect on the bikes handling, compared to non-hub bikes.
Matt Schumacher had just started developing high-end RC motor drives that established a new standard of performance and quality. Deec discussed all the options and decided to use an Astro Flight 3210 motor with a dual reduction. The primary reduction would be turning very fast, so Matt recommended that Deec use a belt and pulleys for that, which would run much quieter than a chain.
Deec wanted to keep the drive fairly small, because he didn’t know yet exactly where it was going to be mounted, but he did already know that his Kona Stinky full-suspension frame didn’t have much room anywhere. The primary reduction 60T driven pulley was the smallest one that would allow a White Industries ENO 22T freewheel to fit inside the pulley face (after the 22 teeth are machined off, the factory circumferential slots provided perfect mounting points).
The 17T drive pulley provides a ratio of 60:17, which equals 3.5:1
This left the secondary reduction to sort out. The drives’ freewheel allows the pedals to drive the bike without also needing to back-drive an un-powered motor. Since the motors’ freewheel was located in the primary drive, this freed-up the secondary chain drive to use the smallest possible drive sprocket.
Deec and Matt had settled on using high-quality #219 Kart chain for the secondary. The secondary reduction turns at a lower speed, but feels more torque, so a narrow and strong chain is appropriate.
The chainring was going to be the maximum diameter possible regardless of the chain pitch, and…the small pitch of the #219 chain allowed the drive sprocket to have the minimum needed 11 teeth in a smaller sprocket than a bicycle-pitch chain (11T+ to avoid the high noise from the “polygonal effect” when using sprockets with 10 teeth or less), and the 80:11 ratio provided a significant 7.3:1 reduction in RPM’s.
The combined primary and secondary reductions (3.5 X 7.3) results in a 25.5:1 reduction in RPMs between the motor and the crankset. The motor chosen is a radial-flux in-runner Astro 3210 “10-Turn”, which has a Kv of 135-RPMs per volt. If the bike is unloaded (with the tires in the air), a Kv of 135 X 48V = 6,480 RPMs!
Ultra-high RPMs are one method to getting high power from a small motor. As a result, these Astro motors make a high-pitched turbine noise when at full power (Here is a video to show the sound). It’s not too loud, but it is definitely not silent. Running 6,480 RPMs through a 25.5:1 reduction will result in the crankset spinning 254 RPMs.
When a bike is driven on the road, it is a rule-of-thumb that the RPMs will be roughly 20% lower than when it is unloaded, but…even at around 200-RPMs (at full throttle) this system was still going to be way too fast to be able to pedal along with it.
The Tiny RC Controller
The largest RC motors are surprisingly powerful when you consider their tiny size, but the RC component where this size difference is the most notable is the controller. Getting peaks of 3,800W from a controller this tiny is expensive.
Deec had decided to use the tiny and well-regarded RC Electronic Speed Controller (ESC) from Castle Creations. The 50V max HV-110 (High Voltage, 110A) looked like it would work well, since he intended to give the motor the use of the bikes gears (as opposed to the higher amp-draw peaks from a direct-chain “one speed”). That not only kept the motor RPMs high in order to help with the slow-speed off-road performance, it also reduces the peak amps that are drawn by the controller and pulled from the battery.
When we talk about a 48-volt battery, that number is sometimes called it’s “nominal” rating. Deec had chosen to use the very safe LiFePO4 chemistry, and in 2008, the high-current cell to get came in a large cylinder from Headway. He arranged for ES member GGoodrum in the USA to build a pack for him to fit the small triangle of his favorite 2004 Kona Stinky frame. But how many cells and how to arrange them?
The average LiFePO4 pack that is called “48V” uses 16 cells in series (16S). If we use the charge profile of 3.6V per cell when fully charged, a 16S pack will actually start at 57.6V, which would damage the 50V ESC as soon as it was plugged in (if you set the Low Voltage Cutoff / LVC at 3.0V per cell, the battery will actually stop at 48.0V). This led GGoodrum to recommend the odd arrangement of using 15 cells (15S), resulting in a user profile between 45.0V when low, up to 54.0V when fully charged.
There is a small voltage safety margin in the design of the ESC he had chosen, and 15S of LiFePO4 (equalling 54V max) has worked out so far (though we don’t recommend it due to voltage spikes). Deec added more low-ESR capacitors to the ESC input to help suppress voltage ripple to hopefully keep the ESC as safe as it could be.
Once the custom battery was built for the central triangle of Deec’s Kona, that severely limited the options concerning exactly where he could mount the drive on the frame. This may have seemed like putting the horse before the cart, but…this type of design was very new in 2008, and enthusiasm drove everyone involved to make the first steps before the entire plan was solidified.
The Freewheeling Crankset
There is a type of bicycle called a trials bike. For a variety of reasons, many of the riders of these acrobatic bicycles (like Danny MacAskill) have embraced the option of having the bicycles chainring freewheel instead of the rear wheel. Because of this…there is a readily available selection of right-side crankarms with freewheel threads machined into them.
These last two years, mid drives in this configuration have become common, but back in 2008…this was an uncommon solution to making a non-hub E-bike. If two chainrings are mounted to the freewheeling crankset, a motor can drive one of them, while the other one drives the rear wheel. Deec considered a variety of options (like the dual-parallel right-side drive on Roy’s eCortina), but eventually made the commitment to a freewheeling crank with dual chainrings.
Since the drives secondary reduction was definitely going to be #219 Kart chain, a custom adapter disc was machined to connect the 42T bicycle chainring and 80T Kart chainring to the flanged White Industries ENO freewheel. This is the same heavy-duty brand that was integrated into the primary reduction driven pulley seen above.
Mounting the drive
The obvious place to mount the drive was just in front of the bottom bracket, but Deec initially feared that when crawling over large rocks and logs, that the drive might be damaged. This led to weeks of discussion over various other contorted possibilities. He briefly considered mounting the drive behind the seat-tube, but that required a switch to a smaller 20-inch wheel (which was OK), but the smaller wheel meant that the pedals were closer to the ground. In the end, he had to finally admit that the location just in front of the bottom bracket was the best compromise.
However, when contemplating an all-new frame design, Haibike has recently addressed this concern by designing an off-road frame that wraps under the mid drive.
Giving the motor three speeds
When you have a motor-driven system with a broad power range, you don’t need as many gears to dramatically improve the motors’ performance, but how many is optimal? If your user-profile involves using only the motor most of the time, giving the motor 9 speeds might involve frequent shifting to get to the desired gear at any given moment, but…too few gears will limit your options.
Miles’ eMoulton and Roy’s eCortina have both decided that giving the motor three gears is the best compromise, and Deecanio agreed. There have been several builds where someone had used an Internally Geared Hub (IGH), but a sudden application of high power will sometimes break them.
Miles’ build was designed around efficiency and lightness, but Roy wanted high power, and Roy found that using external sprockets (with a common derailleur) can take more abuse than an IGH. Deec built up a freehub rear wheel, but instead of the 9-speed cluster, he used three single-speed cogs (along with spacers) to contrive a 3-speed sprocket-set.
Applying too much power might still break some of the parts, but the splined track cogs are much more affordable to replace (compared to a new IGH), and high-powered builds have posted that excessive power only causes the chain to loudly “skip” over the cogs, which makes a “ratcheting” noise. Be aware that if you want to try this, you might need to adapt the derailleur to accept a slightly wider chain (1/8″ vs 3/32″, referring to the width of the chain rollers and the sprockets)
The drive as shown here used a 42T drive chainring, with 34T/22T/11T sprockets on a freehub-equipped rear wheel (vs a common multi-sprocket freewheel). Splined “single speed conversion” sprockets can be found on the web, and Surly is one popular source. The bike is geared slow (by using a smallish 42T chainring), and the top speeds in each gear are only 17, 23, 28-MPH (27, 37, 45-kph). These lower speeds work well for Deecs off-roading style.
“…I was hitting peaks the other day of around 80A and 3850W. These peaks lasted just a few seconds, but I repeated it often…maybe 10 times over a 5 minute period. I didn’t have my temperature widget with me, but my “calibrated” thumb said about 170F (76C). The controller was not hot at all…
…This is with the PWM [Pulse Width Modulation in the controller] set to 16kHz and the timing set to “Normal”, which means the advance will vary from about 5-10 degrees. I think “Low” timing is 0 to 5 degrees of advance. Bob says his motors like about 10 degrees advance in order to get the max power out of them without excessive heat. I previously had mine set to “Low” timing, and the peaks were lower (about 72A), but the motor temp only got up to 108F (42C), up until I broke the master link on the [bicycle] chain…
… I will leave the PWM set to 13Khz. The difference between 13 and 24khz really was vast as far as temp is concerned – initially a [continuous] 30 second blast [uphill] saw the ESC soar to 176F (80C) whereas now…it barely goes over 140F (60C), and that’s with continuous bursts”
Here is the epic discussion thread on this awesome build. As wonderful as it was in the form shown here, Deec continued experimenting with this build and others. GGoodrum had an Astro 3220 that was far too powerful for the 20-inch wheel it was driving, so Deec and GGoodrum swapped motors and Deec upgraded to the Castle 160A controller (among other experiments…)
Written by Ron/Spinningmagnets, June 2014