Custom Build – Roy’s eCortina

Written by Ron/Spinningmagnets, April 2014
Originally published here:


This months featured build has had three major changes over three years, and each version has been so well done…they could each be featured by themselves in separate articles. But…rather than stretch them into three articles, I found the back-story of the evolution of this bike to be just as interesting as how it ended up.This impressive builder is Roy ( username: RWP), who lives in Ventura California. He decided that he wanted to experiment with a Cortina Triton DS hardtail bicycle (Cortina Is based in nearby Santa Barbara, California)…and this reasonably affordable frame is made from CroMoly steel. The round cross-section down-tube, seat-tube, and top-bar allow for an easy construction of custom bracket mounting clamps.

If you like this shape of hardtail frame, and you want it in CroMoly steel, and also with a slack head-tube angle like this…these are sometimes called  a “Dirt Jumper” frame.



 V1, May 2009

Back in 2009, Roy became fascinated by the E-bikes being built at the endless-sphere forum using the small and light-weight Radio Controlled (RC) model aircraft components. He was already familiar with RC models, and felt that if he could design a compact dual reduction system, he could get significant power from a very small motor that was spinning ultra-high RPMs.

He had seen Bottom-Bracket (BB) drives already, and their benefit was that they gave the motor the use of the bikes gears, so Roy calculated that by also giving the motor some gears to use (compared to a “one-speed” hub motor), he could then select the absolute smallest possible motor.

What makes a BB-drive easily possible is that there is a type of bicycle called a “trials” bike, which has a freewheeling BB, rather than the common method of using a freewheel at the rear wheel. This allows the rear wheels’ solid sprocket to be the smallest possible, and that in-turn allows the BB chainring to also be as small as possible (while still maintaining the desired ratio), and by doing all this, you can obtain the ultimate ground clearance. Because of trials bikes, freewheeling BB components were readily available.

So…when you have two chainrings on a freewheeling BB (which was taken from a trials bike), the motor can drive one chainring, while the other chainring drives the rear wheel. This allows the motor to drive the rear wheel while your pedals are not moving, but…you can still add pedaling when you desire.


Because Roy wanted to experiment, he designed a dual reduction that could pivot at the central jackshaft which connects the two reductions. This allowed him to change it’s shape to a “V”, an “L” or in-line so it could be long and thin. He settled on an L-shape, so it would fit snugly inside the frame triangle. Even at this stage, the quality of his work really knocked me out.

After custom fabrication of the aluminum cases that enclose the belt-reductions, Roy had them black-anodised, and the results are nothing short of fantastic. They have an almost military look that provides a very professional finish.

Roy openly confesses that his work back in 2009 was directly inspired by the beautiful drives made by ES member recumpense (Matt Schumacher), and everyone here at has also been a fan of Matts work from the very beginning.

“…Building this bike has been great fun and a personally rewarding experience. I built my first running E-bike with relatively simple tools and some outside machine shop work. It’s an attractive machine that can go faster than I am comfortable riding. It’s a great test bed for future projects, and it’s easy to remove the drive unit from bike to work on it…”


Towerpro brushless outrunner RC motor, 63mm diameter, 235-Kv

Castle Creations HV-110A Electronic Speed Controller (ESC)

10S-LiPo battery (36V), 10-Ah, later reconfigured for: 5S-LiPo (18V), 20-Ah

Battery enclosed in a water-proof Pelican case on a rear cargo rack.

The throttle is a Magura that is wired into a servo-tester to provide a signal to the RC-ESC

drive belts are 5mm-pitch GT2 X 15mm-wide

Drive unit ratios:
15T/56T primary belt drive, motor to jackshaft
20T / 56T secondary belt drive, jackshaft to drive output shaft

15T / 56T = 3.7:1
20T / 56T = 2.8:1
3.7 * 2.8 = 10.4:1 reduction between the motor and the drive output freewheel

Then, 16T / 48T = 3.0:1 between the drive output and the crankset chainring
10.4 * 3 = 31.4 total reduction between the motor and the BB at the crank

The Towerpro RC motor is rated at 235 rpm/volt
235 * 36 (volts) * 0.85 (efficiency factor) = 7191-RPM’s at top speed

7191 motor-RPM’s / 31.4 reduction ratio = 229 rpm @ the crank

Shimano Nexus 3-speed Internally Geared Hub (IGH), with a disc brake, and a Surly chain tensioner

Tires are Bontrager Hank 2.2 slicks for both the front and back

“…I had to replace the secondary belt with a #25 chain and a tensioner. The belt had started to delaminate. It made this loud low pitched “thumping” noise at slow speeds. I still have a 5mm-pitch 15mm-wide belt for the primary drive…”

“…I took the voltage down from 36V to 18V and now the motor is not getting overheated unless I hold WOT [Wide Open Throttle] for about 5 minutes – then it gets hot. I tend to ride about 15-20mph most of the time while under electric power which is about 2/3 throttle position in top gear. It tops out at about 25-MPH now which is OK with me. I originally wanted to go faster [40-MPH at 36V], but I am quite happy with this performance…”

“…For my next project I will use a larger motor and higher voltage. I probably will not make another through the crank BB-drive system. Don’t get me wrong…it is working very well on the Cortina. I just want more options for keeping the cranks and final drive ratios separate…”

“…My main problem areas have been the slipping secondary belt and going through a lot of worn freewheels, and also the small Towerpro motor is running hot. Not sure what the actual temp is but at times it’s too hot to keep my hand on. It’s hard to access the drive unit internals for easy repair, and the [secondary] belts do not seem to hold up to the torque, except for the primary drive reduction, its fine…”


 V2, April 2011

After the first version was completed, Roy had set a high bar for himself with the quality of the build, but…he wanted to improve the several issues that had been revealed.  I chose the pic of the V2 for this article header because that was the first version I saw, and it is also the version that has been published in several magazines. A central battery, combined with a low and central mid-drive that gives the motor the use of at least 3 gears is a configuration that is really proving itself to be popular these days…and Roy was doing this starting in 2009!


The most immediate visual change that I noticed in the second version is that…Roy moved the battery from a case on the rear cargo rack to an absolutely awesome triangle enclosure. Three metal tabs were welded into the corners of the frame triangle, and small plates connected these tabs to an aluminum sheet that forms a central backbone, and it also divides the pack, with half on each side.


After the first glance, the second thing that jumps out is the completely new drive unit. Roy consulted with Matt Schumacher and moved up to a larger Astro 3220 inrunner, using a single reduction with a chain instead of a belt. Using a chain as a high-RPM primary reduction might be slightly louder than a belt, but it does allow for a greater reduction in one step because of a small drive-sprocket. In spite of any additional noise that may be caused, a primary chain will not slip, which might be an issue with a high-powered belt-drive.

Also, this drives output shaft is connected directly to the rear wheels’ sprocket cluster. A more common method would be to reduce the drives RPMs down to the BB pedal-speed, and then immediately gear the bicycle’s main drive chain back up from the BB to the rear wheel…which is normally about a 1:4/3:1 ratio. Between the drive bypassing the BB and also the very large motor reduction, Roy could achieve his goals with a single reduction.







Motor: Astro 3220 (5 turn)

Primary Drive Assembly: Matt Schumaker

Tube Clamps: Self made

Torque Limiting slip-clutch: Dalton OSD-225

Motor Drive Freewheel: Try-All 108.9 16T

Controller: moved up from the HV110A to the HV160A + 2 additional Capacitors

Battery: LiPo – 44V 10-Ah, 12S / 2P, 25C, 6S/5000-mAh in each brick

Electronics: Matt put the servo tester and BEC into a small electronics project box

PreCharge anti-spark resistor: 10 watt, 1-ohm

Throttle: Magura

Rear Hub: Shimano Nexus 3-Speed

Sprockets at rear hub: Combination of Gary Goodrums adapter [], Big Cheese adapter modded to fit the 16T Nexus sprocket, a 50T chainring and Shimano 18T Freewheel

Chains: Pedal and motor powered drive chains are both Wipperman 108, primary drive chain is #25

Bottom Bracket Cartridge: Sick Bike Parts, 153 mm wide


Bike Frame: Cortina Triton DS CroMo []

Forks: Marzocchi Bomber

Rims & Tires: 26-inch CST Cyclops 26 X 2.4

Weight: 59 pounds

Primary chain reduction: 12T/45T  3.7:1
Secondary chain reduction: 16T/50T  3.1:1
Overall reduction: 11.7:1

“…Goes like a bat out of hell geared for 45-mph. So far I have only had it up to 35-MPH and it feels very stable. Issue: The triangle is too small, I wanted to get 8 packs in it, but…I only have four now with the possibility to add another two. The torque-limiter seems to grab and not slip smoothly – feels a bit like a slipping timing belt. Still working out how much pressure to apply, after every ride I adjust it some. This thing gets hot, I measured over 160 degrees F after a short ride. I think this means its slipping a lot…not good”

“….I took off the torque-limiter because it was causing trouble, but then…the Nexus 3-speed croaked. First gear is inoperable while I still have 2nd & 3rd…The 3220 is running at about 170F (77C)…I am going to rewind a [larger]  80 X 85 and try it out.

A very interesting bit of news is that the V2 cortina was raced at Willow Springs Karting race course in California in 2011.


V3, May 2012

The single reduction chain on the previous V2 managed to eliminate any slipping, but chains are noisier than belts. For this version, Roy went back to a quiet belt-drive, but since the previous 15mm wide size that he used earlier on the V1 would slip too much (damaging the belt), he moved up to a 25mm wide Gates Powergrip GT belt.

However…even with the wider belt, Roy needed to add an idler that would not only take up any slack, it would also “back bend” the belt to increase the number of drive-pulley teeth that the belt engaged (to eliminate all slippage).

Primary Drive Assembly by Roy (RWP)
Primary stage is 95T / 20T, or 4.7:1

The stock Turnigy 8085 motor I am currently using is 170kv (80mm diameter, 85mm long)

“…It’s geared to top out in each of the three gears at 25mph / 31mph / 39mph – so attempting to add pedaling is a joke. The top pedal-gear is the only useful gear for pedaling, since the bike accelerates so quickly…”

After more research, Roy moved up from the Castle Creations HV-110 to a HV160 on the V3.

Batteries: six 25C 6S Turnigy batteries, 44V / 15-Ah.

13T ACS-Crossfire freewheel on Matt’s shaft adapter.

Stand alone high amp Cycle Analyst.

Because both chains are vary strong BMX, I modified the derailleur to allow for the wider chain and I used only three sprockets on the cassette, with two spacers between each sprocket to accommodate the wider chain. Shifting is 2-clicks of the trigger shifters for each gear.

Roy switched to smaller 24-inch wheels so he could swap up to fatter tires, and he now is using Hookworms. The lower 170-Kv of this motor means that Roy can use 44V, but still avoids needing a second reduction. The RPMs of the drive were low enough that he could go back to using a “trials” freewheeling BB, which simplified the adjustment of his chainline (this is the number of millimeters between the centerline of the bike, and the center of the drive-chain to the rear wheel).




In the pic below, you can see the bright red “kill switch” that he has mounted within easy reach, and every E-bike should have one of those. Notice that Roy has changed the rear wheel to external sprockets with a common derailleur, since these can take more power than the available Internally-Geared-Hubs (IGH’s), and…this version of his Cortina definitely puts out plenty of power.

For more pics of the V3 click here.