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Custom Build Gallery, Mark’s Cromotor Phatrod

August 19, 2015

This E-bike’s design is not subtle or sophisticated, it’s very simple. Take a large rear hub-motor, and feed it a huge amount of watts. It is a hot rod for the street, and it is all about power and style.

If you are interested in something similar, the first consideration is to find a bike frame that you like, and that also has someplace in the frame to mount a very large battery pack. Here, an endless-sphere.com forum member Mark from from Spokane, Washington (username: “Brake“) chose the Electra Straight 8 as a starting point.


Let’s get started!

After the frame question was settled, he had to decide what motor and voltage would provide him with the kind of performance he wanted.



Motorcycle rims, tires, and spokes are much stronger than typical bicycle components.


Mark chose the Monster Cromotor, from Zelena Vozila (“Green Vehicles” in Croatian). The rear wheel has a 17-inch  by 1.6 wide rim from the rear of a 1982 Honda XL 250R dirt bike. The rear tire is a Full Bore M-66 King Tour 100/80-17, originally spec’t for the front of a Harley Davidson (and rated for 150-MPH). Read our article on how to use moped rims and tires on a large rear hub. And…if you are interested in a rear hub that is a little bigger or a little smaller than this one, read our article on large hot rod hubmotors.



The new wheel next to the stock Straight-8. Notice to ventilation holes in the side of the Cromotor to let the heat out.


One of the things Mark liked about the original Straight 8 was the checkered flag pattern on the rim, along with the distinctive red spokes. He ordered custom-length spokes from John Rob Holmes, and after they arrived, Mark used a very tough powder coating (stronger than paint) to make them a fire-engine red. Mark also found a vinyl “wrap” with a black & White checkered pattern. The wrap was difficult to heat and form, but the results were well worth the effort.



Both sides get their own beefy torque-arm, with each having two clamping bolts, and made to be as thick as possible.


Since the frame is made from aluminum, Mark decided to make the custom drop-outs from aluminum to make the welding options simpler. In order for aluminum torque-arms to work on a motor this powerful, they had to be very thick, and also have clamping bolts built-in. He bought a TIG-welder, and got to work.



Mark made a temporary alignment jig, to hold the parts straight when TIG-welding a new rear frame section on. The pink bike in the background belongs to his battery engineering consultant (seen below).


As long as Mark knew he was going to have to weld-up new clamping drop-outs to the rear of the frame, he also decided to take this opportunity to make an entirely new frame section for the back half that was stronger and longer. The added length will help to tone down the tendency of the stock bike to wheelie, when using this much power.



The new rear frame section used larger and stronger tubing, and that also allowed Mark to run the motor phase-wires through the lower left chain-stay arm. The red arrows show where the wires enter and leave.

Something very interesting that Mark was able to do with the larger diameter tubes in the newly fabricated section, is to run the motor phase wires through the hollow chain stay. This type of custom detail really knocked me out, and I really appreciate Marks extra effort.



A quick test fitting of all the parts before final prime-and-paint.


This may seem like a lot of time and effort to extend the frame just four inches, but I think the results are well worth it.



Mark purchased a front hub that had the mounting flange on the left for a disc, and laced the front rim to it.



After a disc-hub was laced to the rim, Mark can now see where the caliper-mount needs to be welded.


He chose the popular Avid BB7 cable-operated caliper, and the disc is a 203mm diameter Shimano Ice. The frame is definitely aluminum, but…the fork is cromo steel, so he machined a mount and properly welded it on. for the rear, he has been using regenerative braking (regen) as a magnetic brake.


Battery Pack Housing

One of the major reasons Mark chose this frame is the generous space in the center of it, to allow for a significant battery pack. He decided to make top and bottom housing supports from curved aluminum. For this, Mark dusted off an old body-panel forming trick from decades ago, used in the European custom sports-car prototyping tradition. He made and shaped wooden “styling bucks”.



Even in this modern age, some custom “one off” parts are rapidly designed and created using old methods. Take a flat sheet of aluminum, and pound it over a wooden shape.


“…I made wood forms. Annealed the 6061 T6 and beat it into shape. Had to anneal several times. This was tough…”

There was a time in England and Italy, when skilled craftsmen were very affordable to hire, and wealthy clients could order a unique “one off” custom body, made to bolt onto a specific frame. King Leopold III of Belgium once purchased a 1953 Ferrari 342, but had a custom cabriolet body made to his design by the Italian studio Pininfarina, so that…his Ferrari didn’t look like any other.



Fitting the various pieces of the battery housing before TIG-welding them together. The vertical box just behind the seat-tube is the Addapto Max-E controller.


After all the effort required to do a good job using the wooden styling bucks, Mark decided to buy an English Wheel to try his hand at forming compound-curved aluminum fenders with that.



Its pretty amazing that he started out with a flat sheet of aluminum.



A custom-built rear fender is an impressive feature. Hey Mark, whats under the tarp?



The custom front fender is even more impressive with an awesome (but understated) paint scheme.


I love the shiny-black next to flat-black styling theme on the front fender. It reminds me of the beautiful black-on-black native-American Martinez pottery from New Mexico.


Building a Battery Pack



The box at the top left is an order of high-quality cells, just like the ones used in the Tesla electric car. At the bottom right is a wooden form that matches his bike frame triangle, so he can see how many it’s possible to fit.




After determining the maximum number of cells that can fit, Mark and his battery pack consultant (shown here) used “hot glue”to bond the cells together so they can determine the proper shape of the electrical bus-plates to create the series and parallel groups. These Panasonic cells can be purchased with blue or clear plastic shrink-wrap.



Here, Mark is spot-welding the nickel bus-plates onto the cells.


If you look closely, you can see the white fiberglass insulating cloth between the parallel groups to reduce chafing wear from vibration over time. On an 18650-format cell (18mm diameter, 65mm long), the entire bottom and sides are the negative electrode (anode), and only the small button tip is the positive electrode (cathode). Some of these cells look like their sides are bare metal, but they have a clear shrink-wrap on them.



86V X 165 peak amps = 14.2 kW of acceleration power.


“…This 3-kWH Tesla-cell pack is a huge improvement, my range is now somewhere between 50 and 100 miles…”

This is a 21S / 12P pack, using 252 of the 18650-format cells. If you use the recommended charging protocol of 4.1V per cell, a 21S arrangement provides 86.1V when the pack is fully charged.  The Tesla-Panasonic NCR18650B cells are 3400-mAh each, so a 12P pack provides a whopping 40.8-Ah of range.

The Watt-Hours (Wh) of a battery pack are Volts X Amp-Hours, so 86.1 X 40.8 = 3512 Wh


A few acessories

The tail-light, and “Cycle Luminator” headlight are from Grin Technologies in Vancouver, Canada.

Mark also chose the 2-speed Metropolis Patterson” bottom-bracket crank-set. Since the second gear is an overdrive, this compact unit equals a 60T chainring, to make pedaling along with a fast E-bike a little easier.



The 2-speed Metropolis Patterson crankset is growing in popularity.


I would be reluctant to run this much power through a hardtail frame (even with such fat tires), and in order to take some of the jolts of the road out of the ride, Mark upgraded to a suspension seat-post. we like these too, so read our article on the various suspension seat-post options.



A fat cruiser seat mounted on the Thudbuster “Long Travel ” (LT) suspension seat-post.


The Inspiration

Back in March of 2013, an ES member “willow” posted a beautiful Electra Straight-8, and the hand-made leather battery enclosure really caught my attention.  Mark saw this too, and decided that he wanted to use this same model as the basis for his own version.



Another custom E-bike using the Electra Straight-8 frame


“… The bike has been fantastic, I absolutely love it. There is no way I could imagine living without an ebike now….”



This street hot rod is not only stylish and powerful, it runs almost silently through the surrounding hills.


Here’s a 6-minute video. At 2:12, he does a 0-50 MPH run up in about 3 seconds! and no…its not for sale.



Written by Ron/spinningmagnets, August 2015




Grew up in Los Angeles California, US Navy submarine mechanic from 1977-81/SanDiego. Hydraulic mechanic in the 1980's/Los Angeles. Heavy equipment operator in the 1990's/traveled to various locations. Dump truck driver in the 2000's/SW Utah. Currently a water plant operator since 2010/NW Kansas


  1. Will they ever make them for sale? Approx how much would it be in USD?

  2. Nice ride!
    What spot welder did you use?
    Cheers … Chris

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