The Revel Propulsion Lightweight 450W Mid Drive

July 27, 2019
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This is a new product, but it’s coming from someone we have known for a long time. Paul Daniel was formerly an engineer at Brammo electric motorcycles, and he was racing ebikes back in 2011. He also built a custom ebike that we wrote about back in 2014 (click here).

Sharp readers of electricbike.com might recognize that name from the picture of Paul brazing a custom frame, which is the pic we used as the header for our “40 custom electric bikes” article (click here). So…let’s quit the chatter and get to the kit!

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The Motor

Paul raced the prototype at the Sea Otter Classic in 2017. racing is something that shows that a company is serious. This, way, the designers know exactly how capable the kit is, and where the limits are if it is pushed too hard. This is vital to getting the controller programming dialed-in so you can get the best possible performance and efficiency without any fear of damage.

The 2017 prototype that was raced at the Sea Otter Classic

A quick glance at the pic is all you need, to see that the custom motor case is CNC-machined, the aluminum mounting bracket is water-jetted, and the Electronic Speed Controller (ESC) has a cover that was made on a 3D-printer. These three custom production methods are all a part of “Computer Aided Design” (click here), and they allow a designer to rapidly produce several improving prototypes, instead of waiting for a fabricator in another company to finish making your parts for a design upgrade.

As you can see in the header pic, the secondary chain from the drive output to the bottom bracket is a very strong #415 that is normally used on Kart racers. It has a smaller link than standard bicycle chain, roughly 2/3rds the size, and this allows the sprockets to be smaller while still providing the desired reduction in RPM’s.

Inside the housing, the primary reduction uses planetary gears. Paul chose helical teeth to help the drive remain as quiet as possible. This dual-stage RPM reduction allows the lightweight motor to provide a surprising amount of torque.

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The Freewheel

The additional freewheel of a mid drive allows the motor to drive the wheels, while the pedals are stationary. This is important because if it was a solid connection (like the stock sprocket and crank assembly), the pedals would always be spinning when the motor is on. A rider being in that situation is an issue when the rider is leaned over in a turn, and the pedal on the low-side spins around and hits the ground. A pedal-strike can send the ebike and rider flying, causing an unnecessary injury, or possibly losing a race.

The 3D-printed prototype of the proprietary freewheel.

I was very surprised that Paul put the time, money, and effort to develop his own freewheel, when there are several freewheels that are commonly used, and readily available (White-Industries ENO, ACS Crossfire, etc). Paul stated that they liked the impressive Sempu torque-sensing bottom bracket a lot, and no existing freewheel would attach to it (most mid drive freewheels attach to the threaded boss on a BMX pedal arm).

But something else popped-out at me. As long as you are designing a completely new freewheel, the diameter on this one is larger than normal. Most bicycle components are typically designed to be as light and as small as possible. A smaller freewheel would also have slightly less friction. The freewheels I listed above were never designed for electric bicycles. They may have proven over time to be the strongest of the available options (through trial and error), but…they were designed for the 250W legs of an athletic bicyclist.

One of the engineering tidbits I picked up when I researched “Internally Geared Hubs” IGH’s (click here), is that the farther away you locate the pawls from the center of the hub, the stronger the freewheel will be, while still using the same components found in common hubs. Notice in the picture above, the 3D-printed parts are the custom components, but the pawls and springs are off-the-shelf bits (why reinvent the wheel?).

I have no way to prove it just yet (years ago I worked as a tech in the load-testing of aircraft parts, both lifetime endurance load cycling, along with peak loads until failure). But at a casual glance, I suspect that this is likely to be the strongest mid drive freewheel currently available, due mostly to it’s larger diameter. That being said, I am not an engineer, nor do I play one on TV.

The Revel propulsion freewheel and 104-BCD spider.

This proprietary freewheel is designed to bolt onto a Sempu torque-sensing bottom bracket, while still being the largest diameter that can fit inside a common 104-BCD spider.

The Sempu torque-sensing bottom-bracket with the proprietary Revel freewheel on a 104mm spider

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The Battery

Revel is making their own battery packs. The initial reason is because nobody is making one that meets their size and shape requirements. Paul wanted the base-model pack to attach to the common “water bottle” mounts on the downtube of a frame. He also wanted the pack to be reasonably small so it would be light, and would also fit into a variety of frames.

The 36V pack is configured as a handy 10S / 3P using the well-regarded MJ1 cell from LG-Chem is South Korea. This is one of my top-three cells to recommend when asked about building a pack. At 3500-mAh per cell, a 3P size will provide 10.5-Ah of range. Also, the factory rates these at 10A per cell, so you can depend on getting 30A peaks from this pack without overheating the cells. 36V X 30A = 1,000W, and that’s important because of the last feature I want to mention…

This pack is fully potted. That means that all of the components are surrounded by a thermally-conductive and flame-retardant polyurethane, so there is no airspace left in it. This provides the most extreme water-proofing and shock-resistance that is possible. Zero motorcycles was the first manufacturer I can think of to take this bold design move (at 102V nominal), and they have enjoyed great success with it. The Luna Cycles V2 Wolf pack is another example (at 52V). There’s one more ebike battery provider that is potted, but they have requested that I do not mention them.

If you are only drawing 450W from a pack that is capable of 1,000W, then the battery pack will never get hot…

The orange square is a rubberized water-proof cover over the on/off button, and the five white dots are LEDs to indicate the state of charge. Fully potting a pack is a bold move, but this method is the most robust that you can make a pack.

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Revel Propulsion

Pauls initial prototype was designed to be a smooth and intuitive addition to an off-roader, where technical trails sometimes need a precise touch. He also felt that a lightweight road bike would also be a big market for a drive like this. This is why the primary design was focused on a “Pedal Assist Sensor” / PAS (torque-sensing), using using a new unit from Sempu. And to be fair, if you are always pedaling, the battery will last a very long time.

Also, in view of the changing ebike laws, Paul also wanted this first version of his system to be “Class-1” compliant, which can have a few benefits on certain state and federal park trails. Paul and Derek will customize the programming in these kits to match your specified maximum legal speed, which depends on the laws in your location.

As soon as he was in development, a few observers asked for a hand-throttle option. If there is enough demand, Paul mentioned that he may add that feature in the future. Just be aware that if you like to use the hand-throttle most of the time, you will not get the full range that your ebike is capable of.

This is the full kit, with the Sempu torque-sensing bottom bracket
The Revel drive on a Rocky Mountain Blizzard
One of my favorite cargobikes of all time, the Yuba Sweet Curry. I’ve always liked longtail cargobikes, but the smaller 20-inch wheel on the rear of this model lowers the cargo-rack, and it’s awesome.
A carbon fiber Marin Cortina road bike
Salsa Fargo with gravel tires, using Jones handlebars. This location is near Ashland Oregon, USA

Revel Propulsion’s home website can be found by clicking here.

Their Facebook page can be found by clicking here.

Paul is from Ashland Oregon, Just over the Northern California border, and you can contact Paul or his business partner Derek at

info@revelpropulsion.com

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Written by Ron/spinningmagnets, July 2019

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


2 Comments

  1. 350 watt motor on 36V has a mac current of 20A. The battery has a peak of 30A and when brand new it will not really heat. However after a bit of ageing it will start heating up. I will not build a 30A configuration on a bike with more than 250 watt. For a 350 watt I only build minimum 40A.

  2. Hi Tikki, the kit is actually 450W peak (250W continuous), which is a peak of 15 amps (at 0% SOC, less at higher SOC), or 5A per cell. About half the max rating of the cell. Yes, heating will increase as cells age, but there is a lot of overhead, and the potting spreads heat effectively.

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