Dave's 80:1 RC mid drive kit build log

Jun 6, 2015
Seattle, WA

Firstly, amazing community everybody has built here, it’s inspirational.
This thread is for technical and general discussion of the kit offered by Tangent Motor Company, here:



This setup is completely custom, all parts designed and machined in house (by me). What we have is an internal 40:1 reduction and a secondary 2:1 reduction between the drive unit and crankset, yielding our overall 80:1 advantage. The drive unit turns the crankset at a maximum cadence of 130RPM (@50V) which matches nicely to our human pace. Selecting the proper gear is straightforward. The reduction unit weighs just over 3lbs, making it 5.8lbs or so with the Astro.





Bare drive startup: https://www.youtube.com/watch?v=3cwpTn6w3hY
10th gear on-stand test: https://www.youtube.com/watch?v=UTVK2UvUka0

The drive unit also houses an Astro Flight 32XX motor (I’m using a 3210 7-turn). The huge reduction allows for the full 10kRPM range of the Astro, and is it fun. Rated for 1350W continuous, 4000W peak, this thing moves. A Castle Creations HV80 is run sensorless without issues or overheating and can get me started on any hill so long as I can manage to stay on the bike (low gears and a steep hill make for hill climbing fun). The motor has turned several times before the slack is even taken out of the chain- hard riding doesn’t heat the ESC over 120 deg. A Cycle Analyst with shunt provide throttle input (current control) to the ESC. Power is very controllable- I power wheelie that Kona in the pictures in every gear but 10th. The CA allows for easy tuning of the throttle response and power limits.

Range and efficiency are very good. With chunky street slicks I’m averaging 22Wh/mi at 28mph. Top speed is dependent on the bicycle gearing as this unit is able to max out the gearing in almost any situation. 500W pushes the Kona to 25mph without pedaling, 1300W yields 30mph up a steep hill.

The bicycle drivetrain is holding the power well. A quality bicycle chain (and master link) has held up to over a month of abuse. I use a Surly stainless steel chainring on the power side and a quality aluminum chainring on the bicycle drive. I find that I don’t need all 10 gears of the bicycle and normally shift two at a time- I can’t keep up with the acceleration of the drive unit.

Maintenance is easy. A small shot of special grease through the fill port every 6hrs. It’s easier and cleaner than maintaining a bicycle chain. The unit is sealed against contaminants and is completely user serviceable.

For batteries I’m using two 6S Turnigy Multistar 16Ah packs in parallel from Hobbyking. These are 10C batteries and provide an excellent balance between performance and charging convenience. I ride with these in a backpack to keep the weight on the bike to a minimum. I’ve had more than a couple get-offs while riding and haven’t had any problems with the connectors pulling out when they need to. After a few rides, you won’t notice the cable between yourself and the bike anymore, but mount your batteries where you want. Ripple voltage is not an issue (as logged by the ESC) even with the batteries in a backpack due to the reduced current demands of the system.


A taste of what it's like to ride: https://www.youtube.com/watch?v=oZygysNgh_A

The unit runs warm. Heavy abuse will make it hot to touch, just make sure there’s airflow if you’re keeping the wheel in the air like I do. On the street with more speed, I run 1000W for 30mph all day and peak 2500W to keep the acceleration fun and the unit is only warm, no more. If the trail is relatively flat, 700W/hrs of battery keeps me at race pace for an hour, two hours of normal riding. Tight trails I tend to turn the power down while leaving the response crisp for amazing control over front-end lightness.

Its 40:1, about an inch and half wide, fits on the end of the motor. It allows the rest of the kit to be as awesome as it is. Any other question or concern is fair game. BTW, what I'm riding is the 42nd prototype.

won’t be discussing what happening inside the reduction geartrain, sorry. It is 40:1, about an inch and half wide, fits on the end of the motor, and is powered by fairy dust. It allows the rest of the kit to be as awesome as it is. Any other question or concern is fair game. BTW, what I'm riding is the 42nd prototype.

Nice build log....but seriously? fairy dust? its a planetary (edit: epicycloid) reduction.

It is nicely executed & I love the size form...not the hulking gng or derivatives.
another big + is no giant belt & pulley hanging out the side (harking back to the days of the steam driven tractor)

Since your apparently not talking about the Jazz that makes this an interesting product, this is simply a misplaced commercial for another boutique serial drive.

congratulations on mining the forum's for ideas & information. And thanks for taking the time to actually create an account to SELL your whiz bang mountain stomper.
Come & get it consumers.
nutspecial said:
It's not for sale yet, so what's the issue? And if op does sell, I still see no issue.
All societal 'progress' is just building on previous accomplishments imo.

here it is..& "special" pricing for forum members.... :roll:

This is a free forum, kept free by a free thinking individual....for the sole purpose of free idea exchange.
my point is clear. the only thing interesting about this is the reduction unit....& since its a "fairy dust" redesign of any of a thousand planetary reductions...this guy thinks its something special.

it may be the best serial drive to date.....big frocking deal.

keep it in the sales section unless your willing to show the work & have a little faith in your fellow man to do the right thing.
Guys, let's not judge the topic starter for his desire to keep some details secret. The potential product looks great and it seems like a lot of effort and talent was put into it.

While I do understand the point of the op I don't share the approach.

Here are my thoughts:
* The first curious customer will disassemble the reduction gear, make a dozen of photos and the mystery will be over. At least if there will be no factory seal :D
* The real value of the quality reduction gear lies not in a blueprint, but in the precise manufacturing and materials. Only a few of us have required skills and access to the mills/lathes, and even if we've had it all, the question of right materials and thermal treatment still remains open. The art of a mechanical engineering is not only in making the system work in the first place, but making it work in a given conditions for a long time without wear and fatique of metal.

Any design sooner or later is going to be reverse-engineered, but only a few are capable of copying it.
I believe selling to the DIY market requires a different approach to the normal consumer markets. "Proprietary" is a dirty word for many DIYers :)
I like it and kudo's to the build. He has every dam right to market it within copyright law etc. The point is, if you do get out-engineered and can't stay on top with your creation, (this is inevitable for the small business- it is happening to me btw) hopefully you made the best of being part of something bigger. I have a feeling the free-thinkers would agree.

Dave has engineered what looks like a very capable system, and there's no need to get bent even if he can take the market with it is there? Such is life, is it not? All we can ever leave with is our experiences and impact, whether that be positive or negative is up to us.
Ok, I'm out on this subject, not gonna mess up Dave's thread.
Very nice looking clean package.

My bet its a double reduction planetary gear. Too narrow to be a triple reduction and have any decent gear life.

Its nearly impossible to put much intellectual property protection on this. Its certainly not "new art" as required for a patent and copyrights won't be of any real value either. However a well designed package at a reasonable price will be all that's needed for commercial protection.

One of the triple reduction units I designed for trucks was machined up the road in Arlington Wa at Newell Corp is pictured below:

The inside is patent-pending, spent a year and couple credit cards making it work the way it does. I'm betting the first person to buy is going to open it, I made it so it's serviceable. That's fine. Last year I started with a cycloidal reducer and began tinkering. Obsession can pay off.

The important part is what we can do with an 80:1 reduction. Look how simple it makes the whole setup- all the work is done within the drive unit. This is tough as nails too. I beat the crap outta this thing and it loves it. If you turn the power down on the CA, this setup behaves nicely. Compared to the expensive commercial mid-drives I've ridden, this kit can drive just as smoothly but also has the power to be exciting, not just easy pedaling.

With the 10C batteries, even though they are 16Ah, I have to turn down the amperage gain on the Cycle Analyst to keep a smooth acceleration. I have lower capacity 65C packs on the way, we'll see if they can hold the power better. The Castle Creations ESC works infinitely better than a chinese ESC (which have a hard time getting the motor started). Basically, if I can pedal away from wherever I'm stopped, the CC ESC can get the bike started. I'm sure their startup algorithm isn't as sophisticated as it could be either, meaning there's still room for improvement.

Inside the drive is nothing but a geartrain. The Astro takes up almost all the room inside the housing, there's not much tinkering that can be done to it.

Nice work, unfortunately it's way too noisy for the local trails around here. Sounds even noisier or higher pitch than my cyclone, which I rarely take to the trails these days due to the noise.

Motor mount is nicely done, very minimal.
The camera picks up the sound of the drive. It's actually about as loud as knobby mountain bike tires at 20mph when you're at full power. It's almost silent under light load, 300W or so.

kfong said:
Nice work, unfortunately it's way too noisy for the local trails around here. Sounds even noisier or higher pitch than my cyclone, which I rarely take to the trails these days due to the noise.

I would have to agree. The design and machine work is beautiful. It's compact and nicely packaged. But in the video it almost sounds like a gasser. I would surely get kicked out of my trails around here with this. You could potentially have a real winner if you could address the noise issue.
Try posting a video of a drive by. That should give a better indication of sound levels to us. I went with a bbs02 for a middrive trail setup, but if yours is just as quiet. Then I'm sure a lot of us would be interested. It certainly sounds more capable than the Bangfun. My current trail setup these days is the geared BMC motor, which has some noise but only in high loads.

My guess for such a high internal gearing is a harmonic drive, but not sure if they can handle such power levels. Seems too small of a package for a planetary. I picked up a harmonic to do something similar but it didn't look beefy enough for the purpose. I would of wanted it twice the size, but might still try to make a low power compact setup or look for a bigger version.
Same helmet cam, without the plastic crash case:


Here's the guts of the unit, I'd rather not talk about the alterations I've made to adapt the cycloidal reducers to run at 10kRPM and be this tiny for this cost. This is real, it works great. Sorry it's a surprise.


Cool, a real harmonic with some bite. This design has some potential, you should do well. Do you sell just the harmonic for us who have a full shop. PM me.

Bummer, just now was able to view the video. The noise is apparent, have you tried oil filled to reduce it or vibration dampeners so it doesn't use the frame to amplify the sound?
1st, thanks for the responses & video.

I hope i didn't sound too harsh.....but we are bombarded with comercials all day every day.
This forum is solice from that..

Now that the genie is out of the bottle, mad props for getting it prototyped & going into production.

As I stated before:
+1 on the size/weight....very well done.
+1 on the asthetics...it is very good looking.
It elegantly solves most of my gripes with 99% of the other available crank drives.

The only negatives will be the sound it makes (a strike against all us using small-high revving motor users) & ultimatly pricing....
this is obviously a high-end unit, and I am not saying its over priced....but @ a $1400 (thats the discount price iirc) it will be a bit harder to justify for many. At least in my neck of the woods.

Maybe I am just cheap. :mrgreen:(I can't justify the cost of the 3210 alone)
Looking foward to some unbiased reviews if at all possible.

Nice work man. Compact and light weight. Some pretty good craftsmanship went into the design and build. Looks well thought out.
Looking forward to see people start making reports. If you can sell this at a price people accept I am sure customers will their way to your shop. Seems even a 3220 will fit into the drive unit. Think your reduction can handle twice the power from a 3220? Have you noticed any chain/sprocket wear?
On fresh bicycle cassette, chain and chainring I haven't noticed any wear yet after a month of use. The first sprocket I used off the motor was a nice aluminum bicycle sprocket, it lasted two weeks and I had to turn it around once. The stainless chainwheel shows no wear and is nice and shiny.

The unit is filled with grade 00 grease, so light if feels like cold air. When the unit freewheels, it makes the most sound as there's no load to dampen the backlash.

I didn't mean to pitch a commercial in here, that's not why I come here either. Just figured people here would be interested in more power with less weight. Thanks for the understanding, I meant to start a discussion around the motor, controller and batteries now that we have a huge, convenient reduction unit.

Yes, if you already have an Astro Flight motor, I will make you a drive unit. The 3210, 3215 and 3220 will all fit (the 3205 will also if I shorten the housing, no biggie). Contact me on the other thread if interested.

Harmonic drives have been discussed before (very well done, BTW), but I have a question. As odd as this may sound, I am wondering if there is any potential to make a similar drive that is only 40:1, rather than 80:1

Most geared hub motors only have a 5:1 planetary reduction, simply because of gear-size restraints of the planetary configuration. If this reduction works as well as you have posted and truly stands up to long-term abuse (it will take several hands-on customers agreeing to get the majority to believe you), then it may be a principle that is well worth exploring further.

What motors do I imagine a harmonic reduction being used on? That depends totally on the design constraints of that reduction principle. I am very impressed by how compact it is.