Carn Bikes frame #3 - Freeflow e-hardtail

The repairs on frame #2 are in progress, the gravel thing I was working on has gone on the back burner, and the Freeflow Technologies motor system has arrived. It’s time to build a downcountry e-hardtail!

Design goals for this are to keep the riding characteristics of my acoustic hardtail (fast, composed, balanced, playful). Some tweaks have been necessitated by the motor size (longer chainstays) and dynamics of riding a powered bike (keeping the front wheel on the ground under power). I’d love to aim for a 16kg (36lb) AUW but, given budgetary constraints, I think it’ll be closer to 18kg (40lb). Still pretty respectable for an e-bike and a heck of a lot easier to lift over a gate than my current Giant eMTB.

Tubing will be cromoly steel, probably plain gauge subject to what Ceeway can supply me, except the seat tube which has already been sourced from FBS following recommendations from the previous build. Dropouts are Metal Guru UDH cast steel. Motor shell supplied by Freeflow. Drivetrain is SRAM GX Transmission. Forks are RS Revelation 27.5" 150mm travel.

Have at it :smiley:


Would you be willing to share what the Free Flow Technologies system cost you? Motor + battery + cabinet and whatever else you needed.

Freeflow don’t publish their prices publicly that I know of so I won’t do so here either, but prices did seem reasonable to me and they were happy to speak to me as a hobby builder, which differs from other major players. They’re very friendly and easy to deal with too, although I did find there were short delays in responding sometimes. Being a small company, I imagine there’s a lot going on so don’t be afraid to nudge them if they don’t get back to you in a reasonable time.

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In case anyone’s interested, here are some photos of the system as delivered. They offer motor shells in steel and aluminium for custom builders but also publish full design docs on purchase to enable builders in other material or people who want to make their own motor interface. They don’t supply a dummy motor to mount the shell in a frame jig so one of these will need to be built. My dummy motor is in progress and I’ll publish designs here when complete.

The system is designed around a 53mm chainline and has a 5-arm 110mm BCD chainring mount. I’ve bought a 36T Hope Retainer ring for it which claims SRAM Transmission compatibility. I’m going to try it with the default 53mm chainline as that’s within SRAM’s published specs but have the option of fitting chainring spacers if needed.


I’m really into in FreeFlow and your design looks a lot like what I had in mind for my first one. I’ll be watching with great interest, best of luck with the build.
I’ve reached out and had positive responses from them which is refreshing as a small scale guy. I actually feel bad I haven’t had the time I’d like to get stuck into sorting out my own design. But following a quick n dirty copy/ paste into our current designs it looks like integration would be about as simple as it can get (not necessarily actually simple).

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Thanks. I’ve been following you on Instagram for a while and really like your bikes. We definitely come from the same school of MTB design.

Aside from going a bit more over-sized with the tubing selection on this one, the other noticeable difference was needing to extend the chainstays due to the motor shell diameter. My previous build had 415mm stays but that wouldn’t work here so I’ve gone for 428mm which is about as short as I think I can go and still keep sensible tyre clearance. I’m hoping they’ll still be short enough to keep it feeling lively but not so short that the front end floats up when climbing under power. I ordered a second motor shell for a good reason :smiley:

For 29" wheels, the stays would need to be up around 435-440mm so, theoretically, you can build it into your Eponym without making any geometry changes. From what I’ve read, the demo MTB which Freeflow uses is a Specialized Hardrock which had the BB removed and an alu shell welded in. If you’ve got an old Eponym frame kicking around, that’s probably your fastest route to a working bike.


About time for some updates - it’s been a slow plod so far.

I made a dummy motor fixture:

I made a tool to clock the dropouts:

I cut, mitred and drilled the main tubes:

I made the charging port housing:

I finally got set up for silver brazing:

I built the wheels:

They’re Hope Pro 5 Boost hubs, DCR i35 alu rims, Sapim D-Light spokes with Wheel Fanatyk spline nipples, 28H front, 32H rear. 1840g for the set.

A few tweaks to the geo since my first post:

DT goes up to 44mm from 40mm - supplier didn’t have suitable stock of the former. CS goes down a tube size to 19mm - I wasn’t happy with the way the 22mm tubes would interface with the dropouts - and up to 433mm length from 428mm to make sure I’ve got enough room for the tyres. ST gets a little shorter to make room for a 240mm dropper.

Since mitring the front triangle I’ve discovered that my jig’s wooden backboard has distorted under the weight of all the metal stuck to it so I’m going to be getting a metal welding fixture table next week. That’ll need assembling then I’ll check/adjust my mitres and weld the front triangle.

Final chainstay designs are still in progress but I’ve got a clear path to completion, then it’s on to bending. I still haven’t figured out how I’ll fixture them as that was the area which went least well with the last build.


Looks great! Are you drilling holes in the motor cabinet to allow the main tubes through? Or are you welding the tubes to the surface of the cabinet?

I’ll be welding the tubes to the surface of the cabinet with this build although the cabinet will be drilled for drainage and cable access. Someone else (or maybe you somewhere else?) mentioned drilling to allow the tubes to pass through - apparently it helps avoid misalignment. It sounds like it could be a good idea so I may do that next time, especially if the ‘welding to surface’ approach causes me any problems this time.

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I had just seen discussion about the topic elsewhere. Curious about people’s experience with both. Yours looks great so far!

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Hey Sunshine.fab,

Please do email me directly and I can talk you about our pricing, are you looking to build a ebike?




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Following as I may also be interested in this motor.

@FFTmotors: note, it would be neat if you guys provided a BSA mount to use with standard frame jigs


I second that request!

and just because asking is free, any F360 compatible design of your piece to add to my cad process? :smiley:

Freeflow have .step models for most of the system’s components. They dropped straight into Fusion for me. You can see their motor/cranks, shell and external battery models in one of the renders in my first post.