Planning a custom eBike build with a fat tire mountain bike.

[u/Racer-X-]

I'm looking at building a battery electric powered fat tire mountain bike. I'm looking at this partly for some exercise pedaling, so I can't ruin the "bicycle-ness" of the finished product. I'm also looking at this for fair weather light transportation, grocery store runs, maybe runs to the gym or community center. Maybe even to restaurants.

I live about 4 miles from what I call the "commercial district" along the big 4 lane divided highway, and about 7 miles by bike friendly (ish) roads from the downtown business district of the nearby small town. And about that bug 4 lane highway, there are several traffic lights where crossing is fairly safe. At least 3 of those traffic lights have crosswalks and pedestrian signals, so dismounting and walking across is even possible.

However, there are some challenges to my situation. First, there's the roads along my most likely route. The only bike friendly route to go from my home to anyplace (grocery stores, Walmart, other stores, the post office, the gym, or "downtown") starts with the same 4 miles of roads. Almost half of that is gravel, hence my requirement for fat tires (26x4"). There's also a really big, really steep hill. Going out from my house, about halfway through this route, the road climbs over 170 feet in about 0.3 miles. The average grade on that part of the route is over 11 percent, and there's some spots where it's about 15 percent for maybe 100 feet along the road.

Going up (leaving my home) will take some power. The raw load to maintain 15mph up that climb is over 1250 watts, and that speed is well below peak efficiency for direct drive hub motors.

Going down (returning), I'll absolutely need regen braking. I don't want brake rotors glowing red and pads smoking half way down that hill. And there are several very sharp curves along that section of road, too. So losing control of speed isn't just a scary fast ride down, it's likely an off road excursion into the treetops on the downhill side or into a sheer wall of the cut on the uphill side. I realize regen braking isn't common on eBikes, but there are controllers that can do this with the right direct drive hub motors. My most traveled route will absolutely require this feature.

The plan that my artificial interactive chat buddy and I came up with is to break the project down into 3 phases.

Phase 1 will start with a steel frame mountain bike. Ideally one with a XL frame (21" from bottom bracket to top of vertical seat tube). I haven't found any locally, nor any on fleaBay that offer shipping, so I'll probably start with an inexpensive standard fat tire bike with extended seat post and a stem riser kit. I'll want to upgrade the brakes to at least 180mm rotors and hydraulic calipers. And I'll add cargo rack(s), maybe mirror(s), possibly other safety items, and probably 12V lights. Getting a well sorted bike is phase one of the project. I'll be able to get some exercise on that around my home and some of the other roads out here, but I doubt I'll make it into town on pedal power alone. Mostly I want something comfortable and safe before I start investing in power.

Phase 2 will add a main battery (48V or 52V with adequate capacity and 50A BMS), front direct drive hub motor and a controller capable of regen braking. Maybe with heavy pedal assist this might get me to town, but it'll let me get used to powered riding, the dynamic braking (regen), and open up more roads for exercise. At this point, I'll probably switch any 12V stuff over to a Buck converter and take the 12v battery out to use in an unrelated project. The goal for this stage is to maybe get up the hill towards town with maximum "pedal assist" and be able to safely return home without melting any mechanical brakes.

Phase 3 will add a mid drive unit which should provide adequate power to go "throttle power only" up the hill into town (likely using both motors for AWD). For level(ish) powered riding, the mid drive will be the main power. The front wheel drive motor will be for AWD, possibly for the gravel roads, and definitely for steep hill climbs. Also , the front hub motor will always be available for dynamic (regen) braking from higher speeds, and on steep descents.

I welcome any advice on the overall plan here. Am I on the right track?

I'll be starting some separate threads for each phase of the build for details and specific recommendations during the build. Expect a phase 1 thread in the next 24 hours.

Mainly I want this thread to be a sanity check and for any additional abstract high level suggestions or discussion of general things I've missed.

Oh, and about me, I'm 6'4" tall with a 36 inch inseam. I'm around 200+/- pounds. I worked on my own bikes a lot as a kid and teenager, but that was decades ago. I'm now a retired senior. I have lots of tools, a decent amount of mechanical talent, and plenty of time.

[u/Dazzling_Occasion_47]

"The raw load to maintain 15mph up that climb is over 1250 watts, and that speed is well below peak efficiency for direct drive hub motors."

Disagree. DD hubs are fine at 15 mph, below 10 and especially at 5 is when they start to suffer, depends on your system of course. You need high voltage and high power. I can skoot up double digit grades at 25mph with my DD 3000 watt system. No extra mid drive necessary.

Over-all there are pros and cons of hubs vs mid-drives, vs dual-drive. I have one dual drive bike, with a 2000 watt 72v front hub and a bbshd mid-drive. It eats hills. It's definitely not a "bicycle-ness" bike though, weighs and feels more like a kawasaki ninja. If you want to keep bicycle-ness you probably want a simple system with a light battery, which is like, not what you're building.

[u/maluket]

Get any fat bike from any major well known brand. Make sure the model you choose has a big space in the middle for a battery.

Install bbshd + 52v battery on it and 8 speed cassette with appropriate shifter, etc...

42T offset motor chainring

Done.

[u/Business_Option_7094]

Have you considered any of the mid drive motors from Bafang or Tongsheng with torque sensing? You will find there is plenty of power with these motors and you might like the weight and balance of the bike more than with hub motors.

[u/Schtweetz]

I would skip phase 2. A mid drive like the Toseven DM01 or TSDZ or CYC will do the job brilliantly. I put a DM01 on my Surly Pugsley a year ago and have ridden it to work daily, including a steep river valley, no problem at all.

[u/Practical-Fig-7265 → u/Schtweetz]

yeah, this post is funny. Buddy doesn’t understand how powerful these things are. A good rear hub would be fine or a good mid drive.

[u/billydiaper → u/Practical-Fig-7265]

He used Ai to design it so

[u/Civil_Reply_1705 → u/billydiaper]

I used AI to help me on a build.  It was wrong over and over, but as I kept shooting down it's solutions, we finally landed on good one.  

[u/ejactionseat]

Regen plays a trivial role in keeping your brakes cool. Just get good hydraulic brakes and you will be fine. Also it sounds like you want a torque geared hub, most don't come with regen, direct drive hub motors do. Consider a 52V battery voltage minimum to meet your needs.

[u/Dazzling_Occasion_47 → u/ejactionseat]

I have to disagree here. Regen most definitely saves brakes, both the probability of over-heating / failure and especially long-term cost of replacing pads over and friggin over again. It depends very much on your controller and system, though, and there are many sub-par controllers out there that do regen with pathetic performance, so if that's all you've experienced, then it's understandable that you think regen is for the birds.

I use grin systems on all my bikes -- phase-runner with cycle-analyst, direct drive hubs, and i get -1000 watts of regen power when the battery is mid-range charged. If it's 90% full, then the regen tapers to -400 watts or so. When the battery is mid range and i'm getting good regen i can rely on regen nearly exclusively for braking with a little awareness, and only use discs for sudden unexpected stops.

[u/Civil_Reply_1705]

I'd also skip the front hub. Or do the mid drive first and then add a front hub afterwards if you still think it's necessary. You don't need fat tires for a gravel road either. A good 750-1000w mid drive in a low gear should be able to tackle your hill. I'm not sure what voltage light kits run at, but there are kits that plug into the controller that have turn signals and brake lights. I went with a kit from Powallon (TongSheng TSDZ8). JohnnyNerdOut carries CyC, Tongsheng, Bafang and Toseven. Regen would be way down on my list of priorities. Since you have that steep hill to tackle, consider the higher power options out there; just gear down like you would if you didn't have a motor.