Lightweight carbon framed hub-motor build - Goal of 20 lbs or less

[Catnap]

I've been toying with the idea of trying to build a 20-lb or less carbon fiber 700c single-speed e-bike, using the same Keyde P160 All-In-One hub I have on my steel single-speed. To tension the chain without a rear derailleur or tensioner, I want to use a Wheels MFG eccentric bottom bracket. They work with BB30 or PF30. I'd really like to use disc brakes, but since the hub isn't compatible with thru-axles, I have to find a disc frame that uses QR.

Pretty uncommon these days but I found:

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Framed Mallorca Carbon Disc Brake 54cm 700c Black Road Bike Frame / Fork New | eBay​

https://www.ebay.com/itm/335391512101
NOTE: STRIPPED THREADS IN CRANKS AND PEDALS ARE DUE TO THOSE COMPONENTS NOT BEING PROPERLY TIGHTENED. STRIPPED THREADS DUE TO IMPROPER ASSEMBLY / MAINTENANCE ARE NOT COVERED BY WARRANTY. Fork Weight: 520g.
and

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New all carbon fiber internal cable road bike racing frame disc brake opening | eBay​

https://www.ebay.com/itm/204005767944
100% Toray Carbon Fiber. EN and CN Certification. Normal Weight：960g-1100g. New Arrival. Before sending item,we will check all again and again,to confirm it's perfect to be sent. All prices are Ex-Factory.


Next challenge is tire width. The Mallorca frame can fit up to 700x28, while the other is limited to 700x25. I'd like to go a lot wider but I have not been able to find a carbon gravel frame that has discs and QR dropouts. There are 29er frames that meet all the requirements, like this one:

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AIRWOLF MTB Carbon Frame 29er Boost Bike Bicycle Frameset 148mm Ultralight 960g | eBay​

https://www.ebay.com/itm/204377178433
1>Material:Toray T1100 carbon fiber. 2>Carbon Weave:UD. 7>Weight:960g(L Size).


I don't love look of the 29ers, but all-around it might be a better choice if I want wide tires. Most of the frames weigh around 1kg, and the hub weighs 4kg. That's about 11 pounds so far. That means I have 9 pounds left for the rims, spokes, cranks, cockpit, chain, and saddle.

I want to lace the hub to carbon rims to keep the weight down and run the tires tubeless. I haven't been able to find much written on the web about combining hub motors with carbon rims, but did see one or two folks that did it.

what do y'all think? Is it possible to keep this to 20 pounds based on what i've specc'ed so far?

 

[Chalo]

Rim brakes are much lighter than discs, which is relevant to your weight budget.

Wider tires will make it that much harder to achieve your top line goal. Historically average sized road bike tires, 28mm give or take, are available in very light versions.

Carbon rims can work just fine with a low powered/low mass hub motor. Don't use too many spoke crosses, one is enough.

It might be lighter to use a chain tensioner than to use a BB eccentric adapter. Those things are pretty chonky.

 

[2old]

Don't give up at trying for a "magic ratio"; as rare as they seem to be intuitively difficult to achieve, I've had them at least twice.

 

[Chalo]

Don't give up at trying for a "magic ratio"; as rare as they seem to be intuitively, I've had them at least twice.

eehouse.org has a web app called "FixMeUp!" to help with this. It seems to be broken at the moment, but maybe it will be back up sometime soon.

Any significant amount of chain wear makes magic ratios a whole lot less magical in a hurry.

A ghost ring tensioner can be quite lightweight and is usually at least a little bit adjustable. It works best when you can get the chain length pretty darn close, which allows a smaller ghost ring and more potential notches to put it in.

 

[Catnap]

I've never seen a "ghost ring" tensioner before! That's kind of wild - any website with more details you can link?

 

[glennb]

I agree with sprung tensioner and rim brakes for weight savings. Also expands the frameset options.

Without having looked at the geometry of the frame you linked, it’s still safe to guess that a modern rigid 29er won’t work with drop bars or handle well on pavement … seat tube too steep, head tube too slack, top tube too long, chainstays too long, chainstay chainring clearance too narrow, and likely BB too high. (Sort of OK though, with flat bars, very short stem, and post with >25mm of setback. Still the wrong tool for the job though.)

Carbon rims I’d consider marginally worthwhile, certainly not the first parts I’d be looking to buy weight reduction with. You only save 200g for the pair vs light aluminium rims, and for rim brakes they don’t make sense. Even for the cheapest rims via Alibaba you’re looking at $2 per gram of weight saved. I’d only go carbon rims if I’d already bought the lightest cranks, pedals, brakes, bars, and seatpost, and still had money leftover to spend.

 

[glennb]

Regarding ghost rings, i wouldn’t set your heart on it. The chainring, ghost ring/sprocket, and rear cog sizes, and chainstay shape, have to be just so. Even with a wide range selection of rings and cogs in one tooth increments you still might not be able to find a working combination. Or you might get lucky … never know until you try, but bear in mind that a fat carbon chainstay will likely be problematic.

 

[2old]

Any significant amount of chain wear makes magic ratios a whole lot less magical in a hurry.

I don't have enough miles on the Salsa (pictured) to determine longevity, but rode a GT Zaskar off road with a Boonen Ti rear cog and chainring (32/20 gearing); durability of the drivetrain was phenomenal. The two or three times I needed a new chain, I used a Surly tensioner and 32/19 gearing; after a couple of rides, I would replace the cog with the "20". When the chain started to "loosen" (wear) in two or three years, I'd repeat the process. The 8-speed chains cost about $10 then which was very affordable, the chainring and cog lasted the entire 10 years for the set up, and were still in good condition when I sold the bike.

 

[chris667]

For more tyre clearance, you could build 650B wheels. Lots of rim and tyre choices, some quite lightweight. And I will add to the chorus of rim brakes. Maybe even Magura rim brakes! There's nearly 1lb in a set of cables.

Measure the BB drop, but if it is a singlespeed specific frame you're interested in you'll probably find it will be OK.

 

[E-HP]

what do y'all think? Is it possible to keep this to 20 pounds based on what i've specc'ed so far?

Possibly.
Are you going to use a battery? That might add some weight.

 

[LewTwo]

Possibly.
Are you going to use a battery? That might add some weight.

My attempt at a light weight build hit 30 pounds with the battery but it was a really small battery (36 volts and 6.4Ah).

 

[neptronix]

Please don't buy a Chinese Carbon frame..

Carbon frames are already bad enough in that they fail suddenly without any indication by shattering. They're the structurally weakest of all bike frame types. And that goes for major makers who know what they're doing when building these things, use great materials, etc.

And these things are known to be dangerous on pedal power, now add the forces of a motor and higher speed into the equation.

Also consider how good of QC a low end Chinese factory is.. now your frame is even riskier than that.

Is saving 5 pounds on your bike worth the risk to your life? why not optimize for weight in other areas:

- you can get 300whrs/kg cells today.. and even lighter if you look
- two motors exist today that are 4lbs and output 750-1000w

The above two drivetrain choices could easily give you a carbon frame's worth of weight savings - maybe more.

 

[2old]

The best I achieved was slightly < 35 pounds with a GT Zaskar MTB frame, Ritchey full carbon forks, BBS02, 52V 4ah battery and very blingy components.

 

[ebike4healthandfitness]

Possibly.
Are you going to use a battery? That might add some weight.

The battery is in the hub:

P160 ALL-IN-ONE Rear Motor

ALL-IN-ONE: Built-in battery, controller, sensor, connection module, etc.

keyde.com

A different approach compared to something like this:

 

[Chalo]

Putting the battery inside the hub is in the running for worst e-bike idea ever. That doesn't keep self-confident noobs from coming back to the idea over and over, though. But it's like integrating a suitcase with your underwear-- it creates problems but doesn't solve any.

Also, that's a 9 pound hub. Fitting it into a 20 pounds weight budget isn't an obvious thing to do.

 

[The Toecutter]

- you can get 300whrs/kg cells today.. and even lighter if you look

I've looked. I haven't found any for sale in the USA. I could use something like this.