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

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?

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.

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.

2old said:

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.

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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.

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

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.

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.

Chalo said:

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

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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.

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.

Catnap said:

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

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

E-HP said:

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

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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).

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.

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.

E-HP said:

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

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The battery is in the hub:





A different approach compared to something like this:

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.

neptronix said:

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

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

A sub 30 lb ebike is possible. The hard part is finding the right components to build it up. The other option is to just buy an off the shelf gravel e bike with the mahle x20 system ( scott addict, orbea gain, ect), but those are very expensive. I think 1st you need a light frame, of which are all thru axial designs, which is the standard. A budget friendly china carbon frame will work, and there quality is pretty reliable these days. I have been riding a china carbon hardtail frame and fork for 5 years with no issues.
Grins all axle hub motor (thru axle design) would work but is a direct drive motor, so big and heavy, about 9 lbs.
Grin Rear All-Axle CA3 Kit

I found a comparable x20 type hub on amazon for about $400 and weights about 4 lbs, but no idea about quality or how to connect to a controller or bike frame for anti-rotation.
https://www.amazon.com/Thru-axle-Br...rd_r=QX1BKZMTHCE8N8P5WVDK&ref_=ci_mcx_mi&th=1

A mahle x20 from tree fort bikes website is close to $700, claimed torque of 50Nm, but that's supposedly a conversion to a mid drive motor torque, so actually around 30Nm.
MAHLE X20 Ebike Hub Motor - Class 1, 20mph, 55Mm, 28h Straight-Pull, 12x142mm Thru-Axle | Tree Fort Bikes

I am still struggling with how to spec out a BB torque sensor and cadence (PAS) to a controller and hub motor. It doesn't seem straight forward. Most BB torque sensors I find have the old JIS square axle, which limits you to really hard to find old cranks designs. Mahle has a lot of options for bb torques sensors, again tree fort website.
MAHLE X20 TCS Bottom Bracket - BB86, With Torque/Cadence Sensor | Tree Fort Bikes
But getting technical data from Mahle on the electronic of the BB is kind of impossible if your not an OEM.

You are never going to achieve 20lbs with a battery, 40 minus possible, a 36v battery is 10 lbs. You gotta consider hitting a pothole at twice the speed you can pedal, if your ride fails you are toast

There are, in fact, e-bikes with a battery available for sale that weigh less than 10kg. The cost is exorbitant, the engine small and weak, and the battery is barely enough for an hour of riding if that.

Until you need to lift it up, every kg spent will actually increase the bike's range and power to weight ratio as the weigh costs of the motor, frame, wheels, etc. all amortize until the square-cube law starts taking over. This is to say, a heavier e-bike will almost always outperform a lighter e-bike, at least as long as both of them are still rolling and don't need to be lifted over stairs, obstacles etc.

neptronix said:

Please don't buy a Chinese Carbon frame..

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We all ride on motors made in China, connected to controllers made in China via cables made in China. Composite manufacturing in China has already virtually caught up to the west, and I don't see a particular reason to distrust that part of the system any more than the other. I was myself looking at Lightcarbon offerings (which, incidentally, have seen at least a couple of sub-20kg builds on youtube with Bafang M820 and 21700 10s1p batteries) and the products they sell are sold directly, i.e. they make them. They have control over the entire process and can vouch (and show you) how your frame was made, which in the connected world of supply chain of shiny, overpriced brand bikes is something you can only dream of.

I'm afraid that almost all carbon frames are made in China, at least these from the cheaper segment. And the cheapest looking ones also weight more, so should be a bit tougher than the expensive ones

bananu7 said:

We all ride on motors made in China, connected to controllers made in China via cables made in China.

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The problem is that even western manufacturers still make weak ones and China typically cuts corners when they can. Expensive, well made carbon bikes can snap in half while being ridden at bike speeds in non-extreme situations by a light rider.

..that's concerning because we are talking about more weight and speed vs an acoustic bike..

Carbon is super tough along a single axis but relatively weak otherwise. Strength is determined by how they lay up the carbon mats in the tubes. Well laid up tubes won’t snap, but they can be fairly delicate to both forces they’re not designed to withstand and abrasion.

The upshot is it’s hard to say. A carbon road bike frame can last decades, but a muddy day can ruin your brand new carbon gravel bike. Or the opposite.

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Here's a gallery of carbon bike frame failures. These are all non electric: Busted Carbon
The most common failure is for the bike to suddenly shear in half while riding it without any warning signs.



..you won't find a gallery of titanium frames suddenly shattering, because like steel, titanium gives you warning signs.





Truth about these lighter frame materials is that if they're built to the same strength specifications of aluminum bikes, you're only saving yourself 500-1000g of weight on the frame. Anything lighter than that is weaker than a typical bike.

You can make a 1000g weight cut in your controller/battery/motor choice if you're exacting about the selection. That's a much saner choice than picking a frame that's already weight optimized at the expense of longevity/safety for pedal bike duty

neptronix said:

..you won't find a gallery of titanium frames suddenly shattering, because like steel, titanium gives you warning signs.

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Which is probably why Ti, rather than CF, is used for fan blades in jet engines? Even with that....


FWIW, the only vertical stabilizer I know of that came off the plane entirely due to excessive sideways forces*** was held on via CF lugs (vs the aluminum ones in other craft that had probably been subjected to the same forces may times in years past from the same maneuver, and the aluminum ones in this craft and it's titanium bolts that did not fail). It was an A300 that had been flown for almost two decades, until that day it hit wake turbulence and the pilot basically slammed the rudder hard over side to side repeatedly as he was trained in order to restabilize it.


***other planes I know of have lost the VS but not due to a failure of the VS itself, like the Japan Air 747 back in '85 (oddly there were actually survivors from that one).

I don't know anything about planes, but i do know that carbon fiber is strong in one direction, weak in another.. so too much motion in one direction, and it snaps. Turbulence in air can come at any direction, so that's a problem.

Once you make it strong in all directions, you end up with no weight savings vs aluminum
So carbon fiber is usually about riding the edge of structural integrity just right.
..and in this case, the target for 'just right' on a Chinese designed frame is probably someone lighter than OP riding at bike speeds.

Catnap said:

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

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I used to use one on the SB Cruiser,
https://endless-sphere.com/sphere/t...vy-cargo-trike-dog-carrier.67833/post-1022950

But having it in the middle (where it looked "coolest") allowed the chain to eventually fling the ring off as I crossed a bunch of waves in the asphalt at a corner that jounced the trike pretty badly, and I couldn't stop in traffic to retrieve it. By the time I got back there it was gone. :/

If I'd had it at one of the ends, closer to another chainring, it would not likley have been able to move around like that .

neptronix said:

I don't know anything about planes, but i do know that carbon fiber is strong in one direction, weak in another.. so too much motion in one direction, and it snaps. Turbulence in air can come at any direction, so that's a problem.

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Yeah, but it wasn't the turbulence that did it...it was the hard side-to-side rudder inputs from the pilot. :/ (somewhat equivalent to the cyclic flexing a frame experiences between the headtube and seattube, which is where a number of CF bike frames have snapped)

AFAICT all of the CF stuff will eventually fracture the plastics the CF is embedded in from cyclic motion. Once that happens, the CF can just tear right thru, and that's where these things snap. The fractures tend to happen instantaneously, rather than gradually tearing. If they could use a softer or better-crosslinked plastic to embed the CF in, it would have less of this problem, but a softer plastic means the entire assembly would be less stiff and not usable for some things. Better-crosslinked but still stiff would be best.

But I'm not a materials scientist.

neptronix said:

So carbon fiber is usually about riding the edge of structural integrity just right.
..and in this case, the target for 'just right' on a Chinese designed frame is probably someone lighter than OP riding at bike speeds.

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I have a feeling that quite a few frames, regardless of design or material, are actually built to the spec target of BSOs: once sold, it has acheived it's goal, and thus has no further requirement to remain a single functional piece.

That said, I've abused a lot of bike parts over the years in ways they were never meant to be used, and broken some, but most of them have withstood the abuse. But they were all metal, almost all steel...the only aluminum frame I ever rode was a regular bike and that one fractured near the top of a seatstay (quite likely from the stress of the cargo baskets, later rack and pod, but I have another frame given to me after it also failed at a seatstay just from normal riding).


BTW, I wish busted-carbon was still kept updated. It'd be interesting to see how failures have changed over the years (as I'm sure they've changed, if not improved).