Carbon fiber hub motors?

[bdole]

I recently mounted up a Q100C CST eBike system to a mid-level aluminum road frame. It's pretty nice, and the amount of assist is pretty good in that I get a workout but more easily hit a 20mph cruising speed. Maybe a 328-series would have been better (I got the 201 rpm motor) but I can always change that to get a bit higher top end. But I do like how it behaves on hills, letting me ascend at around 15mph, and again, getting at least a bit of a workout.

So, that isn't specifically related to the subject title, but I wanted to explain where I'm coming from.

If I really wanted to shed weight, aside from mounting this to an even lighter bike and maybe creating a half-size battery, does there exist any sort of ultralight hub motor with similar performance?

I figure it could be made partially of carbon fiber. At least the shell and maybe the flange for connecting the disc rotor. I really don't know. There is plenty of necessary heavy elements in a hub, but how far can it be taken?

And, is there something, or some company, currently on the market that can offer this?

Thanks!

 

[Drunkskunk]

There isn't much weight to lose. You might be able to save a few grams by swapping all the aluminum hub parts to Titanium. CF wouldn't be strong or stiff enough for this kind of application. Cost of machining a new Ti body for the motor might go into the thousands.

 

[Ykick]

Have you looked into Kepler friction drives?

He's recently upped the bar - https://endless-sphere.com/forums/viewtopic.php?f=28&t=84905

 

[amberwolf]

What else on the bike is not already made of lightest possible materials? I suspect going to all the existing commercially-avialalbe "weight weenie" type stuff on the bike itself (frame, wheels, components, etc) would take more weight off than custom-making very expensive motor parts for a one-off.

If the point is only to reduce the rotating weight of the wheel itself, going to thin/skinny tubes and tires, and "weight weeny" rims and spokes would also probably be cheaper and easier to do.


But remember that lighter stuff is not always as strong, at least not in all the same directions, so depending on the usage of the bike (terrain/etc) it isn't always better to go lighter.


There are friction drives, like Kepler's, that are possibly lighter than the hubmotor.

There's even smaller systems like the Gruber assist, etc., but the smaller and lighter it is, the less power it makes for the same money. :/ (or the less power it *can* make, period).

 

[dogman dan]

You are looking for the cheater motor, the one that hides in the seat post. I think those are pretty light, and low power. Not cheap though.

Friction drive like the Kepler is likely still the main choice, if you really want to lighten up.

Too light, too small, is death for hubmotors. People will just kill em with too big a wheel.

 

[MadRhino]

There is a hub that is made with some special ceramic stator and composite shell. I don't believe it is available yet but it should be soon. A French company that I can't remember the name is doing some tests with it.

Nevertheless, your requirements are out of hub builds and I agree that a friction drive is your reasonable solution, or a Gruber assist is tailor made for you if you have the budget.

 

[bdole]

There is a hub that is made with some special ceramic stator and composite shell. I don't believe it is available yet but it should be soon. A French company that I can't remember the name is doing some tests with it.

Nevertheless, your requirements are out of hub builds and I agree that a friction drive is your reasonable solution, or a Gruber assist is tailor made for you if you have the budget.

Can you try again to remember it's name or find it somehow? I had no luck.

And yes, understandable that a small motor like the Q100 doesn't have a lot to take out. I guess if it was a lighter rim, with less of a spoke count that could help.

But imagine I have a reasonably light bike already, like a carbon fiber such and such. Maybe a $4000 bike. I want to add a motor to it and not add an extra 8 lbs of motor and battery. Halving the battery size is a quick start.

The Vivax is an option too but I understand it to be a push-button style, and apparently they are kind of finicky. I would prefer at least the robustness of a hub motor.

What exactly encompasses the heavy bits in a hub motor? And can those be made of something else? Maybe instead of steel or brass gears, a material like PEEK or another fiber-impregnated plastic or composite? Blah blah durability and all that can be solved with some development, no?

So, it sounds like nothing is on the market quite yet, except for the possible French motor? Can you try to remember it's name? I didn't have any luck finding it

 

[Drunkskunk]

the heavy bits are the copper windings, the iron laminates, and the neodymium magnets in their iron ring. Those need to be the weight they are to function. reduce the weight and you reduce the capability of the motor. The only part that has a substitute material is the copper, for which silver can be used. However, silver is heavier.

Also, the steel axle. You couldn't reduce the weight of that without weakening it. Materials like Titanium are stronger than steel by weight, not by volume. The axle is the size it needs to be to function. make it out of a lighter material the same size, and it would be weaker. Ti by volume is almost as strong as mild steel. That's not strong enough to be a good axle. It's also not as hard as steel, so a special torque arm would need to be developed. Ti is also not stiff enough to make a good axle.

 

[bdole]

Understood. Kind of like how the battery is 95% cells and only a small amount of ancillary material.

So, how does a mid-drive, by comparison, hope to continue its weight shedding?

Is it just a smaller motor with more gears and reduction? And does that eliminate a bulk of the weight because you're trimming down on the heavy stuff like magnets and windings?

Why can't you do that in a hub motor? Granted, plenty of hub motors are already geared, but can you take it further akin to what mid-drives are starting to do? Don't think Bosch, think some of the newer systems that are maybe 3/4 the size.

 

[MadRhino]

A small lightweight motor can't be a hub. Building for mountain trails with a hub, you need to have a different concept of "lightweight" that is a power to weight ratio. My dirt bike is very light weight at 76 lbs because it is fed 20+ Kw and that is making a power ratio of ~ 83:1 (20 000w peak power / 240 lbs total riding weight). In comparison, a 160 lbs rider on a 26 lbs bike fed 1000w would make a ratio of ~ 5:1 :wink:

The lighter you make a hub motor, the less suitable for the mountain you are making it. Mountain does like big hubs, or big gears.

 

[bdole]

Okay...for the sake of argument (because this is the internet, after all), who makes the nicest 250W-350W hubs at this point? Either front or rear, doesn't matter.