Hub as friction drive

Discussions related to motors other than hub motors.
This includes R/C motors, botttom bracket, roller and geared drives.
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Grantmac   100 mW

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Hub as friction drive

Post by Grantmac » Nov 09 2018 5:35pm

Anyone tried mid mounting a small diameter hub and running it as a friction drive? Not that I'm seriously considering it, but it's an interesting concept.

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spinningmagnets   100 GW

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Re: Hub as friction drive

Post by spinningmagnets » Nov 09 2018 8:53pm

Look at a Conhis, or maybe one of the stand-up scooter hubmotors, perhaps an electric mountain board.

Image

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neptronix   100 GW

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Re: Hub as friction drive

Post by neptronix » Nov 09 2018 9:01pm

You just have to consider the ratio of reduction between the hub shell and the tire.. spinning mag is right; scooter sized motors that are meant to spin really fast are probably appropriate.

A chain drive would be tons better though.. none of the friction drive issues..
My first major build: 8T MAC motor on a Trek 4500.
The new all-arounder: Leafmotor 1500w on a Turner O2 full suspension.
The wheelie machine: 20" Rear Magic Pie II on a Trek 4300 MTB

"The best time to plant a tree was 20 years ago. The second best time is now."- Chinese Proverb

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amberwolf   100 GW

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Re: Hub as friction drive

Post by amberwolf » Nov 09 2018 10:18pm

It might seem odd, but the speed of the friction roller is directly transferred to road speed, simply using hte tire as a giant transfer roller. So if the roller is spinning at "20MPH", then that's how fast you'll go.

So it isn't really a reduction ratio between the two.

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spinningmagnets   100 GW

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Re: Hub as friction drive

Post by spinningmagnets » Nov 10 2018 10:16am

The friction drive system is an area where I actually have some experience. There are many facets that are the opposite of a (hubmotor) driving the core of a wheel.

With a direct drive hubmotor, best torque and efficiency is a large diameter motor in a small diameter wheel.

With friction drive, a small diameter motor works fine, and best hill-climbing torque is on a large diameter wheel (Visualize the Friction Drive At 12:00 On the wheel, and the wheel diameter provides leverage.

I have been able to get consistent results up to 1,000W, using 36V, and it remains a useful option if products from China become restricted due to a trade war.

As Amberwolf said, the road-speed of the bike is the same as the friction roller speed. Sounds odd, But It's true.

Grantmac   100 mW

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Re: Hub as friction drive

Post by Grantmac » Nov 10 2018 12:33pm

Motor surface speed=road speed is very logical. Which does let the motor act as though the rim is just the diameter of its outer casing so a lower diameter/higher KV motor can be used. The inspiration for this was the RC friction drive someone posted.
The main reasons behind my thought exercise was regen and mechanical simplicity without the disadvantages of a hub motor. Plus perhaps being able to slide the motor forward out of contact with the wheel in the event of battery exhaustion.

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spinningmagnets   100 GW

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Re: Hub as friction drive

Post by spinningmagnets » Nov 10 2018 1:16pm

I was living in a 3rd story apartment when I built my friction drive (my first ebike). I attached it to a rear cargo rack that was designed to easily pop on and off of the seat post so I could carry the drive upstairs while the "cheap" bike remained locked up outside. Later, I permanently attached the drive to a seat-post that was easy to remove with a lever-clamp on the seat-tube (I removed the seat, seat-tube, and drive all together as one unit)

My friction drive build
viewtopic.php?t=21365&start=125#p971314

Also, it worked best on a fat beach cruiser tire with a relatively flat tread to emphasize contact with the drive roller. A skinny road tire with a round profile to the tread was the worst for applying serious power. A tiny direct drive hubmotor is likely the most reliable type of system for driving the tread.

A friction drive index
viewtopic.php?f=28&t=14403

Grantmac   100 mW

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Re: Hub as friction drive

Post by Grantmac » Nov 10 2018 6:28pm

I wasn't seriously considering a friction drive for my commuter (20 miles round trip, fairly flat). But it seems reasonably possible.

What about a pair of skateboard hub motors? They seem to have regen, a durable friction surface (tire) and even a wireless controller with cruise control. The bearings (which seem to not live long on a skateboard) should be plenty strong.
Plus they fairly cheap although I question the ratings on many of them.

Grantmac   100 mW

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Re: Hub as friction drive

Post by Grantmac » Nov 11 2018 12:17pm

Running a pair of these in tandem:
https://flipsky.net/collections/accesso ... 67kv-1100w
At about 500w each is what I'm thinking.
Two rollers should have twice the roller to tire traction and the roller surface is replaceable, plus may wear to the same shape as the tire. I likely wouldn't bother having the motors disengage from the tire and instead use regen.

Two controllers is a level of complexity I'd rather avoid, but redundancy is also nice.

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amberwolf   100 GW

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Re: Hub as friction drive

Post by amberwolf » Nov 11 2018 9:32pm

If the shafts of the motors (or their rotors, however they're made) are locked together, such that they are physically paralleled, in a way that syncs their magnetic positions, you can use a single controller, paralleling the phase wires.

IIRC, you can set them up next to each other and parallel the wires, then run a small current thru one pair so they'll self-align, then you can physically lock the rotors together at that position.

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neptronix   100 GW

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Re: Hub as friction drive

Post by neptronix » Nov 12 2018 12:37am

Actually two friction rollers would be a lot better than one, since you'd have twice the traction to the tire. Take your target wattage and split it between two rollers.
My first major build: 8T MAC motor on a Trek 4500.
The new all-arounder: Leafmotor 1500w on a Turner O2 full suspension.
The wheelie machine: 20" Rear Magic Pie II on a Trek 4300 MTB

"The best time to plant a tree was 20 years ago. The second best time is now."- Chinese Proverb

Grantmac   100 mW

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Re: Hub as friction drive

Post by Grantmac » Nov 12 2018 1:19pm

The Esk8 controllers run in parallel and aren't terribly expensive (plus wireless throttle is cool if it's not buggy). So trying to run them in tandem probably isn't worth it.

My thoughts on using two wheels was halving both heat build up in the motors while also reducing traction demands between the roller and tire. The polyurethane rollers might also wear into a shape more closely matched with the curvature of the tire.

The Esk8 stuff is climbing moderate hills, with a gross weight around 95-100kg using zero pedal power. So it must be fairly capable.

Grantmac   100 mW

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Re: Hub as friction drive

Post by Grantmac » Nov 14 2018 11:40pm

Well I've decided to head a different direction since a cyclone just fell in my lap and I'm a hotrodder at heart.

However just as that happened came across this little gem:
http://www.qiroll.com
It says 260w but I'm wondering if that's underrated like a lot of the Chinese stuff. It looks like a very well realized little unit. Something you could put on your kid's/mom's bike without worry.

-Grant

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