Large Chainring w/ Crystalyte Hub motor?

Hello All-
A complete novice at this e-bike thing, so please be patient with my complete and utter lack of understanding of the basics. I've done quite a bit of searching through the forums for a topic such as this, and haven't found it, but please feel free to add links if I've missed something pertinent in the past.

More a hypothetical question; Would it be possible to pair a high-torque, "medium" powered motor with a bicycle that features a large chainring on the crankset? In theory, one would have electric assist while getting started from a standstill and at lower speeds, but be able to provide quite a bit of human power at higher speeds.

(For a very extreme example, albeit sans motor assist, see the Donhou Land Speed Bike: http://www.donhoubicycles.com/land-speed-bike/)


Would this work? I understand that wind resistance becomes a major hurdle for human powered bicycles at higher speeds, but could the motor offset this? And would you risk running against the motor and burning it out?

Please forgive my complete lack of knowledge! Just testing out this idea, as it is very hard to find any precedent for it. Thanks for any information you can help with!
-andy f.

Hello

The Schlumph planetary crankset comes in three models. It is a 2 speed crankset that does away with front dérailleurs and multiple chainrings. Their high speed drive has a 2.5 to 1 increase in ratio. For example it can use a 27 tooth gear in low and when high is engaged it makes it act like a 67.5 tooth gear. I've not used one but I'm thinking about getting one. If diving into the math of gear ratios, rpm, and cadence sounds like fun but might bore other readers feel free to PM me.

Sincerely mrDS

Yes. However there is a problem with the Crystalyte hub for that purpose. There will be a point where the bike's speed being pedaled is higher than the motor's maximum free running speed at it's supplied voltage, and it will be acting like a generator. When that happens, it generates more voltage than the battery, causes the controller to effectively short the phases together, and becomes a rather effective brake.

Its not something that normally ever happens on a direct drive motor, you really have to be going a lot faster than the motor was capable of taking you, but the donhou bicycle would be able to achieve that when drafting behind something.

The answer to that problem is a geared drive motor, which freewheels when not in use. Crystalyte doesn't make one, theirs are all direct drive.

Another point. Having a motor get you up to speed won't effect your top speed. You can't pedal any faster just because the motor got you up to speed. Your maximum pedal speed is governed by the wattage your legs can put out against the wattage needed to overcome the wind resistance of your body and the bike (and a watt or two needed to overcome the tire resistance)
having the motor running will add power and speed to your pedal effort, but once the motor is off, you'll drop down to your maximum pedal speed, regardless of the size of the chain ring.

Perfect info, Drunkskunk. Everything I needed to know. Thanks for your help, you two!

You can put a darn big front chain ring on a bike, and keep up with pedaling up to 30 mph with a typical 48v powered dd hubmotor. So for real world use, don't worry about going too big. You won't be overspeeding the motor heating it up, except for when descending a huge mountain with the throttle off.

I have a 56 tooth front ring on my big cargo bike, which allows me to put in my meager 50-100w pedaling, at any speed the bike can go. In use, it's just as you predicted, I throttle till going about 10 mph, then start pedaling. I could pedal hard from the first foot, but it strains the chain and can wear out the small gear in back fast. So I either faux pedal, or just don't bother till going 10 mph.

At 30 mph, I'm only contributing about 10% of the power at most, but hey, 10% more range is still worth pedaling for. On that bike, the middle ring was swapped for a 48 tooth, so when I go slow (15-18 mph) I can just drop to that ring. Rear cogs generally stay on 14t, or at most the 16t gear. Never get near the big gears in back.

I use a 62t front chainring.
I want the exercise, so I start off in my lowest rear gear and pedal hard from a stop and then up through the gears until I reach the speed/cadence I want.
Works great for me. I can pedal up to 40+ mph this way with the 14t rear cog.
http://hostelshoppe.com has big CRs if you want to go this route.
I did and I love it!
I pedal my fat buns off the whole way on my (50 mile round trip) commute.

-Slo

Good for you. I used to ride more that way before I got sick. Except I just cranked hard from the start in the highest gear, wearing out chains all the time. A habit that dated from my youth, when I never used any gear but 52-11.

I can't afford to pedal hard on starts anymore, I pay too high a price later. But the spinning at 25 mph is still beneficial. For me, I need to get exercise, but only light work.

Since the illness, most of my bikes are normal geared, meaning I can pedal to about 20 mph max.

It is good for me!
I had to do something to get my weight down.
That long duration, medium intensity exercise is just what the doctor ordered.
I don't know the OP's reason for wanting to pedal at speed, but I do it in an effort to improve my health.


-Slo

It's a little spendy, but I'm using a Patterson Metropolis Tranmission 2-Speed internal gear crankset. I've changed to a 39t chainring, which is effectively a 62t ring in high gear.

Even without a rear derailleur, this would let you downshift for takeoffs, and still be able to contribute with the the pedals up to about 40 mph.