Wednesday, September 30, 2015

Two speed motors?

Most hub motors have a hard time climbing steep hills unless they are built like tanks. Big enough to ram more than enough amps through them and they will move up steep hills despite the lack of gears.

In the state of Washington 1000 watts is the legal limit of output for a motor on a bicycle; 1500 watts for a moped. Unfortunately it is harder to get a moped registered than a power assisted bicycle.

1000 watts will drive 400lbs up an 8% grade at about 12-13mph; possibly a little more if pedaling hard, but only if the gears are correct. That is too slow of a gear to drive around in all day if you want good mileage.

The new Xiongda two speed hub motor has a max out put of only 720-740 watts (when run on 48 volts) and will slow down to about 9-10 mph with 400lbs (total combined weight) on an 8% grade; and you may need to pedal hard. I need to test out one of these motors.

If you live in Canada you will probably need an additional gear reduction to the drive wheel at 500 watts, and it will be crawling up the steep hills of Vancouver.

I think the best way to motorize a cargo bike would be to use a Nuvinci infinite gear hub like the Elf car does. But use a two speed gear box to reach the desired gear reduction from the motor. Then directly to the brake side of the Nuvinci with a Gates GT3 belt; thereby using the Nuvinci to help your legs keep up with the motor. But this Xiongda hub motor looks like it has fabulous potential.

Mid-drives are popular, but for a cargo bike with 400-450lbs (total combined weight) they can destroy your drive chain fast, way too fast at 1000 watts! If you’re obsessed with a large motor like the Stoke Monkey you would be better off using it as a mid-mounted motor, driving the rear wheel through a stronger chain or belt. But don’t let me stop you from finding out the hard way; after all if you accelerate slowly and don’t stress the chain out much it may have reasonable life span.

Also be aware that these Chinese two speed hub motors are new and unproven for durability by power obsessed Americans. But keep in mind that the only bicycle power systems that are proven by time are old technology, and have problems that the sellers will never tell you about.

See this review on Endless Sphere forum.  The internal gears of the Xiongda may require better grease.

Some e-bikes have torque sensors that make the motor run intermittently, making a jerky motion. The Xiongda has an independent throttle that you can use at any time and a five-level PAS setting that you set in the LCD. You don't have to pedal to use the throttle.

Much more information on this article

Bidirectional hub motor with unidirectional two-speed output
US 20120302390 A1
A two-speed bidirectional hub motor includes an electrically driven hub, including an electrical motor, oppositely aligned one-way bearings, and a planetary gear system connected to the electrical motor. A fixed shaft is connected to the stator of the electrical motor. The first of two one-way bearings connects the electrical motor to the planetary gear system, and the second one-way bearing connects the electrical motor to the hub casing. When the rotor is electrically activated to spin in the forwards direction, the second one-way bearing engages the hub casing and rotates the wheel forwards at high speed. When the rotor is electrically activated to spin in the reverse direction, the first one-way bearing engages the planetary gearing system, rotating the wheel forward at lower speed by a gear reduction. This design gives two-speed output without a transmission.

Friday, September 25, 2015

Two Speed Gear-box for Electric Cargo Bikes?

With gears you can move the "efficiency peak" around to match your mph. On a

fixed hub motor you are permanently tied to your power-band and this presents a real problem at low speeds. Low mph/rpm on a hub motor places you into the low efficiency areas of the power-band. You can compensate a little if you have an ammeter and are really careful about using even less current than the current limit allows (so you are using almost no voltage down low in order to be more efficient) but you are still fighting a less than desirable situation

At 10 mph the efficiency is running about 50%. The good efficiency areas don't even exist until you are past 20 mph. So unlike with gears where you can "gas it" off the line and be in the good efficiency areas all the time, hub motors have a real "issue" with quick starting. Quick starts on a hub motor are really bad news for battery usage... (much worse than if you are geared right).

An inefficient system (like a small-diameter rear direct-drive hubmotor) will convert much of the 1500W of input into heat instead of torque. A geared hubmotor of the same diameter and width will have close to 5 times the wheel-torque per input watts. A mid drive that gives the motor the use of the bikes gears can have a dramatic torque-multiplication between the input watts and output torque at the wheel (especially in the lowest gear).

There are riders who are experiencing very accelerated drive-train wear on the Bafang 750W BBS02 mid-drive, when unrestricted and running 1,000W. Chains and sprockets breaking after a short amount of time that have been beaten into submission.

Large motors (like a 1500 watt out-put) used as a mid-drive can wear out you drive chain fast. Smaller motors like a 1000 watt out-put (Washington state maximum and still too much power for a bicycle drive chain) will drive 400lbs up an 8% grade at only about 13mpg. That is a very low gear, if you if you want good mileage, you don’t want to drive around in such a low gear all the time.

However if you drive your large motor mid-drive gently enough it will not pop your chain, and will last longer. Hard starts are very bad for your bicycle drive chain as well as the battery and tires.

The way around an efficiency robbing, reliability reducing, maintenance and weight increasing gearbox is to instead invest that money and weight, is the right size motor and gear it properly. Locomotives driven by electric motors don't have multi-speed gearboxes. Telsa engineers initially thought the same as you and tried to make the Roadster a 2 speed and learned better after wasting millions, so now Tesla's are single speeds. Brammo uses or used them on their consumer emoto's, but more to give potential customers something familiar than out of need as riders generally use just one gear, but their racing emoto is a single speed like all the other electric racers.

 This is a gear box for robots, 
I cannot swear that is it adaptable t a bicycle.

"We have run configurations of our shifting gearbox internals with 3 CIM motors in the past, which total up to 1011W of power. The limiting factor inside these gearboxes is the #4-40 screw that holds the shifting dog in place. There's more of an issue with shock loading this via the output shaft, along with the shifting rod, rather than the motors attached."