the ideal and practial motor solution

[dirty_d]

DC brushless direct drive hubmotors are efficient and can be overpowered without burning up because of their mass, but they are only efficient at a narrow rpm range the same as all permanent magnet motors. so why not use gears with them? mount it in some way where the axle is fixed and the sprocket is replaced with a fixed sprocket, you run the chain from one of the gears on the motor to the rear wheel with a derailleur. you can install one of those freewheel cranks for tandem bikes and run a chain from the crank to another sprocket on the motor. the hubmotors are already designed to run at a very low rpm without any gearing so the mechanical losses in the chain will probably be the same as in a regular bicycle and also be very quiet. id like to do this to my bike but im not really ready to spend $200 on a hub motor.

 

[safe]

Need I pull all the charts back out?

Yes, most definitely, if you want better performance with less weight and at a lower price point you use an electric motor that is built into your existing gearing. All the charts came up with these basic approximate results: (that will vary slightly with different motor combinations)

Average Gain in Peak Power over the Speed of the Bike: 0-40mph ~ +25%

Average Gain in Peak Efficiency over the Speed of the Bike: 0-40mph ~ +25%

Average Savings in Heat Creation over the Speed of the Bike: 0-40mph ~ -33%

Assuming an 8-Speed transmission and 1 horsepower rated load.

The actual motor doesn't need to be a hub motor... it's the gears that give the advantage... (and if high rpms are your problem then use a motor with a built in geardown like the MY1020Z3)

 

[OneEye]

Nobody argues with the gearing providing a better match for the motor efficiency at the required wheel torque. There's just a tradeoff involved.

The beauty of the hubmotor is its use as a drop-in solution. No need to source unusual parts, research freewheels, tweak, twist, wrench and hammer a solution together. Other than the dropout size difficulties (and torque arms) it should be a 1-hour project to slap a kit onto a bike and take your first spin. For users who plan on pedal-assisting in the areas where the motor is operating at poor efficiency, the motor will spend most of its time being run at near its peak efficiency anyway. In that case, the amount of benefit on the road doesn't always add up to the effort expended in creating the right gear ratios for their perfect beast. Hills or rough terrain may shift the scales a little in the direction of gearing. Netherlanders and East Anglia riders needn't worry about it.

All that said, "time is money" in a hobby economy is a questionable tenet. It's amazing how much effort someone will put into something that is their passion. As long as you are having fun doing it, the enjoyment of the creation process is as much a part of the experience as the ride at the end.

 

[safe]

All that said, "time is money" in a hobby economy is a questionable tenet. It's amazing how much effort someone will put into something that is their passion. As long as you are having fun doing it, the enjoyment of the creation process is as much a part of the experience as the ride at the end.

"Time is money" when you have no money.

When you have adequate money reserves then "Money is time" or more specifically "Money is 'free' time".

If my "Road Racer" concept evolves as a hobby to a level of mindblowing success (which would be nice) then I'd have to shift to a "time is money" philosophy again if I ever wanted to make it into a business.

Sometimes the pressure to make money works AGIANST the creative passion to build something that people might actually like. This is why we often see products that aren't any good... some things get designed by committee and turn out terrible. (you can think of all the terrible bicycle concepts that have been presented in the past that turned out to be complete failures)

Hub motors are the "easy way" to powering an electric bike right now, but there are also those strap on bottom bracket kits that do a good job of integrating with the gearing...

 

[dirty_d]

The actual motor doesn't need to be a hub motor... it's the gears that give the advantage... (and if high rpms are your problem then use a motor with a built in geardown like the MY1020Z3)

yea but that gearing is adding to the mechanical losses and the hubmotor would have a higher peak efficiency.

 

[safe]

yea but that gearing is adding to the mechanical losses and the hubmotor would have a higher peak efficiency.

The efficiency comes from something being either a brushed or brushless motor. Those cyclone motors are brushless. You could go direct with a small sprocket to a huge sprocket (like maybe 11 - 114) but I suspect that the geardowns only shave off about 5% or less of your efficiency and so it's just easier to use the geardown.

But I see your point... if you use a hub motor that is designed for low rpms you can skip one step in getting the gearing right.

The "big gain" is simply having the gears in the first place, as a percentage that's your biggest area of gaining, but then after that you can go to brushless and then geardownless. (probably in that order)

Don't forget where the "big gains" are:

 

[TylerDurden]

DC brushless direct drive hubmotors are efficient and can be overpowered without burning up because of their mass, but they are only efficient at a narrow rpm range the same as all permanent magnet motors. so why not use gears with them? mount it in some way where the axle is fixed and the sprocket is replaced with a fixed sprocket, you run the chain from one of the gears on the motor to the rear wheel with a derailleur. you can install one of those freewheel cranks for tandem bikes and run a chain from the crank to another sprocket on the motor. the hubmotors are already designed to run at a very low rpm without any gearing so the mechanical losses in the chain will probably be the same as in a regular bicycle and also be very quiet. id like to do this to my bike but im not really ready to spend $200 on a hub motor.

Stokemonkey does something very similar. But it still costs as much or more than DIY w/ a hubmotor.

It's probably not accurate to say that hubmotors can be overvolted or overpowered because of their mass. All that mass takes longer to heat up, but it would hold that heat too, were it not for its large surface area.

Point being: they are just larger motors, and very possibly under-rated.

Gears can be helpful, but you dont need very many with a motor>500W. Humans have a comparatvely narrow power-band compared to motors, so more gears are helpful there. Motors can benefit from gears for those extreme areas like up steep hills and low-drag/high-speed runs. "Stirring the gears" is not going to help an ebike much, unless it has a weenie-motor.

 

[Miles]

Yes, you generally need fewer gears for the motor/pedal combination than if you are just pedalling - unless you want to go at excessive speeds...

The problem with putting the motor and pedal power through derailleurs is that 3/32" chain and sprockets wear so quickly. If you use a hub gear, you can use 1/8" chain, which is much better - then you have to be careful of the torque limit of the hub, though (the smaller the wheel the better).

One practical problem with dirty-d's idea would be getting a viable chain line...

 

[safe]

The problem with putting the motor and pedal power through derailleurs is that 3/32" chain and sprockets wear so quickly.

I don't agree with that.

I've been running a cheapo chain on my bike and pushing 1.36 horsepower (peak) and have gone 2,500 miles with it with no noticeable chain wear. The important thing about chain life is lubrication... if you neglect your chain then it will die an early death. I tend to drop a few drops onto the chain(s) (I have two) about every other ride. (so about every 25 miles) This is easy for me to do since I have a rear wheel bike stand that allows the chain to move freely. (which also makes getting the gears set right very easy)

Chains that are only 3/32" are still plenty strong and last a long time if you take good care of them.

 

[Miles]

But you're not putting pedal power through it, as well, though :wink:

 

[safe]

But you're not putting pedal power through it, as well, though :wink:

That's true.

A constant motor torque will do less damage than an oscillating pedal torque. However, it's my understanding that bicycle chains are essentially overdesigned for historical reasons. In the past they did not have the manufacturing techniques and high quality steel that we have today. So the trend has been to use narrower and narrower chains on the 8-9-10 speed rear deraillers. At some point the narrower modern chains are pushing the limits, but the "fat" normal sized 3/32" chain is very durable.

I wouldn't worry about the chain as a source of problems.

A greater area of concern is the combined torque acting on the rear hub. The chance of destroying a mutispeed rear hub is very real. And in the first 100 miles on my bike I managed to wreck a cheapo 6 speed rear cluster freewheel... so parts can break. (however in those days I was having troubles with the chain falling off and getting jammed)

 

[Miles]

I don't think it's that they were ever overdesigned - more that it became necessary to make them narrower to cram in more cogs at the back. I agree it's not really a big problem - a wider chain will be more durable, though.

Yes, we'll see how durable the hub gears are.... As I weigh 67kg and my bike has 20" wheels, I have a better margin than most.....

 

[safe]

Yes, we'll see how durable the hub gears are......

Torque is all about gearing...

So what you need to watch for is a TOO LOW gear ratio. If you gear the bike too low then the torque rises and that places extra stress on the rear hub. So it's something that you can prevent through knowledge.

The basic number I've seen is 100 Nm. Under no circumstances do you want your rear hub torque to exceed 100 Nm. The typical electric motor produces maybe 5 Nm at the shaft and then if you gear it up 10:1 you are going to get 50 Nm at the rear hub. So that's roughly how things are set up... it's the potential of a combined motor torque and pedal torque peak that could throw the overall value above 100 Nm.

The secret (in my opinion) is to set your normal pedal rpm (about 76 rpm) WELL BELOW the motors natural power peak. By doing this the two peaks are out of alignment slightly and when the motor is getting "weak" then the pedaling is getting strong. When the pedaling is spinning really fast (thus the torque peaks flatten) then the motor is hitting it's best power. So you need to be SMART about this... you can't do something like place your motor power peak below your pedal peak because that's going to invite trouble.

The moral of the story:

"Set your (final gearing) motor power peak HIGHER than your pedal peak. (70-90 rpm)"

 

[Miles]

That assumes peak pedal power output is 70-90 rpm, of course

 

[Dalecv]

It is hard enough to find room on a bike to put batteries to get all of the Ah I want, but to add a large outside case spinning motor just so I have a one to one ratio with the drive wheel doesn't seem to make much sense. I think hub motors are best put at the center of the wheel where they are designed to go.

Please don't pull Safe's chain drive leash or more graphs will poop out!

 

[cadstarsucks]

It is hard enough to find room on a bike to put batteries to get all of the Ah I want, but to add a large outside case spinning motor just so I have a one to one ratio with the drive wheel doesn't seem to make much sense. I think hub motors are best put at the center of the wheel where they are designed to go.

Please don't pull Safe's chain drive leash or more graphs will poop out!

:lol: So where is the mime? You know, a mime is a terrible thing to waste.

 

[safe]

Torque Extremes

Not a true "chart" (see the lower chart) in that it's not spreadsheet driven, but the idea is there. At the lowest pedal rpms your torque swings will be the greatest, but as the rpms rise your ability to deliver torque drops until all your energy is spent trying to get your legs spinning in circles. This is why the peak power tends to be at lower rpms for untrained cyclists since the coordination to deliver real torque at high rpms is not there.

So you can see why this previous chart is valid also:

...and the reason for what's happening:

 

[Miles]

The problem with putting the motor and pedal power through derailleurs is that 3/32" chain and sprockets wear so quickly.

I don't agree with that.

I've been running a cheapo chain on my bike and pushing 1.36 horsepower (peak) and have gone 2,500 miles with it with no noticeable chain wear. The important thing about chain life is lubrication... if you neglect your chain then it will die an early death..........

Chains that are only 3/32" are still plenty strong and last a long time if you take good care of them.

As far as the chain goes, its gotta be stardard chain that fits through a regular bike derailer...we bought the highest performance SRAM chain but it didn't really last that long.

 

[numberonekiwi]

this has been done already here nice set up and I am sure could be fitted to nearly anything

 

[recumbent]

It is hard enough to find room on a bike to put batteries ... Please don't pull Safe's chain drive leash or more graphs will poop out!

This has been discussed before, the end result is chains can be problematic, add two of them, plus another freewheel, more noise and more potential problems.

Hub motors fit the goal most of us seek, we don't mind pedaling a little bit some times, it's a pedal bike after-all. Both systems work well, just depends if you want to tinker with dirty chains more, or tinker with them less :?

 

[safe]

Hub motors fit the goal most of us seek, we don't mind pedaling a little bit some times, it's a pedal bike after-all.

We all don't "seek" the same goal... (but "most" is probably accurate)

In my case my "goal" is a Road Racer bike that has certain odd charactoristics that other bikes don't have:

1. At top speed I DON'T pedal. (staying tight in a tuck makes so much less wind resistance that it equals all the pedal power you might create)

2. I need to be able to climb hills as well as hit my top speed, you just can't do that with a fixed gear. On my present bike I don't even bother with pedals. (though that will change with future versions)

3. When you are allowed only 750 watts (with just a little above) you get a net gain in power of 25% across the full spectrum of bike speeds. You get a 25% improvement in efficiency and a reduction of heat of 33% all because of the use of gears... that's hard to ignore and would make my bikes performance suffer if I went without it.

However... my bikes are all custom built, so for the "real world" most people will choose the hub motor because it takes no real effort to make work. Often one's "goal" is more about a compromise... because other options are simply too hard. (custom bike creation is very hard work)

 

[Geebee]

The problem with your "Bad" example is that it works great, as it gives more assist up uphill and on the flat it is better as your pedal speed rises it unloads the motor sving power for when it is needed.
I have had no problem with the setup on steep (40% +) off road but with a lower powered motor the only negative is that you need a tensioner in the drive side of the rear wheel as it will tend to move forward.
I have never broken a chain with a bb drive and I clocked up a lot of ks on one which peaked over a kw, as pointed out keep the chain lubed and wear plummets.
Tandems put huge peak loads on chains and they work fine.