Author Topic: Optimum rpm and gear inches for low gear  (Read 4117 times)

Optimum rpm and gear inches for low gear
« on: 19 August, 2017, 09:24:31 am »
On a Fb forum with regard to my Bafang 250W motor I was told, "With the flat torque curve of your motor I think you need to figure out the optimum RPM you should be running & figure out your Gear Inches needed in low gear."

Anyone know what that means and what is the way of doing this, please? Links to suitable formula would/may be useful.
Cheers
Never knowingly under caffeinated

Re: Optimum rpm and gear inches for low gear
« Reply #1 on: 19 August, 2017, 03:25:47 pm »
Gear Inches here.

For a human-powered machine optimum RPM is highly variable.  Some folks work better at 100rpm - 120rpm, others at 80rpm or even lower.  For an electric motor -  ???

Re: Optimum rpm and gear inches for low gear
« Reply #2 on: 19 August, 2017, 05:59:24 pm »
Any motor, electric or petrol, can be described by a torque vs RPM curve. The said curve can be approximately flat, but only within a limited range of RPMs.  I may be able to help you if you can show us the curve.

Re: Optimum rpm and gear inches for low gear
« Reply #3 on: 19 August, 2017, 08:17:41 pm »
Gear Inches here.

For a human-powered machine optimum RPM is highly variable.  Some folks work better at 100rpm - 120rpm, others at 80rpm or even lower.  For an electric motor -  ???
Thanks S. That is the only bit of what I wrote above that I understand :)
Never knowingly under caffeinated

Re: Optimum rpm and gear inches for low gear
« Reply #4 on: 19 August, 2017, 08:18:43 pm »
Any motor, electric or petrol, can be described by a torque vs RPM curve. The said curve can be approximately flat, but only within a limited range of RPMs.  I may be able to help you if you can show us the curve.

I've started searching but no luck as yet. Thanks for the help; I may be back to you later! :)
Never knowingly under caffeinated

Re: Optimum rpm and gear inches for low gear
« Reply #5 on: 19 August, 2017, 08:36:21 pm »
Any motor, electric or petrol, can be described by a torque vs RPM curve. The said curve can be approximately flat, but only within a limited range of RPMs.  I may be able to help you if you can show us the curve.

You probably also need an idea of machine weight loaded and probably max gradients envisaged

Re: Optimum rpm and gear inches for low gear
« Reply #6 on: 19 August, 2017, 09:45:17 pm »
Is the motor in the wheel hub, or mid-mounted driving the pedal cranks?

If a hub motor, the gear ratios are not relevant to it, only to your own pedalling speed.

A bottom gear of 20 inches and a top gear of 100 inches is a sufficient range for most purposes and riders.

Re: Optimum rpm and gear inches for low gear
« Reply #7 on: 20 August, 2017, 07:01:04 am »
Is the motor in the wheel hub, or mid-mounted driving the pedal cranks?

If a hub motor, the gear ratios are not relevant to it, only to your own pedalling speed.

A bottom gear of 20 inches and a top gear of 100 inches is a sufficient range for most purposes and riders.
This is a mid drive in the bottom bracket Bafang BBSO1B 250W with a 36V 15A battery

On my previous trice I used a 14" gear up Birdlip Hill from Cheltenham and occasionally a 17" in Welsh foothills but I was usually towing a trailer of camping kit.
Now that I am not a young man, and nowhere near as fit as I was a few years ago, I am hoping to avoid trying to walk the Trice up any hills at all. :)
Never knowingly under caffeinated

Re: Optimum rpm and gear inches for low gear
« Reply #8 on: 20 August, 2017, 08:16:04 am »
I have absolutely no idea about any of it (although I am looking into electric motors for the tandem), but

HELLO NOBBY!

Nice to see you, I've missed you  :-*
Quote from: Kim
^ This woman knows what she's talking about.

JennyB

  • Old enough to know better
Re: Optimum rpm and gear inches for low gear
« Reply #9 on: 20 August, 2017, 08:58:55 am »
Is the motor in the wheel hub, or mid-mounted driving the pedal cranks?

If a hub motor, the gear ratios are not relevant to it, only to your own pedalling speed.

A bottom gear of 20 inches and a top gear of 100 inches is a sufficient range for most purposes and riders.

Quite the reverse, in a way.  With a crank motor, optimum cadence is determined by what the motor likes, so if you prefer to spin especially fast or slowly then you are out of luck. That's not a problem for most people.

I think Knobby's friend was trying to say that an electric motor doesn't need a close range of gears. You're still going to need the same high gears anyway for speeds over 15 mph, so you  all you need to work out is the gear you need to pedal for the steepest hill you anticipate. As a rule of thumb, with motor assistance you can climb the same hill in the high ring as you could unassisted in the low ring (for suitable values of rings - 50/32 works for me).

With electric motors, power is volts * amps. Within limits, the voltage determines the speed and the current the torque. Maximum current is pulled at low speed under heavy load (and a lot of iit is wasted in heat). Motors are designed to run at a particular no-load speed for a given voltage. A typical 36 volt hub motor for a 26" wheel will have a no-load speed of around 200 rpm, so that at 15 mpg it is running at maximum efficiency  and power is taking off.  Maximum power comes at around 8-12  mph, so such a motor isn't  much use on long hills that you would normally struggle up at less than 4 mph.  Riding a hub motor is a bit like riding fixed - you try to hit the big hills as fast as you can.

If you know you want a hub motor for hills rather than speed then you pick a slower motor, or lace it into a smalier wheel. Speed freaks, on the other hand, will either over-volt or put a motor into a larger wheel and step up the current - or both.

For more technical information see this motor simulator.
Jennifer - Walker of hills

Re: Optimum rpm and gear inches for low gear
« Reply #10 on: 20 August, 2017, 09:53:48 am »
I have absolutely no idea about any of it (although I am looking into electric motors for the tandem), but

HELLO NOBBY!

Nice to see you, I've missed you  :-*

Ah, Thank You Butterfly.
It does an old chaps heart good to know he is missed :)
I can recommend Tony Lee at Whoosh Cycles, Southend. Very helpful and good pricing.
Never knowingly under caffeinated

Re: Optimum rpm and gear inches for low gear
« Reply #11 on: 20 August, 2017, 09:54:30 am »
Any motor, electric or petrol, can be described by a torque vs RPM curve. The said curve can be approximately flat, but only within a limited range of RPMs.  I may be able to help you if you can show us the curve.

The graph is on this page here:
https://www.pedelecforum.de/forum/index.php?threads/250w-bbs01-drehmoment-wirkungsgrad-leistungsdaten-von-bafang.38435/

The chap who sells them - Whoosh Cycles - says, "You can hear the 'tune' of the motor - always keep the noise down and RPM up, in the region of 70RPM to 100RPM to keep the motor sweet."
Never knowingly under caffeinated

Re: Optimum rpm and gear inches for low gear
« Reply #12 on: 21 August, 2017, 01:31:53 pm »
Is the motor in the wheel hub, or mid-mounted driving the pedal cranks?

If a hub motor, the gear ratios are not relevant to it, only to your own pedalling speed.

A bottom gear of 20 inches and a top gear of 100 inches is a sufficient range for most purposes and riders.

Quite the reverse, in a way.  With a crank motor, optimum cadence is determined by what the motor likes, so if you prefer to spin especially fast or slowly then you are out of luck. That's not a problem for most people.

I think Knobby's friend was trying to say that an electric motor doesn't need a close range of gears. You're still going to need the same high gears anyway for speeds over 15 mph, so you  all you need to work out is the gear you need to pedal for the steepest hill you anticipate. As a rule of thumb, with motor assistance you can climb the same hill in the high ring as you could unassisted in the low ring (for suitable values of rings - 50/32 works for me).

With electric motors, power is volts * amps. Within limits, the voltage determines the speed and the current the torque. Maximum current is pulled at low speed under heavy load (and a lot of iit is wasted in heat). Motors are designed to run at a particular no-load speed for a given voltage. A typical 36 volt hub motor for a 26" wheel will have a no-load speed of around 200 rpm, so that at 15 mpg it is running at maximum efficiency  and power is taking off.  Maximum power comes at around 8-12  mph, so such a motor isn't  much use on long hills that you would normally struggle up at less than 4 mph.  Riding a hub motor is a bit like riding fixed - you try to hit the big hills as fast as you can.

If you know you want a hub motor for hills rather than speed then you pick a slower motor, or lace it into a smalier wheel. Speed freaks, on the other hand, will either over-volt or put a motor into a larger wheel and step up the current - or both.

For more technical information see this motor simulator.

Not sure that I thanked you for this piece Jenny B. Thank you very interesting and helpful.
Best wishes
Never knowingly under caffeinated

Re: Optimum rpm and gear inches for low gear
« Reply #13 on: 21 August, 2017, 08:21:46 pm »
Any motor, electric or petrol, can be described by a torque vs RPM curve. The said curve can be approximately flat, but only within a limited range of RPMs.  I may be able to help you if you can show us the curve.

The graph is on this page here:
https://www.pedelecforum.de/forum/index.php?threads/250w-bbs01-drehmoment-wirkungsgrad-leistungsdaten-von-bafang.38435/

The chap who sells them - Whoosh Cycles - says, "You can hear the 'tune' of the motor - always keep the noise down and RPM up, in the region of 70RPM to 100RPM to keep the motor sweet."

So you answered your own question!
Let's assume you aim for 100 RPM at 25 km/h on the flat. You will cover a distance of 417m every minute, or 4.17m every pedal stroke. If you had a 1:1 gear ratio, you would need a wheel diameter of 4.17m/3.1416 = 1.33 m. Divide this by 0.0254 to convert into inches, and what you need is a 52 inches gear.  If you want to climb a hill at 10 km/h still at 100 RPM, then you need a 21 inches gear.

Please tell me if I'm not clear enough!

Re: Optimum rpm and gear inches for low gear
« Reply #14 on: 22 August, 2017, 07:57:03 am »
Any motor, electric or petrol, can be described by a torque vs RPM curve. The said curve can be approximately flat, but only within a limited range of RPMs.  I may be able to help you if you can show us the curve.

The graph is on this page here:
https://www.pedelecforum.de/forum/index.php?threads/250w-bbs01-drehmoment-wirkungsgrad-leistungsdaten-von-bafang.38435/

The chap who sells them - Whoosh Cycles - says, "You can hear the 'tune' of the motor - always keep the noise down and RPM up, in the region of 70RPM to 100RPM to keep the motor sweet."

So you answered your own question!
Let's assume you aim for 100 RPM at 25 km/h on the flat. You will cover a distance of 417m every minute, or 4.17m every pedal stroke. If you had a 1:1 gear ratio, you would need a wheel diameter of 4.17m/3.1416 = 1.33 m. Divide this by 0.0254 to convert into inches, and what you need is a 52 inches gear.  If you want to climb a hill at 10 km/h still at 100 RPM, then you need a 21 inches gear.

Please tell me if I'm not clear enough!

Thank you that is an excellent explanation and the results equate to my 7th and bottom gears.
I don't know about answering my own question so much as finding an answer from many of the replies that have helped me understand.
I recognise pi in the formula whereas Cycleman, a long time friend of this parish, who tells me that I am over-thinking the whole electric bike thing, only recognises pi if it has a thick crust and gravy oozing out :)
Thanks again for your help - much appreciated.
Never knowingly under caffeinated

Re: Optimum rpm and gear inches for low gear
« Reply #15 on: 22 August, 2017, 08:01:37 pm »
Meat pie  :o , Apple pie :thumbsup: now you're talking  ;)
the slower you go the more you see

Re: Optimum rpm and gear inches for low gear
« Reply #16 on: 22 August, 2017, 10:11:43 pm »
Meat pie  :o , Apple pie :thumbsup: now you're talking  ;)
Yep, I can see you eating Apple pie with gravy. In fact, I probably have done :)
Never knowingly under caffeinated

Re: Optimum rpm and gear inches for low gear
« Reply #17 on: 23 August, 2017, 08:57:36 am »
Yellow gravy ( custard )  ;D
the slower you go the more you see

Re: Optimum rpm and gear inches for low gear
« Reply #18 on: 05 February, 2018, 08:54:57 pm »
It has taken more than two years in the planning and six months in the fitting, but today I pedalled electric on an upright.
I had it on the Trice, and fitted a Windcheater fairing, but didn't feel comfy about going out on it. Deciding that I am going to have to work my way back to the darkside I tried the motor against three of my upright frames and it fitted none of them.
In the cellar I had a skip recovered Ammaco frame that, besides the top tube being parallel to the ground, is the same dimensions as my On One frame that is a nice fit. The LH thread on the BB was knackered but BB threads aren't needed with the Bafang. Assembly was slow as I stripped bits off other bikes to make this one whole but it went together easily.
Aunty Helen's blog provided the settings to use: five levels of assist and 25kph max speed. (you can have 9 levels of assist and 42kph if you wish) and then I waited two weeks for a day that brought together low traffic, no rain or ice and no man flu. Today was the day.
Only five miles travelled, only one very close overtake, and a very enjoyable little ride after a long time without. I did two significant hills to experiment with gears/assist and was very pleased.
I was normally at level one or two with the assist and level 4 made a very noticeable difference uphill. It gave rise to the thought that each of the Alfine 8 gears now has five levels available if the motor is turned on. Something like 48 gears, or is that too simplistic?
The thumb throttle didn't work because when I pushed it with my thumb the whole thing revolved around the bars. The mechanic hadn't tightened the clamp. I'll have words with him.
Big thank you's to the people who helped out here, and on other threads: Kim, Aunty Helen, JennyB, FrenchTandem among others.
Tomorrow is saddle repositioning and fitting a Thudbuster. Even six years post op low pressure Big Apples and a Rido saddle are not enough.

Never knowingly under caffeinated

Re: Optimum rpm and gear inches for low gear
« Reply #19 on: 05 February, 2018, 09:54:16 pm »
Welcome back to riding in the fully fresh air , well as fresh as it gets in Wolverhampton  :thumbsup:
the slower you go the more you see

Re: Optimum rpm and gear inches for low gear
« Reply #20 on: 06 February, 2018, 08:17:36 am »
Welcome back to riding in the fully fresh air , well as fresh as it gets in Wolverhampton  :thumbsup:
Ah but, 5 minutes to the rurality of S. Staffs, half an hour to S. Salop and 5 hours (at my pedalling) to the Welsh border.
The American definition of Slough is rather apt - swamp!  ;D

I din't include you in my thank you list 'cos I knew you'd include yourself  :D

Have a nice day there and speak soon.
Never knowingly under caffeinated