Bike Forums > Please clarify gear reduction. Thanks.
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 12-08-13, 11:14 AM #1 likebike23  Rides Majestic Thread Starter   Join Date: Jan 2013 Location: Westfield, MA Bikes: 1983 Univega Gran Turismo, 1970 Schwinn Super Sport, 2001 Univega Modo Vincere, Self-Built Nashbar Touring, 1974 Peugeot U08, 1974 Atala Grand Prix, 1986 Ross Mt. Hood, 80's Maruishi MT-18 Posts: 1,357 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 2 Post(s) Please clarify gear reduction. Thanks. I'm contemplating a DIY Ebike build using an EZip500 scooter for parts (24V 500Watt motor). I'm aiming for a low top speed, more torque setup for hill climbing. I'm figuring a 10-12 MPH max speed, does that sound about right for that purpose? To reduce the RPM at the wheel, I'm going to use a jackshaft with a primary and secondary reduction. To calculate the RPM at the wheel do you multiply the primary reduction and secondary reduction then divide the motor RPM by the total reduction? I've seen examples online where people were adding the primary reduction and secondary reduction to get the total reduction then dividing motor RPM by total reduction. I'm a bit confused, can anyone clarify? Thanks, Mike
 12-08-13, 12:56 PM #2 Ypedal  Senior Member     Join Date: Oct 2008 Location: Moncton, NB, Canada Bikes: Posts: 113 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 1 Post(s) Does the current motor get overly hot as it's setup right now ?... If it's relatively cool ( like.. cool enough to keep your hand on it comfortably after a ride) getting a higher power controller and allow more Amperage , V x A = Watts ... Higher voltage would result in higher motor rpm at full throttle, but if you keep the gearing and voltage the same as they currently are, the top speed will remain the same.. more amps just allows more power to the motor at lower speeds for better acceleration.
12-08-13, 02:33 PM   #3
turbo1889
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 Originally Posted by likebike23 I'm contemplating a DIY Ebike build using an EZip500 scooter for parts (24V 500Watt motor). I'm aiming for a low top speed, more torque setup for hill climbing. I'm figuring a 10-12 MPH max speed, does that sound about right for that purpose? To reduce the RPM at the wheel, I'm going to use a jackshaft with a primary and secondary reduction. To calculate the RPM at the wheel do you multiply the primary reduction and secondary reduction then divide the motor RPM by the total reduction? I've seen examples online where people were adding the primary reduction and secondary reduction to get the total reduction then dividing motor RPM by total reduction. I'm a bit confused, can anyone clarify? Thanks, Mike

Yes, you multiply the two reduction ratios of your two stages to get the total reduction and then divide the motor RPMs by that total reduction to get the wheel RPMs which with a known wheel size gets you your speed.

As far as the question you are asking about speed which I would best summarize as: "Gearing for a hill climbing ONLY motor - what speed?"

How fast you normally pedal a bike on the flat in calm air? For me with a regular bike without a cargo load that is about 20-mph a little more or less depending on the bike and road conditions. For most people that number is a lot slower, for some people its faster, for a very few its a lot faster.

How fast do you currently climb those hills? Are the hills all about the same or are some way worse then others? Do you want to plan more for help on the way worse ones?

Answers to those questions and I can help you more with this (I've built hill climbing helper ONLY builds myself) and you also need to realize that the RPM ratings of a motor are usually the free spinning no load speeds and under load you need to plan for less RPM then its rated.

My main hill helper ONLY build I have the free spinning RPM of the motor matched to 15-mph wheel RPMs so that it will give me absolutely zero assistance at any pedaling speed of 15-mph or more which since I usually pedal faster then that on the flat means I have zero temptation to use the motor and it gives me strongest assistance to climb a hill at anywhere from about 7-to-10-mph (47% to 67% free running motor RPM) and the assistance starts to steadily drop off above 10-mph climbing speed until its nothing at 15-mph which is about right for me. But if I try to climb a hill at much less then 7-mph and use the motor for assistance it can overheat the motor because I've pulled the motor so far down on its power and efficiency RPM curves.

Depending on the exact specs. of a motor and what voltage you are running it on and with what controller the peak power output (strongest) point in a motors output curve is going to be at or somewhere above the 50% of free running no load RPM and the peak efficiency output (getting most power for least amount of battery juice) is going to be a little bit less then free running no load RPM (90%-ish is a good guess) but there isn't much power available at that point and the in-between those two different sweet spots (the best power sweet spot and the best efficiency sweet spot) you will get a portion of the motors curve where you are swapping power for efficiency and if you back off on your pedal output and the bike slows down it pushes the motor into making more power to compensate for you backing off and when you pedal strong it makes you go a little faster which allows the motor to spin a little faster and further up its curve where it is more efficient but doesn't produce as much power output.

In that gentle and controlled balance between power and efficiency is right where you want to adjust your reduction gearing to be when the motor is being used. So that the motor and you work together as a team and whenever you back off and the bike slows a little the motor puts out more power to compensate and keep you from bogging down and loosing a lot of speed and whenever you put more effort into the pedals and you speed up the motor puts out a little less and becomes more efficient which combine together to drastically increase battery range.

Right in that zone between the two sweet spots in the motors power and efficiency curves is where you want to set-up your gearing to be on most of the hills you climb to give the bike the best feel and the best performance. This same principle applies to building a drive for high speed as well or a drive for all speeds if you build a mid-drive/crank-drive where the power goes through the bikes variable gearing in which case you just match the human pedaling RPM range and then it works for everything in the bikes gears, but doing that removes the purity of a "hill climbing helper ONLY" build which I certainly appreciate myself and have a couple of myself.

 12-08-13, 02:35 PM #4 likebike23  Rides Majestic Thread Starter   Join Date: Jan 2013 Location: Westfield, MA Bikes: 1983 Univega Gran Turismo, 1970 Schwinn Super Sport, 2001 Univega Modo Vincere, Self-Built Nashbar Touring, 1974 Peugeot U08, 1974 Atala Grand Prix, 1986 Ross Mt. Hood, 80's Maruishi MT-18 Posts: 1,357 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 2 Post(s) The scooter isn't currently running. I got it for free and it needs a battery, but the guy told me it runs. Once I confirm that everything works, I'll proceed with the build. I am going to remove all of the parts from the scooter and move them to a bicycle for a non-hub/mid drive setup. I'm not looking to modify the controller right off the bat. I'm going to concentrate on planning, designing, and fabricating motor/jackshaft brackets and gearing. The goal is to have fun designing/fabricating a working ebike from scratch on the cheap , to commute occasionally, and to run small errands. I plan on pedaling for most of the riding with the motor to help get me up the mountain that I live on. Top speed, acceleration, range are not current concerns. With my proposed setup, any component could be upgraded at a later time at minimal expense. In order to plan the build for what I want it to do, I need to figure out what is a good top speed for torque/hill climbing. The motor is 24V, 28Amp, 500Watts, and runs at 2600RPM according to the sticker on it. The total weight it would be pulling is 220 pounds. I figure that the motor won't be working too hard if it's geared for a 10MPH top speed. I also figure that 10MPH would be a reasonable in town/traffic speed. So, to calculate gear reduction/rear wheel RPM which will determine my top speed, do you multiply or add the primary and secondary reductions to get the total reduction? Do you then divide the motor RPM (2600RPM) by the total reduction to get the rear wheel RPM?
12-08-13, 02:45 PM   #5
turbo1889
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 Originally Posted by likebike23 . . . In order to plan the build for what I want it to do, I need to figure out what is a good top speed for torque/hill climbing. The motor is 24V, 28Amp, 500Watts, and runs at 2600RPM according to the sticker on it. The total weight it would be pulling is 220 pounds. I figure that the motor won't be working too hard if it's geared for a 10MPH top speed. I also figure that 10MPH would be a reasonable in town/traffic speed. So, to calculate gear reduction/rear wheel RPM which will determine my top speed, do you multiply or add the primary and secondary reductions to get the total reduction? Do you then divide the motor RPM (2600RPM) by the total reduction to get the rear wheel RPM?
You multiply the reductions of the two stages, you do not add them.

If you gear so that the motor is running at its full rated 2,600 RPM when the rear wheel is going 10-mph then your strongest hill climbing ability is going to be in the 5-6.7 mph range (1/2 to 2/3 max free running, no load RPM) and you will continue to get some assistance from the motor while pedaling above 6.7 mph up to 10 mph but the closer you get to that 10 mph mark the less strong the motor will be until it isn't doing any of the work and its all on you pedaling at 10 mph or above. Climbing at anything much less then 5 mph will start to make the motor overheat. It most likely will not go 10 mph but something a little less on the flat geared that way. Unless the RPM rating of the motor specifically says that is a loaded RPM then what they are giving you is a free running, no load RPM and you need to expect less RPM while its actually doing work and propelling the bike.

Do you have a speedometer on your current pedal only bike? Do you know how fast you currently go under pedal power only on the flat (and in town) and how fast you go climbing that hill back up to your house?

Last edited by turbo1889; 12-08-13 at 02:50 PM.

 12-08-13, 02:54 PM #6 likebike23  Rides Majestic Thread Starter   Join Date: Jan 2013 Location: Westfield, MA Bikes: 1983 Univega Gran Turismo, 1970 Schwinn Super Sport, 2001 Univega Modo Vincere, Self-Built Nashbar Touring, 1974 Peugeot U08, 1974 Atala Grand Prix, 1986 Ross Mt. Hood, 80's Maruishi MT-18 Posts: 1,357 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 2 Post(s) @turbo1889: Great reply, you totally got what I was asking and provided some excellent insight. I am really not 100% sure how to answer some of the questions I need to ask to refine my design. I will take what you said into consideration when I decide which way I will go. I figure if I choose my jackshaft out gear to be easily changed, then that will give me some room to refine my total reduction cheaply and easily. Thanks again.
 12-08-13, 03:12 PM #7 likebike23  Rides Majestic Thread Starter   Join Date: Jan 2013 Location: Westfield, MA Bikes: 1983 Univega Gran Turismo, 1970 Schwinn Super Sport, 2001 Univega Modo Vincere, Self-Built Nashbar Touring, 1974 Peugeot U08, 1974 Atala Grand Prix, 1986 Ross Mt. Hood, 80's Maruishi MT-18 Posts: 1,357 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 2 Post(s) @turbo1889: I'm not sure if my design I'm contemplating would allow use of the motor as an assist. It's going to be a left-hand drive with a freewheel on the jackshaft out gear. This would allow me to pedal on the flats and moderate hills without having the drag from the motor slowing me down. Does an assist bike not utilize any freewheel on the motor gearing? If that's the case, does the motor create undue drag during pedal only riding such as when you run out of battery or have a failure? Edit: A little research answered my question. Thanks again. Mike Last edited by likebike23; 12-08-13 at 03:32 PM.
 12-10-13, 02:21 AM #9 turbo1889 Transportation Cyclist     Join Date: Aug 2011 Location: Montana U.S.A. Bikes: Too many to list, some I built myself including the frame. I "do" ~ Human-Only-Pedal-Powered-Cycles, Human-Electric-Hybrid-Cycles, Human-IC-Hybrid-Cycles, and one Human-IC-Electric-3way-Hybrid-Cycle Posts: 1,206 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 10 Post(s) At least those are the ways I know of to do it via a roller drive chain (I'm not even counting those cheap nasty sprockets that just bolt to the spokes where they cross on the left side found in the cheap junk gas motor kits that are a good way to mess up a rear wheel fast). There is also the possibility of doing a friction roller drive that torque pivots into the rear wheel when engaged but that isn't the best option for a hill climber helper ONLY motor set-up because of the high torque levels at low speed that are usually involved. Same goes for putting the set-up on the front wheel, unless you have an incredibly beefy front fork the torque loads of a low speed but strong deeply geared down set-up are just too high.
12-10-13, 12:18 PM   #10
likebike23
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 Originally Posted by turbo1889 At least those are the ways I know of to do it via a roller drive chain (I'm not even counting those cheap nasty sprockets that just bolt to the spokes where they cross on the left side found in the cheap junk gas motor kits that are a good way to mess up a rear wheel fast). There is also the possibility of doing a friction roller drive that torque pivots into the rear wheel when engaged but that isn't the best option for a hill climber helper ONLY motor set-up because of the high torque levels at low speed that are usually involved. Same goes for putting the set-up on the front wheel, unless you have an incredibly beefy front fork the torque loads of a low speed but strong deeply geared down set-up are just too high.
Thanks again for the great info. The motor I've got (from a currie ezip500) has a counter clockwise rotation and I'm not sure if I can reverse it by changing the +/- terminals. That is why I'm planning a left side drive. I've also got a set of disk wheels which will be great for left drive.
I saw a cool option for right side drive where you use a trials crank which is threaded for a freewheel on the right arm. You can then use a flanged freewheel on the crank and put your chainrings on that.

12-10-13, 02:53 PM   #11
turbo1889
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Join Date: Aug 2011
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Quote:
 Originally Posted by likebike23 Thanks again for the great info. The motor I've got (from a currie ezip500) has a counter clockwise rotation and I'm not sure if I can reverse it by changing the +/- terminals. That is why I'm planning a left side drive. I've also got a set of disk wheels which will be great for left drive. I saw a cool option for right side drive where you use a trials crank which is threaded for a freewheel on the right arm. You can then use a flanged freewheel on the crank and put your chainrings on that.
If you gear the motor to a freewheeling crank instead of the rear wheel and you have a multi-gear rear end you will then have a full on mid-drive not just a hill climber helper build. Brushed motors are usually reversible but there are exceptions of course.

Disk wheel left drive with a custom sprocket on the 6-bolt disk mount is indeed a popular and effective methodology.