Electric Motorcycle Gearing; Don’t THROW AWAY POWER on Your eBike
So, how the heck do I figure out what size rear sprocket to put on this bike? What size to start with? This is boring. Let’s go on a ride. You’ll see me keep looking at my phone. I’m really not texting and driving. I just wrote a bunch of notes on here. All the stuff I need to know. A motorcycle racer will gear his bike to hit red line at the fastest part of the track. If his bike is geared to go faster than he goes on it, he’s throwing away torque and therefore throwing away horsepower. So bike gearing is very important. If you grew up speaking the King’s language, you thinking horsepower and torque. Torque’s a rotational force. You want to say power, but it’s actually incorrect. So it’s force. Force time distance on a rotational axis. Horsepower is power or the rate of force over time. So your horsepower or your power is just a mathematical product of RPM and torque. It’s torque times RPM divided by 5252. With our electric motors, we should shift our thinking to the metric system. Horsepower is now kilowatt. Kilowatt is power. Torque is still torque. Except in metric, torque is measured in Newton meters and not foot pounds. So the formula turns into torque* RPM / 9549. The point to take away from all that is there’s an inverse relationship between power and RPM for a given motor power just at one point during that power delivery. If you don’t change the power or the kilowatts for that point in time and I’m not talking about motor RPM here, the motor RPM is going to be fixed once it gets to a certain RPM for that given time it’s fixed. I’m talking about the output RPM, you know, going to be the rear wheel or it’s going to be a shaft on the end of a gear reduction drive. If you make that output RPM faster or higher RPM, you’re going to lower the torque. If you make that output RPM slower or lower RPM, you’re going to increase the torque for that given power that that motor is putting out. up, torque goes down. If you want to increase both torque and RPM for a quicker and faster bike, you’re going to have to increase your volts or increase your amps or both. And we can control the RPM of the output shaft with gearing. This is why you can’t take off in sixth gear on your motorcycle from a dead stop. You’ve told the gearing you want a super high RPM, so you have no torque and you can’t even get the thing to move and it just kills the bike. And we’re going to use gears to manipulate the torque and RPM of the output shaft, which is our back axle, to get it where we want it. So, the bigger the back gear, that back sprocket, the more torque you’re going to have, the slower that wheel is going to go. The smaller that back sprocket, the more RPM you’re going to get, the faster that wheel is going to go, but the less torque you’re going to have. So, for a given electric motor RPM, that’s the little gear coming out of your motor, or that’s the shaft going into your gear reduction if you have a gear reduction motor. for that given RPM. We can increase the wheel RPM to get more speed or we can decrease the wheel RPM to get more torque. No, more force. The force you feel is technically not power, but it’s that force you feel that that acceleration, that wheeling, that burnout, that all comes from more torque. It all comes from a bigger sprocket in the back. To figure out what sprocket size we need to start with on this build, we have to take in consideration all the known that we have on this. So, our known are wheel diameter. You can go to a uh tire size calculator here on Google and you can easily find your wheel diameter. I’ve got this 180x 55 * 17 on there. Puts me at 24 1/2 in wheel diameter. The other known is this motor I bought. This GTS 35 came with a 13 to front sprocket. The other known I have is RPM. And if I look at the specs, it shows 10,000 RPM continuous and then a peak of 15,000 RPM. Now, I already know that the FAR driver won’t support beyond 12,000 RPM, unless you get the racing model, which I don’t have. I’m going to use 12,000 RPM for my peak, and that’s the maximum RPM with the four- pole motor. And I’m using peak to do this calculation so I can get a top speed. I’m not going to obviously be cruising at 100 mph, but I want to know what my top speed is. That’s going to come into play here in a second. The other thing we know is I’ve got a 2.526 gear reduction. Or if you have a straight shaft coming out with a sprocket on it, like let’s say a QS138 version one, then you’re going to have a 1.0 for your gear reduction. It’s going to be none. So given those known, I need to determine a desired speed that I want. No matter what variables we throw in here for gearing and motor RPM and everything else, they all begin at 0 mph and they all go to whatever the top speed is we want. So, we can’t really use 0 miles an hour or takeoff speed. We need to start from a top speed and something to consider when you’re talking about top speed and you’re trying to figure out gearing. A gas bike is a little bit different. A gas bike is geared to go very fast, way faster than the bike will go in sixth gear. It might be geared to go 170, but the bike only goes 130, let’s say. And that’s because it’s really annoying to drive around at 10,000 RPM on your gas bike. You want to keep it in that sweet spot between 4 to 6,000 RPM. So you have a transmission in there that you can move around to keep your gearing where that bike rides in that sweet spot. So they tend to gear gas bikes higher than they actually go for speed. With an electric bike, we can ride it at 10,000 RPM or 5,000 RPM. The power curve is very linear, so it doesn’t really matter as much. It’s not annoying to drive at 10,000 RPM on your electric bike, or it’s not annoying to drive at 2,000 RPM. It’s not lugging down. It’s a nice linear power curve. So, what I want to do is I want to find a realistic top speed that I want to go. When I say realistic, keep in mind these electric bikes feel incredibly fast. Because of how the power comes on, there’s not really a curve like a gas bike has or it has to be at a certain RPM to make peak power. These electric motors can make peak power right away. So, even though my motor may only have the power of a much smaller bike, it’s going to feel like a much faster bike until I get up to speed. Now, once I get up to top speed, it’s going to feel like that smaller bike because remember, as RPM goes up, torque goes down. But when you’re starting out with no RPM, you’ve got gobs of torque. The advantage of the electric motor is we know the kilowatts we’re going to put out. Kilowatts is volts times amps. So, if we have 100 volts and I’m drawing 400 amps, then I’ve got 40,000 watts or 40 kW. On this motor, I have the Soon GTS35. It’s rated for 20 kow. It has a peak of 40 kW. So, I can go ahead and use 40 kW as my peak power. And I also know these motors are about 90% efficient. So, what that means is I’ve got about 36 kW peak. You can go into Google and uh translate that into 48 horsepower. So, I just need to find a bike that looks like mine and is shaped like mine and weighs as much as mine that’s 48 horsepower and I’ll know exactly what my top speed should be so I can backm the gearing. Well, I found this Ninja 400. It’s got about the same horsepower, same shape, same weight, pretty darn close anyway. And I know that this bike, just like my little Mad Dog I built, will run off and leave a Ninja 400 at a stoplight. The power delivery is not the same on electric bike as it is on a gas bike. But if you put these two bikes at top speed, my Mad Dog and that Ninja, they’re going to be about the same. They’re going to both be somewhere around 100 110 mph, somewhere in that range there. the gas bike make it a little bit better top speed and I’m going to get to a point where back EMF equals EMF and I’m going to quit accelerating for a gas engine bike it’ll keep accelerating a little bit so they’re getting like 112 an hour even on these Ninja 400s probably equates to about 100 mph on the electric version of it so I’m going to use 100 mph as my realistic top speed and I’m going to figure my gearing out from there I can go to the wheel speed calculator and put my 24 1/2 in wheel in there figure out that I need 1,373 RPM to make 100 mph. From there, I can calculate the rear sprocket size. To calculate that rear sprocket size, I first need to calculate the final reduction for that speed of 100 mph. So, if my electric motor shaft is going 12,000 RPM, and I want my back wheel going 1373 RPM, I can divide 12,000 by 1373 and I get 8.739. That’s my overall ratio, what the rear wheel is doing. So, if I don’t have a gear reduction motor, that single shaft QS138 coming out with a sprocket on it, then my final drive ratio will be the exact same as my overall drive ratio. Take that overall ratio, 8.739, multiply that times that front sprocket you have. Now, I’ve got a 13 to front sprocket on mine. So, 13 * 8.739, and that’s going to give me my rear to sprocket cuz that’s my final drive ratio. And that comes out to 113 teeth. That’s huge. That’s a big old sprocket. That’s why I like these gear reduction motors like the QS138 version two and three or this GTS35 engine. You can actually run a smaller sprocket because we’re going to gear reduce it at the motor and then gear reduce it again at the chain. Call that motor reduction a transmission gear reduction. And if you take your transmission gear reduction and multiply it times the final gear reduction, which is the chain and sprockets, then you get your overall gear reduction, which is your wheel speed. and you multiply them because the one gear reduction has an effect on the other one all the way through it. They tell me the gear reduction of that motor on the specs, 2.526. And I know my overall gear reduction I want is 8.739. So if I divide 8.739 by 2.56, I’m going to be left with what I want for my chain and sprocket, my final drive gear reduction. And that final drive gear reduction would equal 3.459. We can call that 3.46. So that’s what I want my sprocket reduction to be, 3.46. Do the same thing I did before. I know I’ve got that 13 to in front. I can multiply that 13 to sprocket times 3.46 and get what I want the rear sprocket to be equals 44.98. So I’ll put a 45 to sprocket on the rear and that’ll give me that 8.739 overall gear reduction. So you can go back to that wheel speed calculator and you can throw these numbers in there. They’ll have places for your transmission reduction, everything like that. And you can get your speeds at RPM. I figure I want to cruise at 60 65. That’s going to be somewhere in the 8,000 RPM range. And I think that should be okay. So, this will be a plan and it’ll just be a starting place, but you have no idea what size sprocket to get. What do you start off with? You don’t want to waste a bunch of money. These sprockets are anywhere from 60 to 100 bucks a piece. And which one do you get? So, I’m going to start off with this 45. See how I like it. And then I’ll make a change from there. And hopefully he’ll only have to buy one or two sprockets.
Gearing and Sprocket Size for Home Built DIY Electric Mid Drive Motors
