#### gman1971

##### 10 kW

- Joined
- Oct 17, 2015

- Messages
- 976

Anyhow, I am writing this stand-alone thread in response to the multiple emails, questions and general skepticism I get asked, almost on a daily basis with regard to my latest mid-drive build. Its got so bad that some people just think that I am just flat out lying when stating facts that my kit is running so well with the amount of power I am dumping, etc, and they suggest it should be not working, etc. All this, unfortunately, began happening since uploading my Cyclone 3000W video on Youtube.

So, with that out of the way lets begin.

First lets state that there are two schools of thinking on this matter: there are the guys who love the torque, and the guys (like me) who love the RPM. Both are right, but there are times when one way is preferable over the other.

So, why the torque and RPM schools? Well, simple, that's because we all lover POWER, and POWER (among other interpretations) is the product of TORQUE times RPM.

Or in a more "mathematically correct" formula...

Watts (power) = Nm (torque) x Angular velocity (rpm)

So, the most common sense way I see all the time is that more TORQUE equals more power, and this is true. But what a lot of people overlook is that at constant TORQUE an increase in RPM also yields to a power increase at the same rate as torque, and believe me, it is a lot easier to spin things than it is to make them put more force. This is the main reasoning behind people in the cycling world who recommend spinning the cranks over mashing the cranks... and my body tends to thank me (by not hurting too much) after riding 90 miles that I didn't mash the pedals at 60 RPM and instead spun the pedals at 100 RPM.

The same concept also translates to mechanical things.

So if you want to achieve insane power you can either use a massive amount of torque/force and a relatively low RPM... say, a comfortable 60-70 cadence at the crank so you can "pretend" to be pedaling your bike at 30+ mph. Now, before we comment about the other school of thinking, lets expand on this idea: If you have massive torque going through the drive train, what do you need to cope with that? well, lets see, you need a heavy duty chain, heavy duty cranks, sprockets, etc... which in turns ends up being all custom made stuff since most of the off the shelf bicycle gear made nowadays isn't meant to handle massive amounts of torque.

And now, on the other corner of the ring is the RPM school of thinking: have a relative low torque but a massive amount of RPMs at the cranks, in excess of 180 RPM; and even at such high RPM you can still "pretend" that you're pedaling your bike at 30+ mph, but now, because you're running 1/3 the torque at the chain/crank/sprockets over the 60 RPM from the torque school you're saving your drive train on the process.

So, in order to make it clear lets just pretend we are putting 10 kW of power on a mid-drive, and to make things simple we are going to use "figurative units", called "torque units"

So, first lets begin by understanding what the heck is 10 kW of power, or 10,000 Watts of power. Well, in simple terms 10 kW of power means nothing more than given any two random numbers that multiplied together yields a result of 10,000. Like 10 x 1000, or 100 x 100, or 1 x 10000, or even something as crazy as 0.1 x 100000... and as long as the two numbers multiplied together result in 10,000 you'll have 10 kW of power. Why? Because Power (10 kW) = TORQUE x RPM, which are the two numbers that are multiplied together.

But POWER is not only TORQUE times RPM only, POWER is also INTENSITY (amps) times VOLTAGE (volts) so we can say that

TORQUE x RPM = INTENSITY x VOLTAGE or T x RPM = I x V (formula #1)

Since power is coming from the energy stored in the battery, lets assume for example sake that our battery voltage is 50 Volts. Therefore, to achieve 10 kW we must run 200 amps @ 50 volts ( 50 V x 200 AMP = 10,000 WATTS)

Once we have 50 volts and 200 amps running through the wires we need to see the motor, which is what will convert all that electrical energy into mechanical energy.

Without going into much details suffice to say that an electrical motor will take amps and volts and convert them into RPMs and Torque. All electric motors have a constant that's called KV rating. You might've heard of this around here, but a motor KV rating is nothing more than the RPM at which the motor will spin given 1 volt. Or simpler yet, its RPM per volt. So a motor rated at 50 KV when you stick 50 volts it will spin at 2500 RPM. Higher KV will yield lower torque and higher RPM, lower KV will yield higher torque and lower RPM.

Since a motor converts power, and power must conserve we use our formula #1 and we get this:

2500 RPM x T(unknown torque) = 50 Volts x 200 Amps

Therefore, T = 10,000 / 2500, which yields T = 4 units of torque.

Again, I am using figurative units (these should be radians per second and newtons meter to be "technically correct" but the idea is what I am trying to convey here.)

So now we know that for this motor running @ 10 kW it must have 4 units of TORQUE when spinning at 2500 RPM, or in terms of specs:

BLDC: 50 KV, 4 units of torque @ 10 kW.

A motor that spins at 2500 RPM is clear that we can't just chain that to the crank... so in order to bring the RPM down we use these pesky things called REDUCTION GEARS. So, what the heck is a reduction gear? Nothing magical really. A reduction gear is a way to exchange your RPM into TORQUE, and the other way around.

(First I am going to explain the torque school of thinking)

So you want a comfortable cadence, so in order to bring the motor down to that comfortable pedaling cadence, we need to bring the RPM down from 2500 RPM to say, 60 RPM.

So applying our POWER = TORQUE x RPM and knowing that POWER must conserve we have that

60 RPM x T(unknown torque) = 2500 RPM x 4 Units of Torque

We solve for T (unknown torque ) and we get

T = 166.6 units of torque @ 60 RPM.

So what the heck happened here? We exchanged all those RPMs for some massive amount of torque; torque that will have to be transferred through the chain, sprockets, chainring... etc.

So if we are running a heavy duty chain we will be able to cope with that massive torque, say, using a BMX chain which has probably double the torque capacity of a 10 speed chain. Thats cool, you can use a BMX chain with some fancy and usually custom (and expensive) multi gear setup... so you can pretend to be pedaling at 50+ mph at 10 kW... it all sounds very logical to me, right? :lol: Now, we all know what ends up happening when you run this scenario with normal bike gear (no BMX stuff): the chain skips, sprockets get eaten alive, cranks get chewed up too and ultimately the entire drivetrain snaps and fails after a very short mileage...

To summarize this is how my old GNG kit worked: low RPM, massive amounts of torque, and for me (like for most ppl) it didn't fare very well. Thing skipped, stuff broke, snapped, overheated, and even frame twisted under heavy throttle, etc... not good.

Lets look at the RPM school of thinking.

Instead, when I built my Cyclone 3000W eBike I decided I didn't care about pedaling at 30+ mph at all, so I decided that my target RPM was to be 180 in order to reduce the stress on the frame, drivetrain, etc. So, now we work the same math and here is how it plays out: We have 2500 RPM and 4 units of torque from the motor, target crank RPM is now 180 so

180 x T(unknown torque) = 2500 x 4

We solve for T and we get

T = 55.5 Units of Torque.

Wow, so how can you be running so little torque and still have 10 kW? Very simple, my crank is spinning at 180 cadence at 30ish MPH. Therefore I am never on the 11T cog (the chain reaper), I am also running a fairly large chainring (48T) and a 21T sprocket on the rear cruising at 30mph, that coupled with the fact that my chain is running 1/3 the force vs. the 60 RPM example, it has less force and double the amount of teeth engaged at any time than a BMX chain running on the 11T. By lowering the torque inside the drivetrain I can make it last a lot longer than if you run a beefed up drive train, plus its a lot lighter too and doesn't require custom made stuff. And at the level of RPM we are talking on bicycles the heat dissipation isn't as big of an issue as say, a good old V10 F-1 engine that spun to 21K RPM to achieve the power with relatively small torque.

So, in order for this to work I needed RPM, and lots of RPM; and the Cyclone 3000W motor controller happened to have this fancy 3 speed switch which defaulted to mode #2; and once I unlocked mode #3 that gave me the unlimited RPM I was looking for so I could apply all the logic described in this post to my setup to build a 2.2 kW reliable mid-drive commuter.

As of this writing I have 1583 miles on my bike running 1500 watt average, with 2.2 kW bursts @ 30+ mph average and my chain hasn't even hit the 0.5% stretch mark on the ruler.

Hope this helps anyone, and looking forward to seeing folks push 100 mph with mid drive bikes, for those who are daring enough to test this theory!

EDIT: I am now building a tadpole trike with the same exact setup I have on my eBike... once I get something I'll post some pics of that as well.

Cheers!

G.