Are RC motor drives the 'dragsters' of e bikes?

lesdit

lesdit

100 W
Joined
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I've been reading the threads, and have seen some videos of some rc powered bikes, mostly bmx, doing little
bursts of acceleration. It just occurred to me that this is what drag racing is like.
Funny car motors make >4000 HP for a few seconds. They have no radiator.
Now, are RC motors in this dragster category?

My new build is a DH bike setup for trail riding. Uphill. 500 ft elevation gain in one pull.
Are RC motors thermally capable of a herky jerky single track hill climb ?
Right now I'm going with the Cyclone geared motor, but I'm open to all options.
 
Hi,

lesdit said:
My new build is a DH bike setup for trail riding. Uphill. 500 ft elevation gain in one pull.
Are RC motors thermally capable of a herky jerky single track hill climb ?
Right now I'm going with the Cyclone geared motor, but I'm open to all options.
Click to expand...

Depends on the motor and gearing. For an Astro 3210 properly geared, Crank-Drive system (like the Cyclone) the answer is yes.
 
You cant get the power you want on a tight budget but the technology is available.
 
Storm said:
You cant get the power you want on a tight budget but the technology is available.
Click to expand...

Depends on how you define tight. You can get something going for less than the amount someone had to spend for a hub conversion a year ago, but it might be a wiser to spend for quality goods if you can.

The motor could be 200 dollars. You can get some high performance batteries for 150 dollars(lipo). Charger - 100 dollars. Controller 200 dollars.

We're up to 650. Add in the throttle, wires, odds and ends, and it's looking like 800-900 dollars. Okay, so that's about the same as a hub conversion for a 530x type of motor, but it's definitely going to be lighter and a higher performer.
 
I looked at the specs for the Astro 3210, then just for a reality check I looked at the specs for an Etek.
Something does not add up. The Astro weighs 4 lbs and the Etek weighs 22 lbs.
The Astro makes 5 inch ounces/amp = 80 in/lb torque per amp. It's rated at 75 amps continuous.
The Etek makes 1.2 in/lb torque per amp. It's rates at 100 amps continuous. This motor is 80 to 90 % efficient.

Why do these specs differ so wildly, making the Astro, the size of an orange, look like it can power a golf cart ?
 
lesdit said:
Why do these specs differ so wildly, making the Astro, the size of an orange, look like it can power a golf cart ?
Click to expand...
A 6" DC cart motor from the 1970's weighs ~75lbs and produces 3hp. Takes eight 6V batteries that weigh 50lb each, to make a two-seater go 25mi.

Now, a tiny motor and lithium batteries weigh a fraction, take up a fraction of the space and deliver more power, more efficiently.

Ain't tech fun?
 
lesdit said:
I looked at the specs for the Astro 3210, then just for a reality check I looked at the specs for an Etek.
Something does not add up. The Astro weighs 4 lbs and the Etek weighs 22 lbs.
The Astro makes 5 inch ounces/amp = 80 in/lb torque per amp. It's rated at 75 amps continuous.
The Etek makes 1.2 in/lb torque per amp. It's rates at 100 amps continuous. This motor is 80 to 90 % efficient.

Why do these specs differ so wildly, making the Astro, the size of an orange, look like it can power a golf cart ?
Click to expand...


It can power a golf cart.

You aren't looking at the same units though, and similarly, the torque as a quantity is irrelevant. You must look at torque with RPM together, which is unit known as power. The astro makes 1/4 the torque, but also can rev 4times higher. This means you can use a 4:1 gearing reduction on the astro to match the torque of the E-tek.

Ultimately, when comparing motors, it all comes down to efficiency and max power. The other units are needed for setting up a design, but do not dictate the performance potential like efficiency and max power.
 
oOps, I had the math backwards. But to summarize, the Astro, because of the rather high RPM's , is able to
make gobs of power. The Etek turns slower, but is still 90% efficient.
Perhaps an analogy would be an F1 ICE , that spins at 18,000 rpm, but is only 1 liter, and makes hundreds of H.P. vs
a chevy LS4 engine, 6 liter and 400 HP.


Miles said:
lesdit,

5 ounce inch = 0.312 5 pound inch :wink:
Click to expand...
 
So the 'rub' is the speed reduction, as the Astro motor pdf states the peak power is at
above 6000 rpms. Getting the rpm down to a chain ring cadence of 100 rpm or close to it is a challenge.

Gas turbines run 50,000 + rpm, and they get the speed down for traction applications, so there must be a way ;)
A not too expensive way is the stickler. Need a 50:1 ratio , lets say, for a chain ring setup.
 
lesdit said:
So the 'rub' is the speed reduction, as the Astro motor pdf states the peak power is at
above 6000 rpms. Getting the rpm down to a chain ring cadence of 100 rpm or close to it is a challenge.

Gas turbines run 50,000 + rpm, and they get the speed down for traction applications, so there must be a way ;)
A not too expensive way is the stickler. Need a 50:1 ratio , lets say, for a chain ring setup.
Click to expand...
Topic:
Ideal form of speed reduction for powered bicycles?
 
Bicycle tires have such a huge circumference relative to say a kart or scooter. Those, you could get away with a single stage. 10t to a 100t with small OD tires.. no prob even for a high rpm motor. But on a bicycle multistage gets it done.
 
I argue a gear ratio of 6 to 10 is practical enough for modern RC motors when you drive the wheel directly - you just have to have the right low kV.
 
I'm copying my reasoning from another post.

My scooter seems to do just excellently with 8 inch wheels on a gear ratio of 4 with a 200 kV motor. For a 26 inch wheel, that corresponds to a gear ratio of about 12. With a 100 kV motor, that'd correspond to a gear ratio of 6. That's certainly feasible! And it just seems so astonishingly simple in a way that I just can't ignore the possibility.
Click to expand...

It's known that an astro motor is possible down to 59 kV and the hobby city motors are available down to 130 kV. If there's some gaping flaw in my logic, please expose it. I'm sure you'd have no troubles with that, Miles. ;)

And... for Christ's sakes, it's quite easy to check using the simulator. :roll: I should just go ahead and try that.

EDIT: Specific example coming up...

Total weight: 250 pounds
Gear-ratio: 8:1
100 kV
26 inch wheels
60 amp battery limit at 50v
6 amps of no-load current
30 mOhm motor resistance
An 10ish AH a123-like pack's internal resistance, or a typical higher-end lipo pack.


This is on flat ground. Highlights, speed = 45 mph and generated heat ~= 400 watts. The acceleration is pretty good as it goes from 0 to 25 mph in a little over 3 seconds.
screenshot.1.jpg

This is on a 10% hill. Speed is 31 mph, generated heat around 440 watts accelerates to 15 mph in about 3 seconds.
View attachment 1

You can get even better performance by increasing the amps and/or voltage.

And last but not least, here's a generic performance profile of the motor/bicycle over the speed range. You'll notice that the motor doesn't generate dangerous levels of continuous heat until about 15-17 mph at a full 100% 60 amps of throttle suggesting it's "geared properly".

screenshot.3.jpg
 
Hi,

lesdit said:
So the 'rub' is the speed reduction, as the Astro motor pdf states the peak power is at
above 6000 rpms. Getting the rpm down to a chain ring cadence of 100 rpm or close to it is a challenge.
Click to expand...
You got it. An excellent summary.

lesdit said:
Gas turbines run 50,000 + rpm, and they get the speed down for traction applications, so there must be a way ;)
A not too expensive way is the stickler. Need a 50:1 ratio , lets say, for a chain ring setup.
Click to expand...
If $385 isn't too much Matt's two stage (or a DIY version) is an excellent solution. Belt for the high RPM first stage for quiet. Chain second stage for strength (chain is moving slow enough at this point that noise isn't an issue). Something like 5:1 and 5:1 gets you to 25:1. You can easily get another 3:1 between the drive output and the Crank Chain ring for up to at least 75:1.
 
I'm certainly after using the bikes transmission to get more out of the motor in various conditions, from flat fire road to brutal single track climbs. So about 50:1 ratio would be about right for the Astro 3210 ? I'm thinking metal gears for 50:1 and a belt or chain final to the BB, Their pdf data sheet seems to indicate 2.8 HP at 7500 rpm.
 
swbluto said:
If there's some gaping flaw in my logic, please expose it. I'm sure you'd have no troubles with that, Miles. ;)
Click to expand...
Sure. My pleasure :)

From what specific motor is the armature resistance/no load current data that you've used in your calculator example from?
 
lesdit said:
I'm certainly after using the bikes transmission to get more out of the motor in various conditions, from flat fire road to brutal single track climbs. So about 50:1 ratio would be about right for the Astro 3210 ? I'm thinking metal gears for 50:1 and a belt or chain final to the BB, Their pdf data sheet seems to indicate 2.8 HP at 7500 rpm.
Click to expand...

50:1 is not quite enough to pedal with the motor at all times - 65:1 would be better.

Unless you mean 50:1 plus another reduction to the crank?
 
Another calculator Q : You are derating the motor as it heats up during the synthetic hill climbs, right ?
Might be good to plot effective H.P. as well, and motor RPM.

>>>> Unless you mean 50:1 plus another reduction to the crank?

Yes, more reduction via belt/chain to BB.
 
lesdit said:
Another calculator Q : You are derating the motor as it heats up during the synthetic hill climbs, right ?
Might be good to plot effective H.P. as well, and motor RPM.
Click to expand...

Haha. Dude, society is moving AWAY from horses. Let's move on with the kW(It's in watts - just divide by a 1000). As well, you can plot RPM with the graphing utility by selecting RPM instead of MPH. The No-load RPM is about 5000 in my specific example (50volts*100kV = 5000) and the motor is usually going to be somewhere between 2500 to 4500. But, to be clear, I DON'T have a fetish with matching the crank's speed and the additional complexity involved with gearing it down to the cranks, and I'm just considering driving the wheel DIRECTLY.

Anyways, no, the motor wasn't derated as I'm sure you're more than capable of estimating the affect of heat on the size of the motor - 400 watts isn't going to be killing a 3 lb motor any time too soon nor will it be significantly decreasing performance (For an inrunner, check the continuous current rating or some such. Bob does post the continuous current rating for his motors, and you can check the motor current on your graphs to see how able you are to stay within "the safe zone"). Also, if you go with an astro motor, the no load current should be substantially lower which means less heat. You can calculate a custom example using THE SIMULATAH (See below). If you want to derate the motor to account for heat, there's a box where you can put in the armature temperature.

Anyways, the entire point is that it was feasible and almost ideal, not that the specific details were militantly exact in their realism.
 
Haha, Yes, we are moving away from horses, I still like the lower numbers I guess. However society *is* moving to transmissions. I just can't get away from the boost they give. Bike already has a gearbox, shame to bypass it! I wonder if 'Bob' would like to post some measured graphs of the motors operating characteristics to help understand them better.
Sorry if I hit a nerve by pointing out the thermal derating factor, it's not the elephant in the room, but maybe just a pig. Thermal derating is a bitch obviously, because you don't know the airflow, mass and stuff. Never mind trying to predict the effects of sand or silt bogs on an uphill single track. Motor overheaters to the max!
 
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