Understanding Gearing via V4 Reduction Drive

[SHiFT]

OK. So I am now awaiting all my parts and I'm trying to determine if I went the right route with the gears and stuff.

I think I did, but I'm just having a hard time wrapping my head around gears, ratios, and so on. Could you just let me know if I've understood it all properly, and what the drawbacks of my setup could be?

So, Matt's reduction drive has a 4:1 ratio, output:input.
I understand this to mean, the drive reduces the speed of the output shaft by 4, and increases torque by a factor of 4.

Now, the thing that concerns me, is after the reduction drive, Matt uses a small cog to drive a larger cog. This is a further reduction of the output shaft speed, but an additional increase of torque, correct?

But I'm going the other way. I will put the larger cog on the reduction drive output, and the smaller cog on the wheel (like a normal bike setup) This would effectively give me higher rpm, but less torque correct?

Now, Matt has said his "final drive ratio" is a tad under 12:1 (I thought reduction drive was only 4:1, is the rest from the big rear cog?)
My pulley and sprocket ratio will be about 1:2. I'm going 20t in the rear 39t up front.

THe more reading I do, the less I understand how this will work in the sense that, before I thought it'd be OK, cuz it's just like a normal bike after the reduction drive. But, now I'm thinking that it could require too much torque from the motor to turn the wheel, leaving me in a position to burn out the motor and not go anywhere?

Am I going to be able to ride this thing or what?

As I'm starting to understand, I don't see how it'll work.

Thanks for any input you might have.

Few resources I've read/tried.
http://www.nightrider.com/biketech/calc_gearratios.htm
http://www.teamdavinci.com/understanding_gear_reduction.htm
http://www.howstuffworks.com/gears.htm

SHiFT

 

[Miles]

SHiFT,

You've pretty much solved this yourself.

Bicycle gearing reduces torque to gain speed (generally).

Motor gearing reduces speed to gain torque.

So, by copying your bicycle gearing for the second stage you're going 4 steps forward then 2 steps back

To arrive at the total reduction you multiply the stages. So Matt has 4:1 & 3:1 = 12:1

 

[SHiFT]

Thank you for clarifying, Miles.

Just means more stuff for me to do to fit these parts into something resembling a drivetrain heh.

Best,

SHiFT

 

[jag]

SHiFT,
You've pretty much solved this yourself.
Bicycle gearing reduces torque to gain speed (generally).
Motor gearing reduces speed to gain torque.
So, by copying your bicycle gearing for the second stage you're going 4 steps forward then 2 steps back

"4 steps forward and 2 steps back" captures a "through the crank" drivetrain head on I think.

Just to complete the picture: Shift I think you forgot how the motor is typically coupled in a "through the cranks" drive system. Here's the complete drivetrain:
First the 2 stage matt drive gives a 3:1, then 4:1 reduction for a total of 12:1 reduction.
Then a freewheel on the Matt reduction, typically 14t drives one of the front chainrings, say 40t. So we have a a further 2.86:1 reduction.
Next starts the bike drivetrain. You mention a 39t sprocket. That would be a typical 2nd sprocket on a road crank set. This can go to a single speed rear freewheel or a 7/8/9 cluster. Single speed toothcount are from 13 or 14t and up. Clusters are typically 11-32 or 14-32.
For example when running on the 11 tooth smallest sprocket we get a gearing 1:3.5.
On the middle 20t sprocket that you mention we get 1:2

Multiplying all together gives final ratio, e.g. (3/1)*(4/1)*(2.86/1)*(1/3.5) = 9.8 or a 1:9.8 reduction in total between the motor shaft rpm and the rear wheel. At cruising speed the motor will run at about 3000rpm. This is a good rpm for a relatively large motor such as a Mars or the gearless 1kW Cyclone. These weigh 5-10kg, but are unfortunately a bit too big to mount on a matt transmission.

Some choose to instead drive the rear wheel directly, e.g. by putting a sprocket on the disc brake mount. One might choose a 30t sprocket for the rear wheel and get (3/1)*(4/1)*(30/14) = 25 or a 25:1 reduction. At cruising speed the motor will run at 7500rpm. This is an ideal rpm for the small and lightweight (1-2kg) but powerful (2-6kW) Astro motors that fit the Matt drives. Astro rates their motors for peak power at 7500rpm, and they have an efficient range from 5000rpm-10000rpm without much loss of power, so one can go both faster and slower.

 

[rodgah]

also shift,

there is ALOT of advantages to having the motor run in its efficient range. What you are suggesting would not allow this. That is the reason matt reduces even further (to 12:1) to the back wheel, that not only makes the bike a handfull (if running a 3220) to ride 8) fun 8) but also allows the motor and controller to work efficiently and within their limits. Depending on the wind count of the motor determines its top rpm. Generally on a 26" bike with one of matts drives you will see a top speed of 40mph. now with your suggested 1:2 reduction that would give you a theoretical top speed of 240mph. Obviously this isnt possible as the controller and motor would not be able to handle the torque required. Its not as easy as cruising at 10% throttle to keep your speeds at a more realistic 30mph, this still loads the system beyond the point of failure to get to that speed. This is why you gear the bike to do the top speed you would want to do, and in doing so puts the efficient point of the system at a more reasonable cruising speed, and doesnt load the controller and motor more than they can handle. I think the max matt or most people would suggest is 40mph, to keep it reliable.

hope this helps

Cheers

 

[SHiFT]

Wow, you guys are immensely helpful. Thank you.

I totally get it now, although lets just say if it's got nothing to do with money spent or hours worked, a 3rd grader could best me in arithmetic.

So to think aloud for a sec: I will directly power the rear wheel with that combo of cogs. I am thinking I will simply swap the 39t to the rear, and the 20t to the front. By your math, I get 1.95 (3/1)*(4/1)*(39/20), but not sure how that converts to a X:1 ratio... ?

More thinking... smoke emitting now: I will probably want to step that rear cog up to a 45-50t.
With the 20t front cog and a 50t in back, I would lower my RPM at cruising speed. With a 45 (actually 46t) I would be almost on par with 30/14, at a 46/20 = 2.25 versus 30/14 = 2.14.

Putting me up around 7200 +/- 100 rpm.... Or I just increase my cruise speed

I have to wait 8 weeks for the motor to come in, but I will show off the ride I'm building asap. I am sure, by then, I will owe it to you and many other ES members whom I annoyed with rudimentary questions for 2 months. Heh.

Thanks all.

 

[REdiculous]

So to think aloud for a sec: I will directly power the rear wheel with that combo of cogs. I am thinking I will simply swap the 39t to the rear, and the 20t to the front. By your math, I get 1.95 (3/1)*(4/1)*(39/20), but not sure how that converts to a X:1 ratio... ?

1.95:1

Including the first stage at 4:1 that would give you 7.8:1 total.

edit...with 46/20 sprockets you'll have 9.2:1 total.

 

[SHiFT]

Cool.

Thanks guys.

 

[recumpence]

I normally prefer running a second chain to the left side of the rear wheel to separate the drives and to get the ideal ratio without more stages than is needed. My drive system is 4 to 1. The additional 3 to 1 (making it 12 to 1 overall ratio) is accomplished from the drive output to the rear wheel. This ratio seems ideal for most applications.

Matt

 

[SHiFT]

This thing will be a screamer either way, thanks to your e-drive kit. I went ahead and placed an order for the 46t for the rear, and 20t up front, I'll have to get adapters made.

I can't wait to get mine rolling. Wheels came in today. Brakes in a week, and batteries at some point. :|

Would you say a couple 36v lipo packs would suffice, I don't need to ride long distances just yet.

Do you have a specific recommendation for your kit Matt?

Best,

SHiFT

 

[SoSauty]

Now that you see what's involved, use Warren's short cut calculator;

http://www.recumbents.com/wisil/e-bent/rc_drive/rc_drive_calculator.asp

 

[SHiFT]

So, I am also considering maybe going with a nexus 7 internally geared hub, and I'm thinking through how this will work.

Coming from RC cars, a transmission boosts your speed immensely. It's almost like turbo. But with bikes, it goes the other way, gears are not meant to increase speed, but increase torque.

Simply adding the internal hub is not going to give me that much speed then, aside from what little extra I might get from the highest gear?

Keeping the 20t to 46t layout, the hub would only really come into play on hills - I'd shift down to free up the motor to spin more, creating more torque, but reducing speed.

So if I want to add speed by adding a transmission, my only choice then would be to swap the 46t in the rear with a 55 or even 60t. Then I'd add the internal hub and setup the bike to operate on flat ground on say, 3rd gear. I could then "gain speed" by switching to higher gears, but I would lose the flexibility to switch to a lower gear to climb hills, as I now only have a few gears instead of 7. Am I thinking of this properly? EDIT: Wait, I'm confused. To do what I'm thinking, I'd need a smaller cog in rear correct?

Thanks all
SHiFT

 

[spinningmagnets]

Here's a wheel RPM to speed chart, and for me, this is the easiest place to start a build plan. I always recommend to decide what you want your top speed to be, gear your highest sprocket for that speed, and then the remaining lower gears (as many as you have) will help you on increasingly steeper hills.

http://www.endless-sphere.com/forums/viewtopic.php?f=28&t=16114

A common goal might be to attain a top speed of 27-MPH (43-k/h) on a 26" diameter tire, which comes out to 349-RPMs at the driven tire. At the other end of the equation is the volts you plan to use (more volts is almost always better). The variables in the middle are: how much reduction will I need if I use a motor with a kV of "X"? (kV is how many RPMs the motor provides per volt applied). If you apply 22V to a 200-kV motor, you should get an unloaded speed of 4400-RPMs (loaded speed is roughly 70% of unloaded speed).

In this example, the motor would be at 4400-RPM X 70% = 3080-RPMs when you apply a load. The reduction would then be 3080-RPMs at the motor to 349-RPMs at the tire. If the motor gets hot under your usual loads, you may need a larger motor, or...you could try applying more volts (making the motor spin faster), and then use a higher reduction to get it back down to 349-RPMs.

 

[SHiFT]

OK,

Say I want to kill myself at a top burst speed of 40mph.
40x12.93 = 517.2 rpm at the wheel

My Motor, astro flight does:
275kV @ 44.4V = 12210rpm
At load that is roughly
12210 *.70 = 8547

Now I need to reduce that to 8547rpm down to 517.2 rpm, does that mean I need to reduce down 16:1?

My thinking is 8547/517.2 = total reduction.

EDIT:
Just so I understand the concepts:
If I were to decrease the size of the rear sprocket, maintaining a flat RPM of 5000, hypothetically, this would increase speed, and lower torque.
To maintain the same 40MPH I would lower my RPM. This would be less reduction.

Inversely, increasing the rear sprocket size would mean increasing the amount of the reduction, increasing torque, but losing speed. To maintain speed, I would need to rev the motor up more?

Thanks so much for the help.

SHiFT

 

[recumpence]

I would say a 4 turn 3220 would be best. That has a KV of 170. That is the motor I am running in my yellow trike. It is a beast and still runs very reliably. That would be ideal with a drive system reduction of 4 to 1 (my V4 drive is 4 to 1) and 3 to 1 from the drive to the rear wheel.

Perfect.......

Matt

 

[SHiFT]

Yeah, and according to AstroFlight's website, those are $700. Hah, don't think I can do that.

Why is the price hiked up so much more than the 3210, what's the big difference, torque?

 

[recumpence]

It is twice the motor of a 3210.

Matt

 

[Miles]