What MAC Motor to Get for 20" Wheel.

[The Mighty Volt]

I want to be running it at around 16s LiFe or LiPo.

Any ideas??

I have an X5305 but its too heavy for a folding bike, kind of defeats the whole purpose really.

I got a 12T already and put it in a 24" but that is running at 60v in the bigger wheel.

I do like torque but I feel that the 12t in a 20" at the lower volts might be too damn sluggish.

Does anyone have any experience of the MAC in a 20" build?

London where I live is generally flat. South London is a lot hillier.

I still want to get some rapid bursts and a top end of 30MPH.

Thanks.

 

[cal3thousand]

I want to be running it at around 16s LiFe or LiPo.

Any ideas??

I have an X5305 but its too heavy for a folding bike, kind of defeats the whole purpose really.

I got a 12T already and put it in a 24" but that is running at 60v in the bigger wheel.

I do like torque but I feel that the 12t in a 20" at the lower volts might be too damn sluggish.

Does anyone have any experience of the MAC in a 20" build?

London where I live is generally flat. South London is a lot hillier.

I still want to get some rapid bursts and a top end of 30MPH.

Thanks.

You will need to get a faster wind to get that speed in a 20" wheel.

 

[wesnewell]

For 30 mph on a 20" wheel you need a little over 500 rpm. Do they make a motor rated for that at the voltage you want to run? It would probably need to be rated for 600 rpm no load speed.

 

[Architectonic]

A 12T would be lucky to break 20mph on 50/52v nominal (58v charge). There is only one choice for a 20" wheel, the 6T.

 

[teklektik]

My knee jerk reaction is that putting a MAC in a 20" wheel seems a strange plan - very busy little gears although certainly under reduced load. But the fastest winds don't look that bad...

Estimates based on MAC dyno data show that a 7t will do about 27.5mph and a 6T about 30.5mph with a 220lb bike+rider on a mtn bike at 48v. At top speed the 7t will be running around 79% efficiency and the 6T about 77%. Both efficiency curves are pretty flat to the left (under additional load) so things won't go in the dumper too fast on inclines.

 

[spinningmagnets]

Here's a wheel diameter to speed chart. For 20-inch:

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

20-inch tire
1 mph = 16.81 rpm
1 kph = 10.50 rpm

10 mph (16 kph) = 168 rpm
11 (18) = 185
12 (19) = 202
13 (21) = 219
14 (23) = 235
15 (24) = 252
16 (26) = 269
17 (27) = 286
18 (29) = 303
19 (31) = 319
20 (32) = 336
21 (34) = 353
22 (35) = 370
23 (37) = 387
24 (39) = 403
25 (40) = 420
26 (42) = 437
27 (43) = 454
28 (45) = 471
29 (47) = 488
30 (48) = 504

 

[The Mighty Volt]

Thank you guys. I have decided to go with the 6t in the 20" wheel.

Now....is it safe to run this mother at 20s Lipo???

 

[teklektik]

C'mon now - think system design - not piecemeal.....

You need to pick a voltage before you select the motor winding. In a 20" wheel, your MAC pretty much wound out (that's what folks were trying to indicate above) - upping the voltage on the same motor wind is just going to push Watts into heat as you slip down the steep efficiency hill.

This setup could be okay because you said you were only going to visit 30mph from time to time - presumably most of the time you are in a better operating range for the motor (they like to run under 400rpm). These low-turn motors (6T-7T) are low voltage high current motors - at low speeds under load (getting off the line, etc) they tend to draw big current and waste power as heat. If you throw high voltage at a 6T it is just going to drink amps.

That said, if you want to go higher voltage, instead of limiting the amps with a CA or the controller, it makes more sense to use the higher voltage to get a higher wind motor that tops out at 30mph and draws less current from the git-go. This will lower your controller amps and battery C rate.

Here's estimates for MAC 8T and 10T at 60v - looks like the 8T would be the better choice...


Here the waste power for the 8T at 30mph is only 256W - it will run WOT all day on the flat at 60v without overheating. The current draw at that speed is only 21A but a bigger controller (9-12FET) will give you the amps to launch off the line with attitude (you are always controller-limited off the line, so best to buy some headroom for that reason).

 

[worldpax]

I have a 6T MAC in a 20" wheel. I run on 48 volt and have no complaints. Acceleration is good. I easily beat the cars across the intersection. Top speed is around 33-35mph on a fresh charge.

 

[teklektik]

I have a 6T MAC in a 20" wheel. I run on 48 volt ...Top speed is around 33-35mph on a fresh charge.

Very cool - This confirms the first estimate - 6T 30.5mph 48v - your voltage and speed will be a bit higher HOC and I'm guessing tucked to get to 35mph...
Thanks for the post.

Really nice bike BTW

 

[Ben Wilson]

I run a 6t MAC on 36v on my recumbent trike, it tops out at 25 mph.