Motor Swapout.

[tomtag]

I currently have a rear hub conversion with a MXUS XF08C, 36v 11.4Ah battery & KT LCD5 controller. Also a Julet 9 pin motor connector. I would like to have a second rear wheel with a faster motor as an alternative that I could swap with the above as required. Could I build up the below motor into another 26" wheel & use when required?
http://www.topbikekit.com/akm100cst-cassette-36v250w-ebike-rear-driving-hub-motor-20283236-holes-p-728.html

The wheel I have is perfect for riding around the peaks & gives the level of assist I need for hills etc. However I have a 20 mile round trip commute that is much flatter & for that ideally I would like something that can efficiently assist up to a higher speed (~20+ mph) but am happier with a lower level of assist if it can go to a higher speed, but still not draw much more current than the existing wheel to protect the battery.
I have looked at the XF07 on the motor simulator site that is often quoted on this forum (https://www.ebikes.ca/tools/simulator.html?motor=MXUS_XF07&batt=B3614_PF&grade=0&hp=0&axis=rpm&cont=cust_17_20_0.03_V), but cannot find the AKM motor above in there to compare, is there something similar I could look at?
I understand the implications of running a motor over the 15.5mph limit, but would like to investigate if there are any feasible options.
Any advice appreciated.

 

[danielrlee]

Have you considered putting the money towards a 48V battery instead? This would give you a 33% increase in speed and torque availability.

 

[tomtag]

I did consider, but it seems to involve a lot more cost. Really I should have done a bit more homework before buying the kit. I am thinking I could replace the motor wheel for about half the cost of replacing the battery. The main use of the bike is for this trip I mention above, so range is not hugely important & I do put in a fair amount of effort from the pedals. It seemed more practical to change the motor but I may be wrong?

 

[vfr400]

If your controller and LCD are 36v/48v, then it's only the cost of the battery. remember, you need more power to go faster as well. If you fit a small high-speed motor, like the one you suggested, it probably won't give enough power to get to the speed it needs to to run efficiently. It'll only work if you're all-up weight is not heavy and you have a fairly streamlined bike (not an MTB).

How many amps does your controller give?

 

[tomtag]

Do I need more power if I am happy with the equivalent drop in torque if I keep the same power source? (ie. battery & controller). Could I find a motor that will operate at those speeds, and give me a lower level of assist at a higher speed? I am happy to get up to speed via pedals if the motor struggles at lower speeds. What would suit me is having the efficiency at the higher speed range. It is a 5 mile gradual incline at the end of the day that I am trying to overcome, I will be pedaling as well, but want some assist at 16-20mph to keep me spinning up the hill.

 

[tomtag]

I will check the controller output when I get home; I think it is 7w/15w

 

[tomtag]

7A/15A

 

[vfr400]

A 15 amp controller can get you up to 20 mph on the flat, but it'll overheat if you keep it there. The 15A/7A means that it can run at 7A continuously. It will will give 15amps if asked, but it can only sustain that for a short time. I ran a double 328 rpm Q100 motor setup. Obviously, the two motors gave double the power of a single one, so it didn't slow down so much on hills and spent less time at low speed; however, it didn't take long for both controllers to overheat and trip-out.

 

[tomtag]

Thanks for the replies. I am starting to think I should stick with what I have as it has only done ~200 miles & has had no problems. Just one other question though, to get assistance at higher speeds, would it be feasible to limit the controller setting for PAS level currents to the capability of the controller? ie. PAS 3 @ 7A with PAS 5 @ 15A & I then only use sparingly above 3? I would then get a level of assistance at the speeds I want without risking controller or battery damage. Is there a motor anyone knows of that is rated 328rpm @ 36V?

 

[vfr400]

You should have parameter P3 set to 1, which sets the controller to current control, so each pedal assist level in the LCD gives a different current. setting the controller to a lower level of current would decrease the possibility of it over-heating, but the lower power would mean lower speed and at low speed, a high-speed motor runs very inefficiently.

When choosing a motor, you aim for one with a maximum rpm of 1.25 to 1.33 times your planned modal speed, then you choose a battery and controller that will give it the power it needs to run at your planned modal speed. 250w will get you 15 mph and 1000w gets you 30 mph. Once you go over 20 mph, you own efforts to pedal have much less significance, depending on the sort of bike.

 

[wheeliepete]

Another option you could consider if you have a duel voltage controller is a 3S 12v booster battery. This would enable you to run your bike at 48v when extra top speed is required. I've done this myself, but I'm not sure it would be cost effective for you.

 

[tomtag]

As my commute is relatively flat, I am happy to get up to speed myself, so not relying on assistance to get me to the point where inefficiency & wasted heat are a big concern (so 50% of no load speed of a 328rpm motor @26" = 12.5 mph. Then once I am up to speed, I could use some assistance to allow me to maintain 16-20mph for the journey. I average 12mph on the commute on a non e-bike, but am not sure how much assist I would need to raise the average by 4-8mph. It obviously depends on the characteristics of the motor, but assuming I am now in the upper section of the efficiency curve, it then depends on how much current I need for that level of assist & whether my existing battery & controller would be sufficient.
I might look at whether I can buy a second hand motor wheel to just try it out.
Is my thinking flawed?