kalleg said:
I'm about to start a 2-wheel cargo build (Bullit-style Longjohn but in steel), and have a few questions. I understand theres quite a few variables concerning which motor to buy, and I have a hard time getting my head around it. I already have a BBS02 48V 750W system on a different cargo bike, so I would like to stay with 48V for compatibility. I'm gunning for an IGH 3 or 7 gears on a 26" rear wheel. I will transport my two kids and myself which adds up to about 120-130 kg atm. The commute is mostly flat with a long slight incline at each end (I would love a nice combination of torque and speed). I'm looking to buy from PSWPowers german stock, as I live in Copenhagen. I'm planning to run the Open Source firmware with the KT-LCD3.
In your opinion, should i go for overvolting the 36V to 48V, or should I go with a 48V version from the get-go? I also had the understanding that an electric motor runs cooler at higher cadence in regards to overloading. Is this true?
ATM I have my eyes on this one (8-pin version!?):
Also do you have any advice as to how I should set the gearing (front and rear sprocket size/teeth)? Again there's a lot of variables. The Nexus IGH3 seems like a simple and cheap solution:
https://electricbike-blog.com/2015/...-power-speed-nexus-3-igh-paired-with-a-bbs02/
Let me know if you have any advice. Thanks all for your great work and information.
Sounds like a fun project. In your case everything follows a pedal cadence that is comfortable for you. After you have that number you know what motor to order. On your bike find a gear that is comfortable and time how many crank rotations you make per minute. For example count the revolutions you make in two minutes and divide by two. The the motor decision is probably easy. Your natural cadence is probably 80 rpm or less (most people are in that range). In your case that means a 48V motor on a 48V battery. Use the standard 42T front chainring. It gives the best chainline. Personally I would buy 8-pin because throttle/temp sensor can be added later.
If your natural cadence is over 90 you may want to run a 36V motor on 48V. If you are definitely going open source with the KT-LCD3 no issues, buy a 36V system and set the battery parameters with the KT-LCD3. If you are using stock display and firmware for a while you would need to flash stock 48V firmware onto the 36V controller.
kalleg said:
I also had the understanding that an electric motor runs cooler at higher cadence in regards to overloading. Is this true?
It is a factor in a limited technical sense but not worth worrying about.
3 vs 7 speed, the rear cog, the amount of power you will use in assist all interact, but the nice thing about an IGH is the rear cog is easy to change to lower or raise the whole range. If you really want to calculate in advance you need to know:
1. Your natural cadence
2. Grade of the steepest hill you want to climb (you can use google elevation to figure this out for a given climb)
3. Power use on the flat (do you want to go fast and draw 350W? Do you want to save battery and draw 150W?) this tells you the speed to gear to at natural cadence
4. the ratios in the hub for each gear
Use bikecalculator.com if you want to to get a feel about how watts relate to hills, weight, and speed.
It's a lot of calculation (the sort of thing I love), but if you aren't an engineer you can pick a common rear cog and adjust if the range is too high or low.
I live in hill country so I cannot imagine choosing 3-speed over a 7-speed, but in Denmark 3-speeds might make total sense if the jumps between gears are not too large. If it is a wide range 3-speed you could end up only using one gear. If you never see more than a long slight incline you might not need gears at all, just a well-chosen rear cog. Use more assist and climb the long slight hill at the same speed you ride on the flat.