Hi everyone, I have been reading a lot in the forum and I think I’m finally ready for my first build. I have outlined my design below and I was hoping I could get some feedback (I think I know what I’m doing but I’m sure there is a lot of stuff I’m missing). I hope this is also helpful to anyone trying to do a similar build. Thanks for all your help in advance!
PARTS:
- Deck: skateboard from Walmart.
- Trucks and wheels: 83 by 52 mm wheels with longboard trucks, Trucks and Wheels
- ESC: VESC 6 from Trampa, I kind of got excited and went ahead and purchased this (I probably should have consulted the forum before I did that), Trampa VESC 6
- Motor: Turnigy Aerodrive SK3 6364 245 KV, motor
- Batteries: two 4S 6200 mAh 40 C Zippy Compact LiPos in series, Batteries
- Drivetrain: 14 to 36 reduction using HTD 5 mm pulleys and a 330 mm belt (the belt is 15 mm wide), I plan to get these from Torqueboards: Belt, driven pulley, driving pulley
- Controller: I found a cheap controller on Amazon, cheap Amazon controller
- Miscellaneous: I plan to have a fuse, I got a 50 A one from Trampa, between the batteries and the VESC, fuse; XT90 antispark connectors; voltage testers on both batteries, voltage tester; and a power switch connected to the VESC, I saw a post in the forum explaining how to connect one to a VESC: Connect power button to VESC
MOTOR MOUNT AND TRUCKS:
I realize that the trucks in the link above might not be the best to attach a motor mount due to the ribs and oval-shaped cross-section, however, I already owned these and was hoping to use them. I’m going for a single motor build and I used a manual mill to machine the side of one of the trucks to get a square-shaped cross-section (picture below). This should make it easy to attach a motor mount which I am planning to machine out of 6061 Aluminum. I was somewhat concerned about the trucks failing after having completely removed one of the ribs, however, I have been riding the skateboard and the trucks have been holding up.
DRIVETRAIN:
I am going for a 14 to 36 reduction. I settled on this gear ratio because it was the biggest reduction I was able to get while maintaining 6 teeth on the motor pulley in mesh and keeping the centers of the pulleys at a reasonable distance from each other. With a 330 mm belt, I get a center to center distance of roughly 4". I got this value from this calculator that was recommended by onloop: calculator
I am planning to use an SK3 motor, which does not have a keyway, and Torqueboards pulleys, which do have a keyway. From the research I have done, it seems like some people consider keyed shafts to be mandatory while others find Loctite to be sufficient. From my experience, Loctite is really strong, so for now, I am planning to go with that. I would be more than happy to hear your opinions on this.
BATTERIES AND RANGE CALCULATIONS:
I plan to ride on flat paved terrain. I’m in Indiana so no slopes 
. I weigh 70 kg and don’t have a lot of experience skateboarding so I would like to not go very fast. My daily commute is 2.59 km (which kind of makes me think that having an electric skateboard is overkill ) and I was hoping to only have to charge my skateboard once a week since LiPos are a pain to charge so I’m aiming for 10 to 15 km of range. The reason why I’m going with LiPos instead of Li-ion is that I already own a balance charger and my experience with electronics is also pretty limited so I don’t really trust myself to build a battery pack.
From what I have read it is very hard to predict the Wh/mile that I will be getting wtih my build. Until I build it and test it I will basically be guessing. I have seen estimates anywhere between 7 and 40 Wh/mile on the forum. I decided to settle for 25 Wh/mile. Using the batteries I mentioned in the parts section above I get the following range:
Range = (Nominal Cell Voltage in V)(# Cells in Series)(Battery Capacity in Ah) / (Wh/mile estimate)
Range = (3.7 V)(8)(6.2 Ah) / (25 Wh/mile) = 7.3 miles = 11.8 km
This calculation is fully dependent on my Wh/mile estimate and I would be happy to hear your opinions on this. I might be totally off. For me though, 11.8 km of range is more than enough.
The battery is rated to be charged at 2 C but I plan to charge it at 2 A.
The battery is rated for 40 C continuous discharge and 50 C burst. From what I have read battery manufacturers often exaggerate C ratings and it is good practice to assume that the actual C rating is half of what it is. Taking this into account the maximum current my battery can supply is:
Max current = (Battery Capacity in Ah)*(C-rating specified by vendor)*1/2
Max current = (6.2)401/2 = 124 A
I plan to set the max current in the VESC Tool much lower than this, especially since the Trampa fuse is rated for 50 A.
MOTOR:
I honestly don’t have a good feel for what my top speed should be. Especially given that I don’t have a lot of skateboarding experience. 40 km/h sounds like more than enough for my top speed since it seems very very fast but I don’t actually know. I definitely think that I would prefer to have more torque than more speed.
The main reason why I selected the SK3 motor is because of the price. Some other contributing factors were that a lot of people on the forum seem to use them and that Hobbyking’s customer service appears to be pretty good from what I have read. As for my actual numbers, this is the part of the build that I am most confused by and I would be very thankful for some feedback. When calculating ERPM values some people use nominal cell voltage (3.7 V for LiPos) and other people use the voltage of the fully charged cells (4.2 V for LiPos). I realize that you can set the ERPM limit in the VESC Tool but I am not sure if I should be trying to keep both of these values under 60000 ERPM or only the 4.2 V value under 60000 ERPM. Here are my numbers for the motor I mentioned in the parts section:
ERPM = (KV rpm/V)(Cell voltage V)(# Cells in Series)*(# Pole Pairs)
ERPMnominal = (245 rpm/V)(3.7 V)(8)*(7) = 50764 ERPM < 60000 ERPM
ERPMmax = (245 rpm/V)(4.2 V)(8)*(7) = 57624 ERPM < 60000 ERPM
With my gear ratio of 14 to 36 I get the following speeds:
Speed = (Motor RPM)Pi(Wheel diameter in m)*60 / (Gear ratio * 1000)
Speed at nominal voltage = (245 rpm/V3.7 V8)Pi(0.083)60 / (2.571000) = 44 km/h
Speed at full charge = (245 rpm/V4.2 V8)Pi(0.083)60 / (2.571000) = 50 km/h
That top speed seems very high to me, personally. I thought about increasing the gear ratio to get lower speeds, however, I don’t see a way to do that without having less than 6 teeth in mesh. The only thing I could think of was to get a bigger driven pulley and to add tensioners to ensure that there is a minimum of 6 teeth in mesh. However, I would prefer not to add tensioners as this increases the complexity of the motor mount.
Would you recommend getting a motor with a smaller KV so that I have lower top speeds? I was thinking about using the 192 KV SK3 motor. The trade off with this is that I am operating much further below the 60000 ERPM limit and from what I have read this results in a decrease in efficiency. I’m not sure if the greater energy losses actually offset the decrease in speed and increase in torque that I am after. Let me know what you think!
BLDC vs. FOC
I have decided to go with BLDC because the SK3 motors are sensorless. I read a really great forum post on BLDC vs. FOC here: BLDC vs. FOC From what I understood for FOC to work the VESC needs to very accurately know the position of the rotor as it spins. The back EMF of a sensorless motor tells you the rotor’s orientation with an accuracy of roughly plus or minus 30 degrees, which is not nearly enough for FOC to be properly applied. Another reason for choosing BLDC is the noise. I would prefer to give people a heads up as I am approaching them as opposed to being stealthy
SUMMARY AND MAIN POINTS I NEED HELP WITH:
I realize that this is a really long post but I hope that by making it detailed it can be useful to other beginners like me.
The main things I would really appreciate your help with:
- Did the parts I chose seem compatible and adequate for a build?
- Should I stick to a 245 KV motor or should I go for a lower KV motor, like the SK3 192 KV? I would much rather have high torque instead of high speed but I also don’t want to be operating too far below 60000 ERPM and be very inefficient.
Thanks for all your help and patience!
Pictures:
