If you're looking for 50mph plus and a fairly light frame, then you have a few options depending on what you want to do. If hub motors are your thing, then a Cromotor overvolted to 100V should give you a fair amount of speed (or you could move to the Enertrac MHM602 or 603 for a better guarantee of sustained high-power handling. That said, those setups are quite heavy and expensive; an Enertrac motor will run you $1300 by itself before you even start looking at a controller or battery pack. The more typical solution for something like this is a brushless Etek motor like the ME1305 (good for 10kW peak, so a little bit wimpy for a 200lb vehicle but not bad) or the 0913 (good for 10kW continuous at 48V and 30kW peak at 96V). They're both cheaper than an Enertrac (the 10kW peak one is $500 and the 30kW peak one is $825), but they'll spin a bit fast and require gearing down by 5-7:1
Both of those should be drivable by a big Kelly (something like the KBL96251, which can do 250A peak and 100A continuous at 96-100V nominal), and the other option is to build a Lebowski controller (field-oriented control, built and assembled by you). Lebowski will sell you the preprogrammed microcontroller for 25 Swiss francs, and then you send the PCB file he provides to a fab house, assemble the main logic board, and then design and assemble a custom power stage to go with it. It's a fairly time-consuming process, and it works best if you either know a lot about electronics or are willing to learn, but you can build a 120V-capable Lebowski controller that can handle 400A peak and 200-250A continuous for significantly less money than the $900 big Kelly with identical specs. Also, the Motenergy website recommends sinusoidal and field-oriented control schemes over trapezoidal setups like the Kelly or Infineon machines; it claims that trapezoidal controllers may shorten the motor's service life and threatens to void the warranty if they are used.
As far as batteries are concerned, you can get a
lot of Hobbyking LiPo for cheap. A 4.5kWh pack (88.8V nominal, 100.8V charged, 50Ah) can be assembled from 60 of the 4S 5000mAh 20-25C Hobbyking LiPo bricks arranged in a 6s10p configuration, and that will cost you only $1350 plus shipping in exchange for what should be a 30-40 mile range if you ride it hard and something closer to 60 miles if you ride around 20-25mph in stop-and-go traffic. That pack will weigh something like 80lbs completely assembled, and unless you treat it well you run the risk of a massive LiPo fire in your frame, but it should give you range. Furthermore, 50Ah of "25C" Hobbyking LiPos can probably handle 4-500A discharges without it being too hard on them, so that paired up with a 400A peak controller should work well.
That's the quasi-traditional route, using a motorcycle frame and motorcycle-sized motor. There may be another alternative (that I'm looking at now, but don't necessarily have the funds or the time to try out yet) centered around building something similar to, but a bit bigger and heavier than, a Qulbix Raptor 140. The idea I had was to use the Revolt RV-120 in a mid-drive configuration at 72-96V for 15-20kW peak and 6-8kW continuous, then use an 8-speed IGH as a jackshaft to the final gear reduction (effected by a geardown ratio somewhere between 2:1 and 4:1 between the jackshaft output sprocket and the final drive sprocket on the rear wheel). I was thinking of powering that with a 150V-rated Lebowski controller and using the LiPo configuration I just described, then building the rest around a frame based on an upsized DH mountain bike. The goal would be to keep weight below 130lbs, and have top speeds on the order of 60-70mph but with reasonable mixed-offroad capability.
That last option is attractive to me because it lets me save money (the RV120 is $280, and an 8-speed IGH is $150-200 more; thus I can basically have an 8-speed 20kW peak setup for the same price as a 10kW peak direct drive Etek), and the weight penalty for that setup is maybe 12-15lbs, as opposed to a 22lb direct-drive ME1305 or a 35-lb direct-drive ME0913. I'm not sure if it can be done, and I'm definitely still very thoroughly in the iterative planning stage (come up with ideas, kick them to the ES community, see what feedback I get, revise from there) and far from starting to build. I wish you the best of luck with your build, though!
