Announcement

Collapse
No announcement yet.

Aggressively Custom Mountain E-Bike

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

    Aggressively Custom Mountain E-Bike

    I'm designing my perfect version of a mountain e-bike, but am struggling to find resources to answer my questions. Here's an overview of my design decisions:
    1. I want a good balance between torque and top speed. I have found an 8000W rear hub motor with a max rpm of 880 at 72v that I like. I plan to use it with a throttle, not pedal assist. With 21" wheels, this should go about 50mph top speed. https://www.qsmotor.com/product/8000w-bicycle-motor/

      21 inch wheel diameter ≈ 66 inch wheel circumference, which will also be 66 in/revolution
      0.9 * 880 rpm = 792 rpm to adjust for manufacturer's often-exaggerated claims

      792 rpm * 66 in/revolution = 52,272 in/min
      52,272 in/min * 60min/hr = 3,136,320 in/hr
      3,136,320 in/hr ÷ 63,360 in/mi = 49.5 mi/hr

      With a smaller wheel, the motor will have more apparent torque than a larger wheel. Torque is scaled inversely to wheel size, meaning less torque with bigger wheel and visa versa. This will help reduce wear on the motor. If you're worried about hitting higher rpms more often due to the smaller wheel, I will most often cruise around 30mph, not 50mph, so that shouldn't be a big deal.
    2. High-capacity (maybe 40ah) 72v battery mounted as far forward as possible. This is to help with traction during acceleration. I've thought about putting the battery in a basket mounted on the handlebars, but I'm worried that this will inhibit steering. I've also thought about connecting two 36v batteries in series. This is possible with a few diodes to protect each pack's BMS (PLEASE correct me if I'm wrong here). The reason for two batteries is that with a triangle shape, I could have both of them in the frame, just slightly offset and meeting in the middle. This is highly dependent on the frame that I can find, and definitely a lower priority.
    3. I want the gearing for the pedals to compensate for the added weight. The motor itself is 45lbs. The whole bike with battery(s), frame and everything will probably be something like 100lb -150lb. I also weight 165lb, so that's over 300lb that I'd like to be able to move on my own. This is a bit open-ended as I haven't researched this much. I just need some advice on how to choose (or make) a cassette that will allow me to climb hills without severely limiting my top pedaling speed.
    So my primary question is how to find a frame that will support this build. I am 5'9" and like to ride a smaller frame (20"). I'm pretty sure that on a typical frame shape, 21" wheels will cause my pedals to hit the ground when I turn tightly. It already happens sometimes on 26" wheels so I believe it's rather likely. I'm hoping that there's a frame type that I can customize the angle of the rear "axle" (not sure what it's actually called) relative to the frame, to keep the pedals high enough off of the ground. I will probably compensate on the front end with a wheel that's larger than 21".

    Thanks in advance for taking the time to read this.

    #2
    ...perfect version of a mountain e-bike...
    I want a good balance between torque and top speed. I have found an 8000W rear hub motor....
    Why go with a monster hub instead of a mid?

    Comment


      #3
      Three reasons:
      1. Hub motors dissipate heat better than mid drives. They usually have a larger surface area and I think that there is more turbulence over a spinning heatsink as opposed to a stationary one, if that makes sense.
      2. Mid drives are generally geared down for better torque and have much lower max rpms than hub motors. Lots of shifting required to accelerate to top speed.
      3. Mid drives put extra wear on the chain and sprockets, which is completely avoided by a hub motor.

      Comment


        #4
        Also, the monster size is so that I can actually achieve as close to the max rpms of the motor under no load as possible. If the motor can provide much more power than I'll actually need/use, then it should perform pretty well for what I want (essentially dirt bike acceleration up hills). I'm hoping that this will also mean that the motor is more durable for my application, since I will essentially be underpowering it by being very conservative on the throttle.

        Comment


          #5
          1. I ride a mid in AZ summers - it was 47C Sunday and I leave it out in the sun before riding so after over 10,000km on this mid I'm not worried about the heat - I think this is a red herring
          2. The problem with a hub is they are stuck in one gear so they have to not only put out a ton of torque at low speeds but also turn fast at the higher speeds and a DD hub that has enough torque to do slow MTB style rough terrin climbs is sill heavy and the weight is where you don't want it... so you either have a transmission so you can change gears to provide for a adequate speed range or you need this monster weight that will significantly hurt MTB style riding
          3. You'd be surprised... I get around 5000km on a chain these days and they are cheap so who cares?

          Regardless, it sounds like you may be looking for something more along the lines of a small electric motorcycle, not really what most would consider an eMTB... even then I would still do some research on a large mid-drive since the weight distribution will be far better

          Comment


            #6
            Do you plan on pedaling most the time (a la eMTB) or primarily just throttling the motor (a la eMoto)?

            Comment


              #7
              Good points. I'll keep in mind your experience with heat and the motor. The weight of a rear hub motor is indeed in a bad position, but I'm not sure that I'll be able to get the acceleration I want with a mid drive. I'd like to be able to burst from say 20 mph to 40 mph if I ever needed to, and my inexperience tells me that there's a gear shift required there if using a mid drive. The cost of the chain and sprockets isn't that important, I'd just like to set-and-forget as much as possible. I'm lazy ¯\_(ツ)_/¯

              I'd say I'm trying to land between an eMTB and eMoto. A lightweight eMoto that looks like and can operate like a bike, moreso.

              Comment


                #8
                Anything that's going to giddyup and go from 20-40 is pushing towards eMoto and suggests pedaling is not going to happen most of the time (if at all)

                There are ways to get from here to there with a large mid-drive (a la sur-ron) but it is almost certainly an easier build task with a hub

                Have you thought about just purchasing something like a sur-ron?

                Comment


                  #9
                  I really only want to pedal for speeds under 20. I can't really pedal faster than that anyway on an unloaded bike, so faster than 20 would be throttle only.

                  I've thought about buying something pre-built, but I can probably make the build significantly cheaper if I source the parts myself. Also, I haven't found something prebuilt that meets my listed standards. For example, the sur ron doesn't have pedals and is probably classified as an electric motorcycle rather than bicycle.

                  Comment


                    #10
                    What speed you can pedal at is only limited by the gearing - my present mid build climbs anything I got juevos to attempt, goes as slow as ~2.5mph [pedaling], I can pedal at a leisurely rate to 25mph and can still pedal approaching 30mph but admittedly it's very brisk pedaling... It's not a 40mph bike and while it can go >35 it does take a while to get there but frankly on something as light and bicycle-like as it, I find much more than 25mph pretty scary and too old to get all beat to hell wadding it up at those speeds

                    An 8000W hub that can go 40mph isn't going to be an electric bicycle - not even close... you left that pond already - you can get pedals on a sur-ron but I sure as heck wouldn't bother... If I thought I was going to haul my bike more often to the desert or mountains I'd think about getting something like it but I only get out like that every few weeks... my mid is just fine for the local multi-use and good for hauling out to the backcountry so I'm good with it although I'm always thinking about the "next bike"

                    You can likely build the rear hub bike like that for not too much depending on what you start with - personally after owning both mid and hub I'm pretty much done with hubs for the reasons I mentioned [and others] and only really interested in mids at this point... I'm hoping for the best however you go about things

                    Comment

                    Working...
                    X