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    Rivian's TM-B electric bike

    (www.theverge.com)
    188 points hasheddan | 12 comments | 22 Oct 25 18:00 UTC | HN request time: 0.001s | source | bottom
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    dreamcompiler ◴[22 Oct 25 19:10 UTC] No.45673731[source]▶
    808Wh battery and 100 miles of range. These two numbers track with each other and are roughly believable.

    OTOH, with a battery this big, a generator powered by the pedals, and regen braking this thing has to be heavy. I'd expect it to weigh at least 80 lbs. More likely 100. The fact that their "specs" say nothing about weight suggests they're embarrassed about the weight.

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    jeffbee ◴[22 Oct 25 21:22 UTC] No.45675314[source]▶
    Regen braking is how you can tell this was designed by a moron. The energy balance simply does not favor regenerative braking on a bicycle, especially a bicycle that flippantly ignores aerodynamics like this one does. A bicyclist loses roughly all of their energy to air resistance. It's not a truck. There is not substantial potential energy to be recaptured going down hills.
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    1. edaemon ◴[22 Oct 25 21:29 UTC] No.45675413[source]▶
    What do you mean? The regenerative braking only kicks in when you engage the brake lever. It's not going to add much range but it's free, I don't see any downside to including regenerative braking.
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    2. jeffbee ◴[22 Oct 25 21:40 UTC] No.45675508[source]▶
    It isn't free. How could it be free? It requires at least an electronic control system and a pressure sensor.
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    3. LoganDark ◴[22 Oct 25 21:48 UTC] No.45675599[source]▶
    It was already going to have some sensor for, say, the brake light.
    4. 01HNNWZ0MV43FF ◴[22 Oct 25 21:51 UTC] No.45675620[source]▶
    Are there popular e-bikes without electronic motor control?
    5. edaemon ◴[22 Oct 25 21:51 UTC] No.45675625[source]▶
    It's the same control system that operates the motor. The motor is just being used as a generator.

    I'm not sure which pressure sensor you mean, like in the brake lever? E-bikes with hydraulic brakes already have sensors for power cutoff (and in this case for brake lights).

    6. ◴[22 Oct 25 22:32 UTC] No.45676004[source]▶
    7. linkjuice4all ◴[22 Oct 25 22:53 UTC] No.45676175[source]▶
    You're going to have a difficult time pulling a lot of energy out of the back wheel as you're slowing down. The more you decelerate the less weight you've got on your back wheel. Eventually you reach the maximum energy transfer from back wheel contact patch into the motor and lock up the back wheel, and even then you may not have considerably slowed the bike.

    Regen on the front wheel would be most effective - but then you've got two motors or a less-than-ideal front motor that adds unsprung weight and has similar traction issues during acceleration as the front unloads.

    It's a shame - I think a lot of people want ebikes to work, but they're not as convenient as a pedal bike (especially not in small apartments) and usually they're too heavy to really use in blended pedaling/e-assist mode.

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    8. numpad0 ◴[22 Oct 25 23:16 UTC] No.45676361[source]▶
    3-phase motors are controlled by torque commands into the driver. Give it a value and it generates requisite voltages to fill the gap between current state and desired state. Give it a positive value and the driver spins up the motor, give it a negative and it artificially spins down the motor progressively by commanded amounts. So especially off-throttle regen is completely free. IIUC.
    9. cyberax ◴[22 Oct 25 23:26 UTC] No.45676443[source]▶
    If you engineer it properly, it doesn't add _any_ weight or complexity. All you need is a bit different arrangement of power transistors and some software.

    Why existing bikes don't use it? Because you need software or a more complicated controller, and the amount of regenerated energy is indeed not that large.

    10. amluto ◴[23 Oct 25 01:40 UTC] No.45677263[source]▶
    As other commenters noted, rear wheel regenerative braking doesn’t work very well. But there are more problems: most mid-drive e-bikes fundamentally can’t regeneratively brake at all: the rear hub freewheels and cannot drive the motor. Even ignoring that, the chain/belt frequently also can’t drive the motor because that would cause the pedals to drive the motor, and a lot of e-bikes are designed to be pleasant to ride with the motor off, and the rotor has rotational inertia and often has drag as well.
    11. dzhiurgis ◴[23 Oct 25 02:10 UTC] No.45677412[source]▶
    ABS on ebikes exist already, so I'm not buying your locking issue. Also e-scooters like Lime had regen braking forever.
    12. daemonologist ◴[23 Oct 25 03:47 UTC] No.45677972[source]▶
    It's not free in a bicycle - it requires significant design compromises in the drivetrain because normally the rear hub has a freewheel to keep it from being back-driven (this may be part of the reason they went for "pedal-by-wire").