Rivian's TM-B electric bike

Bear with me I don't know much about bikes but I have this honest question: why does it need a chain, and won't place the engine (or whatever gear box it has) directly on the wheel?

So it will appear as a bicycle and be allowed on paths in parks (is what I'm guessing).

The biggest reason is minimizing unsprung mass, the performance of the rear suspension would be much worse with a hub motor.

It's not clear if their design allows for it from the details in the article, but you'd want a chain in case you run out of battery so you can pedal manually.

Putting the motor into the wheel makes for an excellent riding experience, I have a Bionx rear wheel (out of business) that is connected to the chain ring and pedals in the traditional way. The wheel detects torque from pedaling and applies it's own torque to drive the wheel. It can be used with and without power and feels almost seamless and is silent.

Unfortunately it seems to be difficult to engineer and build these wheel motors for reliability and longevity. They significanly increase the unsprung mass of the wheel which leads to increased wear on the hub components.

does that actually matter much on anything except dirt-bike tracks, or trying to go 40mph on a horrifically bumpy track? minus some comfort advantage, of course.

like technically, sure, it's obviously true. but for performance it only really matters when you would get air time with higher mass, and the lower mass stays in contact more. commuter e-biking generally doesn't get anywhere near those speeds or bump-sizes. (trail biking: sure! I 100% believe it's a sizable consideration there)

Dirt bikes and many motorcycles also have chains, this allows you to change gears to adjust torque (alternatives include shaft and belt drives).

E-bikes can have the motors on the wheel (hub-drive) or on the pedal (mid-drive). This choice is largely related to how much you want your e-bike to really be a scooter or really be providing pedal assist. As a consequence hub-drive e-bikes typically have a throttle while mid-drive do not.

A good mid-drive e-bike really makes it feel like you are a super human cyclist rather than riding a scooter. It leads to a much smoother riding experience if your aim is to still be essentially bicycling but you'd like to get moving faster and not break a sweet even on the most extreme hills.

mid-drive e-bikes like this one are generally more expensive also but more efficient than rear hub motor systems. They also provide better overall weight distribution.

> A good mid-drive e-bike really makes it feel like you are a super human cyclist

Of course. But the pedals on this thing aren't even connected to the drivetrain. So what is the benefit in this specific case?

I have a couple of mid-drive e-bikes (Bafang motors) with throttles. Throttled mid-drives let you get the best of both worlds: Great pedal assist combined with the ability to just be a scooter when you get tired. And I agree that mid-drive is the way to go if you want to ride on hills. If you commute in San Francisco for example, a hub drive e-bike will not help you much but a mid-drive will change your life.

Completely agree. They should probably just remove the pedals and generator entirely and save that weight. But then legally it becomes a scooter and probably would require a license and be more limited as to where it could go.

I've never ridden a full suspension with a hub motor so I can't say, but my guess is that yes, it would make a pretty big difference with an aggressive rider or poor quality streets. It's not just keeping contact that matters, its the consistency and quality of contact, especially with a super torquey motor ready to jump at a twitch of your thumb. Its of course not necessary for commuter biking, but neither is basically anything on this premium product aside from the wheels and pedals.

Also to note, they are very much marketing it as a trail bike in addition to a commuter so it's not surprising they would spend a bit to optimize for ride quality and traction.

It's a backup so that if you do lose power, you can still pedal home. Major pain to pedal an 80 lb fixed gear bike, but This is standard on ebikes and doesn't contribute much weight or cost in itself.

But what if the pedals are connected to the wheel only in a "symbolic" way, say, some electrical contact or something? So technically you still have to pedal to qualify as e-bike, just without real mechanical transmission? And if you lose power bad luck, don't commute with an empty battery...

Not all e-bikes have a rear suspension, so the motor will feel the bumps either way pretty much the same, I would guess. Or being placed in the middle halves somewhat the shocks?

I could imagine (not necessarily feasible!) the pedals only moving a dynamo with variable resistance to mimic real world pedaling, and the hub drive doing the drive according to the pedaling speed? Would that be a scooter or a e-bike, according to norms?