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when reading articles about location of battery placement on e-bikes, i don't seem to find seat tube being discussed anywhere. This is however exactly where Ecoride, a Swedish e-bike brand, places batteries that it manufactures. See e.g. the ambassador model places the battery (example model below), but other series too. I think the standard mechanism to remove the battery is the bike seat flipping to the front when a knob under it is pushed.

Seems like an obvious choice to place the battery when aiming to lower bike's centre-of-mass.

What are the disadvantages though? Why is this not a standard battery location?

One is the extension of bike length, making it less maneuverable, how significant are those up-to-10cm (likely not so for city bikes, but some sporty models also have same battery placement location)? Is moving of the rider away from the back wheel axis is significant in particular in any way?

Ecoride Ambassador X-AXS-H-9 2021

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  • with a longer wheel base it is more difficult to do wheelies! If you cannot do wheelies, why do you need an e-bike? (I am kidding, at least for the question).
    – EarlGrey
    Apr 15 at 10:19
  • I think i remember having read that in germany, with a battery mounted like that, you are not allowed to attach a child seat to the pannier (for whatever reason, though, i don't know.)
    – Burki
    Apr 15 at 15:38
  • For that matter, why not place the batteries inside the tubes? Maybe with a removable cap on the end of the tube, to service the batteries later. Apr 16 at 0:12
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    @DannyBeckett inside the tubes has both pros and cons for several reasons, More aero and more discreet, harder to swap out, and if it catches fire then its harder to quick-release. Also won't take a "standard" battery and needs a custom one, and makes the tube very chonky. Some bikes do this, like electric-assist road bikes.
    – Criggie
    Apr 16 at 2:36
  • One little advantage of having it sitting above a frame rather than vertically along one, is that less dirt and water will splash upwards to the battery connector. Looking at how dirty my own bike tends to end up behind the chain guard (it has similar mud guards as this one), that's perhaps not the best place for a connector in terms of dirt.
    – Lundin
    Apr 18 at 14:46

4 Answers 4

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I could see some arguments to consider the battery in the downtube as a better placement, so the reason is that it is not the best alternative:

  • Structural reasons: especially on step-through bikes, having a wide down-tube is better for structural reasons - but that is a trend that is also visible on gravel bikes. Having the battery in the down-tube is then a "two birds with one stone" situation: by having a wider down-tube, you gain structurally and there is more space for the battery.
  • Volume: batteries needs volume. The downtube is longer than the seattube, and has a more stable length across bike sizes. In smaller sizes, I can easily imagine that it's more difficult to design a mechanism/shape for a seat tube battery that doesn't interfere with the seat stays and/or the saddle.
  • Weight distribution, especially for utility-city bikes used with panniers: very upright position and panniers increase the weight on the rear wheel, having the battery in the downtube, at a similar height, slightly moves the weight to the front, which is not a bad thing.

On the two arguments "against" that were given in the question:

  • longer chainstays: city bike usually have longer chainstays, even without battery at this location, for a simple reason: foot interferences with panniers.
  • lower center of mass: the height on the bike is not that different for a seat tube and a down tube. Because of the angle, you could potentially a slightly lower center of gravity with a slanted tube than a straight one.

The other occurences I've seen of such battery placement is Cowboy (a brand that uses a lot of custom parts), and longtails (compact or not).

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    Good points. I think "Structural reasons" sounds pretty convincing in itself for a bike manufacturer. Apr 15 at 19:26
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What are the disadvantages though?

The removal mechanisms need a special seat post, the simple knob you describe is a very specific design that needs intensive testing (you need to be sure that it does not release arbitrarily after some years/wear). And even when you have the part, if you have any production issues you are left with no alternatives.

Regarding manufacturers doing that, the early Flyer bicycles (before being greedily bought by a big German retailer) had a compact and low placed battery (see their models line-up). No specific battery release mechanisms needed, fantastic dynamics, well tuned motors, they were ahead of their time (even with the look, that was quite ugly before ugly would became the norm :D ).

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  • Good point, i will pay more attention to the mechanism when i next see the bike. Would be funny for the seat to be catapulting the rider after some use :D And yes i can find some more manufacturers placing battery on the seat tube now. I figure that it is more easy for large battery manufacturers such as Shimano or Bosch to make one-size-fits-all batteries for down tube or luggage carrier that can be fit by bike manufacturers than for the seat tube. Apr 15 at 19:20
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Downside - this makes the wheelbase longer, so the bike will turn slower compared to a shorter bike at the same front-wheel angle.

This is why race bikes tend to have the rear wheel tucked in close to the seat tube.

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    A longer wheelbase resulting from extended chainstays and seatstays like in this case also increases weight, because you need more chain and more material in the chainstays and seatstays. This is probably not a major consideration for an E-bike though, as the weight difference likely translates to less than 1% difference in effective range. Apr 16 at 1:42
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Another downside, with step though frames, is the battery on the down tube will significantly reduce the clearance the rider has to step through. This can be mitigated with integrated down tube and battery (as seen on most Mountain bikes), but there is a cost premium to this level of design.

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