Rear rack vs downtube battery case — the rack is not the bigger one

Short answer: the box on the back is not the bigger one. The largest rear-rack housing we mould takes 104 cells of 18650; the largest downtube housing takes 200 — and on 21700 it is 80 against 160. A rack is not a capacity upgrade. It is what you fit when the frame triangle cannot take a housing at all.

This trips people up because a rack looks like more room. It is not more usable room: a rack housing has to sit between the mounting points on the deck, clear the wheel and stay clear of the saddle. A downtube housing runs along a tube and can simply be longer — the longest one we mould has a 538 mm opening against 335 mm for the longest rack housing.

What a rear rack actually buys you. Not capacity — placement. Three cases where it is the only thing that works:

• There is no room in the frame. Folding bikes, small-wheel bikes and step-throughs often have a short or curved downtube with nowhere flat to mount a housing. The rack is not a compromise there; it is the only mounting surface the bike has.
• The downtube is already committed. Bottle bosses, a controller box, cable runs — if the triangle is spoken for, the rack is the free space.
• You want the housing out of the way of the cranks and the front wheel. A rack housing never fouls a pedal stroke.

What a downtube housing buys you. Capacity, and a lower centre of gravity. If your build needs more than 104 cells of 18650 or 80 of 21700 in a single housing, no rack mould we make will reach it — you need a downtube or one of the other in-frame formats.

The trade nobody mentions. Weight on a rack sits high and behind the rear axle. On a small-wheel or short-wheelbase bike that is noticeable in the steering, and it is the difference between a bike that feels planted and one that feels like it is being pushed from behind. A downtube housing puts the same mass low and central. If both mounting options exist on your bike and capacity is not the constraint, the downtube is the better place for the weight.

Pick a rear rack when the frame has no usable mounting face, or when the triangle is already occupied. Accept the 104-cell ceiling — it is a placement choice, not a capacity one.

Pick a downtube when you need the longest housing the range offers, when you want the weight low and central, or when you simply have room for it. It is the higher-capacity of the two by a wide margin.

Both series are listed in full — all 17 rear-rack and all 21 downtube housings, with the cell ceiling and opening length for each — on the rear rack and downtube pages. To go the other way and start from your pack, put the configuration into the case selector.