How to measure your bike frame for a battery case

Short answer: measure the mounting spacing and the clear length along the down tube, subtract whatever already occupies that line, then add 30–50 mm for the BMS. Those two numbers — mount spacing and usable length — rule out most of the range before you look at capacity at all.

1. Find the mounting points, not the frame size

Ignore the seat tube and top tube figures the bike was sold with -- those describe the rider, not the battery. What matters is where the housing attaches: the bottle bosses on the down tube, or the rail a Hailong-style case slides onto. Measure from the centre of one mount to the centre of the next. That spacing is the first number, and if it does not match the case, nothing else on this list matters.

2. Measure the clear length along the down tube

Run a tape from the head tube joint to the bottom bracket along the line the case would sit on. Then subtract whatever is already there -- cable guides, a bottle cage boss, the front derailleur clamp. That is your usable length. Most folders and small frames come out under 350 mm, which rules out the long-profile formats before you look at capacity.

3. Check the clearance above and below the tube

A down-tube case needs room on both sides of the tube it mounts to, and the number people forget is the one under the tube -- a case that runs into the front wheel or the fork crown at full suspension travel is a case that does not fit. Measure the gap to the nearest obstruction with the fork compressed if you can.

4. Work out the pack you need, then the length it needs

A cell block needs roughly 18.5 mm of length per series group on 21700 and about the same on 18650 once the holder is counted. A 13S pack is therefore around 240-260 mm of cells before the BMS. Add 30-50 mm for the BMS and wiring -- more on a 13S or higher build, because those boards are longer than the 10S one the mould was drawn around.

5. Confirm against a real interior, not the exterior

Listings quote outside dimensions, which is not what you measured. Ask the supplier for the interior length after the end caps, the width of the BMS compartment, and where the cable exit sits. Those three answers decide whether the case closes. There is a longer piece on what to ask for in what internal dimensions to ask a supplier for.

What to do with the numbers

Mount spacing tells you which family you are in. Usable length tells you which moulds inside that family. Feed the second one into the size finder and it lists the housings that provide at least that much opening, shortest first. If you would rather start from the pack, the pack calculator takes the configuration and works back to the cell count.

Where people go wrong

Using the exterior. Listings quote outside dimensions. That is the wrong end of the measurement: you measured the space you have, and you need the space inside the case, which is smaller by whatever the end caps and connector take. Ask for interiors, every time.

Forgetting the BMS. The single most common reason a build that measures correctly still does not close. A 13S board is longer than the 10S one these moulds were designed around, and that difference is usually about the size of the margin you left. See why your 13S5P pack will not fit.

Measuring with the forks extended. If the bike has suspension, the clearance under the down tube at rest is not the clearance under compression. Measure it compressed or you will find out the hard way on the first pothole.