How to Measure Centre Distance on an E-Moto
Two of the three numbers in a chain length calculation are printed on the parts. The third one you have to go and get. It takes a tape, a stand and about five minutes, and it is the difference between a link count that fits your bike and one that fits a photograph of someone else’s.
What centre distance actually is
Centre distance is the distance from the centre of the countershaft sprocket to the centre of the rear axle, measured along the straight line between them. Not along the swingarm, which on most bikes is not parallel to that line. Not along the top run of the chain. The line between the two shaft centres, and nothing else.
It matters because it sets the first and third terms of the chain length formula. The first term is simply how much chain it takes to span the gap twice. The third is a small correction for the fact that a chain running between two different-sized sprockets does not run parallel — the two runs converge slightly, and the closer the sprockets sit, the more that correction bites.
Why it is a range, not a number
Your rear axle slides in an adjustment slot. That slot exists precisely so chain tension can be set and reset as the chain wears. Which means your bike does not have a centre distance. It has a band of them, and the width of that band is the length of the slot.
So measure twice. Once with the axle pushed fully forward in its slot, once with it pulled fully rearward. Write both down. Those two figures are what the chain length calculator asks for, and the range it hands back is the honest answer: these are the chains that will tension on your bike, and here is where the axle ends up with each one.
How to measure it
A steel rule or a decent tape is fine. You do not need a caliper for this; a millimetre either way moves the answer far less than the axle slot does.
Put the bike on a stand with the suspension unloaded and the rear wheel off the ground. A compressed rear end changes the swingarm angle and therefore the centre distance, and it is the single most common way to get a figure that does not reproduce.
Measure to the centre of each shaft, not to any edge or face. On the countershaft that means the centre of the output shaft the sprocket sits on. At the rear it means the centreline of the axle, not the outer face of the axle nut, not the edge of the adjuster block. If you can see the axle end, measure to the middle of it. If you cannot, measure to a repeatable feature and subtract half the axle diameter — and note what you did, so the second measurement matches the first.
Do it on both sides of the bike. If the two sides disagree by more than a millimetre or two, use the shorter figure, and understand what the disagreement is telling you: the wheel is not square in the swingarm. Straighten it before you buy a chain, because a wheel that is out of square will also eat the chain and the sprocket.
What changes centre distance, and what only looks like it does
An extended swingarm changes it directly, and by a lot. That is the whole purpose of fitting one. Any chart-derived link count is void the moment one goes on.
A chain tensioner changes the effective wrap — the chain has to take a longer path to get around the tensioner wheel, so a tensioned setup consumes more chain than the same geometry without one. The centre distance is unchanged; what the chain has to do between the two sprockets is not.
Wheel diameter does not change centre distance. A 19 inch rear and a 16 inch rear on the same swingarm, with the axle in the same slot position, have the same centre distance. This surprises people, but the axle is where the axle is regardless of how big a circle is bolted to it.
A wider tyre also leaves centre distance alone, but it can force you to dish the rear wheel away from the chain to make room. That changes chain line — the lateral alignment of the two sprockets — without changing the distance between their centres. Chain line problems and chain length problems feel similar from the saddle and have completely different fixes.
Why sprocket size is not on that list
Changing a sprocket does not move the axle, so it does not change centre distance. What it changes is how much chain gets consumed wrapping around the sprockets, which is the second and third terms of the formula rather than the first. That is why a bigger rear sprocket needs a longer chain even though nothing about the frame moved.
What to do with the two numbers
Feed both into the chain length calculator and read the range it returns. If two or three even link counts fall inside your adjustment band, any of them will fit; the calculator shows where the axle sits for each so you can pick one that leaves you room to take up wear later. If none fits, it will say so rather than rounding its way out of the problem.
The honest limitation
This site holds no measured centre distance figures for any swingarm, stock or aftermarket, and it will not publish guessed ones. Publishing an estimate would recreate exactly the problem this page exists to solve — a plausible number, detached from any particular bike, that people would use instead of measuring. The tape is the answer.