The arithmetic is a seesaw
Treat the bike as a beam resting on two supports, the front and rear contact patches, separated by the wheelbase. Every mass on it — rider, frame, each bag — sits somewhere along that beam, and how much of it each support carries is a straight lever calculation: the share taken by the front wheel is the distance from the mass to the rear axle divided by the wheelbase.
A mass exactly over the front axle puts all of itself on the front wheel. A mass at mid-wheelbase splits evenly. A mass over the rear axle puts nothing on the front. And a mass behind the rear axle produces a negative front figure, which is not a quirk of the formula but a physical fact: it levers the front of the bike upward, and the front wheel carries less than it did before that bag went on.
This is why the placement of the last few kilograms matters more than their weight. Six kilograms in low-mounted front panniers ahead of the front axle adds more than six kilograms to the front wheel. Nine kilograms in rear panniers a few centimetres behind the rear axle subtracts a few hundred grams from it. Same bike, opposite signs.
Where the rider goes
The rider is usually more than half the total and cannot be positioned by measurement, so this page takes the unloaded front-wheel share as an input and works backwards to an equivalent position. A seated rider on a touring bike is commonly around 40 percent front, more in a low aggressive position and less sitting upright.
If you want the real figure it takes two minutes and a bathroom scale: put one wheel on the scale with the other on a book of the same thickness, sit on the bike normally with a wall for balance, and read it. Do the same for the other wheel. The two should sum to your total, and the front reading divided by that sum is the percentage. That single measurement improves every number this page produces, because the rider dominates the sum.
Four ways to carry the same 20 kilograms
| Arrangement | Effect on the front share | Character |
|---|---|---|
| All in rear panniers | Falls, sometimes below 35% | Light steering, prone to weave when tired or fast |
| Split front and rear, low | Roughly holds, often rises slightly | The classic four-pannier setup, stable and slow to turn |
| Frame bags between the axles | Barely moves | Handling closest to unloaded, least capacity |
| Bar bag and rack top | Front share may rise but the mass is high | The split looks fine and the bike still feels wrong |
The last row is the reason this calculation is necessary but not sufficient. Height does not appear anywhere in a static balance about two axles, and yet it dominates how a loaded bike feels. A kilogram on top of a rear rack and a kilogram low in a pannier at the same point along the wheelbase produce identical numbers here and very different bikes, because the higher mass raises the centre of gravity and slows every correction the rider makes.
What goes wrong, and when it goes wrong
A rear-heavy touring bike usually feels acceptable at 12 mph on the flat and reveals itself at 35 mph on a descent. The failure mode has a name — speed wobble, or shimmy — and it is a resonance between the frame, the steering and the loaded mass rather than a fault in any one component. A light front wheel makes it easier to start and harder to damp.
Names for the hazards, without procedures: a rack or a fork mount failing under load does so at the front of a bicycle in motion; a load high and behind the rear axle changes descending behaviour in a way that is not obvious at low speed; and load ratings for racks, forks and frames come from the people who made them and are not something a calculator supplies. If a fork was not built to take a rack, that is the answer regardless of what the rack is rated for.
Questions people ask
How should a touring load be split front to rear?
The calculation gives you the split you have rather than the one you should want, and preferences differ genuinely between riders and bikes. What it does show clearly is the direction of travel: a rear-only load pushes the front share down, and moving weight into low front panniers pushes it back up faster than the same weight moved anywhere else, because it sits ahead of the front axle.
Why does a rear pannier show a negative front number?
Because it sits behind the rear axle, so it levers about the rear hub and lifts the front of the bike. The front wheel carries less than it did before the bag was fitted. It is a real effect rather than an artefact of the arithmetic, and it is the main reason a rear-loaded bike develops light, vague steering.
Does this account for how high the bags are?
No, and that limit matters. A static balance about two axles only sees position along the wheelbase, so a kilogram on top of a rack and a kilogram in a low pannier at the same point produce identical figures. Height raises the centre of mass and makes the bike slower to correct itself, which is why low mounting is the usual advice even when the numbers say the split is fine.
How do I measure my own unloaded front-rear split?
Bathroom scale, one wheel at a time, with the other wheel on something of the same thickness. Sit on the bike in your normal riding position with a hand on a wall, and read the scale. Repeat for the other wheel. The two figures should add to rider plus bike, and the front one divided by the total is the percentage the calculator asks for.
What load can my rack actually carry?
Whatever its maker says, and the same goes for the fork or frame it mounts to — the lower of the two governs. Nothing on this page supplies a rating. The comparison only runs when you enter a figure, and it groups the bags crudely by position, so on an unusual setup read the per-item table rather than the summary line.