Tire drop, and where the 15% rule stops working
Tire drop is how far a loaded tire squashes. Roughly 15% is the published figure almost every pressure chart is built on, and this one is too.
What matters is what you do with it after that — and where it stops being the thing that limits you.
The number under almost every pressure chart
Tire drop is the amount a loaded tire squashes, as a share of its own height. Put a rider on a bike and the tire flattens a little; drop is how much. Frank Berto measured it, published tables built on it, and settled on roughly 15% as the figure that balances rolling resistance, comfort and the risk of hitting the rim.
It is a genuinely good idea and it is the foundation of this calculator too. Baró’s engine is built around the same 15% target — we did not find a better constant and we are not going to pretend we did.
The target is the start, not the answer
A drop target tells you what the tire should be doing. It does not tell you what to pump, because the pressure that produces 15% drop depends on things a flat chart never asks about.
Around the target the tire deforms enough to follow the ground and not so much that the rim is at risk. This is the band the whole curve aims at.
Get the pressure that lands in this bandSo the 15% survives as the goal, and the layers on top of it are what turn a goal into a number. At 190 lb on a 40 mm tire the engine lands at 14.5% front and 16.7% rear — around the target, split by which wheel carries more of you.
Follow it out to a mountain bike and it falls apart
Drop assumes the thing that limits how low you can go is the tire deforming too much. On a road tire that is true. Keep widening the tire and it stops being true, because something else starts binding first: the rim hitting the ground, and the tire rolling off its own bead in a corner.
Those are different limits with a different shape, and they arrive well before a flat drop rule says you should worry. Follow 15% out to a 2.6 inch tire and it hands you a number in the mid-teens. The table on the MTB page says 21 / 23 psi for that rider, and that is the number people actually ride.
Which is why a mountain bike gets its own curve here rather than a discount applied to the road one. Not because the drop idea is wrong, but because out there it is no longer the thing doing the limiting.
We kept the constant we could not improve
It would be easy to present 15% as ours. It is not. It is published work we adopted because it holds up, and the useful thing we added is everything that sits between that target and a pressure you can pump.
If you want to see the rest of it laid out in order, the method page has the whole stack — including the figures we chose ourselves with nothing published behind them, which is a shorter list than you might expect and an honest one.