Formula
The pressure is read off Frank Berto’s 15% tire-drop chart, the one reprinted with wheel load on the horizontal axis. The six published widths are 20, 23, 25, 28, 32, and 37 mm. Each curve is the straight stroke drawn for that width. Between two widths, the page interpolates only when both strokes cover that wheel load. Past the end of a stroke, or outside 20–37 mm, it stops. It does not fit a new equation.
fill = the lowest of chart, tire maximum, rim maximum, and 72.5 psi if the rim is hookless
72.5 psi = 5 bar, at 14.5 psi per bar
The Bicycle Quarterly caption is the check. A rider and bike of 100 kg, split 45% / 55%, is 45 kg on the front wheel and 55 kg on the rear. The caption prints 125 psi and 155 psi for 20 mm tires, and 45 psi and 53 psi for 37 mm tires. This page’s reading of those strokes is 124.1 / 154.5 psi at 20 mm and 44.2 / 53.7 psi at 37 mm, within 1 psi of the caption. The 20 mm rear stroke ends just at that 55 kg example, so 55 kg is read at the drawn end.
Weighed wheel loads are the measurement Heine describes: one wheel on a scale, the other on a block, both level, you in your riding position. The 45/55, 40/60, and 35/65 splits are the published approximations for a randonneur, a racing bike, and a city bike. They are not a second correction on top of a weighed load. Berto’s 2006 article also described a total-weight graph, then subtracting about 10% in front and adding about 10% in the rear. That graph is a different axis. This page does not evaluate it, and it does not apply that 10% on top of the wheel-load chart.
The fill pressure is never the chart when the chart is above a limit. A 20 mm hookless tire at the caption loads still has a chart reading near 154.5 psi in the rear. The fill pressure is 72.5 psi. Frank Berto wrote that neither he nor a tire company will say exceeding the sidewall maximum is safe. A knobby mountain-bike tire is refused: Berto wrote that 15% drop is not a valid criterion for those tires, and the minimum graph on his page 6 is a catalog compilation, not this chart.
Worked examples
100 kg, 45% / 55%, 20 mm, hooked, tire maximum above the chart. Chart and fill are both about 124.1 psi front and 154.5 psi rear. The caption prints 125 and 155.
The same loads at 37 mm. About 44.2 psi and 53.7 psi. The caption prints 45 and 53. Wide tires do not need the pressure a narrow tire does.
The 20 mm caption loads on a hookless rim. The chart is unchanged. The fill pressure is 72.5 psi on both wheels, because both chart readings are above 72.5 psi.
A knobby tire. No pressure. The 15% chart is not used.
Reference
Published widths and the wheel-load span of each stroke. Pressures between the ends are the straight line. A load within 0.05 kg of an end is read at that end.
| Width | From | To |
|---|---|---|
| 20 mm | 27.0 kg, 69.5 psi | 55.0 kg, 154.5 psi |
| 23 mm | 27.0 kg, 58.5 psi | 65.9 kg, 154.3 psi |
| 25 mm | 27.0 kg, 48.2 psi | 70.2 kg, 141.2 psi |
| 28 mm | 27.0 kg, 42.4 psi | 70.2 kg, 117.1 psi |
| 32 mm | 27.0 kg, 33.5 psi | 70.2 kg, 90.9 psi |
| 37 mm | 28.3 kg, 28.5 psi | 70.2 kg, 67.9 psi |
Questions
What pressure is 15% tire drop?
On the published chart, a 100 kg rider and bike at 45% front and 55% rear is 45 kg and 55 kg on the wheels. A 20 mm tire reads about 124.1 psi front and 154.5 psi rear. The caption of that chart prints 125 psi and 155 psi. A 37 mm tire reads about 44.2 psi and 53.7 psi, against the caption’s 45 psi and 53 psi.
Why is the fill pressure lower than the chart?
The chart is a 15% drop reading. The fill pressure is that reading or the tire maximum or the rim maximum, whichever is lower. A hookless rim is also capped at 72.5 psi, which is 5.0 bar. The page will not tell you to exceed any of those.
Does this work for a knobby mountain-bike tire?
No. Frank Berto wrote that a 15% drop is not the right test for a knobby mountain-bike tire. His minimum-pressure sketch for those tires was compiled from catalog recommendations, not from the 15% chart. This page will not turn that into a number.
Should I measure the tire?
Yes. The chart is actual width, not the number on the sidewall. Berto’s note on the chart is for a rim of an appropriate width. A weighed front wheel and a weighed rear wheel are better than the 45/55, 40/60, or 35/65 approximations.
Sources
- Jan Heine, “Optimizing Your Tire Pressure for Your Weight,” Bicycle Quarterly, reviewed by Frank Berto. The chart caption reads “Tire inflation for 15% wheel drop in relation to wheel load and actual tire width” and “Source: Frank Berto.” The worked example on that caption is 100 kg, 45%/55%, 20 mm at 125/155 psi, and 37 mm at 45/53 psi. Note 4 limits the chart to a rim of appropriate width. The rough-road and smooth-road sentences in the conclusion are qualitative. This page does not assign them a percent.
- Heine, Vande Kamp, Wetmore, and Spence, “Optimizing Bicycles for Carrying Heavy Loads,” Bicycle Quarterly Vol. 5, No. 3, 2007, p. 37. That is the source of the 45/55 randonneur, 40/60 racing, and 35/65 city approximations printed with the chart.
- Frank Berto, “All About Tire Inflation,” 2006. Optimum drop is about 15% of section width, or about 20% of the height from the ground to the rim. His own graph in that article is total weight, and the printed 200 lb example is 136, 126, 106, 88, 76, 60, and 46 psi from 19 mm through 37 mm. He then says a sport tourist on the drops is about 45% front and 55% rear, and suggests subtracting about 10% from the front pressure and adding about 10% to the rear. This calculator uses the wheel-load chart instead of that total-weight graph, so it does not apply the 10% again. He writes that neither he nor a tire company will say exceeding the sidewall maximum is safe.
- Frank Berto, “Under Pressure,” Australian Cyclist, March/April 2004, p. 48, as cited by Heine. Berto’s mountain-bike section says the 15% criterion is not valid for knobby tires, and the minimum graph on his page 6 is compiled from Continental and Michelin catalog recommendations. This page does not use that graph.
- Hookless rims are capped here at 72.5 psi (5 bar), using 14.5 psi per bar. The tire maximum and any rim maximum you enter still apply when they are lower. A measured width and a weighed wheel beat the labeled size and the split table.