I’ve extended the graph a bit, because I intend to gain a bit of weight this summer. Now I can use it to figure my tire drop when I weigh 275 with a 25 lb bike (180 lbs on the back wheel).
Now if someone could just figure out the line for 45mm and 54mm tires!
The latest issue of Bicycle Quarterly had a short article (by their standards) about how to optimize your tire pressure to get the ideal 15% ‘drop’ front and back. The drop is how far your tire deforms when you get on the bike.
If your tires are too hard, they bounce on road irregularities and rob you of speed. Also, it’s less comfortable. Too soft, and there’s too much drag.
The BQ tests corroborated a Frank Berto test that indicated 15% is the ‘sweet spot’ between too soft and too hard, and included Mr. Berto’s chart.
I even weighed my bike, with me on it, with the tires on a doctor’s scale and a decorative cinder block (which was the perfect height) to get the weight biases fore and aft. It came out 40/60 with some random (but light) stuff in the basket.
So I plotted the tire pressures, made sure it was also a 40/60 ratio, and pumped up the tires to 55 front and 75 rear with my hugely aggravating SKS Kompressor pump that probably needs a new red rubber part to grab the stem.