By Tom Jow — Last month while I was searching for some new tires on the internet I found something new. A tire pressure recommendation chart. It was not a surprise to me that the chart existed. What surprised me was how much I have been overinflating my road bike tires all these years. Not just a little mind you, but by approximately 20 psi! Wondering how I could be so far off, more research followed. Not only did I learn why tire pressure matters, but I found another method for determining the proper tire pressure; the online tire pressure calculator.
Previous to this, determining my road bike tire pressure consisted of choosing some fraction of the maximum recommended pressure. If it wasn’t unbearable, and I did not get pinch flats it was good enough. Many athletes I have worked with over the years, and I am sure many recreational riders as well, just choose the maximum recommended. This always seemed to make sense, because it seems that a harder tire would have less rolling resistance. Not so, says Jan Heine, of Rene Herse Cycles. In his blog post “Tire Pressure and Performance” (March 2022), he explains the theory of “suspension losses”. To make a long, interesting story short, overinflated tires vibrate while rolling over the road surface. Have you ever noticed a buzzing feeling while riding at a high rate of speed? My interpretation of that is the tire bouncing up and down in micro amounts over the not so perfect asphalt. Not only is rolling resistance increased by all that vibration, the rider must absorb some of that vibration as it is transmitted through the bicycle.
If this is true, there is a good case for using lower tire pressure. What then, are the benefits? One, it is more comfortable. At lower pressures the tire casing flexes to absorb vibrations and imperfections of the road. The rider is forced to absorb less vibration and impact. Second, rolling resistance is reduced because the tire casing can now flex and absorb imperfections from the road. Instead of being bounced around by small cracks in the road, the tire conforms to the road surface thereby producing a smoother and faster ride. However, there is a risk with using lower pressure, pinch flats.
Pinch flats occur when force compresses the tire enough for the rim to “pinch” the tube between it and the road surface. In order to reduce pinch flats with lower pressure, a wider tire can be used. According to the Silca.cc blog “part-3b-faq-and-putting-it-together-so-far” a 28mm tire at 87 psi can withstand 50% more force before suffering a pinch flat than a 23mm tire at 116 psi. In addition, between two similarly constructed tires, the wider tire also has a more compliant ride. Now we have a wider, more comfortable tire with lower rolling resistance, but we still don’t know our optimal tire pressure. How do we attain our optimal tire pressure then?
One method of setting our optimal tire pressure is the 15% drop method. As described by Frank Berto in “Under Pressure”, (March/April 2004), 15% compression of the tires total height under the rider’s weight is optimal. Other methods (with less math) of determining tire pressure include using a printed chart or an online tire pressure calculator. The online calculators can be simple, requiring only tire width and combined rider and bike weight such as the calculator by Rene Herse Cycles. On the other hand, they may ask for more detailed information like intended surface, tire casing type, tubeless or with tube as the calculator by the tire maker Vittoria. After data input and clicking calculate, the results are returned as recommended pressure for front and rear.
These results, as with all manufacturer recommendations, are a starting point. No calculator or chart can adjust for all the factors of a bike and rider rolling down the road. Also, it is rare for any multiple of pumps/gauges to give the same reading. This is relative, however, meaning that if we use the same measuring device each time we will, in the end, be able to have consistent settings. In addition, tire casings make a difference. A tire with a more supple casing is more compliant (absorbs more vibration) than a lesser casing at the same pressure. Finally, as Silca describes a sharp increase of impedance (suspension) losses above optimal pressure, they state, “ Better to set your pressure a few psi below the BreakPoint (optimal) Pressure than to have it a few PSI above the BreakPoint Pressure”.
Testing the Calculators
I compared the results from four tire pressure calculators: Vittoria, Silca, SRAM and Rene Herse. My base set up is 700 x 42c Specialized Pathfinders mounted to Roval 38 carbon rims. My seat of the pants tire pressure set up was 40 psi rear, 38 psi front. When given a choice I selected dry, rough/old asphalt.
- Vittoria: This calculator asked the most questions. Bike type, tire type, tire size, insert yes/no, tire feel reactive/balanced/supple, rider + bike weight, surface type, surface condition wet/dry. Recommendation: 42 psi front, 43.8 psi rear. (https://vittoria.com/pages/tire-pressure)
- Silca: Similar questions with different wording. Rider + bike weight, surface, tire width, wheel diameter, tire type, average speed, weight distribution (bike type). Recommendation: 37 psi front, 38.5 psi rear. (https://silca.cc/pages/pro-tire-pressure-calculator)
- SRAM: This calculator only asked for the basics with two exceptions, rim type and internal rim width. Recommendation: 36 psi front, 38.3 psi rear. (https://axs.sram.com/guides/tire/pressure)
- Rene Herse Cycles: This calculator asked the least number of questions. Rider + bike weight and tire width. It was also the only one suggesting the same pressure front and rear, and a firm and soft range (presumably for rough vs. less rough conditions). Recommendation: Soft, 28 psi, firm 36 psi. (https://www.renehersecycles.com/tire-pressure-calculator/)
Because most of the recommendations were pretty close to my baseline, I only tested the “soft, 28psi” result (one ride). I rode one of my regular routes out to the west, past the airport. Through the industrial zones there is plenty of dirt and rough, broken pavement. Also along the way are train tracks, years old unbroken pavement and decades old pavement.
What I experienced was kind of eye opening. The tires did not feel slower at 28 psi than at 40 psi (avg. speed 15 mph). Out of the saddle going up the overpasses the tires did not feel squishy at all. Going down the overpass or speeding along with the wind at my back the road surface did not feel buzzy. I floated across the tracks. And finally, the rough, broken roads were much less rough than before.
Verdict?
The calculators are an excellent way to find a starting tire pressure quickly. These would have saved me some time getting my tires set up the way I like. The 28-psi recommendation by the Rene Herse calculator really surprised me. Not only that it even suggested it, but that it also seems to work really well. My ride was more comfortable than with higher pressures, and just about the same speed. I will have to test it more, but so far the results are promising.
Got a bike question? Email Tom at runnerrunner.rider@gmail.com
