Tire Size, Pressure, Aero, Comfort, and More!

'23mm' Tire Measuring 24.89mm Wide at 6Bar (87psi)

Part 1: How We Got to Now

No other single component affects the comfort, handling and efficiency of a bicycle like the tires.  Tires are the sole connection to the ground, they are the sole transmitter of drive force to propel the cyclist forward, and they are the sole means of gripping the road during cornering.  They are the most dominant spring in the bike/rider system which means that more than anything else, they control comfort.  They are the sole component which will (ideally) ever have to resist the abrasive contact of asphalt, concrete or gravel with minimal damage.

In future segments, we will show data discussing the Aerodynamic, Comfort and Rolling Efficiency of tires, but for starters, we will be looking at something seemingly so simple, yet not simple at all.  Tire Width.

Years ago, tire width was quite a simple affair.  Tubular tires were sold in different casing widths, which they maintained even if not mounted to a rim.  A 21mm tire would measure with calipers at 21mm +/-0.5mm.  With clincher tires, this became more complicated as the interface between tire and rim became a factor in the tire size discussion.  Manufacturers were led by the ETRTO to recommend which tire sizes worked with which rims, and the conventional wisdom of narrower tires being faster kept everybody in check for some 30 years as racing rims measured 13mm between beads, road rims measured 15mm between beads, touring rims were 17mm, mountain rims were 19mm, etc.  

Fast forward to now and things are much more complicated.  ENVE just released their 7.8 TT/Tri wheel set with 19.5mm inner bead width.  That's 2mm wider than the Zipp 303 we developed for the cobbles of Paris-Roubaix 7 years ago and 6.5mm wider at the bead than the original 808 TT wheel I designed in 2004.  

Many of these changes have come in a stepwise motion over the last 10 years with first the availability of slightly wider racing tires (23mm as opposed to the previous 21mm standard), which led aero wheel companies to make wider wheels to try and offset the aero penalties of the wider tires.  Then athletes took advantage of the wider wheels and began trying (and liking) even wider tires.  This has gone on in 1 and 2 mm increments for some years now resulting in the world we live in today where the fastest racing tires are being launched with the smallest sizes at 24 or 25mm width!

As this was happening in Road, gravel was gaining popularity as were fat bikes.  In many ways, gravel riding has further pushed road wheel development toward wider format rims, while fat bikes have pushed Mountain to think wider as well.  Today we even have Plus sized mountain tires that are significantly wider than anything we would have imagined riding 20 years ago.

For this first part of our study, we have focused on road wheels and tires, but the learnings here are applicable everywhere.  The main lesson we want to convey is that tire size is no longer accurately linked to the number on the sidewall, but rather, needs to be measured based on the rim you are mounting it on.  These measurements will give us a foundation for future discussions of aerodynamics, Coefficient of Rolling Resistance (Crr) and other topics. 

Many road tires are still given their widths based on the old ETRTO fitment standards, so that 23-28mm tires are still often based on fitment to 13mm inner bead width rims, something you are likely only to find at swap meets anymore, and mounting these to wider rims will net you an effectively wider tire.  

This chart was inspired by my good friend Damon Rinard, former development engineer for Cervelo and now Engineering Manager at CSG (Cannondale).  I've borrowed (stolen) his format outright, and populated with data collected using Zipp, ENVE and Continental components.

This chart is important because it shows that the same tire can be a lot of things depending on what rim it is mounted on, generally none of which are equal to the number on the sidewall!  We didn't measure on a 13c rim, but I can imagine that the 23 would be more of a 23mm width on the 13c rim as would be the 25 and 28, but on a 15, 17.5 or 19.5mm rim, they are all much wider.  

Also note is how dramatically the tire pressure affects both width and height of the tire.  This will be a big deal when we start to talk about aerodynamics later on.  The width also plays a large factor in determining the optimal pressure for the tire which we will discuss in the next post in this series.

Simply saying '110psi is optimal for 23mm tires' suddenly has little meaning anymore.  Which 23?  On what rim? What does it measure?  Not easy!

In our next post we will be looking at the vertical compliance of these tires on these rims and ultimately the rolling resistance and aero performance which gets even more interesting as the tires all change size with air pressure!

In the mean time, take a look at what size tire you're riding on what size rim and see if it matches the number on the casing.  You might be surprised!