June 08, 2016 8 Comments

'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!




8 Responses

Brandon Tyler
Brandon Tyler

April 24, 2018

Very helpful piece of information. I am happy that you simply shared this blog with us. For newbies who always look for useful tips and updates, it will definitely a big help and a wonderful resource to bookmark. Thanks!

Edward Pascht
Edward Pascht

October 06, 2017

Where is part 2 of the series?


September 08, 2017

Insightful. Anecdotally, it matches my own experience. I’ll be experimenting with some of the variables you discuss.


April 06, 2017

1) Width of bead have profound effect in the shape of tire casing… (more bulb shape or U shape)… of course it will change the height depending on where it is mounted as we are reshaping it… (flatter side wall will increase height)
2) Again, the air pressure required changes significantly as now the air volume as per same tire mounted inside increases significantly… for same pressure, we do have a lot more volume of air in same tire…
Of course, looks like I am going ahead and next few articles probably will explain this in finer detail.
Good stuff and looking forward to completed version.
Giving some recommendation toward end of the articles based on combination of factors would be nice way to wrap up this as most of this type of articles lacks a little on real day to day recommendation… (IE, if you have 23mm tire with 19.5mm bead width, and weight about 160lbs… start around 85PSI front and 90PSI rear, etc…. just somewhere to guide people with…)


March 09, 2017

Two things:

1) It doesn’t make sense that tire height should vary so much with rim width alone for a given tire and pressure. Geometric models do not explain that. Height will vary with rim width, yes, but over the range of rim widths for these tires it doesn’t vary much. I question whether this data is valid. I’ve not measured road tires but have measured MTB tires and they don’t behave this way at all.

2) While simply saying ‘110psi is optimal for 23mm tires’ has little meaning, the fact is that it never did and nothing in this article has changed that in any way. Optimal tire pressure depends on a great many things, rim width not chiefly among them.


David Ward
David Ward

January 22, 2017

This is great. Thank you so much!

I would have thought as width increases, height would decrease because more tyre us being used to cover the wider base, it has to be taken from the height.

For example, I would have thought a 23mm tyre on a 15mm ETRTO rim would have a lower height (but more width) than the same 23mm tyre on a 13mm ETRTO rim as the latter would squeeze the base and thus “push” the height up.



September 20, 2016

Thank you for that info, As rims change with new production every year, I need to go back and measure some of my older ones because the data on the current websites doesn’t reflect my 2-4 year old parts.

Another variable in the equation is how much tires “stretch” over time, I have some 28mm clements that I’ve been running on HED Belgium rims that have less clearance now after a few thousand miles than when new, I’m sure the tread has worn down some, but the casing has expanded even at the modest (70-80psi) pressures I run them at.

Damon Rinard
Damon Rinard

June 08, 2016

Hey Josh, glad to see you adopt my chart format. Thanks for adding to the data! This is important info to help us understand more about tires and their performance. Cheers, -Damon

Leave a comment

Enter to Win the Ultimate SILCA Prize Pack!