The argument for wider tires

Those of you who have been riding road bikes for a while "know" that the fastest tires are 23c tires, filled to 110psi, right?  In fact, for time trials, you may even have used 20c tires filled to even higher pressures.     That was the way.

In theory, it is correct - but recent studies are increasingly showing that this is generally true only in ideal cases.   Tires filled to higher pressures are faster but only on really smooth surfaces, like indoor tracks.     Narrower tires are also more aero, but again, only in zero yaw (when the wind is coming head-on).   In the real world of imperfect roads and wind coming from various angles, this is no longer true.

Some of these conclusions fly against what many of us have "known" to be true for all these years.  In addition, there is a lot of misinterpreted versions of this info online as well.   This article is meant to be a primer to help you cut through the noise.    Resources for additional information are provided at the end of the article.


WIDER TIRES HAVE LOWER ROLLING RESISTANCE

When you sit on your bicycle, your tires deform, creating a contact patch that touches the road.     The area of this contact patch is such that it offsets the weight of the rider+bike.   When two tires are filled to the same pressure, the area of this contact patch is the same - after all, the contact patch is supporting the same force (weight of rider+bike) with the same pressure.

Tire Contact Surface (Source:  DT Swiss)

So what does this mean?  We know that Area = Length x Width - so for a wider tire, the length is going to be lower.

This, in turn, leads to lower tire sag and results in lower rolling resistance.     Obviously, this holds true to equal air pressures - ie, a 28mm tire will have lower RR than a 23mm tire at the same pressure.     Drop the pressure too much, and rolling resistance increases.

Here is an example of tire size, pressure and RR, as measured on a drum:

Source:   BicycleRollingResistance.com (Link at the end of the article)

As you can see - wider tires have lower rolling resistance.   But dropping the pressure does result in an increase in rolling resistance.   

However, keep in mind that this is based on a test done on drums.   When this experiment was replicated on various road conditions, it gave rise to the next point, namely...


LOWER PRESSURE IS FASTER ON IMPERFECT ROADS

I don't know where you live, but in most parts of India, you are not likely to have perfect tarmac.    And when the tarmac is imperfect, tires filled to high pressure will cause the bike and rider to bounce up and down - this doesn't have to be extreme:  even the force that causes small vibrations transmitted up your frame and body is energy that is coming from your pedaling.       Compounded over the course of a long ride, that's a fairly significant loss of energy.   

This loss of energy is referred to as impedance - you can think of this as the extra force when you get off smooth tarmac onto bumpy asphalt.    Then there is also casing losses - the loss of energy caused by your tire rubber compressing and expanding when your tires or wheel bounce - or even micro-bounce - over rough surfaces (which is not perfect in energy retention - some energy is transferred as heat)

Here is what the graph of total resistance (CRR + impedance + casing loss) looks like for different surfaces:

Source:  Silca (link at the bottom)

As you can see, as tire pressure increases, total resistance does reduce - up to a point, but after that, it rises up very significantly as well.    So in each case, there is a "sweet spot" which represents the ideal tire pressure - and in this case, for, say, rough asphalt, that is a fair bit lower than the 110-120psi that many people are used to.

Keep in mind that the efficiency losses are not symetrical - take a look at this chart, which shows a change in resistance when you change the tire pressure by 10psi over optimal:

Source:  Silca

As you can see, if you err on the side of too low pressure, there isnt a very significant change in wattage - 1-2W loss only with a 10psi reduction.   On the other hand, increasing the pressure beyond optimal results in a pretty significant increase in watts required. 

So if you aren't sure about what kind of roads you are going to encounter, it is better to be err on the side of lower pressure than higher.

The one downside to lower pressure used to be (and still is) the risk of pinch flats - go over a pothole at too high a speed and you flat your tire.       However, with the advent of tubeless technology, you are now able to get the benefits of riding at lower pressure without the risk of pinch flats.


WIDER IS AERO - KINDA

The early days of aero involved V-shaped airfoil shapes (NACA profiles), based on lessons in aerodynamics learned from planes.    But the thing is -  bicycles don't move at aeroplane speeds and without getting into the details of aerodynamics, they are more prone to facing apparent sidewinds (aka, wind coming from an angle or yaw).     This results in differential air pressures on either side of the airfoil shape - which is great on a plane (it lets it fly!), but is not so good on a wheel, as it results in sidewards force. 

Anyone who has ridden deep sections wheels on a windy day knows what that feels like - the wheel may be fast in no wind, but the moment there is a wind, it becomes a struggle to hold it steady, which defeats all the aero gains.

So the focus shifted from aerodynamics at 0 yaw to aerodynamics across a range of wind angles (0-15 or 0-20 degrees, typically).   This resulted in rim shape evolving from V-shaped to the more commonly-used U-shaped or toroidal shapes in vogue today.     The purpose of this was to improve the range of wind angles at which a wheel would provide an aerodynamic benefit.

(Note my use of the word "apparent wind angle" - that refers to the angle of wind as seen from the rider's perspective.   It depends on two things-  the actual speed and direction of the wind, and also the speed of the rider.   Faster riders generally see wind at a lower range of angles, whereas slower riders see wind at a larger of angles)

The advent of disc brakes - and the fact that it allowed manufacturers more freedom in rim shape - has changed the goals of wheel design these days.   Manufacturers are now increasingly thinking of overall speed - not just aerodynamic speed - and factoring in tire width and lower pressures into their wheel design.     After all, what is the point of building a wheel that is 2W faster with 23c tires, if you lose 10W in impedance losses in real world use? 

So yes, looking purely at aero, a narrow wheel with thin tires would indeed end up being faster in a wind tunnel.     But they would be very twitchy in crosswinds (poorer drag characteristics and also greater side force) and in addition, suffer from the increase in resistance described in the earlier sections. 

So optimizing speed basically involves balancing aerodynamic benefits (which depends on a rider's speed) vs impedancy/RR losses.   And this, in turn, depends on how a given wheel is designed.   For example, DT Swiss has the following recommendation for their wheels:

Roval also recommends using 26mm tires with their CLX64 wheels, even though the 24mm tires test a little faster in the wind tunnel.

So in practical terms - wider = better, although the exact point of "how wide is wide enough" depends on your speed and road conditions.

Do note, however -  in order to get the maximum aero efficiency out of your wheelset, the tire width should be no more than 95% of the width of the rim at the brake track.     This creates the most aerodynamic profile and results in improved aero gains across a range of yaw angles.   A tire that has a bulbous profile (much thicker than the rim) has a very heavy aero penalty.    Thanks to disc brakes, bikes can now take wider rims, which in turn allow wider tires.

And while on the subject of tire width:  even regular 23mm tires, when mounted on wider rims, will measure out wider as they get spread out more.      Eg, on my 21mm internal width rims, a pair of tires with a stated size of 24mm actually measures out to 28mm.     So don't go by the nominal size measured on the case - actually measure the tyres to see where you are ending up.


SUMMARY & PARTING THOUGHTS

So in summary, all this means that you need to find the right balance of tire width and pressure - and for a lot of people riding on less than perfect roads, that means wider tires/lower pressures.

A few guidelines to  consider:

• The ideal tire pressure depends on your weight and surface conditions
• The rougher the surface, the lower your ideal tire pressure
• It is better to err on the side of slightly lower pressure than slightly higher pressure
• If you are about maximizing aero, make sure that the outer width of your wheel, at around the brake track, is 105% of the measured width of the tire

At the time of writing this article, 28mm (measured width) is around the optimal width for  uncompromised speed on smooth tarmac, whereas 30-32mm would be the real-world "optimal" width that balances speed, comfort and efficiency.     Keep in mind these are just ballpark estimates, not hard-and-fast rules. 

There are tradeoffs, of course.    To get the benefit of these tire widths, you may need a disc frame - and the extra weight/maintenance issues that entails.     If you are running lower pressure, you may also need to run tubeless to minimize pinch flats - and that is a whole different can of worms.    And of course, if you live in a place with really smooth asphalt, you are good with thin tires pumped to high pressures.   Or if you don't deal with a lot of wind, you can go with narrower-profile aero wheels.

What is missing from all the analysis so far is the qualitative effect of being less fatigued on a  rider's ability to maintain power.      Does the fact that you havent gotten beaten up by the roads for the last 2-3 hours make you fresher and more able to maintain power?

Brands like HED, Zipp, Enve, Roval and more are all releasing wheels that measure 30mm or wider (outer diameter) with an inner diameter of 21-25mm makes it very clear that wide is here to stay.       While we can always debate the magnitude of the benefits, no company would deliberately release a product that was worse - there are too many independent tests being conducted these days to risk that.      So that fact that almost all the big brands have moved in this direction (HED has been banging the drum of wider rims for nearly a decade now!) supports this as well.

Personally, having ridden wider tires, I find the benefits to be very much worth it.     I do not notice any increase in required wattage in going from 23/25mm tires to 28mm, but i definitely find an increase in comfort.      

Hell, I have even started doing some group rides using Panaracer Gravelking 35mms, which measure a whopping 37mm on my wheels - I am sure it is a little slower than my normal race-oriented tires, but that difference is not as much as one might imagine.      For sure, on tarmac, they dont feel as fast - but  "feeling fast" and "being fast" are 2 different things:   the softer tires dont feel as fast because they deform around road imperfections, as opposed to bouncing on them - and that is the very same reason they actually are fast.     Certainly, the difference in power required to hold the same speed is lower than my ability to notice.    And if the road gets even slightly rough, there is a huge advantage. 

I am convinced enough to make 32mm my "normal" riding tire width, with 25-28mms reserved for races or fast group rides where I am barely hanging on for life.      In fact, I have just sold off my Cervelo R5 (one of the nicest bikes I have ridden) to get a Factor LS because I want to make 32-35mm tires my daily riding tires.

Note, of course - all this depends on the wider tire also being equally (or almost equally) fast-rolling.   That is not always the case.      In my experience, Panaracer Gravelking Slicks are very nice, fast wider slicks, as are Continental GP5000s.   Rene Herse also makes very fast wide tires (no personal experience, however).    So don't expect to put on touring/commuting tires with a lot of puncture protection and expect the same kind of speeds as a race-oriented thinner tire.

Questions?   Feel free to ask on our Facebook group, BikesZone Reborn.


FURTHER READING

Here are some links that do a very good job explaining the rationale.   Note that they are all manufacturer sites, but I have chosen them because they have made the science more accessible to laypeople.    You can dispute the magnitude of those numbers, but the science is solid.

BicycleRollingResistance - a review of CRR, tire width and pressure
Zipp and Total System Efficiency
DT Swiss on Endurance and balancing Aero vs Efficiency
Silca Blog on Rim and Tire Width - Effect on Aerodynamics
November Bicycles:  Wider is Better - Until It Isnt
Roval - Designing Aero Wheels Around Wider Tires
Slowtwitch article on Tire Pressure and Rolling Resistance
Silca Blog on Tire Rolling Resistance and Impedance
Active - Are Wide Wheels Faster
Enve - Real World Fast