Tech FAQ: Again, bigger tires roll faster!

Lennard Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn. 

I wasn’t kidding last week; bigger tires DO roll faster!

Dear Lennard,
In your recent discussion about rolling resistance and tire width you repeatedly stated that rolling resistance for a wider tire is lower at the same tire pressure. But in the real world we don’t run two tire sizes at the same pressure. When I swapped my 700x23s for 25s they beat the snot out of me so I dropped the pressure until the ride was equal to the 700x23s (from 105psi for 700x23s to 85 psi for 700x25s). In this real world application, is the rolling resistance higher, equal or lower for the wider tires?
— Robert

Dear Robert,
The reduced tire pressure drops the rolling resistance even more. You’re getting a rolling resistance reduction for the larger tire, and additional rolling resistance reduction for the lower tire pressure.
― Lennard

Dear Lennard,
Before I go out and buy myself some fatter tires, what testing have you done with aerodynamics? Does the increased wind surface area with the larger tires cancel the positive effects of lower rolling resistance?
— Brad

Dear Brad,
Very good question. As you’re probably aware, the aerodynamic data put forth by Hed and Zipp in support of wider rims for improved aerodynamics shows that it definitely depends on the rim. In general, if you ride very fast, the aerodynamic drag of the fatter tires will cost you more in speed than the rolling resistance benefit will gain you.

In an email, Zipp lead engineer Josh Poertner said:

In general, a wider tire of (the) same construction will have lower rolling resistance for exactly the reasons (you stated). Ironically, the best description and data on this comes from studies done in Britain in the 1800’s looking to optimize the width and diameter of wheels for locomotives.  There is also a lot of great info related to this in “Bicycling Science” from MIT press, as well as Paul Van Valkenberg’s writing on racecar tires.

Generally, though, the decrease in rolling resistance becomes smaller as the tires get bigger. So for example, going from a 19mm to a 20mm may save 1 watt, from a 20mm to a 21mm may save 0.8 watt and from a 23mm to a 25mm may save 0.3 watt. There is great data on this in “Bicycling Science,” using old Avocet Fasgrip tires, which were available from 18-32mm. The 28mm and 32mm were nearly identical, but moving from 18mm to 25mm saved a few watts.

What they are missing is the aerodynamic piece. We have data from the Zipp 303 launch showing the 303 with different width tires (see graph). The figure tells the story of how you can really optimize for tires below a certain (width) number, but eventually the tire really dominates the airflow and ruins everything.  In general, our wheels are optimized around 23mm tires, which means that 21mm tires usually run about equal, maybe a fraction of a watt faster, but don’t change the behavior of the wheel.  Moving to a 25mm adds drag, but can also change the stall behavior of the wheel. And by the time you are at 27mm, you have something that behaves quite differently.

The question really needs to be in regards to the balance of lower Crr (coefficient of rolling resistance) from the wider tire against the aero penalty. The 303 was designed to be as good as possible with 23mm tires, and as a result, its rim is 28.5mm wide. To behave similarly with the 25mm, it would likely have to be at least 2mm wider.  In the graph you see how the 25mm tire has the same curve shape as the 23mm tire on the X45 (code for 303FC clincher). The 27mm tire is on the 285FC (code for 303FC tubular), and you notice that not only is the drag higher, but the curve shape is completely different. In fact, the curve shape looks more like the Easton or Mavic. This is indicative of the rim not being able to clean up the dirty air behind the tire. Ultimately, the offset should be Crr watts vs. Aero watts. In this case you have grams of drag on the left; every nine grams is one watt, so from 23mm to 25mm, you have nearly no penalty up to 10 degrees, and then three-to-six watts at the higher yaw.  With the 27mm, you have something like no penalty to five degrees, and then a five-to-eight watt penalty after that.

Ultimately for the Specialized I would say that the 0.2 watt (0.3 to 0.8 watt) of rolling resistance does not overcome the zero-to-six-watt aero penalty.

Last interesting note: we have been working with Jordan Rapp on this since he noticed that his ‘training Firecrest’ wheels with 25’s were ‘twitchy’ compared to his race wheels with 23’s… we thought this might be largely aerodynamic, but the shorter contact patch (you) discuss is actually the culprit; the longer contact patch serves to resist steering input and adds a slight damping effect to steering inputs. By lowering tire pressure to increase contact patch, the effect could be eliminated, even though the aero properties of the wheel remain the same.

― Lennard

Dear Lennard,
In regards to wider tires and lower pressures, I’m totally on board.  However, I’m wondering just how wide and just how low the pressure should go before it’s too wide or too narrow.  Specifically, for road racing applications, if we are now certain that 25c tires roll faster than 23c tires, are 28c even faster?  What about 32c or 35c?  I suppose then we are looking at significantly heavier tires and more wind drag.  I realize there are dozens of factors influencing overall speed, but for rolling resistance alone, do we yet know the sweet spot for a 160lb rider?
— Peter

Dear Peter,
I think Josh Poertner’s answer above sheds some light on your question. The aerodynamic drag and weight penalty of the 32mm or 35mm tire would be much higher than you could ever gain from rolling resistance improvement, and the rolling resistance improvement for a given increase in tire width drops off as the tires get bigger.

A literal answer to your final question is that, for rolling resistance alone, the larger the tire, the lower the rolling resistance (tire construction remaining equal). See the attached table on the rolling resistance of Schwalbe Durano tires ranging from 23mm to 32mm.
― Lennard

Dear Lennard,
Enjoyed your column as always.

Low pressure in the spare tire will set off the tire pressure warning light, e.g. in your rental car.
— Dave

Dear Dave,
Ah. Good point. Maybe that’s what was triggering it. In case it’s not clear from Dave’s note, the sensors do check all of the tires.
― Lennard

Dear Lennard,
This past weekend I was at a crit race and it began raining prior to the start.  On the start line several racers bled some air pressure from their tires.  This was my first race in the rain, and I didn’t do this.  Early in the race, on a straight stretch of road, a couple of guys went down in front of me.  I thought I had room to maneuver around them but when I tried my bike shot right out from under me.  I’ve been wondering if my fall would have been avoided if I had let some pressure off of my tires.  I was using 23mm Continentals clinchers (Super Sonics) with 95 psi front and rear. I weight 155 lbs. What is your take on tire pressure in the rain?
— Tom

Dear Tom,
Reduce your tire pressure in the rain to increase your tire contact patch and its ability to keep rubber on the road and hence your tire grip.
― Lennard