So I had written a post earlier about the argument for wider tires and posted it, among other places, on BikesZone Reborn. A few people raised some very interesting questions and that made me dig into things a bit deeper and email a few wheel makers.
While the argument is sound, I have found what appear to be a few discrepancies which apply mainly to those who are looking to optimize speed and get every marginal gain possible. Please note this last statement, because it provides context for the rest of this analysis (which is going to be very focused on very minor differences).
Before we get into that, let me recap the argument for wider/softer tires:
- For a given pressure, rolling resistance (RR) decreases with tire size
- Increasing the tire pressure actually reduces rolling resistance (atleast on smooth surfaces)
- However, in the real world, surfaces are not smooth and as you increase tire pressure, vibration losses start to increase and beyond a certain point, they go up significantly.
So in short, there is an optimal tire pressure where the sum of rolling resistance and vibration losses are minimized. And if in doubt, it is better to have too low a pressure (and deal with 1-2W of increased RR) than have too high a pressure (and deal with 5-10W of increased vibration losses).
Before we go further, one thing to keep in mind, however - the difference in rolling resistance with tire sizes and pressure is quite small - less than a watt when you go up a tire size, and maybe 1W or so for every 10psi pressure change. See this table from the previous article:
As you can see, a wider tire has less than 1W of difference in terms of RR for a given pressure.
So what exactly is the problem here?
Well, the problem is two-fold. Let's start with the graph of rolling resistance and vibration losses as discussed in the previous article. It looks as follows:
Let's look at just the 25c tire for now - its rolling resistance is depicted by the line in blue and decreases with tire pressure. On the other hand, the vibration loss for the tire (the green line) increases with tire pressure. At some point, there is an optimal point where the sum of the two - ie, the total energy losses - are minimzed.
Now, if you take a 28c tire and plot its rolling resistance curve, it will be the line in red. That line is going to be slightly below the RR curve of the 25c tire, as you might expect (RR for a thicker tire is lower for a given pressure).
However, based on info I got one in reply to a question sent to one of the top makers of aero wheels in the world, the vibration curve does not change too much by tire width - for a given surface, that curve is dependent almost entirely on the tire pressure. So the vibration curve remains the same - the line in green.
That means that the wider tire will indeed have lower total resistance. This is what the brands have been telling us so far, right.
HOWEVER - because the curve of the 28c tire is very close to the curve of the 25c tire (remember - less than 1W difference in RR between the two sizes), the optimal point for the 28c tire is going to be very close to the optimal point for the 25c tire. Heck, even the optimal point for a 32c tire is going to be very close to the optimal point for the 25c tire.
And that is where the problem begins. If you look at the Silca chart I had referenced in my earlier article, the optimal point for a 25c tire is in the 90-110psi range for asphalt:
So by extension, the optimal point for the 28c or even 32c tire should also be in the same ballpark. While we can expect some improvements, those improvements will not be huge, simply because the curves don't change that much for 25 vs 28mm tires.
Also, this improvement will be in the same ballpark tire pressure range as well. Let's be generous and assume 10psi lower - that means an optimal range of 80-100psi. However, the new wheelsets from Enve and Zipp are all coming with a maximum recommended pressure of 70psi. So what gives, there?
That is problem #1.
And now, let's get to the second issue - aerodynamics. We haven't considered the effect of a wider tire (on a suitable optimized rim) vs a narrower tire (on a correspondingly optimized rim). I have searched high and low, but have been unable to find any data comparing the two.
The closest I have come is to something from DT Swiss's website, which looks as follows:
According to this, wider tires do have a disadvantage in aero, especially as speeds increase (whether that cut-off is 35kph, a bit higher or a bit lower doesn't matter so much).
So in short - when we go to a wider tire, we are looking a rolling efficiency improvement that is less than a 1W for a given tire pressure. But when speeds increase, there is a larger aerodynamic penalty.
So what exactly ARE we gaining with lower pressure, especially at higher speeds (ie, the very sort of speeds for which we buy expensive carbon wheels)? Comfort? Sure. That is important. If these wheels were marketing as "endurance wheels", balancing speed and comfort, I would have no problems with things.
To be fair, Zipp and Roval are both calling their new wider wheels "fastest allround" wheels, implying they will be the fastest for a wide range of surfaces - and for rougher roads, like unfinished concrete or chipseal, lower pressures make a lot of sense as the point of optimal pressure is a lot lower. And Silca's studies show that it is better to be lower (a lot lower, even) than optimal pressure rather than even a little higher. So trading off a couple of watts for all these benefits makes a lot of sense for most of the riding.
But what does that mean for people who want to squeeze out the maximum speed for their watts and so want the absolute fastest wheels for riding on good tarmac - eg, for ITTs or triathlons? Does that mean that narrower rims/wheels are still faster? There is no data that clearly shows this to be the case.
So what's going on here?
Let me be clear: I am going to rule out "it is a conspiracy to sell more wheels". I'll leave such simple-minded nonsense to 5-year old Youtubers with a questionable track record of telling the truth. It does not work that way. While the bicycle industry certainly has a track record of hyping very marginal gains and selling snake oil, their claims also rarely *contradict* science (as opposed to merely being hard to prove/disprove). Even if one company were to falsify science and put out an inferior product, it would leave them vulnerable to independent tests and to competitor products. The market does not work like that. When Zipp, ENVE, etc release a wider rim, it could be that the benefits of this are marginal or even non-existent - but it is definitely not going to be WORSE (atleast not as per their analysis).
What options are left, in that case? A few things to keep in mind:
- The above analysis is cobbled together using analysis from different sources. Differences in methodologies could account for the difference in numbers, and they may not be directly comparable
- The analysis by Zipp, ENVE, etc comes up with a different optimal point from the one shown on Silca's blog. Silca's blog uses a total rider+bike weight of 190lb - a different number shift the curve and also potentially change its shape
- Differences in tires and rider weight could also change the shape of the curve
- I have not factored in the physiological cost of reduced comfort and its effect on ability to put down power. Eg, my titanium bike was never the most exciting of rides, but over longer distances, I have done some of my fastest rides on it. Would the comfort gains realized from giving up 1-2W in rolling efficiency result in the ability to put down 20-30W more later? I don't know - but I do think that if the manufacturers had analyzed this, they would have referenced it. As far as I know, no one has.
- In the real world, the odds of consistently encountering smooth roads - even during races - is fairly low. So as earlier, taking the 1-2W hit in overall rolling efficiency may be worth the trade-off of not losing 10-15W when the road gets rougher. So it isnt worth making a wheel optimized just for very smooth roads.
Either way, I think this is still developing science and we haven't reached steady state yet.
I also suspect that for absolute, optimized efficiency, there is an upper limit to how wide/how soft we can go. 32mm may be great for everyday riding, but if you want to save those 1-2W, you may be better off in the 25-28mm range and north of 80psi.
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