Tire Size Reëxamined

Like that umlaut? The Economist does it, so I figure it's probably correct.  Anyhoo... all of this hoo-ha about tire sizes and inflation and tire size versus aerodynamic speed has had us thinking about this quite a little bit lately.  

Step 1 was to to further examine the effects of tire size on aerodynamics.  Ideally, we would have been able to naturally (i.e. without artificially manipulating bead seat width) set two of the same model of tires up to the same inflated dimensions on wheels with different bead seat widths, to test the slope that we'd earlier established that had the 404 and 52 leap frogging each other at inflated width. The 52 with a 23 is, inflated size-wise, a bit smaller than a 404 with a 25 and moreso bigger than a 404 with a 23.  It is a little closer in speed to a 404 with a 23.  What would happen if you put the same tire inflated to the exact same dimensions on a 404 and a 52?  Unfortunately, that's an unanswerable question, but we were able to test a good approximation.  

Continental makes the Attack 22.  It's a different tire than the 4000s II, but its inflated dimension on a 52 closely replicates the inflated size of the 404 with a 4000s II 23 - the Attack 22 on a Rail is .2mm narrower than the GP4000s II on a 404, and about .6mm shorter than the GP4000s II on the 404.  There are tread differences, which are known to influence aerodynamic speed, but net of everything it's the closest we could get to being able to measure exactly what we wanted to.  This was part of a totally separate trip, primarily to do some totally different testing which we can't yet talk about, and we shoe-horned this bit in.  We ran the 52 first with the GP4000s II, and then with the Attack 22.  With this testing reaching the point of diminishing returns and threatening to turn into a bottomless money pit, we kept it to just the two runs.  Since we don't directly compare different tests from different days, we used the Rail 52 with 23mm GP4000s II as the baseline, rather than the Pacenti SL23 which was the baseline for the other round of tests.  Semantics, we know, but it counts.  



As predicted, the narrower tire gains some speed.  Whether it's the same speed gain we'd get from the theoretical GP4000s II that set up to the same dimension that the Attack 22 set up to, we can't say.  We're getting well into the realm of splitting hairs here.  

So now, the million dollar question: which tires should you be using?  We've now examined the relationship between inflated dimensions and effective pressure, and the relationship between aerodynamic speed and tire dimension, and the fact that people don't want to ride around on the road at 60psi, and a few things show up.  The first is that you should ride the tires that you like, at the pressure that you like.  There's no "right answer."  That said, it seems to make little sense that a 110 pound triathlete should choose a 25, on which she'd have to knock the pressure down pretty low to get a smooth ride (also taking into account the rims on which she's mounting them), when she could instead get more aerodynamic speed, better protection against pinch flats, and a comfortable and secure ride on a 22.  Conversely, a 200 pound guy doing performance recreational rides - group rides, gran fondos, etc - on beat up roads can choose a bigger tire to advance his priorities.  

Varying tire size, type, and inflation pressure gives you an array of tools to tailor what's between you and the road more specifically to your priorities.  Rather than try and drill down into making you feel as though there's a specific answer of what, how much, and how wide, what we've tried to do is to give you the tools and information to help you discover your own perfect ride.  

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I really like the last paragraph when you say weight of the rider should really be taken into account !Really resistance testing (except for the Continental graph on GP4000S II in different sizes) often has a "shortcut" : all tires tested at the same pressure -> in this case bigger tires always end up showing better (lower) rolling resistance but a 25 tire at 110psi is a lot harsher than the same tire in 23 at 110psi, thus for a rider to get the same level of comfort in the 25 tire he must go down to let's say 90 psi and quite often if you compare rolling resistance at the same lever of comfort the math is more : RR of the 25 tire at 90psi = RR of the 23 tire at 110psi.Of course those numbers are probably wrong but I think the idea is not too far from the truth…I think for RR testing tire pressure should be adjusted to tire size. Maybe charge a wheel with a weight and find a pressure to have let's say a 1mm compression of the tire, and adjust the pressure on a different tire size to have the same 1mm compression before doing RR testing. There is still the problem of TPI, different casing materials and angle but we would be on step closer to reality even if not perfect ;) .Thanks again for a nice article, only 1s faster for the Attack is very small… I was hoping for better ;-) . This confirms that eyeball wind tunnel is not a good indicator ;-) . There is still the weight saving of Attack (40gr per tire) at similar RR so on a hilly day 80gr total saving (two wheels) + slightly improved aero at = RR is still a small advantage !Good luck with the rest of the analysis of your findings !!!


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