Yes. What front wheel are you using?

A 23 or smaller on the front is a good idea for crosswind stability IMO, even if it's a hair slower. And usually it isn't slower.

Like trail said, gotta know the rim and what races you are doing.

I'm using some locally made rims. Wide rims though. 26.2mm wide at the brake track, 17.1mm internal width.

23

http://www.trainingandracingwithapowermeter.com/...inny-vs-fat.html?m=1

What rims are you using?

See post #4 above.

23mm. The 23mm TT already isn't exactly a great aero performer. The 25mm would be a disaster and, contrary to popular belief, the 25mm tire will not have a lower crr.

Post #4 doesn't answer the question.

best for aero is 23mm
not gp TT but GP4ks2
you need the air to flow and TT is flat, without any thread. TT on rear is perfect, 25mm

I switched to 25 front and back on my 404s a couple weeks ago after riding 23 for a couple years. I don’t have any objective data on the change, but I enjoy the ride so much more that I don’t really care if I fall from 52nd to 55th in my age group.

Really? Did they screw up the GPTT25 they way they appear to have scewed up the SS23?

OP, the best option for the front if you want some puncture resistance and and low Crr is probably the Attack23. It has "aero grooves" oriented the right way now.

Pressure? The spring rate of the tire is dominated by pressure. The lower limit is set by pinch flat worries... and if you use latex tubes, you can run low pressure. If you hit something hard enough to pinch flat you are probably wrecking the rims (or about to). I once hit a pothole hard enough to dent both rims, but the tubes were fine.

Still experimenting with pressure.

Happy to get any recommendations.

I weigh 175.

85psi felt a little low.

90 felt pretty good.

Ran 105 on my 23s.

I ride in Florida in midday heat so I am a little skittish on putting too much air in the tires, whether justified or not.

I was just going to make a point along these lines. The roads in my area have deteriorated so badly that I can't be confident running 23 mm tires much anymore. Running larger tires and lowering the pressure makes a lot of difference in how much I get beat up. The OP might want to take that aspect into account if they know their courses. Aero and RR might not necessarily be the top considerations if the pavement is less than optimal.

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Stay aero my friends.

Nope, crr is primarily driven by compound and tire construction. The whole "wider is faster" myth is because CRR tests are normalized around tire pressure instead of casing tension. As a result, wider tires have a higher "spring rate" by default. You wouldn't run a 29x2.0 MTB tire at 90psi (it would be rock hard) so why would you run a 25mm tire at the same pressure as a 23mm tire? You wouldn't (or shouldn't). Once you adjust for pressure, the size of the contact patch and the amount the tire deforms does not change meaningfully. As a result, crr is driven primarily by compound and casing ("spring material" if you will).

Wide tires ≠ more comfort either. Wide tires allow you to obtain a more comfortable ride by running a lower pressure with less risk of a rim strike on account of having more volume.

A narrow tire and a wide tire of the some construction will have the same spring rate at the same pressure. I think Josh Poertner and Damon Rinard have both measured this. If you run them both at 100 psi, they'll ride about the same and the wide tire will have lower Crr. If you run the wide tire with less pressure, the Crr may be the same, but then you gain more comfort with the larger tire.

90 psi should be plenty for 23s. You could probably get by with 80 or even less if you use latex tubes.

The casing tension of the wider tire will be higher and thus its “spring rate” (not a 100% accurate term here but close enough) will be higher. Damon chimed in on this last year when Flo released their preliminary casing tension study saying that normalizing for casing tension was indeed the correct approach. TomA has mentioned this as well. Hopefully he joins this thread. I’ll find and link Damon’s comment when I’m back in front of my laptop.

Casing tension has been missed by most in the industry because 1.) its difficult to measure and 2.) humans are wonderfully prone to placebo effect. BikeRadar has a story last year where they compared the GP4000S II in three different widths. Even though they ran all three tires at the same pressure (95psi if memory serves) they stated that the 28mm tire was “smoother” and all the other usual marketing gibberish.

GreenPlease wrote:

23mm. The 23mm TT already isn't exactly a great aero performer. The 25mm would be a disaster and, contrary to popular belief, the 25mm tire will not have a lower crr.

Huh? I though the GP TT was gernally accepted as the fastest choice in tyres? Is that not the case?

I've been using 2 x 23mm GP TT with latex tubes on my cervelo for ages. Have I been disadvantaging myself? My new bike comes with a 23mm GP Attack III on the front and a 25mm GP Force III on the back. Is that a better choice than 23/25mm GP TTs?

It's one of the fastest tires from a CRR standpoint. It is not that great of a tire from an aero standpoint. However, to quote Tom A "a low crr can make up for a lot of aero sins". See TomA's testing of the Turbo Cotton (which is an aero brick). The 23mm GP Attack III should be a very fast tire. If it follows in the same footprint as the Attack II it's likely just a touch thicker than the GP TT and thus rolls a touch slower *but* it's likely more aero than the GP TT given its tread pattern.

Thanks. Very informative. I didn't realise tread pattern was such a factor. I guess I just assumed that the smoother a surface, the lower the air resistance.

Not all tread patterns are created equally. The GP4000 SII was an accident as far as we know. Schwalbe did some active development for the tread pattern of their "Ironman" tire. I'm not sure if they used the same pattern on the Pro One.

The Attack 3 is bigger than the old Attack by quite a lot. It's wider than a 23mm Michelin Comp. But it only weighs 165-175g IRL and the grooves are oriented properly for some potential boundary layer tripping, so I'd guess it's as good as the GPTT 23 for Crr and better for aero.