Hai ragione su tutto. Ma io ho semplicemente chiesto come può essere più scorrevole un 28 di un 23. Non quale è più comodo.
Non ho parlato di marketing, di strade belle o brutte o di aerodinamica del complesso bici/ciclista.
Quando sento parlare di maggiore scorrevolezza di un 28 resto sempre perplesso. Ok la comodità, la tenuta in curva, le strade brutte, il peso del ciclista e la sua posizione sbagliata e non aerodinamica ecccc. Ma la scorrevolezza secondo me no mentre molti parlano sempre di sta cosa.....
Qui sotto, il primo articolo che mi è capitato tra le mani.
You can see where this one comes from. In cycling, smaller things are lighter and lighter things make you go faster, right? Well, no, not for tyres. Countless measurements have demonstrated beyond doubt that rolling resistance of tyres is lower if the tyres are wider, as long as the construction — carcass thickness and materials, tread rubber and depth etc — is identical.
But is that the whole story? What about weight and aerodynamics?
As discussed above, weight, even rotating weight, has a much lower effect on performance than people think, so the few grams difference between 23mm and 25mm tyres is immaterial.
We’re not aware of any detailed modelling of the aerodynamic effects of fatter tyres, but let’s have a bit of a stab at it. Aerodynamic drag arises from an object’s frontal area and its drag coefficient.
Comunque, come dice
@Enea96 , parliamo di pippe mentali, a mio parere. Ed i numeri lo confermano
Drag coefficient depends on an object’s shape and how air flows over its surface. A very aerodynamic shape such as a smooth wing might have a drag coefficient of 0.005, while a brick’s is more like 2.0.
Multiplying the drag coefficient by the frontal area gives you the aerodynamic drag, so drag force increases as, say, a tyre gets wider.
According to
CyclingPowerLab(link is external), the frontal area of a cyclist in the drops is about 0.36m². The change from 23mm to 25mm tyres adds 0.001436m², an increase of 0.4%. That’s the increase in power you’ll need to maintain any given speed. It takes 102 watts to maintain 18 miles per hour in this scenario, which increases to 102.5 watts with the fatter tyres.
According to
BicycleRollingResistance.com(link is external), there’s a 0.3 watt difference in rolling resistance per tyre at this speed between 23mm and 25mm versions of Continental GP4000s II tyres at 120psi. The half-watt increase in aerodynamic drag is therefore almost exactly countered by the decrease in rolling resistance.
The problem here is that you’re not going to get the other benefit of fat tyres – a softer ride – if you keep the pressure the same. If you do reduce the pressure, then the rolling resistance goes up too, and you end up with slightly more total resistance.
With 28mm tyres it turns out you have a bit more leeway and can drop the pressure a little. At 100psi our 28mm GP4000s IIs have 0.5 watts per tyre less resistance than 23mm tyres at 120psi, and one watt more aerodynamic drag.
Narrow tyres, then, faster or slower? The answer, it turns out, is “it depends.” The total aerodynamic and rolling resistance depends on tyre size and pressure, and which is faster changes with how you fine-tune those variables.
An extra complication we haven’t mentioned yet is speed. As you go faster aerodynamic drag increases more than rolling resistance. At finishing sprint and time trial speeds, you’re almost certainly better off with narrow tyres.
If you don’t race, though, you might have noticed that we’re talking about small differences in resistance. A 28mm GP4000s II at 80psi has the same rolling resistance as a 23mm at 120psi. Does the extra watt of air resistance matter? It’s definitely not a difference you can feel (the threshold for that is 5-10 watts depending on the individual) and it’s going to make a tiny difference to your ride time even on a long ride. You might well decide the comfort is more than worth it.