Tyres

[derrick]

Hired a couple of bikes in Spain last week. The first had 25mm tyres, and I could keep up with the guy I was riding with going down hill, two days later I swapped the bike for one with 28mm tyres on it, going down the same hills I had to put a lot more power through the pedals to stay with him, I feel 25mm tyres are faster. Anyone else had similar experience.

 

[Cycleops]

Too many variables to assume it's just down to the tyres. Gearing, drag in drive train, aero factors and the biggest one, pressures.
Worth noting that the pros now seem to prefer 28s to 25s.

 

[ebikeerwidnes]

Could be the tyre pressure or even the make/model of tyre - softer rubber is better on corners but has worse rolling resistance


or you might have eaten a bigger dinner
(or smaller - is it weight that makes you faster downhill?? I know nothing!)

 

[Ajax Bay]

Yes, you need to share the make/model and pressure of both tyres for this comparison to be useful (assumes very similar bikes).
Have a skim of this Bicyclerollingresistance article on 5000s:
https://www.bicyclerollingresistance.com/specials/grand-prix-5000-comparison
"A bigger road bike tire (at least the Continental GP 4000 and GP 5000) provides a lower rolling resistance at the same air pressure. You do have to realize that at the same air pressure, a bigger tire will provide a less comfortable ride.
"At 15% tire drop air pressures, which are very close to the recommended air pressures for a given size, the tables are turned and a bigger road bike tire will have a higher rolling resistance. The higher rolling resistance of the bigger tire is because it provides more comfort at the 15% tire drop air pressure."
And for the 4000sII: https://www.bicyclerollingresistance.com/specials/conti-gp4000s-ii-23-25-28
This is rolling resistance. Having a 28 (cf 25) on the front will cost the athlete riding at 30kph average plus in aero drag (rear far less loss).