Rob3rt
Man or Moose!
- Location
- Manchester
Some numbers for you from my field testing:
Changing the front tyre from 22mm to 19mm exhibited a Crr increase of 2.48% on average. The CdA decrease for the same change was 4.22% on average. Figures quoted are for the entire system, i.e. me, on a time trial bike, in full race kit, the rear wheel has a 22mm tub on it. Of course being a relatively low drag system (full aero kit) the impact of the tyre on total CdA would have a higher relative impact than it would on a higher drag system.
Using these as inputs to model a time trial on the Cheshire J2/1 time trial course (keeping the course, weather inputs and power inputs, as well as the rider weight etc constant and only varying the Crr and Cda values):
The Crr increase would result in going 1 second slower
The CdA decrease would result in going 7 seconds faster.
So the net result is that the change provides a net gain of 6 seconds.
For @I like Skol 's interest, adding 1lb to the bike (I can't account for where exactly it is added so I have just added an exaggerated chunk of weight), that is a much higher weight difference than you would expect from changing tyre width
would result only in going 1 second slower. As such even if it saves 1 second from the weight saving and 1 second is saved from the lower rolling resistance, overall it is still 5 seconds slower, due to the aerodynamic cost.
The above is only supposed to be illustrative of my prior point, not exhaustive or universal, it is for my setup only! Ultimately, the effect of changes will vary, tyre to tyre, wheel to wheel, bike to bike and rider to rider, if you care enough, you need to evaluate the effect on your own setup.
Changing the front tyre from 22mm to 19mm exhibited a Crr increase of 2.48% on average. The CdA decrease for the same change was 4.22% on average. Figures quoted are for the entire system, i.e. me, on a time trial bike, in full race kit, the rear wheel has a 22mm tub on it. Of course being a relatively low drag system (full aero kit) the impact of the tyre on total CdA would have a higher relative impact than it would on a higher drag system.
Using these as inputs to model a time trial on the Cheshire J2/1 time trial course (keeping the course, weather inputs and power inputs, as well as the rider weight etc constant and only varying the Crr and Cda values):
The Crr increase would result in going 1 second slower
The CdA decrease would result in going 7 seconds faster.
So the net result is that the change provides a net gain of 6 seconds.
For @I like Skol 's interest, adding 1lb to the bike (I can't account for where exactly it is added so I have just added an exaggerated chunk of weight), that is a much higher weight difference than you would expect from changing tyre width
would result only in going 1 second slower. As such even if it saves 1 second from the weight saving and 1 second is saved from the lower rolling resistance, overall it is still 5 seconds slower, due to the aerodynamic cost.The above is only supposed to be illustrative of my prior point, not exhaustive or universal, it is for my setup only! Ultimately, the effect of changes will vary, tyre to tyre, wheel to wheel, bike to bike and rider to rider, if you care enough, you need to evaluate the effect on your own setup.
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