rosshm wrote:
I nerded out a bit on the physics here.
According to the internet the coefficient of rolling resistance (Crr) varies from ~0.004-0.007 for typical tires (lower is better). The power you have to produce to overcome rolling resistance is:
Proll = Crr*m*g*v
m = mass of rider + bike
g = gravitational acceleration
v = speed of bike on road
If m = 80 kg then
"good" tires save you a little over 1 W/mph. So depending on the hill, probably about the same power savings as shaved legs on a flat TT! Over an Everest-length effort I think you'd be very happy to not have to put out those extra 10-15 W on the last few climbs.
Except we're talking "good tires" vs "great tires." A 25c GP4000 with butyl tubes has .crr of ~.004 @ 90psi. A same-size GP5000 with a latex tube has a .crr of ~.003. So more like ~0.4W/mph difference.
You can nerd out scenarios to your heart's content on the
Gribble Cycling Power Model. Using all the default values except .crr and grade, it showed the difference in speed between the two tires on a 7% grade at 200 watts was .09mph (7.19mph vs 7.10mph). If you prefer a watts:watts comparison, it would take ~202.7W on the GP4000's to hold the same speed as the GP5000's @ 200W. On a less steep grade of 3.5%, it would take 204.4 watts on the GP4000's to maintain the same speed (12.03mph) as the Gp5000's @ 200 watts.
OP, I'm going to assume (maybe incorrectly) that on an "Everesting" attempt, you'll be soft pedaling or coasting on the descents to recover. So the higher .crr will cost you a bit of time on the descents where you're freewheeling, but won't have any metabolic cost. How much time is more difficult to quantify. Descents are harder to model as .cdA is a significant factor, and that can vary considerably from person to person, bike to bike, and position to position. Using the default values other than .crr, the model predicts a difference in terminal velocity on a 7% descent of less than 1%; 37.11mph vs 36.84mph. That percentage doesn't vary a lot even if you assume a pretty slick (for a road bike) 3.0CdA.
So back to the question at hand. Is it worth it? I'm going to assume that your effort's going to be the same on either tire. So it's not a matter of saving watts, but saving time. Best I can figure, changing tires will save you roughly
5-7 minutes on an ~8 hour ride (1-1.5% faster). Maybe 3-5 minutes of that time would be under power; the rest lost while coasting descents. If this were a race, the 1-1.5% would undoubtedly be worth the effort. For a personal solo effort? If it were simply a matter of swapping wheels, I'd do it just because. If I had to change a cassette as well? It's a toss up. Dismounting and mounting a set of 4000's once and wresting a set of 5000's with latex tubes off and on a set of rims twice?* I'd much rather pedal another 5 minutes.
*(once to go on to the road bike, and then again to go back to the TT bike)
"They're made of latex, not nitroglycerin"