Tire geekery and the back of an envelope
I've been trying to figure out about how much difference it makes to use good tires and tubes, and I've done a back of the envelope calculation. Curious to see what you all think of my admittedly very broad analysis. As per your intuition I'm sure, it is more helpful for less powerful riders, but the difference between "average" or even "good" tires/tubes and "the best" appears to be real and meausrable. (Less powerful riders get a bigger correction because a few watts is a larger percentage of their total! Sorry, big guys!)
I looked up the "watts consumed" column in the bike tech review literature that we've all seen before. As my "best" standard, I chose the Vittoria Evo Corso tires that are widely recommended around here. As my "good" standard, I chose Michelin Pro Race 3s, because that's what I've been riding. :-)
From those data:
- Vittorias plus latex tubes consume 12.3 W in rolling resistance each.
- 23c Micheline Pro Race 3s with latex tubes consumbe 15.1 W each
- The use of butyl tubes instead of latex adds about 5 W total
- Going from Pro Race 3 quality tires with butyl tubes to Vittorias with latex thus saves you ~10 W in RR. The difference would be larger of course, if you were not using decent race tires to begin with..
For the perspective of improving speed, the "best" case is that you are going very slow, so wind resistance doesn't matter and there is no gravity correction. In that (very unrealistic) case, the correction would be linear. Riding into the wind, we can make tha assumption that your extra watts are "diverted" into taking on the wind, and the correction goes about as the cube root of power. However, most people observe that overall, their speed goes roughly as the square root of power, which makes sense...it's a gross approximation of linear (RR and hill), square (applied wind) and cubed (self-induced wind) terms.
Ok, so now to figure improvement of use of butyl tubes, the best tires, or both, I round the improvements over average tires with butyl tubes to 5 W each, and therefore a total of 10 W if you do both.
Next, I assume four different target wattages, from low (120 W) to fairly high (240 W) in increments of 40 W, and comparing performance to n+5 or N+10 W. I assume that relative velocities are proportional to {(n+5)/n]^0.5 or {(n+10)/n]^0.5. Finally, I applied this factor to a race time of 6 hours (360 minutes). If you take longer, the correction would be proportionately longer, and shorter ---> less correction in direct proportion.
So here is the table:
120 W 160 W 200 W 240 W
+10 W 1.041 1.031 1.025 1.021 velocity factor for improving tire AND tubes, i.e., 10 W
? (min) 14.7 11.1 9.0 7.6 minutes saved for 6 hour ride
+5 W 1.021 1.016 1.012 1.010 velocity factor for improving only tire OR tubes, i.e., 5 W
? (min) 7.6 5.8 4.3 3.6 minutes saved for 6 hour ride
OK, so what I take from this is that the numbers are really pretty large, considering these are virtually free improvements...especially for our smaller peeps. After all, both tubes and tires are commodity items and you have to have both anyway. So if you pay a few bucks more for great tires or tubes, it's really only a few bucks!
If these data are anywhere near right, I'm looking at a ~5 minute improvement, will I would gladly take, over what I've been riding before....almost for free.
Whaddya think?
PS I didn't take into account anything about the aerodynamics of your tire choice here because I don't know how big those factors are. I assumed that we are comparing apples to apples on that.
Comments
http://www.biketechreview.com/tires_old/images/AFM_tire_testing_rev9.pdf
The Vittoria Open Corsa w/ latex is listed with a crr of .00250. I couldn't find the exact match of a Pro 3 with butyl, but a Michelin Pro 2 Light with Bontrager Standard Butyl is listed at .00322.
For a 85 kg bike/rider combo on level ground with a cDa of .25 at sea level, it takes 174.1 watts to go 22.37 mph (10 m/s) with the Vittoria Open Corsa with latex (.00250). The same parameters with a crr .00322 is 180.1 watts, or a difference of 6 watts. This is a difference of 213 seconds over 112 miles.
I'm thinking the best tire out there is a Bontrager R4 Aero. It's got a crr within .00001 of the best clincher combo, it's very light (not that that matters at all on the flats), and it's arguably more aero by filling the gap between the tire and rim. I've got that up front with a Specialized Mondo Open Tubular in back, which is a bit heavier, a little less aero, probably more durable, and 1 mm wider.
The Continental GP 4000 is even worse. Even with latex tubes, the crr is .00384. This takes 185.3 watts to go 22.37 mph (10 m/s). If you use those 185.3 watts on the Vittoria Open Corsa w/ latex, you'll go 10.23 m/s, which saves 405 seconds over 112 miles.
Continental GP 4000
@Mike - I do the same thing... train on standard wheels with butyl tubes and gatorskins for most of the year then put on my Zipp 808s with Vittoria's and latex tubes the week before the race. I am instantly noticably faster on the bike and it is a great confidence boast right before a race.
I was using the same data sheet as you.... the Mich Pro Race 3 (23) with a latex tube are on the second page. The "watts consumed" column that they generate, is almost undoubtedly generated using either (literally) analytic cycling or the same equations.
In any case... you're getting a little bit shy of 4 minutes gained for a rider at 174 W, and interpolating my table, I would have quoted 5 minutes, so we're in the same ballpark, even if your number is a bit smaller. Very difficult to do the real control experiment! :-)
As they say: "Consider the spherical cow..."