A question that is frequently asked is “should I ride my road bike or TT at Race X?”. A few weeks ago, I used some math to address course difficulty, and I will use a similar approach to address this question. The idea here is to quantify roughly how much time difference you could expect between the two bikes, in order to help make a decision. Again, this analysis is not meant to give an exact answer, but to put the issue into perspective. I.E. Is the difference likely to be seconds, minutes, or hours?
(The post on course difficulty can be found here → http://forum.slowtwitch.com/cgi-bin/gforum.cgi?post=2815444;search_string=compare%20bike%20course%20b;#2815444
The approach I will use is to model the time is takes to complete an IM bike course, given some inputs like weight, frontal area, climbing, etc. The course is modeled as an out and back, where the course is a constant uphill grade on the way out, and (the same) constant downhill grade on the return.
Editorial note: It is not entirely clear to me why this question comes up so frequently, so I will take a few guesses and offer my opinion.
- Your TT bike is not as comfortable as your road bike, so if the time differences are small, you’d prefer to ride your road bike. My answer to this is that you need to make your TT bike comfortable given the distance you are to ride. The TT bike was NOT invented as some form of torture device. If you need to raise the bars, shorten the reach, move the seat back, then do so; make yourself comfortable.
- Your road bike is lighter than your TT bike, and the course has a huge amount of climbing. You think the weight save will be significant. My answer: See the results later; it is unlikely that the road bike is faster due to weight savings.
- The course you are to ride has signficant descending of a technical nature, and you are more confident descending on your road bike. My answer: It is possible that your preceived descending difference is due to your TT bike tires being narrower and/or higher pressure. Try your road bike tires, at the same pressure, on your TT bike. If that does not solve the problem, you likely have too much weight too far forward; consider a position change on your TT bike for that course only.
- Your alternate bike is a travel bike, like a Ritchey Breakaway. The bike is much less likely to be damaged during travel, replacement cost is substantially less, and the bike flies for free on most airlines. My answer: This is the only good reason I can think of to ask the question; and is the reason I put this together, as I frequently race my Breakaway cross.
I will now answer the question, using inputs that apply to me. You can either use these as a rough guide, or use a tools like the analyticcycling.com to dial in a better answer for yourself. For the first case, I am going to assume that me, on my road bike, results in 15% more frontal area than me on my TT bike. My own testing indicates that this is in the ball park when the following are assumed: similar wheels on both bikes, road helmet worn while riding the road bike, aero helment worn while riding TT bike, road bike position was optimized within the constraints of the road bike. This last point is important. When I race my road bike as a TT bike, I move the seat forward and up, remove all headset spacers, and even use a negative angle stem to get the bars down a little lower. An unchanged road bike setup will be considered later.
TT bike - left three columns, at three different grades
Road bike - Right three columns, at same three different grades
Difference in cases - The “road bike” assumes 15% more frontal area; 0.575 m^2 for the road bike and 0.5 m^2 for the TT bike.
http://i41.tinypic.com/2vacu2d.jpg
PowerInput (Watts) 180.0 180.0 180.0 180.0 180.0 180.0
Distance_(Miles) 112.0 112.0 112.0 112.0 112.0 112.0
TotalWeight_(Pounds) 175 175 175 175 175 175
grade 0 0.015 0.03 0 0.015 0.03
Crr 0.004 0.004 0.004 0.004 0.004 0.004
Temperature_(F) 70 70 70 70 70 70
RelativeHumidity 0.27 0.27 0.27 0.27 0.27 0.27
BarometricPressure(inHg) 30.2 30.2 30.2 30.2 30.2 30.2
StationElevation_(ft) 2000 2000 2000 2000 2000 2000
WindSpeed_(MPH) 0 0 0 0 0 0
A_(area,m^2) 0.5 0.5 0.5 0.575 0.575 0.575
TimeOut 02:27:42 03:14:02 04:20:08 02:34:17 03:19:54 04:24:36
TimeBack 02:27:42 01:58:18 01:39:27 02:34:17 02:04:43 01:45:28
TotalTime 04:55:25 05:12:20 05:59:34 05:08:34 05:24:37 06:10:05
VelocityAvg(MPH) 22.7 21.5 18.7 21.8 20.7 18.2
Total_climbing(ft) 0 4435.2 8870.4 0 4435.2 8870.4
Time_difference - - - 00:13:09 00:12:17 00:10:30
Result - Depending on the average grade, you’ll be 10.5 - 13 minutes slower on your road bike, assuming a 15% frontal area difference.
I assumed a 15% increase in frontal area for the road bike versus TT bike in the above. Specialized wrote a white paper comparing a road setup (road bike, road helment, 32 spoke box section wheels, NO aero bar, rider in drops but with little to no elbow bend) to a TT bike setup (deep wheels, aero helmet, much flatter torso position). In that analysis, the TT setup reduced drag by about 30% at 40KPH. So I re-ran the above, but with a 30% increase in frontal area. Those numbers are below.
http://i40.tinypic.com/2ag0kno.jpg
PowerInput(Watts) 180.0 180.0 180.0 180.0 180.0 180.0
Distance(Miles) 112.0 112.0 112.0 112.0 112.0 112.0
TotalWeight(Pounds) 175 175 175 175 175 175
grade 0 0.015 0.03 0 0.015 0.03
Crr 0.004 0.004 0.004 0.004 0.004 0.004
Temperature(F) 70 70 70 70 70 70
RelativeHumidity 0.27 0.27 0.27 0.27 0.27 0.27
BarometricPressure(inHg)30.2 30.2 30.2 30.2 30.2 30.2
StationElevation(ft) 2000 2000 2000 2000 2000 2000
WindSpeed(MPH) 0 0 0 0 0 0
A(area,m^2) 0.5 0.5 0.5 0.65 0.65 0.65
TimeOut 02:27:42 03:14:02 04:20:08 02:40:16 03:25:23 04:28:49
TimeBack 02:27:42 01:58:18 01:39:27 02:40:16 02:10:41 01:51:02
TotalTime 04:55:25 05:12:20 05:59:34 05:20:33 05:36:04 06:19:51
VelocityAvg(MPH) 22.7 21.5 18.7 21.0 20.0 17.7
TotalClimbing(ft) 0 4435.2 8870.4 0 4435.2 8870.4
TimeDifference - - - 00:25:08 00:23:44 00:20:17
Result - Depending on the average grade, you’ll be 20 - 25 minutes slower on your road bike, assuming a 30% frontal area difference.
Last, I’ll consider the case for a very light road bike. Lets say you have a super road bike, and it weights 10 pounds less than your TT bike. (This is really unrealistic, but I am emphasizing the number just to show how little affect weight has in the results. Lets say you also only suffer a 15% frontal area increase on this road bike. Those numbers are below.
http://i44.tinypic.com/vfectv.jpg
PowerInput(Watts) 180.0 180.0 180.0 180.0 180.0 180.0
Distance(Miles) 112.0 112.0 112.0 112.0 112.0 112.0
TotalWeight(Pounds) 175 175 175 165 165 165
grade 0 0.015 0.03 0 0.015 0.03
Crr 0.004 0.004 0.004 0.004 0.004 0.004
Temperature(F) 70 70 70 70 70 70
RelativeHumidity 0.27 0.27 0.27 0.27 0.27 0.27
BarometricPressure(inHg)30.2 30.2 30.2 30.2 30.2 30.2
StationElevation(ft) 2000 2000 2000 2000 2000 2000
WindSpeed(MPH) 0 0 0 0 0 0
A(area,m^2) 0.5 0.5 0.5 0.575 0.575 0.575
VelocityAndTimeOut 02:27:42 03:14:02 04:20:08 02:33:43 03:15:56 04:15:12
VelocityAndTimeBack 02:27:42 01:58:18 01:39:27 02:33:43 02:05:44 01:47:04
TotalTime 04:55:25 05:12:20 05:59:34 05:07:26 05:21:41 06:02:16
VelocityAvg(MPH) 22.7 21.5 18.7 21.9 20.9 18.5
TotalClimbing(ft) 0 4435.2 8870.4 0 4435.2 8870.4
TimeDifference - - - 00:12:01 00:09:21 00:02:42
Result - Even on a course with A LOT of climbing (8870 ft.) the road bike is still slower by almost 3 minutes; and that is assuming a huge 10 pound weight difference, along with a drag difference that may not be achievable. A more realistic weight difference of 5 pounds results in a time difference of 6.5 minutes, in favor of the TT bike, at 3% grade. The road bike and TT bike will be about equal when the road bike weighs 5 pounds less, results in a 15% frontal area increase, on a course with about 5% grade (14,700 feet of climbing!).
Conclusion - If you have a TT bike, use it. If you are on a course with more than about 5% average grade, a lighter road bike with good position may be faster.
