Actually, my point was that it seemed unlikely that a TT frame on a 5 mile, 5% grade TT would yield a 40" difference.
By your own math, a 10w difference would only yield a 8" difference for a 5 mile TT. So in order to save 40", you would need a frame that saves you 50w.
Correct me if I am wrong, but the time savings of 10w = 1 sec / KM saved is also based on a 30 mph test (or maybe 40 kph?) The speeds for this uphill TT would be substantially less, correct? So doesn’t your time saved / KM also drop?
Actually, my point was that it seemed unlikely that a TT frame on a 5 mile, 5% grade TT would yield a 40" difference.
By your own math, a 10w difference would only yield a 8" difference for a 5 mile TT. So in order to save 40", you would need a frame that saves you 50w.
Correct me if I am wrong, but the time savings of 10w = 1 sec / KM saved is also based on a 30 mph test (or maybe 40 kph?) The speeds for this uphill TT would be substantially less, correct? So doesn’t your time saved / KM also drop?
Well…that’s where we start getting into the true differences between people’s positions on road bikes and TT bikes…heck, I’ve seen my drag on my TT bike go up by the equivalent of ~20W just if my bar height is too high by 5mm.
The ROT is based on typical TT speeds, so yeah, it would be de-rated for this sort of TT…which just goes to show the best thing to do for these decisions is to model the whole thing up. It’s not that hard
And with the speeds the OP was talking about (12-13 mph), I doubt many people are holding an aero position.
Anyone riding a TT who isn’t holding an aero position at 12-13 mph is giving away time to the competition. In a short TT, the only reason to be out of aero position at those speeds is if you are one of those folks who can’t generate power in the TT position. In a longer TT, 12-13 mph may be your opportunity to stretch and relax a bit - but you’re still giving up time.
Eb - ““My TT position has pretty low drag (CdA .21-.22), but my road bike position is fairly upright””
That’s me as well (not saying anywhere near your CdA, just in comparison to road bike posistion) , I have a pretty good TT posisition I can hold, but roadbike is fairly upright, that’s why I mentioned in orginal post that there is, more often than not, a decent tail wind.
On my road bike I would benefit more from that tailwind as will be presenting more surface area to get blown up the hill?
Ok - so I know with my weight I’m gonna be well towards the back of the field and much much slower than the top guys but was genuinely interested in getting the best performance for myself so thanks for all the replies.
Being more upright would be a benefit with a tail wind if and only if the tail wind is faster on average than your average speed. If the tail wind is slower than you, the TT bike is aerodynamically advantageous, although by less so than with no wind or a headwind.
And with the speeds the OP was talking about (12-13 mph), I doubt many people are holding an aero position.
Anyone riding a TT who isn’t holding an aero position at 12-13 mph is giving away time to the competition. In a short TT, the only reason to be out of aero position at those speeds is if you are one of those folks who can’t generate power in the TT position. In a longer TT, 12-13 mph may be your opportunity to stretch and relax a bit - but you’re still giving up time.
We can argue what people should do at what speeds all day, but the reality is what I described…most people are out of aero well before 12-13 mph.
And with the speeds the OP was talking about (12-13 mph), I doubt many people are holding an aero position.
Anyone riding a TT who isn’t holding an aero position at 12-13 mph is giving away time to the competition. In a short TT, the only reason to be out of aero position at those speeds is if you are one of those folks who can’t generate power in the TT position. In a longer TT, 12-13 mph may be your opportunity to stretch and relax a bit - but you’re still giving up time.
If you extrapolate down from tests done at 30 mph in a wind tunnel possibly, but on the road at those speeds, are you sure?
You, of all people ?! I really hope you’re pulling my leg! Because if you truly don’t get it, then you are leaving time on the table, and you of all people should not be leaving time on the table. If you were in my age group I’d tell you to go ahead and ride your road bike! ;^>
The answers are “Yes”, “No”, and “It depends”. I’m only being the tiniest bit flippant.
Enter the following: Note: everything is identical except CdA (“Frontal Area” * “Coefficient Wind Drag”). You have to be a little careful with the calculator because it apparently rounds those two inputs to one significant figure. I used values of .24 (0.60.4) for a TT bike and .28 (0.70.4) for road bike. In most cases the difference in CdA between road bike and TT bike would be greater than .04, so this is a conservative estimate.
Data Parameters Parameter Standard Rider Test Rider
Frontal Area 0.6 0.7 m2
Coefficient Wind Drag 0.4 0.4 dimensionless
Air Density 1.226 1.226 kg/m3
Weight Rider 65.0 65.0 kg
Weight Bike 8.0 8.0 kg
Front Wheel
Weight 1.300 1.300 kg
Ic 0.0900 0.0900 kg m/s2
Cx0 0.0500 0.0500 Dimensionless
Radius 0.337 0.337 m
Rear Wheel
Weight 1.800 1.800 kg
Ic 0.1000 0.1000 kg m/s2
Cx0 0.0491 0.0491 Dimensionless
Radius 0.337 0.337 m
Rear Shelter 25. 25. %
Power 300. 300. w
Coefficient of Rolling 0.004 0.004 dimensionless
Grade 0.05 0.05 decimal
Initial Speed 0.0 0.0 m/s
(sorry about the format)
The results? “At the end of 8000m Standard Rider is ahead by 19.91s and 124.97 m.”
Go ahead and run it yourself. That’s a 5 mile, 5% grade climb. Speed ends up being a bit over 6 m/s (22 km/hr).
That’s pretty similar to Bear Mtn near where I live.
Tri bike all day, every day, and twice on Sundays. There is a definite aero benefit, even at those speeds.
(like Jackmott, I don’t lose any watts on my TT bike)
