On Saturday I conducted a brief experiment to see if I could measure a difference between my old bike ( Kestrel 200 EMS with Syntace C2’s) and my new bike ( Cervelo P3 SL). I did a roll down on a small local hill over a distance of ~ 200 metres, measuring time and max speed. I had my son balance me at the start (on the downslope) for a slightly wobbly zero mph start. I freely admit that my testing protocol leaves a lot to be desired. I attempted to control as many aspects as a I could, trying to keep the same body position on each trial (harder than you would think) randomizing the order of the trials (only 5 on each bike). But I had to time it myself holding a watch in my hand since I couldn’t impose on another person to help out with my silly test. I also made the assumption that the 2 speedometers were accurate (same model, same size wheel and tire, 120 psi) but could do a calibration check.
Now, I would like to conduct a similar test to see if I could measure a difference between 2 helmet styles, aero and standard vented. How would you design this study? What would you do differently? I think a simple test like this, if accurate and repeatable, might be handy in evaluating helmets, wheels and body position without going to a wind tunnel.
BTW the P3 was faster, as tested, averaging 26.58 max speed and 24.97 sec compared to the Kestrel, 25.64 mph and 25.25 sec.
Dude, I admire your ingenuity.
Nothing like doing it Macgyver style 
The most significant problem I see with your study design is the influence of weight of the bike on how fast it will accelerate down the hill.
I think short of the wind tunnel, the best method is using a power meter, that way you eliminate grade, wind & rolling resistance variables.
Short of that, I think you could get a reasonable estimation if you pick the least windy part of the day (measure wind to make sure it’s constant on two trials), same flat course (or velodrome), same bike, same resting HR (with at least 3 days of rest beween trials). Try and hold a constant speed over a fairly short course (ideally race pace) and monitor heart rate to ascertain difference in physiological output between the various positions/equipment choices.
Hope this helps,
Alan
Same wheels?
Victor
The weight of the bikes will influence your results. However, assuming that they are pretty close to the same weight (and, obviously weight won’t be an issue for your other testing, if you use the same bike), I have a couple of suggestions…
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Find a longer hill, or maybe even a longer hill followed by a flat section or rollers. 200m (or 25s) is not a very long distance (time) for measuring very small differences in aerodynamics (which is what you’re looking to measure).
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Instead of starting from 0mph, try to hit a certain speed at a certain point at the top of the hill and tuck (stop pedaling) from that point. You’re likely introducing alot of error from your wobbly start. When you’re only measuring 1/4s difference, this is likely overwhelming your results.
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Coming from a totally different perspective… Find a long flat stretch with a smooth surface on a windless day (velodrome?) for the test. Again, certain speed by a certain point and then tuck/coast from there. Whichever one coasts further is more efficient. Weight & line are issues with this method, however.
*NOTE: When I say hit a certain speed by a certain point, I mean to actual hold the speed constant prior to that point so that you are not ac/decelerating when you start coasting.
Steve
I would like to conduct a similar test to see if I could measure a difference between 2 helmet styles, aero and standard vented.
My advice would be to not waste your time.
Since I was testing to quantify the overall difference between the two bikes, not just the frames, I did not control for the wheels. I just made sure the tire pressure was the same. There was no perceptible wind but the road was still wet from overnight rain. I do not have and am unlikely to buy a powermeter. That is why I wanted to do a no pedalling rolldown. I think the inconsistent (wobbly) starts probably affected the time measurement but not the max speed. A rolling start at a constant speed might work better but be difficult to execute. I am curious why Andrew Coggan says don’t bother with a helmet test. Is the protocol not stringent enough to measure a difference? If not, could it be modified? Or, is it a waste of time because an aero helmet would not be faster? Just wondering. Unless I’m way off base here I think a simple do-it-yourself type test like this could be useful and kinda fun.
I am curious why Andrew Coggan says don’t bother with a helmet test.
Because 1) it would require an inordinate number of trials/amount of time to be able to confidently detect a difference of the magnitude that is likely to exist, and 2) there’s little, if any, reason to attempt to do so in the first place (since you already own the aero helmet and there’s really no reason not to use it regardless of what such testing might show).
i agree with AC. plus the differences between your data points (bikes) is minimal and unlikely to be statistically signicant even with a larger sample size. the power meter is your best choice, pick a stretch of road and ride each for the same average watts, then compare elapsed time.
Is your position the same on the bikes? I recall that the 200EMS is a slack angle road geometry with a long headtube.
the power meter is your best choice, pick a stretch of road and ride each for the same average watts, then compare elapsed time.
I’ve got a powermeter and access to a nearly perfect venue for testing, but I still wouldn’t waste my time trying to detect the difference between a regular helmet and an aero helmet (assuming that I already owned both).
i agree, i wouldn’t waste my time comparing helmets. i was thinking more of bike comparisons and in particular position analysis as described in your book. makes you wonder how much time is wasted with insignificant “tests” that would otherwise be more useful for actual training…
Yes, the bikes are quite different, though I have the seat all the way forward on the Kestrel. But what I was intending to test was the difference between the bikes, as they were set up, to help justify to myself buying a new bike (which I did). I wasn’t trying to measure the difference in aerodynamics of the frameset but the whole bike, rider, geometry, wheels etc. The question about the helmet was more theoretical as I do not own an aero helmet, but could convince myself to buy one if the difference was measurable. The same with wheels. Doing the short test was kind of fun and I did have to ride back up the hill for each trial.
I am curious why Andrew Coggan says don’t bother with a helmet test.
Because 1) it would require an inordinate number of trials/amount of time to be able to confidently detect a difference of the magnitude that is likely to exist, and 2) there’s little, if any, reason to attempt to do so in the first place (since you already own the aero helmet and there’s really no reason not to use it regardless of what such testing might show).
I have to disagree with 1). I’ve done a series of many, many rides on the same courses with, and without, aero helmets. If the power is even close, the aero helmet is faster - much faster.
In the wind tunnel, the difference between the LG Rocket and and LG Chrono was .017 of CdA – say around 15 watts at TT speeds. Hell, you don’t even need a powermeter to detect that. Just get a regular bike computer and go by perceived exertion.
– jens
In the wind tunnel, the difference between the LG Rocket and and LG Chrono was .017 of CdA – say around 15 watts at TT speeds. Hell, you don’t even need a powermeter to detect that. Just get a regular bike computer and go by perceived exertion.
It was the “obviousness” of the benefits that stopped me from bothering with formal tests. The Rocket was clearly faster than my regular helmet, so I didn’t try to quantify it exactly. I just started using it and didn’t look back. Plus, I had your wind tunnel results to refer to.
I have to disagree with 1). I’ve done a series of many, many rides on the same courses with, and without, aero helmets. If the power is even close, the aero helmet is faster - much faster.
But the OP doesn’t have a powermeter, but instead is relying on coast down testing on a relatively short, shallow hill while hand-timing himself.
Jay will fit you with motion capture reflectors, plot your position with multiple IR cameras, and maximize your position in terms of power and drag.
So is all this done in a wind tunnel?
why not rely on the data generated by the companies and folks who regularly do wind tunnel testing (ie. Cobb)? data from various groups has suggested that aero helmets are measurably “faster” in a wind tunnel compared to regular helmets. so why not justify the purchase of the aero helmet on these data vs. the insignificant data you may generate. save your precious time and get out and train. building a bigger engine will make you faster than the purchase of any helmet. you say you won’t buy a powermeter, but IMO this is the best purchase you can make. you will train more effectively, pace yourself better on the bike leg, and subsequently run better…all resulting in a better overall result (much more than could be achieved by the purchase of a new helmet).
the aero helmet is faster - much faster
Really? I’m curious what “much faster” is because everything I’ve seen posted here by Cobb et al seems to point to it being 7-10 seconds in a 40k tt… and I don’t know that I’d call that “much faster” 
This testing is a great idea - dont listen to all the naysayers (watch what happens if you try telling them the world is round, or your seat angle is a bit sharp…) I’m planning to do some similar tests with a few small differences. Long down hill section to roll out (about 4km). There will be two of us, two bikes, two wheelsets and three different helmets (Limar Crono, Rudy Proj and a standard Limar) and we plan on doing the roll out many, many times. Yes I know that this is not a perfect lab environment (much like a race course?) and that wind direction etc will impact but, if on a still day, with mutiple rollouts we should be able to get some meaningful numbers (ie the aero wheels, helmet and bike should with the same rider be just a bit faster than the non aero combo). The expensive aero stuff (wheels, frames, helmets) should provide some measured advantage - otherwise why buy it?
Oh and the other thing - its fun.
This testing is a great idea - dont listen to all the naysayers (watch what happens if you try telling them the world is round(…)
Actually the naysayers are on the “the world is round” side, people like you are on “the world is flat and at the end of the sea you’ll just fall over” side.
But I don’t want to spoil anyone’s fun here, I’m all for going downhill at great speeds, even if it’s in the name of pseudo-science.