TT Bike vs Road Bike

[JaRok2300]

I've seen the question come up a few times and always been curious myself, so having got a power meter recently, I decided to conduct my own very basic little experiment.
I created a segment in Strava on a local lap I regularly use. It's 2km long and virtually flat (8m drop over the 2km). I've only had the power meter since the beginning of May so have about 15 data points for my VN Euros. I put the wheel in my Litespeed Blade last night and did 2 laps for comparison.
I plotted them on a graph this morning, expecting a complete mess but actually the VN points formed a fairly reasonable line which was almost parallel to the 2 points for the Blade.
The difference was approx. 5kph across the range with all points (barring one outlier) between 160-240W and 28-37kph.
What this says about my aero prowess is another story altogether, I am the proverbial brick sh*t house but I'll keep adding points as I get more to see where it leads.

[JaRok2300]

[explorerJC]

you need to do the ride both ways
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[JaRok2300]

Thanks EJC
If I was trying to arrive at a Cda value or similar I realise I would have to take a lot more steps to remove variables including reversing the direction, wind, clothing, position on road bike etc.
As a rough like for like comparison does this hold up or am I missing something fundamental?

[explorerJC]

do the ride both ways on both bikes; same power, record HR and split

but there is much to take into account such as specific training, fatigue etc...
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[JaRok2300]

Agreed, there are a lot more factors in the real world and if the TT bike position means I'm working harder to put out the same power then it's a false benefit in the long run.

I was just trying to arrive at a like for like comparison such as:
If I ride that stretch of road at 200W I do 31kph on a road bike compared to 36kph on the TT, all other things being equal. (Which I realise they aren't)

[explorerJC]

you need to train on the TT bike...
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[JaRok2300]

I need to train.

[Wheezy]

Nice work.

To add to EJ comments, I believe you can also factor in air pressure data, if you want to get into that much detail. Will see if I can find where / how to do it. I take it you were using all the same clothing for both tests? Probably in the long run it would be more benefit to use the power meter to test clothing / helmet / wheel / position options on the tt bike rather than the difference between two bikes?

my road bike is a Euros. I love it but it's got the most enormous round down tube which is an aerodynamic monstrosity. means I feel the benefit a bit more on my tt bike!
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[JaRok2300]

To be honest, I'm such a lump, I think any drag created by the bike itself is relatively insignificant.
I think the vast majority of any gain (whatever that might be in a proper scientific test) will be down to my position.
Clothing would be very similar, I don't train in tri suit, aero helmet etc.

[Andy916]

Bear in mind that your ability to produce power on the TT (or any) bike will change over a period of adaptation. Would highly recomment Adam Topham's book; there's a case study in there of going ever-higher on the saddle to flatten the back, then dropping both front and rear which turned out to be more aero. At first power was compromised by the low saddle, only to return a couple of weeks later.
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[chickenboy]

[JaRok2300]

Based on the comments so far I went back and checked my HR for each of the segments above.
While the data is less clear cut than the speed/power graph, both TT segments were near the bottom of the scale so there's nothing to suggest I'm working harder for any given power output on the TT bike, in fact just the opposite.
To clarify the segments came form all kinds of rides, only the 2 highest Van points were all out efforts, hence lots of reasons why the HR data would show little pattern.

[Tin Pot]

Nice work

It's good to see people doing their own testing
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[JaRok2300]