Sunday, March 08, 2009

Old Skool

Time trialling is a popular part of the sport of cycle racing. The "race of truth" is deceptively simple. Ride a set course by yourself and try to finish it in the shortest time possible. Fastest time wins. Easy.

As always with this sport, there are a number of elements that go into riding the fastest time you can. But in essence, they come down to the three P's:

1. Power to the pedals
2. Piercing the wind
3. Pacing the course

If you don't address all three "P's", you are riding sub-optimally from a "least time" perspective.

Power is all about the training you do to improve your fitness and your ability to ride harder. The power and training part of the equation is something I've written lots about on this blog.

Piercing the wind is all about the aerodynamics of your bike and especially your position on the bike. Given this is about the physical elements that resist our forward motion, I also include a few other items, like having the best tyres for the lowest rolling resistance and an efficient drivetrain so power is not "lost" between the cranks and the rear wheel.

Pacing is the skill part of the equation and is all about knowing how hard to push yourself at various times on the course. Pacing is something I've also written quite a bit about (but not here on my blog), and for which I have developed some mathematical modelling. I'll write a bit here about that one day.


Anyway, what's all this fancy pants maths got to do with "Old Skool"?

Well last weekend I rode a TT. It's our "local" TT run by the NSW branch of the Australian Time Trials Associations (ATTA) which is held on the first Sunday each month on the Calga course, about an hour's drive north of Sydney.

I decided to get back to doing some TTs for a few reasons. Firstly, since they are a race against the clock, once you have set a time then you have a benchmark for improvements. In essence you are racing only against yourself. They are an excellent fitness marker, especially when you are measuring your power output.

Secondly, I have also decided to race the paracycling national road championships (late April-early May), which includes a road TT (and a road race). So getting some similar length TTs under my belt seems like a good idea.

Finally, as I keep on with my return to the bike, I need to rediscover the type of competitive outlets that will better suit my new "body". So going back to riding many different types of events is one way to re-learn all about that.

Anyone who's ridden a TT, or even just watched one, will know that many riders use special bikes which are designed to improve the aerodynamics of the rider and bicycle. Much thought goes into this as it can result in big savings in time for the same physical effort.

However, some riders decide to ride TTs "Old Skool" style, meaning they use a standard road bike with standard wheels and helmet. No special effort is made to improve aerodynamics. That's what I did last week.

For some fun, below is the graphical summary of my ride and pacing analysis.



There's a bunch of stuff on there I won't go into today but in essence it shows the course elevation profile (it's an out and back course, with the first half mostly climbing) with elevation data obtained via two methods I won't describe here now, and two lines showing power - the yellow line is my actual power output (a 40-second rolling average) and the green line is a theoretical optimal power output for that segment of the course. By comparing the two lines you can see how well or otherwise I dosed out my effort along the way. So thats the Power and the Pacing elements.

There are also a range of other numbers shown, one of which is an estimate of my Coefficient of Aerodynamic Drag and Effective Frontal Area (CdA), which is a measure of how well (or not) I slice through the wind. Thats the "Piercing the wind" element.


OK, so how did I go?

Time: 42:55 @ 35km/h.
Average Power: 264 watts
Normalised Power: 268 watts

Not too bad given it was on standard road bike, helmet and wheels and the constantly up 'n' down Calga course. That placed me 25th out of 45 riders.

I was hoping for more like 270-280W (my first 20-min were at an average of 277W and holding myself in check) but I struggled with my prosthetic leg getting loose after about 15-20 minutes which saw 2nd half power fall somewhat and quite a bit of stump pain in the latter stages. My stump was very sore afterwards.

But that was exactly the point of the exercise - to find out the issues I face and need to fix when doing this sort of an effort. On a course like Calga, which is never flat and the power is somewhat more variable as a result, then the leg does tend to work loose a bit more quickly than say on a flatter road or on the ergobike where the effort is far steadier.

Main thing is to work on a solution for keeping my leg packed firmly in the socket during a longer hard effort. Otherwise it hurts and more importantly, you lose power. Maybe I needed to use more foam packing (which is what I'm doing at the moment). I probably didn't put enough in. Then there is the socket itself, which my rehab doc and prosthetics specialist says no longer fits properly (I have lost weight and my stump has changed its shape and volume again).

Of course I am also getting a special cycling leg made up, thanks to the generous donations and fund raising by the track cycling community here in Sydney. That is now being planned. I have the appointment to recast my stump on 2nd of April and from there we can begin the production of a new leg socket and a new general purpose leg. I can hardly wait!

So at the Calga TT next month I may be able to borrow a TT bike and see what difference it can make to my time. The analysis above shows a CdA of 0.33m^2 (square metres), which is very high for anyone doing a TT. Certainly riding on a TT bike I would expect that number to be much lower. Even at a modest 0.28m^2 (certainly readily attainable on a TT set up) I could expect to ride that same course in the same conditions (wind, temperature & air pressure) at the same power about 2-minutes faster.

We'll see.

7 comments:

Buttsy said...

I would be interested in the difference a time trial bike makes as I have just bought one and really want to get better at time trialling....havent got your fancy power stuff, I have polar but dont use the power much. love looking at your graph...interesting and you certainly have some interesting reading about power etc. I did ITT on the weekend 30km and it was all about when to power upand when to conserve....

Anonymous said...

how do you go about calculating the CDA number out of it. I don't want to know the math, because its way over my head, but given that you have power, elevation, wind, air pressure, etc, obviously it can be calculated. So did your number come from a proprietary software or what? I do a lot of TT's and would like to be able to get an average number on a given day. Thanks.

Alex Simmons said...

There are two methods I use:
The regression method and the Chung method. Chung method can be done fairly easily from simply riding a course with a known elevation profile and not much wind about.

See this post for details:
http://alex-cycle.blogspot.com/2008/03/funky-things-with-power-meter-77.html

No need for special software, it's just some home brew Excel spreadsheets which do the maths for you. I created a couple of versions for each of SRM and Powertap data files. They are located at the wattage forum on google groups.

Harry said...

I would like to hear more about how to have an efficient drivetrain and what is an excellent, good and fair drive train loss. I think I'm seeing about 10 watts of loss in my drivetrain and that's what I've heard others report.

In your article you said; "an efficient drivetrain so power is not "lost" between the cranks and the rear wheel."

Alex Simmons said...

Well Buttsy - I'll be doing some TTs in the not too distant future on a TT rig, so I'll have a reasonably good comparison to share.

A generous offer to loan me a TT bike (from a formaer client) will get me started until I can get my own rig sorted (which is a project underway).

Smith said...

How did you calculate "optimal power"?

Alex Simmons said...

Well the concept of "optimal power" is discussed in detail in a paper I published last year. The process is a little too complex to explain in a blog comment.

If you sign onto Google groups wattage forum, you can view the discussion and see the paper here:
http://groups.google.com/group/wattage/browse_frm/thread/87ba9c8913f77ebb/


As for the comment on drivetrain losses, well apart from making sure all the components in the drivetrain are working effectively and are clean and properly lubed, then the only other item to consider is the size of the chainring used, as this does affect transmission efficiency.

Typical losses on a geared road/TT race bike are in the order of 2-3%, so 10W is quite a plausible loss.