I did twice when I first started tri's. I raced for 10 years on the track and road and always trained in the winter with a fixed gear. Around here, in PA it is not too bad to ride a fixed gear, but for TT's and such, I would only ride a fixed gear on the flattest courses with the calmest weather. My PR's for the 40km, 20km and 10 mile were all set on a fixed gear on the road. My kilo and pursuit training set me up for the speed and intensity very nicely (might be a good idea to do a couple of TT efforts followed by a run to get a feel for it). For the tri, it was dead flat, 20km, with no wind. I rode a 50/15 with a disc and trispoke, (both 700c) I posted a great bike split, something like a 28:20, then got off and ran a great 5km (for me, like 19:00). I felt great because of the high cadence and the fact that the fixed gear seems to carry momentum through the pedal stroke smoother. The cornering is probably the only thing that could give you problems, not to mention a healthy dose of practice mounting and dismounting (the cyclocross dismount DOES NOT work unless you want to look like a kangaroo). I rode look pedals, and my usual track cranks (165's) and didn't scrape a pedal once, but I don't think the corners were very challenging. Preview the course, and give it a shot. I did it again on a different course, but the weather didn't cooperate, and I rode into a 20mph headwind for the second half of the ride. Crappy story, undergeared for the first half, overgeared for the second, dead meat on the run. I think if you can keep your cadence on the bike somewhere near your run cadence, you'll rock!

BTW, I never went that fast for 20km in a triathlon since. I can do it if I am doing just a tt, but that day was too perfect to happen to me again. Take a chance, you never know, the day may be perfect for you! You've inspired me to go out to the shed and pull down the ol' Schwinn Madison. If I can only find those scott 100k aerobars...

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"Maybe you should just run faster..." TM

Funny you should ask: see: http://www.bicyclesports.com/forum/ShowPost.aspx?PostID=891

I would go with at least a single gear with a freewheel because the weight gain is negligable ona flat course. You'll be able to (dis)mount and corner easier. Plus (heaven forbid) you take a few seconds to stretch out your legs during the ride.

In summary, weight gain is a non-issue with flat courses. Although fixed gears can improve efficiency, remember there ARE some (not so fun) consequenses (mentioned above) to riding a fixed gear.

IMHO I'd take the fixed gear out for some training rides instead.

$0.02

-gc

I am currently designing a "one design" bike for tri racing. The bike will be avail in several sizes, maybe two and you will enter the race in the one design class. The bike will not be super light or have more than one gear. I plan on racing a prototype for my next two races. I will let you know how it goes.

Shawn

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"Nothing in life ever just happens. Calculated progression insures your strength."
Shawn Spencer
Bike Cycles
Wilmington, NC
http://www.bikecycleshop.com