It's been my experience that the hillier the course, the greater the time difference between a CT course and the real thing. On a flat TT I've found it to be relatively close, but on a hilly course (such as the Lake Placid IM or Columbia), you're getting zero assistance from gravity on the CT on all th downhills. On the CT, you still have to put forth some effort on the downhills simply to keep moving. If that same wattage output was applied outside on an actual downhill, you'd be going MUCH faster than when on the CT. Essentially when you're racing a CT course, there are uphills and flats but really no downhills.

I would be curious to hear others' feedback as well.

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Brian Shea
http://www.PersonalBestNutrition.com
Open-Water/Masters Swimming at the Jersey Shore:
Monmouth County NJ Ocean Swim/Masters Workouts

I'm slower on the CT and would agree that the hillier the course thebigger the difference.

FWIW, I do a few 30 minute time trial type efforts on the CT each year and usually have an average speed of 23.1, 23.2, somewhere in that neighborhood. In 3 different tris last year with bike legs ranging from 16 miles to 25.8 miles, I averaged 23.8 mph on each.

Though I've never ridden the same roads outdoors that I've ridden on the CT, there are embarrassingly big differences between my performance on CT vs. real life - 6+ mph. I'm always faster on the roads, probably for some of the reasons Brian mentions - however, even on a flat road, I'm much slower on the CT. That may be part of the reason why I grow to hate riding on it over the winter. By the time spring comes I can't wait to get away from that thing.

I think that it is hard to answer this question in absolute terms. The reasons:

1. It depends on the accuracy of the course(I have ridden two versions of the IMLP course, and one is horrendously inaccurate/difficult, featuring 15% climbs where the actual grade is 5%).
2. How well you have the system calibrated to the wheel, your weight, etc, can vary.
3. Differences in weather conditions.

That said, I rode the IMLP course loop on the CT in training several times, and usually finished one loop in 2:48-3:11. When actually doing the race, my loop times were 3:08/3:21. Part of that was weather, part of it was loss of concentration on the second loop, and part of that was that I rode easier during the race than in training(what with the marathon and everything).

This year I was disappointed to find that I am faster on the CT than on the road, contrary to what I had been led to expect(conventional wisdom says you are faster in life than on the CT). Perhaps I have some calibration issues that I need to work on. Perhaps I need to minimize my drag coefficient. ;p

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...but on the other hand when you are going up the hills you are not really working against gravity in the way that you would be in real life. I have a power to weight ratio that is nowhere near good enough so I always feel that the CT makes me look better on the hills than I actually am (ok, I know it does!).

It is true though that the fact that you can't just sit and let your legs spin to recover on the down-hills does contribute to you feeling more tired - which has to hit your ability to keep up a high speed over a longer course.

Blackwater Eagleman Computrainer: 3:05
Blackwater Eagleman real life: 2:37 (and I did not blow up on the run)

Lake Placid Computrainer (1 Loop): 3:45
Lake Placid Real Life (Loop 1): 3:09
Lake Placid Real Life (Loop 2): 3:15

Thanks.

On balance it does seem that we go faster in real life than on the machine - at least that is a source of encouragement during long boring based training sessions....

No contest, the CT is harder on every course that i have had the opportunity to ride both on the road and on the CT - IMNZ and IMC (both really accurate) But it was harder (maybe not faster) on the road @ IM Moo b/c the Moo course has turns and I think this makes it more challenging then a relatively straight course due to constant change in speed.

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Simple Simon
Where's the Fried Chicken??

I think it is safe to say that the CT will always be slower just based on the fact that there is no gravity assistance on the downhill. Pure logic. However, I would be curious if a totally flat course would be the same time.

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Adrian in Vancouver