I'm doing the same thing this spring. I have also asked this question on the bicyclesports website and got an answer of 30 seconds for 40K.

Dave from VA

30 seconds over 40k, www.timetrial.org/aerodynamics.htm

I would be surprised if it was as much as 30 seconds. Both the sites mentioned above, along with others, say that the difference between 32-spoke box rims and a disc/trispoke combo is about 2 minutes, 2:30 tops. Everyone agrees the front wheel is more important (at least 2/3 of the savings) because it hits clean air. That means disk vs. 32-spoke box rim rear is probably about 40 seconds, maybe 45. HED says that the difference between its trispoke and its disk is 10-20% (depending on crosswinds). That would make the difference between a trispoke and a disk 10 seconds or less.

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Anyone who tells you they're as fast now as they were when they were 18...
sure wasn't very fast when they were 18.

According to Jim Martin/John Cobb in Chapter 10 "Bicycle Frame, Wheels and Tires" in the book "High Performance Cycling" by Asker Jeukendrup PHD, the difference between a rear disc and good composite wheel (tri spoke) is 12 seconds over 40 kms. I've noticed on Cobb's forum he is now claiming about thirty seconds, which contradicts what he and Martin wrote in this book.

The tri spoke is a bit more versatile for those very few races that don't allow disks or for those really, really hilly races. I don't think your going to be wrong with either wheel. IMO if I'm saving only 12-20 seconds its not worth the hassle to switch wheels, tires, cogs etc. I've been in only maybe 6-7 tris or duathlons out of 120+ where 20 seconds made a place difference. Only 1 where I missed prize money which would not have covered the cost difference between the wheels.

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Brian Stover USAT LII
Accelerate3 Coaching
Insta

Shoot, the cool whoosh whoosh sound is worth a few second by itself... ;-)

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1*

John Cobb was a panalist at the science of speed conference in Kona. He stated that a disc is faster than any other wheel in ANY triathlon on the planet.

"John Cobb ...... stated that a disc is faster than any other wheel in ANY triathlon on the planet."

No arguement, but as I've already stated, he seems somewhat unsure how much faster a disc really is.

The point was to respond to an earlier post suggesting that a disc would not be faster on a "hilly" course. Cobb seemed to think that there were no tri courses hilly enough to negate the advantage of a disc.