I am thinking about buying a rear disc, for a sprint distance I can avg about 22-23 mph and for a 1/2IM about 21mph, does a rear disc make sense at these speeds? I know it also depends on the course, but just assume a rather flat course.
Thanks
Jeff
Nah,
Unless you can average 35 mph uphill, a disc isn’t necessary 
Seriously, a disc can give you some ‘free’ speed regardless of how fast you are. Only you can determine if its worth it. Go to analyticcycling.com and play around with the models to give yourself an idea of how much time you can expect to save.
If it was my money, and I only had a set of 32 hole box section rims, then its a no brained. If I had a pair of Hed3’s (which I do) then it gets a little more difficult to justify buying a disc when I could spend that $500 or more on something else.
I think that if you average 15mph or more a disc is a better choice. If you are a 15-18 mph rider then you will be out on the course longer and you can ultimately burn less calories by having more effective wheels. I do think that at lower speeds wheel weight is a real issue so the balance of weight vs watts needs to be considered.