I've seen triathalon setups with either 650c or 700c wheels. What are the pros and cons behind both wheel sizes?
They are primarily used for high-performance road bikes for smaller riders. There comes a point where a proper fitting frame for a smaller rider will have strange handling using 700c wheels. 650c wheels are designed to address that.
Smaller riders can also have road bikes built with 26" wheels.
So if you are of average height there is no significant advantage to 650c wheels.
I have been considering this question lately. Here's my input...
I understand that 650 wheels have advantages because:
I understand that 650 wheels have dis-advantages because:
I'm in the USA and mostly ride on 700's so that is the angle from which I see this.
700C wheels are larger and will require slighty less energy to make them go round and round. If you have a really small frame (like an XS), though, these 700's may be large enough to cause toe overlap. In this case, you're hitting your toes on the tires when going around corners. Not good. Smaller frames often have 650 wheels for this reason and also because the smaller wheel allows a slightly lower stand-over height.
700C wheels are the most common size road wheels in the USA. Elsewhere, my understanding is that 650's are more common. Some touring bikes will have 650's even in larger size frames for this reason--if you need a new tire in Romania, it may be eaisier to find 650's than 700's.
The real cons of the 650 is that they are less common in the States and are slightly smaller and therefore not quite as fast.
If you are doing Tri and you are doing it just in the USA... I think that 700's are the way to go.
Since this might have really been a question about 650, I am going to answer if from perspective of why use 650B.
650B is common in Randonneur style bike, currently one of the more popular types of bikes being requested in the handmade bicycle market.
650B allows the use of 42 width tires with full fenders, big tires even at low pressure with 650B become about same size total diamteter as 700's with typical racing widht high pressure tires 25 width for example.
650B rando bike with compact crank designed for front load carrying may the fastest overall bike for practical use, see Bicycle Quarterly magazine for detailed analysis.