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I've seen triathalon setups with either 650c or 700c wheels. What are the pros and cons behind both wheel sizes?

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intersting answer's! I see it as about 3/4 to 1" lower in the front. with the potential for less wind restance! wind restance/drag is the bigest problem any bike rider has for going faster. maybe a 29 in the back too? shorter cranks, anything to get you out of the wind and still be able to ride well. –  MR NORM Dec 17 '13 at 5:00
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4 Answers

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.

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You appear to be confusing 650c (571mm effective rim diameter) and "26 inch" (559mm ERD) or 650b (584mm ERD) wheels. 650c is relatively uncommon everywhere, and the lack of wide-ish tire choices make it unsuitable for touring bikes. –  lantius Nov 5 '10 at 2:20
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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.

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Related to triathlon, there are varying theories on seat tube angle steepness. Using 650c wheels in some cases can help put together a frame that gives you the desired seat tube angle. –  darkcanuck Nov 9 '10 at 5:07
    
With respect to height, is there a threshold at which a rider should be looking at 650C instead of 700C? –  Soo Wei Tan Jun 28 '11 at 17:03
    
This blog post (rodbikes.com/blog/?p=355) suggests 5ft 5in or under. –  JamesBradbury Jan 9 '13 at 16:09
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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.

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Except the question specifically asked about 650c on Tri bikes. –  WTHarper Jan 19 '13 at 3:12
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I have been considering this question lately. Here's my input...

I understand that 650 wheels have advantages because:

  • Less weight, less inertia. The less weight there is the easier it is for the wheel to accelerate, turn and climb.

I understand that 650 wheels have dis-advantages because:

  • Less popular than 700 & 26" with retailers, so potentially harder to source.
  • Smaller diameter means bumpier ride on uneven surfaces.
  • Less weight, less inertia. Less inertia means more prone to small speed fluctuations caused by things like uneven pedal stroke, wind gusts, and road bumps/fluctuations.
  • Most likely less aerodynamic. Generally speaking the longer something is in relation to its width then the more streamlined it is.
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As has been shown elsewhere, the weight/inertia difference has an inconsequential effect on acceleration. –  Daniel R Hicks Jan 29 '12 at 8:42
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Compared to 700c tires/wheels of equivalent width and structure, 650c tires/wheels have been measured to have less aerodynamic drag but more rolling drag. –  R. Chung Jan 29 '12 at 15:21
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