650C vs 700C?

I am 5'2" and did a lot of research on this last year, when I was in the market for a tri bike. Do you have any data for 650 vs. 700Cs? Thanks!

"If the shoe fits, wear it!"

Question

What is your opinion on 650C vs. 700C for smaller cyclists?  I noticed your 48cm tri bike is on 650Cs.  I realize that you have to make some design sacrifices to fit larger wheels on smaller frames.  I am 5'2" and did a lot of research on this last year, when I was in the market for a tri bike.  The online consensus seemed to be that 650's used to be a trend, but for long distance riding, they are not smooth and provide more of a power surge/less efficiency.  Shop owners agreed.  So I went with one of the only tri bikes small enough for me that had 700C's, only to later discover that most of the competitive women cyclists in my area are on 650's...even ones who aren't as short as me.  I've enjoyed your technical write-ups on crank lengths and Young's modulus.  Do you have any data for 650 vs. 700Cs? Thanks!

Answer

You may have noticed our new P3 uses 700C wheels in sizes down through 48. And we’ve added a new smaller size 45 (with 650C wheels) to better address all fit options.

8% Smaller

Before addressing your specific questions, let’s put some numbers on the basic difference in wheel size. Based on ETRTO (European Tire and Rim Technical Organisation) dimensions, 650C wheels are smaller than 700C wheels by about eight percent.

Wheel size:

  700C  

  650C  

 Difference, mm  

 Difference, Percent    

Bead seat diameter, mm       

622

571

-51

-8%

Overall diameter, mm

667

616

-51

-8%

Overall radius, mm

353

334

-26

-8%

Tire circumference, mm

2095

1935

-160

-8%

Table 1 650C wheels are about 8% smaller than 700C wheels.

This basic 8% difference in size helps inform just about every question about 650C wheels.

Let's look at some pros and cons of 650C wheels, followed by a few common myths which should help answer your questions.

Reasons for 650C wheels

1)  Better fit below a certain frame stack. Fit is by far the number one reason for 650C wheels. Why? The frame’s head tube stands on the fork, and its length has a practical structural minimum, currently around 70mm or so. Shorter than that and the loads go up significantly in the frame, headset bearings and fork. As an alternative to reinforcing these parts (which adds weight), the smaller 650C wheel and the correspondingly shorter 650C fork make lower frame stacks structurally easy. Good fit and no special structural problems. 650C for the win!

2)  Aero drag. This ties in with the previous point, fit. The athlete’s body is responsible for up to 80% of the aero drag of the bike+rider system. The lower frame stack possible with 650C wheels, if that’s what your fit calls for, means your body can be in a good, aero position.  If you raise your handlebars from this ideal position to fit on a higher-stack frame to use 700C wheels, you are instantly significantly slower.

Lower isn’t always faster, but usually, it is. We confirmed this by measuring the drag of pro athletes in various wind tunnel measurements. For nearly all of them, lower is faster. 


650Aero 650 chart

Figure 1 Change in aero drag as a function of changes in arm pad stack height. 2009, LSWT.

The large spread in this data clearly indicates there are other variables affecting drag besides arm pad stack height, but in general riders saved roughly 2 Watts per centimetre reduction in arm pad stack. The 51mm smaller diameter of 650C wheels can directly reduce arm pad stack by 5 centimetres, suggesting roughly a 10 Watt savings if that’s the position you need.

Another aero advantage of 650C wheels is that the wheels themselves have less drag. Wind tunnel testing of wheels shows about 8 to 12% less aero drag for 650C wheels.

The last but not least aero advantage of 650C wheels is that they’re less susceptible to the buffeting effects of gusting cross winds, making them easier to steer than equivalent depth 700C wheels. That’s because the same side wind produces less torque about your steering axis. There are two factors to this torque, side force and lever arm, and both are reduced about 8% with 650C wheels. With 8% less surface area, the force is about 8% less. And with 8% smaller radius, the lever arm is about 8% less. Multiply these, and the steering torque you have to fight is about 16% less. 

Emma Pooley 1

Figure 2 Emma Pooley riding her Cervélo P3 with 650C wheels to victory in the 2010 World TT Championships. Photo: VeloNews.

As an example, thanks in part to her excellent aero position, Emma Pooley rode fast enough to win the 2010 world time trial championships on her Cervélo P3 with 650C wheels.

3) Toe overlap. Less likely with 650C wheels.

Reasons against 650C wheels

1) Selection. Poor selection is by far the biggest negative of 650C wheels. Wheel, tire and tube selection and availability is limited compared to the wide variety with 700C. But used 650C wheels, even nice race wheels, are often quite inexpensive. And for top of the line 650C race wheels, Zipp and Hed have just made their newest wheel designs available in 650C versions.

2) Interchangeability. For training, it’s nice if the wheels on various bikes in your household are interchangeable, and for most households that’s 700C. However for racing, it’s not really an issue in triathlon, since no outside support is allowed. But in pro road racing, neutral support’s spare wheels are all 700C. (Interestingly, this is not an issue for pro time trialling – your individual follow car simply carries the right spares for your bike.)

3) Rolling resistance? Theoretically perhaps, but we’re not aware of any data definitively demonstrating this. As a guess, even a 4% increase over the rolling resistance of a good 700C tire is less than 1 Watt (Al Morrison’s Roller Crr data). For most riders for whom this is a question, this is likely far smaller than the power you’ll save in your optimum body position on a correctly fitted bike with 650C wheels.

4) Aesthetics. Beauty is in the eye of the beholder. While some people see the smaller visual size of 650C wheels as a minus, others see the proportionality of 650C wheels as an aesthetic plus on the right size bike.

Myths

1) Spare tubes. Of course, you carry your own 650C tubes. But in a pinch you can fold a 700C tube. After a year or so it might rot at the fold enough to finally go flat. No, you don’t feel any lumps while riding.

2) Gearing: The smaller wheel has to spin 8% faster to go the same road speed. But that’s no problem, just change to a harder gear. Changing your gear works because the wheel radius is just one lever in a system of levers between your foot and the road. Crank length, chain ring and cog sizes are the other levers. The 650C wheel is 8% smaller, so just choose an 8% higher gear. For example, install a cassette starting with an 11 tooth instead of 12 tooth cog to get 8% more gear. (Such a cassette might also have a 23 instead of a 25; that’s an 8% difference too.) 

Wheel  size    

  Chain ring  

teeth

  Cassette   

teeth

  Hardest gear,  

inches

  Easiest gear,  

inches

Comment

700C        

39-53

12-25

117

41

             (Baseline)

650C

39-53

11-23

118

41

Change the cassette only, get virtually the same gearing

Table 2 Gear inches comparison. Simply changing the cassette can maintain virtually the same gears with 650C wheels.

3) Smaller wheels, axles closer to the ground - will my pedals hit? No. Frames designed for 650C wheels are designed with the bottom bracket raised back up to the appropriate height, perfectly canceling the lower axle height of the smaller wheels. 

Your two main questions actually are answered in the next two myths.

Not smooth?

We’re talking about road irregularities, surface roughness, etc. Basically, bumps of various sizes. Consider how small or big the bumps might be.

  1. Infinitely small: On perfectly smooth surfaces, all wheels sizes (tiny skateboard wheels to giant earth movers) roll smoothly.
  2. Infinitely big: Hitting a wall, all wheel sizes stop.
  3. In between: Bump size as a fraction of wheel radius (together with road speed) determine vertical acceleration. For these middle bump sizes, the difference in smoothness (the exit angle, or the trajectory of the bike coming out of a pot hole) between 650 and 700C wheels is proportional to half the difference in the radius, or 4%.  A 650C wheel has about 4% steeper exit angle.

 Wheels 1

 Figure 3 Exit angles for 700C and 650C wheels on a 4cm bump.

What are the exit angles for some different bump sizes?

Wheel size:

  700C  

  650C  

Difference, degrees  

Difference, percent  

Exit angle, 1cm pot hole, degrees

14.1

14.6

0.6

4%

Exit angle, 2.5cm (1”) pot hole, degrees

22.5

23.4

0.9

4%

Exit angle, 4cm pot hole, degrees

28.4

29.5

1.2

4%

Table 3 Exit angles for different depth pot holes. 650C wheels have 4% steeper exit angle.

For bumps less than one inch (2.5cm), the difference in exit angle is less than one degree. Even a deep 4cm pothole gives a difference of only 1.2 degrees (as illustrated above). For all these bump sizes the difference is about 4%.

But is a 4% difference important? Can you even notice it? This 4% difference is much less than the “Just Noticeable Difference” (JND) of even the most sensitive riders.

Just Noticeable Difference (JND), %    

    Which subjects detect JND 

5

(None)

10

A few sensitive subjects

15

Most

20

All

Table 4 Just Noticeable Difference (JND) in vibration input to tire. (Unpublished research in Cervélo’s Technical Development project TD09)

In a study conducted as part of our scientific vibration transmission and isolation research project, the most sensitive test subjects could just notice a difference of 10%. (None could notice the 5% difference we tested.) Most riders could just notice a difference of 15% and some “insensitive” riders could barely notice a difference as high as 20%.

Bottom line: Yes, probably there is some difference in smoothness, but it’s not noticeable. Consider how rough your roads are, and also remember to balance this theoretical detriment against the real benefits of 650C wheels. 

More power surge/less efficiency

Your phrase “more power surge” might be referring to a theoretical reduction in a 650C wheel’s angular momentum, due to less mass at the wheel’s periphery.

Lighter rims and tires of any size reduce the “flywheel effect” (angular momentum is less for smaller radii at the same angular velocity, but this effect is mathematically canceled perfectly by the requirement to compare momentum at the same tangential velocity). Lighter wheels, at the same average speed, change instantaneous speed more during each pedal stroke: both more acceleration and more deceleration. 

Whether such surging is good or bad depends on your physiology. Do you want less surging, meaning more momentum? Only a few crazy riders want more momentum badly enough to actually add weight to their wheels. For the rest of us, the (typically) lighter wheel weight of an otherwise equivalent 650C wheel is a performance advantage, not a disadvantage.

Bottom line: “Power surge/less efficient” is a red herring, a potentially-true-but-negligible-in-practice reduction in rotational momentum that might even be beneficial.

I think the above answers most your questions. The last one you asked is whether 650C wheels are trending up or down.

650C: Trending up or down?

Currently, 650C wheels are trending down. However, at the Hawaii Ironman at least, 650C wheels grew in popularity to a peak sometime around 1999, then decreased. See the chart below showing the trend among Kona triathletes.

650 at Kona

Figure 4 650C wheel test at Hawaii Ironman, 1992 to 2002. No data from 1996-1998. Source: Dan Empfield, “Are 650c wheels relevant today?” 2004.

The dotted line labelled “Poly” is a polynomial trend line for the 650C data. As you can see from the downward trend, the market has spoken – most people don’t want to buy bikes with 650C wheels. But trends and markets aside, if you’re interested in good performance, you want to know how wheel size affects your speed or energy savings, regardless of whether other people are choosing 650C wheels. See the pros, cons and myths above. 

Bottom Line

Get the frame size that lets you fit into your best riding position. In terms of athletic performance, whether it takes 650C or 700C wheels to do that is far less important than optimizing your body’s aero position. “If the shoe fits, wear it!”

 

Sincerely,

Damon Rinard, Cervélo engineer

 
Comments (7)
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  • Frank

    I have a P2 with a 650c wheel. For myself the only problem is the choice of wheel we have in that size. We Can't buy a zipp wheel more than 404. I realy like my bike. I would prefer a 700c for the choice of wheel and when we can have a deal on used wheel we never have it on the 650c. So it's bad in that case.

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  • Nitipat

    Found the brakes calipers with longer drop now (either Tektro r539/559) but would the larger gap between seat tube and rear wheel compromise the aerodynamic of the bike especially if the rear wheel is a disc? Would the gain on aerodynamic from the disc be offset by the aforementioned gap?

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  • Nitipat

    I have a 2013 P2 51cm, I am wondering if i am to use 650c rim, how do i deal with the brake issue? I currently have ultegra calipers, which specific brand and model i need to replace them with in order to use smaller wheels? Thank u very much for your advice.

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  • Nitipat

    I have a 2013 P2 51cm, I am wondering if i am to use 650c rim, how do i deal with the brake issue? I currently have ultegra calipers, which specific brand and model i need to replace them with in order to use smaller wheels? Thank u very much for your advice.

    Report this comment.
  • Damon Rinard, Cervélo Engineer

    Hi David, We were able to reach an even lower frame stack with 700C wheels (505mm in the R3, for example, http://www.cervelo.com/en/bikes/r-series/r3.html ) than we did with the RS model that used 650C wheels (520mm), so there was no fitting requirement to use 650C wheels. On the other hand, triathlon and time trial bikes, fitted with aero bars and lower riding positions, need correspondingly lower frame stack. The smaller sizes can only get low enough using 650C wheels. Cheers, -Damon

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  • David

    Great article, I own a 48cm P2 with 650C wheels. l would have a very difficult time achieving my current position on a frame with 700C wheels. Given all the pros above, I'm wondering why Cervelo doesn't make a road bike model with 650C wheels anymore? The now discontinued RS in the XS size seemed to be a road bike that could fit a lot of shorter riders.

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  • Sherwick

    Well written! Slowtwitch's chart has often been used to demonstrate the popularity of 650C vs. 700C framesets and wheels. The possibility yetl exists that (assuming the pairing of smaller-framed riders with 650C remains constant) the trend may instead showcase the variability in body size among Kona competitors over the years: the % of smaller athletes qualifying for Kona peaking in 1999. It would be good for Empfield et al. to also survey height of athlete along with frameset size...just a thought.

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