Re: [CR]Why 650B?

(Example: Framebuilding:Brazing Technique)

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Date: Wed, 7 Jan 2009 14:27:05 -0800
To: Jon Spangler <>, <>
From: "Jan Heine" <>
Subject: Re: [CR]Why 650B?
cc: Jerry Moos <>

At 10:22 AM -0800 1/7/09, Jon Spangler wrote:
>Jerry and all,
>Your post brings to mind an issue I've never been able to understand
>very well: why are there so many wheel diameters and and tire sizes
>within such a small range? It's almost overwhelming.

Originally, the reasons were simple: Before caliper brakes were popular, makers could use the same frame with different tire widths, as long as they kept the outer diameter of the wheels constant. Even early caliper brakes clamped to the fork blades and seatstays (see the brake history in the latest Bicycle Quarterly), so they were easy to adjust. That made mass-production a lot easier.

However, the wheel sizes persisted even after modern brakes became popular in the 1930s. I think there is another reason.

When you look at bicycle history, you see that an outer wheel diameter of about 68 cm has been pretty constant since the invention of safety bicycles. There have been lots of attempts to change it, but none have made a breakthrough, whether the many folding bikes of the 1950s and 1960s, Moulton's "revolutionary" machines, or Cinelli's ideas of smaller-wheeled bikes. On the other hand, the recent popularity of 700C mountain bikes (29ers) is giving way to using 650B wheels instead.

The dimensions of a bicycle don't scale - you can't just shrink the blueprint for a 60-cm tall bike by 16% to make a 50-cm tall bike. If you did, it would not handle the same as the 60-cm bike. You can see that with children's bikes - I have not yet seen a 16"-wheel children's bike that is both stable at 20+ mph, yet has little wheel flop to make low-speed riding easy. For a big-wheeled bike, these two goals are easy to combine on the same bike.

I suspect that this is due to the gyroscopic forces of the front wheel. Gyroscopic forces have been dismissed sometimes, because they do not keep the bike upright (which is what some people thought). However, gyroscopic forces do stabilize the bike - when you turn the front wheel, it tries to turn back to straight.

It stands to reason that if you get gyroscopic forces that are too small, the bike will not be very stable. In fact, many small-wheeled bikes appear to be that way. If the gyroscopic forces are too large, the bike may be too stable and not lively enough.

We hope to test this at some point for Bicycle Quarterly. In the mean time, from riding a variety of bikes, wheel sizes and tires, I can say that I have not yet ridden a bike with 650B wheels and tires narrower than 35 mm that I thought was perfect. I also have not yet ridden a bike with 700C wheels wider than 35 mm that I thought was perfect. And I have not yet ridden a bike with 26" wheels that I thought was perfect (the latter may be due to a limited sample size.)

So I'd recommend using 700C if you plan to use tires narrower than 35 mm, 650B for tires 35-42 mm, and perhaps (pure conjecture here) 26" for tires that are wider than 60 mm.

Jan Heine
Bicycle Quarterly
140 Lakeside Ave #C
Seattle WA 98122