Dave Moulton

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Monday
Mar282016

Smaller Wheels

I was recently sent a picture of a Fuso track bike I built around 1990. It is different in that it has smaller 650C wheels. At the time this smaller tire size was becoming popular with triathletes with much talk of them being “Faster.”

People in the know, such as myself and Terry Shaw, owner of Shaw’s Lightweight Cycles in Santa Clara, just south of San Francisco, knew there could be little or no advantage. However, Terry Shaw thought that if there was an advantage, it might be in the initial jump during a sprint race on the track.

Smaller wheels have the effect of lowering the gear ratio, so this would have to be compensated with more teeth on the chainring, or less teeth on the rear sprocket. And so if you have two bikes with the same gear ratio, where is the advantage? Except that smaller wheels and tires have to be marginally lighter.

It interested us both enough that I agreed to build Terry Shaw a track bike that he would ride himself. The only way to test the theory was to actually race on it.

Building a bike with smaller wheels changes the whole design of the frame. The front fork is shorter, so the head tube is longer as you can see in the photo above of a 60 cm frame. It looks larger because one is used to judging frame size by the length of the head tube.

With the wheels being a smaller radius, the angle of the down tube and chainstays has to be altered to in effect raise the bottom bracket. Smaller wheels also means less trail, so to compensate the fork rake or offset was shortened to increase the trail. This was a scant 20 mm or roughly ¾ inch, and is also evident in the above photo.

For readers who don’t fully understand the concept of “Trail” and how it affects steering, here is a drawing.

Draw and imaginary line through the steering axis and it reaches the ground at a point in front of where the wheel actually contacts the ground. The wheel therefore pivots about that point of contact.

This provides a castor action and the wheel trails along behind the steering axis, Hence “Trail.”

Looking at the drawing you should be able to see why smaller diameter wheels makes for less trail.

A steeper head or steering angle also makes for less trail. More rake or offset means less trail, less rake, more trail. Which is why this frame had a far sorter fork offset.

The drawing is for a road bike, the trail makes for a bike that will hold a straight line, and also gives some self-steering characteristics when cornering. A track bike typically has less trail, because it is designed to be ridden on a banked velodrome, where the banking has the effect of riding in a straight line.

The rider needs a quick handling bike, one that the rider can physically steer and change direction easily. It calls for a bike with a steeper head angle and less trail than a road bike.  

So now I have explained what went into the design of the frame, how did it perform? Terry Shaw raced it for a whole season, but then went back to his conventional track bike with 700C or 27 inch wheels. He reported the bike was faster in the initial jump, but was harder and required more effort to keep it rolling once top speed was attained. So no real advantage.

The one pictured above may or may not be Terry’s actual bike, because I built one other for a customer of Shaw’s Cycles. Soon after this I did design and built a TimeTrial/Triathlon bike for a trade show. I can’t show you a picture because it was later sold to a bike store in Del Mar near San Diego, and I never saw it again. I have no idea where it is now and would love to know.

It was an interesting design concept. It had a 650C front wheel with a fork with even less rake than the track bike above, it also had a steeper head angle. With the rear wheel there was the option to use either a 700C or 650C wheel. There was an aluminum adapter plate that bolted on to special braze-ons on the seatstays to accommodate the different brake heights.

When the larger rear wheel was used it of course raised the rear end making the frame angles steeper, including the head angle. This changed the trail and the handling characteristics. To compensate the front fork brake mount was made so the fork could be turned and the fork blade was curved backwards like a motor-pace or stayer bike.

This increased the trail to compensate for the loss of trail due to the steeper head angle. The head angle and the fork blade offset was designed for optimum handling in either set up. In test runs it handled beautifully, but I only ever built the one.

My thinking was to use the larger wheel for flat and straight courses, and the smaller rear wheel for technical courses with a lot of twists and turns, where faster acceleration out of corners might help.

 

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Reader Comments (5)

Very interesting, Dave. I had no idea that you'd built any 650B frames.

I have several friends from the 'C&V' world who like 650B wheels. One guy, who loves them, insists that they ride 'softer' and more comfortably. The lower trail should make the bikes more twitchy, but I bet the real difference is minimal.

Anyway, different strokes for different folks. I stick to 700C on my bikes for the simple reason that it's easier to carry parts that are interchangeable between them.

March 28, 2016 | Unregistered CommenterYohann

Yohann - Dave's post was about 650C wheels, not 650B. The 650B wheel is designed to use larger tires, so that the outside diameter of the tire is roughly the same as a 700C tire. Think balloon tires.

650C has a 571 mm rim (bead seat) diameter, whereas 650B is 584 mm. And the tires used in 650C are narrower than 650B. The consequence is a wheel+tire combo that is significantly smaller than 650B or 700C.

Sheldon Brown's website gives a good overview:
http://sheldonbrown.com/tire-sizing.html

March 28, 2016 | Unregistered CommenterBrian Ogilvie

I only ever rode a 60cm with 650C once, and I had thought that my Alan was twitchy.
Makes for a quick handling bike, but they are better left for the small frames where it helps let you keep 'standard' geometry.

March 29, 2016 | Unregistered Commenteredstainless

Brian - Thanks for catching that! I read the whole article with '650B' in my mind. :)

I'll have to read the article on Sheldon Brown's site. I still regret not going down to the basement in Harris Cyclery when I had a chance to meet him. He got me into fixed-gear bikes.

March 29, 2016 | Unregistered CommenterYohann

In 1946 age 13 when I started racing, most bikes had 26" wheels (700s?) Then there was a switch to the larger 27" that we all rode and raced on for years. WHY? The general thought was that 27" rode better? I have a 1962 Legnano that I will be riding at Eroica in Paso Robles in April, that was sold, by the Italian Legnano.co,from the factory WITH 27" size wheels. WHY you would think, that by then the Italians had all 700c wheels.Also in the 60s and 70s most American bikes seemed to have 27"wheels. My self, I did not find much difference, in 26 to 27 wheels. Of course now everything for road bikes is 700c By the way Yohann, Dave and my self started out riding only single fixed gear bikes. Twiddling we called it!

March 30, 2016 | Unregistered Commenterjohn crump
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