The Chainlink

Machined Vs Non-Machined

Anyone have any disagreements with Sheldon Brown or Rivendell Bicycle Works? Does a machined rim really make a difference? Does it help breaking? Does it degrade the rims quality?

Sheldon Brown
What you hear and read is mostly marketing hyperbole, but machining rims has its reason, and it isn't for your benefit. If you inspect a machined rim closely, you'll find a surface that looks as though made by a thread cutting tool. The purpose is not to get a flat braking surface, but rather to produce a series of fine grooves to prevent brake squeal on new bicycle test rides.


Rivendell Bicycle Works
A rim starts life as a straight extrusion; and then it is rolled into a circle. Before it's rolled, all's well with the braking surface (sidewalls). But in rolling it round, the sidewalls get wavy. That's because the inner and outer circumference travel a different distance, in the same way as the runner in the inside lane of a track runs a shorter distance than a runner in the outside lane (assuming they start and finish on the same line). If the rim material were stretchy, maybe the outer portion (the part nearest a mounted tire) would stretch and get thin. But it's not stretchy, so instead, the inner portion develops waves, so that despite being on the inside, it still travels the same distance. It's like the runner in the inside lane running a curvy line to make up for his advantage. Is this making sense?

In theory, wavy sidewalls are bad, but only in theory. In fact, the degree of waviness is so small that it hardly matters. However, it is undeniable that perfectly flat sidewalls are the ideal, and have less of a tendency to draw attention to themselves on an initial test-ride. That's the whole deal, right there.

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Permalink Reply by Doug Haynes on March 10, 2010 at 12:40pm
Both.

Any time you bend an extrusion without stretching and shrinking the material it is going to get waves, look at the breaking surface of a new non-machined wheel and you will see wear marks that demonstrate this. You will also see that a good portion of the pad still has full contact with the rim and that the waves are very small; it will still stop just fine.
Permalink Reply by Jessica on March 10, 2010 at 6:07pm
Ha... the brakes can still squeal with machined rims, believe-you-me. It has nothing to do with what the rims are made of, and a whole lot more to do with how the pads touch the rim. Ideally, you want a little bit of an angle so it doesn't come to the rim and lay flat on. This causes that glorious vibration what we all know as 'brake squeal.' You want it to toe in *juuuust* a little, so that it lands one end first, then the rest of the brake come in contact.
Permalink Reply by Tank-Ridin' Ryan on March 10, 2010 at 9:45pm
That is - FRONT end first, for anyone who's wondering which end.

Jessica said:
Ha... the brakes can still squeal with machined rims, believe-you-me. It has nothing to do with what the rims are made of, and a whole lot more to do with how the pads touch the rim. Ideally, you want a little bit of an angle so it doesn't come to the rim and lay flat on. This causes that glorious vibration what we all know as 'brake squeal.' You want it to toe in *juuuust* a little, so that it lands one end first, then the rest of the brake come in contact.
Permalink Reply by Jessica on March 11, 2010 at 7:13am
Whoops, thanks Ryan! =)

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