[CR]Alternative to shimming 27.2 to 27.4


Example: Events:Eroica
Date: Thu, 28 Feb 2002 19:41:39 -0800
From: "Grant Petersen" <gp@rivendellbicycles.com>
To: Robert_White@notes.ntrs.com, classicrendezvous@bikelist.org
Subject: [CR]Alternative to shimming 27.2 to 27.4

Short background: A typical single-butted seat tube for a typical road frame is 0.8 or 0.9mm thick at the butt, and 0.6mm at the top. The diameter is 28.6. So, 28.6 minus twice the wall thickness (that's 0.6 x 2, or 1.2mm) equals 27.4mm.

Brazing distorts and effectively shrinks the tube, after reaming, to pretty darn close to 27.2, which is why so many frames take 27.2mm posts.

BUT: Some Reynolds seat tubes are 0.55, and sometimes silver brazing doesn't seem to shrink the tube as much, so the reamed and finished seat tube really takes about a 27.3 post.

In 7+ years of Rivs, Herons, and Atlantises, I've seen this happen 4 times (in about 1,800 frames).

Shimming is the practical solution, but sometimes, some folks who have a hifalutin' frame think shims are low-brow solutions (I am not among them; I think they're PERFECT.)

An alternative: knurl the seat post, using a Stein tool made just for this purpose. It bites into the seat post and raises the surface just enough.

You knurl the part just below the top of the lug. Depending on the shape of the seat lug and where you stop the knurling, a bit of it may show; but it'll never again slip.

Grant Petersen
Walnut Creek, CA