Date: Fri,
14 Jul 2000From: alex wetmore
Subject: RE: [hpv] Online calculators?
List-Id: Human Powered Vehicles Mailing List <hpv.ihpva.org>
On Thu, 13 Jul 2000, Shaw, Byers Jr. wrote: I wanted to order it with 11-34 and 62 while he insisted 12-34 and 60 would give me plenty of gear inches. Of course, I had measured the actual diameter of the wheel and tire (Primo Comet 1 3/8) and obtained 18.5 inches. The 60-12 gear combo he claimed yielded 100 inches [ > (60/12)*20 ], while I insisted it gave 92.5", an unacceptably low set up. Your spreadsheet proves my point, viz. that on the road, I was spinning out at about 30-33 mph (your chart shows 120 rpm at 33 mph).
This is a very common mistake, even with larger wheels. A lot of people assume that a 26" mtb wheel with 1" slicks is still 26" (or close enough when doing gear inch calculations) when it is really closer to 24".
I usually use 24.5" as the wheelsize for MTB wheels with slicks, and 18.5" for BMX (406) rims with slicks. The true size varies a little from here, but these are much better estimates than 26" and 20".
One simple formula that gets close is: (rim diameter + (2 * tire diameter)) / 25.4
So (406 + (2 * 37)) / 25.4 = 18.9
(that is for the Primo Comet).
The true measurement is often a little less, but this is close.