Re: [CR]Figuring TT length - position


Example: Component Manufacturers:Avocet

From: NortonMarg@aol.com
Subject: Re: [CR]Figuring TT length - position
To: monkeylad@mac.com, Steve@sburl.com, classicrendezvous@bikelist.org
Date: Mon, 16 Sep 2002 12:09:07 EDT

In a message dated 9/15/02 7:35:13 PM Pacific Daylight Time, joebz@optonline.net writes:

<< Slack angles on touring bikes require moving the seat forward to get the same net position relative to the crank axle. In other words, only the seat lug moves back to net a longer top tube, the saddle stays fixed relative to the bars. For example, going from 74 to 72 degrees: sine 2 degrees X 57 cm = 2 cm longer top tube. >>

Top tube length makes more sense when you divide it in two, by establishing a mark on the top tube using a plumb bob over the bb center. If you don't do that, you cannot tell if it's longer because of a "more slack" seat angle or if the frame has a longer forward "reach". Of course you can do this with a precision angle finder, but most people don't have one. Make sure the spot of floor you are using is level. Once you know the forward reach of the frame, THEN you can simply add/subtract from a known stem/frame-reach combination that works for you and it's all the same. For the seat fore and aft, you can put a drafting triangle at your mark, and measure back to the saddle tip. If you have the same saddles on all your bikes, this can be amazingly precise. You may have to check the height and fore/aft a couple of times to get it right. A rule of thumb: 1 degree makes about 1/4" difference over 1'. So on a 24" frame, 1 degree of seat angle difference translates to about 1/2" of top tube length, all in that rear portion BEHIND the mark, without affecting the reach of the frame at all.

The concept of dividing the top tube this way is the idea behind the "Fit Stick". You can spend $139 and get that, or get a level and a plumb bob. The important thing is to always think of the top tube as two dimensions, not one. It makes fitting much easier. Stevan (been doing it this way over 25 years) Thomas Alameda, CA