Re: [CR]The definitive frame sizing........


Example: Events:Eroica
Date: Sat, 16 Jul 2005 19:04:47 -0500
From: "John Thompson" <JohnThompson@new.rr.com>
Organization: The Crimson Permanent Assurance
To: classicrendezvous@bikelist.org
Subject: Re: [CR]The definitive frame sizing........
References: <456a55af800e47f1b6aec482edcb37c0@htc.net>
In-Reply-To: <456a55af800e47f1b6aec482edcb37c0@htc.net>


Ken Wehrenberg wrote:
> [F]rom a comfort and efficient riding position standpoint, I now
> feel there is a "third rail" which I will touch briefly on: bike
> handling. Just like a sportscar, it is a worthwhile goal to attempt to
> attain a 50-50 weight distribution front to rear. For each body weight
> type, this attempt to equalize can mean drastic geometry alterations.
> It can, for lighter climber sorts such as I am, mean shoe overlapping
> the front wheel, which must be brought back under the rider more.
> Finally all this was put together for me when I was fitted by the Seven
> Cycles system. I received rather drastically forward body position
> relative to the front wheel, but the noticeably improved handling was
> something the likes of which I never experienced. One more thing: Stem
> length. It takes around an 11cm or so to achieve this in order to move
> the weight forward enough.

About 12 years ago I built myself this frame:

http://www.os2.dhs.org/pictures/gallery/bikes/dsc01454

This bad boy has a 74deg head tube, 78deg seat tube, 40cm chainstays and a 99.5cm wheelbase. It has the same standover height as my 57cm TS Isaac frame (http://www.os2.dhs.org/pictures/gallery/bikes/isaac0001), but because of the steepness of the seat tube, the center-to-center measurement is only ~54.5cm. I didn't want the wheelbase to stretch out too long, so I cut back on the top tube as far as I dared without creating a dangerous amount of pedal overlap: 54cm. This makes it about 3cm shorter than the top tube on my Isaac, and I compensated with a 3cm longer stem (12cm vs 9cm).

The end result was...interesting. The weight distribution on this bike is completely different from that on the Issac -- much more weight on the front wheel than I was accustomed to having. I'm not sure what the distribution is, but I suspect it is something like 60% front and 40% rear; the first couple times I rode it I almost face-planted when I hit the brakes.

I've since become quite accustomed to its peculiarities, and enjoy riding it as much as my other bikes. It does highlight a couple things, though: first, there is a lower limit to tube top length below which you need to consider smaller wheels to avoid toeclip overlap. Second, changes in geometry, riding position, stem length, etc. can have a profound effect on how a bike handles -- something to keep in mind when adjusting things.

--
John (john@os2.dhs.org)
Appleton WI USA