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visual bike geometry question(10 posts)

visual bike geometry questionET
Aug 16, 2001 11:07 AM
Suppose your view to the hub when on the hoods appears visually as +/-epsilon (so, e.g., if epsilon is zero the hub is obscured, if positive, you are past the hub visually by that amount, and if negative, you are before the hub visually by that amount). If you remove, say, a 1-cm spacer from your stem, is it obvious which way visually the hub will go?

To use a particular example, suppose visually your bars appear 1/2 inch past the hub when you're on the hoods. If you remove a 1-cm spacer, will the extra vertical drop to the bars cause one's vision to get closer than 1/2 inch by bringing your head slightly forward, or will the slight increase in diagonal distance from seat to bars win out, causing the bars to appear even more than 1/2 inch past the hub?
And your point would be...?Huh???
Aug 16, 2001 11:25 AM
Simple geometry...Cima Coppi
Aug 16, 2001 11:33 AM
You did not give us your head tube angle which is criticle in calculating the horizontal measurement you seek. I ran a calculation based on a 73 degree head tube angle, and a 1 cm drop in stem height. The horizontal distance for reach gained by droping the stem 1cm is .29cm. This is calculated using the following:

cos (head tube angle)= adjacent(unknown)/hypotenuse (1cm)

Adjacent=.29cm

CC
Your better off changing the length of your stem by 1cm (nm)Cima Coppi
Aug 16, 2001 11:45 AM
not so Simple geometry...ET
Aug 16, 2001 12:05 PM
This is visual geometry, not actual, and there are practical considerations. As your arms come down, your head goes forward. Sure, the amounts might vary among individuals, but I'd bet what happens is pretty consistent one way or the other. This consideration is much more important than the relatively miniscule head tube angle differences between frames for a 1-cm drop.
Real geometry plays the roll...Cima Coppi
Aug 16, 2001 12:49 PM
Your head and arms will adjust to the numbers dictated by the measurements I gave you. They will move down 1cm and forward .28cm. You're correct that they are small amounts, and this is all your body will adjust as well.

What you are inferring is the body will make some sort of radical adjustment to a 1cm drop of the stem, and this just will not happen.

Your better off changing the stem length to get the desired effect you seek.

CC
just to clarifyET
Aug 16, 2001 1:07 PM
Are you claiming it will lengthen perceived visual distance of bars past hub or shorten them?

No need to keep saying I am better off changing stem length; the numbers are hypothetical. But I may indeed knock off a spacer.
The numbers you are talking about are so small...Cima Coppi
Aug 16, 2001 1:27 PM
I don't believe you will notice any perceived difference in the location of the hub relative to the tops of the bars by removing a 1cm spacer. If anything, you may perceive the bars being slightly more over the hub.

CC
No longer validMB1
Aug 16, 2001 11:37 AM
That is an old racing style setup that got outmoded when frame angles and top tube lengths started changing. The idea was that in your normal riding position you would look down and your handlebar would block your view of your front hub. Now fitters are looking at seat height, thigh angle, back and arm position, and on and on and on.

Whatever works for you.
I can still ask the question, can't I?ET
Aug 16, 2001 12:10 PM
Aside from curiosity, if one uses the hub test as a general proxy for fit and finds he's moving rather far way from the hub, it may at least be an indication or warning of improper fit. Fitters can look at a million different things and still be off. You gotta ride for a while to really know.