positioning on the cross bike

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11.4
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Joined: Tue May 23, 2006 4:33 am

by 11.4

Briscoelab wrote:
Marin wrote:The head tube on the CX is only shorter because the fork is longer - stack will be similar.


This isn't necessarily true. Most CX bikes have a slightly to dramatically higher BB. So that can negate the increased fork height.


Some have higher bottom brackets but not all. That's more a Euro thing. There are plenty of top major brands and top custom cross bike builders who are running quite low bottom brackets for greater stability. The old argument used to be that you needed extra height to be able to ride on the backs of pedals without the toe clips dragging, but even that was a bit of a stretch. Watch old videos of the Belgians in the 70's riding cross and they were back into their clips almost instantly. Only awkward amateurs would cruise the course with pedals upside down. The issue back then was more to give clearance in turns, on off-camber, or going over barriers. Today riding techniques and bikes have both changed so much that needs have changed but myths continue. For example, you can do a racing dismount so fast on a bike if you can come off with one hand on the top tube that you then can carry the bike in that hand rather than shouldering it -- runs are shorter than in the past and the bikes are so much lighter that you don't need to shoulder them. Same kind of changes in the sport for bottom bracket height and its relevance or not.

It's correct that IF your bottom bracket is a centimeter higher, the head tube will be a centimeter higher as well if the overall position is to be unchanged. But then we need to add in whether it's an inset headset which puts the stack height of the top race inside the head tube and increases the head tube length, and so on.

I still happen to like higher bottom brackets, but partly because I ride small frames and can use anything that gives a decent fit without running out of head tube. And because I like to dismount with one hand on the top tube and then pick the bike up that way, I am fine with a longer actual seat tube so I get a more horizontal top tube. I've never really been able to shoulder a 49 or 50 cm sloping top tube cross frame anyway -- there's no room inside the triangle. But higher bottom brackets are relative and if you get used to the higher bottom bracket, you don't miss any theoretical loss of stability. Plus, you may be able to lower your bars a few millimeters and more than offset any change in center of gravity.

Cross racing anyway is so intense that you don't really get the luxury of riding like a tourist with your head up and arms in. You have to go so fast in the straight and flat sections that you have to consider those sections and the speeds you need when you are designing a cross frame. And higher bottom brackets are marginally more aerodynamic because the bottom bracket, cranks, pedals, and feet are not hanging down as much in the wind. This has been worked over in detail in track cycling and the effect is quite considerable.

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