One of the first considerations of an off-road-going bike is getting the rider's weight more aft of the front axle. This can be achieved in several ways: 1) A big frame with a shorter stem. 2) A longer top tube. 3) A slackened head tube angle. 4) *A STEEPENED seat tube.
The 4th way requires a bit of explaining. The steeper seat tube moves the top tube, and hence the front axle, foreward, and inevitably the rider will adjust the saddle to their preferred position relative to the bb, UNLESS the seatpost doesn't allow enough rearward movement of the saddle rails. So, depends on the parts used (saddle, post) and whether the rider prefers a relatively rearward saddle position in the first place.
An off-road-going rig will usually benefit also from less trail, to enhance stability.
Chainstays need only to be long enough for tire clearance, as more length impedes rear wheel traction and transfers weight foreward, preventing the rider from unweighting the front over bumps, holes, etc.
No need for a higher bb for CX in the age of clipless pedals, as a lower saddle height makes for surer and quicker re-mounts.
Also no need for Century comfort, as most 'cross races are an hour or less, so a rearward seat is not be the best way toward getting the rider's weight aft of the front axle.
Older bikes usually have more clearance for chunky tires, tho I've raced alongside riders using Trek OCLV road bikes for their 'cross weapon.
My own 'cross racer, being a Cheltenham Pedersen, is both heavy and featuring a most rearward of saddles. It rides well enough though for amateur racing and can be thrown over the barriers without taking hands off the grips on the highest of handlebars.
David Snyder
Auburn, CA usa
> Gang:
>
> While being inundated by the fixed gear crowd I have seen another
> outcropping of the fad. That being the conversion of an old road frames
> and
> converting it to some sort of cyclocross rig. And no I don't mean what
> Fisher and Ritchey were doing in Marin Co. making MTB's. I was to believe
> that the geometries of the two were quite different or maybe slightly so,
> a
> la BB heights. Can the gang shed some light on this for me. I am sure
> his
> eminence Mr Sachs can chime if he is checking the list today.