Checking In

I'm almost done creating an odd format racer.  It's taken lots of thinking, but I think its dialed in correctly. 

By odd format, I'm referring to the fact that the seat height is 770mm, w/ a 520mm effective toptube.  It's a big short bike - to fit a long legged rider with limited reach.  The drop is about 3.5 inches, so that's not a problem - just limited material between his hips and shoulders.

So how would you design this frame?  Note that the rider does road races and crits in souther CA (although he's currently on his way over to ride the Stelvio).  He likes both, and is probably best at the crits - he doesn't have a classic climbers build.  On the other hand, he seems to put out pretty good watts - and can sprint pretty hard.

Based on power, weight and height, I chose a 2X oversized steel tube frame.  35mm downtube, 31.7mm seat and top tubes, 36mm headtube.  All fairly light, but large diameter and stiff.  The rear triangle is similar - and looks strong. 

Unlike bike manufacturers, I have the luxury of building chainstays to the length I chose, not what's fashionable.  While short, the chainstays are 410mm - the minimum I think will offer good shifting over the range of gears.  Let's face it, everyone ends up in cross-over gears sometimes, and for races that may be even more true as they need to be selective about when to do a double (front and back) shift.

The BB drop is about 70mm, but I don't think it would make much difference if it were 65 or 75 mm.  The 70mm, combined with a 6 degree sloping top tube will make sure he has comfortable top-tube clearance. 

I don't like lots of spacers topped with an upward pointed stem - it doesn't look pro.  So he has a long headtube (about 175, plus a conventional threadless headset).  If needed, he can about a centimeter off the headtube - but this is computed to work with his preferred drop and and a downward (relative) pointing stem. 

But the real key to all of this is front end geometry.  Depending on the source, there is a limited range of appropriate trail and/or head-angle and/or rake.  Others maintian front center distance is the critical element to handling.  Most all hold that different  head-angles require different trial numbers.  But some emphasize widely varying trail via a constant rake, while others insist that's poor design and adjust rake to limit the range of trail figures.  And some limit the range of head-angles to avoid this question.

My rider excels at crits, but has to ride some very technical descents in the LA area.  So, stability and predictability trump quickness for his front end.  Some builders argue that there's shouldn't be a difference between a crit and road course bike.  I won't go so far.  I'll say that one bike can be good at both, but that the disciplines do push different handling dynamics.  Moreover, the kings of road course design are (IMO) the Italians.  That doesn't mean a Pinarello can't compete well at a crit, just that I expect it to be at its best on a road course. 

I didn't want the front center squished by the short top-tube.  Which means that the head-tube has a relatively shallow angle.  That's a given to keep the fundemental handling solid.  But where I originally was thinking of a longer rake to manage the trail, I've looked closely a Colnagos, De Rosas, and Pinarellos, and how they handle a similar front end.  Based on this, the fork will have 43mm of rake - which is leading to lots of trail - which should give that stable, predictable, handling on descents.  And it'll still be a great crit bike.

So it's back to the frame - which still needs a brake-bridge and chain-stay bridge before its cleaned up and painted.  Hope to have some pix soon.