Rosso Corsa

I am wondering if anyone has insight regarding the geometry of the Fuji SL1. Everything looks pretty normal, although a 47cm seat tube and a 52.5cm TT seems like it would afford a pretty big drop, so I should err on the side of a smaller frame as this will prevent the reach from becoming too much, right?

The strange thing is the head tube angle. I have read amazing things about the Spooky Skeletor's handling, so I compared the two bikes' geometries. They both have a steepish 74 degree seat tube angle for a 52, but the Fuji has a head tube angle of 71 degrees. The spooky, and the other frames I checked out (Tarmac, CAAD9, Xenith SL) have a steeper and more logical 73-74 degrees. Doesn't this mean the SL1 would have slower, more stable steering - a feature out of place on such an 'all out race bike'? IS there a good reason for this?

Classic steel racing bikes generally had 73-73 parallel geometry, right?

Sorry for so many questions, don't feel like you need to answer all of them, my biggest concern is the HT angle.

Rosso Corsa

Nobody can tell me what Head Tube angle does?

Short version: SL1 has 71 degree while other race bikes all have at least 73. Why?

adriano 

a shallower head tube will handle less quickly and more stably.

DaveSSS 

A quick google serach for steering trail will brings up lots of info. A slack HTA increases the steering trail and slows the steering. The seat tube angle work in conjunction with the TT length to result in frame reach.

The formula for steering trail is (R/tanH) - (fork offset/sinH), where R is the tire radius and H is the head tube angle.

It is common for larger frames to have steeper HTAs. In part, it is an effort to decrease the wheelbase. The smallest size may have a 71 degree STA to do the opposite - increase wheelbase, slow the steering and provide a little more toe clearance.

https://www.phred.org/~josh/bike/trail.html

I couldn't find any info on the model you mentioned. It never hurts to include a link to the bike you're asking about.

adriano 

cervelos, neuvations, and jamiss have a 73 seat tube angle along the whole size range. i have read some people questioning the handling characteristics of such a seat angle in the smaller frames, but they do not elaborate. is there any reason that a 73 degree seat tube angle would provide poor handling in the smaller frame sizes? i am not concerned about toe overlap.

DaveSSS 

Seat tube angle has nothing to do with handling. It mainly affects the amount of seat post setback you might select, since each degree move the seat rail clamp forward or back by 1-1.3cm, depending on the saddle height. In my frame size (51cm or next to smallest in most brands) the STA is most often 74 or 74.5 degrees, but some brands will use 73. A 73 degree STA moves the seat rail clamp back about 17mm. That's not enough to allow me to change from a 32mm setback to zero, but changing to 20-25mm setback would work.

The only dimensions needed for fitting are the TT length, the STA and the head tube length. IF the HTA varies by more than 1 degre, I might also include 1-2mm per degree change in reach for the HTA.

For handling differences, I look at the chainstay length, the front-center and the steering trail. If the trail is not listed, you need the HTA and fork offset to calculate the trail.