Hida Yanra

Alright - I've got a geometry question to ask all you lovely people. (three of them actually, "a", "b", & "c" - bolded through-out the post)
I bought a 52cm Redline Disc-R from another BFer recently, and I'd like to learn how to calculate for stem/seatpost, setback, etc.

As a bit of background, the Disc-R is a road/cross hybrid with disc brakes (I live in the rainforest, it will be nice). I have the numbers from a very good fit session about a month ago - but they are in stack/reach, and I am not confident with my conversion math/drafting yet. The new bike is more slack than the Allez I usually ride/race and I'm okay with that as it will be a commuter/rain bike - but I will be training with it during the winter so I'd like a similar fit.

I've built up bikes before, but they have always been standard-ish road bike geometry 56cm, and this one isn't. FWIW, I prefer a 54/55cm road bike, and a 53 cross bike - and my flexibility is improved to the point where I think I can handle going a bit smaller. I'll post my numbers, and I'd love to know:
a) What stem/setback numbers I should be looking for
b) How to arrive at stuff myself - I tried, but I'm unsure since the angles are a good bit different.

I'm an architect and good with autocad so I could draft it up, but I don't know what/where to make allowances for measuring differences vis. Center-Center vs Center-Top, etc.

New Bike Specs: Disc-R - 52cm (unsure on year, '05/'06?)
Seat Tube Length, Center to Top 520
Top Tube Length (Horizontal) 535
Chainstay Length 435
Seat Tube Angle 73
Head Tube Angle 71
Head Tube Length 107
Rake 45
Wheel Base 1005
Standover Height 77.3
BB height 26.5


Current Bike Specs: Specialized Allez '06
Size 56
Seat Tube Length, Center to Top 520
Top Tube Length (Horizontal) 565
Bottom Bracket Drop 69
Chainstay Length 405
Seat Tube Angle 73.25
Head Tube Angle 73.5
Head Tube Length 165
Rake 43
Trail 56
Front Center 591
Rear Center
Wheel Base 986
Standover Height 765
Stack 59
Reach 38


My personal fit coordinates -
Handlebar Width 42
Stem Length 80mm for 56cm (at 16deg flipped down)
Crank Length 172.5 (C: I've got a 170cm crankset for the new bike - I assume this gets adjusted for in the seat height?)
Seat Height 91.8
Setback 21.1
Frame+Stack 59.0cm
Reach 44.6
Drop 6.5 (Per the fitter's recommendations I'll be moving to 8.5cm in the next day or so)
Cockpit 62.4

iansir

Can't wait to see that Redline build

eippo1

I understand the need to look at the numbers, but the best way to check using a bike fit thingy like the Serotta one or just try aggressive vs. relaxed geometry bikes and compare them to your Allez. Both the Tarmac and Roubaix for instance would feel different than the Allez.
That said, you shouldn't bother with CAD, go with Sketchup or Rhino. I'd say Revit, but then you'd be out on a ledge after just trying to model the top tube.

old and new

Why not consult Competative Cyclist ? They've a very good on-line fit.calc. They ad. here. Their ad. is omni-present, as I type. I see an ad. to click-on.
I don't see what you meen.. crank length/ seat height. Crank length may be addressed by the calc. Lately longer cranks are favored. Nothing wrong with 170 just that med./lg. to lg bikes use 172.5s. Very much a personal pref. choice. This is apparent in new bike ads. DO use the on-line calculator.. it's FUN !!

ridethecliche

Don't overthink it.

Saddle tip to bars.

Setback relative to bb from saddle tip. Saddle to BB. Match them up and you're good to go.


Or you could do this.



Take a huge sheet of paper and put it on the wall. Find a few 'fit points' on your good bike and mark them on that paper. I.e. trace out your bike. Take the new bike and make it match.

Done.

DaveSSS 

Comparing two frame for fit is not difficult, but it's odd to be trying to make two frames with such large differences in TT length and head tube length fit the same - if that's what you're trying to do. Most often, the differences will not exceed 2cm. In that case, take the frame with the steeper STA and add 9mm per degree to the TT length, then take the difference to get the difference in stem length. HTAs make some difference in the fit, but it's usually only 1-2mm per degree and the HTA are not usually that much different. In your case, the Allez may require about a 35mm shorter stem, since the TT is 30mm longer, the STA is a little steeper and the HTA is also a little steeper. Both of those add to the reach.

The diference in the STA is very small. The seatpost would only need 3-4mm of additional setback. This is calculated with the formula: (cosA-cosB) times the saddle rail height.

To further complicate things, using different bars or saddle can change the seatpost setback required or the stem length.