a design, finally

this is damn close to what i'll be aiming for...with the understanding that all this is theoretical, the finished bike may be off a bit, but better to design precisely and build imprecisely than to design imprecisely and build imprecisely. and yea i'll be trying to make it precise.
the area that needs work now is the front end/steering geometry...i don't have the headset in hand yet, so i need to find out its exact lower stack height before locking anything down and building a jig.
the top tube is a compromise between fit and toe clearance. this is one area where i found an enormous amount of conflicting info. Paterek claims front center determines toe overlap--that 58cm is the magic number over which it's not a problem. R. Sachs says not to worry about it--that fit is infinitely more important. a recent discussion on the framebuilders list went on for a week.
i tried to strike a balance between the two Rivendell models i'm copying, the Atlantis and Ramboulliet, and my TT length is about mid-way between the TT lengths of their 58cm sizes. ditto for the chainstays, which are shorter than i had originally planned, but should make the bike a little more maneuverable.
i'm going with a 71.5° HT, a half degree shallower than planned, to allow for a short TT and decent toe clearance. this stretches things out on the front end a bit, so the chainstays got shorter to make up for it in handling. hopefully the wheel flop will be OK. i kept trail 58-59 cm with a 30 mm tire by increasing fork rake 2 mm. the front end is by coincidence very close to the cannondale touring model, which i'm planning to test-ride before building.

i'll attach a drawing soon; it's stuck on my home computer with no WWW access.

in cm unless noted
this was all done in BG 5.1 by Martin Manning, so the info below will be most applicable to users of that program.

me:
inseam 85.0
sternal notch 148.0
torso 63.0
arm length (avg) 65.75
femur (thigh) length (avg) 52.2
foot length (avg) 26.35

calculated:
saddle height, BBc-saddle top 73.64

design specs:
virtual ST 58.7
virtual TT 57.0
HT angle 71.5°
ST angle 72°
CS length 44.4
BB drop 7.75
fork rake 5.2

@ 700x30 tire
wheelbase 104.15
front center 60.93
BBc height 26.35
trail 59.3 mm
standover, mid-TT 82.18
TT angle to horizontal 2.5°

TT c-c 56.26
DT c-c 63.12
HT length c-c 16.23

fork:
front brake reach 64.0 mm (designed for cantis, long calipers or centerpulls possible)
HT upper free length 20 mm
HT lower free length 6 mm
fork length on steerer axis 38.75

tubing:
seat tube Deda ZeroUno 28.6 mm
down tube Deda ZeroUno 31.7 mm
top tube Deda ZeroUno 28.6 mm
head tube Deda ZeroUno 31.7 mm
steering column TT Verus 25.4 x 240 mm
chainstays Deda ZeroTre 29.9 x 16 mm single bend
seat stays Deda ZeroUno 16 mm
fork blades TT Verus 27.6 x 20 x 400, 1.1-0.6 mm

frame parts:
seat lug Long Shen 74° long pt
top head lug Long Shen 74° long pt
bottom head lug Long Shen 60° long pt
BB shell Long Shen 60.3° x 62.3° x 7.3° x 30x1
fork crown R. Sachs/Long Shen Newvex
top eyes/seatstay caps Silva 16 mm
rear dropouts Tecnociclo 70° 2-eyelet
front dropouts Long Shen stainless 2-eyelet plug-in
canti bridge
canti pivots
chainhanger Columbine Quikchainger
brake and chainstay bridges 1/2" 4130 CrMo
bottle bosses stainless, dead-ended
rear rack mounts
lowrider bosses

equipment:
cranks 175 mm 26-36-48t
cassette 9-speed 11-32t
rear hub spacing 135 mm, 2.5mm narrower if possible
steerer 25.4 mm threaded
stem 80 mm Nitto Technomic 26 mm
bars 45 cm Nitto Randonneur 25.4 mm
nitto shim 26.0->25.4 mm

i assumed a lower headset stack of 14 mm and an upper stack of 19 mm based on info in Paterek and specs on the Harris Cyclery site. this is until i can get a measurement of the actual headset to be used, likely a Ritchey Logic.

I decided to design for a mid-length stem, so in the event that I'm off on the TT length, I'll have a few cm leeway available in both directions.

2 comments:

Anonymous said...

A couple of thoughts to share regarding your design....
1) You're using relatively fat tires which offer you more options. You should pick up the next to last (I think) issue of Vintage Bicycle Quarterly with it's discussion of pneumatic trail. Bottom line, you don't need the shallow front angle and may well produce a better handling (less wheel flop) frame without the shallow angle. The lose of geometric trail related to a long rake and steep fork gets made up for by the pneumatic trail. Must Read.
2) You spec an 80 mm stem, which is quite short. According to your statement, you've designed for a "mid-length" stem. By this I assume that you designed for a 100-110mm stem. This is a much better length to use when climbing as it helps keep weight on the front wheel for better traction/control.
3) I seem to recall that this is intended to be a practical bike. Carry things, carry you, rain or shine, etc. Carrying weight and going slowly make toe clearance more important. BTW, I take seriously all of e-Richie's advice. The bike should stll fit you properly, even if you need to avoid toe overlap. So, what's a guy to do (besides very slack angles)? Especially if you need a shorter top tube? One of two things:
a) Make the seat tube steeper bringing it closer to the bars. I have a 58 CM ST c to c with a 56.5 CM tt c to c. A 74 degree seattube and 73 degree head tube. No overlap.
b) Use a smaller tire (650B or 26").
Chose one of these courses and you can have fit, handling, and no overlap.

Martin Manning said...

Re Anonymous' comment a) about using a steeper seat angle to shorten the cockpit- That doesn't work. The saddle goes where it goes relative to the pedal arc regardless of the seat angle.