I played around a little in BikeCAD this afternoon, after getting my frame fitting results back. I'm trying to shoehorn my 6'5", 94cm inseam body around a fast commuter frame to fit 700x38c tires+fenders (and an EBB).

Does this geometry look reasonable? It's 72.5 head angle, 63.5 C-C seat, 63 C-C top. Using a Dimension cross fork (400mm A2C, 45mm rake) the trail ends up being about 62.5mm, the Jones -2.4, and the front projection .067.

This is my first frame design, so I really don't know what I'm doing. Any advice would be welcome!

Looks ok to me!

Braze 'er up and tell us how it rides!

Front looks to be a bit steep in the head angle and might be on the twitchy side. That is just my personal preference, I like a slacker head angle.

Looks good, should work fine. Although I think you'd be better balanced if the chainstays were a bit longer.

I would go with 71.5 or 71.7 degree head tube angle as this is going to be a BIG frame and with correspondingly longer chain stays in the back, my 2 cents worth

Thanks all, I appreciate the advice. I'll add 10mm to the chainstays, since I'll be riding it semi-upright most of the time and will probably have lots of weight on my butt.

What I really want to get is the 'carvy' feel in making tight turns in small city streets at 20km/h. Sort of like a criterium, but with less pedaling in the turns. That's why I put in the 72.5 head angle. Fingers crossed!

That's what I prefer too- stick w/the steeper HA, you'll be glad you did.

I'm assuming this is a steel frame....


Frame Angles
*Headangle = 72.5∞
*Seatangle = 72.5∞ (71.5 if using a straight post)

Major Lengths
Wheelbase = 1088.5549 mm
*Seat tube length (c-t) = 690.0 mm
Seat tube length (c-c) = 638.6374 mm
Top tube length (effective) = 632.68585 mm (Might be a tad long for a commuter)
Top tube length (c-c) = 630.00476 mm
*Head tube length = 200.0 mm (215)
*Bottom bracket height = 271.5 mm
Bottom bracket drop = 77.5 mm (70)
*Chainstay length = 440.0 mm
*Front Center = 660.0 mm

Fork and Headset Specs
*Fork offset = 45.0 mm (48-50mm would be better with 72/72.5 HTA)
*Fork length = 400.0 mm (might be a tad short)
This is a rigid fork
*Lower stack height = 11.0 mm

Stability Info
Trail = 62.855465 mm
Jones stability criterion = -2.4148805
Relative front projection = 0.06759858

Tube Diameters
*Seat tube diameter = 34.9 mm (31.8)
*Top tube diameter = 31.8 mm
*Head tube diameter = 38.1 mm (36)
*Down tube diameter = 50.8 mm (38.1)

Seatstay Info
*Seatstay diameter = 22.2 mm (16mm if using caliper brakes)
Direct distance from rear axle to center of seatstay at seat tube junction = 566.4608 mm
Sideways angle of seatstay = 6.1623745∞
Seatstays are straight
*Distance from top of seat tube to center of seatstay = 40.0 mm
*Location of center of seatstay at seat tube junction (0∞ = rear of seat tube, 90∞ = side of seat tube) = 45.0

Bottom Bracket Info
*Bottom bracket length = 68.0 mm
*Bottom bracket diameter = 57.2 mm (38.1)
Chainstay angle relative to horizontal = 10.356807
Seatstay angle relative to horizontal = 67.59641

Welding Clearance Info
*Head tube extension above top edge of top tube = 8.0 mm (10mm)
*Head tube extension below bottom edge of down tube = 10.0 mm (15mm)
Gap between bottom of top tube and top of down tube at head tube junction = 90.58825 mm
*Seat tube extension above top edge of top tube = 40.0 mm (20)
Extension of outer face of BB from side edge of chainstay = 3.7928886 mm
Gap between inside edges of chainstays at BB junction = 15.585777 mm


My 2c

Thanks Thylacine. I've shortened the seat tube extension above the top tube to 30mm. The 77.5 BB drop and 57mm shell is because I'm using an eccentric - I figure I'll lose about 7-8mm from rotating the eccentric upwards to tighten the chain.

As for more fork rake... well, it's steel, I'll just have to test-ride it and bend it if necessary.

Followup: The frame rides great, it's just as carvy as I hoped for. I did get a bit of pedal strike when riding home drunk, and the top tube is definitely 1cm too long, but them's the breaks. Thank you for your advice.

How about some pictures!?!

As you wish! (it's a bit ugly, I built it up with cheap used parts from the local bike co-ops)

nice!
where did you order such a custom frame? how much?

Three years later on my XACD chinese Ti frame:

* I specced the BB too low, top tube too long (as I mentioned before)
* XACD specced the tire clearance symmetrical on top and sides. This resulted in unneccessarily bent chainstays (didn't need 1cm side clearance)
* Cable guides not ideally placed - rear brake cable guide too close to caliper, Rohloff chainstay guide doesn't secure cable around bends (could be solved be straightening chainstay)
* Top tube too high - straight top tubes are too 1970s. Should have sloped the top tube 10-15 degrees, and gotten a taller seatpost.
* Fork was too short. I replaced it with a 410mm carbon fork which works great.

Other than that, it's been good!