The rear triangles are made of the seat stays and the chain stays. The chain stays connect the rear wheel dropout to the bottom bracket, and the seat stays connect the dropout to the top of the seat tube. The distance between the dropouts is dependent on the wheel and gearing that are desired, and 130 mm is a fairly standard width. My design calls for a 130 mm distance between the two dropouts.
The chain stay length is determined by the size of the wheel and any other accessories, i.e. a fender. With the 700c wheels I've chosen and the desire for a fender, BikeCAD tells me the chain stays need to be 425 mm long. The bottom bracket drop (the vertical distance between the rear wheel axle and the center line of the bottom bracket) for this design will be 73 mm; this is calculated as the difference between the height above ground of the rear wheel axles and the height above ground of the bottom bracket (the bottom bracket height). The BB height on this design is 267 mm and was determined by BikeCAD based on my lower body length, length of the cranks, and to provide for adequate ground clearance in a turn.
Since the seat stays do not fit into a socket at the top of the seat tube, their lengths are not quite as critical as the other tubes. I plan to size them during construction and cut to length. Some more trigonometry yields a seat stay length of 519 mm or so.
The seat stay and chain stay lengths are measured from the axis of the rear wheel - when these tubes get cut, I have to take into account the size of the dropout as well as (as with the other tubes) the length that gets inserted into the lug sockets.
Chain Stay Dimensions provided by BikeCAD
The fork is the final part of the bicycle to be sized; the critical lengths here are the fork blade lengths, the rake, and the steerer length.
BikeCAD provides the length of the fork blades; this length is again determined by the size of the wheel and the included fender. This length is defined as "the straight-line distance between the front wheel axle and the front brake hole" (Talbot). BikeCad provides a dimension from the top of the fork crown to the vertical height of the wheel axle - 375 mm in my case.
The rake is the offset distance from the front wheel axle to the center line of the fork blade. This offset affects the ride quality and steering; less of an offset provides quicker turning but a stiffer ride. I opted for a 50 mm rake, hopefully for a relatively smooth ride with adequate steering quickness. The rake length is a design choice.
The steerer tube connects the fork crown - where the fork blades connect and the (caliper) brake assembly is mounted - to the upper half of the headset. It must be long enough to fit into the crown, through the bearing between the crown and the head tube, through the entire head tube, and the upper bearing assembly of the headset. At this point, I need to decide on a headset and measure the depth of the hole in the fork crown for the steerer; since I haven't done this yet I don't have a dimension for the steerer. I have a set of calipers on order and will measure the crown when they arrive.
Fork Dimensions provided by BikeCAD
That's the entirety of the design process for now. Talbot suggests making a half-scale or full-scale drawing, but I think that having the BikeCAD model on a computer in the shop should be sufficient for my needs. I may sketch out full-size drawings of the joint areas if I think I need them for fitting the tubing together.
I'm reading you post all the way from the beginning.
ReplyDeleteI just got the pro version of BikeCAD and having some fun around with it too. A TIG-ed 'cross frame to come too.
I know this is an oldddd post but I cant help commenting about your geometry.
Its a surprisingly nimble and racy geometry. considering that it's the same geometry used by some of more distance oriented bikes like the Roubaix. It's an awesome geometry!