Factors in Frame Design

The type of frame I am building is pretty simple, but there are still a number of factors to consider. Many of these techniques would be applicable to building any recumbent. Some of the factors will be inputs into the design and some will be outputs. Because I am building my first frame, I am considering the existing work that is out there, particularly from Volae and Bacchetta, with the expectation that if I am near them in terms of geometry, I'll have something that works. Having said that I am designing this frame from scratch and thinking about all the trade offs.

Lets look at the factors in detail:

• Weight Distribution: I'd like to design this bike so that between 45 and 50% of the total weight is carried on the front wheel. Equal weight distribution is rumored to be nearly optimal in terms of handling. I don't know this from experience but it sure sounds good on paper so I'll go with it as a design criteria. In order to figure this out we need to find our center of gravity, which is dependent on several other things.

• Seat Angle: I need to consider seat angle in order to calculate my center of gravity. I've fixed this for the purposes of this process at 28 deg from horizontal. I built a mockup seat that I could change the angle of and found that to be a comfortable angle for me. Using the same seat that will ultimately be used on the bike would be better, but I don't have it yet.
  

• Seat Reference: The next thing to consider is where we are measuring the seat from. I decided to use the lowest point of the seat as my reference. I will be able to transfer this to the final seat I use later.
  

• Bottom Bracket Height: This is calculated relative to seat height and it's something I don't have a good way to calculate. I could let it be an output of the process and optimize some other variables... but I decided it's easier to just pick a number and design around it. In this case I just copied the bacchetta aero dimension of ~9"

• Wheelbase: This will be an output of the process in my case. I'm not going in with any preconceived notions of what it should be other than to say it should be in the same ballpark as the common highracers on the market. For reference the Aero is in the range of 1240mm and the Volae Team is around 1195mm.

• Seat Height: I wish I could say that I arrived at this through some clever method, but really I just looked at what is out there and picked 24". That's in between the Bacchetta Aero which is around 23" and the Volae Expedition which is around 25".

• Distance from BB to Seat Reference: AKA leg length. This was determined experimentally using a method I'll describe in a later post. It's a very important measure if I want to optimize weight distribution since I'm going with a sliding seat type of build. Using an adjustable boom would make this less important but would be more complex in other areas.
  

• Steering Geometry: This really deserves a whole post of it's own so I'll pretty much say that what I'm going for is something between the Aero and the Volae bikes in terms of geometry. The trail is the most meaningful number when describing different geometries (all other things being similar). For reference the Aero has a trail of 71.5mm and the Volae Team has a trail of 50mm. Taken in a vacuum this would suggest that the Aero is more stable almost to the point of being dead while the Volae is a bit quicker steering. Overall I'd favor stability but I'll probably bring the trail in a little in my design. I'm going to let this mostly be an output however as long as it's within "normal" ranges.
  

• Heel Clearance: I've defined this as the distance between the arc defined by the pedal spindle on a 175mm crank and the top of the front tire. This is not a standard definition AFAIK but it's easy to measure from pictures and works well for designing in CAD. For reference the Aero has a HC of 60mm and the Team of 86mm. I split the difference and rounded up a little and decided to go with 75mm. Again, this is something that only experience will tell me what works well, and I don't have experience so I'll just guess but keep it within established ranges.