Frame building theory is about as different as the builders themselves. Jalon starts out making a clear point that the main purpose of the frame is to support the rider and the bicycle’s components. From there it has to provide a riding quality as well as tracking capabilities that follow the rider’s style of riding. If all we were concerned about was the first point, we would probably be still making frames out of hardwood. However, when riding quality is entered as an element, this is where frame builders separate in philosophy.
Geometry plays the next important factor then, for regardless whether the finest materials are used, the overall ride can be catastrophic if the bicycle doesn’t fit the rider. Frame geometry then is determined by fork rake offset. This measurement is taken off the centerline of the fork steer and is directly proportioned to the length and rise of the stem. Over steer and under steer is direct result of variances of this theme. Neutral steering is found when the builder decides on other factors such as top tube , seat tube, and chain stay length; bottom bracket drop; seat tube and head tube angles. All are decided on after an extensive discussion on riding style and body measurements with the customer. Generally speaking, fork rake is usually predetermined by the manufacture. When rake is established, the rest of the frame geometry can then be decided. Jalon believes all frames should have neutral steering-with the exception of crit and trials frames- and should steer from the seat of the pants; that is, steering should be caused by leaning either one way or the other on the saddle. This is the result of an understanding of perfect balance. Experience is the only source for understanding these perimeters, and Jalon has over 100,000 miles of fat tire riding, and 176,000 miles of road riding to help fit his riders to their “dream machines”.
Perfect balance is caused from proper stem length which is somewhere between 120mm to 135mm depending on fork rake and riding style. Stem rise should be kept from 0deg. to 10deg. depending on rider style. Top tube length is decided on by seat tube and head tube angles and desired wheel base, all which are proportional to rider’s torso and arm length. Depending upon frame use, these areas can change vastly. For instance, one may be interested in a “mountain bike”. This could translate to “ I am going to use it for slow speed single track( short wheel base, neutral angles), occasionally do some down hill racing ( long wheel base, shallow angles), some long distance touring (medium/long wheel base, neutral angles), and commute to work (medium/short wheel base, steeper angles). EH?
Bottom bracket drop (not height) is the correct term for locating bottom bracket position. Tires vary in size so much -especially in ATB- that your bottom bracket height could change over an inch! Drop refers to distance the bottom bracket axle centerline is in relationship to the wheel axle centerline, which always remains constant. This distance is critical to center of gravity -balance- and clearance for pedals when cornering. Chain stay length plays a factor in over all wheel base as well as character of ride. On ATB frames, chain stay length decides how well the rear tire will grab on hill climbs and plays a roll (bad pun) with stability on descents. On road frames, the tighter the rear triangle the stiffer it becomes which translates to a more efficient -but less stable- machine. Touring frames typically have longer stays to offer more heal clearance for packs while offering better stability by increasing wheel base.
Head tube and seat tube angles are two separate frame building considerations. The head tube angle affects steering while seat tube angle positions the rider over the bottom bracket. The steeper the head tube the faster the steering. The seat tube angle affects rider balance over the top tube, while becoming a factor in which leg muscles are used during the pedal stroke. Again, extensive discussions about intended use as well as rider’s style determine these factors. Now let us move onto the tubing choices!