Frame Design

The modern bicycle frame is designed around two triangles connected by a single side, the seat post. The rear triangle is made up of the seat stays, chain stays and seat tube. The forward triangle is actually a quadrilateral made up of the seat tube, top tube, down tube and head tube. Depending on the style of riding a bike is designed for the seat tube angle, head tube angle, fork rake, chainstay length, and bottom bracket drop vary within a small range of values that greatly affect the performance of the bike.

I would break up types of bike frames into two general categories, road frames and mountain frames. I would then break the category of road bike frames into the following categories: road racing, endurance road, road touring, track/fixed-gear, cyclocross and finally time trial. Mountain bike frames I would break into the following categories: race, cross-country, cross-country trail, downhill/freeride, all mountain, 4x/dirt jumper, BMX, and hybrid. The real difference between all these different types of bicycles is in the geometry.

The seat tube angle determines the leg position in relation to the crankset, as Todd Downs points out in his book, generally steeper angles are better for a higher cadence while shallow angles allow for comfort and a slower cadence.

The next important measurement is the head tube angle coupled with fork rake and trail. Starting with head tube angle, shallow head tube angle increases ride stability while having a sharper head tube angle sharpens handling, Downs makes the generalization that racing bikes will have sharper head angles that touring or trail bikes. But the head tube angle is not the only important measurement when it comes to handling. Trail is a combination of head tube angle, fork rake, and wheel diameter which Karl Anderson describes as, “ the extent to which the front wheel of a bicycle follows its steering axis” in his essay on bike geometry and handling. Trail is found by drawing an imaginary line down the center of the head tube to the ground, now the distance from this point to directly below the axle of the wheel is the bike’s trail. But how does this affect handling? Anderson goes on in his essay describing the effects of trail which in short are, the longer the trail the softer and more stable the handling of the bike is, the shorter the trail the sharper the bike’s handling will be.

Chainstay length also affects the handling of a bike. Downs generalizes in his book that shorter cahinstays can make the bike handle more sharply, due to added rigidity in the rear triangle as well as shortening the wheelbase, while longer chainstays allow for softer handling.

The last aspect of frame geometry I am going to talk about is bottom bracket drop. Downs outlines in his book that a larger bottom bracket drop lowers the saddle and the center of gravity for the bike, which provides a more stable ride, however race bikes often have smaller bottom bracket drops to allow for faster handling as well as allowing for racers to pedal through turns. Mountain bikes have smaller drops to allow for clearance for rocks and other things they may roll over.

• Anderson, Karl. "An Introduction to Bicycle Geometry and Handling.." dclxvi.org. N.p., n.d. Web. 17 Mar. 2010. .
• Boston, Bill. "Bicycle Fit." Cutting Edge Bicycle Technology. N.p., n.d. Web. 17 Mar. 2010. .
• Brown, Sheldon. "Revisionist Frame Sizing--by Sheldon Brown." Sheldon Brown-Bicycle Technical Information . N.p., n.d. Web. 17 Mar. 2010. .
• Downs, Todd. The Bicycling Guide to Complete Bicycle Maintenance and Repair: For Road and Mountain Bikes(Expanded and Revised 5th Edition). 5 Rev Exp ed. Emmaus, Pa.: Rodale Books, 2005. Print.
• Nicol, Scot. "Ibis Cycles, Inc :: Materials 101." Ibis Cycles. N.p., n.d. Web. 17 Mar. 2010. .
• Wilson, David Gordon. Bicycling Science, 3rd Edition. 3 ed. London: The Mit Press, 2004. Print.