DESIGN OF LOW PROFILE FRAMES (Jim Cook MCEI)

 The final part of the whole process is to look at the frontal area to the induced airflow; “ DRAG IS THE SQUARE OF THE SPEED “we are all well aware of a head wind and a tail wind so we are not talking rocket science. About 75% of the rider’s output is used to overcome the effect of drag when riding on a flat road. The obsession of getting as low as possible is not the answer; it has the same detrimental effect as being too high. Is your Aero Helmet blending with your back shape or working as an AIR BRAKE, To prove this try a simple roll down test, you can relate either time / distance to check the effect of your position. . The position of a high diver at the time of entry into water is a perfect example of reduced frontal area. (The SUPERMAN POSITION.) Arms should be extended in a straight line to reduce frontal area, don’t clench your fists it increases the area, if you think this is nonsense drive at 30 mph put your arm out of the window and feel the effect OK! The forward saddle position also increases the separation between the natural scoop effect of leg and chest; this gives the airflow an escape route reducing form drag. Quite a number of riders have remarked that they cannot feel the wind on their back or chest, a secondary effect is their breathing is unrestricted due to the free movement of the diaphragm. In the normal folded position the thigh comes into contact with the abdomen.

Measuring method with rider on diagnostic rig and normal bike on turbo. (It takes about 2-3 hours) beware of the five minute fitting, it produces a standard frame.

• 1. The top tube with Tri-bars will be shorter by approx. 35 mm than the rider’s normal road frame.
• 2. Saddle height to B\B centre is normally 3-6 mm higher, this follows the rule, back and DOWN forward and UP. ie, the saddle height is a radius from the bottom bracket centre.
• 3. Now the fine-tuning can start. Height of the Tri-Bar pads will determine head and top tube height. This is critical, can the rider look up. .
• 4. Saddle and Tri-bars are adjusted fore and aft along the top rail fine tuning the saddle nose to bottom bracket centre line position. An 80 degree seat angle is the most common ie, nose of the saddle approx. 6 cm ahead of B/Brkt C/Line. ( weight is transferred to the forearms)
• 5. The upper body is now supported by the arms; this reduces pressure on the lumber area. ( many riders report that back problems during racing have been eliminated with a steeper seat angle) there is a secondary effect in the rotation of the pelvic area, numbness can result therefore saddle quality is a vital part of the design.
• 6. Break for a short period of say 15 mins in the measuring procedure, the rider should take a ten-minute ride on his normal framed bike on the road. It is essential to have this pause to remove the confusion in body and mind caused by the change in riding position.
• 7. The guiding principle is to relax the muscles not directly involved in propulsion of the bicycle. Pulling on the bars turns T/T cycling into a wheeled version of Nordic skiing. ( check the riders pulse rate )

A frame I designed for Sean Yates in 1997 followed this design route. The starting point was his Track bike, which had a seat angle of 76 degrees, this was his best position up to that time, his body found that an 80 degree seat angle was more efficient and comfortable. This moved the saddle nose forward by 13 cm compared with the standard stock 71 degree T/T frame he was riding at the time.

General notes on frame design.

The idea of using a curved seat post on a road frame to move the saddle forward places the rider’s weight further over the front wheel; the result is an unstable position. A steeper seat angle effectively moves the B/Brkt backward, this maintains stability.

The Head angle will be 72 or 73 degrees with a fork offset of 50/46 mm to produce a trail of 56--55 mm with 700c wheels. Steep head angles transmit more road shocks to the rider. These transmitted micro vibrations contribute to rider fatigue. For a smoother ride get your self a set of “Columbus Muscle” carbon forks they are light at 360g and the best thing since sliced bread, the ride quality is superb without side deflection. The fork offset of 43 mm is compatible with 73-74 head angles.

A stem length of 100 mm places the Tri-Bar mass in a fairly neutral position, steering in a straight line hands off is the norm with this head geometry and Tri-Bar configuration. A shorter stem will be twitchy; a longer stem slows it down.

 It is quite noticeable that frames with road seat angles are subject to PIO’s (pilot induced oscillations) when used with a Tri-bar set up and a trail that is not matched to the head angle. With the saddle nose about 70 mm behind the B/B centre line the rider is basically trying to push the cranks forward, he counteracts this by pulling on the bars, the induced oscillation generated is magnified by the wrong fork trail setting. The trail works as a feedback control system that is quite sensitive to over correction from the mean steering track, result, a steering oscillation in time with the pedalling cadence. YOU ALSO RIDE A GREATER DISTANCE.

 Wider Tri-Bar armrests will improve the steering control and straight-line stability, they also reduce strain on the neck muscles, one of the main causes of neck pain, the rest is plain common sense. Listening to the rider is a vital part of the design process, making suggestions but not transplanting your Ideas into a willing victim. 

The final stage is to produce a full size drawing of the frame so the rider can comment on the finer points to produce his dream frame, I still believe that the look of a frame has an effect on the riders performance.

 Remember that the riders body shape designs the frame. Not the builder. They would rather supply a standard frame; it takes less time to build.