DESIGN OF LOW PROFILE FRAMES (Jim Cook MCEI)

 The major change in design of a typical Lo-Pro frame is the steep angle of the seat tube, it looks like a funny bike because we are conditioned to the look of a typical road frame with a 73 degree head angle with a 72 degree seat tube. The dedicated low–pro frame is a completely different animal, it is almost a different discipline to conventional cycling.

 The penny drops when we see a rider on a dedicated frame, rider and bike move as one smooth unit, it is immediately apparent that the pedalling has a more fluid action. On the modified road frame the rider appears to be sat on top of the bike fighting it all round the course with a position that is made worse by the addition of Tri-Bars. As a direct consequence the rider suffers back pain and assumes that if it hurts he must be going faster.

The rider on the dedicated Low-Pro returns an improvement in performance of 4—9 sec per mile over a tri-bar modified road frame. Of course good legs, heart and lungs help.

 What follows is the result of 10 years designing and building dedicated Low-Pro frames, none of this would have been possible without the feed back from riders using dedicated frames. I would also include the work of Ian Garside BSc, at JM Liverpool, his work on the effect of seat tube angle and GME in a series of tests on nine riders in his research project endorsed my conclusions found by empirical methods.

 Steeper seat angles are not a recent innovation, during the thirties continental track riders used seat angles of 76’ degrees +.  In 1937 a British frame builder and track rider (Dick Swann) on a visit to a Belgium track meeting asked a rider why he rode a bike with a steep seat angle, the answer was, it’s faster.

 We’ve all ridden on the rivet, all gone through the pain of the last mile of a ‘25’. On the rivet had moved our position forward by approx 6 cm. A rough guide is 1 degree on the seat angle = 1 cm on the saddle nose. They knew in 1937, we knew it was faster in 1950.

 Then came Graeme Obree, with a stroke of Genius he changed the whole concept of riding position restricted by conventional frame design.

Two factors that contribute to faster times on two wheels were now obvious. Reduced frontal area, (the tucked position). and Greater Mechanical Efficiency, Old faithful had an 85 degree effective seat angle this increases the output of the pedalling action (equal wattage at a lower pulse rate).

Design objectives.

• 1. Is the rider comfortable in the new position?
• 2. Has the pedalling efficiency improved? GME.
• 3. Is there a reduction in the frontal area profile to the air- flow?
• 4. Can the airflow escape the scoop effect between upper body and thigh?
• 5. Has the rider’s output wattage increased for the same pulse rate?
• 6. Will the frame transmit the rider’s output with the minimum of loss due to frame flexibility?
• 7. Is the tubing type matched to the rider’s weight?
• 8. Will the finished frame give the rider pride of ownership?
• 9. Will it hold a steering line with the minimum input from the rider?

Design Procedure

  The method I use to find the optimum position is by comparison and measurement of output. I mount the rider’s current bike on a turbo trainer, preferably one with a magnetic resistance unit. Then transfer the key measurements to a dynamic simulator consisting of a base unit with 700c wheel and computer controlled resistance unit. (COMPUTRAINER) The system highlights the change in pedalling action when the saddle nose is behind the B/Brkt C/Line and the increase in GME with the saddle nose forward of the centre line.

 In other words steep seat angles increase the mechanical advantage; your legs are a simple system of levers and pivot points. The pedalling force is now more vertical to the B/Brkt centre. (as in a reciprocating engine configuration)

 A top rail with slides carry the saddle and Tri-Bars, these are fully adjustable in the X& Y planes. With adjustments of 43--66 cm in seat tube and top tube length the simulator accommodates all road or low-profile frames.

 The rider can compare the original position with the new simulated position under load; repeated changes between bike and rig highlight the effect of changes to the key measurements. In other words good old common sense prevails. Whenever possible the rider’s saddle and Tri-bar set up is installed on the rig, this produces accurate measurements for the new frame.

  Newton’s third law of motion states that for every action there is an opposite and equal reaction, if your saddle is behind the b/bracket and you push hard on the pedals the opposite reaction is to pull on the bars. Trying to bend the bars will not increase your speed, only your pulse rate will go faster. That’s why moving the saddle forward to a position that you push down on the pedals without pulling on the bars increases your GME. (Check your pulse rate) blood flow is restricted in muscles under tension, therefore return flow to the heart is reduced. ( remember you only have one pump to power the whole hydraulic system , why misuse it )

 A recent test on a top rated rider showed a reduction of 7 bpm when the effective seat angle was increased to 81 degrees from 72 degrees. There seems to be a relationship between pulse rate reduction and seconds per mile, i.e., 6 sec = 6 BP. It may sound too glib but it has been noted during debriefing sessions on numerous occasions to be a simple coincidence.