Why lower your pedaling stack height?

In anticipation of the launch of its revolutionary new shoe-pedal system, Q36.5 and SRM worked together with Oliver Elsenbach of Solestar to compile a comprehensive first analysis of the biomechanical advantages of a lower pedaling stack height and what the rider can expect to gain from adopting the new system.

Effects of Lower Stack Height

By reducing stack height the foot is positioned closer to the center of the circular pedaling motion. The surface of force transmission between foot and body is also larger, which improves stability, reduces shear forces in the shoe sole, and enables more direct power transfer. The rider’s center of gravity shifts closer to the ground, allowing for a more aerodynamic position.
The trend toward shorter crank arms has the disadvantage that the rider’s center of gravity shifts upward (away from the ground). This often leads to a frame stack-height issue — too little stack height — which reduces bike handling and confidence in descent. For a crank length reduction of 7.5 mm (from 172.5 to 165 mm), on average, a saddle height increase of about 65% of that value (approx. 5 mm) is necessary. With the use of the new pedals, the stack height adjustment is on average 4–7 mm (saddle lowered). As a result, the saddle height increase caused by shorter cranks is essentially compensated for.

Test riders’ subjective evaluation after uys use shoe pedal system

Better seating position on the bike, a feeling of being more “inside” the bike.
More comfort in the shoulder area.
Power transfer: especially under high-intensity loads (climbing accelerations, criterium sprints out of corners), significantly higher cadence at high loads.
The feeling of “more torque”; at high wattages, it feels easier to ride higher cadences.

Tests on the cyclus 2 ergonomenter

With fixed watt and Newton loads testing showed a higher cadence (under watt load) and higher power output (under Newton load). However, there are still too few tests/data to confirm this with statistical significance. Additional tests with O₂ and lactate measurement should be conducted in the near future to assess the rider’s physiological adaptation.

One pattern was consistently observed among our test subjects:

Left graph: Comparison before — DA pedal (red) vs. Q36.5 pedal with saddle lowered by 5 mm (green). The body shifts approx. 20 mm rearward.
Right graph: The rider is overlaid at the hip point, clearly showing the significant changes in rider position.

Conclusion

Quite simply, the farther away I am from the center of force generation, the more shear forces occur — and the more power is lost to propulsion. The optimum for perfect physical power transfer would be a pedal axle running directly through the foot, with no stack, or even a negative stack of about 5 mm.

All stories