At cycling speeds, air is the wall you never see. Roughly 80 percent of a rider’s effort goes into pushing it aside, which is why a streamlined shell transforms the same legs into startling speed. The physics is old; the application is radical.
This article explains drag, frontal area, and laminar flow in plain language, then shows how velomobile shells exploit each one. You’ll understand why the shape works, what the numbers mean, and where the limits are. Physics is the reason the shell wins.
Drag Is the Enemy
Cycling speed is a war against air — and the velomobile wins it with geometry. The physics is unforgiving and beautiful: halve the drag, double the speed on the same watts. This guide explains why the shell makes you faster.
1. The Drag Equation: The Governing Law
F = ½ρv²CdA: the equation (force = half × air density × velocity² × drag coefficient × frontal area; the everything). The v²: the tyranny (double the speed, quadruple the drag — the cyclist’s wall; the 15 mph barrier). CdA: the beatable (the only term you control — the shell’s mission; the 10× reduction).
2. CdA Compared: The Numbers
Upright cyclist: 0.35–0.45 m² (the baseline — the sail; the worst). Recumbent: 0.15–0.25 m² (the halved — the position; the better). Velomobile: 0.05–0.10 m² (the order of magnitude — the teardrop; the best). Car: 0.6–0.8 m² (the ironic — the “aerodynamic” car is the draggiest; the mass + area).
3. Speed on Watts: The Payoff Table
100W: upright 15 mph → velomobile 23 mph (the free 8 mph; the shell’s gift). 150W: 17 → 27 (the commuter’s; the car’s pace). 200W: 19 → 31 (the trained; the fast). 250W + motor: the 35+ (the assisted; the motorcycle’s pace on bicycle watts). The rule: every halving of CdA is ~26% more speed (the cube root — the power law; the math).
4. The Teardrop: Why the Shape Works
Pressure drag: the wake (the blunt rear’s turbulence — the teardrop’s tapered tail; the flow reattached). Skin friction: the surface (the smooth shell — the laminar; the polished). Frontal area: the small (the reclined — the low; the narrow). Crosswind: the trade (the side area — the gust’s push; the design’s compromise).
5. Rolling Resistance: The Other Force
Crr: the coefficient (the tires — the 0.004–0.008; the pressure’s role). Speed: the linear (unlike drag’s v² — the rolling grows gently; the high-pressure win). Three wheels: the extra (the third tire — the 50% more rolling; the aero still dominates). The balance: aero above 15 mph (the crossover — the shell’s domain; the tires below).
6. Real-World: The Theory Tested
The century: the 100-mile (the velomobile’s party trick — the 5-hour; the upright’s 7). The commute: the 20-mile (the 45-min — the car’s time; the bike’s energy). The hill: the weight (the 65-lb — the climb’s penalty; the motor’s erasure). The wind: the shell’s day (the headwind’s defeat — the upright suffers; the velo cruises).
7. Designing Faster: The Engineering
The tail: the longer (the extended — the wake’s shrink; the practical limit). The nose: the rounded (the stagnation — the smooth; the no-sharp). The seams: the sealed (the gaps — the turbulence; the tape). The rider: the still (the pedaling smoothness — the body’s motion; the drag).
Final Considerations
Drag grows with velocity squared (the tyranny), the shell cuts CdA tenfold (the weapon), 100W becomes 23 mph instead of 15 (the payoff), and aerodynamics beat fitness dollar-for-dollar (the economics). The velomobile isn’t faster because of the motor — it’s faster because of the shape. Physics doesn’t negotiate; the teardrop just obeys it better.