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Effect of Wind Drag on Speed

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Most Betterbikes use, or modify, off-the-shelf drive systems. They represent the state-of-the-art and can offer a no-pedaling speed of 20 mph for 20 miles. Other Betterbikes offer speeds up to 30+ mph and distances exceeding 100 miles. This increased range and speed (compared to conventional e-bikes) results from streamlining + extra batteries. Reclined seating reduces the wind drag which is - by far - the biggest hurdle to faster e-bike speeds.

On a regular upright bike pedaled at 20 mph, about 80% of the effort to maintain speed is spent overcoming wind drag, i.e. pushing air out of the way. This graph shows how much energy (watts) is needed to overcome wind resistance (major), rolling resistance (minor), and transmission losses (negligible) on a standard bike (Source: http://www.xsystems.co.uk/machinehead/powercal.html ):

Betterbikes are better because they're safer, more comfortable, and longer-ranged than conventional upright e-bikes. Increased safety comes from being closer to the ground (hurts less when you fall) and having your feet in front (no head-first crashes). The bigger battery load increases possibilities in both range and speed. Combining higher speeds with Barcalounger comfort enables most folks to cover twice the range they would go on a conventional e-bike.

For example, Joe Couch Potato will find that, even with an electric assist, a ride on a standard bike is only comfortable for about 40 minutes. That should take him ten to twelve miles. An electrified recumbert can cover the same distance in 30-33 minutes (21 - 23 mph). That's a factor 1.3 - 1.4 improvement in average speed. When you put Joe in the lounge-chair comfort of a skeeter, you'll find that an hour's ride is practical for Joe in terms of comfort, time, and range. That increase in ride length from 40 to 60 minutes (factor 1.5) combined with the increase in speed (factor 1.3+) yields a doubling of range.




Sent: Tuesday, March 29, 2005 9:10 PM
To: aerobody@yahoogroups.com
Subject: [aerobody] Aerodynamic vehicle coasting speeds (2.5% downgrage)

Where do aero vehicles really show their stuff? Coasting, of course. I have calculated some coasting speeds for a few interesting aerodynamic vehicles (with their drivers) on a mere 2.5% downgrade. On this gentle slope, a conventional racing bicycle coasts at just 24.6 mph with rider in an aero crouch, which is a speed that a strong club rider can maintain on this bike for about one hour on flat ground, before exhaustion.

Vehicles and coasting speeds on a 2.5% downgrade:

Landspeed recordholding streamlined bicycle (Varna Diablo) 95.0 mph! GM EV1 electric car 78.7 mph Toyota Prius gas-electric hybrid car 54.8 mph Honda Insight gas-electric hybrid car 49.6 mphLightning F-90 2004-RAAM-winning "GT class HPV" bicycle 31.3 mph. Conventional racing bicycle with rider in aero crouch 24.6 mph.

To those of you who find it hard to believe that a bicycle exists that can (theoretically) coast down a 2.5% (1.375 deg) slope at 95 mph, I invite you to check that result for yourself. The relevant vehicle parameters, along with pictures of Varna Diablo and information about the bicycle, can be found at http://www.fortebikes.com/Diablo.htm. I used these vehicle specs in my own program for calculating aerodynamic drag and rolling resistance, combined with the standard formula for rolling down an inclined plane. I did not use high altitude thin air in my model. The atmospheric density used was 1.184727451 kg/m^3, based on the Standard Atmosphere Model at sea level with 25 deg Celsius (77 deg F) air temperature.

The improved Varna Diablo described in Diablo.htm subsequently set a new record of 81 mph at Battle Mountain in 2002. In 2003, Diablo blew a tire and crashed at over 80 mph as it entered the timing traps, but the rider, Sam Whittingham, walked away shook up, but with just a few very minor scratches! Links to the 2002 and 2003 Speedbike events can be found on http://www.recumbents.com/whpsc2005.htm, as well as a must-see link to a video of the WHPSC 2004 event.

Gerry Pease

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