Saturday, October 4, 2014

A Speed/Wattage Comparison Tool for GPX Routes

Cycling Calculator for a Specific Route 

The side effect of me now having a much better handcycle, aerodynamically speaking, is that I have new questions:
  • What should I expect for average speed given stop signs and wind for a given route?
  •  If I expend a given wattage of work (let’s say a constant 125 watts) over a route with a zero wind speed, what should I expect for bike speed if the wind is xx miles per hour ?
  • What happens to speed when I add stop signs to a route?
  • If I know my bike’s aerodynamic coefficient, the wind speed, and the number of stop signs for a give GPX route, can I calculate my average wattage?

One reason I want this information is, I am thinking about doing the 24hr Sebring.  I have completed the 12 hour Calvin’s Challenge for which I rode 193.5 miles at 16.4 mph average (with an aerodynamically awful bike).  The handcycling WR for 24 hour is 403 miles at just under 17 mph average.  The Sebring course is VERY flat whereas the Calvin’s Challenge is far from flat.  So an enhance bike calculator can show me the expected difference.

I could not find a cycling calculator that answers the above questions given a GPX route – so I wrote my own.  This calculator takes a GPX route and does all the hard math along the many waypoints.  Most of the GPX routes (like those from MapMyRide or RideWithGPS), show a latitude/longitude/elevation for every 10 meters or so along the route.  With the lat/long/elev, one can then calculate the distance and slope from waypoint to waypoint.  With a known wind speed and wind direction, and the above information, one can then calculate the average speed for the route for a given constant energy (wattage) output. 

With all the waypoint-to-waypoint and wind information, the calculations become “interesting.”  Let’s say from one waypoint to the next waypoint, there is an upward slope. In order to maintain the constant wattage (again for this example – 125 watts), the cyclist is going to slow due to the climb.  But when the cyclist then moves on to the next waypoint that is for this example, flat, the cyclist speeds up.  But the acceleration from one speed to another requires work. That work might limit the speed in order to maintain a constant 125 watts.  Also, the rolling resistance will increase or decrease with speed.  All of this can be calculated. 

Acceleration and speed are manipulated all along the route in order to maintain a constant 125 watts of power.

With my calculator, one can also throw in any number of stop signs for the route.  For each stop sign, the cyclist is slowed to 0 mph and then then cyclist must accelerate up to 125 watts.  The average speed drops at the stop and then moves up during acceleration. The slope may help (or hinder) acceleration.

All of the above calculations take into account the wind direction, cyclist/bike weight, rolling resistance and drive-train efficiency. 

So what is the result?  For my bike, I found that the aerodynamics is so good, that in order to improve the performance of the bike, the bike must lose weight!  I am 6”-3” and 190 lbs.  I can lose a few more pounds – but not many.  As well, relative to handcycling, the amputee handcyclist is always going to beat the legged one – unless the amputee is really fat – given that everything else is equal.

Here is the result for standard workout without wind and without stop signs.

If I add the 15 stop signs:

What if I use the same input data applied to the Sebring 3.6 mile course with zero elevation change:

So, as you can see from the above, I “should” be able to go about a 1.66 MPH faster on the Sebring track with the same effort as my nightly workout.  Of course I do not expect to average 19 MPH.  So what do can I expect for average output if I were to maintain 17.4 MPH on the Sebring track?

If I were to change the front fork on my bike in order to accommodate a regular derailleur (I have a heavy I-Motion 9 transmission now), I will drag around far less weight.  Currently, I use the transmission so that I can shift down at a stop sign.  I have over-stressed my shoulders to many times by getting caught in the wrong gear at a stop with fast cross traffic.  But I should not have that problem on the Sebring track.  Here is the calculations with less weight:

As you can see, the weight difference (10 lbs) allows me to go about a ½ MPH faster (or use less energy) on the Sebring track.  That is a huge change!  Anyone that has done long distance events, know the difficulty of gaining just a tenth of a MPH increase over many hours.