Hello craigas2 and All, As with a rock in a water stream - downstream draft depends the velocity. direction, and the size of the object providing the draft and the properties of the fluid medium.
Also from a practical standpoint cross winds (which can dissipate and/or change the angle of a bicycle draft) can put the draft out of reach sometimes.
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Monday, 09 August 2010
WTC changes drafting rules for IM WA A 12-metre draft distance will be enforced for the first time on the Ironman Western Australia bike course this December.
World Triathlon Corporation has initiated the rule to ensure an equal race for all competitors by stamping out the drafting issues created by the old seven-metre rule.
“The 12 metres will be measured from the rear wheel of the leading bike to the front wheel of the following bike,” WTC Australia general manager Shane Smith said.
and from the Athletes Guide IM Kona 2010:
Age Group Athletes:
a. Absolutely NO DRAFTING of another bike or any other vehicle is allowed.
b. Athletes must ride single file on the far right side of the road near the white line except when passing another cyclist. Side-by-side riding is not allowed.
c. Cyclists must keep a 7-meter distance (approximately 4 bike lengths) between bikes except when passing.
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On the show Mythbusters drafting behind an 18-wheeler truck was tested and results showed that traveling 100 feet (30 m) behind the truck increased overall mpg efficiency by 11%. Traveling 10 feet (3.0 m) behind the truck produced a 39% gain in efficiency.
Of course, they warn that this type of driving "is insane" because the truck's blindspot is in that area, and if the truck stops quickly there is much less time to react.
Truckers are not fond of the extra stress this puts on them, worrying about cars on their tail. Additionally, it can be very dangerous for the following car if one of the truck's tires (or their Retread delaminate as the chunks of ejected rubber can be large enough to cause serious harm, even death, to a driver following too closely.
http://www.inlandempirecycling.com/...nes.htm#_Toc48401232 DRAFT ENVELOPE
The Draft Envelope in a pace line of fewer than six riders is about six (6) feet long (about the length of one bicycle). If you are further behind than six feet, you are out of the draft. Larger packs of riders riding in parallel will create wider and slight longer drafts. The same is true for one or more tandems. Cross winds change the position of this envelope. If the winds are coming from the left, then the envelope is moved to the right. When riders in a cross wind situation ride in a diagonal formation, it is known as an echelon. For winds coming from the left, then it is a right echelon. (Winds from the right generate a left echelon.) Often on the many narrow roads in California , it is difficult to safely ride in a left or right echelon with the usual moderate to high traffic densities.
WINDS
Winds will generate several problems for a pace line. Head winds or head winds within plus or minus 30 degrees of front will require that the period of rotation be shorter. For example, ten mph head winds would require the number of pedal strokes to be decreased from a range of 60 to 80 to a range of 50 to 60. Head winds of 20 mph could decrease the number of strokes to 30 to 40. At 30 mph winds the range could get to down to 15 to 20 strokes. When winds get above 10 mph, pace lines of two or three riders deteriorate in the speeds that they can maintain as the wind speeds approach 20 mph. As winds become more of a cross wind (that is, from 30 to 120 degrees from the front), then the draft pocket begins to move to one side of the rider ahead of you. In fact, the draft pocket can be at the side and slightly back (about a foot or so) of being adjacent (parallel) to the rider ahead of you under the most extreme situation.
This depends on the wind speed and the direction of the cross wind (typically 15 to 25 mph and wind directions of 70 to 110 degrees from the front). When cross winds are present, then the pace line forms an echelon. If the winds are from the left, then the current leader rides as far to the left as safely as possible (depending on road conditions and traffic) with the rest of the pace line staggered to the right of the leader. This is known as a right echelon. Similarly, if winds are coming from the right, then the leader rides as far as to the right and the rest of the pace line forms a left echelon. For a right echelon the leader always rotates by pulling to the left and dropping back. With this right echelon the old leader begins to move to the right while continuing to fall back.
Once the old leader has cleared the new leader, then the new leader gradually moves to the left where the old leader was and the rest of the pace line does the same. This creates a space for the old leader to pull into on the right-hand side. In extreme cross winds, the old leader needs only to drop straight back and move diagonally to the right. When pace lines are riding in a right or left echelon, then there will be an overlap of front and rear wheels. However, there is a significant difference. Here the lateral separation between adjacent riders is typically two to three feet and not two the six inches. Since the pace line riders are to one side, the riders have a clear view of the road conditions ahead.
Can you tell which direction a cross wind is coming from? Yes. Note which side of your body is cooler or which side is warmer. If your left side is cooler (or your right side is warmer), then the cross wind is coming from the left. The reverse suggests a right cross wind. Of course, a flag on a flag pole, tall grass, or trees (if present) is even better.
Remember, Right and Left Echelons require lots of room, but traffic/road conditions may not allow its safe deployment, unless roads are closed to traffic.
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Keeping in mind that the effects of drafting decrease with separation between bikes and the following is for normal close drafting:
From
http://bikerackheads.blogspot.com/search?q=watts Excerpts:
Upright: 260 watts
Aero: 215
Drafting a cyclist: 180
Drafting a car: 120
Drafting behind a normal-sized rider at 20 mph saves you 80 watts; drafting behind a 6'6" rider saves you 81.48 watts.
This is crude and assumes a lot of things remain constant.
Still, this suggests that drafting behind a larger person does save a small amount of energy. Over an hour or two, this adds up.
Also, at higher speeds, a big booty will offer even greater savings in power output. My guess (commoner for "extrapolation") is that at 30 mph, drafting behind a large person compared to a smaller one is probably at least a 20-30 watt advantage.
Cheers,
Neal
Cheers, Neal
+1 mph Faster
