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I commute and want to draft another bike, but still have room to stop when someone opens a car door in front of the rider in front of me. At what distance does drafting significantly reduce drag? Are there factors other than following distance that influence drafting efficacy (wind direction, etc)?

This is not a question about whether the drafting while riding through urban environments is a good or bad idea. I know that it is probably a bad idea.

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Are you talking about drafting another bike, or a car? If a bike, you have little choice; you'll have the be directly behind them. If you're talking about drafting a car, I'm closing this question as blatantly unsafe and illegal behavior. –  Neil Fein Oct 14 '10 at 21:00
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Clearly, I am referring to bicycles. See first sentence. –  Precipitous Oct 14 '10 at 21:10
    
It may be that the conclusion from the answer "Effective drafting stops a x feet." is that effective drafting is not possible with a following distance. I don't think that this is a legal question in my area: Laws for bicyclists behavior in my area don't go farther than wearing a helmet and using a light at night. –  Precipitous Oct 14 '10 at 21:14
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That's fine. I found your opening sentence to be unclear, though; I added "another bike". I suppose the "the rider in front of me" should have tipped me off! As odd as it seems, there are many, many cyclists who do draft cars, trucks, and buses. (Also see Ben's answer.) –  Neil Fein Oct 15 '10 at 0:44
    
It depends on how good he/she looks in lycra –  mgb Jan 25 '11 at 23:22

3 Answers 3

up vote 22 down vote accepted

My opinion:

I don't believe you can effectively draft and have enough time to stop. I don't know the exact aerodynamics (and I suspect it's affected by speed and wind), but if you watch any paceline or peleton, they're never more than a couple feet (about half a meter) behind the rider in front, often only a few inches (centimeters) behind. With a crosswind or larger groupings, they're even likely to have wheel overlap (where the front part of the front wheel of the following cyclist is next to the rear part of the rear wheel of the lead cyclist).

You can really only draft safely with the cooperation of the lead cyclist, who needs to signal back to you before starting to stop.

Do the math:

Let's put it another way:

  1. If you're going 15 miles/hour (25 km/hour)
  2. That's also 22 feet/second (6.9 m/second)
  3. And if you're following 3 feet behind (1 meter) (a bit far for drafting)
  4. You have 0.136 seconds (0.145 if you take the metric rounding version) to react to something that causes the lead bike to need to brake suddenly to avoid colliding.
  5. If you're 2 feet (2/3rds of a meter), you have less than a tenth of a second. If you're 2 feet and 20mph (32 km/hour) you have about 1/14th of a second to react.

Less than a sixth of a second to figure out that the lead bike is braking and to start braking, and that's assuming your brakes are as good or better than the lead bike's brakes. If the lead bike has slightly better brakes (or wider tires, or grippier tires, or even just pulls the brake levers a bit harder than you do), you may simply have no reaction time available and a collision will be unavoidable.

In other words: you definitely can only safely draft with the full cooperation of an experienced lead cyclist who won't brake unless they've warned you or being rear-ended by you is better than whatever they're about to run into. If you were going to go by hazards, they'd need to warn you about upcoming hazards and you'd need to pull back to 10 feet (3 meters) or more.

Avoid the door zone

Additionally, you should simply stay entirely out of the door zone so that opening car doors aren't much of an issue. It's not unheard of for car doors to open very fast with no warning, so that nobody has enough time to avoid hitting the suddenly opened car door. I haven't checked the laws of every single state in the US (and definitely not outside the US), but in general the area that parked car doors can open into (the door zone) constitutes a hazard that you're not required to ride through. When a bike lane is in the door zone I'll either ride the line on the left of the lane (if that's outside the door zone), or simply ride in the right wheel track of the car lane next to the bike lane.

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Excellent write-up. The emphasis on full cooperation is important. The rule is basically: Before drafting someone, make sure they know you are there. –  bikesandcode Oct 15 '10 at 23:19
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I'll add that if you're drafting someone at an effective distance (1-3 feet), you will hit them if they brake hard without warning. This is why braking in a paceline is a big no-no. So if the rider in front of you gets doored (no warning) then you are going to join in the fun! –  darkcanuck Oct 17 '10 at 1:37
    
Nice comment about avoiding the door zone - if only that was possible round here. One of the roads on my commute is so narrow that the door zones from the 2 sides of the road are only about a handlebar's width apart, and where the bike lane runs in the door zone deliberate dangerously close passing to try to force bikes into the lane that shouldn't be there becomes commonplace. –  Chris H Mar 3 at 10:08

Depends on the size of the thing you are trying to draft.

The distances listed below are distance between objects at normal road biking speeds. Bigger distance for higher speeds I may (or may not have) drafted a lot of things this is what I've found.

  • A Bike: 1-2 Feet
  • Small Car: 2-4 Feet
  • SUV: 3-5 Feet
  • Truck: 3-5 Feet
  • 18 Wheeler: 4-8 Feet
  • Bus (my favorite): 8-12 Feet

Be very careful when drafting anything, watch brake lights and cars up a head. If you are not paying attention you will run into the object in front of you.

EDIT: I am in no way condoning or suggesting you draft any of these moving objects.

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I'll note that there is a significant benefit to chasing another cyclist (even a safe 10+ feet): if not physics and aerodynamics, motivation. –  Precipitous Nov 14 '10 at 0:18

First, at around 10-12 MPH and below you are fighting rolling/frictional resistance enough that the drafting benefit is negligible in still air. If the you have a headwind of 10+ MPH or are traveling at 15+ MPH then there is some benefit to be had.

Assuming that there is no wind or a direct on head wind, the extension of the 'drafting bubble' is about 5 - 6 feet beyond the front rider. Any further back than that and neither rider experiences any benefit. Obviously the closer you get the better, but that requires the lead riders cooperation and that the two of you communicate.

Since wind is rarely straight on, many times you will be drafting slightly to one side or the other. The impact again is a little less, but you may be safer, as long as you are careful not to overlap wheels.

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