What can you tell me about drafting?

[actwithclarity]

I've taken up drafting every day on my commutes.... For me M/G is not so important as M/G*M/T, so drafting is far better than the other techniques since it doesn't take you extra hours to get to the destination..

Can anyone tell me approximately, if I'm doing 70mph, 20ft behind a semi, how fast I would be driving without drafting to get the same mpg?

Also, is it really unsafe? Do I need to be 10-20 ft behind or is farther back OK?

Also, if there is a cross wind from say the right side of the road, am I better off behind the semi and to the left (in other lane if there is one)? I imagine say if the wind is, say 30mph, and we are travelling 70mph, adding the vectors together would give the direction of the stream I want to be riding in. Practically speaking probably in the same lane just shifting far over to the side as I can.

[ecomodded]

Holy crow your brave, I stay further back from semi's and what not, at least 40 ft. away and I find that dangerously close!
I am not sure how much benefit I am losing by staying back so far but I am sure not going to go any closer to find out.. I do notice the draft effect if I change lanes I start to lose speed. Give that a try and you will find your gains or as it may be losses in mph. That's bout all I suggest, I am sure somebody has figured out the actual gains made by drafting at different distances but its not me.
10-20ft. sounds too close to react to a sudden brake lock up from the drafted vehicle, 40ft. may be too close as well..

[Bdorion1]

I need one more post to be allowed to post links :/ hopefully this will do it and will be followed by the link I found haha.

[actwithclarity]

My door seal or whatever is not tight, and I hear the air whooshing through it when I go fast down the highway. I noticed that sound does not go away until I get very close, probably <20ft, from the truck in front. I'm sure there is some drafting effect if I'm further back, but I really hate that noise so I figured I'd just go as close as possible.

Your idea of switching out of the lane to see how far your speed drops when air resistance comes back is interesting. Can we assume that gasoline burned is constant?

[Bdorion1]

I'll just leave this here.

Mythbusters: drafting 10 feet behind a big rig will improve mileage 39 percent &mdash; Autoblog Green

The chart at the top is useful (Do what you want with the commentary below it). Granted its at 55mph not 70 so the percentage gains would likely only increase for your purposes. Quite likely drafting at 10-20 ft would boost your mileage beyond any slower non drafting speed.

However from another site I read that at 55mph at 10 ft you have .143 seconds to react. Not sure how they calculate that. I mean tractor trailers cant stop as fast as a car. Either way I hope your paying very close attention when you do this and bring a clean pair of underwear just in case.

[actwithclarity]

It would take 0.123 seconds to travel 10ft at 55mph. However since a truck cannot stop instantly (lol), saying you only have that long to react is kind of dumb. The main concern is you have to be able to decelerate faster than the truck. I know my little geo metro has pretty weak brakes, so if a trucker really slammed on it I don't know if I'd survive that.

[ecomodded]

Your idea of switching out of the lane to see how far your speed drops when air resistance comes back is interesting. Can we assume that gasoline burned is constant?[/QUOTE]

If your foot stays the same on the gas pedal the gasoline/ fuel used will be the same, only the air resistance and consequently your speed will change.
on a side note, If you had it on cruise control it would lay off the throttle.

[actwithclarity]

Drafting with cruise control... now THAT would be dangerous.

[euromodder]

Originally Posted by actwithclarity

It would take 0.123 seconds to travel 10ft at 55mph. However since a truck cannot stop instantly (lol), saying you only have that long to react is kind of dumb. The main concern is you have to be able to decelerate faster than the truck.

10' back, you'd cover the gap in about 1.3 second if the driver decelerates only 5mph.


50 feet back, though still too close, you still get half the max. benefit.

[SentraSE-R]

You folks don't get it. 0.15 seconds is an excellent reaction time. That means from the time you notice the big rig's brake lights to the time you jam on your brakes, you've already eaten the big rig's rear end. The only time you should be within 1-2 car lengths of anyone at highway speeds is if you're in the lane next to them.

From riding an underpowered motorcycle on highways, there is a maximum drafting benefit 10' behind a truck, where he actually sucks you along with him. But you can get much of the same wind blocking benefits with much more safety in the adjacent lane, or 100 feet back.

Considering the overall scenario, even on an hour-long commute, it's better to drive 15 mph slower, and leave 15 minutes sooner, than to risk being lunch meat jammed underneath the frame of a tractor-trailer. I've seen the blood on the windshield. The frame comes across at neck height, just right to decapitate you - a benefit to end your suffering instantly. Bumpers drop lower, though, to keep you alive and push your engine block into your knees. It's not worth it.