What can you tell me about drafting?

[jkv357]

"What can you tell me about drafting?"

You will get better mileage.

It can be dangerous.

Truckers will hate you.

If you are close enough to get a significant benefit you are too close to be driving safely.

It's not a great idea unless you are on a racetrack.


Jay

[basjoos]

The only use I find for drafting is that it can give me a quick indication of a vehicle's aeromod potential, that is, the mileage I get while drafting is the mileage I would get at that speed after I had dropped my vehicle's Cd down into the 0.1's. I get a very minimal mpg increase when drafting in the aerocivic since my Cd is so low and so is not worth attempting.

In addition to possible rear ending of the drafted truck, you also have to worry about road debris either being straddled by the truck or being kicked up by the truck's tires. I've seen a 2x4 thrown up into the air by a truck's tires running over it and wouldn't want to have that coming through my windshield.

One of the tricks I use when being tailgated is to aim for and then just barely miss a box, board, chair, or other bulky road debris in the hopes that my tailgater would not see it since they are so close that they can't see past me and then not be able to miss it or at least give them a good scare, so they back off. I could see truckers using the same technique to get rid of an obnoxious drafter.

[California98Civic]

Originally Posted by 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.

You mention two issues: is it safe and what's most effective. Drafting close is never safe and if you do it realize you are making a safety choice that could cost you. It is true, however, that trucks, big semis, cannot stop nearly as fast as a car. A sudden breaking event would leave you with the advantage. And a truck will tend to plow through what it hits.

But from people on here and from Hucho's "aerodynamics of road vehicles" I have learned that driving close behind other vehicles is not necessarily the best FE/drag strategy. I now follow three-to-four lengths behind and it seems to be working. Trucks have a huge wake, and you can get into their slip-stream and cruise in it. There can even be a benefit to letting that truck tailgate you, as the higher pressure at the front end of the truck will "push" you and I think reduce your drag. Weird, huh?

Remember to make your own safety choice though. You own the car. You pay the repairs...

[Fat Charlie]

You don't have to be aggressive about it. On one EOC the other day I gained almost 10 mph over my usual speed and I wasn't all that close.

Originally Posted by Fat Charlie

...I don't draft closely. If something's big enough to be worth drafting, it's big enough to punch such a hole in the air that you can follow it at normal distances and still get a real benefit. What makes it drafting is that I understand the help that's there and I'm behind the rig on purpose to get that bonus rather than just being helplessly stuck behind a truck.

Treat the truck like you would any other vehicle, except don't pass it or let it get away. Just let it stay in front of you and you'll see a benefit. That benefit varies, but it's there (unless you're basjoos- he makes his own aero benefits).

[brucey]

1 car length for every 10 mph, double that at night or rain, is how I was taught to drive. I never break this rule (unless someone cuts in front of me, then I just slow down again)

There isn't enough of a benefit with drafting to out weigh the risk anyway, I get better mileage at 65 mph than I do at 70 mph and drafting.

[PaleMelanesian]

Originally Posted by actwithclarity

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?

I tested this with an Odyssey. 70 mph Drafting 2 seconds behind (much more than 20ft ) gave the same mpg as driving solo at 65 mph. I don't draft since I found that.

[ChazInMT]

A math constant I came up with which is useful useless knowledge is 1.5 feet per second for every mph, so 2 seconds at 70 MPH is 70 x 1.5 x 2 or 210 feet. give er take.

[larrybuck]

Thank you california 98civic!

Now when a mean trucker rolls up Movie Dual style right behind me; I'll stay calm and happy because he's helping me! This is for real. It was not meant to sound sarcastic!!

[Fat Charlie]

He's filling your wake. Now you've got a full boat tail and can still parallel park!

[RobertSmalls]

I can tell the difference when I'm drafting a trailer at two to four trailer lengths. It is easier to hold lean burn behind the truck than in the lane next to it. However, my car is very twitchy when it comes to MPG.

There are a few issues with drafting, which means I'll pass up lots of trucks before I find one worth drafting. For one thing, you will have a hard time seeing what's in front of him, so you'll miss out on potentially fuel-saving information about terrain, speed reductions, etc. For another, if the truck is going too fast or too slow, your desire to draft him can lock you in to a suboptimal speed. If he's going just the right speed for you, he's unlikely to pass you and vice versa. Also, his hill climbing strategy might be incompatible with yours.