Drafting a big rig

[bestclimb]

Originally Posted by k.civic.f4i

people also argue that the trucks can stop abruptly but its highly unlikely that they could slow down faster than whatever you are driving(passenger vehicles)

The problem with the concept that a car can stop quicker than a truck is that by the time that you get the first indication that the truck is going to stop/slow quickly the truck driver already has his foot on the brake and is applying pressure.

A truck does not have to come to a complete stop to create a large difference in speed between it and the car following. Depending on how much room the driver of the following car gave himself to see, react to, and avoid the developing hazard will go along way in deciding if he needs a tow truck.

[k.civic.f4i]

to draft a semi you need to be at nearly the same speed.

bestclimb; i could see an accident happening in that situation if you really weren't paying attention but otherwise, i think reflex's would be enough to make you hit the brake once you see the semi's lights turn brighter red.

[cfg83]

k.civic.f4i -

Originally Posted by k.civic.f4i

to draft a semi you need to be at nearly the same speed.

bestclimb; i could see an accident happening in that situation if you really weren't paying attention but otherwise, i think reflex's would be enough to make you hit the brake once you see the semi's lights turn brighter red.

I try to follow the 3-second rule (in the early 1980's I used to quote the 2-second rule). I could probably "push it" closer to that when I am in hyper-vigilance mode (aka right foot hovering over brake pedal), but that's too tiring for long term driving.

CarloSW2

[bestclimb]

Originally Posted by k.civic.f4i

to draft a semi you need to be at nearly the same speed.

bestclimb; i could see an accident happening in that situation if you really weren't paying attention but otherwise, i think reflex's would be enough to make you hit the brake once you see the semi's lights turn brighter red.

Kinda depends on how close you are.

[SentraSE-R]

Most of the time, it's hard to find a big rig going slow enough to justify following it. I can almost always get better mileage going slower. On those rare occasions I find a really slow truck, I'll follow it 2 seconds back. At that following distance, we're often passed by an endless conga line of cars following so closely, three of them can often fit in the distance between me and the truck in front of me.

[ilusnforc]

Originally Posted by roflwaffle

The problem w/ a ~125-150ft interval IMO is that it can take ~130ft before someone even starts to slow down.

If you are 130 ft behind a vehicle that begins braking and it takes you 130 ft to react and begin braking as well, you will begin braking at approximately the same point as the vehicle in front of you, if you have better braking capability or shorter stopping distance you will still be well behind that vehicle when stopped as the vehicle in front of you will be incapable of an instantaneous stop (except in the instance of something influencing the abrupt stopping of the vehicle in front of you such as the already mentioned reinforced wall, car hitting a stopped big rig, etc).

Assuming that average human reaction time from seeing a situation to applying brakes takes 130 ft, I'd say that 130+ ft is safe for a following distance. Of course that distance will vary with speed, so basing your folowing ditance on something fixed like time will allow your distance will vary appropriately. 3 seconds following distance generally considered safe (the "three second rule"). By my calculations, at 70mph you can cover 100 ft per second, putting your safe following distance at 300 ft or 100 yards (a football field length minus the field goals). At 55mph that is reduced to 240 ft or 80 yards.

I have drafted big trucks before, and have seen significant gains in efficiency. Once I drafted a typical big rig with a box trailer, can't say the exact following distance as I'm not good at guessing distance in numbers but a good number of car lengths back. I had a 2000 Honda Insight at the time, 5-speed, A/C off, driving from west Texas into New Mexico drafting the truck at 55 mph for half the drive, the other half solo and I average 74.6 mpg and easily put around 600 miles on a single tank. I was stopped at a border checkpoint station where most cars were simply waved through and suspicious ones stopped, just so the guys could check out the car and ask questions about fuel economy. In normal everday driving I would typically get 52-56 mpg and when really trying being able to get up to 60-62 mpg per tank.

The Insight was replaced with my current 2006 Prius, which I have drafted an big rig that had an oversized load, it was just the perfect drafting truck, it had some odd shaped object on a large and very low to the ground flatbed that was all tightly wrapped in white plastic and the back end of the object had a nice taper and because the trailer was so low the ground there was little chance of any rocks or other objects getting thrown and it was going rather slow at about 55 mph heading east on I-10 from San Antonio to Houston, I was able to draft it for nearly the last half of the drive into Houston (mind you the entire distance is gradually downhill to the gulf coast) averaging just over 60 mpg where I normally average about 56 mpg.

[Lazarus]

Originally Posted by pstrbrc

Actually, there are risks that go beyond braking distances.
#1. Trucks have a huge draft, and they suck up/throw up a lot of trash.
#2. Trucks can drive over road hazards that'll take your undercarriage out. So if it's heavy enough for the truck's draft to leave on the ground, it's gonna break something going under your car.
#3. One word: "Gators". Semi tire treads.

When riding motorcycles, (no windshield or fairing) drafting behind a semi just made the trip easier. Until I saw a gator take out a Crown Vic's front fender. Maybe with a very "aero" car one could expect minimal damage, but what about the undercarriage? .

+1 Braking is the least of your concerns

[Mifunego]

Originally Posted by RobertSmalls

I'd like to meet the brick wall that can stop 20 tons at 65mph. But I would not like to meet that wall at 65mph.

Something like that.

[Chalupa102]

Originally Posted by SentraSE-R

Most of the time, it's hard to find a big rig going slow enough to justify following it. I can almost always get better mileage going slower...

That's why i almost never do any drafting. Pretty much all the trucks around here go well over the psl on the highways.

[underitall]

Over here(UK) it's often called Slip-Streaming, but I know it more as Drafting.
Also, over here, it's the 2 second rule(At least), which is quite irrelevant when following a Truck(Rig), because there's a saying over here on the back of Trucks, "If you can't see my mirrors, I can't see you", well, that'll have to be about 4-5 seconds back, so pretty much blows out the idea of Drafting.
But, I have tried it, followed a few Trucks for about half an hour, before I/they turned off, and there's a big difference in engine revs, when up close behind them.
I have got really close(IMO), that is about 1.5 seconds distance, with left foot hovering brake pedal...It really didn't feel safe, so if I do draft, it's more just following with a decent safe-ish distance, where I can just about see their mirrors/whatever happens to be in front of them.
Thanks,
Tom.