Does following (drafting) a semi really save gas?

[Gary Allan]

Originally Posted By: meep
Man, I really respect you guys, I do - my post was a condemning one and your responses are genteel, but I still think we are "full of ourselves" here.

Please- someone define the distances for drafting. I'm imagining 1 car length, and that's where my arguements stem from. If we're talking about 3-4, that sounds like typical Atlanta beltway density where, BTW the accidents can get pretty gruesome but you can't avoid it--lots of folks on the road.

"-the truck stops more slowly than your car. All things being perfect, you'd never hit the truck."

driving conditions are never perfect. Are you a perfect driver? also requires follower to be perfectly attentive. Never glance away, pick up the cel, etc.

"-you aren't affecting the truck drivers ability to drive safely at all- he can't even see that area behind the truck. So it makes no difference at all whether you are there or not."

I'm not arguing that. I'm arguing that for the sake of his day and humanity in general, he has a right to not wish to be in an accident. He's seen his share. It's formed his opinion, and he has a right to want to maintain safe practices. Pulling an injured or lifeless body from a car really changes your outlook on traffic accidents.

"-as for side drafting,"

I have no issue with side drafting as long as traffic isnt being blocked.

"It is more dangerous for the car to do either thing, but only for the car. If the driver is aware of the danger, seems fine to me. "

If you tangle with a semi, snap a steering rod and careen into another vehicle, who gets hurt? Just you? At 65 mph, your car isn't just going to slide to a straight and perfect stop. It's going to become a "pinball." I watched a tire, just a tire, come off a car at 65, roll across a median and bounce into an oncoming car. If I recall, it killed the driver.

"...but meep, you seem to confuse drafting with tailgaiting ..an important distinction if you're earnestly evaluating the content of those who are comfortable with the concept. "

And this is where we run into the plain truth that people have different talents, and some people are better drivers than others. Some people could probably do this much more safely than others, just as some can handle 95 mph more safely than others. Is 95 mph considered acceptable for everyone? No? It's not? Then I don't see why drafting could be seen as an "acceptable" practice for everyone. And you can't say that it's ok for me to do it and not you, because you base that on the whole "I can bend the rules for myself because I'm a better driver than..." That's the thinking that says "I can cut line because I'm in a bigger hurry than a stranger," or "I can speed in the parking lot because I can stop faster," or ...I'm allowed a privilege because I'm better than the next person."

"I still think it's mostly an emotional issue ... with very little facts or logic to support that this common practice is a danger to anyone."

Definently emotional. But what kind of facts do you need? Recall the incidents that lead to the addition of 3rd brake lights in cars? Whole bunch of folks driving close together on I-75 in the fog. Low visibility, just like following a truck. Several hundred cars rear-ended each other.

"...Little logic to support this" ???

I watched this happen: car was following a truck very closely. Without warning, truck changed lanes to avoid an accident ahead. The following vehicle didn't see until too late and plowed right in. We can argue that the truck probably should've slowed down, should've been paying better attn;, but still-- HE avoided the accident.

Here's some hard data.
It takes 0.15 seconds for sensory information to reach the brain. It takes 0.15 seconds for that command to reach the muscles for movement. There's processing time in the middle. If you're alert (driving is congnitive and not instinctual) let's say 0.5 seconds to process and make a decision, doesn't include a mirror check. That's pretty quick. You've lost 0.8 seconds before even reaching brake application.

at 65 mph, you're traveling 95 feet per second.
traveling at 2 car lengths, you've got 30 feet clearance
Assuming a 60-0 stopping distance of 160 feet, a semi slows 3.63 seconds to zero yielding a decel of: 24fps^2

Semi slams brakes. in 0.8 seconds he's drpped to 76 fps, and you've closed the gap by 8 feet by the time your foot first moves. you lose 0.2 seconds for foot travel and brake engagement.
He's down to 71.6 fps, you're still at 95fps, and you just lost another 4.3 (call it 4) feet to 18 feet clearance.

so now, he's stopping from 71.6 fps, at 24fps^2, you from 95 fps in an accord (117' 60-0 distance, 33.2 fps^2 decel rate).

semi to stop: takes 2.98s over 107' additional
accord to stop: takes 2.88s over 136' additional

Result: you will hit the semi 9 feet before coming to a stop, best case.

Me, I'd prefer to stay in control of my vehicle and stay [censored] away.

M


That mainly proves to me that 90% of all statistics can be made up on the spot ..50% of the time

You've constructed an ideal collision resulting condition. I'd also like to see your semi truck deceleration database. There's plenty for cars. You just plug in the weight.

 

[meep]

Now now... poo poo my one assumption all you want, it's the first post to actually demonstrate some hard math/physics.

I'm being countered by unfounded statements of "logic," "common practice," "perfect" driving conditions and a whole bunch of other opinion-stated-as-fact claims. Even if my single assumption, which I tried to make as accurate as I could figure, is off, it still proves a point.

I've raised the bar. To do better, bring your stats/numbers/data.

Game on!

Mikey

 

[meep]

By the way, that condition was not constructed. I started with the 65 mph scenario and started from there. Had to guess at 160'. the accord distance is documented, the rest is physics. no tables. no way of knowing that it'd support my point.

deltaD=(v1-v0)xT
a=(v1-v0)/t

M

 

[BrianWC]

Plain and simple, my accident would NOT have happened if I hadn't been 3 car lengths or less behind a truck. I could not SEE the accident he was heading straight into. He slammed into a stopped flatbed at 60 mph. I had enough time to see his truck buckle in mid air and slam on my brakes. Then, best part, he got to sue ME for his injuries....

 

[Gary Allan]

Originally Posted By: meep
By the way, that condition was not constructed. I started with the 65 mph scenario and started from there. Had to guess at 160'. the accord distance is documented, the rest is physics. no tables. no way of knowing that it'd support my point.

deltaD=(v1-v0)xT
a=(v1-v0)/t

M


Well, you had to pull the 3.6x seconds from 60-0 out of somewhere (for the semi). If it was from your behind, I'd like you to include footnote acknowledgments in the text.

(envision me hitting the bell on a chess match to signify "your turn"

)

 

[meep]

ok,

it's a multi-step math:

given 160' stopping distance D from 60mph, and starting speed of 88fps V0, 0 fps ending speed V1

Good 'ole Rate x Time = Distance V x T = D.
and ((v1+v0)/2) x T = D solving with avg velocity during T

Avg speed from 88 to 0 over T = 44 fps

So: 44 x T = 160. Solving for T, we get T=3.6 seconds to stop from 60mph.

Now we have D, T and V.

Solving for rate (acceleration) A we use: a=(v0-v1)/t
a=(88-0)/3.6. 88/3.6 = 24.4 foot per second^2 or fps^2.

I'm not going to write out the whole darn thing-- that's about 20% of the full scenario... but you get the idea. You then keep A (accel) and that becomes your constant, but with the actual V numbers used for the scenario.

I aced high school physics and engineering dynamics....

I'm going home. Drive safe!

M

 

[Gary Allan]

Okay ..so @ 40 ton or empty (or for that matter a semi or a Civic)..there will always be 3.6x secs of decel from 60-0..only the distance will change? Time to stop will be 100% the same in either case?

(there has to be some missing link here ...and I may be it ..but

)

 

[Gary Allan]

Okay ..let's say it another way. A semi will stop in no shorter or longer distance than any other vehicle on the road. Loaded, unloaded ..6% grade ..12% grade (just throwing that in there for fun) A stop watch will be needed between a Civic and a semi in time to stop. All vehicles will come to a complete stop from 60 in 3.6 seconds and all in 22x feet once brakes are applied (134ft being reaction time).

That about it?

 

[Bror Jace]

Thanks meep for being civil and elevating the discussion with some numbers ... even if we aren't agreeing upon all of them just yet.

I'm in a hurry to close down for the night but am also having some difficulty following your math.

Can you tell me the total stopping distance for the rig in your scenario above?

That's the part I hang a majority of my hat on. I did some searching on-line and found distances starting around 100 yards and going up to 300 yards to stop a semi (depending on a lot of factors ... mostly load) ... which is at least double to several times the stopping distance of a typical car.

Rate of deceleration should be similarly proportional (I would think).

If it isn't safe (a relative term, for sure) for me to follow a semi at, say, 50' (which is the closest I'd be for any length of time) then it isn't safe for me to follow ANY vehicle that close ... and those of us in congested areas do this all the time during rush hour. If you tried to use the 2 second rule, you'd drop steadily back as vehicle after vehicle would cut in front of you ... and people behind you would be honking in frustration ... and an accident would result after a short while, no doubt.

 

[meep]

I am not a "semi" expert. Obviously if the stopping distances 300 feet then the end results change. Regarding the question of whether distance changes with load, yes and no.

Ignoring the imperfections of tires-to-road contact, stopping distance would remain equal. The formula is force (from friction) = coefficient of friction times normal force... Ff=uN. Normal force is the weight upon the tires. What this means is while you may have more mass to stop, you also then have more traction to stop with. That increased traction allows for increased braking force. Therefore, the math suggests that the stopping distance remains unchanged; the converse of course is less weight to stop equals less weight over the tires and less traction potential. The reality however, is a little less perfect because at some point brake temperature comes into play, tires began to come apart, etc.. I would think that in reality a fully loaded truck will have a longer stopping distance, but assuming optimal braking, the difference won't be as great as you'd think.

Interestingly enough, it is my understanding that when the roads are wet, the loaded truck stops much more quickly than a passenger car. This is because the extra weight forces the tire into much more complete contact with the road surface.

now, if you'd like -- we could play with the 300 foot stopping distance and show that an absolutely attentive driver could stop without hitting the truck, but that the driver that hesitates for a half for a second or so hits...

Gary -- that 3.6 second is only specific to a vehicle with a 60-0 stopping distance of 160 feet. You have to repeat the same math inserting a different distance to get the stopping times for other vehicles.

For example,the Honda Accord with 117 foot stomping distance requires 2.7 seconds to stop from 60 mph, with a deceleration force of 32.2 feet per second squared.

How am I doing?

Mike

 

[Gary Allan]

Quote:
Gary -- that 3.6 second is only specific to a vehicle with a 60-0 stopping distance of 160 feet. You have to repeat the same math inserting a different distance to get the stopping times for other vehicles.

For example,the Honda Accord with 117 foot stomping distance requires 2.7 seconds to stop from 60 mph, with a deceleration force of 32.2 feet per second squared.

How am I doing?


Ok, you lost me with your second line with your progressive text (original assertion) in how you came up with the 3.6 ..but I'm totally mathematically undisciplined. I need to reason most things and it's counter intuitive ..which many aspects of physics can be (for a simplistic example, the velocity of falling objects in a vacuum) ..but the supporting dialog (what I got out of it) was:

"A car following a semi by 60ft, going 60mph, regardless of mass (of either vehicle), will hit the semi due to reaction delay".

This puts a whole lot of "given" into a highly variable situation that I've experienced. A fully loaded 15 passenger van ..or my 1 ton loaded C20 had tremendous difference in stopping distance ..etc..etc.


Now, if your point is that the perceived buffer zone is much less than most will think, then there's little argument. At higher rates of speed ...or even traffic jams ..the (in electronic terms)"propagation delay" creates the "slinky" that cuts both ways.

..but the original post is "can you save fuel" ..and the answer for most is "not in a practical manner for most". Our morphed examination of the practice has shown a dislike of semi's to be followed ..period.

We then evolve to our self created justifications for the annoyance. "Pulling a dead, mutilated, corpse from the wreckage .. and costing him many hours making his living..."

Well, I don't like other cars on the road, they can "get in my way" and inconvenience me by causing traffic jams ..and having accidents ..and, occasionally the idiots have fatalities and I'm just going to employ countermeasures to inhibit them from getting in my way to and from work "because I can". The nerve of all those people ..purposely ..with premeditation ..getting in my way and upsetting my "driving experience" in my quest to make a living and provide for my family. They should be banned from the road. Now I do my part to "fix them" ..but it's a never ending battle for truth, justice, and exercising "right of weight".

I'd like to hear of the frequency of "pulling mutilated corpses" by semi-drivers ..and how many hours the average driver (that kicks up stones on trailing cars) have lost due to said corpse pulling. I venture it falls to a very low percentage compared to the vast number of cars struck by semi's that are sliding on ice or whatnot (whether due to "idiot drivers" interations/influences ..or not).


I admire your skills. I wish I had them