Does following (drafting) a semi really save gas?

[ffracer]

Of course, it does. I'm a racer and instructor and there is an easy way to tell:

See if the rpms drop in the draft. It does.

The issue is, it really isn't a good idea, except on a track with trained drivers.

 

[rpn453]

Yep, it can make a big difference. But my paint, windshield, and safety are worth more than the gas I'd save.

Originally Posted By: ffracer
Of course, it does. I'm a racer and instructor and there is an easy way to tell:

See if the rpms drop in the draft.


That won't work for most people. There aren't many cars without lock-up torque converters these days!

 

[Bror Jace]

rpn453, I read ffracer's post and thought the same thing. At a given speed and assuming no slip in the clutch, the RPMs at a given speed should be the same, regardless of wind and rolling resistance.

However, load on the engine decreases ... and (I thought) there's where your savings lie.

The front of my car is pretty clean and chip-free and while doing 62mph and following a truck, bus or minivan, I feel I'm being a lot safer than the fools in the fast lane doing 75mph ...

 

[ffracer]

Originally Posted By: rpn453
Yep, it can make a big difference. But my paint, windshield, and safety are worth more than the gas I'd save.

Originally Posted By: ffracer
Of course, it does. I'm a racer and instructor and there is an easy way to tell:

See if the rpms drop in the draft.


That won't work for most people. There aren't many cars without lock-up torque converters these days!



Actually, it is the opposite. I was assuming a manual transmission.

With a lock-up, it should decline in rpms if the air resistance is lower, since it is almost direct drive. Without lock up, the automatic would get confused and slip.

 

[ffracer]

Originally Posted By: Bror Jace
rpn453, I read ffracer's post and thought the same thing. At a given speed and assuming no slip in the clutch, the RPMs at a given speed should be the same, regardless of wind and rolling resistance.

However, load on the engine decreases ... and (I thought) there's where your savings lie.

The front of my car is pretty clean and chip-free and while doing 62mph and following a truck, bus or minivan, I feel I'm being a lot safer than the fools in the fast lane doing 75mph ...

If the load is lower, the rpms should be lower. Sometimes those "fools" know better what they are doing than the ones not paying attention.

In Europe, there are plenty of high speed drivers that are FAR safer than US drivers.

This in no way impying anything personally.

Speed does not kill, lack of judgment and using the wrong equipment for the speed does.

 

[rationull]

With an MT the crankshaft is mechanically connected to the drive axles with no slippage. How can a different load possibly result in different engine speed at the same wheel speed?

 

[Carmudgeon]

One of the auto rags, C/D I believe, tested this many years ago and confirmed it works.

But as already pointed out, staying inches behind a larger vehicle that blocks your view isn't exactly a safe nor desirable practice.

 

[Black Bart]

Originally Posted By: rationull
With an MT the crankshaft is mechanically connected to the drive axles with no slippage. How can a different load possibly result in different engine speed at the same wheel speed?
IT CAN'T CHANGE NOT POSSIBLE

 

[saaber1]

I suppose a CVT transmission would get the most benefit such as in the Prius, etc.

 

[Bror Jace]

ffracer, I didn't mean to imply all/most people traveling at 75mph are foolish ... in my mind I was thinking of people driving that fast in dense traffic ... often zooming passed the rest of us occupying the right and middle lanes. The disparity in speeds between 55-60 and 75-80 means someone is likely going to have a collision sooner or later. It's not so much the speed, but the rate of closure between the various vehicles.

But I tend to agree with rationull and Black Bart: with a mechanical link between the tires and the engine, RPMs should be the same in a given gear at a given speed. Period.

Load is a separate issue. You can have different loads at the same RPM ... and to maintain the same RPM under varying loads you have to open or close the throttle more. More load = more throttle to maintain speed = more fuel consumed.

Taking this back to the original topic, while 60-70' feet behind a large truck, you may experience 20-25% less wind resistance which will decrease the load on your engine and you'd use less throttle (and thus, fuel) to maintain the same speed.

If someone can point out where I'm going wrong, please do so.

 

[Gary Allan]

Well, it sure works in my Wrangler ..and you don't have to get TOO close to realize the gains. You can feel getting inside the envelope. Now on "open road" it would appear a bit close just due to all the space ..but it's nothing that any trucker or other motorist doesn't experience almost daily. It's merely perception that alters it. Then again, I've got a high square frontal area and lots of under chassis space too, so my risk and advantages are not typical.

..but I have a comment about the stone thing ..as in "doing it on purpose" to get someone off your tail. "What's it to you?" that is, just what makes you hate this so much that you'll, with full malice and premeditation, damage someone else's property for your "preference". How many accident reports have you filled out ..and how much do you estimate the tab is for the damage that you, by nothing more than your own standards, cost other people??

Just the other side of the coin, pal(s).

 

[Black Bart]

Gary if you are stupid enough to tailgate me I'm stupid enough to make you look for another target.
I'm not talking about following too close I'm talking about driving 20 feet from my bumper.
I don't want any part of extracting you from under my trailer.
If you don't like it the solution is simple don't tailgate me.
What makes you think it is your right to tail gate.
If the police see you doing this he will explain to you that you do NOT have a given right to tail gate and give you a $150.00 safe driving award.

 

[Gary Allan]

Again, I'm not judging you on how you cope with it. I'm merely asking how you rationalize damaging other people's property in your annoyance (read that AGAIN before you answer what you THINK you're reading). Sure, you can say it's for "their safety" ..but you probably do very little for people's safety where you're not annoyed and you have to go out of your way (as opposed to damaging their property with the mere swing of the wheel).

..and..what do you do in dense traffic @ 65 mph (most trucks are governed there now) and your in "dense pack" with people all around you and there's nothing you can do? Do you have a "stone broadcaster" that deploys a perimeter defense screen??

 

[Black Bart]

Use either the CB or cell phone to contact a Smoky also works well.

 

[leftlane]

Truckers sure dont have any problem riding my bumper 3ft away flashing their lights and engine braking loud when Im doing 75mph in the left lane to where they pass at 80mph when I get the chance to get over in the right lane. This is not a rare occurance with me and Ive seen them do it with other cars also. It is for drivers like that why many people carry batteries ball bearings and other goodies that will do damage when thrown out of the window and in this day and age when people pull guns and start shooting for the slightest provocation too I wouldnt tail gate, slow down or brake at tailgaiters, or throw rocks up with the tires on anyone tailgaiting me.

 

[Gary Allan]

I'll also say that people shouldn't tailgate, but I hardly see the semi driver feeling "at risk" in the whole affair. At the most, he could lose time ..and I have seen plenty of "professional drivers" forming a wagon train in very close proximity to each other. The day a semi can stop faster than any passenger car will never occur until a few laws of physics get repealed. That leaves it down to reaction time and the assumption that the passenger car has an incompetent driver ..and the assumption that the semi's all have drivers that can always pay attention if the situation occurs where they all have to stop short.

Again, most passenger cars aren't lacking enough in aerdynamic enhancements to take any sensible advantage of drafting a semi and the risks just in random stone damage and retread loss surely is something to think about.

..and while we're whining here, how many times have you put lives in danger or caused damage due to having a cap let loose? Oh ..it's all part of having trucks on the road ..live with it ..or just don't drive.

Just to reaffirm that I'm not calling you anything ..but I am giving you a different perspective that you may consider ..like being on the other end of stuff that you may do to annoy others and don't think twice about

Happy and safe motoring. GOD loves you!

 

[Bror Jace]

Black Bart, I'm with Gary here.

I wrote a smarmy post on another board. I'll leave off the attitude for here but (essentially) why does this bother you (and other truckers) so?

It's not a safety issue from your perspective as my Civic isn't gonna damage your truck, let alone threaten your life. On the other hand, if you tailgate or drift out of lane, you have the real possibility of killing someone ... possibly even a whole family of someones.

And how close is too close? I'm usually somewhere between 50'-70' behind a truck when I 'draft.' Is that too close for comfort? What does the law say? I understand that at that distance, you might not be able to see me all the time ... but is it worth getting upset over?

I just don't get it. Aren't there other, more important things to worry about while on the road?

 

[shoz123]

A vehicle with a locked TC or a MTX transmission will always maintain a certain engine rpm compared to road speed. This will not change with load. The fuel injector duty cycle will change and possible the timing too.

 

[Black Bart]

Originally Posted By: Bror Jace
Black Bart, I'm with Gary here.

I wrote a smarmy post on another board. I'll leave off the attitude for here but (essentially) why does this bother you (and other truckers) so?

It's not a safety issue from your perspective as my Civic isn't gonna damage your truck, let alone threaten your life. On the other hand, if you tailgate or drift out of lane, you have the real possibility of killing someone ... possibly even a whole family of someones.

And how close is too close? I'm usually somewhere between 50'-70' behind a truck when I 'draft.' Is that too close for comfort? What does the law say? I understand that at that distance, you might not be able to see me all the time ... but is it worth getting upset over?

I just don't get it. Aren't there other, more important things to worry about while on the road?

Well you and Gary don't have to worry about me throwing stones I'm retired sold my trucks.
In the 42 years that I drove I never had the first accident. I drove millions of accident free miles but you guys sure made it difficult.
From my first post it has been changed from a two lane to a three lane and then you placed my truck in the middle lane with traffic on both sides.
You make the scenario what suits you rather than what I stated but if you spent the time on the road that I have and helped pull people out of their crumpled car you would better understand that it is not something that you enjoy doing.
I have been one of the first on scene of accidents waaay too many times it is not fun.

Like I told Gary if you don't want to get stones DON'T tailgate.

I guess I could take your attitude and say it is not going to hurt me and let you run under the trailer and dial 911 and wait for the medics to pull you out from under the trailer but who pays for my lost time spent filling out the reports.

If I'm delayed and can't make scheduled delivery I have to make another appointment. If that results in me loosing a extra day who is going to pay me for my lost wage.
Gary wants to know who pay for a $50.00 chip on his car well who is going to pay me for the $1000.00 that I lost as a result of you desire to tailgate me.
I'm not joy riding I'm trying to make a living and time is money

As to what I think is too close go back and read my previous post. Look at what I actually said not what others tried to make it into.
In case you never noticed their are no stones on the interstates just emergency pull over lane that is pavement.
As to your advice on how I should have drove my truck I probably drove more miles just backing into a dock to unload than you have going forward
I have millions of accident free miles under my belt can you say that.

 

[Gary Allan]

My thing, BB, is that you could just slow down and have the person pass you out of lack of interest ..but, and again, far be it from me telling you how to configure your disposition toward your fellow man. If you want to hold them in contempt and "be in command" and force them to suffer the consequences of your dominion over them and their defiance of "your will"..well so be it. I don't pretend to live hold dominion over you.

I drove professionally ..and I was an owner operator. I didn't drive 18 wheelers. I drove my own cars/trucks. I did 125k/year for 5 years. I've seen enough, I suspect. The biggest problem that faces the driving public today, aside from incompetence and not taking the practice seriously, is the "me" that wants to force others to drive the way they think others should. It's just the main ingredient in road rage.