Over GVWR? Slow Down

[Mike Ellis]

There have been a lot of debates on slide-in campers and payload on various camping websites. It is easy to over GVWR with the larger campers, and a certain amount of paranoia exists regarding the ability of the trucks to handle the weight. Opinions seem to vary from those who think anybody 1 pound over GVWR should be sent to the electric chair, to those who are comfortable hauling a 5,000 lb camper on a VW bug.



I have done a bit of thinking on this topic, and it seems to me that if you are within the load-carrying abilities of your axles and tires, the remaining critical aspect of on-the-road safety is braking. Some folks think crash-worthiness is an issue as well, but since trucks do not appear to be crash tested with full loads in bed, much less with large bulky loads that approximate the loadout of a truck camper or such, it seems unlikely that this is a major player in how GVWR is set.



The braking system must be capable of bringing the vehicle down to a stop from legal highway speeds in some distance that the engineers think is acceptable. What stopping distance constitutes "safe" for this purpose? Is there a standard? It would be nice if every vehicle on the road could stop in the same distance as a new Corvette, but since they don't you have to wonder about how the makers determine what is "good enough" for the different trucks.



Since we don't know what stopping distance / time is considered critical by the engineers, we could scratch our heads a long time. We can, however, quite easily figure out much energy a vehicle of a given mass has at any speed - and we know that to stop that vehicle, that energy has to be dumped into the brakes as heat.



Suppose you have a truck at 10,500 lbs GVWR zipping along at 55 mph. This is in the neighborhood of 25 meters / sec, with a resulting kinetic energy of 1. 4 million joules. Mumbulating the numbers, we see that a guy with the same truck overloaded by 20% but travelling 5 mph slower will have almost exactly the same 1. 4 million joules of KE. So, by driving a bit slower the driver should be placing no greater demand on the braking system than that imposed by the truck at GVWR.



On the other hand, a guy who is fond of driving fast can push his brake system pretty harshly. Increase the speed of the truck at GVWR by 25 mph to a zooming 80 mph, and the KE is now more than doubled - 3 million joules. The guy with the 20% overloaded truck will still be well below this number (2. 7 million joules) even zipping along at 70.



On paper, it would seem that speeding at legal loads presents more danger than overloading at moderate speeds...

 

[klenger]

An exhaust brake can also offset the braking problem.

 

[dresslered]

Excellent write-up, Mike!



I too was ready to pass on my Bigfoot 3000 after taking it for a "test try" and having it weighed only to find that I had already exceeded my 11,000-lb. GVWR with an empty camper. What I did discover, however, was that this truck handled much better than other trucks that I had driven when empty. With the Jacob's brake, moderate traveling speeds, and some margin between my actual load and the axles and tires, I feel that I am traveling at least as safe as the other guy. Happy trails!

 

[jponder]

Originally posted by Mike Ellis

There have been a lot of debates on slide-in campers and payload on various camping websites. It is easy to over GVWR with the larger campers, and a certain amount of paranoia exists regarding the ability of the trucks to handle the weight. Opinions seem to vary from those who think anybody 1 pound over GVWR should be sent to the electric chair, to those who are comfortable hauling a 5,000 lb camper on a VW bug.



I have done a bit of thinking on this topic, and it seems to me that if you are within the load-carrying abilities of your axles and tires, the remaining critical aspect of on-the-road safety is braking. Some folks think crash-worthiness is an issue as well, but since trucks do not appear to be crash tested with full loads in bed, much less with large bulky loads that approximate the loadout of a truck camper or such, it seems unlikely that this is a major player in how GVWR is set.



The braking system must be capable of bringing the vehicle down to a stop from legal highway speeds in some distance that the engineers think is acceptable. What stopping distance constitutes "safe" for this purpose? Is there a standard? It would be nice if every vehicle on the road could stop in the same distance as a new Corvette, but since they don't you have to wonder about how the makers determine what is "good enough" for the different trucks.



Since we don't know what stopping distance / time is considered critical by the engineers, we could scratch our heads a long time. We can, however, quite easily figure out much energy a vehicle of a given mass has at any speed - and we know that to stop that vehicle, that energy has to be dumped into the brakes as heat.



Suppose you have a truck at 10,500 lbs GVWR zipping along at 55 mph. This is in the neighborhood of 25 meters / sec, with a resulting kinetic energy of 2. 8 million joules. Mumbulating the numbers, we see that a guy with the same truck overloaded by 20% but travelling 5 mph slower will have almost exactly the same 2. 8 million joules of KE. So, by driving a bit slower the driver should be placing no greater demand on the braking system than that imposed by the truck at GVWR.



On the other hand, a guy who is fond of driving fast can push his brake system pretty harshly. Increase the speed of the truck at GVWR by 25 mph to a zooming 80 mph, and the KE is now more than doubled - 6 million joules. The guy with the 20% overloaded truck will still be well below this number (5. 5 million joules) even zipping along at 70.



On paper, it would seem that speeding at legal loads presents more danger than overloading at moderate speeds...

YES!YES!YES! I agree totaly. People ripping down the interstate at 21K doing 70MPH are way more dangerous than a 30K persondoing 55. Their energy would equal at the 30K guy going 58. 56 MPH but he would still be safer because he would travel less during the reaction time if a situation occured. That is if you have tires to handle the load. Those guys that think that GVWR or GCWR has some magical value that will make them safe if they stay under it are living in a fantasy world. Those numbers are just to protect the manufactures from the lawyers. Your energy increases linearly with weight ; its a parabola with speed. I say we all slow down to 50 and check our Mileage

 

[zari]

Hmmm...

I don't think those running around over GVWR (or any G*WR for that matter) should be "sent to the electric chair", but I do believe it to be irresponsible to drive around knowingly overweight. You guys do realize that you're standing a good chance of your insurance company denying your coverage in the event... ?



I bought my truck to fit my 5th wheel - and weighed the whole thing to be sure. I am below all G*WRs, and believe that is the <em>only</em> responsible thing to do.



Alain.

 

[EricBu12]

I just bought a 28' Toybox you might have seen in another post...



I had it weighed and I had firewood in the bed and 2 other adults. I think I will put the firewood in the trailer next time, it had ample weight room.....



I was slightly over on GVW but the trailer wasn't over on it and I was at 19,400# total. ..... 600 under my GCVWR but with a Jake E-brake and 4 wheel disk on my 01 and with slotted rotors and carbon Kevlar pads, it seems to stop pretty well. The Kinetic energy is really felt even with my Trailer brakes but I am not pushing hard and I let the Jake do much of the braking on the off-ramps. I do drive reletively slower so That I have more reaction time. Good write up on that Figuring the #'s

 

[jponder]

You know Mike I had another thought. Instead of a speed limit we should have a maximum kinetic energy allowed limit. We can bust all these "I'm one lb under the GCWR so I can do 80MPH now", Idiots. If we assume a 70MPH 20K truck is okay energy then we can figure max speeds for every other combo. Man that means a 3K VW Bug has a speed of 180. 7 MPH Yikes can those cars go that fast?????



I once posted on one of those RV newgroups and got the kind of responses you are talking about. Some guy was asking about exceeding GCWR and I told him exactly what you said. SPEED is Way more dangerouse than weight. I told him I had pulled a D4H with my truck for 50 miles and They all went nuts because that would put me rolling at somewhere between 35K and 40K. They could not get the idea that at 40K going 50MPH was exactly the same energy as 20K going 70MPH. Could not get it, was like some kind of space alien talk. I only went 45MPH with that load. It sure was heavy

 

[klenger]

Looks like it's time to switch from an oil war to a weight war.



Here's a pic from irv2.com of someone that took the weight issue seriously. BTW, it's an F450 pulling.



#ad

 

[RustyJC]

Yeah, but Ken, did ya notice that he doesn't have a Hensley, or a PullRite, or WD bars, or sway control? That there trailer's probably gonna start swayin' an spit that ole truck right off in the ditch!



Rusty

 

[QRTRHRS]

I remember one member who used to freak at anything other that legal rating. Personnaly, I think it's a calculated risk and knowing your equipment and your own ablilities. On the occasion I am over, I am more concerned with DOT than anything. It makes a difference where you are and if your familiar with the area. Of course, there is a point where an equipment upgrade is in order.



This reminds me of an incident I percieved to be a potential hazard the other day. Some dude is pulling a tandem camper trailer with an Astro van. He's tooling along about 80 in a 55 with this thing. It looked managable at a reasonable speed but not 80! I am thinking, man, one good bow wave and your out of the game bud.

 

[MGordon]

Interesting analysis. As I've said before, there is not a pickup made that can legally carry the largest truck campers.



When I have the camper and trailer loaded, I personally limit my maximum speed to 65MPH on the interstate and follow the posted speeds elsewhere. I feel comfortable with being able to control and stop the truck at those speeds.

 

[Mike Ellis]

QTRHRS,



That kind of incident is what prompted me to look at the numbers the other night. During our recent vacation, I had RVs a plenty of all descriptions passing me like scalded dogs on every type of road, and I was driving the speed limits at the time.



zari,



My truck is right at GVWR with camper loaded, so I am not in the horrendously overloaded camp, but at the same time I am conscious of the extra weight and stopping distances required. In fact, at this very moment my truck is at Cummins Southern Plains having 60 psi exhaust springs put on in prep for an exhaust brake - not for the camper but for our big gooseneck horse trailer. Overloading is a big problem for those who buy truck campers, so most people who do so take extra pain to be careful when driving.



The point seems inescapable though - a driver and his passengers are at more risk from speeding in a loaded rig than they are from moderate overloading in the same rig at legal speeds. Unfortunately since the mfgs do not tell us what speeds the trucks are designed to be "safe" at, while towing at GCVWR or loaded at GVWR, we can only presume that they are safe at posted highway speeds. Even that is a risky assumption though, since it wasn't too many years ago that we had a national 55 mph speed limit. Did the mfgs design for safety at 70 or 80 mph back then? Seems doubtful, since nobody designs in extra safety margin unless they have to. When the limit was relaxed back to 70 mph again, were vehicles from the 55 mph era suddenly unsafe? Only the mfgs know and they aren't telling us anything they don't have to.



In reality, the very concept of safety on the roads is completely subjective. 100,000 pound trucks are allowed to drive at the same highway speeds as small cars and trucks, yet have much greater stopping distances, larger turn radius, bigger blind spots, etc etc. Why aren't there different rules for trucks vs cars in most places? Quite simply, because of money - we need stuff moved by truck, and they pay large amounts of road use tax - so they are allowed to drive at the same limits even though they can't stop / turn as fast as a car in an accident. Likewise, in some states you can have your trucks GVWR / GCWR increased simply by paying a higher fee - no change to the truck at all mechanically, or any change in the limits you are allowed to drive etc - just a magic "wave of the hand" and now you are legal and good to go.



All I know, is if I am sitting at a stop sign on a country road around here, I would be more comfortable with Farmer John coming up behind me with a 50,000 lb load of hay on a farm trailer behind his 6500 lb Dodge at 20 mph than with Weekend Wally coming up with the same truck and a 7,000 lb toybox at 50 mph. Why? Hint: Wally's brakes have to soak up 50% MORE energy than Farmer John's.



How about Joe Speeder in an unloaded truck at 80 mph? His brakes have to soak up 80% MORE energy than Farmer John, but people tend to be more worried about John's overloading than Joe's speeding. Seems illogical to me...

 

[jponder]

Mike,

Now don't you get wishy washy on us. You presented a strong argument AGAINST the "G*WR" Police AND in Favor of the "Horrendously overloaded" group, hereafter referred to respectively as GP and HOG. I would like to say right up front that us HOG's will admit you into membership unconditionally due to your understanding of Newtonian mechanics.

GP's on the other hand will scurry like roaches in sunshine when you start talking Physics. They will be blinded by the light of Newtons laws and will scream about Lawyers and Insurance companies and all sorts of stuff just to try to shut off the light! Truth confuses them and they will run to their OEM manuals and read them and feel safe. They will not look at the light

Over time 1/2MV(squared) will wear them down just like the cummins has worn down the ford boys and they will see the light. We need more Physics and less Lawyers and Insurance in our lives. I am glad that you are not trying to put everything in Joules any more, too much conversion. You know in the interest of Physics I will Pay $5 to anyone who can email me in the next hour and tell me the ENGLISH Unit of MASS. Just to be clear I did all my calcs as I'm sure Mike did with Lbs which is a force vector of course mass would be it divided by gravity(g). Everyone knows kilograms are metric unit of mass so what is English? Okay start your engines I need the answer and your adress for the 5dollar bill and I'm capping it at $50 bucks total. If a self professed GP knows the answer then they get 6. 50$. time is 12:49 and my email is -- email address removed --

GO!!!!! HOGS!!!!!!

 

[RustyJC]

Kinetic Energy

If there were a kinetic energy meter and governor on our trucks, I might become a HOG. Since neither is present, I'll remain a GP. And, yes, I'm conversant with Newtonian mechanics - something to do with Physics 101, Statics and Dynamics back in engineering school, I believe.



Rusty

 

[jponder]

Okay its over! the only person that got the money was J. C Creekmore. He gets 6. 50$ as he is a dogone GP man. Mike Ellis did chime in with the correct answer but 30 minutes late. Okay I'm mailing your price JC

 

[dresslered]

Hmmmm... . Cobwebs in the brain... . isn't it the slug?

 

[Mike Ellis]

jponder,

Wishy washy?? After so many years of politically correct indoctrination at a corporation, and experience as a manager to boot, my spine is so limp and flexible I'd have to take calcium supplements for a year just to have a PRAYER of being "wishy washy" again!

It would sure be interesting to get some of the automotive design engineers on the stand and start quizzing them as to what speed ranges a truck at GVWR can travel at and be considered OK. When you think about it, it is interesting that they don't have a blurb in the owner's manual de-rating GVWR / GCVWR versus speed.

 

[RustyJC]

Originally posted by jponder

Okay its over! the only person that got the money was J. C Creekmore.

Also known as RustyJC.



And Mike, after 30 years in engineering and technical management, I agree... Bring on the calcium supplements. :{



Rusty

 

[jponder]

Yall know good and well we aint going to get any OEM engineers to say anything. The lawyers and the insurance guys wont let them out tha HOUSE



All we got is sir Isacc Newton and hes been dead for 400 years!

 

[Mike Ellis]

Doh! I just *had* to go off and read about the blackout, if I had come back to TDR a bit quicker that $5 would have been mine