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-   -   Gcwr (http://www.airforums.com/forums/f463/gcwr-108704.html)

MrUKToad 08-20-2013 11:02 PM

Gcwr
 
A general question, does anyone know how a vehicle's GCWR is typically arrived at?

ROBERT CROSS 08-21-2013 05:17 AM

Dartboard, GM

Dice, Chrysler

Wheel of fortune, Ford

Everything else, London Ont.

Bob
:blink:

MrUKToad 08-21-2013 05:48 AM

Quote:

Originally Posted by ROBERT CROSS (Post 1343521)
Dartboard, GM

Dice, Chrysler

Wheel of fortune, Ford

Everything else, London Ont.

Bob
:blink:

Really? That scientific, eh? :sorcerer:

Who'd a thunk it?

dznf0g 08-21-2013 07:09 AM

CGWR is a number arrived at by the strength/durability of the weakest link in the longitudinal force affected components. Such things as all the driveline components, brakes, frame... Usually the vertical force components are not involved (like axle housing, springs, front suspension, etc. That is why they have separate specs, like rgawr, fgawr, payload, Gvwr...these are vertical load affected components. There may me some overlap of components which limit GCWR and one or more of the others...like rear springs which ( on a rear drive vehicle) sustain both kinds of loads. That's the elementary version of the answer.

Cooling system can often be the GCWR limiting factor as well

MrUKToad 08-21-2013 08:29 AM

Quote:

Originally Posted by dznf0g (Post 1343583)
CGWR is a number arrived at by the strength/durability of the weakest link in the longitudinal force affected components. Such things as all the driveline components, brakes, frame... Usually the vertical force components are not involved (like axle housing, springs, front suspension, etc. That is why they have separate specs, like rgawr, fgawr, payload, Gvwr...these are vertical load affected components. There may me some overlap of components which limit GCWR and one or more of the others...like rear springs which ( on a rear drive vehicle) sustain both kinds of loads. That's the elementary version of the answer.

Cooling system can often be the GCWR limiting factor as well

OK, but longitudinal forces are variable depending on what you're towing. Trailers of identical weight will exert vastly different longitudinal forces dependent on the shape and size of the frontal area that trailer presents to the oncoming air. In that case, surely to express the GCWR as a simple measurement in pounds is, at best, misleading.

Similarly, if you modify (improve!) something like the transmission cooling, doesn't that blow the manufacturer's stated GCWR out of the water?

I'm not disagreeing with you, by the way Rich, just exploring the issue.

dznf0g 08-21-2013 10:30 AM

All those forces are taken into account when arriving at the Max rating. A mod MAY change things....but even I don't have access to what component(s) is/are the limiting factor. Maybe stock trans cooling could support 5000#s more than stated max, but frame tips can't take any more calculated forces than stated, as a hypothetical example.

If trans temps are high, I put on a cooler....not to increase GCWR.

RickDavis 08-21-2013 11:03 AM

Each manufacturer looks at the competitors numbers and adds a couple hundred pounds. Probably the last ad writer wins

MrUKToad 08-21-2013 11:39 AM

Quote:

Originally Posted by RickDavis (Post 1343727)
Each manufacturer looks at the competitors numbers and adds a couple hundred pounds. Probably the last ad writer wins

This where I was going with this.

I liked (the other) Rick's mention of longitudinal forces as it makes sense to a degree, especially when considering individual components, but those forces are so variable that it would be impossible to put any realistic and finite limit on it in terms of the weight of a trailer. It would be simpler for the manufacturer to simply not quote a GCWR at all, which is in fact what Toyota have done with my vehicle, at least not on the door post. The manual says that the GCWR is the GVWR plus the weight of the trailer, by which I assume they mean the maximum weight permissible or the tow rating. On that basis, to use a tow rating figure in the calculation of the GCWR renders it entirely meaningless as the tow rating is a figure arrived at by marketing departments and not engineers, which is what you've said in your post Rick D.

I only used the example of improving the transmission cooling to show that that these things are variable, not expressly to improve the GCWR.

All this came about when a guy at a campground (in Canada) became quite agitated with me and told me that I should never tow my trailer in the US as the combination of TV and trailer surely exceeded the GCWR. The police would impound both vehicle and trailer at the first opportunity, apparently. This was patent nonsense as we'd just arrived back from a week in the US and have travelled in the US quite a bit and never so much as raised an eyebrow. Quite why the police would be any more interested in the US than in Canada was another point, but he didn't want to listen. Anyway, it had me thinking about the GCWR and how enforceable, if at all, it was. Given that it seems to incorporate the unprovable tow rating, the answer has to be that it's nothing more than a marketing exercise.

Axle and tire ratings are another matter of course; directly measurable and finite.

panheaddale 08-21-2013 11:59 AM

Quote:

Originally Posted by MrUKToad (Post 1343740)

All this came about when a guy at a campground (in Canada) became quite agitated with me and told me that I should never tow my trailer in the US as the combination of TV and trailer surely exceeded the GCWR. The police would impound both vehicle and trailer at the first opportunity, apparently. This was patent nonsense as we'd just arrived back from a week in the US and have travelled in the US quite a bit and never so much as raised an eyebrow. Quite why the police would be any more interested in the US than in Canada was another point, but he didn't want to listen. Anyway, it had me thinking about the GCWR and how enforceable, if at all, it was. Given that it seems to incorporate the unprovable tow rating, the answer has to be that it's nothing more than a marketing exercise.

Axle and tire ratings are another matter of course; directly measurable and finite.

I pull trailers commercially and their are laws in the US regarding GCWR. 26,000 is the limit to tow without a cdl license. If you exceed 26,000 pounds their are big fines to pay. They fine you for each pound you are over. I do think that anyone pulling a trailer should be held accountable to the same rules a commercial drivers that is what gets me so agitated about Can Am and their setups. I know you Canadians thinks he is the greatest thing since a pocket on a shirt because you get to use whatever tow vehicle you want to and get told that it is ok to pull with it. While the manufacturers GVWR rating might be a somewhat arbitrary number it is not going to be some completely random number pulled out of a hat. The engineering that has gone into vehicles over the years and even today has merit as to what the vehicle is capable of towing SAFELY. While you can easily tow more than the vehicle is rated at doesn't mean it is safe to do. REMEMBER Just because you can pull doesn't mean to can stop it or control it.

andreasduess 08-21-2013 12:00 PM

Quote:

Originally Posted by RickDavis (Post 1343727)
Each manufacturer looks at the competitors numbers and adds a couple hundred pounds. Probably the last ad writer wins

Alternatively, they just fabricate the number to suit the vehicle.

As an example, a Toyota Tundra comes with a tow rating of close to 10,000lbs, but the total payload of 1330lbs is actually lower than that of a Toyota Sienna minivan at 1513lbs, but with a published tow rating of 3500lbs.

andreasduess 08-21-2013 12:02 PM

Quote:

Originally Posted by panheaddale (Post 1343745)
because you get to use whatever tow vehicle you want to and get told that it is ok to pull with it.

That's not at all what happens when you consult with CanAm. Turn up with a vehicle that can't safely tow the trailer of your choice and they will tell you immediately.

RickDavis 08-21-2013 12:20 PM

Pretty much what panheaddale said.
While I claim no legal expertise, I have about 40 years experience with travel trailers and 10 years of commercial work delivering them.

I have never known any one to be ticketed for exceeding the GCWR of rating of their tow vehicle. Of course if you exceed 80,000 lbs that would be different.
It also irks me that a half blind 100 year old can drive a 40,000 lb motor home , but I can't legally deliver it to him because I don't have a CDL but that is another issue
I too strongly disapprove of the CanAm approach, but I do have a Canadian friend who tows his 34 with a Chevy Minivan and so far both he and the Minivan are still alive.
I have seen dealers park them with a riding lawnmower but it doesn't mean you should go on the road that way..
How the Insurance companies would react to a severe over GCWR is another question. I have heard stories and speculation but have no hard facts

Gene 08-21-2013 12:31 PM

It looks like manufacturers tow ratings are not believed. SAE came up with standard methodology to do the ratings and manufacturer's lowered (in most if not all cases) their tow ratings. I think the Tundra's went down a few hundred pounds from 10,000. Of course, it has been argued that SAE's engineers are lackies for the manufacturer's.

I'm not about to hire an engineering firm to calculate this for me.

So, my understanding of Gross Combined Weight Rating (GCWR) is what Toyota says, but I say it in simpler form—the total of the Gross Vehicle Weight Rating (GVWR) of the trailer and truck. The word "combined" tells it all. The tow rating is different—it means how heavy a trailer you can tow with a specific truck or SUV or car. There are also ratings for each axle and hitch receivers are rated too. Beef up the frame and hitch receiver of the tow vehicle (and maybe more), and you can increase its ratings. That is what I understand CanAm does and there is nothing unusual about people modifying vehicles just like CanAm does.

I think if US cops were stopping people for being over the GCWR we'd have heard about it on this Forum. No state requires people towing travel trailers to stop at weigh stations to check this and I doubt any state wants to go there. They don't have the money to operate enough weigh stations for commercial trucks as it is.

Gene

andreasduess 08-21-2013 12:34 PM

Borrowed from another poster, BradB, but worth posting here too - a 1987 Airstream brochure:

http://farm3.staticflickr.com/2872/9...8987b702_z.jpg

I wonder what the tow rating on this TV would be, happily married to a 34' triple axle. What I do suspect, strongly, is that my 2008 Honda is superior in any way to the car on the Airstream brochure - payload, engine, brakes.

(I didn't grow up in North America, so I have no idea what car this is. Perhaps somebody else can dig up the numbers)


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