Why discount truck tow rating

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SuwanneeDave

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I see a lot of recommendations to not exceed 80-90% of a trucks towing capacity. Why is this? Wouldn't the truck maker rate them conservatively anyway? If they wanted to limit it to a lower percentage wouldn't they have stated so? I assume they test on grades as well as flat roads. Just for the record I don't plan to tow at the max, just wondering about the basis for the recommendation.
 
SuwanneeDave said:
I see a lot of recommendations to not exceed 80-90% of a trucks towing capacity. Why is this? Wouldn't the truck maker rate them conservatively anyway? If they wanted to limit it to a lower percentage wouldn't they have stated so? I assume they test on grades as well as flat roads. Just for the record I don't plan to tow at the max, just wondering about the basis for the recommendation.
That's a very valid question.

The "Tow rating" is a rating that the manufacturer tells you is the maximum amount of weight that the vehicle can safely tow and normally with the minimum amount of passengers and cargo. In other words, they are saying, "A single person can can tow a 5,000 boat (for example) to the river for a day fishing". However, that same vehicle may be over taxed when 4 passengers, the dog and 500# of gear are loaded in the vehicle. It must be kept in mind that ALL ratings must be considered when towing. Tow rating, on it's own, means little when one is thinking of loading a rig for a camping trip. There are many other ratings that come into play, especially when one gets close to the max tow rating.

The other thing to keep in mind is that the vehicle is rated in "New" condition. As the vehicle ages, parts become worn and that rating should be discounted.

If a parachute has a maximum weight rating of 300#, would you want to jump out of an airplane with that parachute if you weighed 285# and you knew the chute was 2 years old????........ Same rule applies.

I forgot to answer the 2nd part of your question....... manufacturers rate capacities very closely to be able to compete with other manufacturers and maintain market share and keep costs in line. There is no upside to over-estimating. The ratings are done by strict engineering calculations. The way that it is done is actually different than one might think. The marketing dept decides what the vehicle should be rated at and the engineering staff designs the vehicle to meet that guideline, keeping costs in line at the same time. Warranty costs and legal costs are all part of the calculation.
 
I'll elaborate a bit on Wavery's excellent answer.  If you operate anything at its maximum limit as the normal mode of operation, you are always running at 100% stress. The tow vehicle isn't going to last as long as it might at, say, 70-80% of max, nor is it likely to handle as well as it might closer to the mid-point of its performance envelope. An analogy might be running the vehicle continuously at its max rated speed or peak horsepower. It can do it, but don't expect everything else to last. That's why race car engines get rebuilt after every race.

The rest of it is simple arithmetic - you have to allow for the extra weight of passengers and gear and even the trailer hitch. Typically 5-10% of the tow capacity gets used for carrying something other than the trailer.
 
SuwanneeDave said:
I see a lot of recommendations to not exceed 80-90% of a trucks towing capacity. Why is this? Wouldn't the truck maker rate them conservatively anyway? If they wanted to limit it to a lower percentage wouldn't they have stated so? I assume they test on grades as well as flat roads. Just for the record I don't plan to tow at the max, just wondering about the basis for the recommendation.

I don't believe that derating is necessary or sensible and have posted at length on my reasons in the past.

 
Jammer said:
I don't believe that derating is necessary or sensible and have posted at length on my reasons in the past.
It's sort of a personal choice and it boils down to how much risk one is willing to accept (or lower)....."Risk Management" and how much one is willing to pay in maintenance.

None of us can "tell" others what to do but I prefer to err on the side of safety when making recommendations to others. What I do personally, is up to me and what risk I am willing to accept.

IMO..... the ultimate towing machine will have a tow rating double what I am towing.

Here's a good example of the "Ultimate tow rig":


 

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Wavery said:
The other thing to keep in mind is that the vehicle is rated in "New" condition. As the vehicle ages, parts become worn and that rating should be discounted.

If a parachute has a maximum weight rating of 300#, would you want to jump out of an airplane with that parachute if you weighed 285# and you knew the chute was 2 years old? ??? ........ Same rule applies.
I don't think that modern vehicles degrade much for many 10's of thousands of miles if maintained correctly.
As far as the parachute, yes I would jump if it was rated at 300. Where does the guessing stop; at 280lbs, 270, 260...?



Moderator edit: fixed broken end-quote tag
 
I don't consider what I suggested to be "de-rating". The rating is exactly what it says - the maximum towed amount assuming a 154 lb driver, a tank of fuel and nothing else in the truck. If you don't match those assumptions, the rating is different (lower).

As for staying inside the performance envelope rather than its very edge, that is a matter of operational choice.
 
I will give the Tim Allen response MORE POWER.

If you tow at the max, your performance is at the minimum, You may have "Issues" going up a long hill or a steep hill.

Cut it down 10 or 20 percent you get better performance and hill climbing.

There is also a Monty Python video of a little car and a big boat going up/down a hill.
 
SuwanneeDave said:
I don't think that modern vehicles degrade much for many 10's of thousands of miles if maintained correctly.

A human starts dying the day that he is born. Some die young, some die old. A lot has to do with the way that we treat ourselves (wear & tear), a lot is genetic...... sometimes we just hit potholes.......

The same goes for any vehicle. Some are just built better than others but they all suffer from wear & tear and metal fatigue from the 1st time that they are driven until that one last part fails that puts it in the junkyard. Anyone that thinks that a vehicle with 100K miles on it has the same individual part longevity as a new vehicle is mistaken. Brakes fail, springs weaken, shocks, u-joints, steering linkage, transmission parts, all fluids.... heck, at some point the welds on the frame weaken from fatigue. None of that happens all at once. It's a slow wearing process. That process of wear & tear slowly eats away at the vehicles ability to perform as new in every way.
SuwanneeDave said:
As far as the parachute, yes I would jump if it was rated at 300. Where does the guessing stop; at 280lbs, 270, 260...?

Moderator edit: fixed broken end-quote tag
Notice....... I purposely said "and you knew the chute was 2 years old"...... same rule applies as above....... one would assume that a 2-year-old chute had been used many times. The parachute is rated in "New condition". You may want to re-think that or we might have to rename you, "SwanDive"....... ;)..... sorry......... that was mean  :-[.... I just couldn't resist.... :p
 
Some of us tow in very high elevations too.  On a gasoline engine, power is reduced by about 3% per 1000' of incline.  Thus, my horsepower is up to 30% less than those of you with a comparable gas engine towing at sea level.  And coming down steep grades, we want an extra margin of truck capacity.  You're one blown fuse away from no trailer brakes....can the truck stop both itself and the trailer quickly enough coming down an 8% grade if the need arises, or is the trailer going to be shoving you down the hill in a white knuckle thrill ride? 

Then there's stability....is the truck robust enough to control the trailer if you get a sudden crosswind, or will it allow a sway that may turn ugly?  Again, more capacity is better than being marginal.

I estimate I am at about 75% capacity.  My truck tows the trailer just fine even in the high country....but I wouldn't want any more weight without upgrading the truck....and that's just a seat of the pants thing.  Flat land, I barely know it's back there.  Going from 4000' to 9200' in 16 miles, the old F250 earns its pay. 
 
Gary RV Roamer said:
As for staying inside the performance envelope rather than its very edge, that is a matter of operational choice.

I don't believe that the manufacturer's published ratings are the edge of the performance envelope.

 
Wavery said:
The other thing to keep in mind is that the vehicle is rated in "New" condition. As the vehicle ages, parts become worn and that rating should be discounted.

Why do you believe that?  It seems to me that any capacity rating should remain valid for the useful life of the vehicle, with proper maintenance.  That's the way it works in trucking, aviation, etc.
 
Frizlefrak said:
Some of us tow in very high elevations too.  On a gasoline engine, power is reduced by about 3% per 1000' of incline.  Thus, my horsepower is up to 30% less than those of you with a comparable gas engine towing at sea level.

What does this have to do with safety?

And coming down steep grades, we want an extra margin of truck capacity.  You're one blown fuse away from no trailer brakes....can the truck stop both itself and the trailer quickly enough coming down an 8% grade if the need arises, or is the trailer going to be shoving you down the hill in a white knuckle thrill ride? 

But this has little to do with tow rating.  In current production trucks you can find 1/2 ton trucks with higher tow ratings than 3/4 ton trucks; the 3/4 ton trucks have larger brakes. 

Then there's stability....is the truck robust enough to control the trailer if you get a sudden crosswind, or will it allow a sway that may turn ugly?  Again, more capacity is better than being marginal.

But that too is only roughly related to weight.  It's more closely related to trailer wind area, weight distribution, and the location of the pivot point relative to the truck's rear axle.
 
Jammer said:
Why do you believe that?  It seems to me that any capacity rating should remain valid for the useful life of the vehicle, with proper maintenance.  That's the way it works in trucking, aviation, etc.
It would "Seem" that way and we would all like it to be that way but that's not reality. Parts ALL break down over time. That's why we have junk yards.

Commercial trucking is strictly regulated by the DOT Federal Motor Carrier Safety Admin...... not so light personal trucks......

Aircraft must have closely regulated yearly inspections to satisfy the FAA.......nothing like that for light personal trucks......

The best that one might hope for is that the manufacturer's ratings may be good for the life of the warranty if they are not abused by violating any one of the many ratings that the vehicles has (beside the tow rating). Although.... the fact is... stuff breaks and wears out even while the vehicle is under warranty sometimes.

BTW.... to answer your question, "Why do you believe that?"....... I don't just believe it.... I know it...... I graduated from General Motors University and worked for GM for a few years before taking a job as Service Manager for a large Chevrolet Dealer for many years..... I lived and breathed this stuff....
 
Tow ratings have been used as marketing one-upmanship for so long, they've reached the point where the maximum ratings have only a passing relationship with reality when towing a conventional or 5th wheel RV trailer, IMO.

Tow ratings are usually set using a small frontal area trailer that largely slipstreams behind the tow vehicle.  Think horse trailer, utility trailer, etc.  Not high and wide RV trailers with vastly more wind resistance.

When you're towing at the vehicle's maximum rating, by definition you'll have performance that's barely acceptable in one or more areas.  If the limiting factor is available power or drive train stresses, adding the extra wind resistance of a high, wide trailer can push you into unacceptable stress or performance areas.

Limiting the towing weight to less than the published maximum gives better performance, so when you run into an unexpected situation (strong headwind, long upgrade, extra cargo in the tow vehicle) you have some performance in reserve to handle it.

 
Lou Schneider said:
Limiting the towing weight to less than the published maximum gives better performance, so when you run into an unexpected situation (strong headwind, long upgrade, extra cargo in the tow vehicle) you have some performance in reserve to handle it.

Perzactly.  Better to have it and not need it than need it and not have it. 

But hey....nobody is stopping you from towing at the ragged edge of what your truck is capable of.  If you feel inclined to do something that's ill-advised, nobody is going to stop you.  You may get lucky and get away with it forever.  I'm not a big fan of luck when it comes to my family's safety. 

 
Lou Schneider said:
Tow ratings have been used as marketing one-upmanship for so long, they've reached the point where the maximum ratings have only a passing relationship with reality when towing a conventional or 5th wheel RV trailer, IMO.

I believe that Lou's comment is insightful.

When tow ratings were first introduced, they were, in most cases, artificially low and used by truck makers as a tool for selling powertrain upgrades.  (Want to tow a trailer? You'll need the big engine to be safe.)

Recent years has seen the ratings used as a point of competition between manufacturers, at least for pickups.

I personally will not be seen towing an 8,000 pound trailer through the mountains with a 1/2 ton truck, even if the manufacturer claims it will tow 11,000 pounds because it has the V10 or the diesel or the 4.10 gears.  That's lunacy.  But by the same token I think it's safe to tow the same 8,000 pound trailer through the mountains with a 3/4 ton truck that (like my 1997 Chevy) is rated for less than that, because of the entry level power train.

You can't just take 10%, 15%, 20% or whatever off the capacity and expect that to provide a more meaningful guide.

Tow ratings are usually set using a small frontal area trailer that largely slipstreams behind the tow vehicle.  Think horse trailer, utility trailer, etc.  Not high and wide RV trailers with vastly more wind resistance.

Right, and one of the nuances that gets lost in these discussions is that it is safer to tow a 34' airstream, which sits low to the ground, than it is to tow a 34' 5er that will barely clear a 13'6" bridge.

When you're towing at the vehicle's maximum rating, by definition you'll have performance that's barely acceptable in one or more areas.  If the limiting factor is available power or drive train stresses, adding the extra wind resistance of a high, wide trailer can push you into unacceptable stress or performance areas.

Limiting the towing weight to less than the published maximum gives better performance, so when you run into an unexpected situation (strong headwind, long upgrade, extra cargo in the tow vehicle) you have some performance in reserve to handle it.

By that logic a Ford Mustang is safer than a Ford Escort because it has more performance reserve, but the statistics bear out the opposite.  That aside, there is nothing to indicate that 10%, 15%, 20%, or any other particular figure is the right amount.  In absolute terms we're safer if we stay in a hotel.
 
I think that some people forget what comes 1st... the chicken or the egg?.....

Manufacturers do not build vehicles, then assign a tow rating.... it just doesn't work that way.

The marketing dept decides what the vehicle should be rated at and the engineering staff designs the vehicle to meet that guideline, keeping costs in line at the same time. Warranty costs and legal costs are all part of the calculation.

Manufacturers rate capacities very closely to be able to compete with other manufacturers and maintain market share and keep costs in line. There is no upside to over-estimating. The ratings are done by strict engineering calculations. The way that it is done is actually different than one might think.

As others have stated. it's a matter of the "weakest link" that determines any rating. The weakest link is sometimes the most expensive part to design and/or replace.

The other thing to remember, engineers do not concentrate on performance (again that's a marketing function). They concentrate on failure. Engineering could care less what hills you can climb. They only care that you can get back down the other side without a failure that could cause an injury or fatality. Marketing tells them what HP to build the engine to. That has little to nothing to do with safety.
 
New vehicles now get their tow rating set in conformance to the SAE standard J2807, a vast improvement.  It even includes frontal area as a factor for heavier loads, a substantial hill climb and a passenger onboard! I believe all 2013 pick-up truck tow ratings use the J2807 standard rating, and most models in 2012 did as well.

Here is an article that gives an overview of J2807.
http://www.automobilemag.com/features/news/0912_sae_tow_ratings_finally_pass_sniff_test/
 

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