What limits tow ratings for unibody vehicles?

[slowzuki]

Similar to the sedans, the first generation of suzuki sidekick had a 2500 lb tow rating here in Canada, I'm guessing in europe it was 3500 lbs with braked trailer. In the US I believe the tow rating was reduced to 1500 lbs, and I think the GM branded models maybe even 1000 lbs.

Some of it is marketing, in europe, the hp to weight ratios are always lower. No one expects to do 0-60 in 7 secs in a family car, and they expect slow hill climbs with trailers. People seem to want 300 hp commuters here, with an ignorant driver and a trailer that is going to damage drivelines and over heat engines that were never designed to operate at peak power output for long.

In US trucks, we have seen the climb of tow ratings in the last 20 years, to match how people have actually been using them. My 2001 F350 superduty originally came with about a 5000 lb tow rating similar to the previous models. To get the higher 12,500 tow rating, all I needed was the larger motor c/w with matching radiator. It had the same 4r100 auto trans, the same driveshaft, same axles, brakes, frame, transmission cooler, etc etc. The truck is very happy towing 16,000 lbs despite the original rating and you see the they had revised the factory rating to much higher even before the bigger brakes of 2005.

 

[PhilY]

Growing up my dad did a lot of s/w design for transmission controllers. I was told the main limiting factor for unibody vehicles are that most of them have FWD transmissions that can overheat from too high a load. That's why most trucks come with towing kits, ie. the trans cooler in front of the radiator. So, if you have a trans temp guage like I have in my truck, you can pull as heavy a trailer as you want as long as you can still stop it, which is the next limiting factor in the equation besides tongue weight. On a side note, you can overload a trailer quite a bit to as long as you know what you are doing. The payload capacity is just a guideline that helps cover things like hitting pot holes at high speeds where the force of impact from the hole could be the highest. So, if you take it slow and limit the bumps I have seen trailers take some mighty loads. I usually just load to the point where the leaf springs still make somewhat of a smile. If they curve down like a frown, you no things aren't going to be happy.

 

[Renze]

With that out of the way, I bet the limit is how much trailer you can put on it before the entire car begins to warp. As you know, the load-bearing system of a LeSabre is nothing like a ladder-frame truck. You might think you have plenty of brakes to stop that thing in a pinch, and maybe you do; but the car might not be straight afterward!

I remember my brothers Nissan Bluebird (with a lame 70hp 2.0 naturally aspirated diesel) opening the trunk lid when driving through dirt roads with a horse trailer... you could actually SEE the car deflecting sideways. My volvo is rated for 1850kg max vehicle weight, and 1600kg tow rating, but the same engine is used in a Volkswagen LT46 with a 4.6 ton gross vehicle weight, and 7.5 ton combined weight rating... I actually drag raced a Ford Transit yesterday, whilst being loaded close to the tow rating, a 900kg Toyota Starlet on a 500kg trailer behind it... And could have won if i wanted to take risks at an upcoming roadcrossing.. which of course i didnt.

Problem i have is weak rear springs. even with 3 people on the backseat its too close to the tarmac. I think i'll swap with stiffer rear springs from an 850 wagon or a V70 to be able to put some more towbar weight on it... being able to put weight on the towbar is what gives you stability, and most unibody construction cars dont have stiff enough springs to carry half a ton of drawbar weight because they are designed to give a smooth ride to the occupants. Thats what limits it.

When i brake fully, even at bone dry asphalt at 30°C summer temperatures, the ABS kicks in. Braking power is not a problem. But having the spring stiffness to control sway, or the spring capacity to carry the trailer drawbar load, is what limits the tow capacity. And for some cars like that Nissan Bluebird, its the structural integrity that limits the tow capacity...

 

[Renze]

On a side note, you can overload a trailer quite a bit to as long as you know what you are doing. The payload capacity is just a guideline that helps cover things like hitting pot holes at high speeds where the force of impact from the hole could be the highest. So, if you take it slow and limit the bumps I have seen trailers take some mighty loads.

Point is, You might know what YOU are doing, but you dont know what the road is doing. If you do hit a pothole, most likely the bearing balls push a dent in their race, then the bearing itself is riding over potholes, wearing it out fast.

There is a reason why manufacturers build in safety margins: Mostly against abuse by ignorant users

 

[zzvyb6]

A lot of interesting speculation on here so far, but the fact is that the U.S. EPA pretty much defines what a truck is:

"Federal regulations define a light-duty truck to be any motor vehicle having a gross vehicle weight rating (curb weight plus payload) of no more than 8,500 pounds (3,855.5 kg) which is ?1) Designed primarily for purposes of transportation of property or is a derivation of such a vehicle, or (2) Designed primarily for transportation of persons and has a capacity of more than 12 persons, or (3) Available with special features enabling off-street or off-highway operation and use.

That being stated, subsets of the law also reference ground clearance specifications, and that coupled with the ride travel requirements (max suspension deflection) pretty much rules out the use of unitbodies (intended for passenger compartment loads and fuel economy optimization with smaller motors, lightweight suspension pieces, etc. and unique emissions test results requirements) for 'heavy' trailer towing. A minivan driver is not likely to be the best trailer driver, either, so the Insurance Institute can give these vehicles a thumbs down for towing.

A Honda Odyssey is one vehicle I am aware of that is designed to certify for these 'truck' requirements. There are probably others. You'll notice that many states also have peculiar tag decriptions for such vehicles: Truck, station wagon, van, etc. For example, my Michigan registration calls my Chevy Tahoe a 'station wagon'. So its not a 'truck'. Go figure.

 

[Renze]

A lot of interesting speculation on here so far, but the fact is that the U.S. EPA pretty much defines what a truck is:

The topic isnt trucks, its unibody vehicles... So if you want to get to the point, please tell us what the law says about tow ratings for unibody vehicles ?

 

[JDgreen227]

The topic isnt trucks, its unibody vehicles... So if you want to get to the point, please tell us what the law says about tow ratings for unibody vehicles ?

To my knowledge there are basically no LAWS regarding towing with either body-on-frame or unibody vehicles here in Michigan. The company that builds each vehicle for sale makes SUGGESTIONS and supplies INFORMATION AND RECOMMENDATIONS for the consumer which includes tire loading, weight distribution, maximum payload, recommended equipment, etc. but I am not aware of any LAWS regarding tow ratings for vehicles used by private individuals. As a former CDL-B holder here in Michigan I cannot think of any specific laws regarding tow ratings except for a few that define those who tow what is defined as a "recreational triple" which is commonly known as a truck/fifth wheel/boat on trailer combination and I am sure the only regulations there are the ones that define maximum overall length and licensing requirements.

 

[Diamondpilot]

A Honda Odyssey is one vehicle I am aware of that is designed to certify for these 'truck' requirements. There are probably others. You'll notice that many states also have peculiar tag decriptions for such vehicles: Truck, station wagon, van, etc. For example, my Michigan registration calls my Chevy Tahoe a 'station wagon'. So its not a 'truck'. Go figure.

Same thing with the PT Cruiser. Dodge/Chrysler had to do something to combat the early Hemi awful gas MPG to keep the CAFE rating in check for the fleet so they took a Neon and made it meet the requirements of a truck and you ended up with a PT Cruiser. The public loved them but what a POS. I started refusing them as rental cars and my uncle who owns service stations/gas stations on the north side of Cincy/south side of Dayton says they are by far the worst vehicle built in the last 20 years.

Chris

 

[dodge man]

First off, it doesn't have anything to do with unibody. Chrysler has been building unibodies since the early to mid 60's. In my opinion, the limiting factor is the brakes, transmission, and suspension in that order. Imagine going down a long mountain road, using the brakes a lot, will they fade? When your going 60 mph on a windy day and a truck passes you, will the trailer start to sway? Pull a long steep hill, what will the transmission be doing at the top of the hill?

 

[Renze]

To my knowledge there are basically no LAWS regarding towing with either body-on-frame or unibody vehicles here in Michigan. The company that builds each vehicle for sale makes SUGGESTIONS and supplies INFORMATION AND RECOMMENDATIONS for the consumer which includes tire loading, weight distribution, maximum payload, recommended equipment, etc. but I am not aware of any LAWS regarding tow ratings for vehicles used by private individuals. As a former CDL-B holder here in Michigan I cannot think of any specific laws regarding tow ratings except for a few that define those who tow what is defined as a "recreational triple" which is commonly known as a truck/fifth wheel/boat on trailer combination and I am sure the only regulations there are the ones that define maximum overall length and licensing requirements.

Well then USA is the same as here.
There are trailer manufacturers that take a loophole in the law (meant for overweight heavy transport) and take a test to get a 3.5 ton small drivers license vehicle, approved for 15 ton GVW.... its a death trap, even with the standard manufacturers rating of 10 ton GVW you cant survive a small accident in an Iveco Daily (basically a van) when the trailer hits you...

So from next year we get the law that small drivers license means max 3.5 ton, and 3.5 ton trailer. However the max ratings for vehicles, are all up to the manufacturer. If Mini decides to approve such compact for 3.5 ton trailer weight, its their choice....

Regarding brakes for trailers vs. brakes on unibody vehicles: The disk brakes on 1800kg torsion axles, are WAY WAY smaller than the disks on my Volvo.... Its because my Volvos brakes shouldnt fade when i take it down from a 200kmh top speed.... Where the trailer only needs to brake from 100kmh down....
Also, a 300x100 brake is used for highway trailer axles up to 5.5 ton. The very same brake is used for agricultural axles up to 8 ton with much higher wheels (means more brake torque required)

In other words, passenger car hi-speed brakes have quite some overcapacity... Pickup trucks need bigger brakes because they run tires with a bigger rolling radius, means more brake torque to apply the same brake force to the asphalt....

I dont think brakes are such a big issue: its mostly the suspension difference. At my former employer we converted Mercedes Sprinters to 5th wheel tractors (the idea is that no CDL is required) and we had a rental vehicle, a 3.5 ton which swayed and sagged with a 7 ton 5th wheel behind it. The customer complained, so we put the 3 leaf suspension of a 5 ton (which were leftover after an air ride conversion) under the 3 ton chassis. The vehicle was overloaded according to the original manufacturers rating, but within the load rating of the tires. But it had absolutely no problems with the 10 ton combined weight with a 3.5 meter high, 10 meter long curtainsider trailer... Thats all suspension.... unibody vehicles are built for handling, smoothness and comfort, which doesnt really go together with towing. Unless you install air suspension which can be adjusted according to need....