What makes a car/truck capable of towing whatever weight it's rated for? How is the tow rating determined? If I build some evil contraption in my garage, how do I know how much is safe to pull with said contraption? Now I tow with a 2WD V8 Dodhe Dakota, but it's not the most stable. I want to put a 7.3 power stroke diesel in a late 70s behemoth Ford (or Lincoln) sedan (or wagon). How will I know how much is safe to pull with it?
It's a combination of chassis, brakes, tires, suspension, power, handling, and durability. You need all of them to tow a set amount, if you're missing one, you're screwed.
If I , for example, put a 7.3 power stroke diesel, an E40D automatic & a 9" rear in a 76 Mercury Grand Marquis, shouldn't I be able to tow a house with it?
Yes, with appropriate brakes, springs, shocks, a better hitch, and the correct tires.
But can you stop it? Those brakes barely stopped those behemoths without any weight behind them.
I'm pretty sure those were only rated for 2k back in the day. The frame just isn't up to it.
M030 wrote: If I , for example, put a 7.3 power stroke diesel, an E40D automatic & a 9" rear in a 76 Mercury Grand Marquis, shouldn't I be able to tow a house with it?
Old full size big block cars were once the go to tow vehicle. The 92-96 Fleetwood could be optioned to tow up to 7000 lbs, stock.
Like Tom said, its more of what the chassis can handle. The frame on the car is nowhere as stout as a 3/4 or 1-ton truck, plus the truck rear axles can carry more load than a 9" by quite a bit (they are full-floaters). I'm not sure the full-size car will be more stable than a full-size pickup, but probably better than the Dakota. Another issue is cooling the Diesel. The radiator in my old 91 F350 7.3L IDI was quite a bit larger than the one a 460 used. It would be nearly impossible to get that large of a radiator in a car, especially for towing use.
I want to tow a 20' airstream with a late 60's Tempest (convertible). Neither of these is very close to fruition now, but I think they're legit. Trailers have brakes for a reason. As long as you aren't irresponsible, have some sway control, and don't jump on the damned brakes, you should be ok.
Having said that, I love the idea of putting a modern cummins or similar in a 60's or 70's boat. E36 M3, if Chris Ledoux can sing about a horse trailer on a cadillac, then you should be good.
It has been a pipe dream of mine for some time to stuff a Silverado 3500 frame with a Duramax/Allison combo under a '70s Caprice with a Class 5 hitch to tow a nice bumper-pull enclosed car hauler. Assuming the wheelbase modification is done properly to the frame, you wouldn't lose any capacity from the truck frame/running gear.
I just wouldn't trust the car frame for that kind of duty.
And lawyers...
For some amusement, compare the tow ratings for cars in the UK and the same model sold in the US.
Ian F wrote: And lawyers... For some amusement, compare the tow ratings for cars in the UK and the same model sold in the US.
Over here people believe you need a 3/4 ton diesel to pull a horse trailer. In Europe you see them hanging off the back of a Volvo wagon. I wouldn't do it, but they seem to get along ok.
It's not really a combination of anything. It's the weakest link. You CONSIDER all the factors, but the weakest link trumps the others, no matter how strong they are.
If we are talking OEM, it's (the weakest link) x (the strongest lawyer) to the fourth power. 
If you are building your own contraption, you will not have an official rating at all. That's not necessarily a bad thing, but you gotta consider safety first. You need to test it and know your limits. If you can't do the engineering (like me), then plenty of controlled testing before assuming anything.
I would worry first about brakes, then suspension, then weight distribution, then cooling.
If you are wrong, the formula will be (the strongest lawyer) x (everything you own) - (your insurance company disowning you) x (# of kids injured) to the tenth power.
I'd rather tow a house with a 4-cylinder F350 than a car with a 7.3L. Engine power and transmission capability are only a tiny part of it. Sure, you can accelerate fine, but one stiff breeze and you'll be in a ditch. Its the whole package; brakes, massive leaf springs that keep the axle centered, tire sidewalls that don't flex as much, enough weight to anchor the trailer... its a big thing.
Tow ratings are often times complete BS. A good example: a 96 Caprice with the 4.3L and the 7.5" rear axle and 2.56 gears is rated to tow more than my 96 Impala SS with a 5.7L and the 8.5" rear and 3.08 gears. That is marketing to the intended consumer. Lowering the tow rating means they can deny more warranty claims on the SS and the consumer doesn't care because market research suggests SS owners aren't buying it to tow.
Tow ratings are also BS from the standpoint of trailer factors. A 10,000 lb flatbed with a tractor on it is a lot different than a 10,000 lb travel trailer with cheap axles that catches wind like a billboard. A 30' triple axle car hauler will behave much differently than a 30' double axle enclosed trailer. Not to mention torsion axles act differently than trundle/spring axles.
Tow ratings are also BS from the standpoint of different drivers. I used my 73 Impala to tow a 9500 lb trailer a few times for short distances. The trailer had excellent brakes, I wasn't going far, and I know how to drive an automatic so that it doesn't fry itself. Contrast that with someone else who might get in their new F150, put in OD, hitch up a 9500 lb trailer and put their foot to the floor.
The manufacturer gives you tow ratings as a guideline. They are basically saying that if the tow rating is 5000 lbs and your transmission explodes while you're towing 5001 lbs, they can tell you to go to hell. They are also protecting themselves from class-action lawsuits. If 15% of their buyers experience (let's say) rear axle bearing failure while towing an 8000-lb load, its wise that manufacturers under-rate the tow capacity to prevent legal losses.
I am building a 66 Bonneville with a Duramax. It already has 3/4 ton front spindles and brakes, a 10.5" full-floater rear, and massive brakes all around. Spring rates are modest, but backed up with airbags. In a straight line, it will tow three houses uphill with an anchor dragging behind. But, right now it has a modified version of the factory 4-link trailing arms with rubber bushings. It also has the stock 4" channel frame and no room for truck tires. You can't compare that to an 8" x 4" fully boxed ladder frame like what comes under an F250 with big leaf springs and E-range 30" diameter tires.
I will be towing with my Bonny. I plan to get something like a vintage airstream or Spartan trailer and fix it up, but regardless of how heavy-duty the parts are that I bolt on, its still a 4000-lb flimsy car - not a 6000-lb box-frame truck.
Tom Suddard wrote: It's a combination of chassis, brakes, tires, suspension, power, handling, and durability. You need all of them to tow a set amount, if you're missing one, you're screwed.
More than that, cooling. By far, the #1 part of towing capacity. The rest is normally there in spades. At least for vehicles made in the last 15 or so years.
Just read the door panel sticker and roll with it. Its the safest way.
M030 wrote: If I build some evil contraption in my garage, how do I know how much is safe to pull with said contraption?
Experience. My bonneville will easily be capable of towing 10k lbs, but i won't get on the interstate with it like that. I would tow my buddy's flatbed across town, but I wouldn't tow a 10k lb travel trailer cross-country.
I want to put a 7.3 power stroke diesel in a late 70s behemoth Ford (or Lincoln) sedan (or wagon). How will I know how much is safe to pull with it?
If you could comfortably tow 5000 lbs with it before the PSD swap, you will be able to comfortably tow 4700 lbs after the swap. (since you'll be adding 300 lbs worth of iron). Engine is probably the tiniest part of tow capacity.
Think of it this way... I loaded my 26' box truck to the gills (well over its 26k GVWR) and hitched up a flatbed with the Bonneville on it (total weight around 9000) and had no trouble keeping up with traffic with a 185 hp 6-cylinder. Sure, you want lots of torque and power so you can pull weight, but its so insignificant in the grand scheme of towing capacity.
I had a Cherokee I did a lot of towing with. Using a dolly without breaks and 3000 lb car. Flat level ground fine. Moutains on i75 little scary. I now have a 3500 6.5 t crew cab. With the same rig no stress at all.
curtis73 wrote: I'd rather tow a house with a 4-cylinder F350 than a car with a 7.3L. Engine power and transmission capability are only a tiny part of it. Sure, you can accelerate fine, but one stiff breeze and you'll be in a ditch. Its the whole package; brakes, massive leaf springs that keep the axle centered, tire sidewalls that don't flex as much, enough weight to anchor the trailer... its a big thing. Tow ratings are often times complete BS. A good example: a 96 Caprice with the 4.3L and the 7.5" rear axle and 2.56 gears is rated to tow more than my 96 Impala SS with a 5.7L and the 8.5" rear and 3.08 gears. That is marketing to the intended consumer. Lowering the tow rating means they can deny more warranty claims on the SS and the consumer doesn't care because market research suggests SS owners aren't buying it to tow. Tow ratings are also BS from the standpoint of trailer factors. A 10,000 lb flatbed with a tractor on it is a lot different than a 10,000 lb travel trailer with cheap axles that catches wind like a billboard. A 30' triple axle car hauler will behave much differently than a 30' double axle enclosed trailer. Not to mention torsion axles act differently than trundle/spring axles. Tow ratings are also BS from the standpoint of different drivers. I used my 73 Impala to tow a 9500 lb trailer a few times for short distances. The trailer had excellent brakes, I wasn't going far, and I know how to drive an automatic so that it doesn't fry itself. Contrast that with someone else who might get in their new F150, put in OD, hitch up a 9500 lb trailer and put their foot to the floor. The manufacturer gives you tow ratings as a guideline. They are basically saying that if the tow rating is 5000 lbs and your transmission explodes while you're towing 5001 lbs, they can tell you to go to hell. They are also protecting themselves from class-action lawsuits. If 15% of their buyers experience (let's say) rear axle bearing failure while towing an 8000-lb load, its wise that manufacturers under-rate the tow capacity to prevent legal losses. I am building a 66 Bonneville with a Duramax. It already has 3/4 ton front spindles and brakes, a 10.5" full-floater rear, and massive brakes all around. Spring rates are modest, but backed up with airbags. In a straight line, it will tow three houses uphill with an anchor dragging behind. But, right now it has a modified version of the factory 4-link trailing arms with rubber bushings. It also has the stock 4" channel frame and no room for truck tires. You can't compare that to an 8" x 4" fully boxed ladder frame like what comes under an F250 with big leaf springs and E-range 30" diameter tires. I will be towing with my Bonny. I plan to get something like a vintage airstream or Spartan trailer and fix it up, but regardless of how heavy-duty the parts are that I bolt on, its still a 4000-lb flimsy car - not a 6000-lb box-frame truck.
why can't you put some 30" tall load range E tires on that Bonneville? it probably came with 28" tall tires stock and the wheelwells were absolutely massive back then. hell, an entire floater rear end out of a 3/4 ton truck might just slide under there with a little bit of fabrication. a few 1/8" steel boxing plates added to the frame and you've got yourself a car chassis that's every bit as good as any 3/4 ton truck chassis.
Interestingly, there is now an SAE spec for tow ratings. Things like having to accelerate up an X perfect grade to Y MPH in under Z seconds a certain number of times, or do a certain number of standing starts on a certain percent grade so many times without frying the clutch or transmission. IIRC there are also braking tests as well.
As for the 4.3 Caprice being rated higher than the SS Impala... I can totally see that, if only because the 4.3 is straining the cooling system less. Plus, the SS has higher compression, which isn't the hottest thing for towing.
novaderrik wrote: why can't you put some 30" tall load range E tires on that Bonneville? it probably came with 28" tall tires stock and the wheelwells were absolutely massive back then. hell, an entire floater rear end out of a 3/4 ton truck might just slide under there with a little bit of fabrication. a few 1/8" steel boxing plates added to the frame and you've got yourself a car chassis that's every bit as good as any 3/4 ton truck chassis.
Or start with/find an ambulance-spec chassis.
I also agree that people need to forget about the engine when thinking about towing capability. It has almost nothing to do with it. If you think you need to be able to accelerate as if you're not towing, you're probably more likely to cause an accident than a guy with a smaller motor.
novaderrik wrote: why can't you put some 30" tall load range E tires on that Bonneville? it probably came with 28" tall tires stock and the wheelwells were absolutely massive back then.
The wheel wells are pretty big, but unless I jack up the car I won't have any suspension travel. It came with 26.8" tires stock. I have 225/65-16s on it now just because I had them sitting around. I'm hoping to at least fit 235/70s but that will be a stretch.
a few 1/8" steel boxing plates added to the frame and you've got yourself a car chassis that's every bit as good as any 3/4 ton truck chassis.
Well, i wouldn't say as good as, but I do plan to box the frame and add a K-frame or X-frame to it. This is the first build where I'm actually not concerned about adding weight.
hell, an entire floater rear end out of a 3/4 ton truck might just slide under there with a little bit of fabrication.
It does, and I did. 10.5" full floater with a posi and 3.21 gears.
Full-floating rears create great happy feeling in me.
I keep eyeing what it would take to put a bombproof rearend in the RX-7 and I keep going back to this: I could buy a Toyota 8" rear for a few hundred, clean it, replace all of the bearings, have the axles plug welded/turned/drilled, hunt down a limited slip unit, etc. Or. I could get a full floater 9" circle track rear and a spool, already have new bearings and axles installed, comes with brakes and car-friendly bolt pattern, built with the driveshaft on-center (which is why Explorer 8.8s are not an option), and the best part is I can just pop the axles out if I want to flat-tow the car.
Knurled wrote: Interestingly, there is now an SAE spec for tow ratings. Things like having to accelerate up an X perfect grade to Y MPH in under Z seconds a certain number of times, or do a certain number of standing starts on a certain percent grade so many times without frying the clutch or transmission. IIRC there are also braking tests as well. As for the 4.3 Caprice being rated higher than the SS Impala... I can totally see that, if only because the 4.3 is straining the cooling system less. Plus, the SS has higher compression, which isn't the hottest thing for towing.
from where do you get your info about the SS having a higher compression ratio?
the LT1 is bigger than the L99, but i think they were both 10:1 motors.. and the SS got the same tune as a regular LT1 powered Caprice- including the cop cars- but the cop cars had a higher speed set for the fuel cutoff point due to being cop cars.
the lower tow rating on the SS might have been due to the higher speed rated lower profile 17" tires more than anything.. what was the tow rating on a 9C1 Caprice that was mechanically identical to an Impala SS other than springs, shocks, and the wheel/tire combo?
I inferred V6 when you said 4.3. The 4.3 V6 gets used and abused in truck duty, pulling around a Caprice and trailer wouldn't faze it.
Maybe GM felt that 3.08 gears were too short when towing with Overdrive locked out like you're supposed to. Or maybe it's suspension tuning, like how the Syclone only had a 100lb cargo capacity despite it being otherwise a 4wd S10. (Maybe it was 50lb. Or 200. But either way - laughably low)
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