what to tow with? - Page 5 - Fiberglass RV


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Old 03-16-2008, 08:38 PM   #57
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what exactly are the factors that go into determining a vehicle's towing capacity...?
FWD/RWD/AWD, engine horsepower, torque, vehicle weight, chassis design...?
--- steven
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Old 03-16-2008, 08:46 PM   #58
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what exactly are the factors that go into determining a vehicle's towing capacity...?
FWD/RWD/AWD, engine horsepower, torque, vehicle weight, chassis design...?
--- steven
This is a start. It is a quote


Why are some vehicles rated to tow 6000 pounds, some much more, and some vehicles only 1000 pounds capacity? Contrary to what most owners believe, it has very little to do with maximum horsepower.

Motor vehicles have more than enough power to pull a trailer. Even the smallest engines can do it if the driver uses low gear. I have even seen a motorcycle tow a tandem axle U-haul trailer, which is a very foolish thing to attempt.

Some drivers think that the transmission determines how much a vehicle can tow. Manual transmissions are sometimes thought to be better than automatic transmissions, because the torque converter in an automatic transmission produces a lot of heat when it is working. Lock-up converters are used in virtually every automatic transmission these days, so as long as the converter is locked, it doesn't produce any more heat than would be generated in a manual transmission. Therefore either will work for towing.

So what does determine how much a vehicle can tow? I had the opportunity to talk with a couple of Dodge truck engineers while road testing the Dodge Dakota pickup. This vehicle, even when equipped with a V6 engine, has a towing capacity that matches many full size pickups. The engineers tell me that they base towing capacity on the ability of the vehicle to tow a load from a stop up an incline that a driver might expect to find on a steep mountain road. In other words, it isn't horsepower that is needed - it is low r.p.m. torque.

A seven per cent grade is about the maximum grade you will find on most major highways anywhere in North America. For Dodge, the maximum load a vehicle can get moving up that grade determines the towing capacity. Automatic transmissions are better able to get a heavy load moving due to the torque multiplication provided by the torque converter. Therefore, if a vehicle is only offered with an automatic transmission, it may have a higher towing capacity than one that also has a standard transmission available.

There are many factors that determine how much low r.p.m. torque is put to the ground. Drive axle gearing is often different on vehicles that tow: installing a 4.10 to 1 axle ratio instead of a 3.70 to 1 ratio increases torque to the wheels by about 10 per cent. Changing tire sizes also makes a difference. Larger diameter tires reduce torque, while smaller diameter tires will increase torque. Most drivers don't change axle gear ratios or tire sizes when towing. Instead, they change the transmission gear they drive in.

Both manual and automatic transmissions are built with overdrive gear ratios in their top gear, and sometimes in the top two gears. An overdrive gear ratio turns the driveshaft faster than the engine crankshaft. With engine r.p.m. lower, fuel economy is usually better and engine noise and wear are lower. However, an overdrive transmission gear ratio reduces torque output to the wheels. Using a lower gear is usually recommended for towing.

With five or six-speed automatics or manual transmissions, using fourth gear is often recommended for towing heavier trailers. With four-speed automatics, towing in third gear works better. Using a lower gear increases torque to the wheels and reduces the load on the engine. With the engine not working as hard, the driver can apply less throttle. Even though engine r.p.m. will be higher, it will typically use less fuel than one that is under constant load.

Tow/Haul mode can be selected on many current pickups and SUV's with automatic transmissions. This mode modifies transmission oil pressures and shift speeds so the vehicle uses the lower gears more. On some vehicles, it also modifies transmission shifting so it remains in lower gears on downhill grades allowing engine braking to help the wheel brakes.

Once the engineers have determined what transmission gear ratios, axle ratios, tire sizes and engines they are using, they can determine the towing capacity. Now they match wheel bearing capacity, axle shaft strength, radiator cooling capacity, braking capability and even frame design and strength to the towing capacity. As you can see, there are many parts of a vehicle that contribute to the towing capacity. Just adding a bigger trailer hitch isn't enough to make it capable of towing a heavier trailer, even though you may have the power to pull it.
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Old 03-16-2008, 08:59 PM   #59
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Good summary Ches!

I recall reading an article about Dodge testing the big pickups (and maybe the little ones) on a paved hill they have in Arizona. It's pretty steep, but I don't recall the grade. One of their tests is to pull a load of what they think the capacity is part way up the hill, stop it and then get it going again. Too much slippage or inadequate clutch spring on manual trans or too much heat on auto trans and they go back to drawing board or lower rating.

Part of the rating is basic to the vehicle, wheelbase, rear overhang, weight distribution between axles, etc.

If it was just horsepower and fuel economy, we'd see a lot more RVs being pulled by big motorcycles...
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Old 03-16-2008, 09:15 PM   #60
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Hey Pete---I find something good once in a while
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Old 03-16-2008, 09:29 PM   #61
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^^^
thanks, good info...
looks like i'll be building a custom transmission...
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Old 03-16-2008, 09:48 PM   #62
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That Dodge test reminds the test they use for the M-1 Tank.

They drive it up a 30 degree incline, stop, then start up the incline again, stop, and drive down the incline. That's only half the test. They then back it up the 30 degree incline, stop, then start up the incline again, stop, and drive down the incline.

All this with an automatic, electronic controlled transmission. Of course they have HP to spare and a huge transmission cooler.
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Old 03-18-2008, 12:30 AM   #63
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after reading Pete's posting in the Kia thread about GVWR's, GCWR's, GAWR's, WDH's & other acronyms, i thought i'd ask y'all's opinions on my TV/TT combo...
first, i'm rebuilding my 13' Burro, & i'll be doing everything i can to keep the weight down, so it'll have a loaded weight under 1400 lbs...
the tow vehicle is my '92 Toyota Tercel 2dr., which will be modified for autocross racing... while i had very little trouble towing the "gutted" trailer with the stock motor/tranny/chassis, the Tercel isn't really rated to tow much more than a utility trailer... i'm hoping that the modified version will be much more capable... a Tercel isn't really designed to climb Pike's Peak, but i plan to do it anyway...
the Tercel starts out at 2100 lbs... racing modifications will include a 6-point roll cage, chassis seam-welding, rollcage to hitch reinforcement, upgraded brakes, custom transmission gearing w/helical LSD, heavy-duty clutch, front & rear swaybars, fully adjustable coilover suspension, & somewhere between 150-250hp... fully modified, it'll be about 2300+lbs, empty...
the trailer will have brakes, & an adjustable-length tongue... i won't be towing faster than 55-60mph...am i being over-confident about the TV...? the old Burro catalog shows it being towed by a compact 70's car that i'd compare to a Toyota Corolla...
--- steven
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Old 03-18-2008, 12:42 AM   #64
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I'm not going to touch the official tow rating, presuming one exists...

Clearly you are going to have more HP (hopefully not at screaming RPM), better clutch and beefy suspension. You might have to give some thought to the differential ratio, esp if first gear is high.

Can't do much about wheelbase, but mount the hitch so the ball is as far forward as possible. Get some extra weight up front, tire on top or something.

Extended tongue is a good thing; keep weight off trailer rear.
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Old 03-18-2008, 11:30 AM   #65
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thanks...
i've read that tow capacity is 1000-1500lbs, depending on the source... the previous generation tercel is rated at 1500lbs... the car had the hitch on it when i got it, & the previous owner had a 10ft utility trailer he used to haul firewood... he claimed he had no problems...
the engine will be tuned for low end torque, with good power before 3500rpms...
i have a wide choice of gearing, & will be cryo-treating the gears, differential, & axles for added strength...
during towing the car will have ~200lbs of tires/tools/parts in it, & room for driver/navigator...
i think my biggest issue will be trying to keep the tongue weight down... there won't be propane tanks, but it will have a plastic utility box on the front...
thanks again for your help... i'll figure this out better when i get further down the road on the project...
--- steven
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Old 04-07-2008, 04:05 PM   #66
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I had very good results towing a 13 ft Scamp with a Subaru Forester. No problems with power even in the Adirondacks. Handling was excellent. On several trips the car averaged 20mpg towing the Scamp. Unfortunately the Forester was totaled in a rear end collision. We replaced it with a Subaru Outback which will be our new tow vehicle. Hope to get it out for the maiden trip soon, tow hitch and brake controller are on order.
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Old 04-07-2008, 04:12 PM   #67
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Hi Mike,

Can I ask why are you choosing an Outback over the Forester for a replacement? We are looking at both models right now and I asking around to see what people prefer.

Thanks!
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Old 04-08-2008, 12:06 PM   #68
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My wife uses the Subaru to commute to work, about 35 miles each way. We drove both the 2008 Forester and the Outback. It was a toss up. She liked the Outback a bit more due to the fact that the seats and the ride felt a bit more comfortable. I could definately tell the difference, the Forester felt more like my pickup (Ford Ranger) Prices were similiar, gas mileage is similiar. I think the Outback even has a slighlty higher tow rating that the Forester. It was also nice to get something a bit different after driving the Forester for about 4 years. We had an Imprezia before that for about 7-8 yrs (150,000 trouble free miles)
Have not towed with the Outback yet, but I have heard very good things about them.

Good luck in your choice, I don't think you can go wrong either way.
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Old 04-08-2008, 12:40 PM   #69
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We own a Subaru Outback for commuting to work. We have never towed with it and get a little better than 32 mpg on mixed driving.
The Outback and Forester are built on the same platform. They change the Outback model every 5 years, with the last time in 2005. I feel the changes in 2005 model year makes looking at 2005 or newer worthwhile.
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Old 04-08-2008, 05:23 PM   #70
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I own a 97 Outback but have not towed with it. I enjoy it because of its combination of the good mileage, AWD, ground clearance, reasonable price and smooth ride. I am considering getting a small trailer, but I don't think I would use the Outback as TV for anything larger than a 13 footer. It is rated for 2,700 lbs, and the Forester for 2,400 lbs.

As my Outback is getting nearer to replacement (170,000 miles), I have been reserching a variety of alternatives with better towing capacity, but I haven't found the right combination yet.
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