why do tow vehicles need those axle ratios?

[stillsearching]

Perhaps someone with knowledge of towing who happens to frequent here could enlighten me on something...

Every time I look at pickups set up for towing they often use low gear ratios, as low as 5.13 in some cases. Needless to say this kills economy when you arent loaded. What I don't understand is why low ratios are so necessary...

What I mean is, is it some kind of cost cutting measure by the companies (cheaper than a new transmission design or a more powerful engine rating) or is there some other advantage i'm not aware of?

I fully understand the physics - using a 4.56 ratio instead of a 2.29 ratio means almost twice the torque to the tires. But that's what transmissions are for. After you are out of first, or perhaps second gear, does it really matter? Short of situations like ratios too widely spaced is there any reason why either a better transmission, or a more powerful engine, couldn't solve the problem equally well?

I'm confused by modern tow ratings, in part. That a given model of frame, with a certain braking and suspension setup is rated for a given weight I take to mean that it's fully competent to handle that. On certain setups or past a certain point of upgrades already otherwise made the only difference seems to be a more powerful engine and a lower axle ratio. The only two cases I see this as being critical is starting from a dead stop going uphill, and maintaining normal (excessive) speed of traffic under a high aerodynamic load with a headwind. Those of us who don't even live in the hills or are content to drive 55mph (or slower) under certain load conditions or who have streamlined flatbed loads I would think should be able to do alot more with absolutely the same level of safety of someone who didn't get the 6 liter with 300+ hp and the 5-something axle.

If i'm wrong in some other area please correct me - whereas I agree of a danger of a load you can't even start in 1st gear uphill on hills within where you actually do drive in case you had to stop suddenly, and that you should always have enough to do that without clutch slippage or transmission damage, is there some other safety hazard i'm missing?


Put another way i'm trying to design a diesel tow vehicle around numerically low ratios for mpg while unladen. Diesel = not excessive power levels of 100-200hp favoring the midrange of that, looking for a less expensive to buy diesel instead of a power monster because this will see very intermittent use at best. Tow = 5 tons plus on a superduty chassis. Axle ratio = as low as 2.29 if I can get it, probably something more commonly available though.

I see two ways of insuring I can start in 1st gear. One way is by just using an SM465 "dump truck" granny gear transmission - 1st gear is 7.05:1 meaning torque multiplication to the axle is at least twice as much, and torque to the tires is likely comparable to running that 5.13. The second way is if I have a 4x4 all of those have a Lo range anyways, intended for creeping off road usually, but here it would be an unexpected on road stop in steeper hills until say i'm at the top of one, can stop again, go back to high range, and use the downhill to get moving in normal cruise ranges. All the sudden my worst case 2.29 axle turns into somewhere from a 4.56-5.13 right there, heck maybe I could even leave it there unless there is some problem of staying in low range. (just like I don't know why staying in 2nd at 55mph with the normal ratio would be a problem since that about compares to drive with the normally low ratios expected) Now maybe I wont be running 70mph pulling a fiver in the Rockies but that's why they have passing lanes if I happened to find myself there sometime.

Another option is just plain having more off idle torque and power, such as with a low rpm turbocharger added instead of another gear. (such as for my RWD fullsize sedan discussion in another thread)

[oil pan 4]

Get a 2 speed rear end or a 6 speed manual transmission.

A lot of people that didn't do the math when coupling a 6-speed to a strong engine with 26 to 28 inch tires found that the closer to 4:1 you get with the rear end gearing the more useless 1st gear becomes.

[Duffman]

You are part on the mark, with a multi speed transmission the only relevant ratios are the overall in the lowest gear and the overall in the highest. You are again correct about having adequate gradability, but it is not as trivial as you make it out to be. While a lower (numerical) gear may successfully pull away from such a grade it may generate excessive heat while riding the torque converter (or clutch slippage). Also many do not think of the accelleration advantage gearing does give you until you do real world comparisons (just try starting in second gear with a manual to illustrate).

The other factor is the high gear, I know of no OEM transmission manufacturer (including HD truck) that offer a significant spectrum of gear ratios, so to get the appropriate startability you have to live with some measure of comprimise on high gear. Also when pulling a heavy load you do need more RPM in high gear to generate additional power.

[thomason2wheels]

One other consideration is the torque load on the driveshaft and ujoints. For a given towed weight on a given incline, the torque the ujoints must support to move the load is much less with a 4.00 axle than a 2.00. I once saw a tractor trailer blow upp its u joints trying to pull a load from a stop going uphill....it wasnt pretty.

[Frank Lee]

or is there some other advantage i'm not aware of?

If you're talking about new pickups vs older ones, it's because the wheels are so ginormous now.

[oil pan 4]

Originally Posted by Frank Lee

If you're talking about new pickups vs older ones, it's because the wheels are so ginormous now.

In the early 1980s the standard truck size was 235/75 on a 15 inch wheel.
Thats like a 28 inch tire. On my suburban they look like clown car tires.
I put 31''s on it and they fit great, no rubbing anywhere.
Now the standard tire size is big and all on 16 inch and up wheels.

[slowmover]

I seconded the "nomination" by a seasoned, well-informed and experienced Dodge CTD owner who would like to have a manual transmission in our 1T pickups which featured two overdrive ratios. The lower one for when towing, and the higher for when running the highways solo. Our trucks come stock with 3.73 rear gears, and while that man (living in mountainous Idaho) might want 4.10's, I'd want 3.42 living at sea level. Both of us have the same goals after a fashion in that this would work for us both.

Your questions are all valid, OP. But you need to be specific about what work you plan for the truck in order to come up with a reasonable plan. That does mean you'll have to narrow the range of what is possible. Climate and terrain speak directly to what is possible for a given horsepower demand (and the length of time it is called upon in full). Gearing is only half of the question as cooling system capacity is the other. Both, together, delineate what is reasonable [possible] for a given locale per work definitions.

How to specify trucks is a full-time occupation in the departments of commercial vehicle sales and manufacture. It bears no resemblance to car dealers.

The order for vehicle performance (FE among them) is:

1] Truck specification
2] Climate
3] Terrain
4] Truck use

I recommend your working them backwards to arrive at number one.

I would also recommend working from a well-known set of drivetrain choices already proven to work together. The Cummins 4BT and New Venture NV-4500 transmission are one such set (aiming at FE). Tire height and axle gearing then tend to fall into place accordingly (with top speed and weight limitations, etc, per cooling system capacity).

If you will search for discussions of the SAE J2807 Towing standard (online) of how and where testing was conducted, you'll see some of what is entailed. There are ways around this, but in main it will serve.

.

[stillsearching]

Originally Posted by oil pan 4

Get a 2 speed rear end or a 6 speed manual transmission.

They cost more than i'd like, especially if I have to buy new vs what I can get rebuilt, though yes that's one of the option. Also to the next, a 4bt and NV4500 i'm aware of and would love, but don't know if it meets my budget. (the amount more i'd spend would never pay for the better fuel efficiency, ever)

The transfer case range box is another way of getting a 2 speed rear - I haven't seen inexpensive (non racing) 2 speed rears, nor ones made for towing that aren't sized for a dump truck. This lets me use common rear axles including cheap replacement if I ever screw one up.

Someone argues for the spread of ratios, the TH400's are fairly close ratio for nonOD and what not - then the range box would mostly choose whether i'm in highway unladen mode, or crawling thru the rockies at 45mph mode.

Originally Posted by slowmover

Your questions are all valid, OP. But you need to be specific about what work you plan for the truck in order to come up with a reasonable plan. That does mean you'll have to narrow the range of what is possible.

The order for vehicle performance (FE among them) is:

1] Truck specification
2] Climate
3] Terrain
4] Truck use

I recommend your working them backwards to arrive at number one.

and 5) Budget.

Okay here goes: The specification is to stretch "universal use" as far as possible on the lowest budget. To simplify things assume that cooling and such are all up to spec - the only underspecced issues are engine and axle vs whats normal. Afterall the easy answer is to have plenty of power, a close ratio 6 speed, and a dual axle ratio. I'm trying to get by with far less cash outlay. The easy alternative is to not care about mileage and just gear low to tow the load. Were this a full time tow vehicle that might make sense, but it's not.

What I need is a runabout for a farm/ranch situation with the lowest total cost of ownership over the long run, to take advantage of craigslist free or cheap type deals for things that are useful to the farm/ranch. I'm not towing so often to justify the expense of a Cummins (never pays for itself, dont know if a 4bt would either with how prices have gone up so much), yet not so little that I can just rent a truck from someone. Several dozen loads over the next few years, maybe even 50 or 60 of them, saving $100 or more each time I need to use a truck over some length of whatever will pay for itself. The wider spec range I can move with it the more opportunities I can take advantage of where it's something like "this works if I dont have to rent a truck but to rent a truck makes it too expensive to get" situations, or alternately "this works if I can do it in two trips but not three".

Because I have more time than money, because most loads are infrequent or sporadic, I don't mind going slow. 70mph uphill is not essential. That makes me want to design for mileage most of the time. This is an all purpose backup vehicle in normal use to insure I can get to work if my main one is broke for instance or for a 2nd vehicle when the first is in use.

The desire to keep expanding tow capacities is because the more that I CAN do with it, when opportunities present themself, the more it pays for itself. And chances are decent it wont cost much more to spec out a truck that can tow 13k as one that can tow 8k when were talking something a decade or more old to begin with. So if a friend calls needing me to flat tow his 4x4 I can do it, if there's free scrap steel on craigslist I can pick up more of it instead of less, or if there's a deal on some piece of industrial machining equipment I maybe can bring home four pieces instead of two because it was 500 miles away and one day auction only type of thing that had to be picked up today.


Since for instance the M35 military trucks could move something like 5-10 tons on the land despite as low as 127hp, just not very fast (like 48mph) i'm trying to do something similar, just without buying an unreliable (after a life of abuse) military truck. :P Old farm trucks were similar, a 120hp 235cid inline six moving many tons in back, again just not very fast. I don't need to go very fast for the special heavy loads.

Climate - anything from subfreezing to hot summer, I live in the midwest, where 100 degree summers alternate with alberta clippers 60 below. But i'm thinking most hauling will be spring/summer/fall. I plan an oversized radiator for the worst of summer though under load, no worries, I think that's the only season-specific issue to consider..?

Terrain - on road 97% of the time, except possibly the last few feet to pick up or drop off certain things might be dirt, but probably not in the wet. Minor hills in a 500mi radius from where I plan to look for stuff, nothing like the rockies, no long grades - just hills, coast down one and ride up the next for maybe 30-40 seconds uphill. I might eventually see the rockies in the future - if that requires derating what I normally pull thats fine. I'm not designing it to haul there. Yet at some point it's possible a long distance move could have me pull trailers through there on my way to oregon or something and take several trips doing it.

Truck use - unladen MOST of the time, 1-2 dozen hauls of something per year, each up to about a day maximum driving distance. (ie 16hr day)

[slowmover]

Several dozen loads over the next few years, maybe even 50 or 60 of them, saving $100 or more each time I need to use a truck over some length of whatever will pay for itself.

This is where you want to come to an understanding of ownership and operational costs: cents-per-mile.

1] Projected length of ownership
2] Projected miles in that period

The IRS standard mileage deduction is the place to start. The "game" is whether these miles are approved.

Once you are able to show the true cost-per-mile then your budget to modify a vehicle is outlined. Show it as a cpm calculation to gauge whether the vehicle is meeting your goals.

.

[oil pan 4]

BT4 motors aren't cheap.