Tow Rig Mods...Increasing Torque (regearing)

Ray, yes, I feel that it's not practical for me to spend the money to modify the rear axle on my '93 F-150. My truck's stock, 4.9L straight 6 engine has enough torque to tow our 3124KS unhesitantingly at highway speeds on most roadways. It's only when we're going over mountain passes, such as "The Grapevine" on California's section of Hwy. 5, that I have to downshift to 2nd to maintain 45 mph. I just keep to the outside lane so that the speedsters won't be too irritated.

What do you think of the "Under/overdrive" in the website pointed to by ripp1202?

[RockyMtnRay]

Originally Posted by Larry_Loo

What do you think of the "Under/overdrive" in the website pointed to by ripp1202?

This device is very effective, particularly with engines that have been tuned to have a very sharp torque curve (narrow power band). You might note that nearly all the testimonials on the website are from folks who've built highly tuned performance cars. Not a single quote (or review) about usage on a tow vehicle. Hmmmmmmmmm.

But it's also a very costly solution...the unit itself runs between $2500 and $2700 for typical 1/2 ton to 1 ton trucks and there's also an additional $550 to $650 for replacement driveshaft and install kit. And unless you're a pretty good "wrench", there'd be another couple of hundred in labor expense. All in all, the total would come to somewhere between $3100 and $3500.

Most truck engines have a fairly broad power band, so the device, while useful, doesn't provide the same bang for the buck that it would on a hot rod car. By using the over or under drive with the appropriate transmission gear, you certainly can get the same effect as regearing the axle and have a lot more flexibility as well. But the flexibily comes at a very steep price.

I actually did investigate the device until I noticed they do not make a unit for any import truck such as Toytoa or Nissan...only Ford, GM, and Dodge.

[Bill]

Ray -

Did you consider an electrically-shifted dual-speed rear end? Do they still make such things? It would let you use a low range when needed, but go back to "standard" or "economy" range when you are not towing. This is sort of the poor man's version of the GearVendors Under/Overdrive that RIPP1202 pointed out.

Long long ago, I spent a year driving a gravel truck that had one of these, and it was really great when starting up a hill with a full load. As I recall, when the truck started to lose momentum, you pushed a big red button mounted on the gearshift lever, then momentarily released the throttle. With the load removed, it would shift almost instantly and you could mash your foot back on the gas - so quickly that very little additional momentum was lost. But with the new gearing, the engine RPMs would come up, and you could remain in the same tranny gear. Shifting the other way was the same. This wasn't intended to be used as a split-shifter or gear-doubler, like the GearVendors' thing - you didn't use it on every shift of the (manual) truck transmission.

Since it is manually-operated (not tied into the automatic tranny control computer, and no computer of its own) and factory-assembled with correct alignment and tolerances, it might be less expensive than a one-time gear change in the rear axle - and if you have 4WD, the cost might be significantly less than changing the gearing in both axles. It would be easy to electrically lock it out when 4WD is selected.

Just a thought ...

Bill

[RockyMtnRay]

Originally Posted by Bill

Ray -

Did you consider an electrically-shifted dual-speed rear end? Do they still make such things? It would let you use a low range when needed, but go back to "standard" or "economy" range when you are not towing. This is sort of the poor man's version of the GearVendors Under/Overdrive that RIPP1202 pointed out.

Long long ago, I spent a year driving a gravel truck that had one of these, and it was really great when starting up a hill with a full load. As I recall, when the truck started to lose momentum, you pushed a big red button mounted on the gearshift lever, then momentarily released the throttle. With the load removed, it would shift almost instantly and you could mash your foot back on the gas - so quickly that very little additional momentum was lost. But with the new gearing, the engine RPMs would come up, and you could remain in the same tranny gear. Shifting the other way was the same. This wasn't intended to be used as a split-shifter or gear-doubler, like the GearVendors' thing - you didn't use it on every shift of the (manual) truck transmission.

Since it is manually-operated (not tied into the automatic tranny control computer, and no computer of its own) and factory-assembled with correct alignment and tolerances, it might be less expensive than a one-time gear change in the rear axle - and if you have 4WD, the cost might be significantly less than changing the gearing in both axles. It would be easy to electrically lock it out when 4WD is selected.

Just a thought ...

Bill

Man that brought back some really ancient memories. When I was teenager back in the early Jurassic (actually the mid 1960s ), I lived on a wheat farm in Montana and we had two grain trucks with dual range differentials. One (a 2 1/2 ton 1956 Ford F750) had an electric actuator, the other (a 1 1/2 ton 1948 International) had a vacuum actuator. So, yes, I'm familiar with the concept.

I just did a Google on a variety of terms relating to differentials and the most I could turn up were links to locking and limited slip differentials. So I strongly suspect this is an automotive technology that has fallen by the wayside, replaced by the secondary transmissions like the over/under drive unit. I faintly recall that these dual range differentials were troublesome devices and had a fairly high failure rate....subject to a lot of road shock, directly exposed to a lot of water/slush spray, and internal alignment problems.

And even back in the day when such devices were being made, they were only available on fairly heavy duty trucks (like the Ford F750s)...never on light duty (1/2 ton to 1 ton) pickups. For one thing the differential housings were really big, way too big to go under a pickup.

For another, they're way too heavy with a lot of hard to control "unsprung" mass that's a killer for the comfy "car-like" ride that the manufacturers desire for modern day pickups...and certainly a killer for the various SUVs that are based on the similar light duty trucks.

In any event, such devices were never created for or made by the Japanese companies....certainly not by Toyota for their line of light duty trucks. Taking things a bit further, one of the facets of my Tundra that I had mixed feelings about discovering was that Toyota is actually using a Tacoma differential in it. I say mixed feelings because it raises concerns about strength margins on the one hand; on the other hand it made it possible for me to do the regearing with some excellent condition used Tacoma R&P gear sets that cost me a mere $150 (vs around $1300 for new OEM gears).

So, no, I didn't even consider a dual range differential. Even if such a differentials were still being made (extremely doubtful)...and one were being made for my truck (incredibly unlikely), such a replacement would probably involve replacing the entire rear axle because the dual range differential needs a much larger and differently shaped cavity (aka "pumpkin"), a shorter driveshaft, and most likely also rear springs and shocks with different spring/dampening rates to control the greater unsprung mass. Total cost would probably be more on the order of $5000 to $6000...or about 4 to 5 times as much as it cost me to regear both the front and rear axles...and anything but a poor man's version of the GearVendors Under/Overdrive.

[RockyMtnRay]

Had a mid-morning brainstorm and recalled the proper name for these was "2 speed axle". So more Googling and found no references to installations after about 1982 (indicating this technique is no longer used)...and all installations were on medium to heavy duty trucks. No pickups whatsoever. Also ascertained that they were implemented using either a planetary gearset or secondary ring/pinion gear (with some form of sliding shaft mechanism) inside the differential which would dictate a much larger differential housing.

Also found that they always involved a small transmission device for the speedometer that had to shift at the same time the axle shifted so that the speedometer would read correctly with both axle ratios. Since virtually all, if not entirely all, vehicles made since the arrival of OBDII in 1996 have electronic speedometers that function off the vehicle's Engine Control Unit (ECU), there would have to be an interface into the vehicle computer.

Basically, the more I looked, the simpler and cheaper straightforward regearing looks, particularly for light trucks and SUVs that had optional factory axle ratios.