(first posted 2/3/2014) Mack’s legendary R-Series had a well-deserved rep for toughness. We praised them effusively here. But if one needed an ever tougher Mack, to take on all sorts extreme jobs off and on the road, there was the RM and DM (with offset cabs) Series.
This AWD Mack RM that I found sitting along Hwy 58 in Oakridge has “Fire District” written on its door, and was rather likely used in some related task on the vast forest lands in the area, perhaps with a water tank on the back, or some other function. Whatever it was, it must have involved rugged conditions, because this truck was built for that.
Of course the cab is shared with the R-Series, but other than that, these DM and other related special-duty Macks were pretty different from the ground up. Their most distinguishing feature is the very old school free-standing radiator, not just a shell. This is the real thing, and that radiator cap is very much functional. The only thing missing here is of course the iconic bulldog sitting on the cap. Maybe they got tired of replacing it, since it unscrews so easily, unlike the ones fixed to the tilt-up fiberglass hoods of the R-Series.
Let’s take a look at what goes into a 4×4 Mack RM, starting on the ground floor. It’s not like one has to get down very low to see what’s there, like this front axle. Beefy.
Here’s the transfer case, just behind the cab. Power goes in on the upper level, in front, and then sent to the two axles on the lower level.
The rear axle is supported by no less than three sets of leaf springs. Bet it rides a bit stiff without a load.
Here it is from the front. Tough stuff, all of it.
I should have popped open the hood, but there’s little doubt that it was anything but Mack’s turbocharged six-cylinder Maxydyne under there. From this side, I saw what I took to be the shift lever for the transmission, and some other control poking out of the that quadrant.
But from this side, it was clear that there was no clutch pedal, and that the quadrant was for an automatic transmission.
Sure enough; the control for an Allison five speed automatic, no doubt. Allison, formerly a GM division, developed the world’s first heavy-duty automatics near the end of WW2 already, probably based on Hydramatic principles. They made units to work in tanks as well as the Budd diesel railcar. And of course, the Allisons were (and still are) the go-to choice for automatic truck transmissions.
I drove Ford Super Duty dump trucks and cement mixers for Metro Pavers in Iowa City in the summer of 1974, and their whole fleet was Allison-equipped. It was my first exposure to them, and they certainly made the job…less challenging; or a bit more boring, as there was no opportunity to keep my clutchless-shifting skills up to snuff. The Allisons shifted mighty firmly, not unlike an old Hydramatic under full throttle. Nobody would call it a slush-box.
I’m still a bit perplexed about the big lever coming up from the floor. It’s probably an auxiliary transmission, presumably only shifted while in neutral, for low range(s) suitable for some serious off-roading.
While five speeds for the main transmission might sound like too few, the Mack’s engine was unusual in having a much wider torque band than average, and was often paired with a five-speed manual too, without getting bogged down.
The RM/DM Series trucks were essentially all custom trucks, tailored to the specific job expected of them. Our featured truck is about as small as a RM as one is likely to find, with only a single rear axle. Most are at least tandems, and used in heavy haulage of all sorts, including prime mover duty for pulling really heavy loads. Over its axle sits ballast, so as to not spin its tires on take-off. This one here is pulling a 140 ton load, but that’s on the light side for serious over-the road heavy haulage.
If you’re from New York or New Jersey, RM/DM-Series Macks may be familiar to you, as they seem to be particularly popular, if not dominant, for the construction, concrete mixers and other hauling jobs there. They were everywhere in NYC back when I used to visit regularly, and in the early morning hours they could be seen hauling big garbage dumpsters and such. And they were all painted that same dark shade of green. And of course the company names painted on them were all Italian. The ultimate Mafia-mobile. Maybe Mack had a special deal with them that they couldn’t refuse?
Here’s one with a few sticks on its back. Wherever there was a tough job to do, there was a Mack willing to take it on.
Yes, this twin-axle RM truck is looking downright petite. Maybe it’s going to get a pickup bed mounted so someone can show up all those jacked-up diesel Ford F-350 4x4s.
At the first shop that I worked at back in the 90s we had a Mack 4×4(X4?) wrecker that we called The Disaster Master. It was a beast!
Quite a machine! I suspect the extra lever sticking out of the floor is to control the transfer case.
That’s a Mack Transfer Case, TC-15/150/25/250 Series, I believe. It has an inter-axle differential (you could get them with ratios appropriate to 4×4’s and 6×6’s) so it can operate with full-time AWD on the highway. You can also lock the diff. Mack made these for a long time. Here’s an illustration of one, actually a special out of an SAE paper from 1986 “The need for a 4×4 Tractor, Fiction or Friction”, with a special automatic lockup clutch in addition to a manual lockout of the differential.
Here’s one of 2 relevant pages from an operator’s manual
Here’s the second one.
What’s with the “down plow up” on the side of the quadrant? Maybe that’s just an old automatic transmission control repurposed to control the hydraulics for a plow?
There is no clutch so the truck is an automatic. The “plow” labeling is puzzling.
The 4×4 single rear axle configuration tells me this most likely started its life as a Municipal Plow or Off Road Water Truck. Most plows have extra frame hardware on the front that is typically not removed when the vehicle is reconfigured for its 2nd job.
The air brake pods are sticking out of the front axle at an odd angle.
That front axle is a Fabco SDA-23. It has wedge brakes, chosen in order to clear the large Single-Cardan Joint, necessary because of the single reduction design. The Wedges are installed at an angle, again for clearance.
Perhaps there were once plow controls mounted beneath it, for a plow long since removed from the truck? Or (less likely still) the control box was a replacement, pulled from a truck whose plow controls sat there? I can’t see the nose of the frame, so hard to say whether it once wore a plow or not.
You never can tell on some old fire rigs. I’ve seen some downright baffling things punched on label tape and stuck in the cabs of such trucks.
There is a lever between the gearshift stand and the passenger seat that could be a plow control. This leaves the question of what drives and controls the hydraulics. Most dump trucks use a transmission driven PTO to power the hydraulics and have a shift lever somewhere. This truck may have an engine driven hydraulic pump like a cement mixer, or an air shift PTO with a a switch on the instrument panel.
It appears that these had quite a long life. It looks like the DM came out in 1965 (the same as the R model whose cab it shared) and went until just a few years ago. This seems to have been the last truck to use the old R model cab. An interesting truck that I knew almost nothing about. There is going to be a lot of that this week, I suspect.
what a cool truck. Great find, Paul…
Always have had a fascination with Macks, from seeing specially equipped flatcars carrying bare R, U, and DM cabs from Sheller-Globe in Norwalk Ohio going past my great aunt’s house in Lakewood on N&W freights to building several of the AMT 1/25 scale R Models and the incredible Ertl DM 1/25 scale dump truck kit as a young’en. Also, Cleveland Builder’s Supply had a large fleet of both models and could be seen everywhere in northeast Ohio. I greatly miss seeing real Macks these days… sigh.
In the close up pic of the AT shifter, it looks like another lever on the other side, right in-line with the word plow… i assumed it was a hydraulic control lever.
Thats only a 4wheeler truck so it was likely a snow plough in a previous life rather than a big load carrier
A nearby trucking company, Haukes, was a loyal Mack customer. Macks were assembled in the Netherlands, way into the sixties.
This is a Mack DM600 6×4 tractor with a semi-trailer to haul bricks. The rig has 6 axles, enough for the (legal) max. GVW of 50 metric tonnes. (about 110,000 lbs)
More historic pictures of Haukes’ trucks (Mack, Tatra, Volvo and others)
Interesting that the rear axle isn’t Mack’s top loader. With the drop down of the transfer case, it may have been impossible to manage the driveline angles.
I was for about two years a contractor for Mack in the late 90’s while they were still owned by Renault. The headquarters was still in Allentown, PA, and the one plant in Macungie PA (which is still operating) is the plant that filled all contracts for these “work trucks.” The plant in South Carolina made most of the over the road trucks. My favorite during one of my plant tours was the cab over, dual steering wheel, four wheel drive truck designed for NYC as a garbage / plow truck (if necessary.). Sorry, no pictures were taken at the time.
If you remember the Steven Segal flick “Fire Down Below”, a would-be assassin tries to take Segal out with a Mack DM 800.
Ah, good movie. Levon Helm, Kris Kristofferson and the great Steven Segal.
That truck is the Mack Truck of Mack Trucks.
It’s a little hard to see but I believe the white 4x that you posted at the top is an RM and not a DM. The set back axle is the biggest giveaway from what I can see. Also, the cab on the DM was not shared by the R model as the DM was an offset cab. But the RM cab was the same as the R model cab. Had that truck been a tandem axle it would have been an RMM.
The lowest spring leaves in the rear only extend from the front spring hanger to the axle and act as an axle-locating radius rod, not a real load carrying element. Also, many vehicles of that era used what are known as wedge-type air brakes, rather than the more common cam-type brakes. The wedge brake air diaphrams generally are mounted nearly at right angles to the brake mechanism, leading to their rather peculiar protruding appearance.
I came across this one, recently. Not AWD, but what the heck. The 8×4 dump truck was assembled by the Floor company, some 50 years ago. I don’t know if there were factory, tridem axle Macks with a liftable tag axle back then, or that this was an aftermarket conversion done by Floor. But I’m sure someone can tell.
Source and way more: https://magazine.bigtruck.nl/08-2020/mack-trucks-in-nederland/
All interesting information for an old truck salesperson. Thanks, gents. I was not on board with Curbside Classic in 2014 so I am happy to see this brought to the fore.
Would love to get that thing moving in 3rd and throw it in to 5th 0r 6th and floor it and feel the acceleration.