Designed by Ford, the M151 1/4-ton 4×4 vehicle — originally called the Military Utility Tactical Truck (MUTT) — replaced the venerable Korean War-era M38 Jeep in the 1960s. However, the M151’s swing-axle rear suspension led to so many rollover accidents that the Department of Defense and National Highway Traffic Safety Administration actually barred civilian sales of surplus M151s. Here’s why these long-serving little trucks were considered unsafe for public roads.
Although it was produced in large numbers and remained in service for over 30 years, the M151 seems to be more obscure than the older MB, MC, and MD military Jeeps it replaced. Military veterans of a certain age may be familiar with the M151 (possibly more than you wanted to be!), but unless you drove them or worked on them in the motor pool, you might not realize how different the M151 was from its WW2 and Korean War-era predecessors.
As I’m sure the Jeep legal department kept trying to remind people, the M151 was not a Jeep, except in the lower-case generic sense the brand’s lawyers have been trying to stamp out ever since their trademark registration was approved back in 1950. People called the M151 a jeep anyway, and it did LOOK kind of like earlier military Jeeps, save for its horizontal grille bars. However, the M151 was a very different vehicle — both more advanced and more troublesome.
The M151 arose from a 1951 U.S. Army program to develop a successor for the M38 military Jeep, which was essentially a militarized version of the civilian CJ-2A. By 1952, Ford Motor Company had come up with a mostly clean-sheet design with unitized construction, an all-synchro 4-speed gearbox, and fully independent suspension. This was approved, and the first production examples were delivered in 1960. Ford built most of the early M151 vehicles, but the U.S. government owned all rights to the design, and some were also built by Kaiser Jeep and later by AM General.
The M151 unit body was designed so that it could be made in either aluminum or steel. Even in steel, a complete M151 was over 300 lb lighter than a M38A1 Jeep and strong enough to take a 5g vertical impact (a requirement for air drop deployment). Early M151s even had lightweight magnesium wheels, although these were replaced early on with heavier but much cheaper steel wheels.
The M151 independent suspension used coil springs all around. The front suspension was by double wishbones, carried on a detachable front crossmember. There was no front anti-roll bar; the front roll center was high enough to make one superfluous.
Where things became more interesting, and more troublesome, was in back. The M151’s rear differential case was bolted to the body, driving the rear wheels through swing axles. Each rear wheel was located by a wide stamped steel control arm. The later M151A1, introduced in 1964–1965, added stiffer rear springs and rubber bump stops.
If you know anything about swing axles, you can probably guess how this worked out in practice. Off-road, the compact, lightweight, short-wheelbase, independently suspended M151 was as agile as a mountain goat. On-road, it suffered pronounced oversteer in sharp turns that made it very tipsy even at moderate speeds. The operator’s manual warned sternly:
Extreme care should be used when driving M151 series vehicles. They have more responsive steering and acceleration than other vehicles. Watch speed, especially on turns. A full right or left turn at speeds above 20 mph (32 km/h) can cause any vehicle to go out of control and/or turn over.
A similar warning was eventually added as a dashboard sign, and some M151 speedometers were color-coded with yellow warning bands in the 30–50 mph range and red at over 50 mph.
These warnings didn’t do much good, and even with additional driver training, the accident rate of the M151 series was very high. In the 1967 fiscal year alone, the M151 was involved in 3,538 accidents, about 36 percent of them rollovers, resulting in 104 deaths and more than 1,800 injuries.
There were a couple of reasons for this alarming accident rate. The M151 was short (132.7 inches long), fairly narrow (about 64 inches wide), and tall (71 inches). Since it was a 4×4 off-road vehicle, it had “cross country” tires and lots of suspension travel (a total of 8 inches front and rear). With the swing axles, this meant the camber angles of the rear wheels could vary by as much as 18.5 degrees, 2.25 degrees more than a 1964 Corvair. Not only would the M151 oversteer dramatically, its rear suspension was prone to “jacking”: In a hard turn, the inside swing axle would act as a lever, lifting the rear end of the vehicle and potentially causing the inside rear wheel to abruptly tuck under the body.
“It looks like a meek, docile little vehicle on the road, but a sharp swerve or turn will lay it on its side, or roll it,” warned the editors of Air Force Driver in 1973. After attending the Army’s new M151 operator training course at Ford Ord in California, they reported:
We were told the biggest menace the Jeep [sic] holds for its driver is that it gives little or no warning when it approaches the rollover point. You can’t rely on a “seat of the pants” feel as you can in a conventional vehicle. The driver’s first indication of approaching trouble usually occurs when the scenery starts to revolve. …
Things get wild fast as the turn rapidly gets tighter. At this point the untrained Jeep driver gets into trouble. He will usually tramp down on the brakes, because all his driving instincts are pressuring him to slow down—now! When the brakes go on, the Jeep goes over.
Part of the reason this kind of accident resulted in so many injuries and fatalities was that the M151 offered no protection at all for occupants if it tipped over. Even lap belts weren’t commonly fitted, although they were theoretically available in the military supply chain by the ’70s.
As with the Chevrolet Corvair, some wits continued to stubbornly insist that the M151’s handling problems were due to a lack of skill or training rather than any intrinsic limitation of the vehicle itself. However, the Driver editors pointed out:
Not even the Army’s skilled instructors are immune to the M151’s tricks. One sergeant, attempting to show his students on the motor pool parking lot just how far he could force the Jeep in a turn before it rolled, lost it—and over he went. He got out of it unhurt, but shaken.
The Department of Defense was aware of the problem, and while the armed forces were content to blame most of the accidents on operator error, they did acknowledge that the early M151/M151A1 suspension “has not been found totally acceptable under all driving conditions because of oversteer characteristics as well as the lack of driving feel during dynamic steering conditions.”
In 1966, Army Mobility Command went to Ford for a solution. Ford’s first attempt, a modified M151A1 with a solid rear axle on parallel leaf springs, completely failed its durability testing even after mid-test reinforcement. The engineers in Dearborn went back to the drawing board and finally came up with a heavily revised independent rear suspension. The rear wheels were now on semi-trailing arms, which reduced the amount of rear camber angle change as the wheels moved up and down, and mostly eliminated the previous “jacking” tendencies.
Unlike the live axle prototype, the semi-trailing arm suspension didn’t flub its durability tests, so it was put into production in 1969–1970 on the latest M151A2 models. However, despite Congressional pressure, the Army decided that it would be too costly and troublesome to retrofit the new suspension to older M151 and M151A1 trucks.
The M151A2 was a big improvement over the earlier variants. It would still oversteer, but less severely, and with more warning before it went belly up. However, M151A2 drivers could still get themselves into trouble fairly easily. In the late 1980s, surviving M151s were fitted with a rollover protection system (ROPS), which included shoulder belts, a three-piece tubular roll cage, and side door nets. This didn’t prevent rollover accidents, but it made them less lethal when they did occur.
In 1972, the NHTSA blocked an Army proposal to sell 17,000 surplus M151 trucks to the public, arguing that their rollover danger and touchy handling characteristics made them a hazard to highway safety. The Defense Reutilization and Marketing Manual was updated to declare that “the only authorized dispositions of both serviceable and unserviceable M151 vehicles are for DoD users and for sale to friendly foreign governments under the FMS program.”
Although M151 powertrain components could be salvaged for other uses, official DOD policy was that before any other disposal of an M151 vehicle, functional or not, the body and suspension must be “shredded, crushed, or mangled in such a manner as to completely prevent rebuild into a usable unitized vehicle.” This requirement applied to all variants, not just the early swing-axle versions.
If that was true, how is it that there ARE now some civilian M151 trucks, which show up from time to time at auto auctions like Mecum and Barrett-Jackson? There were a lot of these vehicles, and quite a few eventually ended up in either foreign military forces or second-line units that weren’t too particular about the disposal procedures. A 1991 report by the federal General Accounting Office (GAO) warned:
The Army considers the M151 jeep [sic] to be one of the most dangerous vehicles in its inventory because of its tendency to flip over when being driven on paved roads. As a result, DOD has been trying keep the jeeps out of civilian hands for 20 years. However, GAO found that M151 jeeps are not being effectively controlled when being transferred from military units to disposal offices, a situation that increases the chance of theft. GAO also found that disposal offices are not following internal control procedures intended to safeguard M151 jeeps. Further, the Defense Logistics Agency has not been following DOD’s policy requiring that M151 jeep parts be mutilated to prevent the jeeps from being restored to working order. As a result, commercial rebuilders have been reconstructing M151 jeeps and selling them to the public.
So far as I know, it’s never been illegal to own an M151, or even to sell one, although it might not pay to delve too deeply into the circumstances of a specific vehicle’s original disposal. (An M151 reimported as military surplus from some NATO country is probably no big deal; one that was stolen on its way to a disposal office years ago might be stickier, although the statute of limitations on most such crimes probably ran out years ago.)
Beginning in the mid-1980s, the M151 was phased out of U.S. service in favor of the M998 HMMWV, although some M151 variants lingered through 1999. Some foreign military services used the M151 into the 2010s. With total production of more than 430,000 for all variants, there are still a fair number in salvageable condition, and they don’t seem to be terribly expensive.
As a collector vehicle, an M151A2 is probably no more hazardous than an early Jeep CJ or any other old roadster, especially with the ROPS kit installed, but a twitchier earlier M151 or M51A1 would still demand caution. As Driver warned back in 1973, “It’s not as harmless as it looks.”
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Interesting, don’t recall ever seeing one but to my surprise Canada did use some 900 of them and there’s a few in museums.
My first thought on the suspension problems would be to whap a copy of the front suspension into the rear. I’m sure someone else would have had that thought, so there must be a good reason that wasn’t done.
The front suspension was more expensive, heavier, and had almost 15 percent more unsprung weight than the swing axles. I don’t know how the semi-trailing arms compared in unsprung mass; I’ve read the Army testing documents, but they were more concerned with whether the new suspension was going to break.
It is interesting that it looks like it should be more stable than a CJ 5.
I certainly spent a good bit of my life thinking that it was the driver’s fault with all these roll-over prone vehicles. Samurais, Bronco 2s, CJ 5s, VWs and Corvairs. I rode a Honda ATC for years even.
I think a lot of that changed for me in a very short time when I was driving fast to the Airport to catch a plane and hit the brakes while changing my mind about a turn at the last moment in a 92 Explorer. Luckily, I didn’t roll over but pretty sure I was on three wheels for a moment. Right about then I decided that evil handling was not a good trade-off for anything.
It can lead to dangerous thinking the other way of course. Like deciding that I should have a low center of gravity Subaru.
As a teenager I drove an S10 Blazer, and my folks Jeep XJ because I was the only one who could keep it running with its finicky carb. But I’ve been primarily a car guy and never actually owned anything like that until my Geo Tracker and own Jeep XJ. Owned as in driving the same road to work and back daily, thinking today let’s take this familiar corner a little harder. Unlike a low center of gravity car that makes a drama show first, I feel like the tipping point is a fine line that is not well defined. Like you can keep pushing the envelope and nothing happens, until all of a sudden it does. While I’ve never tipped one, I’m mindful of it. And they have their purpose, I won’t trade them for anything.
I’m not surprised that this is a Ford, an early attempt at an independent suspension 4×4. From the same company that made the Twin I Beam, then the Twin Trac Beam well before the other American truck makers had an IFS 4×4. And the same company that gave their whole FWD carline 4 wheel independent suspension, even the bottom end Escort, where GM and Chrysler kept many a solid rear axle for a long time.
During my early years as an Army MP, I used the M151 under all manner of conditions in America, Africa, Asia & Europe . It was part of the Army TO&E (Table of Organization & Equipment) for combat MP companies.
By the early 1970s, our 151s were all A2 versions. Though not immune, this version was less likely to “tuck under” during sharp cornering. MP 151s were normally fitted with a center mount for the M60 machine gun. This raised the center of gravity somewhat but also provided some crush protection in case of a roll over.
Depending on the base, M151s might be strictly for tactical use or also be used for regular garrison patrol duty. In Korea, we even occasionally used them for Highway Patrol duty when added vehicles were needed beyond the normal Highway Patrol fleet.
Over the years we did roll a few. Never during highway use though. Each case I personally knew of involved rather extreme terrain where a misstep would likely roll any vehicle. The small size of the M151 created route possibilities my MPs likely would never have attempted with larger vehicles.
Over many thousands of miles, I never considered the 151 A2 version to be more of a roll over risk than small civilian vehicles such as a Suzuki Samurai. I’m aware many including the NHTSA will disagree with me.
Safety consciousness prevails as the Defense Department has long required all M151s to be mutilated before disposal. Specific direction for the 151 begins on p. 191 of the linked reference.
https://www.esd.whs.mil/portals/54/documents/dd/issuances/dodm/416021_vol4.pdf
A cute little mini Jeep .
Glad some survived .
Now, please do a comprehensive write up on the Mighty Mite .
-Nate
I’ve always been led to believe that BOF vehicles universally make better off-roaders. I get that the military was looking for lightness, but I’m left wondering how many of the unibodies were twisted into pretzels under extreme use.
Never heard of it happening. The 151 specs required it to be air droppable. During my time at Fort Bragg, this was periodically practiced. Unless something extreme happened, such as a 151 landing in a tree or a parachute malfunction, you could count on driving these off the DZ seemingly none the worse for the wear. Where the 151 was vulnerable was rust damage. We cleaned these religiously, but there were little nooks & crannies that collected dirt and started the rust process. The older aluminum bodies were reportedly relatively immune to rust issues. The A2s I had were always steel.
Jeep XJ Cherokee’s are excellent off-roaders. They are unibody (Jeep called them uniframe). XJ Cherokee’s are still highly sought after for off-roading. XJ Cherokee’s were my daily drivers for 35 years.
There’s nothing stronger than a well designed unibody. Many extreme military and off road vehicles are unibody, as in the Army Humvee and the Puch Pinzgauer and others. A well designed unibody offers more strength for the weight and can be designed to be as strong as needed.
The reasons that so many light trucks and off-roaders have frames are because it is cheaper and more flexible to offer various body styles, not because it’s inherently stronger.
If Ford didn’t *technically* own the rights but had all the tooling, it would be interesting to know whether or not a modified M151 chassis was ever considered in the 1966 Bronco program and how far in they got before solid axles and leaf springs were subbed in. We already know why – they’re cheaper.
The Bronco was larger and it was cheaper and easier to integrate with Ford’s other light trucks by making it a traditional BOF construction. The M151 was a very specialized military vehicle.
I remember reading up on these probably ten years ago or so, and wondering why you never saw them, unlike surplus Willy’s Jeeps. It all makes sense now, although its predecessor also would never have been allowed into civilian hands whole if they’d hit their disposal period in a post-NHTSA world. Accidents involving Jeeps accounted for a pretty shocking number of casualties in WWII and Korea. The short wheelbase and high center of gravity probably contributed more to rollover danger than the swing axle alone – but of course, that’s gas on that fire. I’m not sure who deserves blame, or how much is deserved, but the DoD would have done extensive testing and I doubt they pulled the trigger unaware. It’s all too likely they called it an acceptable risk, found the cost of mitigating it too high, and pulled the trigger. Either way, it’s kind of sad these mostly got scrapped when they could have been a great utility vehicle for farmers and ranchers and such – maybe designated as off road only vehicles. Unlike the general public, farmers and ranchers are aware and experienced at driving unforgiving vehicles and equipment. It’d be a cool vehicle to own; preferably the A2, but either one. Anyway, great write-up about a largely forgotten vehicle.
We had an A2 version at my first USAF base in upstate NY in the late 70s – we used it to patrol the back woods. It had plenty of warning stickers and we had a special course we had to complete before we could be certified to drive.
Consequently, I was too scared to get it much over 30 mph on the road – but it was a real nimble mountain goat once off, however without a roll bar you had to be constantly aware of tip angles. In Korea a couple years later, I discovered USAF had transitioned to CJ7s, which I preferred.
Good training for the Ford engineers who designed the similarly “roll-over-play-dead” Ford Exploders.
Who needs competent suspension design?
And our Ford Zephyr/Zodiac IV which had swing axles designed to look like STAs – Just like the subject vehicle!
It’s surprising that it’s not a lightly restyled Model-A, knowing Ford…
My only exposure to these is in Vietnam war movies ….and I now ponder how the studios got them… that said I was vaguely aware of their tippy quirks and their unique trait of not being sold for public consumption unlike surplus MBs that preceded them and Humvees that succeeded them. They’re an interesting design though, both internally and externally, but they’re about as flawed as the USA’s involvement in the war it was most notably present in.
Why wouldn’t a set of 38 monster mudders
And 2” spacers fix the handling of these
I don’t get why they used swing axles. The drawing of the back end in the article clearly shows a UJ at the diff and at the wheel each side, so there was no need to have the lower wishbones (or whatever they’re called) set up as swing axle geometry. Strange and dumb, but then, the world of defence materiel often seems to be.
Swing axles are for one cheaper, with two fewer universal joints and lighter because they need simpler suspension elements – so cheaper to make better at what they are designed for (rough terrain) and helps with logistics and supply chains in a war setting (fewer parts, mechanics can fix some stuff faster). Nothing dumb about it, just a different set of design specifications than in highway and civilian applications.
That’s all very well, but my point is that there ARE those two extra universal joints, that is, the expense/complication of the more complex suspension, but then there’s the swing arm geometry of the lower control arms, which is strange. And it does not reflect well on miltary testing that the resulting dire flaw in the handling – which clearly killed and maimed – was not picked up in testing.