In May 1968, Ford Motor Company announced that it would shortly offer an electronic anti-skid brake system to automatically prevent a car’s rear wheels from locking in a sudden stop. Called Sure-Track, this two-wheel ABS was the first of its kind in actual production, available on the Lincoln Continental Mark and later the Ford Thunderbird and Lincoln Continental. Sure-Track was offered through 1979 in two different versions, but it’s now poorly understood and often forgotten. Here’s how it worked.
The antilock brake system Ford called “Sure-Track” was developed by Ford and Kelsey-Hayes, using an analog control unit designed by Texas Instruments. This system had a protracted development process: Ford had been experimenting with antilock brakes since 1954, Kelsey-Hayes since 1957. Although experimental antilock systems had shown up over the years on a number of concept cars (like the 1968 Ford Techna) and a few racing vehicles, Ford engineer Robert H. Madison said when the Sure-Track system was announced that “neither the technology nor the hardware was there until a couple of years ago.”
Sure-Track was not the first antilock system to reach the public. The very rare Jensen FF, announced in 1966, used the “Formula Ferguson” full-time four-wheel-drive system, which incorporated a simplified automotive adaptation of Dunlop Maxaret, an electro-mechanical anti-skid system originally designed for aircraft landing gear. However, the Dunlop-Ferguson Maxaret system in the FF was really only suitable for 4WD vehicles, with a single flywheel sensor that reacted to speed differences between the front and rear driveshafts. Its operation was somewhat crude, and drivers of racing and rally cars equipped with the FF system often preferred to disconnect it.
Unlike Maxaret, Sure-Track was electronically controlled, with an analog control module operating a vacuum-hydraulic actuator to modulate the rear brakes. Basically, the system would detect when the rear wheels locked up and then automatically “pump” the rear brakes to get the wheels rolling again. It was comparable to the way a human driver might pump the brakes in a hard stop, but it was much faster and required no special driver skill.
Each rear axle shaft had an electromagnetic sensor that measured the rotational speed of the rear wheel and transmitted that information to the control module as a series of A/C voltage pulses. The analog control module would read the frequency of these pulses; if the signals indicated that either or both rear wheels had stopped rotating, the control module would energize a solenoid valve in the brake actuator to reduce pressure to the rear brakes. The solenoid wouldn’t energize if vehicle speed was below about 6 mph.
The above schematic shows the early Sure-Track actuator in normal mode, with no lockup detected and the control solenoid de-energized. In this mode, both sides of the diaphragm (the dark shaded area) were exposed to engine vacuum. Since vacuum was the same on both sides, the diaphragm was suspended (held stationary) and the displacement rod (highlighted in purple) remained in its right-most position. This pushed the check ball (highlighted in pink) to the right and allowed hydraulic pressure from the master cylinder to reach the rear brakes normally.
With the control solenoid energized, the vacuum-air valve (highlighted in red) closed off the vacuum line to one side of the diaphragm and opened that area to atmospheric air pressure (the lighter gray-shaded area). This caused the diaphragm and displacement rod to shift to the left, which released the check ball and closed off the check valve leading to the master cylinder. With the check valve closed, no additional brake pressure could reach the rear brakes. At the same time, the leftward movement of the displacement rod allowed more volume for the fluid in the rear brake lines to expand, reducing pressure at the rear brakes. Once the rear wheels were both rolling again, the control module would cut power to the solenoid, causing the spring-loaded vacuum-air valve to return to its normal position and brake pressure to the rear brakes to resume. Later in production, Ford added a time-delay mechanism that would blow a fuse to the solenoid if it remained engaged for more than 60 seconds at a time, effectively disabling the system and reverting to normal brake operation if the solenoid valve or diaphragm got stuck.
In a hard stop on a slippery surface, the rear brakes would likely lock again almost immediately when pressure was reapplied, so the system would repeat the process up to four times per second to keep the rear wheels turning. Like many later antilock systems, operation of the early Sure-Track system was signaled by a pulsation through the pedal and a distinctive mechanical sound of the actuator cycling.
Although the earliest Sure-Track systems had a separate sensor for each rear wheel (replaced for 1971 with a single speed sensor on the driveshaft), Sure-Track could only control both rear brakes at the same time. Also, it did nothing to prevent the front brakes from locking in a hard stop.
The big question everyone asked when the Sure-Track system appeared was whether it could provide shorter stops than conventional brakes. The answer was “sometimes.” As Bob Madison and Kelsey-Hayes engineer Hugh E. Riordan explained:
On extremely low friction surfaces, such as ice, and high friction surfaces such as dry pavement, 4-wheel lock [conventional brake] and Sure-Track stop distances are essentially equivalent. However on moderately low friction surfaces and at 60 mph on medium friction surfaces, the reductions in stop distance due to Sure-Track are impressive.
Where Sure-Track really showed its worth was in directional control. In proving grounds testing of Mark III and Thunderbird test cars with conventional brakes, locking all four wheels at 60 mph could cause significant yaw change — as much as 15 degrees even on dry, high-traction surfaces, and much more in “split-μ (mu)” conditions, where surface friction was much lower on one side of the car than the other. Sure-Track minimized yaw changes and provided much better directional stability even on glare ice. There could still be some fishtailing if surface traction was uneven, but the driver had a much better chance of keeping the car in its lane than with conventional brakes.
Engineers from GM and Chrysler, which were working on their own anti-skid brake systems, noted derisively that the rear-only Sure-Track system wasn’t as beneficial as a four-wheel antilock system would be. Ford and Kelsey-Hayes acknowledged that, but contended that developing a workable four-wheel system would take several more years of development and would cost a lot more. Madison and Riordan explained:
Our position is that the advantage of early introduction of two-wheel control far outweighs any slight, eventual performance edge that might be obtained with the four-wheel system. This is not to say that four-wheel control is impractical or undesirable in the long run. We simply believe that at this time, two-wheel control represents the best solution from the viewpoints of both the public and the automotive industry.
General Motors apparently came to a similar conclusion, introducing its own rear-wheel antilock system on the Oldsmobile Toronado and Cadillac Eldorado in February 1970. Chrysler and Bendix subsequently topped both GM and Ford with the world’s first production four-wheel electronic system, Sure-Brake, which was introduced on the Imperial for 1971.
In the meantime, Ford had a head start. Sure-Track became optional on the Lincoln Continental Mark III in October 1968 and on the Thunderbird in January 1969. Its suggested retail price was $196.80 on the Mark III, $194.31 on the Thunderbird. Lincoln-Mercury made the system standard on the Mark III and optional on the Continental for 1970. Starting in 1972, the system also become optional for most full-size Mercury models with 429 or 460 engines.
For the 1975 model year, Ford switched some of its biggest, heaviest cars from vacuum brake boosters to Bendix Hydro-Boost, which used hydraulic power from the power steering pump rather than manifold vacuum for brake assistance. Sure-Track remained available on some models, but the system was redesigned to work with Hydro-Boost.
As with the later vacuum-hydraulic system, the hydraulic Sure-Track system used a single speed sensor on the driveshaft rather than one at each rear wheel. Although the vacuum-hydraulic system had been available with a Traction-Lok differential, Ford no longer allowed that combination, probably because the limited-slip action could confuse the sensor.
The latest Sure-Track system had a new hydraulic actuator, illustrated in the above schematic. In normal mode, with no wheels locked, the brake expansion piston (purple) was suspended between brake pressure and power steering pressure, holding open a check valve (pink) to the rear brake line. If the driveshaft sensor detected rear-wheel lockup, the control module would energize the solenoid valve (red), venting the fluid in the power steering support chamber to the power steering reservoir. This created a pressure difference that pushed the expansion piston to the left, closing the check valve to isolate the rear brakes and causing rear brake pressure to drop until the wheels unlocked. The system would not energize the solenoid valve below about 4 mph.
Unlike the earlier vacuum-operated Sure-Track, the hydraulic Sure-Track system was normally very quiet when it was operating, so it was reliant on the brake lamp to signal a system fault. (On earlier vacuum systems, the brake lamp would also illuminate if there was an electrical fault, but it was easy to tell by sound if the actuator wasn’t cycling.)
This revised Sure-Track system was offered only with Hydro-Boost. It was standard on the 1975 Mark IV (along with four-wheel disc brakes) and optional on the Continental, Thunderbird, and Mercury Marquis/Marquis Brougham/Grand Marquis. Some modern sources claim that Sure-Track was optional on the 1975 Mark IV as well, but contemporary Lincoln sales literature lists it as standard at least through January 1975. However, by 1976, Sure-Track had become an extra-cost option even on the Mark.
To my great surprise, the Ford factory shop manual asserts that Sure-Track was also available on the 1975–1976 Ford Granada and Mercury Monarch when equipped with Hydro-Boost and four-wheel disc brakes. The Hydro-Boost/four-wheel disc option was apparently added after launch (it’s not mentioned in the early 1975 Granada or Monarch brochures, but is listed in the 1976 ones), but I had never seen any indication that Sure-Track was available on these cars.
Lincoln-Mercury continued to offer Sure-Track on the Continental and Mark V through 1979, although by the time the Mark V debuted, you really had to dig to find any references to it in later brochures or advertising, and it seems to have become very rare. Given its complexity, I suspect that Lincoln and Lincoln dealers were not sad to get rid of it.
In the early ’70s, both Nissan and Toyota licensed the Sure-Track technology from Kelsey-Hayes and offered electronic rear ABS for their senior models, the Nissan Cedric/Gloria and Toyota Crown. As far as I know, this option was limited to the Japanese domestic market; Toyota and Nissan may not have paid for the rights to export it outside Japan. In the ’80s, a later-generation Kelsey-Hayes rear-wheel antilock system became optional on a variety of light trucks, beginning with the 1987 Ford F-Series. This wasn’t the same as the old Sure-Track system, although the general principles were similar.
For Ford and Lincoln-Mercury, Sure-Track was a commendable effort that eventually failed for many of the same reasons as the GM Air Cushion Restraint System: It was expensive ($313 on a 1979 Mark V, $525 on a 1979 Continental) and increasingly under-promoted, so it wasn’t ordered very often. The system’s benefits were also too incremental to make it seem like a must-have safety feature, and it wasn’t until the mid-’80s that Ford followed up with a more effective four-wheel system. Discounting the strange all-mechanical Lucas Stop Control System offered on some European Ford models in the ’80s, Ford didn’t offer antilock brakes again until 1985, when a four-wheel Teves system was added to some Continental and Mark VII models.
Related Reading
1969 Lincoln Continental Mark III, Inside And Out – Unsubtle But Effective (by me)
1970 Lincoln Continental – All-New, Lighter, Faster, But Not Quite Good Enough? (by me)
Curbside Classic: 1968-1971 Lincoln Continental Mark III – Right On The Mark (by Paul N)
1972 Lincoln Continental Mark IV: Bunkie Knudsen Leaves His Mark (by me)
1977 Lincoln Continental Mark V: Bigger Than Ever, This Defiant Dinosaur Was Rolling In Green (by me)
Maxeret must be derived from french – MAXimum ARRÊT. Maximum stopping power. Pointless info but interested from a marketing viewpoint
Ford did announce the availability of Sure-Track & 4-Wheel disc on the Granada. Motor Trend even tested one so equipped, but it never went into production, although the rear discs did, much to the joy of early Mustang fans everywhere. That 9″ rear with discs may be one of the most coveted FoMoCo non-Mustang parts ever made, as it’s a straight bolt-in.
As to why it didn’t make production, I can only guess it may have been deemed too expensive. Both brake options together account for about 10% of the base price of a Granada, almost as much as AC.
On an unrelated note, the aftermarket molding on that first Mark III looks like absolute hell.
Ahh, thanks. I hadn’t seen that test. What issue is that from?
Also, you’re right about the extra molding. I picked that particular car because it was a 1969 that definitely has Sure-Track, which was still optional at that point.
July or August of ’75
other page
I can remember my father being really proud of the Sure Trak brakes in his 70 Mark III. I think his 72 Mark IV may have had it too, but am unsure of the 78 Town Coupe.
I had never been aware that Ford switched to a hydro boost system for Lincoln brakes in the 70s. I knew that Studebaker Hawks used it until the end of production, but thought it had gone extinct on American cars after that. I really enjoy these bits of tech education!
Bendix was heavily promoting Hydro-Boost for truck applications and then for heavier cars in the early ’70s, in part due to new federal rules for better truck braking. The big driver was that emissions-controlled engines didn’t always have enough manifold vacuum to provide adequate assistance for heavier vehicles, and ’70s Lincolns were really heavy. (The curb weight of a 1972 Mark IV was about 5,000 lb, and its GVWR was 6,164 lb!)
Very well explained article as always, and appreciate the pictures. I always had a soft spot for the Marks as my father had a Mark VII.
If my memory is correct, the Mark VII was the first car in the 1980s to offer again ABS.
I find it interesting that the hydraulic update of 1975 was quiet; i vividly recall a number of cars designed in the 1990s (and not cheap ones) that had a quite noisy ABS.
I also recall those who said they “don’t need electronics to drive for them”, disregarding the obvious advantages of ABS…
I think vacuum-hydraulic actuators (which are still used on many modern cars and trucks) are pretty inherently noisy: When the ABS is operating, there are several reciprocating parts going back and forth multiple times per second while pressure inside the diaphragm chamber goes from vacuum to atmospheric and back.
I’ve actually recommended to people that if they get a new vehicle with ABS, they find an empty parking lot or something — ideally on a drizzly day where the pavement is wet — and try hitting the brakes hard enough to engage the antilock function, so they’ll know what to expect in an actual emergency. The noise and pedal feedback can be really alarming if you don’t know how the system works.
The first car I owned with ABS was my 2008 Honda Fit. When the system was actuated, it made all kinds of clicking and groaning sounds. Fast forward to my 2018 VW Golf: I have never once felt the ABS working. I was aware of the early Ford systems, but I have never seen on in person.
As you noted, a lot of pickups from the late 1980s (including my 1995 Ford Ranger) until the adoption of 4-wheel ABS had rear-only ABS. It makes a lot more sense in a pickup, where the load on the rear axle might be anywhere from 1000 to 3000lbs. It’s hard to design an effective braking system for such a widely variable load. That 2-wheel ABS saved my bacon more than a few times.
Back in 1988, I began selling cars for the first time at a Buick, Cadillac, GMC and Honda dealership. ABS was (once again) just coming out and became standard on the Cadillac’s first. Without doing some digging, I think that was in 1989 with the refreshed Deville/Fleetwood (FWD) models. That was a real selling point for us for sure. In those days, most dealers still offered sales people new car demo’s to drive as an extra perk. My first few years I drove mostly Buick’s and 2 Honda’s before moving back to Buicks. My first demo with ABS was a 1991 Buick Regal.
I did well selling ABS as a safety feature, but not having any real experience with it, I was still kind of on the fence. Then one rainy day, I was driving my Regal with the ABS and was following (at a safe distance thankfully) a large work van. I was not able to see what was in front of the van and suddenly the van slammed on the brakes. Then I heard the van hit the car in front of it. Not having much time to react, I slammed on my brakes and there was zero way I was going to stop before hitting the back of the van. However, instead of the brakes locking up and me hitting the van, I was able to slow down greatly while steering around to the right of the van. Thankfully, I missed the van by 2 to 3″ at most and was not involved in the accident. Bottom line? The ABS along with following at a safe distance allowed me to avoid that accident. From that day on, I would never buy any new vehicle without the ABS.
Pickup trucks needed rear antilock more than cars due to extreme variations of rear axle loading. My ’84 Toyota had a crude, but somewhat effective proportioning valve with a lever to measure rear suspension travel.
Rear only electronic ones were on ’93 C1500 and ’94 F-150. Ford used a tone ring as part of the differential assembly. I think GM used the speed sensor at the rear of the transmission.
Yes, Kelsey-Hayes offered both variations for their later pickup truck system. Interestingly, they and Ford had originally tried the transmission sensor for what became the first-generation Sure-Track system, but had found that it produced too much sensor noise.