Packard Torsion-Level Suspension explained and demonstrated.
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- Опубликовано: 6 сен 2024
- We put the 1956 Packard Patrician up on a lift to show you the amazing design of the famous Torsion-Level automatic leveling suspension system.
By 1955, the Packard Motor Car Co. was doomed. Despite a hopelessly optimistic buyout-merger with Studebaker in October of 1954, the company's fate was sealed, unfortunately. But that didn’t stop the venerable Motor City automaker from introducing a whole series of new engineering features, including a highly refined, durable and robust V8 engine with many horsepower and displacement options, an advanced and improved Ultramatic automatic transmission, and an innovative suspension system called "Torsion-Level Ride". Due to the company’s rapidly failing health, Torsion-Level was offered for only two years, 1955 and 1956, so the system isn’t very well known or understood today, so we hope to show you and demonstrate it here for better understanding.
In 1955, the system was made standard on all senior Packards and some Clipper models, optional on the rest of the line.
#packard #barnfinds #automobile #classiccars #vintagecars #classicperformance
I am impressed how well that undercoating worked on this car.
Yes, it did its job very well and continues to do so. Wherever you look under the coating, is factory fresh.
British Motor Company's Australian factory used to spray some sort of rubbery gloop under its cars. It worked well to - my mother's Morris Major was totally rust free at an age when Ford Falcons would be unroadworthy due to rust.
While Packard was late to bring out a V8, they continued to out engineer the Big Three. TL and push button transmission couldn’t save them from problems associated with the 55’s; competition from Cadillac, and the disastrous purchase of Studebaker. The last true Packard (‘56) is a work of art!
You are spot on, correct! We love the '56 in particular over the '55 for the restyled headlight buckets, pod shifter and improvements in the Ultramatic transmission. The car is quality and perfection through and through. Unfortunately, the buyers passed dipping sales from roughly 9k in '55 to a scant 3k+ in '56... then the doors closed on East Grand Avenue.
It was a quality car when it was made and still is. That's an impressive torsion bar suspension that I didn't know about, the Packard engineers had to make that work and I'm sure that took much time and effort to design. Packard was a quality world class car company, that should be remembered.
You are most correct, Marten. I was not fully clear how the entire system worked from front to rear until we had a chance to study it from underneath on the lift. Packard also fully engineered and developed the automatic transmission and OHV V8 all in-house. Most other automakers sourced out engineering, sub components or complete transmission units to companies such as Borg Warner. It is thought Packard might have got in the weeds by being so intent on in-house engineering and manufacturing.
Actually, the Torsion-Level suspension was invented by a Hudson engineer named Bill Allison. Hudson was too broke to develope it, so they let Allison shop it around to other carmakers. Packard's interim president, Hugh Ferry, bought the rights to it.
I also have a 1956 Patrician in a single color (Adriatic Blue met.). I see you answered your question on how TL works. I have taken the control box apart. Inside are contacts and relays, along with a lever that is connected to the main suspension. The timer system is made up of 2 bi-metal contacts wrapped in heater wire, like a toaster. When the car is loaded, a switch sends current into the proper heater wire. The bi metal contact heats up and slowly bends to makes another switch that runs the leveler motor. When the car reaches the correct height, another switch stops the motor. Very ingenious, very simple, and very reliable, as seen by it working here after almost 70 years.
You are most correct! Thank you for the detailed information.
I love the '56...my fav postwar Packard. So classy, and technical. The best thing about the torsion bar suspension is that the bars don't touch the frame at any spot, so the isolation reduces NVH.
Thanks Steve. Good point. And true. Good isolation. Thanks for the interest.
One of the things I remember about the torsion bar system was that if you had a flat tire, you put weight on that corner so the leveling system would raise the car. After it was raised, you turned the system off so the car stayed up and you had less distance to jack it up to change the tire.
Makes perfect sence. 👌 not wasting jacking motion against a soft spring (bar).
The reason the long bars tie the front and rear wheels together is to keep the car level when bumps in the road are encountered. When the front wheels come in contact with a bump and go UP the long bars are twisted and the twisting force go rearward through the long torsion bars and push the rear wheels DOWN. Voila! A perfectly level ride!
Amazing design. Thanks for your input!!
One good part about the torsion bars suspension was the ability to work under the car without the need of the time a lift. If you needed to work on the rear end, you had all the Neighbor kids sit on the trunk lid, you would turn the ignition on car with level itself out you turn the car off and everyone would pile off, and the back of the car would raise up nice and high where you could work on it.
Yes, indeed that is possible. There is a switch to deactivate the system under the dash near the driver's door. I use this process to set a desired ride height too, then turn the system off.
My dad purchased a brand new 1955 Packard "400". One comment I can recall was concerning the beautiful tail lights! My dad said at the time they were shocking to a lot of people who were used to very tiny rear lights on cars!! He used to have to keep the car locked because as a kid I liked to hop in and play the radio and work the power windows up and down! No key needed then!
Thanks for sharing! The rear lights are known as "Cathedral" lights making reference to the shape of them. Very easy to see at night!
I have always loved these cars. Own 4 1956 Packards currently including the twin sister Patrician to yours in Dover White except with the tan interior. In practice the suspension suspension is well designed and robust. Put four new shocks on the car when you buy it and enjoy the ride. Major repair is a bit tricky as you either need to hold the torsion bars in place when you replace say the A-arm bushings or you can unwind / unload the torsion bars. I managed to find the original large dealer tool to do just that.
Only weak spot on the car is the dual action (oil/vacuum) oil pump which should be upgraded with an adapter kit to a late model Mellings oil pump that is spec'd to a GM 455 Buick.
I have done some research on this and know of the potential weak point there. We'll check the Melling option. Oil pressure is very good! Thanks for your interest and sharing your knowledge.
My first car was a '55 Patrician. This was in 1970, and the handling was still surprising by the standards of the day. The self-leveling was effected by spooling up the shorter torsion bars that were about half the length of the springing bars. That way the spring rate of the main bars remained constant. And, yes-- Packards with this suspension did not lean in turns much, compared to Cadillacs and Lincolns of the day. I used to sit on the rear fender, allow the system to raise the car, then shut it off, via a toggle switch under the left edge of the dash. This gave the car a "California rake", when you drove down the street, but it was only a joke, not for driving. BTW, the Packard Clipper, their mid-priced car, came with a standard coil front, leaf spring rear suspension, but torsion bars could be special ordered.
Barry, you are most correct. If you saw in the video we explain and show the shorter bars you reference which are used for leveling compensation. And yes, you can easily get that Cali Rake easily, as we used to do with "Highjacker" pr Monroe air shocks in back. Ahhh, the good old days! Thanks for the info and your interest.
Thank you so much for showing this system off on this beautiful automobile! I have been in love with your Packard from your first video on it, and I am totally in awe of the preserved condition it is in! It is the closest to 1956 we could possibly get without actually traveling back in time! I am a huge Packard enthusiast, and the 1956 model year is my most favorite of all! Packard was a very innovative company, and had they survived, who knows what kind of automotive firsts they might have ended up introducing to the industry! Thank you again kind sir for allowing us to share in the beauty and engineering of this fine automobile!
NYWF, so glad to read your comment. And thank you. Ownership varies. Some like to keep cars hidden in storage away from public eyes which was the case with this car for over 40 years. We and most owners who are our clients, enjoy sharing such great history with others who also appreciate the classics. They all do tell a story, and this Patrician has quite a story to tell.
In 2024, it's all but impossible to find a torsion Packard that functions this way. Never, ever install H-D shocks, or have the exhaust touch anywhere, including the rear bumper. BTW, the lightest-duty KYB shocks allow the "interconnecting" torsion ride system, to function flawlessly. I was amazed, as nothing else that I've found, since the cheapest Sears shocks of the 70's, ever has. The wrong shocks completely ruin the ride quality. BTW, will everyone, with any old car, secure & cushion, all the noisy jack components & other crap in the trunk AND coins in the ashtray!
@@williamlegall2988 Thank you William for all the good information. I do recall Sears had their own branded "Sears Heavy Duty Plus" shocks and also sold and honored Monroe and Tenneco shocks. I suspect the Sears brand was manufactured by either Monroe or Tenneco.
I like the way they utilized starters as the motors doing the work....I have worked on several over the years...
Parts bin engineering efficiency and well under stressed in this application. I bet they never went bad or wore out.
1. The electric motor is only intended to compensate for overloading the rear end. It does nothing for the ride. The motor you see is a Borg Warner starter motor adapted for the job (the push button tranny also used the same motor to move the gear shift cable in and out).
2. The front and back tires are connected by the long torsion bar so that when the front tire moves up or down, so does the rear. There are no such connections side-to-side. I was hoping you'd do a couple of panic stops to show how the car squats down several inches, but does not nose-dive. Strong acceleration causes the suspension to lift the rear of the car. In both cases, the car stays level, hence the name "torsion level."
3. Coil springs are nothing more than torsion bars wrapped in a circle. The wires twist, they do not bend.
4. Citroen had a similar but far more sophisticated system which used air springs and interconnected high pressure hydraulics for front-back and side to side compensation. It was very complex but worked so well Rolls Royce and others adopted it.
Thanks for the great information! We agree with all, 1-4.
Your point 2 is not at all clear how the long bars function to keep the car level. See my explanation of that process in this thread!
Obviously the best way to drive the car was gently without panic stops or hard acceleration. I wonder how many owners took such care of them?
The torsion motor was. It BW unit . It was a Delco Remy unit . I know because I have a 55 400. The puhbutton shift had its own motor to shift gears . That would been impossible to engineer . Everytime you changed gears , would have messed with the suspension . The rest of the push button system was engineered and built by Bendix for Packard .
* Correction - The torsion motor was not a BW unit .
What a beautiful beast!!...wow!... makes my 65 Chrysler 300 look like a lightweight!
We love a Chrysler 300 and the "letter" cars!! Yea, she's a brute!!! You got all your money's worth when you bought one. "Just ask the man that owns one"
Hi from Sydney, Australia. My grandad bought one of these new, he ordered it to his specification, his brother-in-law had bought a new '55 model and had automatic gearbox trouble, so Pa ordered his as a manual with the optional dual carby set up, from the 352-cube engine it was good for 375 horsepower, not shabby for 1956. I learned to drive on/in this car in late 1973, so I could get my license on 24th January 1974. Pa's Patrician was so modern compared to other cars way back then with its poise under panic braking, or its projectile like take-offs when given the "joint" in manual first gear, no bum down, nose up feeling. I enjoyed every moment behind the wheel of this special car. Sadly in 1979 a bus hit it in the rear whilst turning into my mum & dad's driveway. However, it was "rescued" by two brothers who had been members of the Aussie Study/Pack' car club. They bought a scrappy tired California example. swapped chassis, welded on the donor cars right rear guard and rear valance panel and over months put in the hard yards to bring Pa's car back to life after it was written off by the insurance company. The only thing that they did that I hated was turn it from its original black enamel paint, into a turquoise and creamy-white car. All these years later at x'mas 2023 the two brothers still own it, love it & enjoy it.
In September 1982, I bought the next best thing in FACTORY right hand drive, a GM Holdens Aust built '74 Cadillac Fleetwood Brougham. Sent as a completely knocked down bunch of parts to build in GMH's Pagewood, Sydney plant it is a wonderful car, the closest thing to the spectacular '56 Pack' Patrician and driving it now for 41 years as my daily car it has proven to be vastly superior to a lot of modern cars. As it was a virtual full option car with early ABS brakes & traction control called "maxtrac", plus it has GM's early airbag restraints called Air Cushion Restraint System or "ARCS". But best of all as an export CKD Kit, it has a "full power" 8.2 litre export spec' engine with ZERO anti-pollution gear, so you have a genuine 400 to 500 hp car, not a gutless de-tuned USA spec engine. With the '74 Caddy's self levelling suspension it is the closest thing ever to Pa's '56 Packard. Pa was an industrial engineer and he loved that '56 Packard, when I took him with me in September 1982 to look at the Cadillac with a view to buying it, he said if Packard had survived it would have made something like this. In 1974 Mercedes did not have ABS brakes, Mercedes did not have traction control and Mercedes certainly in NO WAY head anything like ACRS airbags but GM had its air cushion restraint system well before any other company.
What a great series of storied, Greg! Thanks for sharing these with us and the group. I have never driven the dual quad setup on a Packard. I use would line to. Wasn't that motor the 374 cubic inch? As for Cadillac, they have always been great cars too! We once had an old Mercedes, from the 70's, it operated more like a truck than a car. As well as an '82 300D (diesel) which till this day, the slowest and most underpowered car I have ever driven. But they were tough! Thanks again! Take a look at some of our other content on the Packard, as well as a nice Cadillac super 60 series. Cheers, Mate!!
Your 1974 Cadillac Fleetwood sounds quite amazing! Perhaps you could do a video on it to show off the features you mentioned! I for one would enjoy watching it! Nice to know that such rare-optioned mid-70's Cadillacs still exist; and being a right hand drive fully optioned 1974 Cadillac makes it pretty much a one-of one in survival existence!!
I bought a well-used '74 Fleetwood in '81 when I was 20. When warm, I couldn't hear or feel the engine at idle. When cold, the pollution controls made it shake so badly, it broke the exhaust manifold bolts on a back cylinder. Great riding car even when decrepit, but I learned to hate leather upholstery.
@ralphl7643 yea, the polution controlls just suffocated cars of that time. We would take all that garbage off. Cut the cat off and straight pipe, block the EGR, take off the A.I.R system and pump, etc and they would run pretty well.
Your "level sensor" is the clamp on the tortion bar on the driver's side. It goes over to the controller box. The little rod has an adjustment on it. If it ever gets moved you can readjust on a level surface. I have to repair mine. It's currently "manual control". Thanks for the video!!
Good to know. I feel the car sits a little low as adjusted. Typically we get the ride level set and just leave the switch in the off position. Unless hauling a load in the trunk or rear passengers there seems to be no reason to have the suspension in the "on" position for regular use with driver only.
American Rolls-Royce, made with disregard to the cost, just to be the best.
It really does seem that way, and of course, more reliable.
I owned a 55 Packard Patrician for a short time back in 1974. in reading up on the 55 and 56 Packard at the time indicated that these cars tended to suffer from the fact that its electric components that controled the Torsion Level Ride, that is the motor, was heavily exposed to the elements and often failed or shorted out after a relatively short period of time. Those Packards equipped with the push button Twin Ultramatic also suffered from premature failure of the electrical contacts in the selector box. Chrysler's push button PowerFlite and TorqueFlite systems consisted exclusively of a mechanical linkage and tended to be much more reliable.
You are correct! I would imagine in climates with winter climates that salt would cause all kinds of problems with the leveling system as well as the Twin Ultramatic shifting solenoids. The Packard push button electric selector would not have been my first choice back then for a shifting system if I was purchasing a car. However, the attractiveness of the gimmick might have captured me. did Chrysler ever use vacuum for the push-button selector on the dash or center of the steering wheel? Thanks for your interest and knowledge.
I just got a Best padlock that was from the Detroit plant. Its 7 pins & I have the control key, Grand master key & the change key. This is a really nice lock. I was born in 1961 so I never
got to experience this EXELENCE in car design. If any of you have these locks here are the the codes. Keys are on "A" keyway & are cut from tip to bow.
Control=6509472. Grand master=8321650. The control key will pull the cylinder out of the lock body & the Grand Master will open all the locks on this system. You would
need to show the locksmith the lock & explain that PACKARD thats stamped on the lock is LONG OUT OF BUSINESS to convince him that its ok to cut these keys. You
would also ask the locksmith to PUNCH the keys to the codes I gave above.
Thank you for the tips, Dave. All good information we'll keep on file.
Great vid love the sound of that Packard would like to see more of that care❤❤❤❤
Thanks for your interest. We used two different cameras and 2 dif mics to try to capture the exhaust sound as it is to the ear, which it does not. On the video it is a bit raspy. To the ear it is smooth, deep and mellow. We feel the addition of an "H" or crossover balance tube between the two sides would be beneficial to more even the pulses. This crossover technology really did not come into play till the 60's and is still used today and to the extreme of now "X" pipes.
seeing this video actually helps me put together mine beautiful car btw incredible condition
thank you and thank you for your interest!
Fantastic car. My brother had one during the early 60’s. The trans went out and no one could fix it.
We agree, a transmission repair would most likely have to be sourced somewhere involving shipping of the unit. Parts new and used are still available, such as clutch packs and gaskets, but would be expensive.
Trans is very easy to work on. However, install government surplus bronze "tank" clutches. Same ones as early Hydramatic, used in WW-2 tanks. Next, install a solid "thrust washer" of correct thickness, in place of the OEM "waved spring washer" in the torque converter. The latter breaks into pieces & destroys the unit!!! Kanter sells direct-drive re-lined clutch plate, that's rugged enough for the enormous 405 pounds of torque. These small changes, make the Ultramatic bulletproof. Such simple refinements that they didn't realize at the time.
The TL system is also wired through the 3 wire brake lite switch. When applying the brakes, the TL system is dis-engaged. You wouldn't want the pitch of the car to suddenly change during a hard brake application.
Interesting and a good point. I felt the 7-second delay was accomplishing that task as well, but this makes good sence. It appears the "level sensor" has to be quite sensitive and precise to "trigger" with a 3-4" change at the rear of the car. It must be triggering and activating quite a bit over bumps in the road and the delay is canceling all the false readings. If just driving with just a driver or additional passenger in the front seat, my theory is to always leave the suspension in the off position. I have also had passengers in the back, let it level set then turn off for the duration of the drive. I can't find any reason or benefit to have the suspension on during normal driving. Others might disagree.
There is a rod clamped to the levelling torsion bar that activates a switch if it twists too far from level either way. The 7 second delay is a bimetal strip (two different metals back to back) with a coil around it for heat activated by the switch activated by the rod. It puts power to the coil and heats the strip that after 7 seconds bends from the heat to contact another switch that turns on the motor that twists the levelling torsion bar. Once the torsion bar twists back to level current to the original switch turns off. All this in a box slightly bigger than a regulator next to the levelling motor. Sadly, subject to corrosion over time. But heck, who needs chips and transistors in 1955/56. Dad sealed the box on my mother's 56 in the early sixties with tar and it works to this day.
Lee, this is good information. I was thinking of a mercury bulb level switch such as in an older home thermostat. The physical bar-arm sensor makes perfect sense. I imagine it is able to be loosened and adjusted to set the "level set" ride heigth as desired. This particular '56 sits "level" just a tad low in my opinion. I have let it rise up a bit then turned off the master switch to give a bit higher ride. In the upload, you can see the delay box and as you said, just a bit bigger than an old voltage regulator box, but similar in appearance. Appreciate your interest and knowledge.
@@leeprice5633 Basically the same device as a turn signal flasher. Ingenious!
Wow that front torsion is controlled by rear lower control arm. Keeping the car flat, as rear compensates for weight
Yes, pretty much the genius design. It's amazing that on lesser models you had this as optional over front coils and independly sprung axles.
So sad that Packard didn't come up with a V8 until 1955. The styling in the early 1950s was rather dated. The 1955 models might have saved the company, but it was already too far gone by the time they were introduced. I believe that the 1956 Packard V8 was the largest of any American car. And stylist Dick Teague did a masterful job on the existing inner sheet metal.
Much agreed, If they had come out with the new design and OHV V8 in '55 keeping up with the tri-five Chevys, or a year ahead, it might have been a game changer.
Packard and imperial have a lot in common also Studebaker
We feel Packard and Imperial might have a bit more in common via product lines than both do with Studebaker. Did you know some Packard and Imperial clubs have actually combined into one?
The levelizer was from a separate set of torsion bars. Not the main set.
Correct. They are the "short" ones headed rearward from the levelizer.
Thanks
most welcome!!
This is what's called making a virtue out of a defect. The car's suspension is way too soft, so they had to add a kludge to keep the car level when there is weight in the back.
I was hoping the presenter would show us what the leveling system does on a hill, which would tell us how the sensor works - does it sense chassis tilt or does it sense wheel extension?
The sensor is not hill sensitive. It is connected to the torsion bar with a time delay. So the bar rotates and then activates the sensor.
How difficult is it to find a replacement motor for the TL system?
We are unsure, and hope to always remain unsure. However, the motor is a common Delco Remy adapted 12V starter motor. Could easily be rebrushed and rewound. We feel the demand and duty cycle is so light, it is very understressed and failure would be rare. The delay box would be of greater concern, as that is application specific.
I think the "timer-system" probably worked by a pressure valve system. I could go into details, but I think my first line explains it!
Others have mentioned it's the voltage regulator styled box just ahead of the levelizer gear box and motor, being a thermal bi-metal delay switch. I have tested that theory and can shorten the delay by over using the system, meaning warming up the box so that the delay time shortens.
Do torsion bars wear out? Loose their springing?
Actually, they don't. Now, they can start to sag a bit past the resting specification but unlike coil or leaf springs, the bar arm stops can be adjusted to compensate for this. If the bars are abused and pushed past the yield point, sure, but unlikely in this application. If they were on a load carrying truck, most likely over time.
I wonder how people got these cars serviced when they were 10-15 years old?
@@1940limitedThe same way auto repair has always happened. You fix it yourself, pay someone else to fix it, or don't fix it. (My boss works on most 50's Era stuff at my shop)
But I wonder who could work on them as they aged. @@jamesgeorge4874
These cars are relatively simple in terms of technology and how they work. Anyone with general mechanical knowledge can service them and even today, parts are plentiful.
I don't think these cars would be too practical for everyday use, especially in bad weather where salt and corrosion could get into that leveling system. This is a nice weather car only. I understand you could order the car with conventional springs if the buyer so desired.
In contrast, these cars were actually designed in detail to easily manage the worst of driving conditions that could come to mind. We'll start with the very heavy undercoating that spans every underside surface to prevent any corrosion from salted winter roads, as well, it acts as a road noise deadening substance at high speeds. Also, the car is equipped with a dedicated front and also rear separate hot water heaters and multi-speed individually controlled fanned blowers with foot ducts for all passengers. You could option for a heated driver seat, fog lights and other dealer options for dark and foul weather driving conditions. The suspension is controlled by a modified starter motor and as all starter motors, low in the belly of the car. The gearbox is sealed as well as other components tar-box-sealed. They thought of it all.
I would still like to buy one new then only take it out in nice weather. :-)@@classicperformance
That was the case with this car. Only ever used for special occasions and 4,800 original miles. See our other content on the car for more information, images and driving.
I've watched all of your videos on this car and enjoyed them very much. Thanks for sharing. @@classicperformance
@1940limited We very much appreciate your interest in The Patrician and your contributions to the discussions! I can assure you the car will never see questionable weather. However, with the effectiveness of the cabin heaters, and the automatic Rochester engine choke, it is a pleasure to tour in on the coldest of crisp winter days, when the sun is shining. We operate the car year-round.
I would get the undercoating removed with dry ice blasting
Jeff, it would be nice to be able to remove the undercoating on the frame structure. We have used dry ice blasting on aluminum suspension components and engine cases in Porsches and Ferraris. I was not aware it could remove something as stubborn as a hardened undercoat. The plan right now it to pressure wash it, then respray with chassis black paint.
@@classicperformance yeah, it can definitely be done to remove under coating.
Thanks for the knowledgeable tip, Jeff. We would like to restore the undercarriage back to factory fresh. It's there, just needs to be revealed carefully and DI is what might be the least invasive and eco friendly. Thanks!
That levelizer motor is making too much noise. The springs are making too much noise as well. I had one and it was nearly silent.
Could be. This car was recently pulled from a 50-year mothball storage with only 4,800 miles on it currently. We are sorting out these issues.
If you keep the gear box that the leveler is in well greased it will work flawlessly. Just ask the man that owns one.
@@swamprat69er OK, I'll as the man. It seems to be well greased by appearance. Is there a way to check the grease or add??? Thanks!
Jay Leno demonstrated it in one his videos, sitting on the back corner. You could hear the motor, but only just. This one is so loud I thought the gut must have put a microphone right on the motor.
1:33 - Aunt Sally getting in & riding to church on sundays.... 🫣
So true. The car only has 4,800 miles since new.
The car was designed to carry 5 passengers plus the driver and carry all their luggage in the trunk and still ride level.
The switch is a mercury switch.
Your timing is set about 1/2° too far advanced.
That is a beautiful example of a 55 Patrician.
My '55 Clipper Super Panama had the same system. I say 'had' because my ex wife got rid of it on me. B*tch.
Good ear on that timing. I bet you heard it backing up on the starter just a bit upon a heat soaked start? The static timing is or seems to be a tad advanced. I have not shot it with a gun yet. I has thought it might be a mercury bulb switch, same as a thermatic coil thermostat.
Sorry to hear you lost your Super Panama, if only you could have departed the ex wife prior to the car! Timing is everything. If only yours was a bit advanced :)
If its just a mercury tilt switch mounted on the chassis somewhere, the car would level itself if you were on a hill, which is stupid. Does it do that? If not, the sensor must sense the degree of spacing between the rear axle and the chassis somehow.