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Cruise Control Decel Function.

wwestwwest Posts: 10,706
Apparently the latest versions, designs, for cruise control have been seriously revised.

Since late in the last century manufacturers have been doing almost everything possible, conceivable, to improve the safety factor of vehicles with automatic transmissions when operating on low traction surfaces, most especially FWD or front torque biased AWD vehicles wherein engine compression braking will be the most detrimental.

It has now become common knowledge throughout the industry that engine braking on the front wheels will oftentimes interfere with the operation of ABS, potentially to the detriment, obviously, of the owners/passengers.

For those with long term stick shifting experience think about how often you wish for a clutch as you drive along in wintertime with your automatic shift transmission, especially a FWD one.

Most new owners manuals state quite explicitly that engine braking cannot be attained absent a manual downshift and in some cases not even then unless you disable, completely turn off, cruise control.

This whole widspread episode of throttle delay, 1-2 second downshifting delay/hesitation has arisen as the result of widespread industry adoption of a new automatic transmission shift pattern/sequence adopted late in the last century.

The technique involves quickly upshifting these electronically controlled transmissions/transaxles upon any FULL lift-thottle event wherein should the current gear ratio be retained would result in a significant level of engine braking. The idea is to improve the safety factor by virtually eliminating engine braking that cannot be overcome by the operator absent a quick shift into neutral, an action currently recommended by the AAA, but of itself fraught with peril.

So, rather than retarding the timing, as was previously done, to reduce the road speed of cruise control, I wouldn't be at all surprised to see the use of rear braking only via the traction control system to initially slow the vehicle.

But it is now pretty clear that applying the brakes, in total, to do this will be potentially safer, overall, than the use of engine braking which cannot be alleviated by the anti-locking braking system should it subsequently be needed.

Even slight engine braking on an extremely slippery surface, an icy bridge deck comes to mind, can easily result in loss of control even on a RWD vehicle, but the potential for loss of control of a FWD in these insatnces rises dramatically in comparison.

Be careful out there....


  • andys120andys120 Loudon NHPosts: 18,660
    What'd he say? :confuse:

    You don't have to be a genius to figure out that disengaging your CC can result in some engine braking and every owner's manual I've ever read cautions against the use of CC in slippery conditions.

    "Tempest in a teapot?"

    2000 BMW 528i, 2001 BMW 330CiC

  • wilcoxwilcox Posts: 583
    "Be careful out there...." --- ;) We can all agree to that!
  • tariktarik Posts: 344
    Maybe I don't fully understand, but are you saying that new programming of the gearbox controllers (or the CC) will shift up if you lift-off quickly enough? Or do you suggest to manually upshift in that moment?

    Well, sure sounds like some bogus last-millenium-advice to me :sick:
  • wale_bate1wale_bate1 Posts: 1,986
    Why would anyone be using CC in inclement conditions in the first place?

    This is some bizarre pseudo-corollary to the old climate control compressor theory, isn't it?
  • rorrrorr Posts: 3,630
    "....but of itself fraught with peril."

    Have we been attending writer's workshop conferences again?

    In all honesty, I'm at a loss as to just WHAT is the villian in all this:

    Is it the cruise control? Is it the programming in current automatic transmissions? Or is it that old bugaboo, FWD powertrains?

    I mean, the TITLE of this thread has to do with "Cruise Control Decel", but you spend your entire post discussing engine braking, ABS, automatic trannies, FWD, and the slippery conditions "fraught with peril".

    Uh, why not just NOT use cruise control in icy conditions? Isn't this the standard instruction in every owner's manual?
  • wwestwwest Posts: 10,706
    Yes, Rule # 1, NO ONE should be using cruise control on adverse roadbed conditions. But then NO ONE should EVER drive with a Blood/alcohol level above 0.08....

    Absent being able to enforce rule # 1 what would you have the automotive industry do?

    Personally I would disable cruise control when raining or if the OAT were below ~35F. And I most certainly would disable it if TC, Traction Control, activated. It would not be re-enabled absent a complete and full stop and an engine restart.

    Big Brother strikes again...?

    The post was intended as simply an advisory of the fact that the cruise control systems seem to be evolving away from the use of engine braking and into the use of actual braking.

    And my guess is that this "evolution" is the result of the fairly "recent" discovery by the industry that old style automatic transaxles and FWD were/are not a good match.
  • rorrrorr Posts: 3,630
    "Big Brother strikes again...?"

    Yes. And no thanks.

    Perhaps if the wiper speed has been set to 'high' I could understand having the CC disabled. But OAT below 35F? Ah, no. Living in rural central Texas I see temps below 35F virtually every morning in the winter - yet we see 'adverse roadbed conditions' which would actually WARRANT no CC maybe 1-2 times a year.

    Personally, I think the best 'solution' (Houston, do we even HAVE a problem?) is driver training.

    "...what would you have the automotive industry do?"

    On this issue? Nothing. How much nanny-engineering do we need? Radios that turn themselves off when vehicle speed exceeds 45mph? Mirrors/seats/steering wheels that CAN'T be adjusted unless the vehicle is stationary? Adaptive throttle controls that restrict vehicle speed based on tire/brake wear indicators?

    In all honesty, I'm not entirely sure just WHERE the discussion should be going on this issue. Are we supposed to be debating engine braking vs. wheel braking as a means by which the CC decel functions? Or is this whole thing just a left handed way to attack the safety aspects of FWD powertrains again?
  • wwestwwest Posts: 10,706
    And what about those bridge decks with an OAT ~35F...??
  • rorrrorr Posts: 3,630
    "And what about those bridge decks with an OAT ~35F...??"

    What about 'em?
  • wale_bate1wale_bate1 Posts: 1,986
    Well, some of 'em need painting something fierce, to begin with...
  • rorrrorr Posts: 3,630
    Yep, they sure do.

    Of course, I'd let it warm up a bit first.....
  • robertsmxrobertsmx Posts: 5,525
    Okay, none of that made sense to me. I do know a lot of vehicles (Infiniti M35/45 is a good example) that have nanny features controlling what can and what cannot. For example, with wiper running, cruise control can no longer be engaged.

    Now this one...
    It has now become common knowledge throughout the industry that engine braking on the front wheels will oftentimes interfere with the operation of ABS

    How exactly does that happen?
  • rorrrorr Posts: 3,630
    "How exactly does that happen?"

    Good question.

    All this time, I thought it was a closely guarded secret and now I find that it is common knowledge...... ;)
  • wwestwwest Posts: 10,706
    At fairly low speeds on a highly slippery surface the engine can provide just enough "braking" such that your anti-lock braking feature, the ability to release the brakes and thereby keep the front wheels turning just enough to maintain directional control, will be compromised.

    Ford was just awarded a patent concerning the use of regenerative braking for hybrid vehicles wherein the primary regenerative source is at the front wheels (where it sorta has to be anyway).

    The patented technique involves substantially reducing the level of regenerative braking if the OAT is near or below freezing, and additionally, otherwise, to disable regenerative braking entirely the instant ABS activates during actual brake pedal pressure.
  • andys120andys120 Loudon NHPosts: 18,660
    ...the "recent" discovery by the industry that old style automatic transaxles and FWD were/are not a good match.

    I am unaware of any such discovery, please enlighten us as to what "the industry" has discovered.

    I wonder if you could also explain what on earth the relationship might be between OAT and regenerative braking. :confuse:

    2000 BMW 528i, 2001 BMW 330CiC

  • oldharryoldharry Posts: 413
    I have owned several FWD and several RWD vehicles over the years, and question Mr. West's assertion that engine braking is not desirable on FWD vehicles.

    That the front wheels are braking without the rear gives a handling situation similar to the RWABS on my 94 C-1500. If this is bad, all the pickups with RWABS are unsafe.

    When I had a stick shift FWD, I frequently used engine braking to avoid spinout from rear wheel lockup during slipperly conditions.

    Driver training could convince you, Mr. West.

  • oldharryoldharry Posts: 413
    wwest wrote:

    "At fairly low speeds on a highly slippery surface the engine can provide just enough "braking" such that your anti-lock braking feature, the ability to release the brakes and thereby keep the front wheels turning just enough to maintain directional control, will be compromised."

    Actually, at LOW speed the engine does not brake the vehcle, but keeps the wheels turning, enhancing the ability of the ABS to keep the front end steerable.

    Your explanations all sound theoretical, Have you ever driven a FWD in winter enough to be familiar with the actual handling?

  • wwestwwest Posts: 10,706
    speaks for itself...
  • oldharryoldharry Posts: 413
    Can you give a link to the patent. I would like to read it myself, and consult with my son who is a mechanical engineer.

  • rorrrorr Posts: 3,630
    Here you go Harry:

    However, I think wwest may be drawing some faulty conclusions (particularly in regard to his interpretation that this is further 'evidence' of the evils of FWD).

    As I understand the writeup (and I'm admittedly not an automotive engineer), in a hybrid drivetrain utilizing regenerative braking, there are several programming maps which the overall braking system consults in a braking event. In 'normal' situations, when the driver first takes their foot off the gas, the system defaults to lift-throttle regenerative braking (referred to as "Compression" regenerative braking because the braking is due to engine compression). Then, as the brake pedal is applied, a secondary component called "Service" regenerative braking is added. The total amount of regenerative braking is the sum of these two (compression + service) components.

    However, the ABS system can only modulate the "Service" portion of the system. And, if the road is sufficiently slippery, only modulating the "service" portion of the regenerative system may not be enough to unlock a locked wheel (since the "compression" portion may offer enough braking force to keep a locked wheel locked). Therefore, in low temps (close to freezing OAT), the braking maps reduce (or eliminate) the "compression" portion of the regenerative braking.

    Now, how all this translates into some sort of global condemnation of FWD powertrains in general, I'll have to let wwest answer. Because I can't find any evidence in the writeup that Compression braking, BY ITSELF, can lock up a wheel.
  • wwestwwest Posts: 10,706
    A slight misread of the Patent...

    Actual engine compression is NEVER present unless the Ford hybrid implementation, like the Prius, has a special "mode" to enable it.

    On normal lift throttle events "light" regenerative braking is used to "simulate" actual engine compression braking. Now as the brakes are applied, more and more regenerative braking will be used as brake pedal pressure is increased.

    And yes, if in freezing conditions, or even in rain, the potential for loss of control increases to the point where it is now being acknowledged that both RWD and FWD vehicles can transition into "unsafe" territory without any obvious actions of the driver.

    ABS will only activate with the most severe braking, itself based on roadbed conditions. Severe, HARD braking, is also the point at which regenerative braking will be at the highest level. So yes, it would be best to disable regenerative braking on activation of ABS regardless of road conditions.

    And also yes, this is an especially serious matter for FWD and/or front torque biased AWD wherein loss of traction at the front means TOTAL loss of control.

    And you don't have to be an automotive engineer to know that the front brakes expend/supply ~80% of the braking HP, and 100% of the stearing capability. So having ABS operate correctly on the front wheels is an especially important consideration.
  • rorrrorr Posts: 3,630
    "And also yes, this is an especially serious matter for FWD and/or front torque biased AWD wherein loss of traction at the front means TOTAL loss of control."

    Whoops! See, that's where you lose me.

    If you have loss of traction at the front in a RWD vehicle due to ice, don't you ALSO have total loss of control? Or are you saying that while the road conditions led to loss of traction in the front of a RWD vehicle that I somehow still have adequate traction to apply power to the rear?

    The odd thing is that I HAVE experienced momentary loss of control due to ice and lift throttle conditions. The odd thing is that this was in a RWD vehicle. Lifting the throttle caused weight transfer onto the nose; the rear-tires lost traction, and the rear-end started coming around. Fortunately I was only doing about 30 so it was a slo-mo tailslapper. I've been in exactly the same circumstances in a FWD vehicle and lift-throttle DIDN'T cause loss of control to the front wheels.

    In fact, usually the opposite occurs: I've taken turns a smidge too fast in cruddy conditions with FWD and the front end starts to plow. The instinctive thing to do is to get off the gas. Hmmm, lifted the throttle, compression braking cause weight transfer ON TO the nose and traction at the front was regained.

    Seems straight-forward to me.
  • wwestwwest Posts: 10,706
    Simple, really...

    Engine braking occurs on the REAR of a RWD vehicle but on the FRONT of a RWD vehicle.

    Which would you rather, assuming both with automatics, be driving downhill on a slippery roadbed?
  • Engine braking occurs on the REAR of a RWD vehicle but on the FRONT of a RWD vehicle.

    Err care to restate that?
  • robertsmxrobertsmx Posts: 5,525
    Which way does weight shifts during deceleration? How does it help or make things worse? Make a case for FWD and RWD. Then we will get to the point of ABS.
  • rorrrorr Posts: 3,630
    "Which would you rather, assuming both with automatics, be driving downhill on a slippery roadbed?"

    Honestly not sure; 95% of my driving history since 1978 has been with a manual.

    However, I've had both a 5.0 Mustang and a Toyota Celica driving downhill on glaze ice (here in central Texas we tend to get freezing rain/sleet rather than nice fluffy snow) and I'll tell you that the Celica (and my previous Hondas) was easier to control.

    When headed downhill, braking (whether by mechanical means or through engine compression) causes weight transfer to unload the rear of the vehicle and load the front. Weight = traction. Therefore, weight transfer caused a LOSS of traction at the rear and an increase of traction at the front.

    The problem with the Mustang was that IF the rears locked, the rear of the car wants to lead. Swap ends. And locking the rears due to compression braking was pretty easy since you are unloading the wheels being braked.

    The Celica on the other hand was much more difficult to induce wheel lockup due to compression braking. The weight transfer would LOAD the front of the vehicle (remember: weight = traction) helping to keep the fronts unlocked.
  • oldharryoldharry Posts: 413
    My current vehicles are both automatic, and my experiences mirror yours. Slowing/braking downhill on glare ice, the FWD has better control than the RWS. The ABS system on the Impala (05) has no difficulty preventing wheel lockup, while the rear ABS on the truck does not stop the front from locking, and steering is lost with ANY application of brakes under extreme slippery conditions.

    I therefore leave a good interval when driving the truck on ice, and slow well in advance, releasing the brakes to reaquire direction when the front slides out of desired path, then braking again as necessary.

    Under cruise control conditions, wet pavement gives no problem, and I am not foolish enough to use cruise on ice in either vehicle.

  • oldharryoldharry Posts: 413
    Thanks for the link rorr.


    The amount of regenerative braking that is applied to the wheels (105) of a vehicle (100) is based on ambient temperature and a lift-throttle event. An ambient temperature sensor (108) monitors the temperature around the vehicle. Based on the temperature, a map is selected (204, 212, 214). If a lift-throttle event occurs, then the map is applied (206, 208). Compression regenerative braking is reduced to zero if an anti-lock braking system event occurs or if the throttle is re-applied, or both (216, 218, 220, 222). "

    This is obviously different in application from a engine driven FWD vehicle, in that, unlike a piston engine, and eletric motor has no 'idle speed' to resist stopping the wheels. Regenerative braking is greater when rotational speed is higher, but unless turned off (per patent) never stops retarding the wheels until motion ceases.

  • wwestwwest Posts: 10,706
    "...never stops retarding the wheels until motion ceases..."

    And this is different from..??

    Put your FWD automatic transaxle in the 1st gear position, accelerate to, say, 15 MPH. Now take your foot off the gas and tell when you ICE stops retarding the wheels.
  • robertsmxrobertsmx Posts: 5,525
    Do you want him to lose control and crash? :shades:
This discussion has been closed.