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Cruise Control Decel Function. - READ ONLY

66 messages,  Last post on Feb 14, 2007 at 9:43 AM

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What is this discussion about? Car Safety

Messages Page 3 of 7

#17 of 66 Coupla questions?? by andys120

Feb 06, 2007 (5:04 pm)

...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.

#18 of 66 I am confused by oldharry

Feb 06, 2007 (5:52 pm)

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.
 
Harry

#19 of 66 further by oldharry

Feb 06, 2007 (6:23 pm)

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?
 
Harry

#20 of 66 The patent... by wwest

Feb 06, 2007 (9:17 pm)

speaks for itself...

#21 of 66 patent by oldharry

Feb 06, 2007 (9:29 pm)

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.
 
Harry

#22 of 66 Re: patent [oldharry] by rorr

Feb 07, 2007 (8:19 am)

Replying to: oldharry (Feb 06, 2007 9:29 pm)
Here you go Harry:
 
http://www.patentstorm.us/patents/6588860.html
 
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.

#23 of 66 Re: patent [rorr] by wwest

Feb 07, 2007 (11:09 am)

Replying to: rorr (Feb 07, 2007 8:19 am)
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.

#24 of 66 Re: patent [wwest] by rorr

Feb 07, 2007 (11:47 am)

Replying to: wwest (Feb 07, 2007 11:09 am)
"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.

#25 of 66 Re: patent [rorr] by wwest

Feb 07, 2007 (4:05 pm)

Replying to: rorr (Feb 07, 2007 11:47 am)
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?

#26 of 66 Re: patent [wwest] by british_rover

Feb 07, 2007 (4:10 pm)

Replying to: wwest (Feb 07, 2007 4:05 pm)
Engine braking occurs on the REAR of a RWD vehicle but on the FRONT of a RWD vehicle.
 
Err care to restate that?

Messages Page 3 of 7

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