Last post on Feb 14, 2007 at 9:43 AM
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#27 of 66 Re: patent [wwest]
Feb 07, 2007 (4:28 pm)
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.
#28 of 66 Re: patent [wwest]
Feb 07, 2007 (5:03 pm)
"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.
#29 of 66 Re: patent [rorr]
Feb 07, 2007 (5:29 pm)
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.
Feb 07, 2007 (5:36 pm)
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.
#31 of 66 Re: patent [oldharry]
Feb 07, 2007 (6:38 pm)
"...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.
#32 of 66 Re: patent [wwest]
Feb 08, 2007 (9:48 am)
Do you want him to lose control and crash?
#33 of 66 Re: patent [robertsmx]
Feb 08, 2007 (10:15 am)
Well no, absolutely NOT..!!
But I suppose I should have added a caution note to not do this test on a low traction surface.
#34 of 66 Re: patent [wwest]
Feb 08, 2007 (12:18 pm)
So when does low traction surface become desirable anyway? Your argument is a FWD will lose control but a RWD will stay planted, doesn't make sense at all. Do you realize that more weight on drive wheels = more traction?
#35 of 66 Re: patent [robertsmx]
Feb 08, 2007 (1:47 pm)
"So when does low traction surface become desirable anyway"
Well, a low traction surface is desireable (to a point) when one starts considering roadbed friction and it's relationship to rolling resistance.
As you know, rolling resistance coupled with air resistance are the big bugaboos when one is attempting to maximize fuel economy. So, while it might be beneficial to tuck in tight behind large trucks to reduce (and possibly eliminate) most air resistance, it is also beneficial to take all reasonable steps to reduce rolling resistance.
The tire/roadbed interface is the prime culprit in rolling resistance. We can take some steps to reduce this by inflating the tires to a high pressure. However, this has it's limitations. One benefit to colder weather (approaching 0deg C) is that the cold temperatures affects the rubber compound in the tires, making the tires harder, and thus further reducing rolling resistance. If you couple this with a slick roadbed surface, you have a combination that can't be beat.
I've found my best hypermiling experiences to occur when tucked 6' off the bumper of semi's with my tires inflated to the maximum sidewall indicated pressures on ice-slickened roadways. Such experiences also give me a great cardiovascular workout without actually exercising (although I usually leave indentations in my steering wheel).
However, I can say that wwest does bring up some valid concerns regarding possible front wheel lockup. However, I've found it to NOT come necessary from lift-throttle conditions (since, as has been pointed out, such conditions cause a weight transfer to the front wheels, increasing traction).
Instead, I've found that sudden engine loads imparted by the AC compressor can bring about front wheel lockup on icy roads; particularly when tucked up tight behind a semi has reduced the aero downforce to the front of the vehicle. This situation seems to be particulary a problem with my Toyota since I have SUCH a problem keeping the windshield properly defogged using the standard defroster settings. I've considered upgrading to a stronger compressor, but the concern over front wheel lockup has precluded me front taking this path.
Perhaps the next innovation to be considered is remapping the AC compressor/clutch engagement software to the OAT sensors to progressively engage as temps approach freezing. Of course, this is just a half measure to address this potentially deadly issue since you still have a system which can unexpectedly impart undesired engine loads to the drive wheels. Perhaps windshield wipers mounted INSIDE the cabin to clear the screen as the OAT approaches freezing is the best solution.
Or maybe not.
#36 of 66 Re: patent [rorr]
Feb 08, 2007 (4:23 pm)
Now I really feel as if my leg is being pulled....
The efficiency of the A/C for helping to prevent windshield condensation and/or help to remove windshield condensation is strictly a function of local climatic conditions.
Throughout most of our world the colder the climate the lower will be the RH, relative humidity. Basically that means that the colder it is where you are trying to use the A/C to help prevent or remove windshield condensation the less likely it is that the will be of any help at all.
The surface temperature of the A/C cooling evaporator will never be driven below ~34F by design. To do so would result in freezing the condensate and eventually blocking all system airflow. So All modern day systems disable the A/C compressor as the OAT declines below ~34F.
What that means to you is that the dewpoint of the incoming fresh airflow must be above 34F in order for any dehumidification of the airflow to happen at all. And that 34F number is under ideal conditions. For instance an extremely low blower speed which will allow the airflow transit time through the cooling evaporator to be long enough for it to be cooled to its dewpoint.
In reality if the OAT is below ~47F, most of the time you may as well have the A/C turned off.
The BEST way to keep your windshield defogged, or remove condensation once it has formed is to HEAT the interior surface of the windshield as much as possible and as quickly as possible.
Turn up the temperature setpoint to maximum, the blower speed should follow, and then switch to defrost/defog/demist mode.
Once the windshield is cleared this method will no doubt begin to discomfort you with all of the heated airflow reflected off the windshield to your face.
Once that happens then turn the blower speed down manually while leaving the heat up and in 3D mode.
I NEVER allow my A/C compressor to operate at any time during the winter months and Lexus has provided two C-best options to facilitate that.
And by the by most newer cars have a variable displacement A/C compressor so the engine load can be varied in a linear way as more or less cooling is required.
Additionally most modern day engine/transaxle control systems, ECUs, will "bump" up the air/fuel mixture feed to the engine just prior to engaging the A/C compressor clutch so that no "drag" on the engine is felt at the drive wheels.