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PaulP
Member Since: 04 May 2007
Location: Barcelona
Posts: 4317
  
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LiftedDisco wrote:In a sick sense, I am actually wondering which 'other forum' is inhabited by such folks..
Actually I think it's great that they're discussing things like this.... 
Most replies so far have been about how the gearbox will explode, the world will end, and you'd slide down the hill out of control until you die, but none of us have really explained enough to know if this is true or why it would happen.
It's obviously a bit of a crazy idea and doesn't sound very mechanically sympathetic, but none of us have been able to explain what would actually happen to the internals of the gearbox etc in this situation.
Topics like this are brilliant because between all the replies we could actually learn something new
EDIT: Snelling Pistakes corrected 2006 Discovery 3 TDV6 SE Auto Buckingham Blue
2007 Golf GT DSG
Last edited by PaulP on 11th Nov 2010 2:46 pm. Edited 2 times in total
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 11th Nov 2010 2:22 pm |
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stapldm
Member Since: 11 Sep 2006
Location: Swine Town
Posts: 2330
 
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I'm sure that in an emergency a standard (mechanical controlled) auto box could be used to brake the car by selecting reverse. It'd cook the oil, cook the box and you'd risk stalling if you didn't keep the revs up, but an emergency is an emergency and if you write off the 'box/engine/car in saving lives then so be it. As a standard practice I'd say you've bitten off way more than you can chew if you believe this is one of the techniques available for normal driving.
On a sophisticated box like the D3 has, it'd say sod off and let you career down the hill. Try driving forwards (on the flat) at about 7mph and dropping into reverse gear ...you keep travelling forwards without any engine braking effect whatsoever until the speed drops (by road friction or braking) below about 3mph...THEN the reverse gear engages and you start to slow - until then you're in neutral. The controller simply won't allow a condition to exist that might harm the 'box.
I'll leave the discussion of whether the 'box should allow this damaging behaviour to another thread; there are times when lorries have crashed after long steep hills (such as in the Alps) 'cos they tried to down shift to slow down (brake fade having left them no option) but the 'box said no, then the 'box changed up each time the revs hit redline.  Dr. Ian Malcolm:
"Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should."
Transgenic tomato anyone?
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| 11th Nov 2010 2:44 pm |
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widget
Member Since: 18 Jul 2010
Location: Marlborough, on the Ridgeway
Posts: 38
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CFB - speaking from personal experience...
Reverse gear descent with an auto does work. But, its pointless off road. I tried it on the 1:1 grade at the Longcross testing facility (Google Maps Longcross), and I thought I was going to die. The surface is high grip ridged concrete. The length is approx 50m. Car was a 90 V8 Defender 50th Anniversary. Nothing blew up, it was all very quiet, it didn't stall (thanks to the throttle stepper) and descended very slowly.
Why did I do this? Well I didn't set out to do it. I actually drove up it first, then having overcome the novelty (and boy it's quite scary) I stopped on the slope to try a restart using left foot braking and blending from brakes to power. Next I explored how much it would roll back and how much throttle it needed to hold to car. Result after some experimentation was that it would roll back, slowly against the engine and torque converter. So then having thought about all the internet theorists who have various opinions about reverse gear retardation, I gave it a go. First by backing up and descending a short distance before building up some courage and doing the whole lot. By comparison doing the same on the brakes in 1st low felt nervous.
In my opinion:
Would I do it on a soft surface? No.
Is it something I'd do again? No.
Is it practical in any form of off road driving? No.
Is it mechanically sympathetic? Yes - the peak pressure is less than under acceleration.
Did it stall? No.
I drove the same testing facility in the V8 Ninety which you bought from me Andy, but only went down it. Best technique on a wet hard surface was brakes and light throttle together to keep the wheels positively turning and resist lock up.
Hope that helps - feel free to link back to here if it helps. JB
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| 18th Nov 2010 9:23 am |
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CFB
Member Since: 02 Dec 2005
Location: Bradford, West Yorkshire
Posts: 6100
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Thanks Widget, pretty much as I would have thought, yes it is technically possible but no real benefit and a risk of more harm than good if you end up out of control 2020 BMW X1 18d XDrive X-Line Auto
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| 18th Nov 2010 10:45 am |
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Simons17
Member Since: 12 Aug 2010
Location: beverley north yorkshire
Posts: 233
 
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Whilst doing my advance course in a D4 the guy showed me that whilst moving forward the D4 would not select reverse anyway even when the shifter is put in reverse. not sure about D3. but with all that compression why do you need reverse anyway. cascade breaking works well when diff lock is not fitted or HDC
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| 19th Nov 2010 7:33 pm |
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Hobgoblin
Member Since: 15 Nov 2007
Location: As always, in the sh*t, it's only the depth that varies
Posts: 1445
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widget wrote: Best technique on a wet hard surface was brakes and light throttle together to keep the wheels positively turning and resist lock up.
That would certainly be my preferred method
Rgs
Paul I'm not as good as I once was........but I'm as good once as I ever was.
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| 19th Nov 2010 8:24 pm |
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