I had an interesting thought the other day as to how automatics respond to emergency braking. In a manual naturally we hit the clutch and hammers almost simultaneously, hence only the gearbox slows down through the transmission - I assume! In an auto however, the torque convertor and engine both would have to deaccelerate very quickly, is that right? It seems to be an enormous stress on the transmission and does it take longer to stop an automatic than a manual given this might be the case?
Simon
_____________________________________
SVPworld (incorporating PSRworld)
www.svpworld.com
|
There are two things to consider with auto's.
Firstly, on older non-electronic control boxes when you throttle off and brake you will reverse the power flow through the box but get little in the way of engine braking, so only the brakes will slow the car. Most of the electronically managed boxes will employ an engine braking strategy by controlling the valves which cause the torque converter to drive the engine. However the engine braking effect will be very slight in an emergency stop.
Secondly, if you are travelling at 30-40 mph and take your foot off the throttle you will notice that the engine rpm drop almost to tick over immediately, 'neutral' has been selected in the box to allow it to coast. This is acheived using a one way clutch within the torque converter which prevents the fluid providing full drive pressure in reverse.
So the short answer is that there will be no difference between manual and automatic transmissions when emergency braking as both are effectively in neutral. However, a modern electronically controlled box will slow quicker under normal braking as engine braking is provided but not to the degree that you get with a manual transmission.
|
|
|
I got considerable engine braking from my Rover 827 auto when lifting off and the engine didn't drop to idle untill the car had slowed right down. This ensured that the transmission drove the engine and made best use of the "fuel cutoff on over-run" feature to help economy.
I noticed no difference in the ability to bring the car to a halt from 100+ with ABS when I changed from my Rover 827 (auto) to my current Rover 820 Vitesse Sport (manual). The vitesse may have a slight advantage, but it is only slight and is more likely down to the Vitesse having 215/45x17 tyres vs the 827s 195/65x15s. Both cars have the exact same brakes.
|
Both Rovers employ control strategies that manipulate the valves to prevent the fluid recirculating and provide engine braking.
When the 75 replaced the 800 series they carried over alot of the auto box operation to the Getrag box because the 800 auto's worked so well.
|
in a manual wasnt there always a debate re emerg stop from high speed about whether you should floor the clutch immediately, or after you have slowed right down
in a manual high speed emerg stop i wouldnt dip the clutch until speed has dropped below about 30
when travelling at lower speeds id dip the clutch anways, since there isnt much in it
in my current auto i can hit the brake and have ABS cut in just about anytime (if i really want to slow dramtically) and i figure that if the ABS is working then the car isnt going to slow any faster whatever the gearbox is doing
|
I'm reliably informed that you should hit the clutch at the same time as the brakes. Apparently this removes the inertial effects of all that rotating and reciprocating mass in the drivetrain. One imagines that the same effect would be had by knocking an auto into neutral, though I never did this in my 827 and, as I said, found no difference in the braking ability between the auto and manual versions of the car.
Why not give it a try and let us know if you find any effective difference?
|
flywheel type inertia would be good if you want to stop in a straight line
however if you have some fancy steering to do then i agree as little flywheel type affects as possible would be best
this is different to the grip at the tyres, which thinking about it if you are in a car which is set up such that you can provoke ABS being on then you are pretty much at the limit of the retardation the grips can provide regardless of what the transmission can be doing ?
isnt the biggest flywheel effect going to come from the flywheel itself ? which yea would be slowed down if engine dropped to tickover by hitting clutch rather than being matched to geared speed - but most of drive train is going to move at speed related to speed of travel regardless of clutch position ?
but then its late and im tired
|
wasn't that argument pre-ABS and linked to the effect that the engine would have in preventing some of your wheels locking up ?
If it was, then I guess with ABS you should hit the clutch at the same tiem as the brakes.
|
|
gyroscope stability provided by spinning parts such as flywheel will add stability is the only real question in my head
|
|
does it make a difference ? I mean a measurable or noticable difference ?
|
absolutely
a fly wheel with that many Rev's on it has dramatic gyroscope properties that most drivers probably arnt even aware of
after all a motorbikes stability and ability to stand up at speed is largely driven by the two gyroscopes with tyres on at each end!
|
|
|
|
If it did, you'd have real trouble going round any corner. There are too many other factors for this to be noticable in a car.
|
|
|
|
|
wasn't that argument pre-ABS and linked to the effect that the engine would have in preventing some of your wheels locking up ? If it was, then I guess with ABS you should hit the clutch at the same tiem as the brakes.
When I was taught to drive some 16 yrs ago, I was told not to press the clutch until almost stopped. As Mark says, it helped prevent locking up the wheels.
|
|
|
|
|
|
Never felt a difference when driving Mum's old Accent auto (horrid car in terms of performance, ok otherwise.) As for slowing down quickly and getting better handling I used to hit overdrive off (drop to 3rd), then select 2nd - driving it pretty much as a manual. Guess it was a naff box because it never kicked down quickly; needed about 5 seconds notice to shift from top to 2nd when above 40!
|
well cyd i think youd be surprized
any gyroscopic experts in ?
|
|
|
|
