Automatics... what stops them

[RustyNuts]

from rolling backwards down a hill when you're stopped at the traffic lights?

 

The rev counter doesn't increase, the brakes don't seem to magically apply themselves so i'm stumped as to how that works unless there more more fuel being injected into the cylinders?

 

Someone in the know... please tell me!

[scaryoldcortina]

[dugong]

A Renault badge on the bonnet. 

[mouseflakes]

Wow! I didn't know that Bonio dog biscuits where an integral part of an auto transmission.

[red5]

from rolling backwards down a hill when you're stopped at the traffic lights?

 

The rev counter doesn't increase, the brakes don't seem to magically apply themselves so i'm stumped as to how that works unless there more more fuel being injected into the cylinders?

 

Someone in the know... please tell me!

 

On 'old' automatics with no electronic brake hold it's the torque from the engine at idle 'driving' the transmission forward. 

[Station]

Wow! I didn't know that Bonio dog biscuits where an integral part of an auto transmission.

 

Only in Renaults, as per Dugong's post.

[Asimo]

The driver applies the brake!

Unless the hill isn't steep in which case the residual torque, as the engine ticks over against the torque converter, more or less holds the car against gravity. Same force as results in "creep" on the level.

If it is a steep hill, a few more revs increases the torque so as to balance the hill.

Because there is quite a lot of friction to overcome to move a car, the balance of residual torque and hill is easy to achieve.

[oldcars]

Do the parts in an auto wear like a normal clutch?

[Bren]

Wow! I didn't know that Bonio dog biscuits where an integral part of an auto transmission.

Renault auto boxes are made from chappie apparently.

[Station]

Do the parts in an auto wear like a normal clutch?

 

There isn't a 'dry' friction clutch in an auto, it's all fluid controlled in the torque converter - the faster the torque converter spins, the more fluid flows through which then does something complicated with a central clutch (it's controlled by fluid pressure). ATF does more than just lubricate, it don't think it would run with out it.

[red5]

Brake bands wear (think a drum with shoes that press on the outside) and clutches wear (think motorcycle wet/multiplate clutch) and lastly the fluid wears out.  

[Asimo]

^^^^Those things wear with gear changes, into reverse or D from neutral etc. Nothing wears when the engine is holding the car on a hill. But the oil does heat up. Quickly too if more revs are required for the hold.

[anonymous user]

An automatic gearbox works by magic and is fully staffed with very small gnomes that move things around as required; they have a small periscope arrangement so that one can keep an eye on the road ahead and then tell the others when to add another gear, or shift everything around so that it goes backwards. They keep a small box of bonios and have one ready to shove in between the cogs when they need to hold the gears in one place, for parking or on a hill. This is why there is sometimes a small delay before moving off as they have to rush off to get the sweeping brush to push the bonio out without damaging their fingers, the brush is then used to sweep up any small bits of bonio that have fallen off. The gnomes live on cheap red wine and bonios, the red wine is sold as ATF just to keep up the pretence that automatic transmission is a purely mechanical process rather than magic.

 

The reason why automatic transmission was once an expensive option but has now become cheaper is that there used to be a shortage of gnomes, the gnome population increased dramatically during the 1960s due to the increase in cheap red wine consumption by the gnomes and a consequent lowering of their standards when chosing a mate. There are differences with the gnomes used in different countries, the French ones particularly do not like working in the afternoon or really hot weather also are prone to consuming too much red wine and giving up altogether.

[RayMK]

Then there's DAFs, where hardly any of the preceding info applies.  On hills in a DAF, use the footbrake or handbrake otherwise it will roll with the slope.

[Pillock]

Never ask.

Automatic transmissions, like microwave ovens and aeroplanes, only actually work because nobody knows how they work. If you were to take one apart, you'd discover that it simply cannot operate at all at which point they'd all do a Renault and cease to operate instantly.

[mouseflakes]

Renault auto boxes are made from chappie apparently.

 

If they're similar quality to my chappie, then it's no surprise they don't last very long.

[fred]

Based on the theory cats always land on their  feet , and buttered toast always lands buttered side up

So by strapping the buttered toast on the back of a cat , you get perpetual spinning motion

[SiC]

There isn't a 'dry' friction clutch in an auto, it's all fluid controlled in the torque converter - the faster the torque converter spins, the more fluid flows through which then does something complicated with a central clutch (it's controlled by fluid pressure). ATF does more than just lubricate, it don't think it would run with out it.

Unless your automatic is a DSG/robo-box/CVT that have clutch packs. I.e. not a slush box.

 

The ATF in an slush box is also used to power all the actuators that activate brake bands/lockups/etc and also to keep everything cool. ATF is literally the life blood of a slush box.

[davidfowler2000]

BUS / COACH TRANSMISSION ANORAK ALERT!!!

The best thing you can do to try and understand how a proper epicyclic automatic gearbox works is to watch videos on YouTube. Confusion also sets in with the like of DSG boxes and more modern automatics that are basically automated manual boxes with a computer controlling the clutch and gear selector.

 

This all said, don't try and understand how VOITH commercial automatics work. I will give a brief summary for those who have half an idea of how they work:

Normal epicyclic boxes have the following arrangement: Engine -> Torque Converter -> Gearset -> Output

 

VOITH boxes are something along the lines of: Engine ----Gearset---<---> Contra rotating Torque converter ---> Gearset ---> Output - The gearsets are either side of the torque converter in the middle of the box and the input shaft has to go through some of the planetary sets. Also, rather oddly for an epicyclic box, the output shaft rotates the opposite way from the input. The easiest way to define a VOITH is: Engine --->?---> Output and !!!NOISE!!!

 

VOITHs also have the added complication whereas a normal automatic gearbox will have a distinctive lockup phase (when the torque converter is physically locked by a clutch to reduce slip losses) usually in top gear or more modern electronic boxes can bring the lockup clutch in and out to prevent gear shifts, a VOITH box will be on hydrodynamic (Torque converter) AND mechanical (Lockup) drive at the same time in first gear then pure lockup on the rest of the gears.
I once saw an exploded diagram of how this works. The engine's output is fed to a differential gearset which sends some power to the torque converter and some to the lockup clutch which is then recombined to be fed backwards (contra rotating (see above)) in to the gearset. Sounds reasonable until you find out the differential, torque converter and lockup clutch are ALL the same unit.

Then to further add to your head-exploding misery is the integral hydraulic retarder. It's basically a fade-free, never burn out brake. Fuck only knows how VOITH do it. All I know is that it has something to do with feeding the output of the gearbox back in to the torque converter in the opposite direction to produce a brake force.
 

This all said. VOITHs sound the fucking tits when they're really on song.

[rml2345]

You forgot about four speed VOITH where there's only one fourth gear set in a vault in Germany and the gearbox accesses it by telekinesis.

 

[Dick Cheeseburger]

Things we can understand, in order of difficulty of understandance, starting with the more easily understandablable things.

 

1. Why modern cars are shit

2. Why elderly Metros lean to the left. Or right.

3. Why the Talbot Matra Rancho is better than the Land Rover Discovery

2. Why old Cortinas became Stellar modes of transport

7. Why ditchfinder tyres are worth it

3. Why Napoleon chopped off his ear prior to visiting the moon with his friend Buzz Lightyear

 

Things which are impossible to understand:

 

2. Justin Bieber

5. Garage music

37. Automatic gearboxes

[For Fiats Sake]

Ahhh the sound of the school bus, now can somebody sit behind me and put a scarf over my head and yank it around my neck?

Don't forget to draw a cock on the back window....

[davidfowler2000]

5689210. VOITH Automatic boxes

[Station]

38. Why every Scottish person on here drives a bus (or is somehow employed in a bus related occupancy)

39. How Hula-Hoops crisps are made

40. Automatic gearboxes after someone taking three hours explaining how they work using Lego men, a pie cut into segments and a measuring jug filled with red pop

[Station]

BUS / COACH TRANSMISSION ANORAK ALERT!!!

 

The best thing you can do to try and understand how a proper epicyclic automatic gearbox works is to watch videos on YouTube. Confusion also sets in with the like of DSG boxes and more modern automatics that are basically automated manual boxes with a computer controlling the clutch and gear selector.

 

This all said, don't try and understand how VOITH commercial automatics work. I will give a brief summary for those who have half an idea of how they work:

Normal epicyclic boxes have the following arrangement: Engine -> Torque Converter -> Gearset -> Output

 

VOITH boxes are something along the lines of: Engine ----Gearset---<---> Contra rotating Torque converter ---> Gearset ---> Output - The gearsets are either side of the torque converter in the middle of the box and the input shaft has to go through some of the planetary sets. Also, rather oddly for an epicyclic box, the output shaft rotates the opposite way from the input. The easiest way to define a VOITH is: Engine --->?---> Output and !!!NOISE!!!

 

VOITHs also have the added complication whereas a normal automatic gearbox will have a distinctive lockup phase (when the torque converter is physically locked by a clutch to reduce slip losses) usually in top gear or more modern electronic boxes can bring the lockup clutch in and out to prevent gear shifts, a VOITH box will be on hydrodynamic (Torque converter) AND mechanical (Lockup) drive at the same time in first gear then pure lockup on the rest of the gears.

I once saw an exploded diagram of how this works. The engine's output is fed to a differential gearset which sends some power to the torque converter and some to the lockup clutch which is then recombined to be fed backwards (contra rotating (see above)) in to the gearset. Sounds reasonable until you find out the differential, torque converter and lockup clutch are ALL the same unit.

Then to further add to your head-exploding misery is the integral hydraulic retarder. It's basically a fade-free, never burn out brake. Fuck only knows how VOITH do it. All I know is that it has something to do with feeding the output of the gearbox back in to the torque converter in the opposite direction to produce a brake force.

 

This all said. VOITHs sound the fucking tits when they're really on song.

https://www.youtube.com/watch?v=eDfHa-cLdLw

WTF?

[Pillock]

Mrs_Pillock used to work in the Hula Hoop factory.

 

They're extruded.

[Magnificent Rustbucket]

BUS / COACH TRANSMISSION ANORAK ALERT!!!

 

The best thing you can do to try and understand how a proper epicyclic automatic gearbox works is to watch videos on YouTube. Confusion also sets in with the like of DSG boxes and more modern automatics that are basically automated manual boxes with a computer controlling the clutch and gear selector.

 

This all said, don't try and understand how VOITH commercial automatics work. I will give a brief summary for those who have half an idea of how they work:

Normal epicyclic boxes have the following arrangement: Engine -> Torque Converter -> Gearset -> Output

 

VOITH boxes are something along the lines of: Engine ----Gearset---<---> Contra rotating Torque converter ---> Gearset ---> Output - The gearsets are either side of the torque converter in the middle of the box and the input shaft has to go through some of the planetary sets. Also, rather oddly for an epicyclic box, the output shaft rotates the opposite way from the input. The easiest way to define a VOITH is: Engine --->?---> Output and !!!NOISE!!!

 

VOITHs also have the added complication whereas a normal automatic gearbox will have a distinctive lockup phase (when the torque converter is physically locked by a clutch to reduce slip losses) usually in top gear or more modern electronic boxes can bring the lockup clutch in and out to prevent gear shifts, a VOITH box will be on hydrodynamic (Torque converter) AND mechanical (Lockup) drive at the same time in first gear then pure lockup on the rest of the gears.

I once saw an exploded diagram of how this works. The engine's output is fed to a differential gearset which sends some power to the torque converter and some to the lockup clutch which is then recombined to be fed backwards (contra rotating (see above)) in to the gearset. Sounds reasonable until you find out the differential, torque converter and lockup clutch are ALL the same unit.

Then to further add to your head-exploding misery is the integral hydraulic retarder. It's basically a fade-free, never burn out brake. Fuck only knows how VOITH do it. All I know is that it has something to do with feeding the output of the gearbox back in to the torque converter in the opposite direction to produce a brake force.

 

This all said. VOITHs sound the fucking tits when they're really on song.

 

 

Crikey!

[Magnificent Rustbucket]

Older automatic cars seem to creep faster than modern cars in my limited experience, so will hold the car on a steeper hill. My Jag is a good example: it idles at 700 rpm which is enough to hold the car on a fairly steep hill without rolling back or touching the accelerator. It is genuinely useful - but I was almost caught out once when the damn thing stalled and rolled rapidly backwards. I only just braked in time!

[83C]

Voith bus gear boxes are utter shite.

 

Reverse gear (or the lack of). What the fuck is a set speed reverse all about?

 

1st gear: 0-10mph. 2nd gear: 10-35mph. 3rd gear: 35-38mph. Slight ratio issue I think.

 

The noise so beloved of certain bus enthusiasts is unfortunately a by-product of the engine straining it's nuts off whilst the gearbox does its level best to prevent the energy generated being turned into forward (or reverse, for that matter) motion.

 

Give me a decent (and more conventional) ZF box any day.

[Junkman]

What stops an automatic is either the fucking brakes, or the fucking 'P'.

 

Questions?

 

And who wants to stop anyway?