Questions 'bout enjins

[turbolag]

Lots of auto drivers routinely use the overrides when driving downhill. You can smell the ones that don't a mile away and see the smoke bellowing from their wheelarches on long and/or steep hills

What's wrong with coasting downhill in 'D' ?

you're not in proper control of the vehicle - while the vehicle is in motion you should be manipulating either the throttle, or else the brakes. No in between.

As my instructor rammed home to me when I did my dibble class I qualification all those years ago "Brakes are for slowing, gears are for going, and never the twain shall meet..."

[steve_earwig]

So you always dip your clutch coming up to the lights and use the footbrake. Man, you must get through some disks!

[DaiRees]

"Brakes are for slowing, gears are for going, and never the twain shall meet..."

What? No changing down for engine braking?

EDIT: Didn't you used to get a warning if someone else had posted when you were thinking about it?

[steve_earwig]

Queue Nelson... I got them twice yesterday (that bluddy Welton geezer ) but I don't think it happens if someone posts while your POST REPLY page is loading.

[Longintooth]

There's a lot of confusion on this subject and mostly lack of fundamental knowledge. I took an advanced drivers certified test some years ago , they advocated that "clutch/gearbox braking" is not to be used. When I delved into what they meant it was when you release the clutch and used that action to slow the vehicle down not the actual engine braking through the gears. So for example the gears speed and engine speed are matched and then the engine braking is OK - as it should be.

Engine braking is a huge asset in braking especially in corners where the differential action shares the brake torque equally between the drive wheels. This action uses the slip angle of the tyres in very useful way since a tyres slip angle decreases when braked and increases when accelerated and this is shared equally between the inside and outside of a circle. It means the understeer and oversteer can be manipulated with the throttle.

If engine braking is used to stop it saves fuel by the fact that the ECU senses that it is decelerating and shuts off the injectors whereas if the clutch is dipped the engine goes into idle mode and uses fuel whilst braking. Also coasting loses some control of the vehicle - coasting as in fuel conservation on down hills will save a considerable amount of fuel even though some is still being burnt in the idle process. If you think carefully about this you can see the difference between that and braking.

[steve_earwig]

I was never quite sure how coasting looses control, does it mean you can't accelerate if an emergency happened behind you (e.g. Mars attacks, thermonuclear strike etc.) without all that tedious and time-consuming dipping the clutch and putting it in gear nonsense, or does it mean being out of gear and reading a newspaper at the same time?

I've found engine breaking very useful on the motorways going up and down the mountains here, (mainly down of course) as well as coasting for 15-20 km when engine braking would slow the car too much. Of course the motorways here are fairly devoid of other traffic, apart from the occasional homicidal maniac.

When I delved into what they meant it was when you release the clutch and used that action to slow the vehicle down not the actual engine braking through the gears.

I think the distinction is between having the clutch released using the compression in the cylinders to slow the car, as opposed to using the engine as a dead weight and the clutch as a brake - think about slowing the car from 70mph using first gear by slipping the clutch.

[turbolag]

So you always dip your clutch coming up to the lights and use the footbrake. Man, you must get through some disks!

Nope, never suffered undue wear because I know how to brake properly, how to modulate on a prolonged braking session, how to release the pressure as the velocity slows. I also never suffer warpage either because I don't hold the car stationary on the footbrake like most lazy boys do - the moment the vehicle is stationary one should punctuate the moment with an application of the handbrake. Even if you're not a an advanced/pursuit/responce/TPaC driver like myself you'll find brakes are a lot cheaper to replace than layshaft bearings and clutches.

Brakes are very robust, able to withstand huge and sudden variances in temperature within seconds. IF used properly.

In extreme circumstances you can use a lower gear to assist the brakes, such as when driving down a mountain, but gears should not be routinely used as a mechanism for reducing velocity any more than the handbrake would.

[Longintooth]

In extreme circumstances you can use a lower gear to assist the brakes, such as when driving down a mountain, but gears should not be routinely used as a mechanism for reducing velocity any more than the handbrake would.

You're on the money with those comments but you may not be aware of the fundamentals otherwise you may have a different view. Using the gears to decelerate moderately ( NOT ramming it into 2nd at 90 MPH) is very useful in that it will share the braking effort at the wheels equally through the Diff. as describe earlier and if double clutching is routinely used there is no wear on the clutch, that's the point about not using clutch braking ie when the RPM drops to idle and you misuse the engine by allowing it race up. Steve wonders how some control is lost when coasting - it's due to the abilty of the engine to apply this shared torque between wheels and this is also where Foot braking cannot do as effectively as Engine braking - however manufacturers are addressing this with variable ABS that controls the angular accelerations on a corner but it still cannot be as flexible as using the throttle (Off) to tighten into a corner ( Oversteer) then accelerate out (Understeer - Front wheel Drive) the opposite on (Rear wheel Drive) due to the ability to change the slip angles of the tyres. This why you see front wheel drives outpacing the rear wheel drives in motorsport. A good driver will be anticipating what gear to be in approaching lights so he will ease off without touching the brakes until the last minute, a lesser driver will be bowling up to the lights in top and then have to use heavier braking to stop. The former will be more economical overall because the driving style will have to change to do it. Every time you use the brakes you are dissipating energy in the form of heat you have induced in the vehicle to get it moving whereas using the engine you have to drive it more sedately and look ahead more. So you are both half right - don't use the clutch to hold it on a hill - use the hand brake and use the engine and gears to slow it down to an acceptable speed before braking. Oh - when I drive my 406 around town I find that third gear is the most flexible at 30mph to accomodate this rather sedate form of driving without brakes. No I'm no doderer a doderer I aint I can take on the rest when needed. I have road tested on Silverstone with many types of vehicles. Another issue would be if you ever have to drive a 44tonne LGV you would not do very well if you relied upon the Brakes to the extent you are suggesting but then more skill is needed to drive one.

[turbolag]

Oh, aye lad. For example, using top to lift off and bleed of some speed from, say, 70 to 55 etc is fine. I think what people are talking about here is using the gears to actually 'brake' the car, such as slowing from 70 to to 20, and changing down through the gears and dragging them to assist the brakes.

Doing that is a waste of time and effort. The correct procedure in such a scenario is lift off and use the brakes appropriately to slow, while remaining in the original gear. When the revs get so low the engine is on the verge of labouring then declutch, but remain in the same gear until you've reached 20, and only then do you select the gear for your new speed, in this case perhaps 2nd. Why fart about wasting time, and stressing the transmission and tyres (and thus potentially also compromise the vehicles stability) by dropping to 4th, then 3rd, and then 2nd? You also still have your original gear still selected and ready to go should the traffic conditions ahead change. You should be reading the traffic and road ahead, and should know before you manipulate the controls whether you're "slowing to go", or "slowing to stop" and adapt accordingly. "Slowing to stop" is much the same, but once the engines close to labouring point in the original gear you declutch, stay in the same gear while you use the brakes to bring you to a stop, and then apply the handbake and select neutral. It's all part of "The System" written for the British Police by a Belgian racing driver (who's name eludes me) in the 1920's, and has been analysed scientifically and reviewed many times over the years and still found to be fundamentally the best method for road driving. Curiously, much emphasis is placed on reducing wear on the transmission during police driver training, and other techniques such as sustained gearchanges, boost gearchanges etc are taught to give a smooth drive, and thus reduce the likelyhood of unsettling the vehicle and losing grip, and extending the life of the drivetrain. As a consequence, in fast road police cars mechanical failures are rare, despite the arduous use to which the vehicles are put (one pundit once calculated that a police cars interior is subjected to 20 times the wear of a regular car).

Additionally, double declutching does nothing on a sychro gearbox - the gears are all permanently meshed so all declutching does is cycle the clutch un-necessary an extra time, and thus presents further wear to release bearing, arm pinion/bearings, cable etc etc.

[teamster1975]

Very interesting reading chaps, thankyou!

[Longintooth]

Additionally, double declutching does nothing on a sychro gearbox - the gears are all permanently meshed so all declutching does is cycle the clutch un-necessary an extra time, and thus presents further wear to release bearing, arm pinion/bearings, cable etc etc.

Intriguing - I don't know which books you are reading but you clearly have no understanding of mechanical issues - doubling the clutch does what used to be done when there was no synchromesh - just crash gears but it worked. Adding synchromesh made life easier for the less endowed of skilled driver and is a very clever piece of engineering, if you understand how it works exactly you would know its short comings. As one who is a qualified engineer with experience in the repair of 1000s of gearboxes of all types and the driving and road testing of same I would not suggest a driving style without good reason. A syncromesh unit will align two rotating gears using friction through a highly pressured cone which has to withstand a lot of abuse, this results in high rate of wear on those parts and the metal fragments remains in the oil acting as an abrasive paste. Now if the strain can be taken off those components by smoothly matching the speeds the sychro has to do practically no work and the upshot is that the clutch life is improved since the shocks have been eliminated by matching the speeds. There is a very small increase in the wear on clutch cables and the thrust race but these components are built to last the life of a plate anyhow. The cost of repairing a gearbox is much higher. As for driving habits years ago we used to refer to those who used accelerator and brake driving style as French Taxi Drivers.

I think what people are talking about here is using the gears to actually 'brake' the car, such as slowing from 70 to to 20, and changing down through the gears and dragging them to assist the brakes.

This is not what I'm suggesting at all and you are correct in the former assumption. When you allow the vehicle to slow down in an inappropriate gear you labour the gear train and the clutch severely and with dual mass flywheels they will not tolerate much of this before collapse - trust me on this. So matching the gear with the speed will allow flexibility without stressing the transmission or the brakes and will return a sensible economy factor. I know you may have been brainwashed in your belief that your style or method is perfect but it's because you are not seeing other issues that you persist. Police driving is not the panacea of general driving, most have no engineering qualifications and are adopted to allow the driver to concentrate purely on the fastest driving style in the safest manner by using just two components Accelerator Brake. He does not worry about the mechanical or economy consequences and that is quite right. On here we are concerned with Engineering and best practice. Oh and the style you mention -

When the revs get so low the engine is on the verge of labouring then declutch,

Should the lights change suddenly there is a scramble for the right gear to get going again, whereas in the other method you are already in the right gear.

[turbolag]

[

When the revs get so low the engine is on the verge of labouring then declutch,

Should the lights change suddenly there is a scramble for the right gear to get going again, whereas in the other method you are already in the right gear.

Longintooth - I.Eng. MIRTE

Er, light's do not just "change suddenly" - there is a well known and predictable sequence to all traffic lights and a skilled anc conscientious driver will know this, and apply their knowledge and observation of the road environment to their driving. Not sure how you passed your driving test without this little gem of knowledge?

Further, to already have a gear selected and immediately available is proven to provide a quicker reaction than the time wasted trying to stir the box for a more appropriate one - far better to be able to add to ones kinetic energy immediately, than in a second or two time. 80 years of police driving instructors, several scientific analyses and one recent study by the Home Office into 'The System' have found it to be consistently superior and correct in almost all regards. As a police advanced/pursuit/response/TPaC and Home Office authorised service test driver, on of less than 30 in England and Wales with this authorisation, I know who I believe, and it isn't someone who may be skilled in engineering but unskilled in it's application to road craft. Hell, I'm qualified to build spacecraft, but that doesn't mean I know jack about flying one.

Regards,

Turboslag, BA planetary studies, MSc Celestial Engineering and Mechanics (soon to be PhD if we're getting into a willy wagging contest)

[Longintooth]

Hi
Lighten up my friend I recognise your abilities and they are not in dispute and reading closely we are not too far apart - you are comparing the abilities of a professional Police High Powered car driver with high concentration levels, trained and educated to Racing Car Standards, not the general run of the mill all round car driver and yes I managed to scrape through my HGV 1 and advanced driving test and have never had so much as a speeding ticket ever in 45 years driving which includes 27 years operating a transport business with a fleet of 40 tonne artics and covering sometimes 2000 miles per week on a regular basis around London and other major cities so I don't speak without thinking hard first and it's all from first hand. I also have a pilots licence which allows me to throw a plane around up to 5 tonnes, so my levels of concentration is not dodery, dodery I am not. I do know what condition Police Cars are in after a very short life in service if the insides are well worn you should see the mechanical bits - I have friends and relatives in the force so it is not fair to compare with every days vehicles.
And er ..... that's what I said - Better to have the appropriate gear selected than scrabbling around for one.

[Welly]

I once went on a day's racing/driving thing at Oulton Park and the instructor proudly told everyone that Police driver training upsets the car under braking as they have to Brake...then select lower gear...then brake again etc.. he said this was contrary to smooth driving where heal and toe action would be far better and ultimately quicker.

I'm not an expert though and I suppose certain styles fit certain conditions but he made a big thing about the car pitching and diving about inbetween braking/gearchanging.

I ended the day with an "excellent car control - should, no, MUST come back"

[turbolag]

Welters, police drivers do not brake and sequentially select lower and lower gears - your instructor was way off beam with that. My previous post details how we are taught to do it with only a single gearchange. It's usually Joe public that drops laboriously gear by gear, causing the front to dive, altering the cars geometry, in turn affecting the effectiveness of steering input and comprosing available grip

Otherwise, his observations with translating skills to the track can be true. I competed in the NHCC championship in 97 and 98, and some techniques work well, but other fast road techniques are a liability, and I had to unlearn a few things for the competition environment.

'The System' is fundamentally designed to work well on a public road - on a track without oncoming traffic, blind corners, pedestrians, traffic lights, potholes, diesel spills, concealed entrances etc etc a lot of it is redundant, but most of the core control techniques (seperating braking and steering being a prime example) are also basic skills on the track too, so it's knowing what to discard and what do carry on with. 2nd in class in my 1st season, pulled out halfway through my second with divorce looming and the need to curb my spending.

Conversely, those who are blisteringly quick on the track rarely make natually fast and safe road drivers either.