Approaching junctions - Gear / Clutch

[R_U_LOCAL]

It's a common misconception that the Roadcraft system of car control requires a complete seperation between the braking phase and the gear phase in every situation. Have a look at the representation of the system below:



Every phase of the system overlaps in some way, apart from the gear phase and the acceleration phase.

Information overlaps all the other four phases, which means you'll continue to take, use and give information whilst going through the other phases.

The position and speed phases overlap, which means that you can, for instance, adjust your position whilst braking for a hazard. The overlap is towards the end of the position phase, so you'd generally start to adjust your position before starting to brake, but there is nothing wrong with starting to brake whilst you're still adjusting your position on the road.

The speed and gear phases overlap in the same way - towards the end of the speed phase. There are, of course, many advantages to seperating braking and gear-changing, but if, for instance, you're on a downhill road and turning left into a junction, coming off the brakes will cause the car to start accelerating again due to gravity, so the best method is to overrlap braking and gearchanging - in this case, one block change down to the appropriate gear - near to the end of the speed (braking) phase.

Heel & toe isn't generally taught as, with most drivers, there is too much to think about and too much potential for things to go wrong. If you have to carry out an occasional greachange without rev-matching, no real harm is done.

Having said that, if you're competent, there is nothing wrong with heel/toe in these circumstances. If I were testing you, I would certainly not mark you down for it.

[GravelBen]

ohtari said:

GravelBen said:

davepoth said:

It's doubly odd because there's almost no engine braking on a diesel engine - you might as well install a freewheel really.

Since when? Every manual diesel I've driven has had more engine braking than an equivalent petrol thanks to the higher compression.

Someone will be along to explain why you're wrong very shortly

Happy to be educated if someone has a good explanation why the perceived engine braking is really an illusion

[jhfozzy]

GravelBen said:

ohtari said:

GravelBen said:

davepoth said:

It's doubly odd because there's almost no engine braking on a diesel engine - you might as well install a freewheel really.

Since when? Every manual diesel I've driven has had more engine braking than an equivalent petrol thanks to the higher compression.

Someone will be along to explain why you're wrong very shortly

Happy to be educated if someone has a good explanation why the perceived engine braking is really an illusion

What he meant to say, in a roundabout method is:

It's not due to the compression ratio.

http://en.wikipedia.org/wiki/Engine_braking#Diesel...

Diesel engines don't have engine braking in the normal sense of closing the throttle like in a petrol engine as they don't have throttles, but most modern diesels can achieve engine braking via other methods (Turbo, DPF, EGR) as explained in the article.

[p1esk]

titian said:

I understand your point Waremark and would wholly agree if I was driving a petrol engined vehicle which is much more tolerant of low speeds in higher gears. However, in a diesel engined car I hate travelling for an extended distance with my left foot flat to the floor on the clutch and my right foot on the brake - it feels unnatural and out of control.

Yes, I hate that too, and I will not do it, though it isn't because of concerns about lack of control. When I reach the stage of needing to declutch, unless I know what gear I'm going to want next, I'll declutch, slip into neutral, come off the clutch pedal, and continue braking to a halt if necessary. If the situation in front changes such that I do not need to stop, I'll engage a suitable gear and carry on.

Many people will strongly disapprove of this way of driving, but I find it works perfectly well and doesn't create any difficulties.

It's the same sort of attitude that leads some drivers to insist on being in a 'responsive' gear at all times, so they engage second gear as soon as they slow down for a 30 mph zone, and go buzzing along intil they emerge into a higher speed limit zone. Almost invaraibly it's quite unnecessary, but they do it as standard procedure, whereas I prefer to adopt a more flexible style that suits each situation as we meet it.

[p1esk]

GravelBen said:

davepoth said:

It's doubly odd because there's almost no engine braking on a diesel engine - you might as well install a freewheel really.

Since when? Every manual diesel I've driven has had more engine braking than an equivalent petrol thanks to the higher compression.

I think the problem of limited engine braking with diesel cars, especially at low and medium road speeds, is caused by the high gearing normally used.

[Toltec]

If you de-clutch during the braking phase are you really coasting? I always took coasting to be synonymous with freewheeling. If you are braking then you are still actively controlling the speed of the vehicle.

[GravelBen]

jhfozzy said:

What he meant to say, in a roundabout method is:

It's not due to the compression ratio.

http://en.wikipedia.org/wiki/Engine_braking#Diesel...

Diesel engines don't have engine braking in the normal sense of closing the throttle like in a petrol engine as they don't have throttles, but most modern diesels can achieve engine braking via other methods (Turbo, DPF, EGR) as explained in the article.

Thats interesting, but the most modern diesel I've driven is my 2007 Hilux work ute and I've spent a lot of time driving 90s (and a few 80s) diesels, both turbo and NA. And none of them had any noticeable lack of engine braking.

Mostly 4wd vehicles though, so probably lower gearing than cars. I have to drop the (turbo petrol) Legacy into 1st to even get close to the low-speed (ie offroad) engine braking of the Hilux in 2nd.

Edited by GravelBen on Monday 16th March 23:54

[SK425]

Toltec said:

If you de-clutch during the braking phase are you really coasting? I always took coasting to be synonymous with freewheeling. If you are braking then you are still actively controlling the speed of the vehicle.

I agree, having the clutch down while braking isn't what I understand by the word "coasting". But it doesn't really matter what we call it - some people are comfortable with it and some people less so.

[RobM77]

SK425 said:

Toltec said:

If you de-clutch during the braking phase are you really coasting? I always took coasting to be synonymous with freewheeling. If you are braking then you are still actively controlling the speed of the vehicle.

I agree, having the clutch down while braking isn't what I understand by the word "coasting". But it doesn't really matter what we call it - some people are comfortable with it and some people less so.

There is a distinction, yes, so sorry I didn't differentiate in my post - that was a mistake. The key principle though is being ready for avoiding action, i.e. cornering, which is more stable with power on at useful revs, or acceleration, which is obviously much easier with useful revs to start with. To be honest though, if you can heel and toe without even thinking about it, the problem dissapears.

[powerstroke]

U

p1esk said:

GravelBen said:

davepoth said:

It's doubly odd because there's almost no engine braking on a diesel engine - you might as well install a freewheel really.

Since when? Every manual diesel I've driven has had more engine braking than an equivalent petrol thanks to the higher compression.

I think the problem of limited engine braking with diesel cars, especially at low and medium road speeds, is caused by the high gearing normally used.

Yes this ,if you are in the right gear for the road speed then you will need just a little application of the brakes to check your speed , a good driver in normal road driving only uses the brakes to stop the car not hurtle up to a roundabout or junction and slam on at the last few yards!!! key is anticipation= good driving!!!

Edited by powerstroke on Wednesday 1st April 07:36

[powerstroke]

Bigends said:

Use brakes for braking - my instructors always emphasised brakes are cheaper than clutches

Where did this crap come from??? Its been doing the rounds for a while now .

[RobM77]

powerstroke said:

U

p1esk said:

GravelBen said:

davepoth said:

It's doubly odd because there's almost no engine braking on a diesel engine - you might as well install a freewheel really.

Since when? Every manual diesel I've driven has had more engine braking than an equivalent petrol thanks to the higher compression.

I think the problem of limited engine braking with diesel cars, especially at low and medium road speeds, is caused by the high gearing normally used.

Yes this ,if you are in the right gear for the road speed then you will need just a little application of the brakes to check your speed , a good driver in normal road driving only uses the brakes to stop the car not hurtle up to a roundabout or junction and slam on at the last few yards!!! key is anticipation= good driving!!!

Edited by powerstroke on Wednesday 1st April 07:36

Whatever difference there might be (and I've no idea if there is or not, because my diesel car and my petrol car are so different aerodynamically that I wouldn't ever notice), is negated by the fact that if you're driving well and you're alert enough, then you barely ever need the brakes anyway, other than to come to a complete halt.

[akirk]

there is a tendency to see brakes as evil - i.e. braking means that the speed was wrong and is therefore compensating for a mistake...
I think this is a fallacy, brakes are a tool as much as the accelerator in how you choose to drive - therefore plan your journey to allow for the use of both - many drivers could carry a higher speed into a hazard and use firmer braking and still be driving at a very high level...

lots of thinking required around this - e.g. being aware of the affect on drivers behind etc. however starting to brake too early can equally disrupt other drivers...

thinking of the brakes as a positive tool (which should still be used smoothly and effectively) can transform driving...
drivers still seem to assume that hard braking will spin the car off into the nearest bush - but with modern cars they can cope far more than expected - in recent testing in my 14 year old z3 doing emergency braking from an indicated 90-110 while throwing the car through chicanes, the car stopped smoothly and easily with occasional ABS use, but no tranction control used... cars are so competent braking really can be used very positively

[RobM77]

I see your point, and I guess this is just a style thing that varies between drivers? If I see something up ahead that'll require slowing, and it won't upset or confuse people behind me, then I tend to just back off early instead of continuing at the same speed and then braking, partly to save fuel and brake pad wear, but mainly as just a psychological trait, perhaps from all the cycling I do. From years of motor racing I'm obviously comfortable with heavy braking and judging speed, deceleration and distance, but when I'm not in a competitive situation I tend to just flow along the road as described without really using the brakes. Obviously I'd change that if I was in a hurry, but I rarely am.

[p1esk]

RobM77 said:

I see your point, and I guess this is just a style thing that varies between drivers? If I see something up ahead that'll require slowing, and it won't upset or confuse people behind me, then I tend to just back off early instead of continuing at the same speed and then braking, partly to save fuel and brake pad wear, but mainly as just a psychological trait, perhaps from all the cycling I do. From years of motor racing I'm obviously comfortable with heavy braking and judging speed, deceleration and distance, but when I'm not in a competitive situation I tend to just flow along the road as described without really using the brakes. Obviously I'd change that if I was in a hurry, but I rarely am.

Hmm, I think I've met Mr Kirk elsewhere and I very much respect what he says, but the way we use braking is a matter for individual choice. I'll press on quite positively when I feel like it, and use the brakes firmly to suit that; but most of the time my braking is infrequent, and applied early and gently when I do use it.

Best wishes all,
Dave.

[Brian Trizers]

Intriguing discussion, hence the urge to pitch in with my first post here. (Hello, by the way.)

I think I belong to the 'do everything once' tendency, which means no downchanges on the way to a stop (or a potential stop.) Like RUL, I see nothing wrong with dipping the clutch at 38, not that my manual car requires me to; as others have said, I'm still exerting control over the speed via the brakes, and I'll be on the brakes either until it's time to stop or till I see it's safe to pick up speed again. Selecting neutral or changing to a lower gear would be a diversion of my attention, which really ought to be fully on the hazard I'm approaching, ready to make that stop-or-pull-away decision.

As for engine braking, yes, diesels can certainly do it and yes, it does depend on gearing. I have a long, steep hill to descend every night on my way home. Starting at 35, my old Volvo D5 (manual) will roll down in third, top out at about 45 and often not require any braking to settle naturally back into the 30 limit at the bottom. My E220 (auto) on the other hand, with its higher low ratios, tops 50 in third and requires a firmish shove on the pedal at the bottom. (I could use second, of course, but the engine would scream unappealingly; or I could use the speed limiter, but that works through the brakes anyway.)

In doing this, I wouldn't say I'm using the expensive transmission to save the cheap brakes, because the drivetrain is preventing me gaining speed rather than actively losing it. So I'm saving brake wear (and maintaining speed control) at no cost to the transmission. And the Volvo's original clutch lasted 114,000 miles; the brakes a mere 96,000.