Avoiding BGOL around town

Topics relating to Advanced Driving in cars
Rainmaker
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Joined: Tue Jan 12, 2016 11:44 pm
Location: Liverpool

Avoiding BGOL around town

Postby Rainmaker » Tue Jan 26, 2016 12:10 am

I do hope this isn't one of those 'done to death' topics. I have read a few posts (including a very helpful one by von on ADUK) but still have a question which I hope doesn't come across as being too stupid. It's a bit hard to explain without being verbose, but please bear with me.

My Mazda 6 has a 2.0 naturally aspirated petrol, but being a 'SkyActiv' engine (14:1 compression and long gears) it generally drives below 2k revs at lower speed limits, and at low speeds the drop between gears can be as little as 250rpm. By that I mean around town I'll generally drive at 30mph in 4th on long straights with little by way of hazards, and drop to 3rd for roads with banks of ATS, more pedestrians etc to give better retardation and more flexibility for acceleration sense.

Please bear in mind I live in Liverpool, and as such the 30mph roads I'm talking about are generally all miles-long straight dual carriageways with two lanes each side, not little single carriageway B roads. Obviously as you get closer to the city centre the roads become more crowded, bendier and often smaller (and demand different approaches), but for the purpose of my question I'm talking about the suburbs and their long D/Cs. Here's an example:

Image

With such long straights it's pointless screaming along in 2nd, and 3rd isn't much better than 4th at these speeds (about 1,800rpm instead of 1,400rpm). The gearing does seem to present me with a problem when it comes to avoiding BGOL. If I've understood von's post on ADUK correctly, using the clutch to prevent stalling/shuddering is only fine if the resulting gear is 1st. Anything else is 'overlap, and not on the grounds of safety'.

So how do I approach this from an AD perspective and avoiding BGOL? Consider this:

  1. I'm in 4th (or even 3rd) at 30mph and the ATS is red ahead, with a car waiting at the stop line.
  2. I ease off the gas (I'm a few hundred yards away) and/or apply the brakes to shave off some speed.
  3. I'm planning to stop but looking to go. If the light stays red, I stop. If it goes green, I select an appropriate lower gear and accelerate again (in theory).

The problem is, as I'm already cruising below 2k rpm (often closer to 1,500rpm around town) I've not much 'play' left in the rev band for slowing down. By the time I'm down to 20mph the car is already shuddering and threatening to stall as I've 'run out of revs', so my options seem to be:

1) Clutch in just before the 'shudder' and continue losing speed. If I end up braking to a stop (light stays red) then all's well as I then select first. This isn't considered BGOL. If the light goes green before I stop then I may well be looking to move down to 2nd (20mph plus). That would be BGOL by Von's definition as I had the clutch in before selecting a non-first gear.

2) Adopt a lower gear between 'spates' of braking (i.e. down through the gears). This obviously isn't System.

3) Drive along shuddering loudly and hope I don't stall before I can select a lower gear and accelerate (lol).

One might suggest I am in too high a gear around town in this case. As detailed above though, short of driving around town in 2nd gear - which is loud, not especially smooth, and uneconomical in the extreme - what to do? I know my friend has a similar issue in his diesel engine, in that it wants to be around 2k revs but that leaves him only 1,000rpm for slowing due to hazards and he then hits the same 'wall' as I do.

On higher limit roads it's not really an issue. The car will happily travel in the same 3rd or 4th gear right the way up to (and beyond) NSL and remain quiet, smooth and pliable with bags of 'space' up or down the rev range. This gives closer to 2,500 / 3,000rpm at a steady road speed, with flexibility to slow and adopt the next gear down where necessary without impacting the drive (and allowing full BG separation), as well as an extra 3,500rpm for acceleration before approaching the red line. Generally then, I'll also be using 3rd and 4th on roads up to and including NSL, dropping to 5th on longer/emptier bits to rest the engine and aid economy, and back to 3rd/4th where required (approaching hazards or busier areas). Sixth gear doesn't really get a look in unless I'm on a long, straight, empty motorway stretch and expecting to cruise for many miles. More often even on the motorway I'm in 4th or 5th gear, such is the gearing.

As such you can see how adopting the same two gears at 30mph is practically already only ticking over, and as such presents me with difficulty when trying to apply system to slowing and down changing without using BGOL (or else driving everywhere in the relatively short 1st/2nd!). So with such leggy gears and the choice between driving everywhere around town in 2nd or seemingly unavoidable BGOL, what am I missing?

Apologies for the long post but it's something I've been struggling to work through alone this week. Does anyone have any ideas or advice? Have I managed to completely misunderstand BGOL? Cheers. :)

waremark
Posts: 577
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Re: Avoiding BGOL around town

Postby waremark » Tue Jan 26, 2016 12:49 am

I recommend your option 1). I am satisfied that it would be quite acceptable to Von now, even though he described it as partial overlap in the past.

The aspect of your use of gears which I would question is what you are doing at higher speeds. For economy I suggest that you should choose gears which keep the revs lower except where you need extra performance. In my manual diesel, much of the time I would use 5th from 40 up, and 6th from 50 up.

Rainmaker
Posts: 72
Joined: Tue Jan 12, 2016 11:44 pm
Location: Liverpool

Re: Avoiding BGOL around town

Postby Rainmaker » Tue Jan 26, 2016 1:00 am

waremark wrote:I recommend your option 1). I am satisfied that it would be quite acceptable to Von now, even though he described it as partial overlap in the past.

The aspect of your use of gears which I would question is what you are doing at higher speeds. For economy I suggest that you should choose gears which keep the revs lower except where you need extra performance. In my manual diesel, much of the time I would use 5th from 40 up, and 6th from 50 up.


Good to know, and certainly makes life easier! I only tend to use 3rd/4th for NSL B roads, where the flexibility is required. I actually get much worse fuel economy in 6th compared to 5th at anything other than a steady cruise simply because the car starts to labour on anything but a long straight road. As soon as there's even a tiny uphill incline or I request a very modest increase in throttle the real time MPG hits the 20s in 6th. Using 5th gives much more consistent MPG as well as more flexibility.

I suppose a lot of it comes down to modern economy minded setups. A few years ago cars would routinely cruise at 2,500 to 3,000 in top gear, whereas now they want you in 5th gear at 30mph... That's great on something with low down grunt (GDI or TDI) but on an NA with a torque curve that's dead until 4k, not so much. :(

ETA: Having switched recently to using 3rd/4th on aforementioned NSL roads (especially when driving for pleasure) I've found no real change in MPG. Bear in mind, as I said earlier, I still change to 5th to rest the engine on straighter bits. I'm still getting about 40mpg with low gear / light throttle as opposed to higher gear / wider throttle. Not bad going for a 2.0 petrol, especially when it'll return 50mpg on the motorway. On a turbo engine I suspect this may not remain the case!

waremark
Posts: 577
Joined: Fri Oct 02, 2015 9:23 am

Re: Avoiding BGOL around town

Postby waremark » Tue Jan 26, 2016 8:32 am

You sound very well switched on to the effects of your gear choice on consumption, well done. My suggestions were related to a gutsy turbo diesel (and also work well for a very large NA petrol).

sussex2
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Re: Avoiding BGOL around town

Postby sussex2 » Tue Jan 26, 2016 8:40 am

'If I've understood von's post on ADUK correctly, using the clutch to prevent stalling/shuddering is only fine if the resulting gear is 1st. Anything else is 'overlap, and not on the grounds of safety'

I'd say that may have been the case before the high torque diesels came along; at low speeds you'll have those torques shoving you along and fighting the brakes.
I'd also say that changing to a lower gear approaching a hazard in order to prevent the vehicle shuddering or stalling is perfectly within the system. You would be changing gear to keep the vehicle driveable.

martine
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Re: Avoiding BGOL around town

Postby martine » Tue Jan 26, 2016 10:28 am

I think you've explored the options well...if 2nd gear at 30 doesn't work for you, then 3rd should - I rarely use 4th at 30.

You mention 1800rpm in 3rd at 30mph...when losing speed what engine revs does it begin to object? In my car all is good until 1000rpm, so on your gearing that would equate to 16mph and depressing the clutch would be very brief before stopping. Perhaps you're extending the slowing period over too great a distance?

At the other end of the scale in my car (Focus ST), it surprises some when I stop in 6th (NSL dual carriageway with roundabouts) - just how late I can leave it before depressing the clutch - and that's being very sympathetic to the drivetrain.
Martin - Bristol IAM: IMI National Observer, Group Secretary, Masters (dist), DSA: ADI, Fleet, RoSPA (Dip)

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Mr Cholmondeley-Warner
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Re: Avoiding BGOL around town

Postby Mr Cholmondeley-Warner » Tue Jan 26, 2016 11:15 am

sussex2 wrote:I'd also say that changing to a lower gear approaching a hazard in order to prevent the vehicle shuddering or stalling is perfectly within the system. You would be changing gear to keep the vehicle driveable.

+1

Just select the most flexible gear. Remember AD is defined as being in the right position at the right speed WITH THE CORRECT GEAR SELECTED. Stop worrying about whether changing down is non-systematic, or fussing about economy, do what the car needs.

What has Automatic Train Supervision got to do with dual carriageways? If you mean traffic lights, say so! :twisted:

Oh, and yes, it's been done to death :mrgreen:
Nick

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ROG
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Re: Avoiding BGOL around town

Postby ROG » Tue Jan 26, 2016 11:39 am

I generally consider being on the brake and at a speed when selecting 1st is not BGOL but giving oneself an advantage

I NEVER have to BGOL for any other gear

If braking too much then the chances are that too high a gear has been selected
IAM observer 1998 to about 2017 when the IMI comes into force
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devonutopia
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Joined: Mon Oct 05, 2015 5:53 pm

Re: Avoiding BGOL around town

Postby devonutopia » Tue Jan 26, 2016 1:26 pm

I had to google that acronym.... I guess it's all in the planning. I am guilty of it at times - usually on an unexpected / unforeseen situation. Enough planning, and it should never be a problem.

sussex2
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Re: Avoiding BGOL around town

Postby sussex2 » Tue Jan 26, 2016 2:48 pm

Mr Cholmondeley-Warner wrote:
sussex2 wrote:I'd also say that changing to a lower gear approaching a hazard in order to prevent the vehicle shuddering or stalling is perfectly within the system. You would be changing gear to keep the vehicle driveable.

+1

Just select the most flexible gear. Remember AD is defined as being in the right position at the right speed WITH THE CORRECT GEAR SELECTED. Stop worrying about whether changing down is non-systematic, or fussing about economy, do what the car needs.

What has Automatic Train Supervision got to do with dual carriageways? If you mean traffic lights, say so! :twisted:

Oh, and yes, it's been done to death :mrgreen:



Exactly!


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