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 Post subject: Re: Coasting
PostPosted: Fri Jan 22, 2010 04:01 
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:welcome: driver67
driver67 wrote:
samcro wrote:
...I’d be interested to explore the pros and cons of coasting, whether it really is dangerous or not, and whether you really don’t have proper control of the car/vehicle or not. Are there situations when it is aceptable to coast, and are there situations where it's dangerous?
...I sometimes coast towards lights if I can see they are red. Does coasting stop me form having proper control of the car?...
Being a Driving Instructor this is one of the faults we have to pick up on quickly, the amount of times the learner turns into a side road to the left or right and doesn't bring the clutch pedal up causing the vehicle to go into the junction too fast narrowly missing the bollard or other vehicle coming out of the road is very frequent.

That would be poor and the result of poor and inexperience of spacial awareness so more concentration is on the important 'where' they are driving. It is of lesser importance to the learner of total car control which takes second place to ensuring they don't crash, so the corner takes more of their attention while learning. Gradually as confidence builds they learn to manage and judge both successfully.
I don't know if you have slalom courses (or any driver private 'parks' or routes), that they can do which can help their steering ability before worrying about the gear and clutch control?
By separating this process I would have thought then once competent at steering the gear and clutch, can then follow and be joined with the steering.
driver67 wrote:
Two reasons for getting the learner to understand this is;
1). It does give more control as said earlier you make use of engine braking which can help your brakes to slow the car down. You would probably agree that when going to a lower gear you will find that the steering will feel more positive as well, which gives more overall control.
You are not actually 'helping' the brakes of course as engine braking tends to be alone in action, possible braking near the end of engine braking as you swap or transfer to the braking action. Coming off the throttle too fast can cause unintended effects too and esp if any steering is in play.
The slower speed gives steering less emphasis, not more. Variable steering feature will vary this of course. Faster speeds make a greater effect to small steering input. Why moving when steering is so helpful whilst parking.
If in second and driving about at approx 30mph then small steering input also has significant effect.
driver67 wrote:
2). The second reason the examiner will pick up on this fault on your driving test, you will get driving faults each time you coast possibly leading to a serious fault which would be a fail or a dangerous fault(again fail).

Do you train for passing the test and also for the road after the test ?
driver67 wrote:
]What types of coasting would be acceptable then? Well if you are approaching a set of traffic lights or traffic that is queuing you would put your clutch pedal down when the gear you are in can no longer cope with the speed, but on the other hand if you are about 50 car lengths away this wouldn't be the correct way because then you should be dropping gears so as to gradually reduce your speed, we all coast a little when changing gears, approaching T junctions but that is acceptable.

Well coasting might be good for a moment to allow better hearing, to conserve on fuel (perhaps trying to make for a garage - not ideal but ...)
experiencing about knowing how it feels when the engine might die.
If you wait for the engine to labour as it 'runs out of that gear' that can damage the engine, surely this is not taught to learners ! ?
I think there can be times to coast in safety while a pause in condition considerations takes place but it is better - far better to be in gear as you look about considering options and judge.
The T junction issue depends on whether you drive so that you are 'ready to go' or 'ready to stop' ..... alters the way you approach and behave. :)
Engine braking or passive braking can be used to best effect when anticipation skills are very high, making braking sometimes redundant. You are unlikely to be coasting at these points. Finding that a brake pad has started grinding makes great & necessary use of engine braking.

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 Post subject: Re: Coasting
PostPosted: Fri Jan 22, 2010 17:56 
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I've sometimes noticed that dipping the clutch over humps (technical coasting) gets less reaction from the transmission/suspension. Just wishfull thinking or ............. :idea: :?:

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 Post subject: Re: Coasting
PostPosted: Fri Jan 22, 2010 23:16 
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botach wrote:
I've sometimes noticed that dipping the clutch over humps (technical coasting) gets less reaction from the transmission/suspension. Just wishfull thinking or ............. :idea: :?:

I often dip the clutch (or at least hit speed bumps on a neutral throttle) to make things a bit more comfy and to be kinder to the car.

Typically, modern suspension systems have a lot of built-in "compliance" so they allow the wheel to move backwards in its wheelarch when it hits a bump. Doing so allows a few extra thousandths of a second for the wheel to start moving upwards - effectively reducing the ramp angle of the speed bump. If the power is on and the driven wheels are pulling (pushing) the car along, their suspension will be compressed towards the front of it's travel and held there against the bump by the torque going through the wheel.

Also, most mechanical systems have a fair bit of friction in them. Friction in the damper piston, suspension pivots and driveshaft joints is magnified when there is a fore-aft load on them as well as a vertical load. The fore-aft load would arise from the engine pasing torque down the driveshaft, through the Cv joint and into the tarmac via the wheel & tyre.


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 Post subject: Re: Coasting
PostPosted: Fri Jan 22, 2010 23:25 
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SafeSpeedv2 wrote:
...You are not actually 'helping' the brakes of course as engine braking tends to be alone in action, possible braking near the end of engine braking as you swap or transfer to the braking action.


I think it's fair to say that engine braking DOES assist the brakes. Any resistive force to the car's motion (e.g. wind resistance) will "assist" the brakes in slowing the car down. Clearly it doesn't actually help the brakes themselves to work better, it just provides an additional resistive force that the car would have to overcome in order to keep moving.


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 Post subject: Re: Coasting
PostPosted: Fri Jan 22, 2010 23:29 
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Was thinking more of the backlash(to put it in crude terms) applied to the CV joints from suspension throw in one direction and forward motion in the other ,and how this could prematurely age/stress things like CV joints .

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 Post subject: Re: Coasting
PostPosted: Sat Jan 23, 2010 03:22 
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Mole wrote:
SafeSpeedv2 wrote:
...You are not actually 'helping' the brakes of course as engine braking tends to be alone in action, possible braking near the end of engine braking, as you swap or transfer to the braking action.

I think it's fair to say that engine braking DOES assist the brakes. Any resistive force to the car's motion (e.g. wind resistance) will "assist" the brakes in slowing the car down. Clearly it doesn't actually help the brakes themselves to work better, it just provides an additional resistive force that the car would have to overcome in order to keep moving.

Ah Glad you agree. :)
Uphill, water rough surfaces helps braking too ! (&so on) And hence why engine braking can help when going downhill prior to making a good smooth downshift.
[Just scene the latest offering from the public info films - invisible cars on water and v/o (voice over) 'When driving on water' pause (and sounds like) "SINK" with tag of THINK ! :lol: Trouble is they are forgetting to say Think - what ? !! :) For those that don't really know it STILL FAILS to teach ! What is wrong with these people - are they really so completely incompetent!]

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 Post subject: Re: Coasting
PostPosted: Sat Jan 23, 2010 04:32 
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botach wrote:
Was thinking more of the backlash(to put it in crude terms) applied to the CV joints from suspension throw in one direction and forward motion in the other ,and how this could prematurely age/stress things like CV joints .

Normal weight transfer (helps retain front grip) I would have thought would not cause damage unduly. Certainly though add a pointless hump into the equation and much damage can be done. I tend to drop to 2nd and go over at an angle to try and stop my poor exhaust from hitting the hump apex ! It is also very gentle as one can be over the hump - the double humps can be driven through the middle and at least the o/s of tyres will show any wear or damage more obviously. I don't coast towards or over them.

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 Post subject: Re: Coasting
PostPosted: Mon Jan 25, 2010 00:46 
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botach wrote:
Was thinking more of the backlash(to put it in crude terms) applied to the CV joints from suspension throw in one direction and forward motion in the other ,and how this could prematurely age/stress things like CV joints .

I believe that's EXACTLY what it does! :)

A CV joint carrying significant torque at the same time as being asked to change its angle (e.g. as the suspension moves up over a speed hump) is bound to wear faster. In addition, as you say, the tractive force acting through the tyre contact patch is loading up the suspension bushes in one direction and then they get a shock load superimposed on them due to the speed bump. Whether either effect significantly reduces the overall life of the components is open to debate, but I believe it must do!


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 Post subject: Re: Coasting
PostPosted: Mon Jan 25, 2010 00:54 
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SafeSpeedv2 wrote:
Mole wrote:
SafeSpeedv2 wrote:
...You are not actually 'helping' the brakes of course as engine braking tends to be alone in action, possible braking near the end of engine braking, as you swap or transfer to the braking action.

I think it's fair to say that engine braking DOES assist the brakes. Any resistive force to the car's motion (e.g. wind resistance) will "assist" the brakes in slowing the car down. Clearly it doesn't actually help the brakes themselves to work better, it just provides an additional resistive force that the car would have to overcome in order to keep moving.

Ah Glad you agree. :)
Uphill, water rough surfaces helps braking too ! (&so on) And hence why engine braking can help when going downhill prior to making a good smooth downshift.
[Just scene the latest offering from the public info films - invisible cars on water and v/o (voice over) 'When driving on water' pause (and sounds like) "SINK" with tag of THINK ! :lol: Trouble is they are forgetting to say Think - what ? !! :) For those that don't really know it STILL FAILS to teach ! What is wrong with these people - are they really so completely incompetent!]



eh??? Just run that by me again please! I wasn't agreeing!

I can see how going uphill assists braking - you're trying to lift the weight of the car through a given height for each metre travelled - so of course, it will.

Water??? I thought braking distances went UP in the wet?! :? Do you mean deep water at speeds low enough not to aquaplane? If so, yes, I agree, it doesn't half help slow you down!

Rough surfaces? Yes and no really. "Yes", they help take energy out of the moving car because they require energy to bash the wheels up and down and that energy can only come from the car. On the other hand, "no", because on a rough surface, the tyre's contact force (and thus it's grip) is varying constantly.

I think engine braking can help in any circumstances - uphill, downhill, on the level... (?!)

I'm confused!


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 Post subject: Re: Coasting
PostPosted: Mon Jan 25, 2010 00:59 
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SafeSpeedv2 wrote:
botach wrote:
Was thinking more of the backlash(to put it in crude terms) applied to the CV joints from suspension throw in one direction and forward motion in the other ,and how this could prematurely age/stress things like CV joints .

Normal weight transfer (helps retain front grip) I would have thought would not cause damage unduly. Certainly though add a pointless hump into the equation and much damage can be done. I tend to drop to 2nd and go over at an angle to try and stop my poor exhaust from hitting the hump apex ! It is also very gentle as one can be over the hump - the double humps can be driven through the middle and at least the o/s of tyres will show any wear or damage more obviously. I don't coast towards or over them.


I tend to dip the clutch as I'm just about to hit them - for the same reason as botach. I like wide cars because they tend to straddle the worst of the hump. As an aside, I developed a new hatred of them these last few weeks. there are a couple on an uphill slope between our house and the kids' school. In the recent snow, I had only JUST enough traction to get up the hill, and the speed humps would either force me to slow down to the point where it was almost impossible to maintain momentum OR I had to hit them faster (which bounced the car enough to break traction anyway). On the way DOWN the hill, the same thing happened with the brakes. I was lucky enough to be able to time hitting them when nothing else was around but I could see how if one WAS forced to brake in snow or ice near one of these things, it would do little to improve one's stopping distance!


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 Post subject: Re: Coasting
PostPosted: Mon Jan 25, 2010 01:24 
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Now ,I don't know if it's just this part of the world that puts bollards/wooden posts beside speed humps .But we have them -simply because ( and this is from conversation with local councillor) if a car hirs the speed bump in icy conditions, then said car could end up on pavement .So hence forth , in this part of the world ,all speed bumps got fitted with wooden posts ,to protect the pedestrians .Now ,if wedidn't have speed bumps -would pedestrians be safer on the pavement :( :o :o :shock: :shock:

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 Post subject: Re: Coasting
PostPosted: Mon Jan 25, 2010 16:47 
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Yep, all of a sudden "safety additions" aren't that safe....what ever makes them think that they aid safety anyway?

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My views do not represent Safespeed but those of a driver who has driven for 39 yrs, in all conditions, at all times of the day & night on every type of road and covered well over a million miles, so knows a bit about what makes for safety on the road,what is really dangerous and needs to be observed when driving and quite frankly, the speedo is way down on my list of things to observe to negotiate Britain's roads safely, but I don't expect some fool who sits behind a desk all day to appreciate that.


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 Post subject: Re: Coasting
PostPosted: Tue Mar 16, 2010 17:16 
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Isn't going squarely over a speed bump better than at an angle? Going over at an angle puts a fairly harsh torsional stress through the car body, unequally loads the suspension, tends to kick steering out of line and results in greater travel of the suspension.

It's either a very low car or a very high speed bump (or a bit too much speed going over it) if the car is bottoming out on it!


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 Post subject: Re: Coasting
PostPosted: Tue Mar 16, 2010 21:36 
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samcro wrote:

It's either a very low car or a very high speed bump (or a bit too much speed going over it) if the car is bottoming out on it!


But you try proving that the hump is too high ,or that ,for that matter ,a chicane is in a dangerous place .You could try ,but in one case that made our local papers ,one local resident had to take the caseof a chicane to our MP to get it removed ,so that he could use his off road parking ( which he'd paid to have installed ,with county planning permission) -he won .And another local community ,place that got nicknamed Bumpington ( Bulkington) formed a anti hump association , backed by councillors to get the decision by County officers reversed .If some County wallah decides it's for road safety -you get it ,and then (like thechicane) the prohibitive cast makes therm baulk at removing it .It's not government of the people ,by the people any more -it's government of the people ,by unelected officials .That's UK democracy .

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 Post subject: Re: Coasting
PostPosted: Tue Mar 16, 2010 22:50 
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samcro wrote:
Isn't going squarely over a speed bump better than at an angle? Going over at an angle puts a fairly harsh torsional stress through the car body, unequally loads the suspension, tends to kick steering out of line and results in greater travel of the suspension.

It's either a very low car or a very high speed bump (or a bit too much speed going over it) if the car is bottoming out on it!


Dunno really. Going over them at an angle would, indeed, put a torsional deflection down the length of the car, and hiting them square puts a bending load down it. My gut feeling is that the torsional load is worse but neither should pose a problem to a modern car in good condition. I also find the torsional case less comfortable as it thwacks my head from side to side - especially if the car has quite a high roll stiffness.

Hitting one square causes both front wheels to try to move backwards and upwards at the same time, giving the suspension bushes a hard time. Hitting diagonally lets them do it in turn. Again, not sure that either is preferable.

I try to hit them to coincide with the "wheel hop" mode of the car's suspension. If you drive a car very slowly over a "washboard" surface, the wheels will rise up over each bump and the body will rise with them. As you hit the bump faster, the body's inertia means it can't rise at the same speed as the wheels, so the spring compresses and the body stays more level. It will start to rise after the wheel has started to rise and as you increase speed, the lag gets greater. For me, the absolute WORST speed to hit the bump is the car body's natural frequency, where body movement is greatest. The wheel rises over the bump, then the body rises, then the wheel falls and then the body falls. You get the two out of phase and the body comes crashing down. At this point, movement of the body is at a maximum and some cars will whack sumps and spoilers when they come down. Faster still, and there will come a speed of approach where the wheel is on its way back down by the time the body starts to rise. This is "wheel hop" and I consider it to be "speed hump nirvana"! Body movement is minimised. For most cars it sems to be 20 MPH (ish) - faster than most people take speed humps and, I'm sure, not without it's own particular toll on suspension bushes. The other problem is that few cars have the same natural suspension frequency at both ends, so if you get it right at the front, it won't be right at the rear!


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 Post subject: Re: Coasting
PostPosted: Wed Mar 17, 2010 23:42 
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I know what you mean about the torsional issues as there is a road close to me fitted with the road pillows from hell. I tend to drive over them alternating left and right sets of wheels. Why? They have quite steep sides and there have been a number of reports of the inside tyre walls becoming dangerously cracked on cars that, apparently, frequently drive straddled over pillows. So I come to a near stop, clutch in easy up the front, clutch feathered with a little throttle to pull the car forward then back in with the clutch for the rear. Back up to 30 for a hundred yards or so before braking for the next, sometimes I use 2nd gear and lug it up from idle and sometimes I just leave it in 1st, which is a bit louder. Neither is particularly good for the car or fuel consumption I suspect.

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 Post subject: Re: Coasting
PostPosted: Thu Mar 18, 2010 12:02 
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Hi Roy Image and :welcome:

I once accelerated hard on my motorbike and fluffed the change up into 2nd gear, with it going into neutral instead and the usual zing of revs. :oops: I wobbled and tried braking which was absolutely dreadful, but that may just be because we are not familiar with braking hard in neutral.

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 Post subject: Re: Coasting
PostPosted: Tue Mar 23, 2010 16:50 
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samcro wrote:
More recently, whilst taking some motorbike lessons, the same theme came up: coasting means you don’t have proper control of your vehicle.


The clutch is a control...

If used to control the vehicle, by definition, you must be in control of the vehicle, but like all the 'controls' it can be used well or badly. We all coast with the clutch fully in/down - especially a motorcyclist when riding at slow speed. There is no other way to control the bike accurately in slow moving traffic.

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