Discussion
I have heard that in modern cars, leaving them in gear downhills over approx. 1700rpm uses no fuel at all, as the fuel pump is turned off until the accelerator is reapplied, or revs drop. Coasting in neutral does use fuel however as the engine has to keep itself running, instead of relying on the momentum. Coasting also greatly reduces control by not allowing engine braking.
I hope thats correct...
I hope thats correct...
Edited by Dyl on Tuesday 10th March 23:41
Hi All
Can someone please explain how coasting at speeds realistic for the public road removes control any more than changing gear does? Each time we change gear we coast while the clutch is down or we move the gear lever through neutral.
Forget about the instant control argument as you don't have instant control while changing gear. The lack of control is a myth at road sensible speeds. Certainly at some time a gear will need to be selected but this can easily be anticipated and selected as necessary.
By the way I am not denying that having the car in neutral never lessens control just that it isn't really an issue at road speeds.
For those of you that may not be aware the RAC rally was won three times in the nineteen sixties by an 850cc SAAB that was fitted with a freewheel device that cut in to prevent engine braking. Surely if freewheeling did cause problems, they would be shown up in competition.
In fact with front wheel drive cars engine braking downhill will transfer weight to the front thus making the back lighter. According to accepted wisdom won't this increase the chance of oversteer, which is a less stable state than understeer, and therefore more dangerous.
On the same topic why are so many drivers worried about optional speed limiters when they may already have a device in the car that SHUTS OFF THE FUEL SUPPLY. What if that didn't switch on again? Where is the outcry about the danger of that?
Hope this helps
Best Regards
Martin A
Can someone please explain how coasting at speeds realistic for the public road removes control any more than changing gear does? Each time we change gear we coast while the clutch is down or we move the gear lever through neutral.
Forget about the instant control argument as you don't have instant control while changing gear. The lack of control is a myth at road sensible speeds. Certainly at some time a gear will need to be selected but this can easily be anticipated and selected as necessary.
By the way I am not denying that having the car in neutral never lessens control just that it isn't really an issue at road speeds.
For those of you that may not be aware the RAC rally was won three times in the nineteen sixties by an 850cc SAAB that was fitted with a freewheel device that cut in to prevent engine braking. Surely if freewheeling did cause problems, they would be shown up in competition.
In fact with front wheel drive cars engine braking downhill will transfer weight to the front thus making the back lighter. According to accepted wisdom won't this increase the chance of oversteer, which is a less stable state than understeer, and therefore more dangerous.
On the same topic why are so many drivers worried about optional speed limiters when they may already have a device in the car that SHUTS OFF THE FUEL SUPPLY. What if that didn't switch on again? Where is the outcry about the danger of that?
Hope this helps
Best Regards
Martin A
Martin A said:
Hi All
Can someone please explain how coasting at speeds realistic for the public road removes control any more than changing gear does? Each time we change gear we coast while the clutch is down or we move the gear lever through neutral.
Forget about the instant control argument as you don't have instant control while changing gear. The lack of control is a myth at road sensible speeds. Certainly at some time a gear will need to be selected but this can easily be anticipated and selected as necessary.
By the way I am not denying that having the car in neutral never lessens control just that it isn't really an issue at road speeds.
For those of you that may not be aware the RAC rally was won three times in the nineteen sixties by an 850cc SAAB that was fitted with a freewheel device that cut in to prevent engine braking. Surely if freewheeling did cause problems, they would be shown up in competition.
In fact with front wheel drive cars engine braking downhill will transfer weight to the front thus making the back lighter. According to accepted wisdom won't this increase the chance of oversteer, which is a less stable state than understeer, and therefore more dangerous.
On the same topic why are so many drivers worried about optional speed limiters when they may already have a device in the car that SHUTS OFF THE FUEL SUPPLY. What if that didn't switch on again? Where is the outcry about the danger of that?
Hope this helps
Best Regards
Martin A
Martin, forgive me if I have miss-understood, but do you advocate, as the OPs mates do, to knock it into neutral going down hill?Can someone please explain how coasting at speeds realistic for the public road removes control any more than changing gear does? Each time we change gear we coast while the clutch is down or we move the gear lever through neutral.
Forget about the instant control argument as you don't have instant control while changing gear. The lack of control is a myth at road sensible speeds. Certainly at some time a gear will need to be selected but this can easily be anticipated and selected as necessary.
By the way I am not denying that having the car in neutral never lessens control just that it isn't really an issue at road speeds.
For those of you that may not be aware the RAC rally was won three times in the nineteen sixties by an 850cc SAAB that was fitted with a freewheel device that cut in to prevent engine braking. Surely if freewheeling did cause problems, they would be shown up in competition.
In fact with front wheel drive cars engine braking downhill will transfer weight to the front thus making the back lighter. According to accepted wisdom won't this increase the chance of oversteer, which is a less stable state than understeer, and therefore more dangerous.
On the same topic why are so many drivers worried about optional speed limiters when they may already have a device in the car that SHUTS OFF THE FUEL SUPPLY. What if that didn't switch on again? Where is the outcry about the danger of that?
Hope this helps
Best Regards
Martin A
My thinking here is that gravity will now dictate the vehicle's speed instead of the driver. Oversteer / understeer will only be an issue if there is a bend at some point down the hill. This will require braking (more than if a lower gear had previously been engaged}thus more weight transfer to the front.
OP, try going downhill in a car fitted with a trip computer, mine will stick to 999mph (foot of the gas) regardless of gear.
Dyl said:
I have heard that in modern cars, leaving them in gear downhills over approx. 1700rpm uses no fuel at all, as the fuel pump is turned off until the accelerator is reapplied, or revs drop. Coasting in neutral does use fuel however as the engine has to keep itself running, instead of relying on the momentum. Coasting also greatly reduces control by not allowing engine braking.
I hope thats correct...
Yes, that is correct. The fuel-cut threshold varies between cars, though it is usually about 1700rpm. This is shown very nicely in any car with an mpg instant read-out - in my BMW E36 it goes to infinity about about 1700rpm when off the throttle, but if you coast in neutral it will just read a very high figure. This not only means it's better to back off in-gear than coast in neutral, but it also means it's better at some speeds to keep the car in lower gears so you get over the threshold, for instance when slowing gradually for distant traffic lights on an empty road.I hope thats correct...
Edited by Dyl on Tuesday 10th March 23:41
Needless to say, even if it wasn't for this technology, coasting is never a good idea. It makes it all the more amusing that people compromise their safety for a practise which actually uses more fuel than simply using the over-run.
Edited by RobM77 on Wednesday 11th March 20:57
Hi All
Gravity will have an effect on the car's speed but this can be countered when necessary by sensible braking. This, I imagine, is what the OP's friends do. Things often seem 'wrong' merely because they are different to our view of the world.
Hope this helps
Best regards
Martin A
7mike said:
Martin, forgive me if I have miss-understood, but do you advocate, as the OPs mates do, to knock it into neutral going down hill?
My thinking here is that gravity will now dictate the vehicle's speed instead of the driver. Oversteer / understeer will only be an issue if there is a bend at some point down the hill. This will require braking (more than if a lower gear had previously been engaged}thus more weight transfer to the front.
I don't advocate it for a variety of reasons. The main one being that people prefer to believe what they want in spite of scientific and experiential evidence to the contrary. I have read stuff about car handling by driving 'gurus' that is contrary to Newton's Laws of Motion and Coulomb's Theory of Friction. When I pressed one about the error of their reasoning they still declared they believed it to be true. Since then I haven't bothered arguing.My thinking here is that gravity will now dictate the vehicle's speed instead of the driver. Oversteer / understeer will only be an issue if there is a bend at some point down the hill. This will require braking (more than if a lower gear had previously been engaged}thus more weight transfer to the front.
Gravity will have an effect on the car's speed but this can be countered when necessary by sensible braking. This, I imagine, is what the OP's friends do. Things often seem 'wrong' merely because they are different to our view of the world.
Hope this helps
Best regards
Martin A
Martin A said:
Hi All
Gravity will have an effect on the car's speed but this can be countered when necessary by sensible braking. This, I imagine, is what the OP's friends do. Things often seem 'wrong' merely because they are different to our view of the world.
Hope this helps
Best regards
Martin A
I couldn't agree with you more, having got into plenty of heated debates with people on this forum who don't understand car handling and basic physics (see the recent thread entitled [sic] "loosing the rear end" for a classic example). Not only that, but I've spoken to one or two track instructors and countless road driving instructors (no names mentioned!) who have their car dynamics wires in a complete tangle, don't understand how cars work in corners and even give out phoney advice.7mike said:
Martin, forgive me if I have miss-understood, but do you advocate, as the OPs mates do, to knock it into neutral going down hill?
My thinking here is that gravity will now dictate the vehicle's speed instead of the driver. Oversteer / understeer will only be an issue if there is a bend at some point down the hill. This will require braking (more than if a lower gear had previously been engaged}thus more weight transfer to the front.
I don't advocate it for a variety of reasons. The main one being that people prefer to believe what they want in spite of scientific and experiential evidence to the contrary. I have read stuff about car handling by driving 'gurus' that is contrary to Newton's Laws of Motion and Coulomb's Theory of Friction. When I pressed one about the error of their reasoning they still declared they believed it to be true. Since then I haven't bothered arguing.My thinking here is that gravity will now dictate the vehicle's speed instead of the driver. Oversteer / understeer will only be an issue if there is a bend at some point down the hill. This will require braking (more than if a lower gear had previously been engaged}thus more weight transfer to the front.
Gravity will have an effect on the car's speed but this can be countered when necessary by sensible braking. This, I imagine, is what the OP's friends do. Things often seem 'wrong' merely because they are different to our view of the world.
Hope this helps
Best regards
Martin A
However, despite sympathising with you on that point, I would still say that coasting in a car is a bad idea. Cars produce more grip and offer more control if they are engaged in a gear at reasonable revs. The 'more grip' statement may seem strange (and forgive me for explaining if you are familiar with this), but the physics of it are that you need a motive force to counteract the drag of the wheels to produce optimum grip in a corner (thus the frequently used phrase "cornering on a steady throttle"). The "more control" aspect should seem obvious, but it goes without saying that in order to balance a car safely below the limit of traction in a corner, or achieve proper control should the limit be breached, one needs control over the car's attitude. That control could be achieved with the brake, but of course the control is only one way if you're only using the brake! The throttle is used to control cars in corners as much as the steering is what I'm trying to say in that last point, and that applies to both sub-limit driving and on-limit driving.
Regardless, you actually save more fuel by leaving the car in gear and backing off, as explained above
Hi All
As previously posted, I am not willing to argue the point. I'm sure Rob77M will agree that the fuel cut off only happens in certain cars at certain speeds. Certainly not in cars with carburettors. I'm sure neither of us will be worried about pedantry. So in general, yes, the car can be controlled better if in gear but the difference it makes is not so significant as to result in a loss of control at sensible, legal road speeds by a competent driver.
Hope this helps
Best regards
Martin A
As previously posted, I am not willing to argue the point. I'm sure Rob77M will agree that the fuel cut off only happens in certain cars at certain speeds. Certainly not in cars with carburettors. I'm sure neither of us will be worried about pedantry. So in general, yes, the car can be controlled better if in gear but the difference it makes is not so significant as to result in a loss of control at sensible, legal road speeds by a competent driver.
Hope this helps
Best regards
Martin A
Yes, you're right Martin. Only modern cars running ECUs can feature the fuel cut off above certain revs. However, I do think virtually all cars made in the last ten years do this.
Regarding the safety of coasting, yes, I agree again. It's really not ideal for proper safe control of a vehicle, but I can think of worse things one could do!! :-)
Regarding the safety of coasting, yes, I agree again. It's really not ideal for proper safe control of a vehicle, but I can think of worse things one could do!! :-)
I would think that the biggest danger freewheeling downhill comes from the people that do not understand just how hot brakes get and how this affects their performance.
Instead of say leaving the car in third and the car just holding a constant speed they are freewheeling with their foot on the brake the entire way down the hill.
Another thought is that with the engine just idling you run the risk of it stalling through placing loads on it with the brakes, power steering, air-con etc.
Instead of say leaving the car in third and the car just holding a constant speed they are freewheeling with their foot on the brake the entire way down the hill.
Another thought is that with the engine just idling you run the risk of it stalling through placing loads on it with the brakes, power steering, air-con etc.
As devient said "Brake Fade" is the main argument against coasting down hills. Even if you believe it don't happen now with disc brakes ( it can in extreamis), it still reduces brake wear.
You only change gear when it's safe to, so that isn't an argument of any sort.
surrendering to momentum in corners (on the level) is dodgy because it only likes going in straight lines!
So you need to maintain gas control for this & as always to accelerate out of dangers path at any instant.
Some of the last carb cars could "shut off" on overun by blockig the idle jet with an electronic valve.
You only change gear when it's safe to, so that isn't an argument of any sort.
surrendering to momentum in corners (on the level) is dodgy because it only likes going in straight lines!
So you need to maintain gas control for this & as always to accelerate out of dangers path at any instant.
Some of the last carb cars could "shut off" on overun by blockig the idle jet with an electronic valve.
Martin, Rob77, thanks for the insight, interesting to hear from this angle, I can grasp the logic behind it.
One other point, as we are talking about public roads. If we are relying on constant, light braking to hold back speed, what signal do we then give to following vehicles if a kid steps out ahead of us?
One other point, as we are talking about public roads. If we are relying on constant, light braking to hold back speed, what signal do we then give to following vehicles if a kid steps out ahead of us?
7mike said:
Martin, Rob77, thanks for the insight, interesting to hear from this angle, I can grasp the logic behind it.
One other point, as we are talking about public roads. If we are relying on constant, light braking to hold back speed, what signal do we then give to following vehicles if a kid steps out ahead of us?
That's a jolly good point. I didn't think of that One other point, as we are talking about public roads. If we are relying on constant, light braking to hold back speed, what signal do we then give to following vehicles if a kid steps out ahead of us?
Hi All
Hope this helps
Best Regards
Martin A
7mike said:
Martin, Rob77, thanks for the insight, interesting to hear from this angle, I can grasp the logic behind it.
One other point, as we are talking about public roads. If we are relying on constant, light braking to hold back speed, what signal do we then give to following vehicles if a kid steps out ahead of us?
No one suggested this be done when there is a chance of such an incident. It isn't appropriate in many situations. When it is appropriate it may or may not save fuel depending on the vehicle and circumstances and when used sensibly does not lessen directional control of the vehicle. That's not to say the control doesn't change slightly by the way.One other point, as we are talking about public roads. If we are relying on constant, light braking to hold back speed, what signal do we then give to following vehicles if a kid steps out ahead of us?
Hope this helps
Best Regards
Martin A
Martin A said:
Hi All
Hope this helps
Best Regards
Martin A
I would strongly contest that (see my posts above). Directional control of a vehicle is a direct function of steering and throttle. The very first thing that I was told by my most recent advanced road instructor and my most recent motor racing coach was that revs = control. If you're languishing down in low revs you have a lot less control ove a car, let alone in neutral!!! This is a well known and established fact.7mike said:
Martin, Rob77, thanks for the insight, interesting to hear from this angle, I can grasp the logic behind it.
One other point, as we are talking about public roads. If we are relying on constant, light braking to hold back speed, what signal do we then give to following vehicles if a kid steps out ahead of us?
No one suggested this be done when there is a chance of such an incident. It isn't appropriate in many situations. When it is appropriate it may or may not save fuel depending on the vehicle and circumstances and when used sensibly does not lessen directional control of the vehicle. That's not to say the control doesn't change slightly by the way.One other point, as we are talking about public roads. If we are relying on constant, light braking to hold back speed, what signal do we then give to following vehicles if a kid steps out ahead of us?
Hope this helps
Best Regards
Martin A
deviant said:
Can anyone actually come up with some figures on how much fuel you really save doing this?
I'm pretty willing to bet that you would save more money by doing a poo and wearing lightweight sandles to reduce the weight of your car.
Did a bit of experimenting on quiet roads over the Pennines this morning. Downhill, various gradients, speeds of between 30 - 50 mph. In 3rd = 999mpg. Knocked into neutral dropped as low as 200mpg. Must dig out my sandles & have another go tomorrowI'm pretty willing to bet that you would save more money by doing a poo and wearing lightweight sandles to reduce the weight of your car.
Martin A said:
Hi All
Hope this helps
Best Regards
Martin A
Hi Martin,7mike said:
Martin, Rob77, thanks for the insight, interesting to hear from this angle, I can grasp the logic behind it.
One other point, as we are talking about public roads. If we are relying on constant, light braking to hold back speed, what signal do we then give to following vehicles if a kid steps out ahead of us?
No one suggested this be done when there is a chance of such an incident. It isn't appropriate in many situations. When it is appropriate it may or may not save fuel depending on the vehicle and circumstances and when used sensibly does not lessen directional control of the vehicle. That's not to say the control doesn't change slightly by the way.One other point, as we are talking about public roads. If we are relying on constant, light braking to hold back speed, what signal do we then give to following vehicles if a kid steps out ahead of us?
Hope this helps
Best Regards
Martin A
It wasn't meant as a criticism of your earlier comments, simply another consideration to be taken into account. Your method of negotiating a downhill stretch would be based on your extensive knowledge of vehicle control & experience. However, I doubt this is the case with the friends of the OP.
Regards
Mike
7mike said:
deviant said:
Can anyone actually come up with some figures on how much fuel you really save doing this?
I'm pretty willing to bet that you would save more money by doing a poo and wearing lightweight sandles to reduce the weight of your car.
Did a bit of experimenting on quiet roads over the Pennines this morning. Downhill, various gradients, speeds of between 30 - 50 mph. In 3rd = 999mpg. Knocked into neutral dropped as low as 200mpg. Must dig out my sandles & have another go tomorrowI'm pretty willing to bet that you would save more money by doing a poo and wearing lightweight sandles to reduce the weight of your car.
I did some maths on this earlier. The difference between knocking it into neutral and backing off is pretty minimal to be honest. However, given how much safer (and easier!) it is to stay in gear and just back off, and also given that it's slightly cheaper (about 8 pence a day for my commute as I worked out!), I'd choose that option every time.7mike said:
Martin, Rob77, thanks for the insight, interesting to hear from this angle, I can grasp the logic behind it.
One other point, as we are talking about public roads. If we are relying on constant, light braking to hold back speed, what signal do we then give to following vehicles if a kid steps out ahead of us?
You're better off never doing that - even lightly applying them heats the brakes a lot making fade more likely. A better approach is to select your gear correctly and brake harder then completely release the brakes alternately to maintain speed. It won't be quite as smooth, but it's a lot safer.One other point, as we are talking about public roads. If we are relying on constant, light braking to hold back speed, what signal do we then give to following vehicles if a kid steps out ahead of us?
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