ADUK Archive

f3racer wrote:I'm not sure the engine is using any extra fule by being used to slow the car down, providing no throttle is being applied.

In gear, clutch up, no accelerator and the engine is using no fuel, zero, ziltch (on modern ECU engines anyway)...until you put the clutch down to change gear...

f3racer wrote:I'm not sure the engine is using any extra fule by being used to slow the car down, providing no throttle is being applied.

It could do, even taking into account Martin's comment about fuel cut-off. If, by selecting a lower gear, you are getting greater engine braking then you are slowing more quickly than if you had not made the gear change. If the extra braking is necessary it is because you have already wasted fuel by maintaining a higher speed for a longer period.

martine wrote:

f3racer wrote:I always deliberately try to avoid the use of brakes for ecomomic purposes and reduce sleed using the gears where possible.

* more balanced (4 wheels are braking, if you use gears to slow only 2 (unless it's 4wd)

Until you're going round a bend, slowing down using the engine, then it's only 1 wheel slowing you down, or 2 I suppose in 4wd.

Octy_Ross wrote:Until you're going round a bend, slowing down using the engine, then it's only 1 wheel slowing you down, or 2 I suppose in 4wd.

Explain more please?

martine wrote:

Octy_Ross wrote:Until you're going round a bend, slowing down using the engine, then it's only 1 wheel slowing you down, or 2 I suppose in 4wd.

Explain more please?

Differentials!

Chris

Octy_Ross wrote:...or 2 I suppose in 4wd.

But four wheel drives have three differentials, don't they? So it would still only be one wheel being slowed?

David

kfae8959 wrote:

Octy_Ross wrote:...or 2 I suppose in 4wd.

But four wheel drives have three differentials, don't they? So it would still only be one wheel being slowed?

David

I was thinking of the Subaru AWD type thing which is cleverer than most. I suspect it would still only be 1 wheel really but... not sure.

Why is anyone slowing down going round a bend ? whether on the brakes or using engine braking you've failed in the entry if your still taking speed off.

My take on it is that we use the brakes as they act on all the wheels, if you are dropping down the gears to use engine resistance to slow you down rather than braking you are likely to unbalance the car and potentially even lock the driving wheels up much the same as using the handbrake you have little control on the amount of braking compared to using the foot brake.

I'd also question if it's such a good technique why don't racing and response drivers do it ? (you can ask yourself that question about any technique thats not taught or used by either of those groups).

Standard Dave wrote:Why is anyone slowing down going round a bend ? whether on the brakes or using engine braking you've failed in the entry if your still taking speed off.

My take on it is that we use the brakes as they act on all the wheels, if you are dropping down the gears to use engine resistance to slow you down rather than braking you are likely to unbalance the car and potentially even lock the driving wheels up much the same as using the handbrake you have little control on the amount of braking compared to using the foot brake.

I'd also question if it's such a good technique why don't racing and response drivers do it ? (you can ask yourself that question about any technique thats not taught or used by either of those groups).

what about trail braking
or
corners that tighten?

Octy_Ross wrote:

Standard Dave wrote:Why is anyone slowing down going round a bend ? whether on the brakes or using engine braking you've failed in the entry if you are still taking speed off.

My take on it is that we use the brakes as they act on all the wheels, if you are dropping down the gears to use engine resistance to slow you down rather than braking you are likely to unbalance the car and potentially even lock the driving wheels up much the same as using the handbrake you have little control on the amount of braking compared to using the foot brake.

I'd also question if it's such a good technique why don't racing and response drivers do it ? (you can ask yourself that question about any technique thats not taught or used by either of those groups).

what about trail braking
or
corners that tighten?

Exactly! Thank you, OR.

In any case, should not a differential share the torque between the driven wheels, whether it is driving torque or braking torque that is being applied?

I appreciate that on slippery surfaces some additional sensitivity and finesse is neded by a driver, but on any normal road surface I reckon it takes a pretty clumsy driver to provoke instability and possible loss of grip by their use of engine braking.

....and Dave, not so far from where I live, there is a bend where, unless you know the road, one would very likely find oneself having to make a further speed reduction after the start of the bend. It's a fairly steep downhill and tightening left hander, and I have my doubts about any driver getting the entry speed right on that one. My guess is that one would need to do a bit of extra braking after getting into the bend, and so long as you don't go in with grossly excessive speed to begin with, it would not be a problem.

Hold on before we get carried away...I can see that using engine braking whilst not straight would mean only one wheel is slowing the vehicle but on a straight it's 2 normally...isn't it?

We're talking slowing for a bend or any hazard for that matter, by coming off the 'gas' and not changing down through the gears. It's better for all sorts of reasons only one of which is fuel consumption.

martine wrote:Hold on before we get carried away...I can see that using engine braking whilst not straight would mean only one wheel is slowing the vehicle but on a straight it's 2 normally...isn't it?

Yes, normally two, whether on the straight or in a bend; unless the onward march of technical progress has made me out of touch on this also.

So a handful of bends tighten to a degree that people feel they couldn't get round them at their entry speed is the curve obscured by trees or a wall why can't you anticipate the exit by the position of the lamp posts, hedgeline or the lights of opposing traffic.

I too am aware of a bend that tightens unnaturally in Derbyshire which most of the police forces in the region specifically drive to to take courses round so thats two nationally we are aware of that don't fit the system exactly, I dare say trying to take such a bend using engine braking rather than the foot brake might cause you some issues. Modern roads or those that are reprofiled wouldn't include such a bend.
I was refering to people who regulaly brake in corners for most people it probably doesn't matter that much as they are so far from the limit of the vehicle that an unbalanced vehicle is of little importance it's when you actually drive quickly for a purpose that advanced techniques matter and you appreciate why they are done the way they are.

Again I'm failing to see the advantages of engine braking by sticking the lever down rather than using the foot brake and also wondering why such a technique isn't used by racing or response drivers ?

Octy_Ross wrote:

kfae8959 wrote:

Octy_Ross wrote:...or 2 I suppose in 4wd.

But four wheel drives have three differentials, don't they? So it would still only be one wheel being slowed?

David

I was thinking of the Subaru AWD type thing which is cleverer than most. I suspect it would still only be 1 wheel really but... not sure.

Probably 1, unless its fitted with diff locks on both axels which most aren't as they are inherently dangerous when used on a road because of transmission wind up! Most modern systems use viscous coupling which activate when the props begin to rotate at different speed caused by loss of traction!

TripleS wrote:
In any case, should not a differential share the torque between the driven wheels, whether it is driving torque or braking torque that is being applied?

Not a fully open diff - if you have one wheel has grip one wheel doesn't - the gripless will spin, hence the introduction of 'limited slip'.