Why is it wrong to brake and down-change simultaneously?

[R0G]

davepoth said:

No, you have that the wrong.

The car is in balance when the centre of gravity of the car stays in the same place. Travelling around a constant radius curve, braking at a constant rate, or accelerating at a constant rate (not going to happen) will all put the car in balance, exactly the same as travelling along a straight road at a constant speed.

Making an abrupt change to a control will take everything out of balance. The CofG will shift, and as the suspension and tyres adjust to that, everything squirms around a bit, which shifts the centre of gravity a bit too. That's what we try and avoid.

Centrifugal forces will be in effect if not in a straight line which is why changing gear is again not recommended but often impossible to achieve

[davepoth]

R0G said:

Centrifugal forces will be in effect if not in a straight line which is why changing gear is again not recommended but often impossible to achieve

If the cornering radius is constant, the car is still in balance in that situation.

The issue is the net effect of the driving technique - and there's no reason why any BGOL technique wouldn't keep balance just as well (if not better) than separating. With BGOL, any engine braking effect of the engine is masked by the braking force - so even if a shift is not perfect the balance will be maintained. If you downshift without overlapping the brakes, a fluffed shift could well knock the car out of balance.

[p1esk]

davepoth said:

R0G said:

When braking the vehicle is 'nose heavy' so releasing the brake makes the vehicle balanced - I think that is a physics fact

No, you have that the wrong.

The car is in balance when the centre of gravity of the car stays in the same place. Travelling around a constant radius curve, braking at a constant rate, or accelerating at a constant rate (not going to happen) will all put the car in balance, exactly the same as travelling along a straight road at a constant speed.

Making an abrupt change to a control will take everything out of balance. The CofG will shift, and as the suspension and tyres adjust to that, everything squirms around a bit, which shifts the centre of gravity a bit too. That's what we try and avoid.

I don't know whether that's the best explanation or not, but it sounds reasonable and I'm certainly not going to dispute it. All I will say is that I work on the basis of how the car feels, and I don't get involved with sophisticated technical considerations.

It therefore seems best to keep the control inputs smooth and progressive, with the aim of avoiding instability. In other words I don't make excessive demands on the car and it, in return, always seems to co-operate with me. I'm inclined to settle for that.

Now where's young Turner when you need 'im?

[R0G]

The question should be - why release one hand from the wheel when braking, conering or steering when it can be avoided?

Two hands on the wheel when the vehicle is doing any of the above has to be SAFER than one hand - that is simple common sense and logic

[davepoth]

R0G said:

The question should be - why release one hand from the wheel when braking, conering or steering when it can be avoided?

Two hands on the wheel when the vehicle is doing any of the above has to be SAFER than one hand - that is simple common sense and logic

If it was unsafe to take a hand off the wheel when changing gear, we would all drive automatics surely?

Nobody should ever be in a situation on the road where having just one hand on the wheel makes a difference to the stability of the vehicle.

Aside from that, as far as I'm concerned it's better to reduce the amount of time necessary to get the car ready for whatever hazard is approaching, giving more time to think about it.

[p1esk]

R0G said:

The question should be - why release one hand from the wheel when braking, conering or steering when it can be avoided?

Two hands on the wheel when the vehicle is doing any of the above has to be SAFER than one hand - that is simple common sense and logic

Agreed, but it depends on the circumstances, so my question would be:

Why insist on both hands on the wheel when safety can be maintained by using only one?

[vonhosen]

R0G said:

The question should be - why release one hand from the wheel when braking, conering or steering when it can be avoided?

Two hands on the wheel when the vehicle is doing any of the above has to be SAFER than one hand - that is simple common sense and logic

How much steering you thinking of doing?
Depends on which is going to be to the greatest detriment as far as safety is concerned. If a small amount of steering is all that is needed & it can be adequately safely done with one hand, that extra margin of safety may be better served by the 2nd hand being elsewhere at that particular time. I don't see how hard fast rules serve hugely variable circumstances best.

[R0G]

vonhosen said:

How much steering you thinking of doing?
Depends on which is going to be to the greatest detriment as far as safety is concerned. If a small amount of steering is all that is needed & it can be adequately safely done with one hand, that extra margin of safety may be better served by the 2nd hand being elsewhere at that particular time. I don't see how hard fast rules serve hugely variable circumstances best.

You used SAFELY whereas I said SAFER

[vonhosen]

R0G said:

vonhosen said:

How much steering you thinking of doing?
Depends on which is going to be to the greatest detriment as far as safety is concerned. If a small amount of steering is all that is needed & it can be adequately safely done with one hand, that extra margin of safety may be better served by the 2nd hand being elsewhere at that particular time. I don't see how hard fast rules serve hugely variable circumstances best.

You used SAFELY whereas I said SAFER

And as I said, there may be a greater safety requirement elsewhere that would result in it being safer to only have one hand on the wheel in order to address that need at that time.
It's safer not to go out on the road if you are really worried about what's safest.

Most of the time it's more than adequately safe to use personal preference than adhere to a 'one rule for all' style.

[R0G]

I prefer to be as safe as possible so if doing one thing is SAFER than another I will choose the SAFEST option

[vonhosen]

R0G said:

I prefer to be as safe as possible so if doing one thing is SAFER than another I will choose the SAFEST option

Which may be taking a hand off the wheel whilst having steering on & braking in some circumstances.

(So you never go out in the car, since that's the safest possible thing when it comes to your risk of crashing.)

[R0G]

vonhosen said:

Which may be taking a hand off the wheel whilst having steering on & braking in some circumstances.

(So you never go out in the car, since that's the safest possible thing when it comes to your risk of crashing.)

THE SAFEST OPTION WHEN DRIVING

[vonhosen]

R0G said:

vonhosen said:

Which may be taking a hand off the wheel whilst having steering on & braking in some circumstances.

(So you never go out in the car, since that's the safest possible thing when it comes to your risk of crashing.)

THE SAFEST OPTION WHEN DRIVING

So you avoid country roads, because statistically you are more likely to have a fatal on them & it's therefore safest to plan your route to only use motorways & built up areas?

The point is you make trade off's all the time.

[R0G]

vonhosen said:

So you avoid country roads, because statistically you are more likely to have a fatal on them & it's therefore safest to plan your route to only use motorways & built up areas.

Now you are just trying to justify your position and agenda so unless this continues on common sense lines then I am not playing

[vonhosen]

R0G said:

vonhosen said:

So you avoid country roads, because statistically you are more likely to have a fatal on them & it's therefore safest to plan your route to only use motorways & built up areas.

Now you are just trying to justify your position and agenda so unless this continues on common sense lines then I am not playing

Common sense is a collection of prejudices acquired by the age of eighteen (Albert Einstein)

As I said, you make trade off's all the time, even if you think you don't. You don't actually do the safest thing all the time, because it serves your purpose to do something that is only slightly less safe but has other advantages.

Edited by vonhosen on Sunday 6th October 19:40

[MC Bodge]

davepoth said:

R0G said:

When braking the vehicle is 'nose heavy' so releasing the brake makes the vehicle balanced - I think that is a physics fact

No, you have that the wrong.

The car is in balance when the centre of gravity of the car stays in the same place. Travelling around a constant radius curve, braking at a constant rate, or accelerating at a constant rate (not going to happen) will all put the car in balance, exactly the same as travelling along a straight road at a constant speed.

Making an abrupt change to a control will take everything out of balance. The CofG will shift, and as the suspension and tyres adjust to that, everything squirms around a bit, which shifts the centre of gravity a bit too. That's what we try and avoid.

It's known as the car 'taking a set'. Everything is loaded up and the car is in equilibrium for its current state of motion.

Letting off the brakes, in order to coast (not particularly desirable) during a gear-change, requires the car to take another, extra 'set'/'balance'/equlibrium before the corner is negotiated and new set taken in the bend. Smooth transitions between sets are preferable.

People need to understand this (not particularly difficult) concept in order to know what a vehicle is doing rather than just accepting the very simplified explanations given by the likes of Roadcraft.

It is fairly obvious to anybody who has tried to ride a motorbike smoothly.

If the IAM/Roadar fundamentalists wish to ignore this, though, then so be it.

Edited by MC Bodge on Sunday 6th October 19:57

[vonhosen]

MC Bodge said:

davepoth said:

R0G said:

When braking the vehicle is 'nose heavy' so releasing the brake makes the vehicle balanced - I think that is a physics fact

No, you have that the wrong.

The car is in balance when the centre of gravity of the car stays in the same place. Travelling around a constant radius curve, braking at a constant rate, or accelerating at a constant rate (not going to happen) will all put the car in balance, exactly the same as travelling along a straight road at a constant speed.

Making an abrupt change to a control will take everything out of balance. The CofG will shift, and as the suspension and tyres adjust to that, everything squirms around a bit, which shifts the centre of gravity a bit too. That's what we try and avoid.

It's known as the car 'taking a set'. Everything is loaded up and the car is in equilibrium for its current state of motion.

Letting off the brakes, in order to coast (not particularly desirable) during a gear-change, requires the car to take another, extra 'set'/'balance'/equlibrium before the corner is negotiated and new set taken in the bend. Smooth transitions between sets are preferable.

People need to understand this (not particularly difficult) concept in order to know what a vehicle is doing rather than just accepting the very simplified explanations given by the likes of Roadcraft.

It is fairly obvious to anybody who has tried to ride a motorbike smoothly.

But not everybody (if anybody at all) can do what's optimal for equilibrium in all circumstances & may have to do the best they can with what they have available at the time.

[MC Bodge]

vonhosen said:

But not everybody (if anybody at all) can do what's optimal for equilibrium in all circumstances & may have to do the best they can with what they have available at the time.

Agreed, but somebody separating everything out for every manoeuvre just because the IAM/Roadar/Police observer/instructor told them to -without an adequate, correct explanation of vehicle dynamics- isn't going to help much in achieving it.

[vonhosen]

MC Bodge said:

vonhosen said:

But not everybody (if anybody at all) can do what's optimal for equilibrium in all circumstances & may have to do the best they can with what they have available at the time.

Agreed, but somebody separating everything out for every manoeuvre just because the IAM/Roadar/Police observer/instructor told them to -without an adequate, correct explanation of vehicle dynamics- isn't going to help much in achieving it.

Agreed

[davepoth]

vonhosen said:

MC Bodge said:

vonhosen said:

But not everybody (if anybody at all) can do what's optimal for equilibrium in all circumstances & may have to do the best they can with what they have available at the time.

Agreed, but somebody separating everything out for every manoeuvre just because the IAM/Roadar/Police observer/instructor told them to -without an adequate, correct explanation of vehicle dynamics- isn't going to help much in achieving it.

Agreed

I'm not going to rag on the "systems". They serve a very good purpose in getting people to think about their driving, but I believe that they should be more result oriented rather than method oriented.