Avoiding Downchanging and Braking. Why?

Avoiding Downchanging and Braking. Why?

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StressedDave

839 posts

261 months

Friday 16th September 2005
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RobM77 said:
The idea when you're turning into a bend is to balance the car front and rear. Most modern road cars are not balanced unless you turn in on the brake or on a trailing throttle.


I'd have said the opposite myself - stability on all road cars is enhanced by being balanced on the throttle, and as anyone who has had the misfortune to be coached by me will tell you, the one thing I say to clients time and time again is 'more power'. As a former accident investigator I'd have said a lot of single vehicle loss-of-control accidents were caused by people coming off the throttle or braking when a nice amount of power would have kept them stable and confident.

It is possible that you could be entering bends a little too quickly thus requiring some additional speed loss as you turn - hence the feeling of more stability with braking/trailing throttle while turning.

RobM77 said:

If your car is not balanced in a bend then if you hit oil, or need to take emergency action, your car is more likely to encounter a handling imbalance, and is more likely to exceed the limit of grip. Turning in after you've braked will set up a balance in most road cars of say 70% of the limit at the front, and 30% at the back. So, if you go 30% faster or the grip gets 30% less, you will understeer. If you've balanced the car properly on turn in then you would be 50% front and rear. 30% extra gives us 50% rear and 80% front - i.e your still in control. Hedge or road, it's your choice!


I'm afraid that the amount of grip used by each end of the car is identical otherwise you'd either plough straight on or spin on every corner. All road cars are designed with steady-state understeer so that the front tyres run out of grip first, but that's a function of suspension stiffness affecting tyre loads and nothing to do with the weight distribution caused by braking

RobM77 said:

As with many advanced techniques, you *need to know what you're doing* (turning in on the brakes is a critical thing - too much and it might end in tears) - but if you do then this is far far safer than turning in after you've braked.

So far as drifts go, in order for a drift to occur, you have to be going fast enough to exceed the limit of grip from the tyres - and you shouldn't really be doing that on the public road anyway. On a track, the fastest way round a corner is in a balanced drift (roughly 4% slip front and rear on a modern tyre), so this is no bad thing!!


Actually, once you're driving above about 30 mph on the road, you're in a controlled drift while cornering.

FWIW I corner in the 'Morris Minor' fashion - it's unimportant how much speed you take into a corner or other hazard, it's far more important how much you carry out the other side.

woodytvr

622 posts

245 months

Friday 16th September 2005
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RobM77 said:
Turning in on the brakes is not dangerous if you do it properly - in fact it is safer than not doing so.



It's also not good practice. More luck than judgement I'm afraid. So by doing it properly I guess you are suggesting not to brake too hard. So I'm coming into a corner on the brakes and I'm just about to turn and a dog runs out into the road - Now what?


RobM77 said:


Usually, this will simply be taking your foot off the brake *as* you turn the wheel into the bend. The two should be linked, much like your accelerator should be linked to the steering unwinding as you come out of a bend.




Only if you are wanting to speed up out of the corner. A car is most balanced under load not necessarily acceleration.


RobM77 said:


The idea when you're turning into a bend is to balance the car front and rear. Most modern road cars are not balanced unless you turn in on the brake or on a trailing throttle.

If your car is not balanced in a bend then if you hit oil, or need to take emergency action, your car is more likely to encounter a handling imbalance, and is more likely to exceed the limit of grip. Turning in after you've braked will set up a balance in most road cars of say 70% of the limit at the front, and 30% at the back. So, if you go 30% faster or the grip gets 30% less, you will understeer. If you've balanced the car properly on turn in then you would be 50% front and rear. 30% extra gives us 50% rear and 80% front - i.e your still in control. Hedge or road, it's your choice!




Yikes, an interesting view but way off mark IMO. By braking whilst you turn you are increasing the load to the front wheels and inducing understeer. Also unloading the rear creating potential oversteer. As before, where is the reserve?


Rob77M said:

As with many advanced techniques, you *need to know what you're doing* (turning in on the brakes is a critical thing - too much and it might end in tears) - but if you do then this is far far safer than turning in after you've braked.



Agree about advanced techniques require you to know what you are doing but braking into a bend isn't an advanced technique. On the road it's a school boy error. Like I said before it has its uses at high speed on a track if you are trying to force some oversteer but has no place on the road for everyday driving, even fast driving.


Rob77M said:


So far as drifts go, in order for a drift to occur, you have to be going fast enough to exceed the limit of grip from the tyres - and you shouldn't really be doing that on the public road anyway. On a track, the fastest way round a corner is in a balanced drift (roughly 4% slip front and rear on a modern tyre), so this is no bad thing!!



Speed has nothing to do with drifting, it all about balance. Disturbing the balance by braking as you turn is one method of starting a drift.

As I said before this subject is far too contentious so I think we should just agree to disagree. Your method works for you and my understanding of the 'System' works for me. I can drive quickly and safely on the road and as sideways as I like on the track (or roundabout if no-one is looking lol) and I'm happy that my techniques are safe.

>> Edited by woodytvr on Friday 16th September 15:56

RobM77

35,349 posts

233 months

Friday 16th September 2005
quotequote all
StressedDave said:

RobM77 said:
The idea when you're turning into a bend is to balance the car front and rear. Most modern road cars are not balanced unless you turn in on the brake or on a trailing throttle.



I'd have said the opposite myself - stability on all road cars is enhanced by being balanced on the throttle,



Balanced yes, but you need to set the balance up in the first place.
StressedDave said:



and as anyone who has had the misfortune to be coached by me will tell you, the one thing I say to clients time and time again is 'more power'. As a former accident investigator I'd have said a lot of single vehicle loss-of-control accidents were caused by people coming off the throttle or braking when a nice amount of power would have kept them stable and confident.


Those accidents are caused by adverse weight transfer. If you back off, weight comes off the rear.
StressedDave said:


It is possible that you could be entering bends a little too quickly thus requiring some additional speed loss as you turn - hence the feeling of more stability with braking/trailing throttle while turning.


Trail braking can be used on track to make your braking point later, yes. However, I am referring purely to the idea of setting a car up for a bend. This does depend what car you're driving of course... My Caterham required very little setting up on the way in, but my present BMW 330 requires a lot more.
StressedDave said:



RobM77 said:

If your car is not balanced in a bend then if you hit oil, or need to take emergency action, your car is more likely to encounter a handling imbalance, and is more likely to exceed the limit of grip. Turning in after you've braked will set up a balance in most road cars of say 70% of the limit at the front, and 30% at the back. So, if you go 30% faster or the grip gets 30% less, you will understeer. If you've balanced the car properly on turn in then you would be 50% front and rear. 30% extra gives us 50% rear and 80% front - i.e your still in control. Hedge or road, it's your choice!



I'm afraid that the amount of grip used by each end of the car is identical otherwise you'd either plough straight on or spin on every corner.


well, that's strictly true, yes - the amount used is identical :-D :-D The amount on offer is a different matter though.

If you haven't balanced a car on the way into a bend, and you're maintaining a steady speed, then you're gonna be limited by the final handling of the car - in most road cars this is understeer.
.
StressedDave said:


All road cars are designed with steady-state understeer so that the front tyres run out of grip first, but that's a function of suspension stiffness affecting tyre loads and nothing to do with the weight distribution caused by braking


The setup of a car is determined by many factors. You're right, yes, suspension stiffness does affect it (generally the stiffer end loses grip first). Also of course you have suspension geometry (camber, castor & toe); and crucially, in a road car, how the geometry changes with suspension deflection. In addition to this, the basic mass distribution around the car will alter the handling (which is what BMW are on about with their 50:50 claim).

All these things will determine how a car handles, but when you're on the move driving the car, the way it responds depends obviously on these things, but also on the weight distribution at the time. Accelerating cars tend towards more understeer, because the front goes light, and decelerating cars tend towards more oversteer, because the rear goes light. Notice I said 'tend towards more' - it depends what the car is doing at the time and you will only add to or take away from that state.

Obviously we also have power or brake application losing grip (a power slide for instance), but thisis out of the scope of this discussion; although obviously an important factor when cornering!

At the end of the day, I'm sure we both agree that if you just turn the wheel into a corner at a steady speed then you will experience the steady state handling of the car. As you previously correctly stated, this is understeer. Now, to be quick (or safe!), you don't want understeer, you want neutral steer (i.e. both ends giving all the grip they can on a track) - so you're gonna have to do what you can to get this. Obviously, you could just change the setup, but for various reasons this might not be desirably; so you have to change the way you drive - this involves playing with the weight transfer. Managing your weight transfer is what motor racing is all about - some call it a 'carving' sport like surfing, wakeboarding or snowboarding.

StressedDave said:



RobM77 said:

As with many advanced techniques, you *need to know what you're doing* (turning in on the brakes is a critical thing - too much and it might end in tears) - but if you do then this is far far safer than turning in after you've braked.

So far as drifts go, in order for a drift to occur, you have to be going fast enough to exceed the limit of grip from the tyres - and you shouldn't really be doing that on the public road anyway. On a track, the fastest way round a corner is in a balanced drift (roughly 4% slip front and rear on a modern tyre), so this is no bad thing!!



Actually, once you're driving above about 30 mph on the road, you're in a controlled drift while cornering.


ok, theroretically, yes, you build slip angles all the time when driving; but I wanted to avoid such talk on an open forum. What I was referring to by a drift is the usual common definition (not the current modern definition that is threatening to take over, as in drifting competitions, that is actually oversteer of course), and that is when you're up on tip toes and the car is *just* starting to slide. A tyre gives its maximum grip at about 4-5% slip, and it is this amount of drift that is fastest.
StressedDave said:



RobM77

35,349 posts

233 months

Friday 16th September 2005
quotequote all
woodytvr said:

RobM77 said:
Turning in on the brakes is not dangerous if you do it properly - in fact it is safer than not doing so.




It's also not good practice. More luck than judgement I'm afraid. So by doing it properly I guess you are suggesting not to brake too hard. So I'm coming into a corner on the brakes and I'm just about to turn and a dog runs out into the road - Now what?

woodytvr said:

You dodge out of the way or stop. My point is that this is much easier if your car is balanced than if you've turned in at teh steady state handling of your car. Surely you agree with that?



RobM77 said:


Usually, this will simply be taking your foot off the brake *as* you turn the wheel into the bend. The two should be linked, much like your accelerator should be linked to the steering unwinding as you come out of a bend.





Only if you are wanting to speed up out of the corner. A car is most balanced under load not necessarily acceleration.



RobM77 said:


The idea when you're turning into a bend is to balance the car front and rear. Most modern road cars are not balanced unless you turn in on the brake or on a trailing throttle.

If your car is not balanced in a bend then if you hit oil, or need to take emergency action, your car is more likely to encounter a handling imbalance, and is more likely to exceed the limit of grip. Turning in after you've braked will set up a balance in most road cars of say 70% of the limit at the front, and 30% at the back. So, if you go 30% faster or the grip gets 30% less, you will understeer. If you've balanced the car properly on turn in then you would be 50% front and rear. 30% extra gives us 50% rear and 80% front - i.e your still in control. Hedge or road, it's your choice!





Yikes, an interesting view but way off mark IMO. By braking whilst you turn you are increasing the load to the front wheels and inducing understeer. Also unloading the rear creating potential oversteer. As before, where is the reserve?
woodytvr said:

The idea is that you are using the grip at the front to create a balance in the car's state. You're not fully on the brake of course - you are just leaning the car into the bend as you bring the steering in. This is standard track technique.




Rob77M said:

As with many advanced techniques, you *need to know what you're doing* (turning in on the brakes is a critical thing - too much and it might end in tears) - but if you do then this is far far safer than turning in after you've braked.




Agree about advanced techniques require you to know what you are doing but braking into a bend isn't an advanced technique. On the road it's a school boy error. Like I said before it has its uses at high speed on a track if you are trying to force some oversteer but has no place on the road for everyday driving, even fast driving.

woodytvr said:

When you say 'create oversteer', what one is trying to do by turning in on the brakes is create oversteer, and the amount that you create balances the car's natural understeer. What you're left with is perfect balance - nice. Far safer than the understeer that the car had in its steady state.




Rob77M said:


So far as drifts go, in order for a drift to occur, you have to be going fast enough to exceed the limit of grip from the tyres - and you shouldn't really be doing that on the public road anyway. On a track, the fastest way round a corner is in a balanced drift (roughly 4% slip front and rear on a modern tyre), so this is no bad thing!!




Speed has nothing to do with drifting, it all about balance. Disturbing the balance by braking as you turn is one method of starting a drift.

As I said before this subject is far too contentious so I think we should just agree to disagree. Your method works for you and my understanding of the 'System' works for me. I can drive quickly and safely on the road and as sideways as I like on the track (or roundabout if no-one is looking lol) and I'm happy that my techniques are safe.

>> Edited by woodytvr on Friday 16th September 15:56

RobM77

35,349 posts

233 months

Friday 16th September 2005
quotequote all
Sorry, all my quotes were messed up there. Hopefully you can tell what is me and what is Woody! :-)

RobM77

35,349 posts

233 months

Friday 16th September 2005
quotequote all
Basically, what this boils down to is me stating that most modern road cars understeer in a steady state (everyone accepts that). If you turn in to a bend in a steady state (i.e. not on the brakes, but instead at a constant speed), your car will obviously be tending towards understeer. It won't actually be understeering, but it is tending towards understeer.

From what everyone has been saying, I think we all agree up to that point. Steady state in a road car is understeer. Turn in at a steady state and you will understeer.

Right, so how are we going to make our car balanced, instead of tending towards understeer? Get a little weight off the back - this can be done with a trailing throttle or by coming off the brake *as* you turn in, not before.

woodytvr

622 posts

245 months

Friday 16th September 2005
quotequote all
I see what you are saying now. You are eliminating the breif amount of understeer which occurs at initial turn in by loading the front of the car. You're not continuing to brake as you actually turn the wheel.

Obviously once into the corner that forward weight transfer has gone as you are now back on the power. There must therfore be a point at which the weight shifts back and the front goes light albiet briefly.

Whilst I agree the theory is good for the track where you can see the exit and everything in between. I'd suggest your are entering a corner too quickly if you require this technique to tame the front of your car at turn in. Modern cars have understeer built in yes but not that much and certinaly not a modern BMW.

GreenV8S

30,150 posts

283 months

Friday 16th September 2005
quotequote all
Although you're all making a lot of sense, not everything that you are (collectively) saying applies as universally as you seem to be implying. In particular where you are talking about the relationship between braking, steering and transient over-/under-steer it's a relatively complex problem and not all cars behave in the way you're describing.

I would say that the main benefit from using trail braking is that it allows the braking phase to extend far deeper into the corner. In a competitive situation this lets you leave your braking later, on a public road it may be your get out of jail card if you find you have left your braking too late or need to scrub off more speed than you had planned.

It's true that trail braking can also be used to control the yaw stability of the car, but the effects on yaw stability are subtle and complex, and I don't think that it is the main reason that people use trail braking.

>> Edited by GreenV8S on Friday 16th September 18:01

StressedDave

839 posts

261 months

Friday 16th September 2005
quotequote all
GreenV8S said:
Although you're all making a lot of sense, not everything that you are (collectively) saying applies as universally as you seem to be implying. In particular where you are talking about the relationship between braking, steering and transient over-/under-steer it's a relatively complex problem and not all cars behave in the way you're describing.


You're telling me! After 5 years as a ride and handling engineer and 9 as an accident investigator, the problem is that it's easy to make generalisations on handling and apply them to all vehicles. I tend to teach to develop a 'universal style' which works on any car rather than having to try a whole load of different techniques to find which keeps you rubber side down on the black stuff.

GreenV8S said:

I would say that the main benefit from using trail braking is that it allows the braking phase to extend far deeper into the corner. In a competitive situation this lets you leave your braking later, on a public road it may be your get out of jail card if you find you have left your braking too late or need to scrub off more speed than you had planned.


We're back to the 'monkeys still fall out of trees' syndrome. On the road proper planning should prevent this - the people who tend to need to 'trail brake' are those who look only a short distance in front of the car.

GreenV8S said:

It's true that trail braking can also be used to control the yaw stability of the car, but the effects on yaw stability are subtle and complex, and I don't think that it is the main reason that people use trail braking.


Proper trail braking involves tapering the brakes off as you turn the steering. Most of the 'trail braking' I see has the brakes on with the steering fully applied - in other words it's late braking not trail braking. You lose enough speed to get around the corner while you're in the corner and you're compromising the grip of the front tyres so you tend to increase the understeer of the car. And I won't talk about the induced drag and tyre stall that often results. People call it trail braking so it sounds like an advanced technique and something planned rather than a survival reaction becuase the entry speed is wrong.

RobM77

35,349 posts

233 months

Friday 16th September 2005
quotequote all
This whole thread has turned into a strange mix of ideas I think!! So many different ideas (many a bit wrong - though I won't be ungentlemanly by pointing the finger) that I don't think I'm going to post anymore if that is ok?

I can qualify my statements up to now by the fact that I have a good degree in Physics and have held a very active interest in car dynamics since I was a teenager. I have read most books on the subject, and even assisted in the Physics behind some car simulation software. I have also raced successfully for 5 years, done many track days, and some informal on track instructing. Absolutely everything I do in a car, whether on track when setting a pole position time, or on the way to work at my usual sedate speed (I drive as safely as I can on the road), is backed up the Physics of what is happening - and it is all relatively simple.

Before I bow out if I may I will make a few statements on car handling:

Weight transfer: in cars of low to moderate performance at speeds of over 40mph, this is the single biggest factor determining the balance of a car in a corner. Decelerate and you will get oversteer, accelerate and you will get understeer. Motor racing is about balance - the best drivers have uncanny senses of balance...

"Break away" (for want of a better term). Braking or accelerating puts load on a tyre and this can overwhelm it and cause break away. This is the other cause of oversteer or understeer. This is more likely at lower speeds.

"Dynamic and static friction" - linked to the above - and thisis why you have to be smooth behind the wheel. A moving object has different frictional properties (mu, if you're into the maths and physics of this) to a stationary one. Therefore, if your car starts sliding, it is capable of less overall grip than if it wasn't. The optimum friction is at about 4-5% slip. The idea is that once you breach grip by being jerky, you're off the curve of grip and you'll need to come back the other way to regain grip. Driving on the limit really is walking a tightrope.

"Steady state handling" (sometimes called 'final handling'): this is what a car does if it is neither decelerating or accelerating. Most modern road cars understeer - indeed most racing drivers prefer slight understeer, at least at higher speeds anway.

"Momentum" - this has a big effect on procedings!! Watch rally drivers at work to watch this in action. This could mean a scandinavian flick, or it could just mean giving the car a twitch on the way into a bend - this will yaw the car and this transient state will continue (on its own momentum) up to the apex to help you round (watch the famous Senna-NSX video that always does the rounds on the web to see this odd technique in action). Momentum is the product of mass and speed, which is why the tail of a 350Z stays out for much longer than a Caterham when you're sideways!

"Turning in on the brakes" - standard technique taught at all ARDS schools at basic Nat B level. This is simply coming off the brakes as you turn in. So, 0% steering, 100% brakes; 50% steering, 50% brakes; and when you're fully turned in you have no brakes on at all. The timing of this can be altered to get a good handling balance. In a front drive race car, very slight oversteer pre-apex may actually be desirable so you can open the throttle earlier.

"Steady throttle" - what you do after you've turned in if you're not trail braking.

"Trail braking" - this is staying on the brakes well into the corner. Jim Clark was a master of this, and Schumacher is a modern expert too. Basically, if you start opening the throttle from the apex onwards, the theory of trail braking says 'why not do the opposite on the way in?'. Requires a lot of skill to perfect at very high speeds. Of course, this allows tweaking of handling balance, but also later braking as you get some braking done in the bend.

"4 wheel Drift": in this situation all tyres have the same slip and the car is actually sliding (to a novice passenger this may feel quite dramatic). A modern tyre produces maximum grip at about 4-5% slip (varies with surface and tyre), so this ever so slight drift is the fastest way round a corner. In the old days, the drift angles were bigger, so watching the likes of Fangio and Moss was a delight!

Hopefully that's all clear - I'm going to bow out of thread now :-)

Pigeon

18,535 posts

245 months

Saturday 17th September 2005
quotequote all
Well, back on the downchanging-and-braking front, yesterday I found myself driving a hired van with a gearchange like pushing a blunt screwdriver through the chest wall, massively over-servoed brakes that would stand it on its nose if you looked at the pedal, and throttle response of the "manana" variety.

Attempting to change down while braking was therefore largely pointless most of the time and I found myself block-changing without much thinking about it.

Didn't get on with it too badly, overall. I found it useful, on approaching a junction, to hold two block change movements in mind at once - a "go" one and a "stop" one - then execute whichever was appropriate once I could see whether it was clear. The bit I was least happy with was the coasting period between coming off the brakes and completing re-engagement of the clutch, though this would have been less of a problem on a vehicle with a more responsive engine (and a more audible engine; for a van it was remarkably quiet and hard to hear the engine note).

As well as a more responsive engine, I prefer a much faster gearchange and brakes that require a decent shove rather than a tap to operate. With such controls I'd still prefer to go down the box as speed falls. But using the block-changing method all day I found it much less of a pain than I thought.