Understanding Weight Transfer
Discussion
willibetz said:
On the road, grip is changing all the time and sometimes dramatically within the course of a single bend. Skilled drivers that I've been fortunate enough to sit with use observation to predict the available grip and then manage the car without troubling those limits (acknowledging that the available grip may disappear suddenly).
I think this great common sense approach also applies to those of us who would not claim ourselves to be skilled drivers. In addition to observing the road conditions, I also pay close attention to the feedback I am getting back from the tyres (a bit like GreenV8 but perhaps less extreme). A small twitch is enough to tell me that these are not the conditions to get too exuberent in so back off.
Personally I think the tyres I use are an important factor in being able to feel the level of grip. Mine are Conti Sport Contact 2 all round. I've stuck with these because I find they have a progressive loss of grip. By this I mean a little too much gas at the wrong time and you can feel them go, a little bit more gas and they'll slide more, etc. Yet, I'm constantly being told that other tyres (particularly Michelin PS2) are much better because they have so much more grip in the wet. That may be so but I prefer a tyre that tells me grip is not good here so slow down rather that a tyre that keeps on gripping until....it suddenly doesn't grip
I know very little about tyres (which is a start, I guess 
), but have been told by people I trust that the conti sport is an excellent compromise tyre for Porsches. I used them on a powerful BMW, and thought them excellent.
As for limit work and understanding how to get tyres to work for you, a session with Don Palmer is highly educational.
WilliBetz
), but have been told by people I trust that the conti sport is an excellent compromise tyre for Porsches. I used them on a powerful BMW, and thought them excellent. As for limit work and understanding how to get tyres to work for you, a session with Don Palmer is highly educational.
WilliBetz
mph999 said:
For normal road driving - I'm sure I'll get shot down for this (gulp) ...
To manage weight transfer just drive smoothly ...
2. Slowing down, use brakes, 3 stage, 1. Gently on 2. Firm braking to get speed off, 3. Gently off.
Job done
To manage weight transfer just drive smoothly ...
2. Slowing down, use brakes, 3 stage, 1. Gently on 2. Firm braking to get speed off, 3. Gently off.
Job done
By no means shooting you down - but to point out for 'enthusiastic driving', in order to effect maximum weight transfer for the most effective braking the technique advised by the likes of Vic Elford is to initially briefly stab/hit the brakes quite hard - whereby maximum weight transfer over the front wheels is effected and thereafter the smooth, firm and progressive braking to get the speed off in the most effective, efficient manner. ( If you are really 'on it' the 'limousine' braking technique you describe, is more prone to wheels locking (or more likely these days-the ABS kicking in) and relatively ineffective braking.
Edited by maddog993 on Saturday 2nd December 18:18
maddog993 said:
mph999 said:
For normal road driving - I'm sure I'll get shot down for this (gulp) ...
To manage weight transfer just drive smoothly ...
2. Slowing down, use brakes, 3 stage, 1. Gently on 2. Firm braking to get speed off, 3. Gently off.
Job done
To manage weight transfer just drive smoothly ...
2. Slowing down, use brakes, 3 stage, 1. Gently on 2. Firm braking to get speed off, 3. Gently off.
Job done
By no means shooting you down - but to point out for 'enthusiastic driving', in order to effect maximum weight transfer for the most effective braking the technique advised by the likes of Vic Elford is to initially briefly stab/hit the brakes quite hard - whereby maximum weight transfer over the front wheels is effected and thereafter the smooth, firm and progressive braking to get the speed off in the most effective, efficient manner. ( If you are really 'on it' the 'limousine' braking technique you describe, is more prone to wheels locking (or more likely these days-the ABS kicking in) and relatively ineffective braking.
Edited by maddog993 on Saturday 2nd December 18:18
I've read this, but my memory of the detail is slightly different...
I recall that he advocated (for Porsches) an initial application of brakes at about 40% of maximum pedal pressure, to allow the weight to transfer without locking up, followed fairly quickly by a decisive increase to 100% of maximum pedal pressure. Lots of caveats, obviously.
Is this the technique to which you were alluding, or is my memory deficient?
WilliBetz
willibetz said:
maddog993 said:
mph999 said:
For normal road driving - I'm sure I'll get shot down for this (gulp) ...
To manage weight transfer just drive smoothly ...
2. Slowing down, use brakes, 3 stage, 1. Gently on 2. Firm braking to get speed off, 3. Gently off.
Job done
To manage weight transfer just drive smoothly ...
2. Slowing down, use brakes, 3 stage, 1. Gently on 2. Firm braking to get speed off, 3. Gently off.
Job done
By no means shooting you down - but to point out for 'enthusiastic driving', in order to effect maximum weight transfer for the most effective braking the technique advised by the likes of Vic Elford is to initially briefly stab/hit the brakes quite hard - whereby maximum weight transfer over the front wheels is effected and thereafter the smooth, firm and progressive braking to get the speed off in the most effective, efficient manner. ( If you are really 'on it' the 'limousine' braking technique you describe, is more prone to wheels locking (or more likely these days-the ABS kicking in) and relatively ineffective braking.
Edited by maddog993 on Saturday 2nd December 18:18
I've read this, but my memory of the detail is slightly different...
I recall that he advocated (for Porsches) an initial application of brakes at about 40% of maximum pedal pressure, to allow the weight to transfer without locking up, followed fairly quickly by a decisive increase to 100% of maximum pedal pressure. Lots of caveats, obviously.
Is this the technique to which you were alluding, or is my memory deficient?
WilliBetz
that's the one willi - it's my wording that is poor; as you suggest, the initial little hard abrupt 'jab' on the brakes is around 40% in terms of braking pressure applied- this takes up the pressure in the braking system and causes an instant weight transfer towards the front wheels- this is followed by the 'squeezing' on the brake pedal which should progressively increase as more weight becomes transferred over the front wheels (ie the more weight over the front wheels, the harder you can squeeze- all the way through to 100% braking pressure)
The 'limousine driver' braking where it is simply very gentle smooth squeezing on the brake pedal is likely to result in wheel lock-up/ABS as without the 'jab' there is no weight transfer over the front wheels and they are consequently relatively unloaded so that much brake pressure cannot be effectively applied.
maddog993 said:
The 'limousine driver' braking where it is simply very gentle smooth squeezing on the brake pedal is likely to result in wheel lock-up/ABS as without the 'jab' there is no weight transfer over the front wheels and they are consequently relatively unloaded so that much brake pressure cannot be effectively applied.
Not sure I understand the explanation here. I'm not disputing the methodology, but searching for a physics explanation. The weight transfer is effected by the car rolling forward and is specifically related to the sprung mass being decelerated.
Why does the sprung mass need a prod to get it going?
Thinking as I am typing...Perhaps it's to do with matching the weight distribution front/rear to match the relative braking abilities of front versus rear to produce most braking. I think for the explanation to hold true, the jab would by necessity of the argument produce a momentary lock up.
Graham
Graham
gridgway said:
maddog993 said:
The 'limousine driver' braking where it is simply very gentle smooth squeezing on the brake pedal is likely to result in wheel lock-up/ABS as without the 'jab' there is no weight transfer over the front wheels and they are consequently relatively unloaded so that much brake pressure cannot be effectively applied.
Not sure I understand the explanation here. I'm not disputing the methodology, but searching for a physics explanation. The weight transfer is effected by the car rolling forward and is specifically related to the sprung mass being decelerated.
Why does the sprung mass need a prod to get it going?
Thinking as I am typing...Perhaps it's to do with matching the weight distribution front/rear to match the relative braking abilities of front versus rear to produce most braking. I think for the explanation to hold true, the jab would by necessity of the argument produce a momentary lock up.
Graham
Graham
Graham, the basic principle is the harder the braking, the greater the weight transfer from rear to front and, through this, the greater the weight transfer that can be applied towards the front of the car, the harder the car may be braked without locking the front wheels - thus through the initial abrupt 'jab' there is a sudden, instant (rather than gradual) weight transfer to the front wheels meaning that the braking thereafter may be heavier than would otherwise be possible without the front wheels locking and the stopping/slowing distance consequently shorter.
It is probably the only instance where anything other than a smooth and progressive approach to driver input is suggested
A consideration that may temper your enthusiasm to jab the brakes, especially of an older 911, is the risk of locking a wheel.
It takes a llooonnnngggg time to recover that situation, and even more space (as they locked on the initial application, at your maximum achieved speed).
WilliBetz
It takes a llooonnnngggg time to recover that situation, and even more space (as they locked on the initial application, at your maximum achieved speed).
WilliBetz
maddog993 said:
Graham, the basic principle is the harder the braking, the greater the weight transfer from rear to front and, through this, the greater the weight transfer that can be applied towards the front of the car, the harder the car may be braked without locking the front wheels - thus through the initial abrupt 'jab' there is a sudden, instant (rather than gradual) weight transfer to the front wheels meaning that the braking thereafter may be heavier than would otherwise be possible without the front wheels locking and the stopping/slowing distance consequently shorter.
It is probably the only instance where anything other than a smooth and progressive approach to driver input is suggested
Thanks maddog, I understand the principle, but I dont think that the weight transfer is anything other that proportional to the deceleration.
from force=mass x acc, the force generated by the mass of the car which produces the turning moment is linear. Thus the weight transfer is as far as I can see linear.
In fact again thinking as I am typing, I would have thought that weight transfer is actually the enemy of efficient braking. At the limit of 100% of the car's weight over the front wheels, there are only two tyres able to work, not 4. That doesnt sound like it should be optimum.
Graham
maybe...
brakes are designed to cope with weight transfer clearly with bigger brakes at the front.
They have to work with varying amounts of weight transfer from 0 to Wmax (at 100% braking force). I'm guessing that they have an effective range probably in the 40%+ bracket.
So until you get to say 40% the front is effectively over-braked. However, this will not cause the fronts to lock as there is only 40% braking being applied.
So the quick dive into 40%+ braking will enable optimum use of unbalanced brakes. That makes me feel a bit better about getting best braking, but I still think that the suggested technique may have more to it than that.
Is the suggestion that the technique makes the max braking force applied before locking higher with a "prod" than with a gradual application?
Graham
brakes are designed to cope with weight transfer clearly with bigger brakes at the front.
They have to work with varying amounts of weight transfer from 0 to Wmax (at 100% braking force). I'm guessing that they have an effective range probably in the 40%+ bracket.
So until you get to say 40% the front is effectively over-braked. However, this will not cause the fronts to lock as there is only 40% braking being applied.
So the quick dive into 40%+ braking will enable optimum use of unbalanced brakes. That makes me feel a bit better about getting best braking, but I still think that the suggested technique may have more to it than that.
Is the suggestion that the technique makes the max braking force applied before locking higher with a "prod" than with a gradual application?
Graham
I think the idea behind the prod is that you apply the maximum braking you can without locking, this means the amount of braking is limited by whichever end of the car has "too little" weight on it (because the weight transfer has not settled out and achieved the steady state weight distribution). If you apply the brakes smoothly and progressively, you remain closer to steady state the whole time (which is good) but you don't exploit all available grip (which is bad if you are trying to achieve the maximum possible braking). This is rather similar to the technique for a perfect standing start, where you apply enough torque to almost-but-not-quite spin the wheels, and then increase it as the weight transfer occurs.
GreenV8S said:
I think the idea behind the prod is that you apply the maximum braking you can without locking, this means the amount of braking is limited by whichever end of the car has "too little" weight on it (because the weight transfer has not settled out and achieved the steady state weight distribution). If you apply the brakes smoothly and progressively, you remain closer to steady state the whole time (which is good) but you don't exploit all available grip (which is bad if you are trying to achieve the maximum possible braking). This is rather similar to the technique for a perfect standing start, where you apply enough torque to almost-but-not-quite spin the wheels, and then increase it as the weight transfer occurs.
I think that we're on the same page now as we're now talking about max poss braking. I think the thing that took me off course before was the talk of limousine driving causing a lockup as somehow it used braking force when the eight transfer hadn't taken place.
Although maybe there is another factor...the pendulum effect. so the initial prod accelerated the car's mass forward against the springs so the weight transfer is not linear. So you can catch it at Wmax and get the brakes on harder before the front locks. With a slow progressive increase of braking force you do get a linear transfer as there is no pendulum effect about the pitch centre (if that is anything like a sensible term).
I wonder what the effective resonant frequency is?
Graham
Weight transfer can have a major effect on the handling of some cars.
I drove a Ferrari GTB in a 12 hour sports car race. I had never driven it before practice, & it was bl@@dy awful.
In the slow corners, it had not bad over steer, but complete rear end breakaway, even at low speeds. My co-driver was having so much trouble with it that he had a little prang, which meant we lost a lot of practice time.
In the race, it was so bad, that I was having trouble getting away from a Datsun 2000 Fairlady, which was 25 MPH slower down the straight. [It was driven by a former Australian F1 GP winner].
For some reason, after 20 or so laps, on one lap, I got off the brakes early, going into one of the slow corners. The car handeled a bit better around the corner.
After another 10 laps, I was getting off the brakes 15 yards before the turn in to the slow corners. I had worked my braking point down from 180 to 90 yards for the slowest corner, & had knocked 3 seconds off the lap times.
The GTB was a production road car, not a race car, but even so, I had never driven a car which was so sensitive to braking unbalance. In fact, I found with many road cars, on the track, like the Holdens, & Falcons at Bathurst, you had to turn in, while under brakes, to get the front end to bite.
I drove a Ferrari GTB in a 12 hour sports car race. I had never driven it before practice, & it was bl@@dy awful.
In the slow corners, it had not bad over steer, but complete rear end breakaway, even at low speeds. My co-driver was having so much trouble with it that he had a little prang, which meant we lost a lot of practice time.
In the race, it was so bad, that I was having trouble getting away from a Datsun 2000 Fairlady, which was 25 MPH slower down the straight. [It was driven by a former Australian F1 GP winner].
For some reason, after 20 or so laps, on one lap, I got off the brakes early, going into one of the slow corners. The car handeled a bit better around the corner.
After another 10 laps, I was getting off the brakes 15 yards before the turn in to the slow corners. I had worked my braking point down from 180 to 90 yards for the slowest corner, & had knocked 3 seconds off the lap times.
The GTB was a production road car, not a race car, but even so, I had never driven a car which was so sensitive to braking unbalance. In fact, I found with many road cars, on the track, like the Holdens, & Falcons at Bathurst, you had to turn in, while under brakes, to get the front end to bite.
gridgway said:
maddog993 said:
The 'limousine driver' braking where it is simply very gentle smooth squeezing on the brake pedal is likely to result in wheel lock-up/ABS as without the 'jab' there is no weight transfer over the front wheels and they are consequently relatively unloaded so that much brake pressure cannot be effectively applied.
Not sure I understand the explanation here. I'm not disputing the methodology, but searching for a physics explanation. The weight transfer is effected by the car rolling forward and is specifically related to the sprung mass being decelerated.
Why does the sprung mass need a prod to get it going?
Graham
Graham
I was just wondering that ...
maddog993 said:
mph999 said:
For normal road driving - I'm sure I'll get shot down for this (gulp) ...
To manage weight transfer just drive smoothly ...
2. Slowing down, use brakes, 3 stage, 1. Gently on 2. Firm braking to get speed off, 3. Gently off.
Job done
To manage weight transfer just drive smoothly ...
2. Slowing down, use brakes, 3 stage, 1. Gently on 2. Firm braking to get speed off, 3. Gently off.
Job done
By no means shooting you down - but to point out for 'enthusiastic driving', in order to effect maximum weight transfer for the most effective braking the technique advised by the likes of Vic Elford is to initially briefly stab/hit the brakes quite hard - whereby maximum weight transfer over the front wheels is effected and thereafter the smooth, firm and progressive braking to get the speed off in the most effective, efficient manner. ( If you are really 'on it' the 'limousine' braking technique you describe, is more prone to wheels locking (or more likely these days-the ABS kicking in) and relatively ineffective braking.
Edited by maddog993 on Saturday 2nd December 18:18
Fair point, I was along the IAM/RoSPA lines where smoothness is the key. For normal road driving it's fine.
Having now though about it, your method will I guess hit maximum braking quicker ...
Martin
mph999 said:
maddog993 said:
mph999 said:
For normal road driving - I'm sure I'll get shot down for this (gulp) ...
To manage weight transfer just drive smoothly ...
2. Slowing down, use brakes, 3 stage, 1. Gently on 2. Firm braking to get speed off, 3. Gently off.
Job done
To manage weight transfer just drive smoothly ...
2. Slowing down, use brakes, 3 stage, 1. Gently on 2. Firm braking to get speed off, 3. Gently off.
Job done
By no means shooting you down - but to point out for 'enthusiastic driving', in order to effect maximum weight transfer for the most effective braking the technique advised by the likes of Vic Elford is to initially briefly stab/hit the brakes quite hard - whereby maximum weight transfer over the front wheels is effected and thereafter the smooth, firm and progressive braking to get the speed off in the most effective, efficient manner. ( If you are really 'on it' the 'limousine' braking technique you describe, is more prone to wheels locking (or more likely these days-the ABS kicking in) and relatively ineffective braking.
Edited by maddog993 on Saturday 2nd December 18:18
Fair point, I was along the IAM/RoSPA lines where smoothness is the key. For normal road driving it's fine.
Having now though about it, your method will I guess hit maximum braking quicker ...
Martin
I'm still searching for an explanation for this. The simple explanations of weight transfer I have found all say it is linear. So it doesnt matter whether you stab or go for max braking very slowly, you get the same weight transfer at any specific brake force.
I have found a paper that is not a simple exposee (ie too hard for my rather rusty maths/mechanics) which suggests non linear activity, but I cant tie it up with the 'stab'!
I will phone a (clever) friend!
Graham
gridgway said:
I'm still searching for an explanation for this. The simple explanations of weight transfer I have found all say it is linear. So it doesnt matter whether you stab or go for max braking very slowly, you get the same weight transfer at any specific brake force.
The steady state is linear (longitudinal weight transfer is proportional to longitudinal decelleration) but the transient behaviour determines what happens as you come on and off the brakes and is rather more complex - essentially, the longitudinal weight transfer has a component that is proportional to pitch acceleration, which is added/removed from the steady state weight transfer. This means the weight transfer lags the acceleration in a complex and highly non-linear way.
I think that lag is the crux of the matter. As far as I can see, the reason for the sharp initial application of brakes is about maximising use of the available grip i.e. brake as hard as possible, limited only by whichever end is closest to locking up. Even without any weight transfer having occured, you can still brake very hard without locking up. It is only the last 15% or so that requires the weight transfer to have settled out in order to have both ends equally close to locking up. I wonder what effect a good ABS has on this. Theory suggests that you can simply stamp both feet on the brake pedal and let the ABS modulate the brakes as necessary to stop any wheel from locking up. There are other transient effects that the ABS can't help with, but as far as getting the maximum braking as soon as possible, ABS seems to rewrite the rule book.
GreenV8S said:
Theory suggests that you can simply stamp both feet on the brake pedal and let the ABS modulate the brakes as necessary to stop any wheel from locking up. There are other transient effects that the ABS can't help with, but as far as getting the maximum braking as soon as possible, ABS seems to rewrite the rule book.
Practical experience (mine, fwiw
) suggests otherwise. I think it's still important to get the weight onto the nose before activating the ABS. Otherwise, the ABS will prevent lock-up but not slow the car as efficiently. Hope that makes some sense, or is interesting, or something.
WilliBetz
A big difference between race drivers mere mortals is in the way they brake. Where road drivers will gradually increase brake pressure as the slow, race drivers do the exact opposite. Maxiumum braking as soon as is possible, then taper off as the car slows. The faster the speed, the harder it is to lock the wheels, and weight transfer "pinning down" the front end also makes it more difficult for the front wheels to lock. As a biker, unless you're fron the US, you quickly learn that the fastest and safest way to brake is to lead with the front, making sure you get the weight over the front wheel.
A rather extreme example would be young Mr Rossi. He may as well be riding a unicycle when he's on the brakes, as pretty much 100% of the weight is on the front wheel, with the rear wagging around in the breeze. Anyone who really wants to understand the practical physics of weight transfer need look no further than a video of The Doctor in action.
A rather extreme example would be young Mr Rossi. He may as well be riding a unicycle when he's on the brakes, as pretty much 100% of the weight is on the front wheel, with the rear wagging around in the breeze. Anyone who really wants to understand the practical physics of weight transfer need look no further than a video of The Doctor in action.
willibetz said:
GreenV8S said:
Theory suggests that you can simply stamp both feet on the brake pedal and let the ABS modulate the brakes as necessary to stop any wheel from locking up. There are other transient effects that the ABS can't help with, but as far as getting the maximum braking as soon as possible, ABS seems to rewrite the rule book.
Practical experience (mine, fwiw
) suggests otherwise. I think it's still important to get the weight onto the nose before activating the ABS. Otherwise, the ABS will prevent lock-up but not slow the car as efficiently. Hope that makes some sense, or is interesting, or something.
WilliBetz
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