Brake / calliper Question..
- Thread starter Big Dave
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Big Dave
New member
This is turning into a good little discusion..[
]
I think Fen got the correct word ,,, NEUTRALISE....the handling..
This is exactly what im trying to achieve....
Yes im probably braking way too late + too hard + turning in too late, which is causing the car to understeer...[]
But if i can at least "neutralise" the "front diving" a little then im sure itl all help ,, in the grand sceme of things...[][][][]. Ta for the responses so far....
Ill let you know how i get on when ive finally fitted + tested the "fronts" on the back...[][]
]I think Fen got the correct word ,,, NEUTRALISE....the handling..
This is exactly what im trying to achieve....
Yes im probably braking way too late + too hard + turning in too late, which is causing the car to understeer...[
]But if i can at least "neutralise" the "front diving" a little then im sure itl all help ,, in the grand sceme of things...[
][][][]. Ta for the responses so far....Ill let you know how i get on when ive finally fitted + tested the "fronts" on the back...[
][]Actually interestingly enough the really quick guys actually brake into and through the early parts of the corner. One of the main benefits of this 'Trail Braking' is in fact to do exactly what you're wanting to do Dave which is put more pressure on the fronts thereby increasing grip/turn in and by reducing pressure on the rears lightens them up which enable them to follow the front end more compliantly. It's a hard concept to grasp and one I've been researching a little on various sites. I've also started a post at the US 968 site on this. Interesting opinions and one technique I'm going to endevour to develop.
http://www.968forums.com/index.php?s=12eeffd9b86ce643c22ab420cfb74f73&showforum=9
http://virtualracersedge.com/trail_braking.htm
http://www.getfaster.com/Techtips/Physics23.html
Have a good read as this is the next step to faster times boys if you're not already using it. This is not to say it negates some of the issues that have been already discussed here, but it's a different way of treating some of the symptoms that Dave is describing.
http://www.968forums.com/index.php?s=12eeffd9b86ce643c22ab420cfb74f73&showforum=9
http://virtualracersedge.com/trail_braking.htm
http://www.getfaster.com/Techtips/Physics23.html
Have a good read as this is the next step to faster times boys if you're not already using it. This is not to say it negates some of the issues that have been already discussed here, but it's a different way of treating some of the symptoms that Dave is describing.
Well one begets the other in a sense. The difficult concept, and one I may need to review since briefly trying my new PFC pads, is that a lot of guys t-brake with their left foot. This is fine if you've practised. (An automatic car is the best way to start) but you have to already be in the gear you need for the upcoming corner. If not, what it means is that you have to downshift/heel and toeing just before you turn in rather than using this as a method of slowing your car. This in itself is hard to separate as by virtue of selecting a lower gear you are going to go slower, but I digress. You still have to be carrying a fair bit of speed as if you slow down too much too early, then by l-f-braking or t-braking you will scrub off too much speed from the car. Conversly, if you go in too hot and try to t-brake you will most likely spin at best, roll or hit a wall at worst. So this is not something to try without practise. The next question from me was where do you practise? I am at the track this Sunday so I might try a bit of this where I can. I already left foot brake and use the throttle at the same time which achieves a similar attitude from the car, but this is when I am already in the gear that I want to go through the corner with not having to rush through the gears to navigate a fast approaching hairpin.
Some brief thoughts, as I've got to go out and am already late.
Bigger brakes at the rear - no point IMHO. The car does the majority of the braking at the front. Switching the bias to the rear will only reduce dive through reducing braking force - the CoG moves forward under braking, as already stated, and it is this which makes the car dive. Switching the balance will, if anything, make the car more prone to spin under braking as there is greater possibility of rear lock up. The other thing to consider is that engine braking is also acting at the rear so there is again more possibility of lock up.
Car manufacturers don't like their cars spinning into ditches when it is wet, so they opt for a safe set up. On a dry track you may be able to achieve more rear braking which will lessen the requirement for front braking. This will allow more available traction for cornering. This does merit having an adjustable brake bias, but only so you can return the brakes to a "safer" or normal set up when wet.
Trail braking is a scary proposition and I tend to agree with Fen that it is used more to move the CoG forward to provide more traction to the front of the car than to slow the car down. It is a very delicate balance as you are trying to achieve more traction to the front tyres at a time when the they are already on the limit of traction through the cornering forces. If you then try to apply additional braking force you are as likely to loose the battle for traction than achieve the result you are looking for.
The other consideration is anti lock braking - obviously the wheels don't lock through braking (well they do but then they don't but then they do etc). But they will still slide if you are already asking them to corner at the limit of what the tyres are capable of any way.
I have more theories which I will bore you with later.
Bigger brakes at the rear - no point IMHO. The car does the majority of the braking at the front. Switching the bias to the rear will only reduce dive through reducing braking force - the CoG moves forward under braking, as already stated, and it is this which makes the car dive. Switching the balance will, if anything, make the car more prone to spin under braking as there is greater possibility of rear lock up. The other thing to consider is that engine braking is also acting at the rear so there is again more possibility of lock up.
Car manufacturers don't like their cars spinning into ditches when it is wet, so they opt for a safe set up. On a dry track you may be able to achieve more rear braking which will lessen the requirement for front braking. This will allow more available traction for cornering. This does merit having an adjustable brake bias, but only so you can return the brakes to a "safer" or normal set up when wet.
Trail braking is a scary proposition and I tend to agree with Fen that it is used more to move the CoG forward to provide more traction to the front of the car than to slow the car down. It is a very delicate balance as you are trying to achieve more traction to the front tyres at a time when the they are already on the limit of traction through the cornering forces. If you then try to apply additional braking force you are as likely to loose the battle for traction than achieve the result you are looking for.
The other consideration is anti lock braking - obviously the wheels don't lock through braking (well they do but then they don't but then they do etc). But they will still slide if you are already asking them to corner at the limit of what the tyres are capable of any way.
I have more theories which I will bore you with later.
I guess I would sum up my desire for more braking most succinctly thus:
It will make the car more "nervous" under braking. By nervous I mean unstable I guess.
In common with with fighter aircraft increased nervousness in a car's chassis equates to increased agility but at the expense of controllability. There has to be a large area between safe road setup and uncontrollability on track which can be experimented with. Having drive to the rear wheels lessens the need in comparison with FWD, where in race trim it looks to me like they are set up to try to swap ends immediately the wheel is turned and are then cornered on the edge of spinning by means of the front wheels pulling the nose forward.
If the rear is slightly unsettled during braking for a corner it will tend to "come round" more readily on turn-in, more quickly getting the car to an attitude where the front wheels are pointing straight ahead (or even a tiny whiff of opposite lock), allowing the car to be driven through the corner on the throttle in a nice, quick, accelerating drift.
A bit more finesse is required than in FWD where I imagine with the aid of an LSD you basically turn in, immediately steer into the spin that that induces and plant the throttle pedal in the bulkhead to try to drag the car out of that spin.
It will make the car more "nervous" under braking. By nervous I mean unstable I guess.
In common with with fighter aircraft increased nervousness in a car's chassis equates to increased agility but at the expense of controllability. There has to be a large area between safe road setup and uncontrollability on track which can be experimented with. Having drive to the rear wheels lessens the need in comparison with FWD, where in race trim it looks to me like they are set up to try to swap ends immediately the wheel is turned and are then cornered on the edge of spinning by means of the front wheels pulling the nose forward.
If the rear is slightly unsettled during braking for a corner it will tend to "come round" more readily on turn-in, more quickly getting the car to an attitude where the front wheels are pointing straight ahead (or even a tiny whiff of opposite lock), allowing the car to be driven through the corner on the throttle in a nice, quick, accelerating drift.
A bit more finesse is required than in FWD where I imagine with the aid of an LSD you basically turn in, immediately steer into the spin that that induces and plant the throttle pedal in the bulkhead to try to drag the car out of that spin.
Hilux
New member
I believe you have it in a nutshell [
]I always see it like this...........................
Imagine a ball on a rod, the lower end of the rod is at ground level and at varying heights or lengths of rod is attached a ball.
Once they are travelling at the same speed (linear) they are fine. Add friction to the rod at ground level and the ball will attempt to travel forward in a downwards arc as it is restrained by the rod. So will a car if it is allowed to.
To prevent this the rod (suspension) is then restrained by varying means and sprung and damped. Rigid it will prevent nose dive (but not the tendency for the greater mass of the car to try and overtake it thereby pulling on and lightening the load on the wheel) but then the suspension is not compliant so the car is undriveable. The intruduced compliance of the rod (suspension) also increases momentarily the weight transfer onto the front tyres thereby improving grip.
To stop a car settling nose heavy there must be link/pivot between the suspension and its mount front and rear which then complicates the springing as this link/pivot then has to be dampened, however the car will still settle under braking albeit evenly.
I have messed around with front and rear brake bias many times and built my own systems and found that even uprating rear pads from standard can have a dramatic impact and greater tendency for the wheels to lock.
I agree whole heartily with John that the rears do relatively little braking and that even small increases in efficiency can have the rears lock and the car will then turn you around before you can catch it
Bigger rear brakes are proportioned regarding the overall retardation needed ie: efficient energy conversion into heat but they are always in proportion to the fronts.
Larger rear calipers (pistons) with the original master cylinder will mean heavier brakes and possibly noticeable pedal travel as the leverage (pressure ratio) is altered.
IMHO fit a brake bias valve in line adjustable (by clicks not levers[
]) to experiment.Most people (including me [&o]) enter corners too fast (to the detriment of corner exit speed) and think that they are late braking when it is semi `panic` braking thereby unsettling the car.
I have definitive article somewhere on front/rear brake proportioning....................somewhere I cannot find [
]*coat on..............waits to be shot down in flames...................*
edh
New member
"panic braking" [][] been there, done that....
First trackday in the 944 - Bedford. Got faster during the day, didn't realise that my braking points from the morning were getting inadequate.
Too fast in, still on brakes, back end out & straight off []
Dave - if you're braking too late, you're probably provoking understeer by coming off too suddenly (and being too fast in) - if you're still on the brakes, (and car is nose down) then you're more likely to oversteer.
Trail braking's great for good drivers. I believe Schumacher is the king of trail braking - bit too subtle for me to try though (I'm just happy to get round the corner reasonably neatly)
I'm planning to do one of those Car limits days in November - they get great write ups on Pistonheads, & are exactly the sort of days where you can learn/practice cornering techniques
http://www.carlimits.com/script/viewEvents.php?type=airfieldtraining&month=20061101
][] been there, done that....First trackday in the 944 - Bedford. Got faster during the day, didn't realise that my braking points from the morning were getting inadequate.
Too fast in, still on brakes, back end out & straight off [
]Dave - if you're braking too late, you're probably provoking understeer by coming off too suddenly (and being too fast in) - if you're still on the brakes, (and car is nose down) then you're more likely to oversteer.
Trail braking's great for good drivers. I believe Schumacher is the king of trail braking - bit too subtle for me to try though (I'm just happy to get round the corner reasonably neatly)
I'm planning to do one of those Car limits days in November - they get great write ups on Pistonheads, & are exactly the sort of days where you can learn/practice cornering techniques
http://www.carlimits.com/script/viewEvents.php?type=airfieldtraining&month=20061101
Remember the old addage "Slow in , fast out".
The thing to consider is the corner as a whole. If you do come in too fast, and are trail braking to the apex, you then have a more violent transition at the point of acceleration. CoG forward under braking then being flung backward on acceleration. Not only do you have the chance of crashing on the outside of the track before the apex, you also have the possibility of crashing on the inside of the track after the apex. [].
One wants to achieve some equilibrium mid corner so the car is ready to receive the power.
My understanding on left foot braking was principally to keep the turbo spooled up so as to minimise lag.
And just to add further confusion: I understand that the BMW Touring Cars were one of the first to use ABS and they had a habit of braking stupidly late, even at the entry to to corner, then use the ABS around the corner. I fear such an approach with a 44 would result in significant brake fade within a very few laps.
I note Fen's comments with regard to "nervous" handling. While this was much the case in the sixties, and before, and on loose surface rallys, I don't believe it holds weight for track driving with modern tyres. Drifting may look spectacular but ultimately overheats modern tyres producing less available grip.
Sliding may look impressive but isn't the fastest way around a track and is equally likely to get you black flagged.
The thing to consider is the corner as a whole. If you do come in too fast, and are trail braking to the apex, you then have a more violent transition at the point of acceleration. CoG forward under braking then being flung backward on acceleration. Not only do you have the chance of crashing on the outside of the track before the apex, you also have the possibility of crashing on the inside of the track after the apex. [
].One wants to achieve some equilibrium mid corner so the car is ready to receive the power.
My understanding on left foot braking was principally to keep the turbo spooled up so as to minimise lag.
And just to add further confusion: I understand that the BMW Touring Cars were one of the first to use ABS and they had a habit of braking stupidly late, even at the entry to to corner, then use the ABS around the corner. I fear such an approach with a 44 would result in significant brake fade within a very few laps.
I note Fen's comments with regard to "nervous" handling. While this was much the case in the sixties, and before, and on loose surface rallys, I don't believe it holds weight for track driving with modern tyres. Drifting may look spectacular but ultimately overheats modern tyres producing less available grip.
Sliding may look impressive but isn't the fastest way around a track and is equally likely to get you black flagged.
I think you're reading too much into my description. I don't mean you should be coming through corners with half a turn of opposite lock like something from The Proffessionals, but that the back should settle into a neutral stance as early in the corner as possible. It probably doesn't apply to F1 cars, but saloons (which includes the 944 in my mind) all corner fast with the axis through the middle of the car slightly beyond perpendicular to the raidius they are travelling through.
sawood12
New member
I believe Stirling Moss was the pioneer of trail braking in order to carry more speed into the corner without understeering off into the armco. It's especially useful for 911 drivers who can use it to load up the outside front tyre and combat the light floaty front end issue that 911's suffer, especially on a low fuel load. I also believe it is very very difficult to master and requires very smooth and precise steering inputs so as not to build up any inertia in the rear end and therefore swing it out.
I think people are getting brake bias and diving under braking unneccessarily intertwined. My thoughts on this are:-
- The rate of decelaration is the one an only component that causes diving under braking. Brake bias is irrelevant.
- It also doesn't stand to reason that heavier the braking always leads to more weight transfers to the front. There is a maximum amount of weight transfer that can occur under heavy braking after which more braking does not result in more forward weight transfer. I don't believe it is possible to raise the rear end of a 944 completely off the ground no matter how much braking force you have at your disposal.
- It is only the suspension that allows weight and CofG transfer and once you have either bottomed out the front suspension or the spring force has balanced with the decelaration force no more weight transfer will occur.
- The more your car dives under braking the less the CofG transfer because the CofG is getting closer to the ground and therfore closer to the effective pivot point of the car. Therefore it is like the law of diminishing returns - the more the car dives under braking a disproportionatly more amount of braking force is required to cause it to dive further - i.e. it is not linear.
- At this point there is still a significant amount of weight over the rear axle (thanks to the dirty great transaxel hanging over it) which can be (and should be) exploited by the rear brakes to achive maximum possible retardation.
- The trick is to size your rear brakes relative to the front so that they can deliver the max amount of clamping force without locking up the rear wheels at this maximum weight transfer point.
- Of course all this goes out of the window in the wet. The best setup is to have a bias valve in the cockpit.
Unfortuately cars are dynamic and therefore no one set of rules apply to all situations threrefore it is not necessarily always right that if you increase braking upfront you have to decrease braking at the back - there is always an optimum rear brake size. The best thing to do is to suck it and see.
I think people are getting brake bias and diving under braking unneccessarily intertwined. My thoughts on this are:-
- The rate of decelaration is the one an only component that causes diving under braking. Brake bias is irrelevant.
- It also doesn't stand to reason that heavier the braking always leads to more weight transfers to the front. There is a maximum amount of weight transfer that can occur under heavy braking after which more braking does not result in more forward weight transfer. I don't believe it is possible to raise the rear end of a 944 completely off the ground no matter how much braking force you have at your disposal.
- It is only the suspension that allows weight and CofG transfer and once you have either bottomed out the front suspension or the spring force has balanced with the decelaration force no more weight transfer will occur.
- The more your car dives under braking the less the CofG transfer because the CofG is getting closer to the ground and therfore closer to the effective pivot point of the car. Therefore it is like the law of diminishing returns - the more the car dives under braking a disproportionatly more amount of braking force is required to cause it to dive further - i.e. it is not linear.
- At this point there is still a significant amount of weight over the rear axle (thanks to the dirty great transaxel hanging over it) which can be (and should be) exploited by the rear brakes to achive maximum possible retardation.
- The trick is to size your rear brakes relative to the front so that they can deliver the max amount of clamping force without locking up the rear wheels at this maximum weight transfer point.
- Of course all this goes out of the window in the wet. The best setup is to have a bias valve in the cockpit.
Unfortuately cars are dynamic and therefore no one set of rules apply to all situations threrefore it is not necessarily always right that if you increase braking upfront you have to decrease braking at the back - there is always an optimum rear brake size. The best thing to do is to suck it and see.
Interesting reading. I'm not sure what to think after having my PFC pads in for a day. Just driving around the streets I am amazed at how quickly they pull the car up. I'm looking forward to this Sunday at the track with expectations and some trepidation as I know these pads will demand that I brake later as my old EBC's were pitifull by comparison. In fact last time at this track a friend in a n/a 944 s2 who is a semi-pro remarked that I was braking too early on some corners. I found this hard to believe as I in fact thought the opposite but now I am curious as to how deep I will be able to go. Also having the new KW's in will be great. So far after one late night drive I am convinced that I will be taking strips off my last times but I will have to revise my technique appreciably. I will definitely be left foot / trail braking on some corners so I'll report back soon.
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