Brake Balancing
Discussion
I accept that ABS is at the moment the safest braking system but that aside if a car had equal size brake discs and callipers would a car handle better under most hard braking manoevres if front and back brakes were adjusted to give an equal amount of braking.
I understand that under normal braking the front brakes do 80% of the stopping and so weight transfer comes into it.
When racing cars have brake balancing done by the driver as he is racing does this mean left and right or front and back.
If weight tranfer and so front wheel braking is a deffinate advantage then why does`nt a car manufacturer design a car with front wheel drive and bigger wheels on the front rather than the back.
And finally if 80% braking on the front and 20% braking on the back is the best way of braking surely dics brakes on the rear of a average car are unnessesary, unless its for cost or weight reasons.
I understand that under normal braking the front brakes do 80% of the stopping and so weight transfer comes into it.
When racing cars have brake balancing done by the driver as he is racing does this mean left and right or front and back.
If weight tranfer and so front wheel braking is a deffinate advantage then why does`nt a car manufacturer design a car with front wheel drive and bigger wheels on the front rather than the back.
And finally if 80% braking on the front and 20% braking on the back is the best way of braking surely dics brakes on the rear of a average car are unnessesary, unless its for cost or weight reasons.
The optimum setup which all manufacturers aim to achieve is where the amount of braking on each wheel is proportional to the grip on that wheel. To all intents and purposes, the amount of grip on each wheel is proportional to the weight on that wheel, so the system has to vary the distribition of braking effort as the weight moves around under braking. This is normally achieved using a bias valve in the rear brake circuit which restricts the amount of braking applied to the rear wheels under heavy braking.
The reason that you will see racing drivers fiddling with the brake balance is that the weight distribution and the condition of the tyres both vary during the race, so the ideal brake balance is a moving target.
The reason that you will see racing drivers fiddling with the brake balance is that the weight distribution and the condition of the tyres both vary during the race, so the ideal brake balance is a moving target.
At rest, cars obviously have their own weight distribution dependant on where the major masses are placed in the car. A Lotus Elise is about 60% over the rear, BMWs are about 50/50 (the rear diff weighs quite a bit and balances things out), and a standard saloon has most of its weight over the front (I've no idea of the exact figure - maybe 65% front, 35% rear?).
When a car slows down, the weight distribution of the car will move forwards (as will your luggage!
), leaning more on the front than on the back (imagine a motorbike braking hard, you can sometimes see the rear wheel start to lift off the ground). The weight pushing down on the road at the front gives more grip (imagine trying to push a heavy wardrobe vs a lighter one), and therefore more braking can be applied to that end. So, under braking, a normal FWD saloon that has a static weight dist of 60% over the front will have up to 80 or 90% over the front, thus the brakes at the front are set to brake much more effectively than the ones at the rear. Out of interest, a Porsche 911 has about 50/50 weight distribution under braking! (I did some interesting simulation on the physics of all this about a year ago with a friend of mine).
Under acceleration, the opposite happens, so the rear squats and the front goes light. It can therefore be seen that an ideal car should put most of its drive to the rear if it wants to accelerate quickly - this is one reason why most high performance cars are rear wheel drive (there are many other reasons, mainly related to cornering). Front wheel drive is certainly not ideal in a car under acceleration as all the drive goes to the end with the least grip!!!
With regard to your question about racing car brake bias adjusters, this is a front/rear bias adjuster and can be changed throughout the race to suit the changing conditions (for instance, in the rain, you can't brake as hard so there is less weight transfer and you can account for this by winding the bias to the back a bit).
You asked about disc brakes on cars. A lot of cars don't really need disc brakes at the rear, you are quite right. The Toyota Avensis was fitted with drums at the back right up to the recent redesign a couple of years ago. Most cars in the 1980s operated discs at the front, and drums at the back. Nowadays, with increased grip available from tyres and various other reasons, braking has improved to the point where a smaller disc on the rear can be used to great effect. (there is also some marketing involved here!)
Returning to my discussion of weight transfer in the car, this obviously affects the cornering as well. If you decelerate in a bend, the front will get more grip than the rear and you will start to oversteer; and if you accelerate the opposite will happen and you will understeer (or tend to these traits, on the road I should hope they never actually happen). Weight transfer is the primary affector during high speed cornering on a race track, and this puts motor racing in the group of 'carving sports', where balance is critical (most racing drivers have an uncanny sense of balance - there is one rally driver who used to be a gymnast, and he is simply superb at it!). Obviously, other physics factors play a part in how a car corners, such as power application or brake application, but at high speed it is mainly weight transfer. I was amazed on a recent thread ("braking whilst changing down"
, at some people's lack of knowledge of this area of car physics and handling - some were even IAM instructors!!! Understanding weight transfer will make you a much safer driver, though of course I should point out that on the road anticipation and planning are far more important!
When a car slows down, the weight distribution of the car will move forwards (as will your luggage!
), leaning more on the front than on the back (imagine a motorbike braking hard, you can sometimes see the rear wheel start to lift off the ground). The weight pushing down on the road at the front gives more grip (imagine trying to push a heavy wardrobe vs a lighter one), and therefore more braking can be applied to that end. So, under braking, a normal FWD saloon that has a static weight dist of 60% over the front will have up to 80 or 90% over the front, thus the brakes at the front are set to brake much more effectively than the ones at the rear. Out of interest, a Porsche 911 has about 50/50 weight distribution under braking! (I did some interesting simulation on the physics of all this about a year ago with a friend of mine). Under acceleration, the opposite happens, so the rear squats and the front goes light. It can therefore be seen that an ideal car should put most of its drive to the rear if it wants to accelerate quickly - this is one reason why most high performance cars are rear wheel drive (there are many other reasons, mainly related to cornering). Front wheel drive is certainly not ideal in a car under acceleration as all the drive goes to the end with the least grip!!!
With regard to your question about racing car brake bias adjusters, this is a front/rear bias adjuster and can be changed throughout the race to suit the changing conditions (for instance, in the rain, you can't brake as hard so there is less weight transfer and you can account for this by winding the bias to the back a bit).
You asked about disc brakes on cars. A lot of cars don't really need disc brakes at the rear, you are quite right. The Toyota Avensis was fitted with drums at the back right up to the recent redesign a couple of years ago. Most cars in the 1980s operated discs at the front, and drums at the back. Nowadays, with increased grip available from tyres and various other reasons, braking has improved to the point where a smaller disc on the rear can be used to great effect. (there is also some marketing involved here!)
Returning to my discussion of weight transfer in the car, this obviously affects the cornering as well. If you decelerate in a bend, the front will get more grip than the rear and you will start to oversteer; and if you accelerate the opposite will happen and you will understeer (or tend to these traits, on the road I should hope they never actually happen). Weight transfer is the primary affector during high speed cornering on a race track, and this puts motor racing in the group of 'carving sports', where balance is critical (most racing drivers have an uncanny sense of balance - there is one rally driver who used to be a gymnast, and he is simply superb at it!). Obviously, other physics factors play a part in how a car corners, such as power application or brake application, but at high speed it is mainly weight transfer. I was amazed on a recent thread ("braking whilst changing down"
, at some people's lack of knowledge of this area of car physics and handling - some were even IAM instructors!!! Understanding weight transfer will make you a much safer driver, though of course I should point out that on the road anticipation and planning are far more important!Sorry, I forgot to answer your original question: If a car's brakes were adjusted to be 50/50 then you'd reach the limit of lockup at the rear and still have a lot of grip left at the front, therefore you wouldn't be able to stop as quickly.
An experiment for you: If you have a push bike, try different braking techniques on that. I once measured the braking distance using the rear brakes only, and then using the front brakes only. The results were in the ratio 80:20 (rear:front) I think.
It may also interest you to know that a tyre produces its maximum grip with a slight amount of slip (this is about 4% for a modern racing tyre), so the best ABS systems will allow the wheels to slow down so they're rotating a little slower than the car's forward speed. This is why in days of old with simpler ABS systems a good racing driver threshold braking (braking on the limit) could beat an ABS system. I don't know if this is still the case. The thing that a racing driver will never be able to do is modulate brakes seperately to all four wheels, as he only has one brake pedal. This is what ABS does and it gives it a big advantage over any skilled driver in an emergency (especially seeing as you often have the left hand wheels on the dirty edge of the road where you can't brake as well).
An experiment for you: If you have a push bike, try different braking techniques on that. I once measured the braking distance using the rear brakes only, and then using the front brakes only. The results were in the ratio 80:20 (rear:front) I think.
It may also interest you to know that a tyre produces its maximum grip with a slight amount of slip (this is about 4% for a modern racing tyre), so the best ABS systems will allow the wheels to slow down so they're rotating a little slower than the car's forward speed. This is why in days of old with simpler ABS systems a good racing driver threshold braking (braking on the limit) could beat an ABS system. I don't know if this is still the case. The thing that a racing driver will never be able to do is modulate brakes seperately to all four wheels, as he only has one brake pedal. This is what ABS does and it gives it a big advantage over any skilled driver in an emergency (especially seeing as you often have the left hand wheels on the dirty edge of the road where you can't brake as well).
ABS would still always beat the driver in the wet though, even back when it was a new technology.
Also look at the rear brakes on some new cars and the rear discs are larger than the front (BMW M3 for example) I believe this is to do with the traction control systems, which uses brief touches to the brake to keep the wheel from spinning excessively. So on a RWD car it will probably be used a lot so it needs extra beef to cope with that.
Also, maybe slightly OT, but do any of the ABS/Traction control or Stability control systems brake the inside tyres more than the outside when going around a corner? If done correctly this could help turn in and cornering. Though the Mitsubishi Active Yaw Control that sends more power to the outside wheels is probably more effective since its not slowing the car down, it can only be used on AWD cars.
Also look at the rear brakes on some new cars and the rear discs are larger than the front (BMW M3 for example) I believe this is to do with the traction control systems, which uses brief touches to the brake to keep the wheel from spinning excessively. So on a RWD car it will probably be used a lot
so it needs extra beef to cope with that. Also, maybe slightly OT, but do any of the ABS/Traction control or Stability control systems brake the inside tyres more than the outside when going around a corner? If done correctly this could help turn in and cornering. Though the Mitsubishi Active Yaw Control that sends more power to the outside wheels is probably more effective since its not slowing the car down, it can only be used on AWD cars.
There's a further factor why, on a standard road car, the back wheels do very little braking and that's public liability. Manufacturers have learned from experience that if the back wheels lock in an emergency braking manoeuvre then Mr Average is likely to spin, lose control and crash. And sue said manufacturer for squillions of damages (especially if he is a citizen of the good old USofA).
As a result, manufacturers make sure that the first axle to lock up is the front. Of course, this pitches the car into terminal understeer but, somehow, this is deemed more acceptable than spinning. I suppose that the remedy is easier, in that lifting off the brakes restores steering control.
I once attended a course where this issue was vividly demonstrated on a car with the ABS disabled. By using the handbrake at the same time as the foot brake, thereby increasing the braking effort at the rear, the overall stopping distance was reduced by about a third.
These days a good ABS system should make all this irrelevant but I suspect that manufacturers continue to set the underlying brake balance heavily to the front.
As a result, manufacturers make sure that the first axle to lock up is the front. Of course, this pitches the car into terminal understeer but, somehow, this is deemed more acceptable than spinning. I suppose that the remedy is easier, in that lifting off the brakes restores steering control.
I once attended a course where this issue was vividly demonstrated on a car with the ABS disabled. By using the handbrake at the same time as the foot brake, thereby increasing the braking effort at the rear, the overall stopping distance was reduced by about a third.
These days a good ABS system should make all this irrelevant but I suspect that manufacturers continue to set the underlying brake balance heavily to the front.
r988 said:
ABS would still always beat the driver in the wet though, even back when it was a new technology.
True, the transition from static to dynamic friction is more sudden, and the ratio between the two is bigger, so it is therefore much harder to threshold brake, but I see no reason why a talented driver couldn't beat a rudementary ABS system in the wet, just as he/she would in the dry - he/she would just need more ability. In practice of course, my money's on the ABS system 90% of the time!
According to the car's handbook, I can turn the ABS off on my road car (a BMW 330) - I've never tried it so I can't verify this. Whilst I'm not the best racing driver out there, it might be interesting to try a braking test or two sometime to compare ABS with non ABS. I should imagine it would be quite hard to threshold brake accurately in a big cosseting coupe.
r988 said:
Also look at the rear brakes on some new cars and the rear discs are larger than the front (BMW M3 for example) I believe this is to do with the traction control systems, which uses brief touches to the brake to keep the wheel from spinning excessively. So on a RWD car it will probably be used a lot
I've only driven my BMW 330 on the road, so I haven't had the opportunity to play too much, but I'm not sure I like the sensation of this, if indeed it is this I'm feeling. When the rear loses grip under power in a tight turn with the DSC turned off (but diff braking still enabled), there is a strange dragging sensation at the rear that is quite different to no LSD or LSD equipped cars. According to the handbook, in this first 'DSC off' mode there is nothing working other than ABS and differential rear braking.
gdaybruce - a very interesting post! I'm fascinated that cars are set up to lock the fronts up first to *such an extent* that you can use the handbrake as well to decrease stopping distances by that much. Sounds like a good demo that chap did on that course, it sticks in the mind! Presumably this isn't the case nowadays with ABS?
Personally (and I'm sure many others will agree), I find that the scariest thing in a car is losing the front end. The thought of ploughing towards something without any steering control is very scary indeed. If the rears are locking first, then at least you retain some control! I feel the same about handling balance in a bend - for me understeer is scary on the public road (oncoming trucks!), and oversteer less so.
I'm thinking of buying an Elise S2 111S, and those skinny front tyres are already giving me nightmares!
I'm fascinated that cars are set up to lock the fronts up first to *such an extent* that you can use the handbrake as well to decrease stopping distances by that much. Sounds like a good demo that chap did on that course, it sticks in the mind! Presumably this isn't the case nowadays with ABS? Personally (and I'm sure many others will agree), I find that the scariest thing in a car is losing the front end. The thought of ploughing towards something without any steering control is very scary indeed. If the rears are locking first, then at least you retain some control! I feel the same about handling balance in a bend - for me understeer is scary on the public road (oncoming trucks!), and oversteer less so.
I'm thinking of buying an Elise S2 111S, and those skinny front tyres are already giving me nightmares!
Rob, The man doing the demonstration was Pentti Airikkala, the Finnish rally champion and the car was a Fiesta Turbo. He demonstrated and then we students had a go. I guess the course was about 10 years ago. I remember thinking that it worked well on an airfield but in a real emergency I could never see myself letting go of the wheel and reaching for the handbrake. Still, it made a point.
I've never driven an Elise (but I do have an ambition to own one!) but from all the press accounts I don't think narrow front tyres are an issue. The Elise is very light at the front and in the wet narrow tyres can be a positive advantage, reducing the tendency to aquaplane. Bit like the skinny tyres rally cars use in the snow.
I've never driven an Elise (but I do have an ambition to own one!) but from all the press accounts I don't think narrow front tyres are an issue. The Elise is very light at the front and in the wet narrow tyres can be a positive advantage, reducing the tendency to aquaplane. Bit like the skinny tyres rally cars use in the snow.
One of the problems with ABS and TC is that the 'optimum' amount of slip depends on the tyre and road conditions and whether you are trying to steer as well as de/accelerate. ABS systems have to be fairly conservative on the slip so they work well and keep the car stable in slippery conditions. This may mean they underperform under very grippy conditions. There are definite advantages to these driving aids but there sre still definite disadvantages. The biggest disadvantage of these ABS systems in my opinion is that they prevent you from deliberately locking the wheels, which in some (admittedly rare) situations is the best option.
gdaybruce said:
I've never driven an Elise (but I do have an ambition to own one!) but from all the press accounts I don't think narrow front tyres are an issue. The Elise is very light at the front and in the wet narrow tyres can be a positive advantage, reducing the tendency to aquaplane. Bit like the skinny tyres rally cars use in the snow.
I recently test drove an S2 111S and was blown away by it. Fantastic! Interestingly, the dealer suggested that I tried an S2 160 for comparison, and it wasn't anywhere near as good, so do try several - they are sensitive to tyres pressures and tracking (the latter was slightly off on the S2 160 that I drove). The Elise isn't fast, and never will be (150bhp/tonne on the standard one, which is the same as an E36 325i!!!, even the 'bonkers' 240R Exige only has 270bhp/tonne, whcih is 50 short of Caterham's entry level R car, the R300); but Elises have the most amazing steering feel and feel through the chassis you'll ever experience - fantastic! I love them to bits and want one!
The word on the PH Elise forum here is that those skinny front tyres do have a tendancy to lock up under braking, and also cause understeer. I've only ever driven an Elise carefully on the public road, so I can't comment on either the locking up or the understeer. Lotus will sell you wider front tyres (like with the S1), which apparently cure the problem (Lotus admit to this problem, but say that the understeer is better for the type of drivers they are attracting now).
gdaybruce said:
I once attended a course where this issue was vividly demonstrated on a car with the ABS disabled. By using the handbrake at the same time as the foot brake, thereby increasing the braking effort at the rear, the overall stopping distance was reduced by about a third.
This is what i was thinking.
Now stay with me on this one, it might sound a bit redical.
If the speed limits 50 years ago were for the average car with average stopping distances, lets say a Ford Pop and may be 100 feet at 30 mph then stopping distances have improved.
The highway code says 30 feet plus 45 feet making it 75 feet so instead of saying the speed limit is 30mph why don`t they say a 75 feet stopping distance.
This way manufacturer might improve 60 mph to 0 instead of 0 to 60 mph times and so distances.
Of course this might have to include a new test as part of the MOT just to prove that your car will stop quickly.
This way you could do 45mph as long as your car could according to munufacturers spacifications stop within 75 feet.
Stopping distance from 70 mph is 315 feet so beef your brakes up and drive faster on the motorway.
No i don`t think it would get accepted either but nice to dream.
One of the advantages of track days is that you get the chance to brake as hard as possible from high speed and to see what happens. Very few drivers ever do an emergency stop from 70+ until they have to and therefore don't realise just how long it takes and how scary it can be! Even track days are misleading since track surfaces are often high grip, all the traffic is going the same way and you don't brake to a standstill. Also, you're not simultaneously swerving to avoid the pile up! Many drivers will think that their brakes are in good order but stopping from high speed really shows up tired dampers, mismatched tyres, uneven tyre pressures and old, contaminated brake fluid in a way that stopping from 30 doesn't.
I reckon that if everyone had to try an emergency stop from 80 every so often, including in the wet, then gaps left between cars would increase and maintenance might improve! If only!
I reckon that if everyone had to try an emergency stop from 80 every so often, including in the wet, then gaps left between cars would increase and maintenance might improve! If only!
gdaybruce said:
One of the advantages of track days is that you get the chance to brake as hard as possible from high speed and to see what happens.
Next time you get chance do a little jink to the left then a jink to the right just as you slam the brakes on, only try this in the wet i might add.
Watch what happens, but don`t bottle out, keep you foot firmly planted.
Stick with it and you should complete a 360 or lift off at 180, but if you intend lifting at 180 get ready to be looking out your back window.
And for my next trick.....
leosayer said:
ABS would still always beat the driver in the wet though, even back when it was a new technology.
Not true, on all the ABS cars I've owned, I can always stop a good deal shorter if I avoid activating the ABS in the wet or dry.
Try it yourself,
I agree with leosayer (
sounds odd doesn't it?! ). I tried with and without ABS driving an (admittedly horrible) Astra. Without ABS in the wet produced shorter stopping distances. Interestingly, cadence braking whilst turning (to avoid an obstacle) was also more effective without the ABS. I can only put this down to a poor quality ABS system in this particular car but it goes to show that many people who have ABS are mistaken in their belief that it is the panacea of braking systems.
GreenV8S said:
Interestingly, cadence braking whilst turning (to avoid an obstacle) was also more effective without the ABS.
Surely cadence braking *with* ABS is a total wft?
yes, quite. What I meant was cadence braking with ABS turned off versus braking (normally) with ABS on.I think now we are getting to the point were the term "Driving by the seat of your pants " comes in.
When i took my car test 33 years ago nobody mentioned pumping the brake peddle if the brakes lock up, but when i took my HGV test 17 years ago they made a big point of it .
Now i would guess that with ABS being the norm even on the HGV test they don`t bother mentioning it.
What i have noticed when you brake that hard (lets say in the wet) that wheels start to lock up, you have to listen and "feel" which wheels are locked and which ones are turning.
If the back is trying to come round on you, the backs locked up.
If the steering is non existant, the front is locked up.
Have you ever expearienced a "tank slapper2 on a motor bike, I have at 90mph going into a curve when i backed off the throttle.
This is caused, so i am told because the frame becomes less rigid and so snakes what you are supposed to do is accelerate out of it to make the frame rigid and so stop the tank slapping motion.
If a simmilar principal is taking place when a four wheeled vehicle is braking then i just thought that rear wheel braking should be a bit more important than it seams to be.
Still its all part of the fun ar driving, I don`t think any of us want too many driving aids, do we?....
When i took my car test 33 years ago nobody mentioned pumping the brake peddle if the brakes lock up, but when i took my HGV test 17 years ago they made a big point of it .
Now i would guess that with ABS being the norm even on the HGV test they don`t bother mentioning it.
What i have noticed when you brake that hard (lets say in the wet) that wheels start to lock up, you have to listen and "feel" which wheels are locked and which ones are turning.
If the back is trying to come round on you, the backs locked up.
If the steering is non existant, the front is locked up.
Have you ever expearienced a "tank slapper2 on a motor bike, I have at 90mph going into a curve when i backed off the throttle.
This is caused, so i am told because the frame becomes less rigid and so snakes what you are supposed to do is accelerate out of it to make the frame rigid and so stop the tank slapping motion.
If a simmilar principal is taking place when a four wheeled vehicle is braking then i just thought that rear wheel braking should be a bit more important than it seams to be.
Still its all part of the fun ar driving, I don`t think any of us want too many driving aids, do we?....
I can't quite see how chassis flex would cause the tank slapper you experienced. It sounds more like weight transfer as mentioned earlier in this thread:
RobM77 said:
If you decelerate in a bend, the front will get more grip than the rear and you will start to oversteer; and if you accelerate the opposite will happen...
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