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The other thing with older cars is lack of power brakes. Steep learning curve in my first Mini (L/1973) was just how much push was needed particulalry laden, I'd previously driven Mum's Renault 5.
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> While it's true that modern cars have wider tyres, they're also much heavier so need more grip.
This thing about wider tyres increasing grip: I seem to remember from A-level maths in the 1980s that friction was a function of the coefficient of friction (for that combination of surfaces) and the reactive force (equivalent to weight on a level surface; weight modified by gradient otherwise) and that the contact area has nothing to do with it. In other words, a car with tyres 100mm wide will produce the same amount of friction as the same one with tyres 400mm wide, less a small allowance for the extra weight of the bigger tyres and wheels.
I can see an argument that wider tyres will provide better contact as the vehicle leans over in a corner, but to argue that wider tyres improve braking performance has to be un-Newtonian. NumberCruncher - are you out there?
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"un-Newtonian"
Tyres are, especially near the limits of adhesion. Otherwise (as you suggest) there would be no point in having wider ones.
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"..the disco distance even made clarkson go quiet!"
Not all bad, then!
I remember annoying our physics teacher with questions about the grip of tyres (and the ability, even then, to exceed 1g) which is not what the classical equations allow, but then they are a special case. The alleged 5g braking of F1 cars is pretty impressive, though.
I assume 4x4's brake badly because of their high centres of gravity (thus transferring more weight forwards) and the fact that their extra weight is not offset by bigger discs. Are there other reasons? Their braking performance does seem to be surprisingly poor, but presumably still better than large trucks, which I suppose would just rip up the tarmac if they had enough grip...
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I doubt if anyone will argue with the view that those of us who grew up with drum brakes were more than happy to see the arrival of (front at least) discs. It was and is a no-brainer. Discs are considerably cheaper to make, stand up to high temperatures much better and are intrinsically self-adjusting. No question of an awful juddering groan as the sharp nose of an ill-adjusted shoe meets a drum not perhaps clean or in the best of shape...
However good drum brakes in good fettle, as big as possible and finned for heat dissipation, were certainly extremely effective, and in the best-designed forms were relatively resistant to fade. But they did need proper attention, not just new shoes slapped in by a chimpanzee (I include myself here I hasten to add).
On this question of the light weight of early cars, I once had the pleasure of a ride on the wooden boards over the chains in a pretty simple and basic chain drive Frazer Nash. This was a spidery vehicle that can't have weighed a ton. Its tiny drum brakes, cable operated on all four I think, were absolutely amazingly effective. But the driver knew the car inside out and wasn't going at all fast.
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I pretty well agree with what Lud said-no one on this planet can sensibly argue that drum brakes are better than disc brakes. Of course drums can be perfectly adequate, but not better.
As far as I'm aware Emergency Brake Assist detects when a driver is performing an emergency stop (speed and pressure of brake application maybe?) and then if the driver isn't braking hard enough, will engage full brakes even if the driver isn't pressing the pedal as hard as possible. Apparently ABS brakes give some people the willies and people either don't press the pedal hard enough or sometimes even release when they feel the pulsing back through the pedal and the ABS noise.
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I pretty well agree with what Lud said-no one on this planet can sensibly argue that drum brakes are better than disc brakes.
Unless you're talking about the ability to do handbrake turns (where rear drum brakes work infinitely better than discs), I agree.
Servo assistance levels make a huge difference to the perception of braking performance as well. I recall driving a Lotus Elise (which of course has completely unassisted braking) and being terrified when I brushed the pedal as I would in a family wagon and precisely nothing happened. Yet once familiarity kicked in, these were among the best brakes of any car I've ever driven. In contrast, a typical modern family car (from VW, Renault or Peugeot particularly) tends to have brakes which can stand the car on its nose at the merest hint of pedal pressure thanks to a high level of servo assistance. Yet I've heard so many people say "those brakes are sharp, so they're better" when in fact it's all artificial and nothing to do with the quality and configuration of the braking hardware as such.
Going from our Scenic to our Mondeo is a classic case in point. The Mondeo's brakes are very good in that they have a firm but progressive pedal, unobtrusive ABS and consistent, fade free response even when fully laden on a hot day. However, they have a much lower level of servo assistance than the Scenic's, meaning greater pedal pressure is required to achieve the same result. SWMBO thinks the Renault's brakes are better because they "feel more powerful". I prefer the Mondeo's because they have more progression and feel. But the Renault stops quicker and for less effort.
I always wonder what is the definition or criteria of "better" brakes?
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Drum brakes;
Can be very powerful in terms of how much braking torque they can produce. They have a great advantage over disc brakes that they are very suitable for parking brakes because the drum cools and contracts, which applies the brake harder.
If drum brakes were so poor, trucks would have been fitted with disc brakes for many years - in fact, disc braked large trucks are a relatively new phenomenon, and many heavy trucks still use drum brakes.
Drum brakes rely, to an extent, on self servo action - where the rotation pulls the brake on harder (for a leading shoe). This is in contrast to a disc brake where the contact force between pad and disc is governed only by fluid pressure and piston size. As the friction material in a drum brake heats up, its co-efficient of friction drops - this means that not only does the brake produce less torque, but also less self-servo action, hence less contact force - a double whammy! Allied to poorer heat dissipation owing to the enclosed drum, this means that drum brakes are more susceptible to sudden brake fade.
Disc brakes:
Do not rely on self servo action, and hence need complex and expensive servos to enable braking force to be produced for reasonable pedal effort. Discs are more open to the airflow, and hence offer better heat dissipation.
Disc brakes are awful for parking brakes because the cooling disc contracts away from the pads.
Brake performance;
For a given tyre to road coefficient of friction, the best way to optimise brake performance is to make sure that each wheel is doing a fair share of the braking. This is acheived by having even front/rear weight distribution, a long wheelbase, and a low centre of gravity, which reduces forward weight transfer under braking to a minimum.
Under (most) conditions of available tyre to road friction (or, so-called adhesion utilisation), the rear brakes should not lock* - this is a legal requirement, both under British Construction and Use regs, and under European regulations, and as a result, various pressure reducing or load sensing valves are used on vehicles to prevent rear wheels locking.
* This is why HJ is wrong to moan that the rear wheels of the car in the speed advert didn't lock - they shouldn't lock, even in a pre-ABS car. Getting the fronts to lock is usually easy, but the rears should always roll, to allow the car to remain controllable.
For many front wheel drive small cars, the weight distribution is already biased to the front. The short wheelbase means that under braking even more weight is on the front wheels. The rear wheels have very little braking to do, and the fluid pressure applied to them is likely to be sharply reduced. In this case, even descending an Alpine pass would be unlikely to put much heat into them!
Fitting drum brakes to the rear of many cars is therefore the *right* engineering solution. You get an effective and safe handbrake, and a safe braking system with disc brakes on the front, where they are needed. On these applications, rear disc brakes can be a bad solution, and can cause more problems than they solve.
One engineering aspect that did drive rear disc brakes onto otherwise unsuitable cars was that it is easier to implement ABS on a disc brake, because the threshold pressure and dead travel of the pistons is much reduced. However, modern ABS controllers can overcome these little niggles easily.
Tyre Friction;
The area independence of friction is correct for non-conforming solids. For these, the mechanics of contact are governed by local contact between asperities. As such, the area of contact between asperities is always much lower than the apparent contact area. For tyres, the rubber does conform to road surface irregularities, and so, there is an advantage to using more area. However, if we temporarily neglect the stiffening effect of the sidewalls, increasing tyre size may not increase contact area, unless you also reduce tyre pressure (in this context, think of tyres as effectively thick balloons, and the vertical load on the tyre is balanced by the product of the air pressure and the contact area - changing the ballon for a larger one, keeping the air pressure the same will not change the contact area)
Number_Cruncher
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As I'm old and gray, I had the privilege and fun of driving lots of cars with drum brakes. Yes: they do fade. yes: older designs were difficlut to maintain. Yes: they are not as good as a well set up all disc or disc/drum setup.
And yet remember that old cars were generally heavier and had far less power than today's cars size for size. For example a Triumph TR3 sportscar (begore my time when new) had 100bhp from a 2 litre engine and front discs and rear drums, but its predecessor had 90(?95?) bhp and all drums... You can forhet 0-60 in under 10 seconds... so brakes were not that stretched EXCEPT on long downhill stretches.
I had a Rover 75 with all drums: super brakes: but in the Highlands going downhill I managed to get severe brake fade and the smell of hot drums and linings still flashes into my mind. Brake fade is trully scary if it's your first time.
(and by smell I mean smoke and smell: quite spectacular.. fortunately the brake fluid had been recently replaced and the linings were new so the fluid did not boil and when cool all was back to normal..
And early 1960s Minis were all drums.. and - when adjusted properly - quite effective.
ABS and Brake Assist are brilliant: especially in the wet.. which is usually when muppets crash into the backs of other cars - or go off the road totally as one car did last week near us... Modern tyre compounds may be good but wet raods and mud or rubber build up can extend stopping distances - discs or drums is irrelevant.
madf
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Good points made above..I will accept that drum brakes make for better hand brake turns, and parking brakes in general.
I prefer very sensitive brakes over the heavier stuff-I drove a new Megane a while back and you only have to sneeze at the pedal and the brakes fly on-I quite liked this. My current drive is an 01 Primera and the brakes on that are pretty good, but the pedal requires quite a hard shove.
I agree that to the uneducated (and by education I'm not meaning degrees in engineering etc, I simply mean an understanding of these matters) that the car with the lighter brake pedal/most servo assistance, will be perceived as having the 'better' brakes. Which may or may not be the case depending on the weights of the vehicles etc.
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I agree with component part. The new Megane's brakes are seriously powerful, you can lose speed with terrific ease. The problem is whether the car behind can do so as rapidly... Before you get used to them in slow moving traffic it is possible to inflict some neck jerking on passengers. I have not experienced such strong brakes in an 'everyday' car since a Volvo 240.
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"The new Megane's brakes are seriously powerful"
The French cars I've owned and driven (including the 2CV) have all had good brakes. Maybe it's an attempt to keep their native owners out of trouble...
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The French cars I've owned and driven (including the 2CV) have all had good brakes.
Yes.
The reason is that the French are not natural mimsers (unlike some nationalities I could mention).
Their cars were made with decent brakes because it was recognised that they would need them. Ours weren't until recently, because no one thought it necessary. It probably still isn't.
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"Disc brakes are awful for parking brakes because the cooling disc contracts away from the pads."
The early Citroen Xantias had a habit of releasing their own handbrakes for that reason (causing André Citroen to rotate in his grave, no doubt) - I saw one in a carpark that had executed a gentle curve from its slightly uphill position into the back of a car parked a few yards away, at right angles to it. Oops!
I blame the Peugeot influence - the GS, which had inboard front discs the size of dinner plates, had separate calipers and pads for parking, and because they were on the front wheels, you could use them to slow down quite effectively, too.
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"Fitting drum brakes to the rear of many cars is therefore the *right* engineering solution."
Agreed. I've had to replace the rear calipers on two cars, not because of a problem with the hydraulics, but because the mechanical fudge required to make it double as a parking brake gave up!
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