>>that drums would not be up to the job on the rear.
I struggle to see how you've arrived at that "obvious" conclusion. Cars much larger, much heavier, and much longer have been more than adequately equipped with drum brakes.
If the rate of wear of the rear brakes is so high, then, I suspect Fiat have used either a very small pad area, or, are using a material which wears quickly. The rear brakes on your car will not be doing much actual work - they can't be, it's a physical impossibility!
>>well-balanced.
The only imbalance in braking you might feel as a driver is left / right - you can't feel the front/rear distribution.
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The editing time out caught and chopped my post up - here's the gist of the rest of it.
I have written about road testing a mkII Cavalier with the front brakes isolated for the purposes of identifying the source of a noise. The meagre braking offered by the rear wheels alone was truly frightening.
Next time you are in an MOT station, watch the gauges on the brake tester, and compare the readings front to rear.
(Please can the editing / posting times be extended by a little? I know I'm not the only one who gets caught out by it)
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OK, Number Cruncher, all I can say, without scientific evidence, is that the brakes work very well and leave me feeling safer on these little cars than anything else I have driven.
There are probably engineering isues on the 4x4 rear suspension that have led Fiat to fit discs rather than drums. Otherwise, the performance of discs in off-road conditions has been a factor.
There's no doubt that the rear discs have a smaller pad area than the fronts. Probably a sensible decision to prevent corrosion on the rear discs.
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Yes, I can imagine that the better self cleaning of a disc is of help in a vehicle designed for off-road conditions.
There isn't any engineering clash between driving the rear wheel and having a drum there per se.
You'll find that the pistons in the rear calipers are smaller than the fronts, and as the area is proportional to the diameter squared, the force they produce for a given hydraulic pressure is quite a lot lower. The area of the pad doesn't affect the brake force itself, but does affect how the pad responds to temperature, and, of course, how quickly the pad wears - in designing the pad, there is usually a maximum power (friction force x rubbing velocity) per unit area for each material which governs the sizing.
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If you want to test Number Cruncher's argument, simply try stopping the car (obviously not on a public road, officer ;-) ) from about 30 miles per hour just by using the handbrake.
I'm sure NC will provide a more detailed and scientifically correct reasoning but AFAIK it's simply the laws of physics in action. Front brakes go on which transfers weight from front to rear, hence the reason why older cars without ABS lock up the rear wheels under hard braking. Simplistic I know but science was never my strong point at school!
Doctorchris, I suspect it's more likely that bigger pads simply are not needed for the reason stated above.
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>>why older cars without ABS lock up the rear wheels under hard braking.
Cars, at least passably modern cars, should not do that, even without ABS. It has been designed against for many years via making the rear brakes weak and/or limiting the pressure appled to them.
For an old example on all drum braked cars, like a Morris Minor, the fronts would be twin leading shoe, while the rear would be leading and trailing. Owing to the self servo effect, and larger wheel cylinder pistons, the twin leading front brakes would provide much more braking force than the rear.
I *think* the ECE regs to prevent rear locking became much stronger in about 1983, at about the same time dual braking systems became the norm. British C & U regs have always said something along the lines of "rear wheel locking should not occur" or something similarly vague, while the ECE regs set out limits for so-called "adhesion utilisation". A car for a British market could get away with very poor rear brakes, while for use in Europe, the rear brakes have to be effective, but still not lock.
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You are of course correct NC. What I should have said was that older vehicles are more *prone* to it.
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You should also have said 'brakes go on which transfers weight from rear to front' - anything else would feel most peculiar ...
Edited by Andrew-T on 18/06/2009 at 10:40
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To go back to the original question, cars equipped with Electronic Stability Control can use the the rear brakes to help keep the car going in the intended direction.
I would think that a car towing a correctly balanced load could make some extra use of the rear brakes as would an heavily loaded car. On hydraulic suspended Citroens the rear brakes are fed off the rear suspension cylinders so that the more heavily the boot is loaded the more force the rear brakes have. As Brompt. noted such Citroens have a powerful parking brake that works on the front wheel discs, the parking brake on my Xantia was so powerful I could stop reasonably well from 30 mph using that alone.
Steve
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>>I would think that a car towing a correctly balanced load could make some extra use of the rear brakes as would an heavily loaded car
Yes - cars which are designed to carry varying loads, like estate cars, and car derived vans tend to have load sensing valves fitted to the rear axle which allow more rear brake pressure when there is more load on the rear axle.
You're also absolutely right to point out that a trailer will add more load to the rear axle of the towing car during braking, again, owing to forwards weight transfer. On a vehicle with a load sensing valve, or the Citroen equivalent, this may allow more pressure to the rear brakes.
Despite having Citroenesque rear suspension, my W124 doesn't have the interconnection between the suspension hydraulics and the brakes.
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Some cars have, or had, suspension geometry that influenced attitudes under braking. I am thinking in particular of the Citroen DS, but also the 2CV and its variants. The front wheels were on 'leading arms' and the rear ones on trailing arms. Under braking, therefore, there was a tendency for the nose of the car to lift and the tail to drop - the opposite of what usually happens. It counteracted to some extent the weight transference under braking and kept the car more level than it might have been with more conventional suspension.
Linkage, either hydraulic or (as with later 2CV variants) mechanical, between the front and rear suspension, was supposed to refine these effects further.
All those cars were given to adopting extreme attitudes in cornering. But they had incredibly good brakes for their day, needed for French-style press-on driving.
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>>it counteracted to some extent the weight transference under braking
Not really. The weight transfer was (virtually) all still happening. What was different was the body's response to the weight transfer.
Weight transfer would still happen if the suspension were absolutely rigid and the body didn't pitch at all. Many confuse the two things as being the same.
I say virtually all the weight transfer was still happening, there is a tiny component of the weight transfer that is prevented by this mechanism. As the body pitches under braking, the centre of gravity of the sprung mass can move forward by a very small amount, depending upon the height difference between the centre of gravity of the sprung mass and the centre of the body's rotation under pitch. All but the most pedantic may ignore this effect completely.
No matter what you do with the suspension, you can't dodge weight transfer, whether rear to front during braking, or, side to side during cornering.
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Thank you NC. The academic filling in my enthusiast's pie. I should have said 'it counteracted to some extent the EFFECT of weight transference under braking'.
However my memory is that the tail of a DS would stay relatively low under very heavy braking, and that must be an effect of the suspension design.
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>>However my memory is that the tail of a DS would stay relatively low under very heavy braking, and that must be an effect of the suspension design.
Yes, that's right, you can acheive similar effects by setting the inner pivots for wishbones at different heights, forming a virtual leading or trailing arm - where the lines produced forwards or backwards from upper and lower wishbone inner pivots cross in side view is the pivot of the virtual leading/traling arm.
On the railways you can see similar on BX1 bogies where the load from the brake pads feeds into the bogie structure rather than the hub. The rear of the bogie sits down, while the front rises up. The odd feature of this layout is that the bogie's primary suspension is shorted out while the brakes are on - which puts very high loads into the brake pad support structure.
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On the subject of brakes, here's a question for NumberCruncher.
Is there a reason why calipers are sometimes fitted on the leading end of a disc brake or the trailing edge or is it simply dependent on the cars suspension and design?
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Fitting a caliper to the rear of a disc means that the braking force when reacted at the wheel bearing subtracts from the bearing load caused by the car's mass. This means that the bearing can be sized smaller, which means that the hub can be made smaller, which means that the springs and dampers can be made softer while still controlling the suspension adequately. It's a good thing, and is why you tend to see it on cars like Porsches.
It can clash with the desire to have the steering linkage behind the front suspension, with the rack mounted on the bulkhead, rather than having a forward or a high mounted rack.
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That's fascinating NC. I would have sworn the position of the caliper made no difference. You learn stuff every day (well, I do anyway).
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Thanks NC. The more I learn about cars, the more intricate and amazing all that engineering seems.
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>>the more intricate and amazing all that engineering seems.
It's the same for me. The more I look into it, the more I realise I don't understand! - phrased another way, it would be nice if more of my designs worked and passed their qualification tests first time!
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Do modern braking systems know when the car is travelling backwards ?
Having spun a car on an airfield - and then travelling backwards rather rapidly, the brakes were pretty useless - the ABS just kept cutting in.
I got the impression that the weak rear brakes would try stopping the car, the 'strong' front brakes would grab - and lock almost instantly. ABS would take the pressure off to get the wheels moving. And so, very little braking effort in reverse.
Haven't managed to spin my motorcycle yet - if I ever do, I hope to remember the back brake by itself :-)
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>>Do modern braking systems know when the car is travelling backwards ?
Without an electrical signal from the reversing light switch, the ABS controller doesn't know - the wheel sensors only produce a waveform, the frequency of which is proprtional to speed in either direction of rotation.
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