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Originally Posted by Jeroen So nice of you.
You’re looking at it from the wrong angle. It is not about disc over drums. It is purely about cost cutting.
I you would find two identical cars, one disc/drum and one disc/disc and you would find the disc/drum would have a larger stopping distance you can not conclude drum brakes are inferior and lead to larger stopping distance.
The appropriate conclusion is they fitted under dimensioned drum brakes! So they were cheap in the first place, to install drums, and then they notched up their cheapness by installing too small ones! It is not so much a technology or engineering or design issue, it is purely a cost cutting issue.
if they fitted really tiny disc at the rear they could perform less than the normal drums too.
On small light cars, rear drums can be perfectly adequate. No problems with my wife’s Ford Fiesta.
My 1974 Royal Enfield bullet has drum brakes, will not stop at all. Not a drum brake issue as such, but it is just impossible to install large enough drum brakes to get a comparable braking to disc brakes.
To put it in a different way: A disc brake gives superior braking performance compared to drum brake, when comparing the weight of the brake to brake performance. That is it single most important criteria. You get better stopping performance per kilogram of installed brake!
For the same weight near your wheel you can get far better braking performance. Or for less weight the same!
Everything else, is accounting.
Jeroen |
I guess we are finally converging towards a similar conclusion.
Lets say for the purposes of this debate we put target Braking distance from 100 - 0 at 40m
If a small hatch back given its light weight is able to stop within 40 m from 100 kmph with a rear drum then we are all happy.
Now i would like summarise a couple of points from your inputs :
1. Disc brakes provide more braking force per kg of brake installed. So for clarity, i would say in very layman terms and hypothetical figures that the braking force of a 50 mm disc brake can be matched by a 100 mm drum. But both will provide the same braking force.
2. With the Royal Enfield example you have also highlighted 2 things:
a. Based on the weight of the vehicle the required braking force may be higher than usual.
b. So if the RE requires X unit of braking force to maintain that same 40 m of braking distance and that equates to a 300 mm Drum brake which is not feasible or installable, means we would have to look at either settling for a higher braking distance with a <300 mm Drum setup or go for a Disc setup which can offer the same X unit of braking force over a 150 mm Disc.
Now the scenario which causes me grief is the following :
Manufacturer's UK Division designs a Sedan with more than 1 ton of kerb weight. Decides to maintain 40 m braking distance following which calculates that in order to maintain the required brake bias, dynamics etc the rear braking force required is X and that can be provided by either a 300 mm Drum or a 150 mm Disc.
Since it's not realistic or feasible for N number of other parameters to install
300 mm Drums on the rear axle, its imperative that in order to maintain the same braking distance the 150 mm Discs have to be installed on the rear axle.
Now the Manufacturer's India division in their conquest to increase profits and cut corners, figures out no one is really bothered about the braking distances in India as long as they throw jargon like ABS,EBD etc at the Mango men. They decide to swap the 150 mm rear Discs with 200 mm Drum thereby immediately reducing the rear braking force from the originally designed X unit to Y unit where Y<X . As a result, the braking distance goes up from the designed 40 m to 50+ m.
Please note all dimensions of the Drums, Discs are purely hypothetical and i have just used them to simplify the message.
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Originally Posted by Turbanator Call me whatever, the first thing I do is to switch off this assistance on the Merc. It's programmed for countries where people know the difference between roads and their backyards, not for us, at least in the current day. The moment it sees a human being coming on a road, it brakes as if it's the end of the world. |
This sounds like those instances I had run into where I saw people run their car with disabled passenger side Airbags saying the slightest impact would result in huge repair charges due to airbag deployment. For someone like me, i would rather have it on than regret later. The sensitivity of all such autonomous or assistance aids are tweakable and we should be able to configure them to give us the required assistance rather than being over intrusive. For eg. I had a fellow TJet owner crib about how mad the rain sensing wipers were, now he is only full of praises about it once I showed him how to set the desired sensitivity levels.
Someone had mentioned about people jumping in the front just for kicks, that's equivalent to saying people hitting the front bumpers and headlights to get kicks out of airbag deployment. The point is once it becomes the norm people won't be too bothered to indulge in such nuisance. And for the odd ones out there. Once the vehicle stops just walk up to them to show the video of the failed Volvo automatic brake test, where the car hits the Top Exec. That will definitely set them straight.
I think these are features which I would rather have and depending on situation either chose to utilise or not utilise, over having stupid gimmicks like sunroofs in a tropical country and bigass touchscreen because firstly that's a real distraction while driving, secondly passengers have their own phones and devices for entertainment. If we have survived so long referring to GMaps using 5 inch phone screens I would say thats enough for its purposes.