Rear Disks are NOT as powerful as drum brakes!


Kozy

Kozy

Bench Racer
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Lots of people keep asking about converting rear drums to disks.

Understand this one point.

Disk brakes alone are LESS powerful, per unit pressure, than the equivalent drum brakes.

What I mean by that is that if you dump 500psi into a drum brake, you might get 250lbs of brake force out of it, but if you dump the same 500psi into a disc brake, you might only get 150lbs brake force out of it.

Changing the rear brakes does not change the fluid pressure in the system, so by fitting disc brakes and doing nothing with the proportioning, you are actually reducing the performance of the rear brakes.

Everyone knows how effective drum brakes are for handbrake turns, and in comparison how rubbish disk handbrakes are at even holding the car on an incline. This is a prime example of the fixed input (arm force x handbrake lever length) vs the brake force applied.

Discs are weak, they need lots more pressure to function correctly. Take some time and do upgrades properly, otherwise you're just wasting your money.:blinx:
 
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As I have just put in the suggestions section......I would be liking this is if I could! another fine post Kozy!
 
Correct me if I'm wrong, but is the handbrake on a rear disc no basically just a small "drum" brake inside the rotor? So your example is just a matter of scale as the dedicated hand brake shoes on a disc brake will be far smaller than the ones in a standard rear drum brake?
You are otherwise 100% correct of course, far too often people carry out mods without doing any research and without thinking it through!
 
oifovo said:
Correct me if I'm wrong, but is the handbrake on a rear disc no basically just a small "drum" brake inside the rotor? So your example is just a matter of scale as the dedicated hand brake shoes on a disc brake will be far smaller than the ones in a standard rear drum brake?
You are otherwise 100% correct of course, far too often people carry out mods without doing any research and without thinking it through!
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Not all cars have a brake shoes inside the discs.
Like the civics don't, but I know evo's do.
 
Aren't discs much better at coping with high temperatures, on track for example? But yeh the handbrake on my EK3 is a lot better than on my EK4.
 
Kipper said:
Aren't discs much better at coping with high temperatures, on track for example? But yeh the handbrake on my EK3 is a lot better than on my EK4.
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I don't think it has anything to do with coping with high temperature (= retaining ability to brake even when very hot) but with heat dissipation - i.e. they will take much more abuse before they get hot. Once very hot they will both start to fade. This can be helped by fitting better brake pads that will work at higher temperatures, which is another reason to fit disc brakes - there are many more options when it cams to high quality pads than high quality shoes (if there are any at all).
 
Kipper said:
Aren't discs much better at coping with high temperatures, on track for example? But yeh the handbrake on my EK3 is a lot better than on my EK4.
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Yes, once you give the discs the right pressure, they can vastly outperform drums.

Heat is one of the reasons manufacturers fit rear discs to performance models, the other is that they have a linear response. If you increase the pressure 10%, you get 10% more force. Drum brakes are not linear, because the shoes grab at the drum, pulling them outwards which creates more force than intended. This is why they are so powerful, the effectively 'self servo' on very low pressures.

The manufacturer will proportion the the rear pressures accordingly, when speccing rear discs.
 
Great info as usual. I think rvm should give you your own special kozy tech section on this forum. :bow:
 
interesting read.

Makes some sense as Trucks usually use drum brakes with air pressure keeping them off, not on, this is why when there is a air leak they cannot move.
 
Some do it for looks and others do it for performance, no matter the reason, do it right or leave it the hell alone.

Another great post from Kozy!
 
Great info! All ways thought the discs were better! I guess they just look cooler haha :)
 
Discs ARE better when set up properly.

Think of it like needing a remap. You can bolt all the bits on your engine you want, but without the remap it's not going to be any better, and may actually be worse. :nice:
 
Interesting read Kozy, is this why F1 cars now all use drums over discs? Them seem to work a lot better for them compared to disks..
 
DOSvtec said:
Interesting read Kozy, is this why F1 cars now all use drums over discs? Them seem to work a lot better for them compared to disks..
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F1 does use disc. Multi disc setups iirc. What you see when the wheels are off is just a housing and heat sheild.
 
not a perfect comparison ive driven the odd classic car with front drum brakes, let me tell you its a very on/ off experience.

however from a power point of view drums on the rear might be better but i wouldnt stick them on due to lack of feel, you wont want rears locking!

might be worth getting ur rear calipers reconed and a set of uprated pads then will surly increase the braking force.

Imy
 
Drums have their place I guess, just not on a performance car.
The lack of pedal modulation with drums is not a good idea.
 
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yollllooooooo
 
I've now updated my brake calculator with rear drum and drum prop valves in the drop downs. You can now select the drums and drum prop, then change the brakes to discs and see what effect it has on the bias and stopping distances. Not good!

The drum brake specs are NOT solid data, it's just based on guesswork.

I know that it is possible to run NSX rear brakes with a drum prop without rear lockup, so set the drum prop valve so that NSX rear brakes would still give front bias, even with upgraded pads.

I then set the drum brake up as a disc brake, but with a big enough piston that it would give the right amount of force on the reduced pressure the drum prop valve supplied.

I had a bit of a guess at the 'disc thickness', making them thinner to show how much hotter drum brakes get.
 
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