A simple difficult question
Thread Starter
6th Gear Member
Joined: Dec 2005
Posts: 5,212
From: Hills of California
A simple difficult question
Why do disk brakes stop a car faster than drum brakes?
Drum brakes have more pad surface area and fade wouldn’t play a part in a 60-0 stop would it?
What are the physics behind this?
Gun
Drum brakes have more pad surface area and fade wouldn’t play a part in a 60-0 stop would it?
What are the physics behind this?
Gun
5th Gear Member
Joined: Mar 2005
Posts: 3,566
From: Marquette Mi
RE: A simple difficult question
Leverage and pad surface. Sure a drum shoe is bigger, but it doesnt have the same contact that a set of disc pads has in relation to where it contacts the braking surface. Also discs clamp on both sides of the rotor rather than just one side of the drum, applying more force over a greater area. I could figure up the math involved, but really I dont want to bother, since its new years eve and all.. 
Drums can stop quite effectively, if set up correctly and no wear is present. More wear means less effectiveness simply due to the design, the shoes move outward and need periodic adjustment, where a disc caliper takes up the wear space automaticaly. Repeated stops will fade a drum very quickly because it cant shed heat as well as a vented disc, once again surface area is smaller than with a disc, as the disc has air running through the middle of it as well as on the surface.
Drums can stop quite effectively, if set up correctly and no wear is present. More wear means less effectiveness simply due to the design, the shoes move outward and need periodic adjustment, where a disc caliper takes up the wear space automaticaly. Repeated stops will fade a drum very quickly because it cant shed heat as well as a vented disc, once again surface area is smaller than with a disc, as the disc has air running through the middle of it as well as on the surface.
Joined: Aug 2005
Posts: 27
From:
RE: A simple difficult question
Thumpin455 is right... discs offer greater stopping performance than drum brakes because they have greater resistance to "brake fade" (caused by the overheating of brake components). Discs are also unaffected by immersion in water--I have experienced this first hand with my 57 F100--I went through some high water on my way to work; my drum brakes were totally useless! Disc brakes are more reliable than drum brakes because of their simplicity; fewer parts compared to the drum brake, and easier to adjust.
Thread Starter
6th Gear Member
Joined: Dec 2005
Posts: 5,212
From: Hills of California
RE: A simple difficult question
Okay more surface area make since but I can still get 4 wheel drum breaks to lock up! Which without thinking would mean that they offer more than enough resistance already.
So the only logical thing I can think of is that disk are able to convert kinetic energy into heat more efficiently so it puts less stress on the rubber of the tire where it meets the road thus increasing your breaking performance, unfortunately that doesn’t sound very logical either. Perhaps it’s on the right track?
Gun
So the only logical thing I can think of is that disk are able to convert kinetic energy into heat more efficiently so it puts less stress on the rubber of the tire where it meets the road thus increasing your breaking performance, unfortunately that doesn’t sound very logical either. Perhaps it’s on the right track?
Gun
3rd Gear Member
Joined: Oct 2005
Posts: 614
From: Sarasota/Orlando, Florida
RE: A simple difficult question
Yes, your drum brakes will lock up... quite easily. When they're cold.
Here's an experiment. Stop from 60-0 as quick as possible without locking your brakes.
Get up to to 30 or so (not too fast.. or you might lose control) and lock the breaks. Good luck on that one. (you probably won't be able to)
My guess is that yes, drum brakes will be equivalent to discs... for the first couple seconds. Even just stopping from 60-0 quickly would cause an immense amount of heat buildup in your cast iron drum brakes
*once again speculation* Disc brakes have a more linear power curve... meaning you can push them closer to the lockup point before they lock up.
Whereas on drums, at a point earlier than discs, the brakes lock up
Also drum brakes have one slave cylinder per wheel while discs have between two and eight (one on each side and four on each side)
Here's an experiment. Stop from 60-0 as quick as possible without locking your brakes.
Get up to to 30 or so (not too fast.. or you might lose control) and lock the breaks. Good luck on that one. (you probably won't be able to)
My guess is that yes, drum brakes will be equivalent to discs... for the first couple seconds. Even just stopping from 60-0 quickly would cause an immense amount of heat buildup in your cast iron drum brakes
*once again speculation* Disc brakes have a more linear power curve... meaning you can push them closer to the lockup point before they lock up.
Whereas on drums, at a point earlier than discs, the brakes lock up
Also drum brakes have one slave cylinder per wheel while discs have between two and eight (one on each side and four on each side)
Thread Starter
6th Gear Member
Joined: Dec 2005
Posts: 5,212
From: Hills of California
RE: A simple difficult question
AH Ha FADE!
So our guess now is that pound per pound (WHEN COOL) drums will stop the car just as fast as disk?
You might be right on with that! So if you combine that with "more linear power curve" (which I interpret as easer to find and hold threshold before lock up) that is where disk get their advantage!
I know how fade works but do you think 60-0 is enough to cause it? I guess it could be, that’s a lot of energy to soak up, if not then why did my Porsche 944 with 4 wheel disk feel like it stopped much faster than the stang with 4 wheel drum?
So the only reason a 1,333 pound F1 car can stop from 124mph to 0 mph in 180ft (or 1.9 sec) at 5G or deceleration is because its 500lbs lighter and has WAY better road traction?
So then ads claming 30% more stopping power by adding rear disk to a car with front disk are leaving out important info or totally BSing? because Im pretty sure the rear drums wont fade during a 60-0 stop
Gun
So our guess now is that pound per pound (WHEN COOL) drums will stop the car just as fast as disk?
You might be right on with that! So if you combine that with "more linear power curve" (which I interpret as easer to find and hold threshold before lock up) that is where disk get their advantage!
I know how fade works but do you think 60-0 is enough to cause it? I guess it could be, that’s a lot of energy to soak up, if not then why did my Porsche 944 with 4 wheel disk feel like it stopped much faster than the stang with 4 wheel drum?
So the only reason a 1,333 pound F1 car can stop from 124mph to 0 mph in 180ft (or 1.9 sec) at 5G or deceleration is because its 500lbs lighter and has WAY better road traction?
So then ads claming 30% more stopping power by adding rear disk to a car with front disk are leaving out important info or totally BSing? because Im pretty sure the rear drums wont fade during a 60-0 stop
Gun
1st Gear Member
Joined: Dec 2005
Posts: 85
From:
RE: A simple difficult question
hey great q dude! the answer is tat the drumss have better levarage on the axle...and so they haul you down most quick...beter than disks for a few stops. But then the drums fade out-out-out becuz the levarage heats em up faster. Cool?
3rd Gear Member
Joined: Apr 2005
Posts: 640
From:
RE: A simple difficult question
ORIGINAL: Gun Jam
AH Ha FADE!
So our guess now is that pound per pound (WHEN COOL) drums will stop the car just as fast as disk?
You might be right on with that! So if you combine that with "more linear power curve" (which I interpret as easer to find and hold threshold before lock up) that is where disk get their advantage!
I know how fade works but do you think 60-0 is enough to cause it? I guess it could be, that’s a lot of energy to soak up, if not then why did my Porsche 944 with 4 wheel disk feel like it stopped much faster than the stang with 4 wheel drum?
So the only reason a 1,333 pound F1 car can stop from 124mph to 0 mph in 180ft (or 1.9 sec) at 5G or deceleration is because its 500lbs lighter and has WAY better road traction?
So then ads claming 30% more stopping power by adding rear disk to a car with front disk are leaving out important info or totally BSing? because Im pretty sure the rear drums wont fade during a 60-0 stop
Gun
AH Ha FADE!
So our guess now is that pound per pound (WHEN COOL) drums will stop the car just as fast as disk?
You might be right on with that! So if you combine that with "more linear power curve" (which I interpret as easer to find and hold threshold before lock up) that is where disk get their advantage!
I know how fade works but do you think 60-0 is enough to cause it? I guess it could be, that’s a lot of energy to soak up, if not then why did my Porsche 944 with 4 wheel disk feel like it stopped much faster than the stang with 4 wheel drum?
So the only reason a 1,333 pound F1 car can stop from 124mph to 0 mph in 180ft (or 1.9 sec) at 5G or deceleration is because its 500lbs lighter and has WAY better road traction?
So then ads claming 30% more stopping power by adding rear disk to a car with front disk are leaving out important info or totally BSing? because Im pretty sure the rear drums wont fade during a 60-0 stop
Gun
yes, but 80% of your stopping power comes from the front brakes.
Thread Starter
6th Gear Member
Joined: Dec 2005
Posts: 5,212
From: Hills of California
RE: A simple difficult question
Right Markstang1969 I knew it was somewhere around 70-80% which means the front should get ~ 80% hotter than the rear and should fade way before.
Jaye I have never heard anyone say that drums stop a car faster but I can see the logic IF the inside diameter of the drum was more than the diameter of the disk I would think the drums would out stop the smaller disk. But that’s not always the case, the disk on my mustang have real close to the same diameter as stock drums but i have yet to test maximum stopping power (new pads, with now properly working distribution valve too)
So again this brings us back to the pound for pound when cool disk offer no advantage over drums.
Gun
Jaye I have never heard anyone say that drums stop a car faster but I can see the logic IF the inside diameter of the drum was more than the diameter of the disk I would think the drums would out stop the smaller disk. But that’s not always the case, the disk on my mustang have real close to the same diameter as stock drums but i have yet to test maximum stopping power (new pads, with now properly working distribution valve too)
So again this brings us back to the pound for pound when cool disk offer no advantage over drums.
Gun
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