Question About Disc Brake Rotors

[HouseTiger]

I have not found this specifically addressed on here . . . . The question: What factors are used to determine the size (thickness as well as overall diameter) of brake rotors on vehicles. I see cars like Mercedes sedans and others in that size class with massive diameter rotors and caliper assemblies front and rear. On the flip side, I've seen trucks in the Dodge Ram 1500 class (Ford and Chevy/GMC as well) and many other sedans that appear to have very small brake rotors and caliper assemblies.

In theory, larger brake rotors matched with larger area brake pads make for better stopping power and shorter stopping distances. If that is the case, then there appears to be a lot of vehicles with undersized brakes. I know there's got to be a logical engineering answer to this . . . what am I missing ??


Thanks in advance

 

[Hall]

If that is the case, then there appears to be a lot of vehicles with undersized brakes.

When it comes to brakes, you can be certain that automakers are not under-sizing them. Can simply be that the fronts are actually over-sized. Or when it comes to trucks with minimal weight in the rear, not as much braking power is necessary.

On my G35, it has dual-piston front calipers that are almost 2x the size of the rear, single-piston calipers. Stopping is not a concern for me... at all...

 

[John105]

I think as time has gone on, rotors have gotten larger and larger. The only downside I can think of is unsprung weight. On a 2022 Chevy Tahoe police vehicle, it has 6 piston Brembos with 16" rotors in front. 16". Not a misprint. Anyone remember in the 90's when a 16" rim was "optional" lol And now the rotor is that size? I forget off the top of my head but my 2007 BMW has 13.7" rotors in front and 13.2" in the rear, which at the time was pretty large

 

[SteveSRT8]

swept area is the stat you need to figure. Outside diameter is not the only number.

 

[bunnspecial]

One of the things that I always keep in mind is that most of the time, your tires are probably going to limit how quickly/how well you can brake more so than a modern 4 wheel ABS set-up.

Of course with that comes the fact that tires are now better than they ever have been(I've been given a hard time before because I have "minivan tires" on my sports car, never mind that I'd put those tires up against what were on the car when it was new) and we also have things like fancy suspension that at least lets the rear wheels do more work than they did in less sophisticated designs.

I've suspected for a long time that a lot of the push to massive brakes on cars is aesthetics. It's amazing how many people I talk to think that slapping bigger rotors on, or otherwise doing things that increase how well the pads/shoes grip the rotors/drums will magically increase the stopping distance without doing anything else.

I am also overlooking something big that may be important for some people, and that's performance over time. If nothing else, for a given amount of stopping, a bigger/more massive disk will heat up more slowly. That may make the difference between your brakes fading to nothing or still working well at the bottom of a steep hill when you're my BIL hauling his 8000lb camper, or if you're on the track. Since I don't tow, though, and also don't live in the mountains, the only time I've personally even seen a hint of brake fade was when I had a sticky brake or was following some aggressive instructions for bedding in pads.

 

[edyvw]

When it comes to brakes, you can be certain that automakers are not under-sizing them. Can simply be that the fronts are actually over-sized. Or when it comes to trucks with minimal weight in the rear, not as much braking power is necessary.

On my G35, it has dual-piston front calipers that are almost 2x the size of the rear, single-piston calipers. Stopping is not a concern for me... at all...

Yeah they do. Japanese vehicles come with rotors exactly for their intended purposes, outside of that it is only downhill.
Toyota Prado I have in Europe has extremely undersized rotors. Sienna I had same. My in laws Pilot same way.

German vehicles on other hand have autobahn cultural bias. It is all about brakes! My 328 that weigh 3570lbs has more rotor surface area than both Prado and Sienna. Same goes for my Tiguan.

 

[edyvw]

I have not found this specifically addressed on here . . . . The question: What factors are used to determine the size (thickness as well as overall diameter) of brake rotors on vehicles. I see cars like Mercedes sedans and others in that size class with massive diameter rotors and caliper assemblies front and rear. On the flip side, I've seen trucks in the Dodge Ram 1500 class (Ford and Chevy/GMC as well) and many other sedans that appear to have very small brake rotors and caliper assemblies.

In theory, larger brake rotors matched with larger area brake pads make for better stopping power and shorter stopping distances. If that is the case, then there appears to be a lot of vehicles with undersized brakes. I know there's got to be a logical engineering answer to this . . . what am I missing ??


Thanks in advance

All German cars come with large brakes. It is cultural bias during design. 70% of autobahn doesn’t have speed limit. You can make anything go fast eventually. But take into consideration that vehicles you are describing are governed to 155mph and tuned can easily shoot over 200mph. You need something to stop that ballistic missile in case there is traffic ahead etc.

 

[benjy]

there are laws, BUT guessing $$$$ determines what gets used! costlier + sportier vehicles usually get better everything braking included while others get just enough! considering how much better most cars + trucks stop than years ago be thankful for modern tech!! or open your wallet to get upgrades $$$

 

[Hall]

Yeah they do. Japanese vehicles come with rotors exactly for their intended purposes

Which is it, they're undersized or they're sized as "intended" ? You're contradicting yourself. Never heard that Japanese vehicles were notorious for getting into accidents because their brakes were too small to stop the vehicle....

there are laws

What laws are these ?

 

[edyvw]

Which is it, they're undersized or they're sized as "intended" ? You're contradicting yourself. Never heard that Japanese vehicles were notorious for getting into accidents because their brakes were too small to stop the vehicle....

Intended purpose: dropping kids to school. Going down Rockie Mountains at 90mph? Problem! If brakes cannot sustain that driving, do not make a vehicle that can go 90mph and pout V6 3.5 V6 that I could hit 110mph with. Put appropriate brakes for it! On top of that, rotor material is absolute POS. Rear brakes are so small that you have to handle pads with care not to break them in hand. And guess what? New Sienna kept the same brakes but added weight (batteries).
My Prado is an excellent off-road vehicle, but the brakes are absolutely abysmal. They do well off-roading. Other than that, absolute POS. And yes, my Prado diesel eventually gets to 160km/h, not that it is a pleasant experience getting to that speed.

 

[skyactiv]

I once had an Elantra Sport with the 201HP 1.6 GDI-T. The car used larger rotors from the non turbo Sonata on the front.
A few auto makers have used the larger brakes from a heavier mid-sized car in their lineup on the smaller one with a high output engine.

 

[BMWTurboDzl]

I have not found this specifically addressed on here . . . . The question: What factors are used to determine the size (thickness as well as overall diameter) of brake rotors on vehicles. I see cars like Mercedes sedans and others in that size class with massive diameter rotors and caliper assemblies front and rear. On the flip side, I've seen trucks in the Dodge Ram 1500 class (Ford and Chevy/GMC as well) and many other sedans that appear to have very small brake rotors and caliper assemblies.

In theory, larger brake rotors matched with larger area brake pads make for better stopping power and shorter stopping distances. If that is the case, then there appears to be a lot of vehicles with undersized brakes. I know there's got to be a logical engineering answer to this . . . what am I missing ??


Thanks in advance

Cost

Vehicle weight, choice of tires, capable speeds. A lot of German cars in the US are governed at 155 mph. Nothing inspires confidence like great brakes and tires.

Cosmetics = large rims need large rotors.

Years ago a HP driving instructor told me that for accident avoidance the smaller looking brakes are typically fine it's just that people typically don't apply enough brake pressure to stop the vehicle. It's the repeated high speed braking or going down hill or along curvy roads etc where brake fade becomes an issue.

 

[John105]

Yeah they do. Japanese vehicles come with rotors exactly for their intended purposes, outside of that it is only downhill.
Toyota Prado I have in Europe has extremely undersized rotors. Sienna I had same. My in laws Pilot same way.

German vehicles on other hand have autobahn cultural bias. It is all about brakes! My 328 that weigh 3570lbs has more rotor surface area than both Prado and Sienna. Same goes for my Tiguan.

Yes, and to the other point made about swept area.....Japanese and American cars routinely have large unswept areas as seen but a rust stripe that follows the hat of the rotor. Germans tend to sweep the entire area with the pads. Then you have the old practice where Germans have semi metallic pads, lots of brake dust, and the pads and rotors are changed together (today they don't always do that). Some Japanese cars are 70k yet they don't even have vented discs in the back.

 

[BMWTurboDzl]

Yeah they do. Japanese vehicles come with rotors exactly for their intended purposes, outside of that it is only downhill.
Toyota Prado I have in Europe has extremely undersized rotors. Sienna I had same. My in laws Pilot same way.

German vehicles on other hand have autobahn cultural bias. It is all about brakes! My 328 that weigh 3570lbs has more rotor surface area than both Prado and Sienna. Same goes for my Tiguan.

Slap some Brembos on that Sienna!

 

[John105]

Slap some Brembos on that Sienna!

and some Bilstein B6's! They are one of the best things on my wife's GM SUV. Same price as OE (a little more on the rear shocks, same on the front struts), but 4X the quality.

 

[BMWTurboDzl]

and some Bilstein B6's! They are one of the best things on my wife's GM SUV. Same price as OE (a little more on the rear shocks, same on the front struts), but 4X the quality.

He'd be the coolest dad at school. Brembos with B6 Bilsteins. Should start a gofundme page.

 

[John105]

He'd be the coolest dad at school. Brembos with B6 Bilsteins. Should start a gofundme page.

I love it when I watch YouTube and they're repairing a SUV and the shocks/struts are yellow with blue boots. There was one like that yet the rest of the Land Cruiser was falling apart lol

 

[JohnnyG]

I can't tell you that Japanese cars have undersized brakes, I've never owned one.
One thing I can tell you is that changing from Cadillac to Mercedes, the MB brakes were better and very hard to get used to.
So hard, in fact, that one of those insurance company "tattletails" was constantly failing me on hard stops, when the stops seemed very normal to me. Even had a few "panic stops" according to the ins. co.,which never happened! I think they called it "hard braking" events.

 

[MrHorspwer]

I have not found this specifically addressed on here . . . . The question: What factors are used to determine the size (thickness as well as overall diameter) of brake rotors on vehicles. I see cars like Mercedes sedans and others in that size class with massive diameter rotors and caliper assemblies front and rear. On the flip side, I've seen trucks in the Dodge Ram 1500 class (Ford and Chevy/GMC as well) and many other sedans that appear to have very small brake rotors and caliper assemblies.

In theory, larger brake rotors matched with larger area brake pads make for better stopping power and shorter stopping distances. If that is the case, then there appears to be a lot of vehicles with undersized brakes. I know there's got to be a logical engineering answer to this . . . what am I missing ??


Thanks in advance

A simple first step is to determine kinetic energy of various vehicles:

KE = 1/2mv2

Kinetic Energy Calculator

If you know the mass and velocity of an object, use the kinetic energy calculator to find its energy in movement.

www.omnicalculator.com

Take a 2020 Ford F150 with a loaded travel trailer at a combined 16,000lbs (nearly maximum GCWR) traveling 83mph (maximum speed for most trailer tires).

That combo has about 4 million joules of kinetic energy that the brakes would need to dissipate.

Now look at a 2020 Mercedes Benz C63 AMG. Only weighs 4,400lbs, but has a maximum velocity of 175mph.

The C63 generates a maximum of slightly over 6 million joules of KE for the brakes to deal with.

This is obviously super simple and there are many more variables, but it does outline the relationship between mass and velocity when it comes to energy.

FWIW, our demo F150 has 350mm rotors and the C63 AMG has 360mm rotors.

 

[edyvw]

Slap some Brembos on that Sienna!

I bought it and then sold vehicle