Brake discs - bigger on the rear!

Magown3

Hi

I replaced the rear discs on my car recently and noticed that they were actually bigger than the fronts on my previous car. Thought it strange as they are in the same sort of class.
On further inspection of the fronts and with a tape measure in hand, it turns out that the fronts are actually SMALLER than the rears.

• Front - 320mm
• Rear - 330mm

I note this is kind of unusual as I've never really seen a car with larger discs on the rear, even after taking note of mine and looking at other cars to see if any others are similar.

Why would this be?
Would larger discs have been ordered from new?

Car is 2009 325D (touring if that makes a difference)
Old car it replaced was an A4 Quattro Sport.

colm_mcm

This car is rear wheel drive and this is probably to do with traction control.

pablo128

Truck brakes can be twice as big on the rear as the front, but I would imagine the 20 tonnes on the back is the reason for that. But to me, having bigger brakes on the rear of a front engined car doesn't make sense.

Hopefully someone comes along to explain it.

Spook_ie

Probably to do with heat disapation on the rear brakes, bigger disk disapates heat quicker.

Phileas Frog

pablo128 wrote: »

Truck brakes can be twice as big on the rear as the front, but I would imagine the 20 tonnes on the back is the reason for that. But to me, having bigger brakes on the rear of a front engined car doesn't make sense.

Hopefully someone comes along to explain it.

Front engine, rear wheel drive

ianobrien

It's more than likely due to weight. The BMW would have more weight at the rear, and as it's a Touring, could have quite a lot of weight in the boot (loaded). Hence big brakes at the rear.

Isambard

are the calipers bigger?

alias no.9

Some setups incorporate both a drum and a disc in the rear brake. The drum is for the handbrake and the disc for the service brake. What you end up with is a big outer diameter but also a big inner diameter to allow the drum to fit inside the braking surface of the disc.

pablo128

Phileas Frog wrote: »

Front engine, rear wheel drive

Like a lot of other cars. But most front engined rear drivers have bigger brakes on the front.

On a normal car, when you brake the weight transfers to the front and the rear gets lighter. There's normally a 70/30 difference in braking so the rear doesn't lock up when it goes light. (roughly speaking)

But as I said, maybe there's a good reason on that model.

pablo128

alias no.9 wrote: »

Some setups incorporate both a drum and a disc in the rear brake. The drum is for the handbrake and the disc for the service brake. What you end up with is a big outer diameter but also a big inner diameter to allow the drum to fit inside the braking surface of the disc.

Yeah my car has that setup. My discs are still smaller on the rear, but it could be the answer to the OPs question.

somebody_else

Looks like it is default config.

Rear 336mm
https://www.mister-auto.ie/rear-brake-discs/bolk-bol-e061016_g900008_a999BOL-E061016.html

Front 330mm
https://www.mister-auto.ie/front-brake-discs/bolk-bol-d011643_g900007_a999BOL-D011643.html

Tom

Magown3

ianobrien wrote: »

It's more than likely due to weight. The BMW would have more weight at the rear, and as it's a Touring, could have quite a lot of weight in the boot (loaded). Hence big brakes at the rear.

I'm still disappointed with the boot space in the touring actually, pretty poor compared to the A4. I'm constantly dropping the back seats to fit things in!
But I see your point re the weight. I think that, coupled with the larger (I presume) differential and driveshafts for the 3.0 litre, the larger callipers/discs will stop the momentum of the diff easier.

Isambard wrote: »

are the calipers bigger?

Callipers are smaller on the rear, but not by much I would say.

alias no.9 wrote: »

Some setups incorporate both a drum and a disc in the rear brake. The drum is for the handbrake and the disc for the service brake. What you end up with is a big outer diameter but also a big inner diameter to allow the drum to fit inside the braking surface of the disc.

I would guess that the inner diameter is larger than the front by approx 10-15mm going by my eye.

I must say tho, it stops unbelievably well. I know it's not a large car or anything but the feel and feedback of the brakes makes it the best car for braking that I've ever owned.

gonko

My recently parted with E46 M3 had smaller discs on the front as standard. And thats a high performance type car, so I guess its all down to car design and brake balance etc.

swarlb

Magown3 wrote: »

Hi

I replaced the rear discs on my car recently and noticed that they were actually bigger than the fronts on my previous car. Thought it strange as they are in the same sort of class.
On further inspection of the fronts and with a tape measure in hand, it turns out that the fronts are actually SMALLER than the rears.

• Front - 320mm
• Rear - 330mm

I note this is kind of unusual as I've never really seen a car with larger discs on the rear, even after taking note of mine and looking at other cars to see if any others are similar.

Why would this be?
Would larger discs have been ordered from new?

Car is 2009 325D (touring if that makes a difference)
Old car it replaced was an A4 Quattro Sport.

Are there rear handbrake (shoes) on that model. That usually explains why the discs have a larger diameter.
Front discs would be wider and vented, and normally have a larger brake pad surface area.
Land Rover Discovery models have larger diameter rear discs than front as well.

Vittu

A 2009 325d is a nice car, not too many on the road. Is that the 3.0d model?

Mjolnir

It's a rear wheel driven car therefore the breaks on the driving wheel need to be bigger as front the mechanics of rear vs front vs 4wd vs and are very different.
Its performance and safety related standard

gctest50

Doesn't matter at all as long as you regulaaaate it

pippip

Mjolnir wrote: »

It's a rear wheel driven car therefore the breaks on the driving wheel need to be bigger as front the mechanics of rear vs front vs 4wd vs and are very different.
Its performance and safety related standard

Well then all rear wheel drive would be but they're not. It's to do with weight distribution under braking and airflow to brakes for cooling.

HappyAsLarE

Where are people getting this rear wheel drive logic from? There is no drive when you are braking.

Basically a car wants to pitch forward under braking, biasing to the front. Having larger rear brakes reduces this a bit, improving the balance of the car.

corkgsxr

Mjolnir wrote: »

It's a rear wheel driven car therefore the breaks on the driving wheel need to be bigger as front the mechanics of rear vs front vs 4wd vs and are very different.
Its performance and safety related standard

Nope nothing to do with it.

It's over the drums in the centre so disk has to go outside that. Also bear in mind its solid disk.

Front has cooled disks and more metal with bigger calipers. So 70% minimum is still from the front. Which makes sense as weight transfer is forward so fronts can handle more as they get more grip as you brake more (obviously to a extent)

Where rear loses grip more force you put through it

swarlb

Mjolnir wrote: »

It's a rear wheel driven car therefore the breaks on the driving wheel need to be bigger as front the mechanics of rear vs front vs 4wd vs and are very different.
Its performance and safety related standard

Sorry, but you are incorrect.
Braking is always greater to the front, regardless of what wheels are driven.
As you brake, the force is to the front, so distribution is always towards the front, otherwise inertia would cause the rear of the car to step out.
A simple example would be a 'handbrake turn', locking the rear wheels will cause the car to turn rearwards first.
A 60/40 split, front and rear would not be uncommon. On racing cars, applying extra braking to the rear, is used to allow the car to turn in quicker, but requires skill to balance correctly, or enforced oversteer.

Mjolnir

swarlb wrote: »

Sorry, but you are incorrect.
Braking is always greater to the front, regardless of what wheels are driven.
As you brake, the force is to the front, so distribution is always towards the front, otherwise inertia would cause the rear of the car to step out.
A simple example would be a 'handbrake turn', locking the rear wheels will cause the car to turn rearwards first.
A 60/40 split, front and rear would not be uncommon. On racing cars, applying extra braking to the rear, is used to allow the car to turn in quicker, but requires skill to balance correctly, or enforced oversteer.

Actually I'm fairly sure 09 e9x have ebd which means that the car distributes brake forces to each individual wheel as required meaning not truly having a % break down. Given that they are rear biased that will usually mean applying breaking force to the rear as in all rear wheel drives they have a tendency to "step out" and lose traction at the rear and have uneven load due to this.
The bigger discs is due to the fact they are rear biased, have an uneven load distribution require better stopping and heat distribution.
Hand brake turns are not comparable to normal breaking.
Excuse me for being too lazy to write all that I'm wrecked ha.

But all that being said its your tires that actually stop you in the long run.

Race cars are a completely different animal and vary greatly depending on the car, are we talking F1 with turns, drag cars like funny cars, top fuel, prostock, drift cars, nascar. Each have vastly different brake set ups

Kenny Logins

Are the pads larger too?

millington

Mjolnir wrote: »

It's a rear wheel driven car therefore the breaks on the driving wheel need to be bigger as front the mechanics of rear vs front vs 4wd vs and are very different.
Its performance and safety related standard

Incorrect. It's because of the handbrake. The rear pads are significantly smaller than the fronts & the discs are also a lot thinner. If anything, the rear would have more braking than a FWD already due to drivetrain losses.

corkgsxr

Mjolnir wrote: »

Actually I'm fairly sure 09 e9x have ebd which means that the car distributes brake forces to each individual wheel as required meaning not truly having a % break down. Given that they are rear biased that will usually mean applying breaking force to the rear as in all rear wheel drives they have a tendency to "step out" and lose traction at the rear and have uneven load due to this.
The bigger discs is due to the fact they are rear biased, have an uneven load distribution require better stopping and heat distribution.
Hand brake turns are not comparable to normal breaking.
Excuse me for being too lazy to write all that I'm wrecked ha.

But all that being said its your tires that actually stop you in the long run.

Race cars are a completely different animal and vary greatly depending on the car, are we talking F1 with turns, drag cars like funny cars, top fuel, prostock, drift cars, nascar. Each have vastly different brake set ups

Reason for distribution is in a perfect world it would be 50/50 split. But as the front dips it moves braking to the front. So you end up with 70/30.


If above theorys were right rwd motorbikes would have much bigger brakes to rear. There tiny in comparison

Pops_20

The outside diameter of the disc on the rear is so large because it incorporates a drum brake for the handbrake on the inside, as others have mentioned already.

What is more important for braking is the width of the contact area of the disc, the thickness of the disc itself, and whether it is vented or not. I'd be fairly certain that your rear brake contact area is a smaller than the front, so the front brakes are still bigger overall.

Leonard Hofstadter

It's not always the case that BMWs have bigger discs at the rear - the E46 most certainly does not for example.

In any case, BMW handbrakes are rubbish even at the best of times:D.

Tazzimus

Mjolnir wrote: »

It's a rear wheel driven car therefore the breaks on the driving wheel need to be bigger as front the mechanics of rear vs front vs 4wd vs and are very different.
Its performance and safety related standard

Very incorrect. Rear wheel drive cars still pitch forward when braking so more weight ends up over the front wheels than the back.
By your explanation, the rear of the car would just lock up and step out every time you braked hard ish as most of the weight would be over the front wheels.

You generally don't want any road car to have braking bias to the rear wheels, FWD, RWD or 4WD will all step out somewhat.

As mentioned already, it's likely due to drums for the handbrake.

Magown3

Vittu wrote: »

A 2009 325d is a nice car, not too many on the road. Is that the 3.0d model?

Yes it's a 3.0L with €390 tax, happy days.

Kenny Logins wrote: »

Are the pads larger too?

No, Smaller, but not by much

Leonard Hofstadter wrote: »

It's not always the case that BMWs have bigger discs at the rear - the E46 most certainly does not for example.

In any case, BMW handbrakes are rubbish even at the best of times:D.

Agreed, handbrake is terrible.
I can confirm that the handbrake is shoe design inside the rear brake disc.


I am planning on swapping the front and rear wheels around for even tyre wear soon. I'll have to take photos when I do so. Don't worry, the tyres are same size, it's an SE model. I'll post a pic of the car also.

Magown3

The day I picked it up in the UK.
My first RWD car.

Edit. Pic didn't attach. Standby

https://www.dropbox.com/s/rh54y22ytq8be03/Photo%2024-07-2019%2C%2012%2037%2012.png?dl=0

Tacitus Kilgore

There are some interesting theories on how brakes work in here :eek:

swarlb

Tacitus Kilgore wrote: »

There are some interesting theories on how brakes work in here :eek:

And even a few on how breaks work... (or don't, 'cos they are broke... or tired...or is it tyred ??)