[westphone]

Hello I am trying to talk one of my friends out spending alot $$$ upgrading brakes (rotors and pads). These after market ones are bigger than the stock ones found on the Maserati Gran Turismo .

A bigger rotor supposedly 380 mm claimed comes with the six-ports caliper up front and four-ports rear. He doesn't need to change rims and hence has the same Michelin Pilot Super Sports. Can you guys confirm that braking distance is depending only on the traction of the tires if the clamps were powerful enough? Fitting an oversized brakes may help fading on track days but otherwise with the same tires and stance it doesn't shorten the braking distance? I guess having bigger clamps and disks will probably lead lockups and ABS cut in earlier?

I imagine he's gonna spend some time face planting into the steering first few days after he goes ahead with the install.

Cheers

[ovekvam]

You are right about bigger brakes not reducing the stopping distance. The purpose is to be able to dissipate more heat. You would pretty much have to be doing track driving to overcome the heat loss ability of the regular brakes anyway. With our 116i, the regular brakes are holding up well even on track days.

You can however easily increase the stopping distance by installing bigger brakes, if the new brakes are not perfectly balanced front/rear. This is very common on "upgraded" cars.

Lockups or ABS cutting in earlier is not really a problem as long as the brake balance is right.

The main disadvantage with big brakes (except for cost and need for huge rims) is the added unsprung (and rotational) weight. It will affect acceleration, road holding and comfort in a negative way.

[SteveC]

You can lock-up the wheels of most cars with drum brakes, but you can't stop very fast

Its about the quality of braking rather than the absolute force applied.

There's a huge difference between applying braking force and simply stopping the wheel turning.

Poor brakes will still lock-up the wheel but will retard the car very little before doing so.

What you want is linear braking, where the relationship between pedal pressure and perceived braking force is linear. Poor brakes will give you exponential braking, where you go from little perceived retardation to locked wheels.

Take a look at the diagram. What's important in terms of braking is the area under the curve. Both lines have the same maxima (lock-up point) but the linear line offers substantially more stopping power before reaching lock-up, and that's what larger diameter ventilated discs with multi-piston callipers will do for you.

In magazine speak, linear braking response is referred to as 'progressive' as opposed to the all-or-nothing exponential curve.

[ovekvam]

SteveC, sounds to me like you have bought into the sales pitch of the aftermarket brake industry. Remember that when you stop as quickly as possible with a modern car, the ABS system is fully active on all four wheels. It is constantly on the limit of locking up.

Adding heavy brake rotors will increase the rotational inertia, causing the brakes to be less responsive around the point of lock-up.

Take a look at the brakes on a formula one car. Not very big, are they?

[mowflow]

Experiment time.

Turn your bike upside down and spin the front wheel. Now stick your little finger in the spokes near the hub. It hurts, doesn't it? And the wheel probably didn't stop very fast. Now spin it again and stick your finger in the spokes near the rim. Let me guess, the wheel stopped quicker and you aren't hopping about in agony. It's exactly the same with the ratio between wheel and ratio disc size. If the disc is large relative to the wheel it'll need less braking effort to generate the same braking force at the tyre. Lever mechanics.

Bigger pistons in calipers mean they will generate the same force against the pad for a smaller force at the pedal. Same as sticking your thumb into the spokes instead of your little finger.

However, so many other factors come into it. Not least of all grip. F1 cars stop on a penny due to the massive amounts of grip and down force afforded by their aerodynamics.

Increasing unsprung weight will have a negative effect but bigger does not always mean heavier. All this said. i doubt without spending mega bucks your friend will see massive improvement over stock on something like a Maserati Gran Tourismo.

[ovekvam]

Yes, the pedal effort is lower with larger rotors, but even with regular (small) F20 brakes, it does not take all that much pedal force to do threshold braking. A lower force could even make it more difficult to modulate the brakes, like on VW, which tend to be over sensitive.

But try to add weight on the outer rim of the bicycle wheel, and then try to stop it with your little finger again...

[westphone]

thanks a lot gents

[westphone]

Found this vid and many more on the web, thanks abunch good time to review some physics too. I will do the wheel spinning experiment with me spinning the wheel and he sticking his fingers in there, har har.

[ovekvam]

Thanks, that video was excellent. He explained what I tried to write in a much better way.

[SteveC]

Originally Posted by ovekvam

SteveC, sounds to me like you have bought into the sales pitch of the aftermarket brake industry. Remember that when you stop as quickly as possible with a modern car, the ABS system is fully active on all four wheels. It is constantly on the limit of locking up.

Adding heavy brake rotors will increase the rotational inertia, causing the brakes to be less responsive around the point of lock-up.

Take a look at the brakes on a formula one car. Not very big, are they?

I don't actually know what their sales pitch is....this is just physics.

Better brakes isn't just about stopping as quickly as possible....its about regulating your speed as responsively as possible.

With good, linear brakes I can take a quick dab before a corner and slow my entry speed. With bad brakes, a quick dab will make no difference...since they respond in a non linear exponential way, its all or nothing. I've either got to stand the car on its nose, or take the corner at my current speed.

The uprated brakes on an M3 CSL made the car more responsive. Their superiority is due to their improved ability to convert work into heat and get rid of the heat more effectively

Regarding rotational inertia, its a bit more complex than that. Increased rotor size and brake pad area exerts greater torque, has increased swept area (greater braking area), converts work to heat more effectively and dissipates the resulting heat better etc. Rotational interia is only 1 element. You'll only see improvements of reducing rotational inertia of a brake rotor if you can maintain the rate at which it converts kinetic energy to heat, which typically isn't possible without costs escalating dramatically.

In F1, the brake superiority is a function of several factors.
Light weight of the car (less kinetic energy), huge contact areas of the tyres, high CoF of the tyres, down force (which increases the force the car exerts on the road without increasing the car's kinetic energy) and the design and materials of the brakes.

Braking is only about converting kinetic energy to heat (and light in an F1 car). The materials in a F1 car are capable of working at extremely high temperatures and have extremely efficient ventilation, thus can convert far greater amounts of kinetic energy in a shorter time than a normal car. Fit steel brakes to an F1 car and you will still be able to lock the wheels, but the stopping distance will increase drastically due to steels inability to convert kinetic energy as effectively or as quickly. Fit F1 type brakes to a BMW and you will decrease its stopping distance, but double its price. Its why ceramic brakes are fitted to cars like a 911 GT3 but add around £7 grand to its price

[ovekvam]

Originally Posted by SteveC

I don't actually know what their sales pitch is....this is just physics.

Better brakes isn't just about stopping as quickly as possible....its about regulating your speed as responsively as possible.

With good, linear brakes I can take a quick dab before a corner and slow my entry speed. With bad brakes, a quick dab will make no difference...since they respond in a non linear exponential way, its all or nothing. I've either got to stand the car on its nose, or take the corner at my current speed.

The uprated brakes on an M3 CSL made the car more responsive, but didn't decrease overall stopping distance vs a standard M3. At full braking, with ABS activated, both MS and CSL have the same stopping power. But at anything less the CSL's brakes are superior due to their improved ability to convert work into heat and get rid of the heat more effectively

Regarding rotational inertia, its a bit more complex than that. Increased rotor size and brake pad area exerts greater torque, has increased swept area (greater braking area), converts work to heat more effectively and dissipates the resulting heat better etc. Rotational interia is only 1 element. You'll only see improvements of reducing weight and size of a brake rotor if you can maintain the rate at which it converts kinetic energy to heat, which typically isn't possible without costs escalating dramatically

In F1, the brake superiority is a function of several factors.
Light weight of the car (less kinetic energy), huge contact areas of the tyres, high CoF of the tyres, down force (which increases the force the car exerts on the road without increasing the car's kinetic energy) and the design and materials of the brakes.

Braking is only about converting kinetic energy to heat (and light in an F1 car). The materials in a F1 car are capable of working at extremely high temperatures and have extremely efficient ventilation, thus can convert far greater amounts of kinetic energy in a shorter time than a normal car. Fit steel brakes to an F1 car and you will still be able to lock the wheels, but the stopping distance will increase drastically due to steels inability to convert kinetic energy as effectively or quickly. Fit F1 type brakes to a BMW and you will decrease its stopping distance, but double its price. Its why ceramic brakes are fitted to cars like a 911 GT3 but add around £7 grand to its price

Of course the ability to get rid of heat is essential when you want to do do a series of hard stops, typically on a track. For stopping once in the shortest possible distance, it is not much of a factor, as most brakes can deal with that heat. The M3 CSL has rather small and light brakes that will give you extremely short stopping distance, but they will fade when you drive hard around a track. That is why most of the CSL owners with track ambitions have installed larger brakes.

The brake balance is particularly important when trail braking into corners. If one end of the car is biting too hard at medium brake load, it will upset the balance of the car.

About the sales pitch, I was referring to the graph with the progressive brake response. I doubt you will find a modern car with brake sensitivity like that. The basic brakes I have on the F20 are nowhere near that graph. The brake response seems very linear. I think fitting a much larger brake system would give a very light brake pedal and somewhat sluggish ABS regulation.

The way the 116i is now, with the small brakes, the braking zone is where it is most competetive on tracks. It is possible to brake very late and deep into corners.

[Honziss]

Bigger brakes doesn't always mean more weight. Some aftermarket brakes are using aluminium rotors with steel discs and also the calipers could be lighter even if looks huge....so I suggest to compare weight and then take decision...

[ovekvam]

Originally Posted by Honziss

Bigger brakes doesn't always mean more weight. Some aftermarket brakes are using aluminium rotors with steel discs and also the calipers could be lighter even if looks huge....so I suggest to compare weight and then take decision...

The BMW Performance brakes are made of lightweight materials to keep the unsprung weight down. High diameter will still add to the rotational interia, though.

[westphone]

So we should
1. reduce moment of inertia --> weight distributed more inside
2. reduce total weight of brake --> hybrid material (light material around the center) suggested while not compromise coefficient of friction. (there's a vid on Williams F1 solving similar problem on their KERs in 2009)
3. Bigger pad --> bigger pad area --> brake applied to as "outside' as possible improved applied torque.
4. Make sure application is liner before buy.
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In conclusion my friend should
1. forget about the whole brake business and practice on his driving, or
2. spend mega $$$ to buy new rims that further lower rotational inertia and weight
3. hopefully he doesn't get into trouble with strangely balanced brake setup
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I personally think unless the car was originally designed for much bigger brakes, the cost for upgrading them is just hard to justify (USD 6000 on the brake upgrades seen in picture), who knows what else have to be upgraded along with the brakes. I would even think picking the correct braking points on a track should be considered for improved lap time before purchasing these aftermarket parts. Drive intelligently to protect the brakes even under hard uses such as on a track. After all, as good as they may be, Jeremy Clarkson experience brake fade even on the Aventador trying to beat the Stig's time in the 458 Italia around Imola

[ttimbo]

For most cars (I can't yet comment on the M135i), braking can be improved (as in better resistance to fade, better braking "feel") with some fairly simple modifications, such as pad material, brake fluid and braided lines. That's where I'd spend my money, initially, rather than bigger rotors and callipers.

[ovekvam]

Originally Posted by ttimbo

For most cars (I can't yet comment on the M135i), braking can be improved (as in better resistance to fade, better braking "feel") with some fairly simple modifications, such as pad material, brake fluid and braided lines. That's where I'd spend my money, initially, rather than bigger rotors and callipers.

That is my experience too, and 90 percent of the potential improvement comes from the brake pads. If you install dedicated racing pads, the brake behaviour is night and day compared to a regular car. They will perhaps squeal, cost a lot of money and damage the paint on the wheels, but the friction and resistance to fading is very impressive.