What are vented discs

thexav

Pro Tuner
Staff member
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357
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2002 Clio 172
A question has cropped up from my other thread on brakes.

What are vented discs? Does this help much with braking? Are they a straight swap for non vented discs?

Which is a better option if you could only have one vented, grooved, dimpled or drilled discs?
 

Dale69

Tuner
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Mackay, Nth Qld
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Charade G102 1,3l
vented, or ventilated discs are ones that aren't solid and increase cooling effectiveness of brakes.
depends on the car as to whether they are a straight swap, some later model vehicle rotors will fit on earlier model solid rotor vehicles but best to ask at a reputable dealer. for my money, not being competition focussed, i would use slotted and or dimpled
 

TCJBOLDIE

Torque King
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662
From
Brisbane
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JB Starion
Weather one can simply swap solid over to vented depends on the disc thickness as the vented discs will generally be thicker and will not fit the caliper.
 

HDi fun

TC ModFather
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637
From
Buckinghamshire UK
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Passat 2.0 TDi
Do many cars have solid discs [rotors] now? My last three cars have all been fitted with vented rotors front and rear. Yet it seems that some cars still use rear drum brakes as service brakes!! Not just parking brakes

With vented brakes it makes sense for the car designer to include some ducting within the lower part of the body to provide some airflow to the brakes and wheel arches
 

TCJBOLDIE

Torque King
Points
662
From
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JB Starion
The only downside I see to rear discs is the E/handbrake is not generally able to hold the car as well as a drum brake

IIRC the early Volvo rear discs incorporated a drum for the handbrake mechanism as well.
 

HuntingTarg

Tuner
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52
From
Cali USA
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'02 Kia Spectra LS
I would not go with grooved or dimpled; these features can help remove dust and increase the surface area of the rotor exposed to air to improve cooling, but also makes the rotor's mass distribution less uniform. If grit or foreign objects cling to the rotor this can produce something called <i>torque instability</i> or <i>dynamic instability</i>, especially at high speeds; it's similar to braking on a warped rotor. Dimpling on the contact surface increases cooling but also DECREASES the contact surface area, which reduces braking power. This also may not be street-legal in some regions.
Drilled and vented are both trade-offs. Dale69 described vented quite well - the rotor is a bit heavier and thicker, and not all OEM calipers designed for solid rotors can accomodate a vented part. It helps increase the rate at which the rotor cools and so helps deal with brake fade. Having more mass means the rotor can absorb more thermal energy and so heats up more slowly. However, more mass also means more rotational inertia, so it can have a modest impact on fuel efficiency, especially when braking and accelerating a lot.
Drilled means the rotor has holes drilled straight through after casting in a symmetrical fashion. It helps reduce the mass of the rotor and so reduce the wheel assembly's total inertia. The holes themselves increase air turbulence, which is bad for turbos but good for cooling brakes; turbulent air heats up more quickly and so helps improve rotor cooling. But by reducing the rotor's mass it also will heat up faster during brake application, so again there is a trade-off.
Not all features are available on all applications; the rotors I installed on my wife's work van are vented AND drilled, so there's pretty much all the benefits without any real sacrifice. Hope you're more informed. Cheers!
 
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HuntingTarg

Tuner
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From
Cali USA
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'02 Kia Spectra LS
Weather one can simply swap solid over to vented depends on the disc thickness as the vented discs will generally be thicker and will not fit the caliper.
Some caliper designs, especially heavy-duty ones, have enough play to admit vented rotors, and in small cars vented ones aren't always THAT much thicker.
It's important to remember that a thicker rotor will decrease petal travel (but not always improve braking performance) so always take a test-drive in light driving conditions when changing rotor style.
 
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HDi fun

TC ModFather
Points
637
From
Buckinghamshire UK
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Passat 2.0 TDi
Some caliper designs, especially heavy-duty ones, have enough play to admit vented rotors, and in small cars vented ones aren't always THAT much thicker.
It's important to remember that a thicker rotor will decrease petal travel (but not always improve braking performance) so always take a test-drive in light driving conditions when changing rotor style.
No it won't. The pads and discs are always in light contact. There are no pull off springs in a brake caliper whether it's a sliding or fix caliper design. The pedal travel remains exactly the same. as the pads wear the pistons gradually move out of the bores to ensure there is no slack.
 

HuntingTarg

Tuner
Points
52
From
Cali USA
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'02 Kia Spectra LS
I'm wondering if auto mfg. laws are different in UK/Europe from North America on this point. The last two cars I got wheels off of there was a gap (small but visible) between the pad and rotor. 1) does that change when the master cylinder is charged? 2) Wouldn't that cause wear whenever the vehicle moves, not just when brakes are applied?
 

gladrags

Torque Junkie
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202
From
UK
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Focus 2.0
Not sure what you mean about the master cylinder being 'charged' but the modern disc brake caliper uses square section seals around the piston(s) instead of the old O ring type. When the piston(s) extend under hydraulic braking pressure they distort the seal outwards ie the seal is tight enough to stop the piston sliding through easily. When the pedal is lifted the square section seal reverts to its original position through hysteresis ergo as Hdi said there are no return springs on disc brakes but the pads are returned to Off by the seals. They will usually allow minimal clearance between the pad and the disc to minimise brake pedal travel.
 

HDi fun

TC ModFather
Points
637
From
Buckinghamshire UK
Car
Passat 2.0 TDi
I'm wondering if auto mfg. laws are different in UK/Europe from North America on this point. The last two cars I got wheels off of there was a gap (small but visible) between the pad and rotor. 1) does that change when the master cylinder is charged? 2) Wouldn't that cause wear whenever the vehicle moves, not just when brakes are applied?
gladrags has given a better explanation than mine. The brakes are the same, whether EU or US. Basically even if you fit a thicker rotor that tiny gap you see will be the same size. Elongated pedal travel is usually a sign of caliper seizing. This can be hydraulic pistons seizing or simply the slide posts in the case of single piston floating calipers.

There can be minimal contact when the brakes are fully released but this does not really impinge upon the wear of pads and rotors. When the brakes are fully applied the forces clamping the pads to the discs are huge. Hundreds of kg.
 

HuntingTarg

Tuner
Points
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From
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'02 Kia Spectra LS
Thanks to both of you for the explanation. I've barely worked on brakes (1 major job and some inspections), so I wasn't aware of the self-adjustment mechanics that gladrags described so well.
 
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