Vented discs

[Andy Moltu]

Having got my brakes warm in the mountains on more than one occasion I was mulling over vented discs.

 

The 4 pot conversions are pricey so looking at fitting a spacer to the calipers (Canley clessics do these pretty cheaply) however I wondered what discs can be used sure I could buy the ECB ones but I assume that there must be a modern car to which these are fitted and thus be relatively cheap from motor factors.

 

Anyone know?

[had17462]

andy ive got some vented discs from a kit ,i can measure them if that helps?

nick

Edited September 18, 2012 by had17462

[cord]

Not sure if these are what your looking for!

[Andy Moltu]

The US spec Cressida ones aren't that easy to get hold of but it would be great to get the measurements off those on yours Nick.

They use Capri ones on the smaller Triumphs like the Vitesse but they are a smaller diameter so less suitable for the bigger wheeled TRs (which means slower rotation at any given speed & thus less swept area)

[john.r.davies]

they are a smaller diameter so less suitable for the bigger wheeled TRs (which means slower rotation at any given speed & thus less swept area)

 

My underline.

 

Andy,

can't get around contradicting you!

Area is irrelevant for brakes. Friction=Coeff. x Frorce between surfaces. the area don't come into it!

The advantage of a bigger disc is that the moment arm of the centre of friction is longer, so the same frictional force has, in effect, a greater leverage to oppose momentum.

 

JOhn

(It ain't rocket science - it's physics!)

 

PS I can thoroughly recommend venteds, but sorry, can't recommend one that will suit a TR. Capris suit my Vitesse fine!

Edited September 21, 2012 by john.r.davies

[Davidw]

John, just asking, but if the disc is bigger then doesn't the outside edge move faster than a smaller disc and therefore have more momentum (force?) for the calliper to overcome?

 

[RogerH]

Hi David,

the amount of momentum that the extra disc diameter gives is insignificant compared to what is going on.

However the extra leverage given does improve the braking.

 

Andy,

the EBC discs have one main benefit - they are available without any maching to do. Sadly they are expensive for what they are.

I found that the replacement discs - they do need replacing every few years - are more expensive than the initial kit with all the extra bits.

 

Roger

Edited September 22, 2012 by RogerH

[Andy Moltu]

As I recall from my dim and distant physics A level work = force x distance.

 

Inside a wheel of fixed size (call it a 15 inch TR6 wheel) the linear speed of the outside of that wheel (relative to the caliper) will be greater than the inside (naturally they are doing the same number of rotations per second)

 

Thus if we assume that the friction of the brake pads on the disc is the force.

If the wheel rotates once - the outer edge will have rotated pi x 15" = 45"

 

The outer edge of a 5 inch disc will have rotated pi x 5" = 15"

The outer edge of a 10 inch disc will have rotated pi x 10" = 30"

The outer edge of a 12 inch disc will have rotated pi x 12 = 36"

 

Thus the work done by the brake pad against the disc is related to the physical distance that the force is applied through. Thus the larger the diameter of the disc the caliper squeezes the more work will be done (ie energy dissipated)

 

Naturally this calculation assumes the coefficient of friction remains constant regardless of speed and disc temperature.

 

Thus size really does matter. At any given road speed the bigger your wheel the slower the rotation thus the bigger disc needed. Smaller wheels means more rotations so a smaller disc will do the same job.

Edited September 22, 2012 by andymoltu

[Grabea]

I think you're confusing acceleration with decelleration which is negative acceleration?

Yes work = force x distance so if the force is exerted for a greater distance more work is done?

Bigger discs = better brakes simples?

Graham

[peejay4A]

Indeed, as John pointed out, it's down to force at a distance so bigger is better. I imagine the consequent ability to use more friction area is useful for heat dissipation, or more probably reduction in heat concentration.

[had17462]

The discs i have here are 11 inches across and just under an inch thick

nick

[john.r.davies]

Daavid,

A disc weighs - 10kgs?

But it's one of a pair that is doing 80%+ of the work stopping a car weighing over 1000kgs.

So the inertia of the disc, smal or wide, is irrelevant in the bigger picture.

 

Andy,

by the same token, the work done by the disc is converting that 1000kgs at 100+kph into heat. Even if a 12" disc does 100% more work than a 5" in stopping the disc, that is piddling when the inertia of the whole car is considered. And even if the friction were the same the moment arm is 50% longer.

 

John

(it ain't rocket sciene -it's physics!)

John

[Peter Cobbold]

The area of the caliper pistons is important in determining the force applied to the disc at a given hydraulic pressure from the m/cylinder.

[stuart]

And dont forget that more braking force applied to the front will upset your brake balance to the rears.

Stuart.

[john.r.davies]

The area of the caliper pistons is important in determining the force applied to the disc at a given hydraulic pressure from the m/cylinder.

 

 

I agree, Peter, but let that not confuse anyone. THis is to about the relative sizes of master and caliper pistons. It has nothing to do with friction, and all to do with hydraulics.

 

John

[Peter Cobbold]

To get back OT. I think I can make the argument that the swept area will make a difference, independent of radius of moment. The larger diameter disc will have a greater heat capacity so the amount of retardation per 100degrees of temperature rise at its surface will be increased in proportion to the greater area swept. Thats ignoring heat dissipation to air or hub. A thicker solid disc will have similar effect, but not in proportion to thickness as the heat has to diffuse into the disc.

Lots of graphs in here: http://ir.library.dc-uoit.ca/dspace/bitstream/10155/15/1/Zhongzhe%27s%20thesis.pdf

 

It will also depend on how much of the energy of retardation is dissipated to air in road driving. For downhill driving I doubt that air cooling makes a lot of difference to fade - wheel and air speeds are too low I suspect. It follows that a vented disc might be less effective downhill than a solid disc of the same weight, the vent-holes delaying heat diffusion away from the surface but not adding much to air cooling.

 

 

Water cooling?

Drilled discs to get rid of gases boiling off very hot pads?

Edited September 23, 2012 by Peter Cobbold

[stuart]

Water cooling?

 

They use water cooled discs in truck racing

Stuart.

[john.r.davies]

I fitted my venteds because I was suffering from severe brake fade when racing. For maximum cooling effect, I also fitted a water spray, duplicating the screen wash pump with jets that were directed into the intake of the disc vents, so that it would not get onto the disc faces. The pump was controlled by the brake light switch, with a master to turn it off for road use. In fact, the venteds worked so well, I never used the water cooling in anger, and to be honest it didn't seem to make much difference in testing. But then I also fabricated air ducts that directed a good air flow at the brakes, so that might be more worthwhile.

 

Drilled discs are another mod with little justification. A properly 'bedded-in' pad should not produce any gas or vapour. Unless the holes penetrate a vent, the other end will be blocked by the other pad, so the volume of expansion they offer will be small, and I've seen more solid discs drilled than vented. The presence of the holes weakens the disc, and cracks can propagate from them, so much that I'm told pro racing teams that use drilled discs will crack test them regularly.

If you really believe in gas venting, then slots in the surface of the disc than can extend beyond the pad edge will allow gas expansion and will not weaken the disc as much as lines of holes.

 

John

Edited September 24, 2012 by john.r.davies

[Peter Cobbold]

Andy's particular application - fade down mountains - is if anything more taxing than racing because the air speed over the discs (even if ducted) will be relatively low, and the rotation speed too. So water cooling might be the best solution here.

This effect might be an issue through when spraying droplets onto a hot surface: http://en.wikipedia....denfrost_effect

( ? dipping wet finger into molten lead...

Edited September 24, 2012 by Peter Cobbold

[john.r.davies]

A-ha! Leidenfrost effect - maybe that's why the direct spray wasn't effective.

Misting would tend to lose heat by evaporation more effectively, and in the air intake too, but droiplets would just 'Leidenfrost' as mch, wouldn't they?

Looking at race trucks that use water cooling, they use VAST amounts;

 

 

I tried to calculate how much a car would need.

Assume a 1000Kg car, stopping from 100mph to 20mph (a slowing of 36m/sec).

Kinetic energy of stop = 1/2 x 1000 x 36^2 = 648000 joules

 

Latent heat of vapourisation of water = 2500 joules/gm

So that amount of energy will boil 260ml of water, that is already at 100C.

 

If the water sprayed is at, say 40C, than it will use up a tenth of those joules to reach boiling point, but that is trivial, when compared with the evaporation effect.

 

So, 260mls each time you slow for, say, the Mallory Hairpin.

20 lap race, that's 5 litres of water, for a single corner. Double that for the whole circuit? Obviously dependent on how twisty is the circuit, so 10+ litres? No wonder a truck needs vast quantities, and no wonder my puny spray was ineffective!

 

John

[Peter Cobbold]

John,

Truck brakes use " 85litres water" so 10litres would be about right. Maybe really tiny droplets would not bounce off by Lf effect, so perhaps could try Aquamist water injection kit (but ££ not good.) - not sure about flow rates......

Peter

[Andy Moltu]

Water cooled brakes were used in F1 for a while until the technical bods noticed that the weight of water used was just enough, when topped up after the race to bring the car up to the minimum weight and whose tanks were empty by the end of the first couple of laps.

[john.r.davies]

_{Thanks, guys, a most interesting symposium!}

_{I hope the OP wasn't outfaced.}

_{If it helps him to know that I was so pleased with venteds to race with, I never tried again with the water.}

My last suggestion would be to consider the brake pad material.

A hotter compound would resists fade on long descents.

I'v efound that Mintex 1155 is 'soft' enough to work well on the road, and 'hard/hot' enough to never fade, even racing, when gripping venteds.

 

John

 

(The font size has gone all odd)

Edited September 24, 2012 by john.r.davies

[jean]

I got the EBC vented disks ( grooved and sloted) on my TR6 for 5 years now and I, m very happy with them. Easy to fit and of exellent quality. Did not use them on long Alps stretches, but never encountered any fading

issue. Contrary to previous brake disks I used, the EBC disks never showed any trace of surface rust after a period of non-use

[Red 6]

I am not a physist or mathmatician and my spelling sometimes lets me down But:

 

(and I am sorry to return to the OP question)

 

I read your brakes are getting hot. Does this mean you are experiencing

 

1. Brake fade? If so change the pad material to one which retains its efficiendy at high temperatures. (pagid rs 42 or RS14)

2. Soft pedal or pedal to the floor? If so change the brake fluid to one which retains its efficiency at high temperatures.(castrol SRB)

 

If however you find that it is taking too long to slow down from high speed then fit larger discs.

 

If after fitting larger discs you tend to lock up easily, you need to fit wider tyres with a softer compound.

 

If you do this you will need to fit stronger road springs to control the sudden weight transfer to the front under braking, which will ruin your shocks so those will need to be upgraded.

 

Trouble is now with a stiff front end you will need to work on your rear so it behaves

 

Now of course with all these modifications you might find the brakes get hot.