Thread: Can you improve RRC BRAKES?

Originally Posted by PhilipA

Then why do nearly all performance car manufacturers have 6 spot calipers and very long pads?
Why not just higher clamping force calipers?
Regards Philip A

In a RRC getting enough force to stop the vehicle with modern hard pads and larger wheels is limited by the amount of assistance the brake booster provides. The size of the pads is not a limiting factor when slowing from say 100km/h. In performance cars that are expected to frequently stop from much higher speeds, bigger pads spread the heat load. Energy dissipated goes up with the square of speed. And 6 pistons can easily have more clamping force than 1 or 2 or 4 because of the geometry of the calipers.

Originally Posted by PhilipA

Then why do nearly all performance car manufacturers have 6 spot calipers and very long pads?
Why not just higher clamping force calipers?
Regards Philip A

Please, it's not "spot", it's "pot". "Pot" being slang for piston just like in engines.

They use more pistons to get more clamp, but due to a lack of space they have to use more pistons spread around the rotor. This makes a longer caliper which needs longer pads.

Originally Posted by PhilipA

We all know that but that is what you are doing by fitting larger diameter calipers and leaving the master cylinder alone. You are increasing the stroke.

No we are not "increasing the stroke", we are using more stroke. About 16% more MC stroke than stock.

Originally Posted by PhilipA

The question is , what is the outcome in an emergency : pedal to the floor?
Regards Philip A

The vehicle stops in a far shorter distance than was previously possible. The pedal moves about 16% more than it used to. Which is not even half way to the floor.

The job of the brakes is to convert the kinetic energy of the vehicle motion to heat energy and dissipate that heat into the atmosphere. Larger diameter tyres increase the mechanical advantage that the wheel has over the brakes, and require the brakes to do the energy conversion through less revolutions of the disc, thus less total swept area. Whereas with standard wheels and tyres, to stop in distance X means that the discs are rotating 100 times during the stop, perhaps now, with bigger tyres, the discs are rotating only 80 times during the same stopping distance. This means that the total area of disc touched by the pad during the stop (albeit the same bit of disc over and over again) is reduced by 20%, so there is effectively 20% less friction area available to do the kinetic-to-heat energy conversion and to dissipate that heat.
Picture a kid on a billy cart with only the heel of his shoe on the pavement for a brake. On your car, the shoe is replaced by brake pads and the pavement is replaced by a rotating disc- like the same bit of pavement coming round and round again past the kid's shoe. Reducing the rotation of the wheels in relation to forward motion - by fitting bigger diameter wheels- requires the kid to create the same amount of energy conversion through lesser distance of pavement. The kid has to put his foot down harder to do this. The heat is generated more quickly, but the kid's foot is only capable of dissipating heat at a given rate. The kids shoe gets hot and is able to do the energy conversion less effectively.
Increasing the caliper piston diameter simply provides more mechanical advantage through the hydraulic system, i.e. enables you to apply the brakes harder with less effort at the pedal (at the cost of greater pedal travel). In other words, the big wheels require the brakes to work harder; bigger caliper pistons enable you to make the brakes work harder. Vented discs reduce brake fade by helping dissipate the heat you are generating by all that hard work. Like putting a fan next to the kid's foot if you like.
Best of all would be to give the kid a bigger, wider foot. He wouldn't have to push down so hard because the friction doing the energy conversion would be spread over a larger area, plus it would dissipate the heat energy more effectively.
Seems to me that putting bigger wheels and tyres on the vehicle has resulted in your brakes being over-worked, and putting bigger caliper pistons on provides you with sufficient mechanical advantage to over-work the brakes in a comfortable and relaxed manner.

I have a 92 rangie vogue se I'm planning on putting 35's on, so I'm interested in doing this brake upgrade. Just to confirm, I'll be able to use my existing booster and MC, the following parts, and get a brake place to match up the lines? (all parts from paddocks for simplicity)

Front calipers
SEB500460G
SEB500470G

Front disks & pads (I assume any defender disks & pads would work?)
FTC902SD
STC2952

The main question I have is what to do with the rear calipers, as rovercare mentioned in this other thread that you should keep the calipers matched?
http://www.aulro.com/afvb/discovery-...h-tyres-5.html

Paddocks has a few different rear calipers listed, but this what I'm guessing I'd use.

Rear calipers
SMC500110G
SMC500240G

Rear disks & pads
FTC1381SD
STC9188M

A couple more quick details, I have braided lines front and rear, and while the RR came with ABS it is currently none functional due to some error and now having rovertracks front axles and CVs.

Originally Posted by POD

The job of the brakes is to convert the kinetic energy of the vehicle motion to heat energy and dissipate that heat into the atmosphere. Larger diameter tyres increase the mechanical advantage that the wheel has over the brakes, and require the brakes to do the energy conversion through less revolutions of the disc, thus less total swept area. Whereas with standard wheels and tyres, to stop in distance X means that the discs are rotating 100 times during the stop, perhaps now, with bigger tyres, the discs are rotating only 80 times during the same stopping distance. This means that the total area of disc touched by the pad during the stop (albeit the same bit of disc over and over again) is reduced by 20%, so there is effectively 20% less friction area available to do the kinetic-to-heat energy conversion and to dissipate that heat.
Picture a kid on a billy cart with only the heel of his shoe on the pavement for a brake. On your car, the shoe is replaced by brake pads and the pavement is replaced by a rotating disc- like the same bit of pavement coming round and round again past the kid's shoe. Reducing the rotation of the wheels in relation to forward motion - by fitting bigger diameter wheels- requires the kid to create the same amount of energy conversion through lesser distance of pavement. The kid has to put his foot down harder to do this. The heat is generated more quickly, but the kid's foot is only capable of dissipating heat at a given rate. The kids shoe gets hot and is able to do the energy conversion less effectively.
Increasing the caliper piston diameter simply provides more mechanical advantage through the hydraulic system, i.e. enables you to apply the brakes harder with less effort at the pedal (at the cost of greater pedal travel). In other words, the big wheels require the brakes to work harder; bigger caliper pistons enable you to make the brakes work harder. Vented discs reduce brake fade by helping dissipate the heat you are generating by all that hard work. Like putting a fan next to the kid's foot if you like.
Best of all would be to give the kid a bigger, wider foot. He wouldn't have to push down so hard because the friction doing the energy conversion would be spread over a larger area, plus it would dissipate the heat energy more effectively.
Seems to me that putting bigger wheels and tyres on the vehicle has resulted in your brakes being over-worked, and putting bigger caliper pistons on provides you with sufficient mechanical advantage to over-work the brakes in a comfortable and relaxed manner.

All correct, but just dancing around the point. Swept area is good for heat dissipation but does nothing for braking force. We could fit a smaller rotor with fatter pads and get the same swept area with lower torque and braking force.

The crux is torque. Your brakes produce torque on the wheels, that torque is resolved to a braking force at the road surface.
To stop harder you must apply more torque at the brake. Bigger wheels require more torque for the same braking force (at the road) than smaller wheels do.

To get more torque from the brakes requires any of the following.
1. Bigger disc rotor.
2. More clamping force.
3. Higher friction pads. (grippier pads)

Since a bigger disc rotor would require even larger wheels, few are interested in that for an offroad going vehicle.
Since higher friction pads are tempermental buggers (squeal, don't work well in the cold, don't work well in the wet etc) they are of limited practical use too.

Which leaves us with clamping force. Improvements in clamping force give a direct improvement in torque on the wheel and braking force at the road surface. It's there all the time and you still have the option of higher friction pads if you wish.

The 46mm pistons in the bigger calipers have 24% more piston area than the stock 41.3mm pistons. If they were fitted all round you would have 24% more pedal stroke. But since they are only fitted to the front the result is 2/3 of 24mm. Which is 16%.

So all bollocks about swept area and bigger pads aside, more clamping force works awesome. I have 6 years on this upgrade.

So if I fitted a longer brake pedal to give me 16% more leverage, would that be a brake upgrade? How about if I did some gym work on my calf muscles so I can press the brake pedal 16% harder- would that be a brake upgrade?
I don't mean the above to be smart-alecky, just an illustration.
I think you've hit the nail on the head Dougal with the fact that 'few are interested' in the effort and expense of fair dinkum upgrades like bigger rotors. Fortunately the RR brakes seem to have enough headroom to cope with the demands of the sort of mods many of us like to throw at them, in normal conditions. Certainly in theory many of the mods we do reduces the built-in safety factors of the vehicle. For instance, I often carry more than the specified 50kg including rack, on the roof, but have yet to tip the vehicle over. However, I know that the threshold for tipping is approached more closely with the heavier load up top.

Originally Posted by POD

So if I fitted a longer brake pedal to give me 16% more leverage, would that be a brake upgrade?

Yes it would. But it wouldn't match the 25% better of bigger calipers.

Originally Posted by POD

How about if I did some gym work on my calf muscles so I can press the brake pedal 16% harder- would that be a brake upgrade?

No, that would be an operator upgrade.