Rear Discs

fubar andy

Moderator & N/W Rep
Staff member
Cna you put vented discs on the rear without changing calipers or is there a better solution/conversion?
In a nut shell, no!

I still run single discs however my upgrades are grooved discs from Black Diamond, Carbotech XP8 pads and braided lines.

I keep looking at adding bigger discs for better braking torque and less heat, but I've not got round to it as its a bit of a faff even though my car spends most of its time on track!
 

Eng1

New Member
All cars have a similar situation :
Maximum braking is when all four wheels are locked up , all cars can achieve this but tyres would obviously last seconds !
So the manufacturers cater for this by sizing things like the master cylinder piston size/travel , caliper piston size and travel so the brake feel is altered , so it takes a Mighty push to lock up .
As the car brakes , weight is moved forwards , the front shocks compress and the rears extend .
Almost all braking load is on the front brakes , hence twin discs and vented discs , rear brakes take much less load and are less effective partly due to the effect of weight reduction on the tyres .
Improving braking is all about heat dissipation and improving feel , hence larger surface area discs , multiple pistons , better compounds of pads , better fluid ,less likely to boil .
The actual force is the same as it’s the same master cylinder being pushed by the same leg but the feel is altered due to multiple pistons in the calipers and larger surface area of pads .
The heavier the car the more heat energy needs dissipating , the faster the car likewise .
An example of weight on Tyres is an aircraft ,once landed , you may notice large panels open on top of the wings , these dump lift pushing the full weight of the aircraft down onto the tyres to allow the brakes to work without these the aircraft would not stop , the brakes would glow red hot but not slow the machine , they use multiple discs , about 10 or 12 and multiple pistons , about 20 ,and the pads are metal (copper alloy) on each wheel for the heat and fans to cool . But the principle is similar
If you drove backwards at 90 mph and then brakes were full on ,I expect it wouldn’t end well
 

Jon Olds

Well-Known Member
I have to beg to differ on a couple of the above points. Maximum braking isn't when all four wheels are locked. If it was, there would be no need for ABS. A four wheel drive brakes slightly differently to a 2wd, in that the rear retardation is 'back fed' into the transmission partly tending to have an averaging effect across the weight transfer. This is why I went to rear 4 pots, and why I can outbrake a similar weight, similar braked, similarly tyred 2wd car.
Again, the above is from experience, not from what the web believes. I will say the GTIR ABS is so crude that I pulled the fuse on my rally car at the end of the first stage of the rally I did after building it, a lot of years ago.
Jon
 

fubar andy

Moderator & N/W Rep
Staff member
A four wheel drive brakes slightly differently to a 2wd, in that the rear retardation is 'back fed' into the transmission partly tending to have an averaging effect across the weight transfer.

I will say the GTIR ABS is so crude that I pulled the fuse
Agreed on the 4WD point, my rear brakes can get very hot when used on track, far hotter than I've seen any of my FWD cars.

Plus I took out my fuse too, feels much better without that "so called" ABS!
 

Eng1

New Member
Interesting discussion , when I say maximum braking , I mean that once all four wheels are locked the brake force on the wheels is at a maximum and tyres now provide the only friction , which of course now allows them to skid and loss of control , hence abs to cater for road surface friction , aqua planing and so on .
The point is if stock brakes can lock up , bigger brakes can only lock up too as a maximum force .
90s jap abs technology was not that good across many cars.
I’m not seeing weight transfer via driveshafts diffs at all , I think this needs more clarity .
Sure there is engine braking but where is the weight transfer path?
Of course bigger brakes /discs more pistons can disperse heat better and reduce gas build up on pads and alter brake bias front to rear , given there is only one master cylinder .
If weight is transferred via driveline components during decelerating them , then the reverse would be true during accelerating them ?
I’m thinking the weight is always via wheels to the ground , and the shift forward of C of G during deceleration via the chassis and springs
 

Eng1

New Member
With a 62/38 split as standard , the question is now how much weight is pushed forwards during deceleration? Let’s say 10-20% ?
So upto 82% of the effective weight is across the front axle
If only 18% is across the rear tyres (9 % each) then the tyre braking ability is low , depending on the brake bias , the ABS will see this as a failure to retard rotational speed ,so apply more braking ( via not intervening) and create a lot of heat in the discs /pads despite not actually improving stopping .
A real world scenario that I have seen :
On an aircraft a wayward baggage trolley hit a cable on the undercarriage (teleflex cable) , not reported so the bent cable failed to operate .
On the next landing the panels that open to reduce lift did not open (it’s automatic) , the aircraft would not slow down , the ABS (anti skid) went to maximum braking , the aircraft nearly went off the end of the runway , all the brakes were glowing red hot ! The heat plugs in the wheels all blew and all the tyres deflated
Glowing hot brakes ,maximum application but not slowing , that is the effect of no weight on tyres !!!!
Why no weight ? Because the aircraft is still producing lift as it’s moving along the runway so 100 ton aircraft effectively weighs much less
The same is true of the back of a car , it rises during braking , effectively weighing less across the rear tyres
 

MarkTurbo

Well-Known Member
If only 18% is across the rear tyres (9 % each) then the tyre braking ability is low , depending on the brake bias , the ABS will see this as a failure to retard rotational speed ,so apply more braking ( via not intervening) and create a lot of heat in the discs /pads despite not actually improving stopping
ABS will only come into the equation if it detects a wheel has locked, otherwise it's just sat there in the background monitoring wheel speed.

I don't really understand your comment about creating a lot of heat without improving stopping? I think the point you're trying to get at is right but you're confusing things by some of what you've said and all the weird aircraft references :tearsofjoy:

The simple way of looking at it is when manufacturers design the braking system on a car they always size the front and rear brakes to make sure the front brake force exceeds traction of the tyres before the rear as it's easier for Joe public to sort out a front brake slide.

So as standard you've got a slightly higher bias to the front, once you fit a big brake kit you are actually making that front end bias worse and under braking the rear. You can either sort that out by fitting pads with a higher friction coefficient, larger discs, different calipers etc at the rear. You just need to make sure you don't go too far and make the rear bias go past the point of locking before the fronts.

Given that only around 15-20% of the braking goes to the rear you'll probably only need 4 pots, vented discs or both at the rear if it's a rally car or you do long track sessions and heat is an issue.

I've got AP racing 4 pots with 330mm discs on the front of my car and I'm using slightly bigger discs (280mm) with standard calipers on the rear.
 

Jon Olds

Well-Known Member
Above, agreed. My tarmac rally car brakes great. I made mods, they work. Nothing brake wise I would change. Would be great if proper pads cost less, but hey ho
Jon
 
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