pads

[Dolly-Nut]

For road use all you need is better tyres. As long as the brakes lock then they a powerful enough, upgrades such as vented discs and pads will only change the pedal pressure required to lock them, as stated. Admittedly for track work vented discs is a must but on the road there are very few situations where they're needed i.e. towing a caravan down a steep hill - if this is what you like to do at weekends then yes, you need upgraded brakes.
Another point is that all pad/disc combinations will have a desired temperature to work at, harder pads will want a higher working temperature than the standard ones of softer material. This means that when you first apply the brakes a harder pad will need slightly longer to reach the peak friction temperature whereas a standard pad will hit it quicker. This will give different levels of response and pressure required at the pedal to lock, it's all down to the driver and what they feel safest with.

[AndyJ]

Completely disagree. It's about what happens before lock up. If you put two cars with exactly the same spec except one with std and one with vented discs the one with vented discs would pull up more quickly and in a shorter distance (before lock up). Of course if you lock the wheels on the same two cars then they will both skid to a similar unseemly halt, but it's about stopping under control and upgraded brakes will do this better and safer. Good tyres can and do improve braking but that's a different point altogether. Improving the brakes has always been the first step in any tuning guide ever as it's no good going faster if you can't stop as well.

[lazeruspete]

i must say this is getting a bit out of hand!



The main process for slowing a car down is converting the forward kinetic energy into heat, dissipating it into the brake discs and out of the centre of the wheel into the free air stream along the side of the vehicle.

brake 'fade' has three main mechanisms:

friction fade; over a certain temperature the friction coefficient of a material varys, increasing initially and then drops off over a given temperature. overheating the friction material essentially. i know this because I did a project on this at university...

mechanical fade; (only really applies to drum brakes). the heat causes the drum to expand away from the pads, causing a longer pedal.

fluid fade; boiling of the brake fluid. air bubbles are created in the brake fluid turning it from an (almost) non compressible fluid into a sponge.

it is a fact that bigger discs dissipate the heat away better from the whole assembly, increasing the total energy you can put into the brakes without fade.

tyres. if you can dissipate more heat more quickly(i.e. using vented/drilled discs) then you can retard the vehicle quicker. sadly if you do not change the tyres, then the limit of grip of the tyres will be found quicker. essentially there is no point fitting a bigger set of brakes if the tyres are not up to it.....stickier the better!

edit: upon inspection it seems i have reiterated what a few people have already said. can we not just play nice?

[AndyJ]

Yes, well said! Happy Christmas one and all.

[lazeruspete]

Yes, well said! Happy Christmas one and all.

*raises glass*

[GrahamFountain]

If you put two cars with exactly the same spec except one with std and one with vented discs the one with vented discs would pull up more quickly and in a shorter distance (before lock up).

Sorry, but the physics of this statement make no sense to me at all. The maximum brake force you can apply to a wheel before it locks is the weight on the wheel from the static weight of the car plus (at the front, minus at the back) the weight that is in effect transferred forward by the deceleration, plus any aerodynamic down force, all multiplied by the coefficient of friction between the road and the tire.

Ignoreing the aerodynamic forces (because not doing is too hard) that is Ff = µ/2 * (Mgr/wb + Mah/wb) at each of the front wheels and Fr = µ/2 * (Mgf/wb - Mah/wb) at each of the back. Where µ is the Coefficient, M is the mass of the car, g is the acceleration due to gravity, r is the distance between the CofG and the rear axle, f is the distance between the front axle and the CofG, wb is the wheelbase, a is the deceleration due to braking, and h is the height of the CofG above the road.

If you apply just less than that much force at all four wheels, the car will decelerate at the maximum rate and thus stop in the minimum time and distance. What I don't see is where the equations that describe these forces have a term in them for vented or unvented disks. So, can you explain how it is that a vented disc produces more braking force than an unvented one without causing the wheel to lock?

[soe8m]

I do miss the front spring rate and front damper setting in his one. Weak springs and soft dampers will transfer more weight to the front during braking so progressively you can add more stopping power.

Vented discs compared to non vented with the same dimensions and pad contact area can only add more stopping power temp wise. It is the friction type what determines what condition is best. For example most cheap pads will work the best with vented discs because at some temp they do not grip anymore due to evaporation. Pads like ferodo ds3000 need some heat before they start grip so these are effective at more temp.

In normal driving a non vented disc with ds3000 pads will be a better combi than a vented because these will not heat up thus stopping less. 1+1 is not alway's 2.

Jeroen

[AndyJ]

This is getting very complicated. The reason I say that vented discs will stop a car faster is because they have a greater surface area (four faces instead of two) and so will lose heat more quickly to the atmosphere. That's how brakes work, energy from car moving forward transferred to the disc as heat and lost to the atmosphere. Of course it's complex if you add in all of the other variables which is why I said two cars the same spec........ The simple fact is that the car with the vented discs will be able to lose heat more quickly and that heat or energy comes from the car slowing so the quicker you lose the heat the quicker the car slows.
How can that be wrong? And if so why are so many pretty mundane modern cars fitted with vented discs as standard?

[GrahamFountain]

Actually, the damper and spring rates will have almost no effect on the loads on the front wheels, it's the deceleration itself that does that. The main effect would be as the body rolls in the long axis, the CofG will move slightly, probably, forward and down. However, I doubt it will be very much at all if the CofG is near the middle of the car, as, generally, the back will go up more or less the same as the front will go down, and the car will tend to roll around the CofG, rather than it move far. I suppose it could be a design goal to make the role centre coincide with the CofG to improve the feel of the car in these cases. However, I also suspect that's a more important issue in lateral roll than longitudinal. It is slightly complicated because the suspension is non linear, having different responses to downward and upward forces because of the asymmetrical responses in the dampers.

However, after the response to the initial impulse (if you jam your foot on the peddle hard, would it be a bit on the "Heaviside Step Function" in LaPlacian transform terms?), and possibly a bit of ringing (assuming the suspension is less than critically damped), the effect of deceleration is a constant offset, mostly on h, and, in proportion, less so on r and f. That initial response will have some effect on the safety margin you want between the actual brake balance and the perfect, where all four wheels lock at the same time (at the highest possible value of µ). I haven't done a proper analysis of the dynamic effects, but I assume that the effects of Mah/wb will be reduced as the car is rolling forward and down, and increased slightly, if it's underdamped, as it "bounces" back a bit.

It should, therefore, be relatively easy to calculate the equilibrium effects of a on the position of the CofG from deceleration, through the spring rates, to confirm whether it is a primary, secondary, or, as I suspect, tertiary effect. Calculating the dynamic effects may be a bit more of a problem, but if the equilibrium effects are small, the dynamic effects can also be ignored.

[GrahamFountain]

Yes, the main effects that vented and or larger discs will have, assuming the standard ones can already lock the wheels for all values of the coefficient of friction that apply, are mostly in better cooling. However, unless the standard brakes would heat to the point of fade towards the end of the first braking event, they won't affect how quickly it will stop the first time. Probably won't have much effect on the second rapid stop either. But eventually, the car with vented discs will continue to stop when that without won't. They will also have some effect on how hard you have to press the peddle to get the car to stop as fast as possible, and thus a little, on response times – less lag.

The other effect that bigger brakes, specifically bigger diameter brake pistons and rotor diameters, have is to move the brake balance further forward, which is worth considering if you put significantly better tires on the car and or drive on better surfaces than the roads in not so sunny Lancs. That increases the maximum coefficient of friction which tends to move the position of the perfect brake balance point towards the front of the car. In which case, with the standard balance, the safety margin over the rear wheels locking first will be eroded. So, by increasing the fronts and, according to the Triumphtune performance manual, reducing the back brake efforts, the safety margin is got back.

The counter effect will be that which bigger brakes have on the unsprung weights. But that's a subject I know begger all about.

[Jod Clark]

This is getting very complicated. The reason I say that vented discs will stop a car faster is because they have a greater surface area (four faces instead of two) and so will lose heat more quickly to the atmosphere. That's how brakes work, energy from car moving forward transferred to the disc as heat and lost to the atmosphere. Of course it's complex if you add in all of the other variables which is why I said two cars the same spec........ The simple fact is that the car with the vented discs will be able to lose heat more quickly and that heat or energy comes from the car slowing so the quicker you lose the heat the quicker the car slows.
How can that be wrong? And if so why are so many pretty mundane modern cars fitted with vented discs as standard?

You seem to miss the point, or the detail of what is going on when you press the brake pedal. Maximum retardartion is achieved at the point fractionally before the tyres lose their grip on the road surface, i.e when maximum kinetic energy is being converted into heat. This is what ABS achieves. If your standard brakes can generate enough force to overcome the friction between the tyre and the road then what will uprated brakes do? Simply, they will make it easier for you to lock the front wheels and once the wheels are no longer rotating then your brakes aren't doing their job any more.

The limiting factor for maximum braking effect on a standard vehicle is the friction between the tyre and the road. Once you have enough grip between tyre and road that the wheels will not lock-up under braking, then you can start worrying about adding more braking force using larger / vented / drilled / grooved discs until you either get back to the point where the tyres lose grip under maximum braking again, the brakes overheat and fade, or the forces involved tear your front suspension off.

[GrahamFountain]

Just as a matter of interest, does anyone have a figure for the braking effort that standard sprint discs can give? I guess it needs to be for a reasonable set of pads, i.e. with a relatively high coefficient of friction to the disc rotors.

[soe8m]

You seem to miss the point, or the detail of what is going on when you press the brake pedal. Maximum retardartion is achieved at the point fractionally before the tyres lose their grip on the road surface, i.e when maximum kinetic energy is being converted into heat. This is what ABS achieves. If your standard brakes can generate enough force to overcome the friction between the tyre and the road then what will uprated brakes do? Simply, they will make it easier for you to lock the front wheels and once the wheels are no longer rotating then your brakes aren't doing their job any more.

The limiting factor for maximum braking effect on a standard vehicle is the friction between the tyre and the road. Once you have enough grip between tyre and road that the wheels will not lock-up under braking, then you can start worrying about adding more braking force using larger / vented / drilled / grooved discs until you either get back to the point where the tyres lose grip under maximum braking again, the brakes overheat and fade, or the forces involved tear your front suspension off.

That is it. If you have a wet road the lock up can be achieved with any brake.

The actual maximum brake power is to the friction material and area. Not vented, not dia. With vented discs you can repeat your braking attempts more because the pads do not overheat so a more consistant brake and safer for today's speeds and number of cars on the road. From then it's the tire/roadsurface that determines how much of the maximum you can use before lock up.

To have as much pressure on the front wheels during braking you need as much weight transfered to the front so as much dive you can get.

Jeroen

[GrahamFountain]

My experience with Sprint brakes is, I admit, limited; however, what I've found with TR7s is that it is usually the servo that's the biggest problem. If this isn't working reasonably well, the brakes will be rubbish. You can get over that by putting bigger, better brakes on the car, but it tends to be a bit more of an expensive solution than getting an overhaul kit for the servo. I remember one 7 where a guy was rabbiting on about the crapness of the standard brakes on a 7, and what he wanted to put on instead. But when Paul Towle (onetime TRDC TR8 reg.) tested the servo, there was no fall in the peddle on starting - you can do a simple test on the servo by pressing the peddle and then starting the engine. The peddle should go down (on a 7) about an inch or so. But it's only a rough measure. If there's no movement, you've got at best (on a 7) HA Viva brakes, which won't stop almost one and a half tons of car and contents so well.

The brakes I have on the current Doly Sprint are, in my opinion, perfectly adequate for road use, but I did have the servo overhauled as soon as I got the car. Though this thread does prompt me to take a wheel off and check if they are standard or not.

[GrahamFountain]

The actual maximum brake power is to the friction material and area. Not vented, not dia.

Actually the brake force from a disc is about the diameter of the piston and the diameter of the rotor and the hydraulic pressure. Oddly, pad area isn't part of the equation - you need to look at Amontons' laws of friction to see why pad area isn't a term. As I remember, that rotor speed isn't part of it either is one of Coulombs'' laws (no "charge" for that last bit of info).

To have as much pressure on the front wheels during braking you need as much weight transferred to the front so as much dive you can get.

Two of the problems with getting as much weight transferred to the front as possible, are that this takes as much off the back wheels as it adds to the front, which leads to the rear end breaking away; and it means putting the CofG as high up as possible, which is not fun on corners. The other issue I have is that this confuses dive with transfer, implying that weak front springs will improve the braking, which is simply not true once the car's stopped tipping up, and probably not true while it is still tipping. Also, as a TR7 driver, all that dive at the front they do is not nice.