"If you can lock up then brakes are good enough" Why false?

[nsi]

I was getting some uprated pads fitted yesterday and was discussing with a mate about how some owners go for bigger calipers and more pistons. He said "well, if you can lock the wheels up with standard brakes then they are good enough".

I stopped him and tried to explain why this wasn't the case but failed to articulate myself! Can someone please explain in simple(ish) terms why uprated brake pads/calipers are better than standard?

I was thinking, maybe its because although you can lock you wheels with standard brakes by giving a sharp "stab" at the brakes, you can't modulate braking force up to the point of lockup as you could with performance brakes.

Anyone else know?

Cheers

[dtmpower]

Well - imagine if you only had brakes that could lock.... you would never be able to stop as quick.

The best 'braking' affect 'mu' is seen when the wheel/brake is at the point just before the wheel locks and the tyre skid.

[900T-R]

If you can lock up the brakes - or more to the point, keep your wheels right at the onset of locking up where brake force transferred to the road is maximised - every time for as long as you need it your brakes are effective enough. Anyone who's ever been on a track day understands the significance of this proviso.

And then, of course, there is the small matter of brake pedal 'feel' and brake balance (the reason TVR Chimaera/Griffith owners tend to go for larger front discs is partly to overcome a balance that is slightly too rear-biased for most roadgoing applications).

Edited by 900T-R on Monday 9th June 09:56

[richardb.jones]

On single application - nobody can realistically argue with that statement. Yes you have factors such as tyre adhension, road surface etc all to take into account - but after all that and you can still lock the wheels, then the brakes 'work'.

Big/upgraded brakes have little to do with the ability to lock the wheels on a single instance - they have the durability to KEEP locking the wheels (nearly). ie where standard brakes would fade / fluid would boil / pads would glaze over or whatever, big brakes can take the punishment.

[Avocet]

I like to think about it in thermodynamics terms. "You can't create or destroy energy - only convert it from one form to another".

So the job of a decent set of brakes is to take the kinetic energy that your car has built up as a result of its speed, and turn it into heat, then get rid of that heat as quickly as possible. Everything about a decent set of brakes should be about turning movement into heat and then dissipating it. The trick is to run them for as long as you can on the point of locking up without them overheating and fading OR locking up. As soon as they lock, they're not generating heat - which means they can't be dissipating it (only the residual heat left over from what they were dissipating immediately before they locked).

If you told your mate that as drum brakes tend to lock up easier than discs, you were going to "upgrade" to drums all round, I think he'd maybe get the message!

[Mr2Mike]

This is a very common misconception. When you lock the wheels of a car under braking, the brakes themselves are doing no work whatsoever, all the energy being dissipated to stop the car goes into melting your tyres.

To stop a car as quickly as possible, you need to keep the wheels on the verge of locking. Under this condition, the brakes are dissipating an enormous amount of energy and get very hot, very quickly. The puny brakes fitted to most standard shopping cars will give you a hard stop from 70 maybe once or twice, before they suffer from terminal fade. And because you are operating the brakes at, or past their limits they are difficult to control precisely to avoid locking the wheels (i.e. modulation).

Bigger brakes can dissipate more energy, so they can stay cool enough to keep working under hard use. They aren't working at their limits all the time, so you get better pedal feel and modulation.

For the average shopping trolley being driven by the average housewife, the standard brakes are perfectly adequate. If you drive the car hard on the road, you can reach their limits quite quickly, and on the track they'd be cooked within a lap or two.

[trackcar]

There's other issues to take into account on the brake sizing other than th all-important balance ..

the larger the rotor the greater the linear velocity of the rotor past the pad for any given angular velocity, this helps prevent brake grab on the point of lock-up, so the final step to keeping the tyre just on the point of lockup is easier.

Also tyres tend to produce their best grip when there is an element of slip between tyre and ground, only a few % is needed but a tyre which is not just slightly sliding relative to the ground isn't on it's point of maximum traction, so you don't want a full lockup neither do you want a tyre which is rotating faithfully relative to the ground (although that is preferable to a full lock up in most cases, snow and gravel being the cases where locked brakes work better in a pure retardation way).

The other aspects to consider are that going too big might mean you never get good temp into a pad/discs because you never have to apply much pressure, a big rotor is heavy so once a tyre/wheel has locked it takes longer to get it spinning again which is bad for trying to recover directional control, and large diameter rotors are more prone to judder caused by inconsistent burnishing of pad material into the disc surface.

There's many more reasons why you should spec brake upgrades properly, but that's just a few to be going on with.

[900T-R]

"Fit the smallest brakes that won't fade" seems a good adagium to me.

[HiRich]

The first thing to remember is that a vehicle has four (or more) contact points. Each has to be working to its optimum to achieve maximum retardation. The first problem here is that if you have all four wheels locked, you have bigger problems to worry about than how quickly you are going to stop. This is because whilst locked front wheels is dynamically stable (the car will plough on), a locked rear axle is dynamically unstable - the back end will come round. Usable if you are Noriyuke Haga, very bad news otherwise. As a result, no properly set-up vehicle makes maximum use of rear wheel braking, and maximum retardation will not be achieved.

The second point, already well-covered, is that optimum retardation is when the wheel is still revolving, albeit with a slight amount of slip (wheel roation speed is slightly less than road speed). If you lock the wheels, the friction drops off significantly, and retardation drops similarly. You want a wide plateau of operation, so you can play with the brake pedal in this area as other factors regarding the friction conditions vary, whether in one emergency application, or in repeated applications (e.g. a hot lap). If the optimum zone is like a knife edge, a reasonable driver will struggle to consistently get the most out of it. If it's a nice wide zone, we can all play safely. This is one area where brakes can be improved.

Third, the parameters vary. As you stamp on the pedal, the fluid compresses. The pads 'light up' and the coefficient of friction between pad and disc changes. The strain on the rubber changes, which puts heat in the rubber and changes the friction between rubber and tarmac. As you continue to move forward, the condition of the actual tarmac varies and the potential friction available varies. And let's not forget the servo totally screwing with the feedback system.
All of these change dynamically. Not just through one application, but each corner is different, and the system may still be recovering from the previous application(s). This would be fine if your brain and the hydraulic system could react at something like 32Mhz, but neither can.
An uprated system can improve the situation in two ways:

• By minimising the variations (both through one application and repeated applications), and much of this can be done by removing heat quickly from the places where it can cause these problems. Note, however, that this isn't just about slavishly removing all heat. You also need to quickly get the components into their working temperatures - if they are ice cold, the initial friction can be very poor and you've slammed into the car in front before they finally light up. On the road, the same has to be borne in mind for all the light applications (easing up to a junction, checking back to the speed limit), which happen much more frequently than hard applications.
• Working back out from the caliper, by creating a feedback system (pedal pressure and movement) that is consistent and informative to the driver. Note for example how if you have to input a huge pressure, it's very hard to fine tune it (just as it's hard to modulate a featherweight input).

So as well as being able to achieve maximum retardation, a good brake system is also looking for:

• Consistency: through one hard application, through multiple applications, and for minor applications (and wet/dry performance).
• Feedback: so the driver can modulate his input according to the instantaneous conditions.

There are several ways a brake system may be upgraded. One important method is the larger diameter. Since the leverage is increased, the application force (pad pressure) can be reduced for the same retardation. This gives you scope in a couple of areas:

• More choice of pad material: you may be able to move to a more stable material with more consistent performance in all situations.
• Lower pad pressure means lower line pressure. In itself this might mean a better feedback to the pedal, but you may be able to reduce the servo assistance as well.

Other options may be:

• Larger (or possibly smaller) pistons, changing the pedal feedback
• Extracting more heat (e.g. vented discs), or better insulation of the brake fluid from the heat source.
• A more rigid caliper, so line pressure is more accurately converted into pad pressure

and more.

So you can see that 'better brakes' is rarely about creating more friction, it's about building in more consistency and better feedback. It's also important to recognise that 'better brakes' means different improvements for different vehicles. Bigger discs may not the way to go with every car. Each vehicle needs to be assessed individually - to see how it performs in a wide range of conditions, to understand what parameters the original designers were working to. You may have different parameters (e.g. track vs road, a willingness to fit larger wheels to accomodate larger discs), and then choose the modifications that bring the performance, feel and consistency towards what you want.