How to Choose the Best Big Brake Kit
In this technical article, we are going to run through some of the fundamental braking theory on which EBC Brakes’ unique Balanced Brake Kit™ is based. We will explain why ‘bigger isn’t better, balanced is better’™ and provide the inquisitive customer with all the facts and knowledge they should be aware of before investing in a performance caliper and rotor upgrade kit.
For decades car enthusiasts have purchased ‘Big Brake Kits’ in a mission to improve the braking performance of their vehicles, yet most Big Brake Kits sold in today’s marketplace supply the hardware to upgrade just one of the vehicle’s axles (either front or rear). The consequence is that the braking torque on the upgraded axle is increased significantly, meanwhile the other non-upgraded axle is often left as stock, with the original brake pads and/or original rubber brake lines fitted. The result is that the vehicle fitted with this ‘Big Brake Kit’ likely now has a totally imbalanced braking system, potentially leading to longer overall stopping distances due to the front & rear tyres not sharing the braking load proportionately.
Contrary to what the ‘Big Brake Kit’ (or ‘BBK’) name implies, fitting a massively oversized rotor or a brake caliper with a much larger combined piston area than stock is not a steadfast way to improve overall braking performance. In fact, this kind of approach to brake system design is potentially dangerous and is more likely to decrease overall system performance rather than improve it. That’s why EBC Brakes has adopted an altogether different approach for the design of our ‘big brake’ kits, one where EBC carefully selects the optimum brake rotor and caliper combination for each vehicle application. Further still, whilst it is common for front ‘Big Brake Kits’ to include pads and sometimes even braided brake lines for the front axle, EBC also provides matching friction brake pads and braided brake lines for the rear axle, at zero extra cost to you. This unique approach is based on core vehicle dynamic principles and an appreciation of the importance of achieving an appropriately balanced brake system to improve vehicle handling and minimise vehicle stopping distances. EBC has developed a range of vehicle specific brake upgrade kits that make it effortless for our customers to realise the maximum braking performance from their vehicle, by giving them every component they need in one complete upgrade package. Alas, when it concerns EBC Brakes the abbreviation ‘BBK’ adopts a new meaning. Introducing EBC’s Balanced Brake Kit™.
Before we go into some of the more involved aspects of braking theory it’s important to acknowledge some of the core principles. Firstly, in order to decelerate a moving vehicle all braking torque must be transmitted through the cars contact patches with the road i.e. the cars 4 tyres. Fans of Formula 1 will recall commentator Martin Brundle’s quote when one of the drivers locks a wheel under braking: “the wheel is only decelerating whilst the wheel is turning”. Well that isn’t quite accurate, since this logic would imply that when a tyre is locked up it has absolutely zero friction with the tarmac, but there is certainly something we should take away from Brundle’s quote here. The exact physical principle Brundle is referring to is: the coefficient of static friction is greater than the coefficient of dynamic friction. What this means is that a tyre generates more grip when there is no relative movement between it and the tarmac (i.e. no slip) than when there is dynamic motion between itself and the tarmac (i.e. the tyre is slipping or is locked up). Without wishing to go into the finer details of why exactly this is (lots of results on Google if you want to read up further on this) what this principle fundamentally states is that a tyre is capable of transmitting more force into the tarmac if it is not slipping. This is easily demonstrated by imagining two identical cars pulling away from a standstill. One driver dumps the clutch and spins away all the engine power, meanwhile the other driver progressively feeds in the clutch to pull away with minimal tyre slip. It’s obvious which vehicle will achieve the better getaway.
The fact that the coefficient of static friction is greater than the coefficient of dynamic friction is also the reason why an ABS system releases the brake momentarily if the wheel locks. Releasing the brake for a fraction allows the wheel to regain the same rotational speed as the vehicle, restoring the condition of zero relative motion between the tyre and the tarmac which in turn increases the coefficient of friction between the tyre and the tarmac. A tyre with more grip can transmit higher braking loads, which ultimately means the car is able to decelerate at a higher rate. This is why an ABS assisted stop is shorter than if the car simply skidded to a stop with all wheels locked up.
Taking into consideration both things we just learnt:
- All braking torque must be transmitted through the 4 tyres into the tarmac
- A tyres ability to transmit force decreases when it begins to slip
We can substantiate that the maximum possible rate of deceleration for any given vehicle is achieved when all the tyres are right on the limit of the available grip, just before any of the tyres lock up. Suddenly the importance of having a properly balanced brake upgrade becomes apparent if lowest possible stopping distances are to be achieved. If the front brakes are locked up but the rears are doing hardly any of the work, all the surplus available grip from the rear tyres is not being put to good use towards decelerating the vehicle. This is the fundamental flaw with front only ‘Big Brake Kits’ and explains how it is possible to spend thousands on a performance brake upgrade but end up with a vehicle that brakes worse than stock…
Now all this talk about being on the limit of available grip and brakes locking up is commonplace if you’re a racing driver, but chances are you won’t be locking up brakes if you’re using your car on the public roads for a trip down to the shops. Hence you might be reading this article and thinking ‘why should I bother with a performance brake upgrade if I don’t drive my vehicle on the limit all of the time?’. Well EBC Brakes do not infer for a moment that just because you fit one of our Balanced Brake Kits™ that you should be pulling emergency stops on the limit of locking wheels every time you hit the brake pedal, but the truth is that having a properly balanced brake set-up reduces the tendency to lock up under braking in the first place. The fact that all four corners of the car are doing their fair share of the braking means that you have more headroom to brake later and decelerate harder in a controlled and safe way before any of the tyres exceed the grip available, whether this be on the public highway or indeed on a race track. Furthermore, having a properly balanced brake system also translates to improvements in the vehicle’s handling, reducing the tendency to oversteer or understeer when braking at corner-entry or whilst trail braking. Quite simply, when it comes to performance brake upgrades: bigger isn’t better, balanced is better.™
With all that extra braking performance on tap, fitting a Balanced Brake Kit™ is like buying a new sports car. Sure you won’t do 0-60 mph in 5.0 seconds every time you pull away from standstill, but on that occasion you find yourself on a clear road (or track) with the sun shining it’s nice to know that you’ve got performance in reserve to have that truly spirited drive.
Anyway, back to the theory. Having already established that the maximum vehicle deceleration is achieved when all 4 tyres are operating at the limit of available grip but without locking up, it seems convenient to define the equation which governs the grip produced by a tyre:
Where µ = coefficient of friction of the tyre and FΝ = Normal force or load acting through the tyre.
The above equation shows that if either the coefficient of friction or the load through the tyre is increased, the maximum force that the tyre can transmit to the tarmac also increases. Fitment of a higher quality/grippier tyre seems like a blatantly obvious way to increase the available grip, but the fact that increasing the load acting through the tyre also serves to increase the available grip is perhaps slightly less obvious. Assuming the vehicle has the same quality of tyres fitted on all 4 corners (strongly advisable by the way!) we can simplify the above equation, disregarding the coefficient of friction term as a constant and concluding that the available grip from a tyre is directly proportional to the loading through the tyre. It now seems like we have the very straightforward task ahead of us in our quest to determine the sweet-spot for balanced braking nirvana. Simply drive the vehicle onto a weigh bridge to determine the weight distribution over the front and rear axles, then design a brake system to match. For example, if we took a vehicle with perfect 50:50 weight distribution, the perfect brake balance would also be 50:50. Seems too easy? Well, this approach would be entirely accurate under static conditions, however when we consider dynamic conditions and more specifically a vehicle decelerating from speed, another crucial factor comes into play: weight transfer.
Weight Transfer Under Braking
Weight transfer is the action of the vehicle’s weight getting thrown forwards onto the front axle under braking. There are several factors that will dictate exactly how much weight is transferred under braking, as we shall see in just a moment, but no matter how much you attempt to minimise weight transfer it can never be reduced to zero. This means that whenever you hit the brakes, the front axle becomes more loaded whilst the rear axle becomes simultaneously unloaded, which in turn has a knock-on effect on the brake balance.
This seems like a good point to introduce the equation for weight transfer:
Hence we can see that:
- A higher deceleration = more weight transfer
- A higher overall vehicle weight = more weight transfer
- A higher centre of gravity = more weight transfer
- A shorter wheelbase = more weight transfer
The most important observation from the above equation is that weight transfer is proportional to the rate of deceleration: hit the brakes harder and you’re going to get more weight transfer. Assume for instance that a car has a perfect 50:50 weight distribution when at a standstill. This same vehicle performing a typical 0.5g stop might have a 60:40 weight distribution biased to the front, whilst the same vehicle performing an emergency stop of 1.0g or greater might have a weight shift as dramatic as 75:25. The diagram below illustrates how the rate of deceleration affects the vehicles weight distribution.
Average stop = 0.5g decel
Emergency stop = 1.0g+ decel
Any increase in weight on the front axle increases the available grip from the front tyres, whilst the lightening of the rear axle in turn reduces available rear grip. An increase in the available grip on the front axle should be matched by an increase in the braking torque for the axle, since the tyres can now be worked harder before the available grip is exceeded and they lock up. Hence we can see that the principle of weight transfer constantly impacts the sweet spot for brake balance. Suddenly the task of designing that perfectly Balanced Brake Kit™ looks a little trickier.
If the driver braked with the same rate of deceleration every time, this would make a brake engineers task considerably easier as they could then set the system up to operate well under those exact conditions, but given that it is entirely unreasonable to assume that the vehicle will decelerate at the same rate every instance the brakes are applied, what we actually end up with is a brake set-up with a certain degree of compromise. This compromise allows the vehicle to perform well under the wide range of situations and conditions that the vehicle might encounter.
Given the numerous factors that we now realise come into play, you might be thinking ‘how exactly do you achieve a balanced brake system then?’. The first step is always to define the desired attributes of the final system. This will in turn highlight where a compromise should be made, or more specifically, whether the vehicles brake system needs a ‘front bias’ (over braked front axle), ‘rear bias’ (over braked rear axle), or a ‘neutral bias’ (even split between front and rear axles).
Generally there is a trade-off between outright braking performance (i.e. shortest stopping distances) and a more conservative balance which favours drivability and forgiveness when approaching the limit. A vehicle with a rear brake bias is unforgiving and rarely desirable since any locking of the rear brakes causes oversteer, which is unduly difficult to control for the inexperienced driver. On the other hand, a vehicle with a front bias is the conservative approach and very common in road-vehicles, since locking the front brakes leads to understeer, but understeer is comparatively much easier to control for the driver than oversteer. Finally, a neutral brake balance gives the best trade-off between controllability and outright performance and is therefore considered the optimum brake set-up if shortest overall stopping distances are the goal.
Despite a neutral brake balance giving the shortest stopping distances, you might be surprised to hear that all vehicle manufacturers set their cars up with a 5-10% front brake bias from the factory. The reason manufacturers adopt this conservative approach is because it makes the car easier and more benign to control for the average Joe driver, however, the trade-off is that some braking performance is sacrificed because the rear axle is not doing as much of its share of the braking load as it could be. Before we continue it should be noted however that a small number of modern vehicles do use a bias valve that sends more braking torque to the rear axle if the vehicle is heavily loaded (more passengers for example) but this device only adjusts the brake bias of the car under heavily loaded conditions. This bias valve should therefore not be confused as having an impact on the general brake bias of the vehicle, the vehicle itself will still be set up with a 5-10% front bias.
The philosophy of an EBC Balanced Brake Kit™ is to supply new components that together make a fine adjustment to the vehicle’s braking system (whether a rear bias valve for loaded conditions is fitted or not). By careful pairing of the components, EBC can move the bias rearward ever so slightly, back towards a more neutral brake balance rather than the 5-10% front bias a vehicle has from the factory. This more neutral brake balance ensures that the available grip from both axles is utilised fully under harder braking, giving better vehicle handling, reduced brake component wear and shorter overall stopping distances. Let’s go into a little more detail around how exactly EBC Brakes select the components of a Balanced Brake Kit™ for a specific vehicle.
Fine Tuning Braking Torque
Lets start off with a diagram and equation for braking torque:
Where µ = friction coefficient of pads, r = torque arm radius of pad on the rotor (which is taken as the midpoint on the rotors pad track), F = force applied by brake caliper (NOTE: the constant 2 comes from the fact that the caliper force F clamps the pads onto both sides of the brake rotor)
From this equation it can be seen that:
- Use of a higher friction brake pad = higher braking torque (EBC’s speciality)
- Fitment of a brake caliper with larger clamping force (i.e. a caliper that uses larger pistons) = higher braking torque
- A larger torque arm radius (i.e. fitting a larger brake rotor) = higher braking torque
Thinking about the brakes on a typical road car for a minute, this explains why we generally see larger diameter rotors fitted on the front axle, usually accompanied by larger brake calipers that use larger pistons. The differences between the braking components used on the front and rear axles are all evidence of vehicle manufacturers tuning the brake balance, fitting larger components on the front axle to shift the brake balance forward in order to account for the effects of weight transfer that increases the load on the front axle under braking.
Now for some myth busting:
Before going any further, notice how the above equation makes no mention to the number of pistons fitted in the caliper, it only refers to the caliper’s clamping force as F. Since Force = pressure X area the number of pistons in the caliper has zero effect on the caliper’s clamping force, the only thing that matters is the pressure and piston area, or in the case of multi-piston calipers, the combined summated piston area. Since the generated system hydraulic pressure is proportional to the force applied to the master cylinder, providing the master cylinder is not changed then any given force on the brake pedal will result in an identical system pressure. We can therefore ignore the pressure term for a given pedal effort, stating that the caliper clamping force F is directly proportional to the caliper’s combined piston area. It is therefore straightforward to deduce that increasing the caliper’s combined piston area will increase the clamping force produced by the brake caliper (at the expense of having a slightly longer pedal travel, due to the higher volumetric displacement associated with a larger piston area).
Quite simply this means that a 4-piston caliper fitted with larger pistons will have the exact same clamping force as a 16 piston caliper fitted with tiny pistons, providing that the combined piston area of both calipers are the same. It is surprising how often customers are miss-sold calipers with higher quantities of pistons, being persuaded through misguided marketing that the more pistons a caliper has, the ‘better’ the braking is. The only arguable benefit of using a higher number of smaller pistons is a potentially more even force distribution across the backplate and a slight improvement in pad cooling (since more of the pad backplate is exposed to fresh air) but in the grand scheme of things these benefits really are marginal and the fact that all EBC calipers use serrated nose pistons as standard totally nullifies this advantage, since serrated nose pistons allow better air circulation over the pad backplate than what could be achieved with several non-serrated nose pistons anyway. Furthermore, the first calculation EBC makes when designing a new brake caliper is to determine the optimal force distribution for the chosen pad shape, which in turn dictates the optimal number of pistons for the brake caliper. For example: take a 132mm width pad, EBC would choose a 4-pot design for this relatively narrow pad, whereas some competitors opt for a 6-pot design. EBC do make a 6-pot design, but it uses a different and larger brake pad that is 20mm wider and 4mm deeper. This larger pad needs 6-pistons to achieve an optimal force distribution, the smaller pad simply does not need 6-pistons. Hence care should be exercised when comparing different brake calipers, just because a caliper uses more pistons does not necessarily mean it is superior. It is quite simply not correct to compare calipers solely based on the piston count, instead more scientific measures should be used for comparison, such as combined piston area and pad area/pad volume.
In truth, the only thing you can guarantee by purchasing a caliper with a high number of pistons is that it will have a high price tag, because caliper pistons are machined to extremely tight tolerances and are therefore very expensive to produce. More pistons means more cost, so calipers designed with an unnecessarily high number of pistons will be very expensive, and if they are not very expensive it probably means that corners have been cut elsewhere, reducing the cost and quality of other components in the brake caliper design to reach a final price point that the customer will accept.
EBC Brakes have no interest in entering the ‘top trumps’ contest with other caliper manufacturers who unnecessarily include high numbers of pistons in their brake calipers solely for the purposes of false marketing. Instead EBC Brakes opt to allocate budget to components that actually make a difference to the performance of our products and add value to our customers, such as all of our brake calipers using stainless steel X-over pipes, stainless bleed nipples, stainless pad wear plates, serrated nose pistons and also hard anodising the caliper body before the final paint top-coat. All these things add up to produce a brake caliper that is of the highest quality and will look great and perform great for years to come.
Now that we’ve busted that myth, lets turn our attention back to the equation for braking torque. The torque arm radius r is determined by the size of brake rotor fitted and generally a brake rotor should be the largest diameter you can accommodate under the wheel rim. A larger rotor usually has a larger swept area which increases the rotor’s ability to absorb and then dissipate heat, a key requirement for high performance brake systems. The fact is however that fitment of a larger brake rotor than stock means that in order to maintain the same brake torque on the axle, the size of the brake caliper pistons need to be reduced accordingly. By careful selection of the appropriate rotor size and piston sizes, it is possible to adjust the mechanical components of the vehicle’s brake system with finesse in order to achieve the desired brake balance across the front and rear axles. This is the balancing act considered during the design of every EBC Balanced Brake Kit™.
Nevertheless, up till now we have ignored the critical µ (pronounced “mew”) term in the equation, the coefficient of friction of the brake pads themselves …
Effects of Pads on Brake Balance
It’s no secret that fitting higher performance brake pads can significantly improve your vehicle’s braking performance, even when retaining the stock brake calipers and brake rotors. Quite simply, increasing the coefficient of friction of the pads proportionately increases the braking torque on that axle. Providing the tyres are of high enough quality to allow this additional braking torque to be transmitted through to the tarmac, then the vehicle’s stopping distances will decrease appreciably.
It’s also fairly obvious to conceive that if you fit higher performance pads on the front axle, but leave the stock pads fitted at the rear, you are going to shift the brake balance towards the upgraded axle considerably. For this reason EBC Brakes always recommend to our customers that when upgrading pads they must not ignore the rear. You should always upgrade the front and rear axles together. Despite this recommendation we regularly hear of people fitting much higher performance friction pads at the front but neglecting the rear, who wrongly assume that the rear pads “don’t do anything anyway”. Unfortunately, we see the exact same enigma being adopted by manufacturers of conventional ‘Big Brake Kits’.
No front ‘Big Brake Kit’ supplied on today’s marketplace contains any components for the rear axle whatsoever. By supplying high-performance pads for the front axle only, the customer often leaves the rear axle with stock pads installed, assuming the big brake kit they’ve spent thousands on contains everything they need to realise significant improvements in braking performance. Another common issue is that because the vast majority of performance brake calipers use racing brake pad shapes, often it can be the case that the manufacturer of the race pad shape does not even make pads to fit the vehicle’s stock rear caliper, making it difficult to match friction profiles for the front and rear.
By not having matching friction pads installed front and rear the overall vehicle brake balance will be affected. This problem is exacerbated when things start to get hot. A stock brake pad is manufactured on a budget and typically begins to fade when temperatures soar past 400 degrees C. When the brake pad fades what we technically mean is that its coefficient of friction is falling. Observation of our braking torque equation above shows that if the coefficient of friction decreases, the braking torque decreases proportionately. This is a big problem for brake balance. The rapidly fading stock rear pads quickly give up when the going gets tough, which in turn shifts all the braking load onto the front axle. This massively overworks the front axle, causing front pad temperatures to increase thus accelerating brake pad wear. If the vehicle continues to be driven hard front pad temperatures may rise to a point where even the high-performance pads begin to fade, leading to brake fade on both axles with vast increases in stopping distances.
For this reason every EBC Balanced Brake Kit™ includes matching friction high-performance brake pads for the stock rear caliper, at zero extra cost to you. This means that the front and rear pads have the same friction coefficient vs. temperature profile, so that as the pads are worked hard their frictional characteristics vary equally in response to the elevated temperatures. This not only results in a balanced brake system from ‘cold’, but also means the vehicle retains a totally balanced brake system at elevated temperatures. This is one of the key factors that differentiates an EBC Balanced Brake Kit™ from a conventional front only ‘Big Brake Kit’ and gives EBC Balanced Brake Kits™ the edge in braking performance, reducing overall stopping distances.
Effects of Braided Lines on Brake Balance
Brake lines are often overlooked when it comes to brake caliper upgrade kits, but the fact is that brake lines also play a crucial role in brake balance. The brake lines form the hydraulic link between your foot and the brake caliper. A vehicle’s brake lines are made up of either ‘hard lines’ (inflexible metal tubes that run through the vehicle chassis) or ‘flexi lines’ (the flexible hose that links the brake caliper to the hard line in the wheel well). To all intents and purposes the vehicle hard lines have zero flex, meaning they do not degrade braking performance. However, all mainstream vehicle manufacturers fit their vehicles with low cost rubber brake lines from the factory, which are prone to flex and thus do play a role in braking performance and pedal feel. It is fairly typical for a front ‘Big Brake Kit’ to be supplied with upgraded braided brake lines for the front axle, what is not common is for the ‘Big Brake Kit’ to also come with brake lines for the rear axle.
Unlike motorcycles which in most cases have a lever for the front brake and a pedal for the rear brake, the hydraulic braking system on your average road car is interconnected and all connects back to a single master cylinder. (Some race cars may be fitted with dual master cylinders and sway bars, but we shall disregard these since this type of arrangement has near infinite adjustability to brake balance allowing the driver to tailor the brake balance precisely to their individual preference). Assuming the vehicle uses a single master cylinder, the system performance is only as good as the weakest link. This means that if the front brake lines are upgraded to braided stainless alternatives, but the rear brake lines are left as the stock rubber lines, the rear lines will be much more prone to flex under hard braking and therefore any money spent on upgrading the front lines is rather wasted. The brake pedal will have as much flex as the weakest link, in this case the stock rubber rear brake lines.
Quite simply, you will never find a car braided brake line kit that does not come complete with all the lines required to replace the flexible hoses on both the front and the rear axles. When a car’s hydraulic braking system is all interconnected, what you do to the front you must also do to the rear. Despite this, all front only ‘Big Brake Kits’ are supplied either with just the front lines, or sometimes without any lines whatsoever. Here lies another performance advantage to buying an EBC Balanced Brake Kit™, in addition to supplying braided lines for the front axle, every EBC kit also includes rear stainless braided brake lines (and pads) at zero extra cost to you. By fitting braided lines to both the front and rear axles, brake pedal feel is improved significantly and the end result is that all the brake fluid pressure is transferred to exactly where it is needed, the brake calipers.
So there we have it, the art of producing a balanced brake upgrade is one that involves fine tuning of the braking torque until the brake bias is proportional to the available grip on each axle. By carefully selecting brake calipers and rotor sizes for front axle, then accompanying them with pads and lines to fit the vehicle’s stock caliper and rotor on the rear axle, every Balanced Brake Kit™ contains all the hardware you need to realise the ultimate braking potential of your vehicle right out of the box. Additionally, every Balanced Brake Kit™ is designed to work with the vehicle’s stock master cylinder and is fully compatible with the vehicle’s ABS systems, allowing maximum braking performance to be realised without compromising on safety features and without the need for the costly replacement of several other components of the vehicles brake system.