The braking system
Power on a kart can range from a lowly 1,5kW on the Bambino kart right up to about 24kW on the DD2 class. The heavier the kart, the more power the engine needs to develop to reach a high top speed, and whilst doing so it develops a hefty amount of kinetic energy equal to the kart’s all-up mass x the square of its velocity. In order to be competitive, you need to be able to dissipate this energy rapidly as you approach the corners, and that’s the job of the braking system. So in short, the more powerful your kart, the larger and better the braking system needs to be in order to do this job effectively.
All karts are fitted with a single brake on the rear axle. One does however arrive at a situation that under heavy braking, the amount of weight transfer that takes place from the rear of the kart to the front is so large that the rear wheels will lock up i.e. the rear tyres are skidding which reduces the amount of available braking effort dramatically. For this reason all the bigger karts are also fitted with two front brake rotors which, although their diameters are physically smaller than the rear, their combined braking area is larger as they are the ones dissipating most of the kart’s kinetic energy.
Mode of operation
Gone are the days of the drum brake which has been replaced by the far more efficient disc brake. Apart from the Bambino class that uses a mechanically operated rear disc brake, all the other classes use hydraulically operated ones. The latter type uses a master cylinder that is filled with brake fluid – when the brake pedal is depressed, a piston with a small diameter forces fluid into the connecting pipes and onto much larger diameter pistons in the slave cylinders of the brake caliper, which in turn push the disc pads into intimate contact with the disc brake rotor attached to the axle or wheels.
The combined surface area of all the slave pistons is much greater than that of the piston in the master cylinder - hence the master cylinder piston travels several centimetres to move the slave pistons by only a few millimetres that it takes to apply the brakes. This arrangement allows a large force to be exerted by the disc pads onto the rotor, in the same way that a long-handled lever can easily lift a heavy object a short distance. The larger karts are fitted with twin hydraulic circuits with two master cylinders, one for the front and the other for the rear.
As mentioned previously, the weight transfer that takes place during the braking phase (particularly on the larger and heavier karts) results in the front brakes having to absorb a lot more energy than the rear ones, i.e. more pressure needs to applied to the slave cylinders inside their calipers. Notwithstanding the fact that there is a master cylinder for the front brakes and one for the rear, there is still only one brake pedal that operates both of them. So, to achieve the higher pressure on the front end, a brake bias adjuster (refer picture) is inserted between the brake pedal and the piston actuating rods of the master cylinders. Turning the knob one way or the other allows more or less pedal force to be transmitted to each individual master cylinder i.e. more or less pressure at each end of the kart.
Braking system differences
Braking systems are unlike the chassis shapes where there are essentially two different layouts. Virtually every kart manufacturer use their own homologated system and there are consequently lots of different calipers, rotors, and pad combinations available out there. Remember also that each of the manufacturers also have a fair quantity to choose from – as an example, CRG have about 6 systems in current use.
All systems use calipers that are either 2-pot or 4-pot. A ‘pot’ is simply another name for the actuating piston/s in the caliper, and 2 or 4 refers to the total number of pistons contained within the caliper. The rotors themselves vary in size and can either be a ventilated disc or an unventilated disc.
Brake pads
Up until about 1970, brake pads were mostly made of asbestos which has subsequently been outlawed because of the health risks associated with the small fibres that tend to fray off. On race cars of the day, these small fibres had to be removed when pitting because when the brakes got very hot, the fibres would ignite and you’d end up having your wheel on fire!! Time marched on, the ‘pointy heads’ got wiser, and today we use much better braking materials that consist of composite materials which include silicon, potassium, titanium oxide, resins, and aramid fibres such as Kevlar. These raw materials are sintered together to form the shape of the brake pad, and in this high temperature and compression process, are firmly attached to a metal backing plate.
Bedding in of the brake pads is of paramount importance if you want them to operate properly. Looking at the pictures, one can easily see the difference between a correctly bedded in pad and one where it was overheated in the process. The surface of the pad on the RHS clearly shows signs of pitting or fissures, where the resins in the structure have been boiled out of the surface due to excessive heat build-up during the bedding in process.
The surface of an overheated pad will also often show up as a glazed surface. These pads should be thrown out because they certainly won’t have much life or performance left in them. You may well be tempted to ‘de-glaze’ them with emery paper because, ‘well, it’s the done thing, isn’t it?’. This is by far the worst thing you can do. In doing so, you have dislodged some of the fine silica particles from the emery paper and embedded them into the small cracks and fissures present in the brake pad. What’s going to happen next, is that when you go out on track and apply the brakes, these silica particles are going to cause havoc on the brake rotor surface by scoring it nicely.
Brake pads, just like F1 tyres, are usually available in three different compounds viz. soft, medium and hard, so it might not be a bad idea to experiment with them to see what compound works best at any given track. If your OEM doesn’t have a range to choose from, then try an after-market supplier such as Frixia.
Fitment and bedding in
When fitting a new set of pads, do so whilst observing good housekeeping principles as follows. Firstly clean off both the outside and inside of the caliper with brake or carb cleaner and a small toothbrush to remove any dirt and other brake debris that may be present. Do the same for the rotor. Next, the new pads need to be cleaned off with a dry cloth before fitting them into the caliper – whilst doing so, ensure that you don’t get any oily residue from your hands onto any of the braking surfaces and that no brake cleaner ends up on the pads themselves. Pump the brake pedal a few times to ensure the pads are nicely centred, and then you are then ready to commence with the bedding in process.
So, the obvious question is why the brakes need to be bedded in. The main reason is to deposit an even layer of friction material onto the surface of the rotor, and in the process to thermally condition both the rotor and the brake pad materials. Whilst this is happening, any small imperfections in the newly made brake pads are removed and the pad surfaces are then correctly profiled with that of the brake rotor. Overheating the rotor or the brake pad materials prematurely can cause the rotor to become soft, or the pad materials to start disintegrating.
To start the bedding in process, go out onto the track and make five light stops from a slow speed and very light pedal pressure to conduct an overall check of the brake system i.e. the brake pedal is operating properly and that there is no loss of brake fluid pressure. Next, increase your speed to up to 50% of maximum revs, release the accelerator pedal allowing the kart to slow and at the same time operate the brake pedal until you feel a very slight resistance - keep applying this pressure for 3 seconds before releasing the brake pedal. Repeat this 10 to 12 times and ensure you don’t exceed 50% revs. Don’t at any time force the brake pads to make the kart slow down - when you are bedding in brake pads you won’t feel the brakes working properly until they are fully bedded in.
Next increase the speed of the kart to 75% of your maximum revs and release the accelerator pedal. Again allow the kart to slow, and with moderate pedal pressure operate the brake pedal until you feel more resistance – keep applying the pressure on the pedal for 5 seconds. Repeat this 10 to 12 times whilst ensuring you don’t exceed 75% revs throughout the process. Don’t keep your foot on the brake pedal at the same time as you accelerate – treat them are two separate actions. After this bedding in process, slowly go back to the pits allowing the pads to cool in the process, and don’t do any harsh braking at this stage. If you wish to remove the pads from the caliper for any reason, first mark each pad before removal so it can be replaced back into the same side of the caliper as it was bedded in on. By adopting this process you’ll find that not only do the brakes work better, but they will also last longer.
Brake fluids and their differences
Brake fluids are not all the same, so you need to know which one to use for your particular kart’s system. Using the wrong type of fluid can lead to disastrous results with the seals, so please keep this in mind. It’s all a question of chemical composition which is identified by the DOT number of the fluid as below (DOT = the US Department of Transportation):
DOT 3 and DOT 4 fluids are glycol based (clear to straw in colour)
DOT 5 is a silicone based fluid (purple in colour)
DOT 5.1 is a hybrid fluid which is mostly composed of glycol (clear to straw in colour).
There are differences in the boiling points of these fluids as shown in the table below, but for karting purposes it isn’t all that relevant. The ‘dry’ boiling point is the temperature at which new fluid will boil, whilst ‘wet’ is once it has absorbed 3.7% water. Glycol based fluids will absorb water but also make for a pretty good paint remover, whilst silicone fluids won’t harm the paint finish. Silicone fluid can be used with any of the seals typically used in brake systems, but that isn’t the always the case with glycol fluid. Providing it’s a glycol based fluid, a ‘higher’ DOT number can replace a ‘lower’ one, but not the other way around. You should never mix the two different base types, so it’s best to always use the fluid specified by your kart manufacturer.
As an example, on CRG the choice of fluid is simple as they have always used DOT 4, and this remains the case on their latest karts. OTK started off with DOT 4 fluids and it was used on all their BS1, BS2, BS3 and BS4 brakes up until about 2005 when the BS5 and BS6 systems started to be introduced – these systems use a DOT 5 silicone based fluid. From 2015 onwards, the OTK brakes use the hybrid DOT 5.1 fluid.
Bleeding the brakes
Let’s start with the CRG brake system. Firstly, remove the cover on the master cylinder and check that the fluid level is up to the mark on the side of the housing as indicated by the red arrow in the picture. Fit a short length of clear tubing to the bleed valve (shown above the spanner in the picture).
Gently depress the brake pedal, and when it has reached the end of its travel, crack open the bleed valve whilst maintaining pedal pressure. When cracking open the bleed valve, either air or brake fluid will start to fill into this tube and the pedal will move downwards in the process. When the pedal reaches the end of its travel, close off the bleed valve and only then allow the brake pedal to return to the top of its stroke. Check the fluid level and repeat the process until such time as the pedal becomes very firm and no air emanating from the system can be seen in the bleeding tube. On some rear brakes you may need to do this on each side of the caliper. Remember to constantly check the fluid level in the reservoir as you proceed.
OTK and similar styled master cylinders come in two types as shown below viz. with or without a reservoir, and also in ‘singles’. Life becomes a bit simpler if you have a bleed tower, otherwise it’s pretty much the same procedure as outlined above. In any event, you need to firstly remove the plug and its O-ring using a 5mm Allen key to allow the tower to be screwed into the body of the master cylinder. Also remove the reservoir caps (if fitted) together with the rubber tank liner and seal. Once completed, screw the bleed tower into the front plug hole, ensure the tower has sufficient fluid in it, and open the tap on the tower. Then, pump the brake pedal a few times to force any trapped air back into the vertical leg of the bleed tower. Next, it’s a case of cracking open the bleed valve on the relevant caliper (usually 8mm spanner, or 3mm Allen key on the side of the older brakes) and then allow gravity to do its job. Retighten the bleed valve when no more bubbles are present at the caliper valve, close off the tap on the tower, and refit the plug on the master cylinder. Refit the reservoir caps together with the rubber tank liner and seal. Empty out the unused fluid from the bleed tower back into the brake fluid container (providing its fresh fluid).
If you don’t have a bleed tower and there isn’t a reservoir on the master cylinder, then you need to be careful when pumping the brake pedal – do this very slowly otherwise you could end up with a face full of brake fluid, and remember to keep the master cylinder topped up at all times during the process. On karts with front and rear brakes, the brake bias must be adjusted at the bias bar after bleeding to equalise the braking between front and rear. With the kart on a stand adjust the bar until both front and rear wheels can just be turned by hand with pressure on the brake pedal – that’s as good a starting point as any until you test it on the track.
Servicing the system
Before you go onto the track each practice or race day, you should inter-alia be checking the condition of the pads, rotor, hoses, and brake fluid level in the reservoir as well as checking for any sponginess or excessive travel on the brake pedal. Replacement of the pads and rotor should obviously take place as and when required.
Taking into account the amount of practising you may or may not do, you should also consider replacing the brake fluid in the system every 500 or so laps because it does absorb some water and then starts to corrode some of the components despite its in-built anti-corrosive additives. Remember that once opened, brake fluid immediately starts to absorb water and you should never keep an opened container for periods longer than 1- to 2-years at best. Also remember that O-rings and seals also don’t last forever, so it’s not a bad idea to strip the master cylinder and calipers every 1,000 or so laps to check their condition and give everything a good clean up in the process. Master cylinder and caliper seal kits as well as more comprehensive re-build kits are available for most brake systems.
Emile McGregor - MSA Technical Consultant