MAXIMIZING YOUR BRAKES
By Mike Burrell
Often overlooked until an issue arises, your kart’s brakes are a key component that needs attention on a regular basis.
Most karters have left our karts in a cold garage over the winter, ran the same brake fluid for two seasons, or used double the manufacture’s recommended brake pad shims to extend the life of the pads. But to what extent are we jeopardizing safety and losing performance by not paying attention to our brakes? We’ll ask a couple of brake experts and go over rebuilding and maintaining the brake system in the remainder of this article.
Robert Sollenskog, former Toyota Atlantics driver and inventor of the full-contact Thor Brake (see sidebar for this story), describes the fundamentals of what is occurring under braking: “Braking is the simple creation of friction between a brake pad and rotor.”
“For any given semi-metallic brake pad and steel brake rotor to “work” properly, the brake rotor must have friction material transferred to it from the adjacent contacting brake pad. Friction between the two like materials during a brake event has the effect of decelerating the vehicle.”
Essentially, kart brakes work the same as any car disc brake or racecar brake. However, there are some differences in pressure ratios, pedal travel, and pad return.
“Brakes are brakes,” declares Paul Martin of MCP Brakes, America’s leading kart brake manufacturer. “Kart brakes are basically the same in design as any other disc brake system, whether it be on your family sedan or your race car. One difference that many kart brake systems have versus auto brakes is the spring return brake pads.”
All shifter karts (and some others) run systems that require dual master cylinders and bias adjusters. These items also need to be inspected for functionality and safe operating on a regular basis.
“Back in the 70’s, the spring return brake pad system became the most common brake because it offered a brake system that didn’t have any brake drag like automotive systems or previous kart brake designs. Larger racecars and your family auto cannot use a system that has mechanical brake retraction because the hydraulic ratios are very different.”
“The master cylinder on your car is nearly the same piston size as our kart brakes but it has to feed four calipers that have very large caliper pistons. This ratio is well in excess of our typical 5:1. If each brake pad pulled back from the rotors .030” of an inch, the brake pedal would have tremendous travel before the brakes would come on. You can see this would never work in the average auto and certainly not a racecar. This does put a little friction in the system but is never noticed because the engines are so powerful. In many karts with only five horsepower engines, this drag can be a big detriment to top end. I call it “free horsepower” because if you don’t take it away with drag, you have it to use to go faster!”
Sollenskog concurs with Martin and continues a step further, “A proper racecar brake has three main characteristics:
1. A stiff pedal that can be used to modulate the brake pressure and thus the stopping force.
2. A short stroke pedal. Perhaps a _” to _” of travel until the brake pads contact the rotor. Traveling at those speeds drivers do not have confidence in their brakes if the pedal needs to travel 2” to slow down. Without confidence they cannot maximize their braking.
3. Brakes are set up to provide good stopping power without losing drivability. By this I mean that the brakes cannot be over sensitive and prone to locking up.
Many kart brakes do not offer any of the characteristics of a formula racecar brake system. The full-contact brakes that we (Elevation Engineering) offer do perform like racecar brakes are meant to: giving the driver a short, stiff pedal allowing he or she to modulate their brake pressure giving them the performance to out brake other karts.”
Common Kart Brake Issues:
But many karters don’t even know the signs a brake problem is about to arise until the brake pedal is pushed to the front bumper with little or no stopping power. However, most brake problems can be prevented with a little preventive maintenance and routine inspections.
“Routine inspection of the brake system should be part of every track day experience, whether racing or just practice,” says Martin. “I start with the obvious mechanical connections starting with the brake pedal and back to the master cylinder. All pivot points should be checked for excessive wear and be sure the safety restraint (cotter pins, quick pins etc.) are in place. I then check the caliper and disc for all physical connections to the frame and axle. Again, looking for loose caliper or disc bolts and being sure the cotter pins are in place.”
Martin continues to explain how this inspection time is also a good time to check for excessive pad wear and leaks, two of any brake system’s most common problems. “This is a good time to look at the brake pads for unusual wear or any cracking or ‘debonding’ of the friction material from the backing plate. Examine all line connections for any signs of leakage. Follow the brake line back to the master cylinder being sure that at no time are the lines able to contact the track or pinch points on the frame. This could cause a catastrophic failure on track. Once back to the master cylinder, again examine the condition of the fittings and be sure all is dry with no sign of leakage. If all of this is in good condition, there is typically no reason to expect any brake problems.”
In addition to routine inspections for leaks, most brake experts will recommend changing the seals in the master cylinder and caliper during the off-season. “I have always recommended that if the kart is used for a full season of racing, the brake system should be part of the winter maintenance schedule,” states Martin. “It would be appropriate to disassemble the system, clean and inspect all the components. I would recommend rebuild kits for both the master cylinder and caliper.
Sollenkog elaborates, “Brake fluid tends to dry the seals out. As a seal begins to dry out it creates more friction in the bore in which it travels. The friction causes the return travel to slow down. A slower brake return can cause momentary reduction in acceleration out of corners. A leaking seal happens at the end of the cycle not the beginning. If you wait until you have a leaking (bad) seal you have already suffered through weeks if not months of ill performance.”
“The second main reason is dirt and dust. Road grime will cause seals to deteriorate much like brake fluid will. At minimum, maintenance between race meetings should involve pulling off dust covers and flushing dirt and dust out with a quality brake clean solvent on both the masters and calipers. A WD-40 chaser is good to maintain lubrication on the piston bore. Replace all dust covers.”
For a heavily glazed rotor, carefully sanding the shiny/glazed look off with a medium grit sandpaper and a sanding wheel can save time.
Occasionally, even with all the preventative maintenance and inspections problems do arise in well prepared racing equipment. One problem that many karters encounter at some point in their racing careers is “glazed brakes.”
Brake “Glazing” and Proper Pad Bedding:
“Glazing of the brake pads is a term used to describe the condition of the friction material when it looks very shiny. This depends largely on the type of friction material. Many years ago, the brake material of choice had asbestos in it,” states Martin. “That type of friction material was very prone to glazing over. When this happened, the brake pads just wouldn’t grip the rotor when the brakes were applied, literally giving the driver the impression he had no brakes at all. As asbestos was phased out and better friction material of carbon metallic compounds became the norm, the glazing problem is not the same.”
Sollenskog continues, “Glazing happens when the friction material on the rotor and pad get overheated. The glazing acts like a surface hardened condensed coating that has reduced coefficient of friction on the contacting surfaces. Brake efficiency is reduced causing brake torque to be reduced as well (ie. The brakes do not slow the vehicle down like they are supposed to).”
“You can see glazing with the eye and feel it with your brake foot. A mild case of glazing can be fixed with sandpaper. By hand sanding or with a small power sander, sand the glazing off the rotor and pads. After re-installing the pad components bleed the brake and bed in the pads and rotor.
For bedding in we suggest:
1. 10 stops from at least 80% full speed using 50% braking.
2. Park and let cool for 5-10 minutes.
3. 10 stops from at least 80% full speed using 75% braking.
4. Repeat step 2
5.You should be ready to use brakes as normal.
If you are an aggressive driver on the brakes or on an aggressive braking track; consider adding a cooling tube to feed air to the brake.”
Paul Martin has never seen the MCP carbon metallic material ‘glaze’ in severe testing, but has had customers over the years tell him they have experienced glazing. “I have always found that when this appears to occur, there were other reasons the brakes were not operating properly. Typically, this has been a poor brake pedal setup that just doesn’t allow the driver to apply the amount of physical force on the system to provide adequate hydraulic pressure to the brake pads.”
“In other words, the driver may be pushing with a tremendous amount of effort, but that doesn’t necessarily relate to system pressure. The entire brake system is nothing more than a means to convert and multiply the drivers pedal effort into the motion that squeezes the brake pads against the brake disc. The multiplication of effort starts with the mechanical advantage of the brake pedal, added to the mechanical advantage of the typical master cylinder, then added to the hydraulic advantage of the master cylinder over the caliper. As an example, the MCP brake system has a normal recommended pedal leverage of 2:1 to 3:1. The master cylinder has a range of 1.5:1 to 2.5:1 ratio. The standard brake system has a 5:1 hydraulic advantage. When multiplied, the system recommendation offers a range of 15:1 to 37.5:1. As you can see, this is a fairly wide range. This allows any driver to find the brake effort and feel he likes the most. The adjustment of these mechanical advantage points is crucial to setting up the brake system.”
“I’m a little off subject, so I’ll get back to the glazing. How to prevent it is to be sure you have the systems setup to provide crisp and relatively easy brake action when the pedal is applied. The previously mentioned adjustments are how to obtain that. Most people would “de-glaze” the brake pads and rotors if they think it has occurred. De-glazing is normally done by removing the pads and sanding them until the “glaze” is removed. Likewise, the rotor can be sanded until it appears dull in appearance. This should offer at least a short-term fix to glazing. I am convinced if the driver would set the system up correctly, this will not happen with today’s brake pads.”
Brake Fluid - There is a difference:
Probably the root cause of the majority of brake problems karters encounter are a result of moisture in the brake fluid (that is assuming the brakes are bled properly and completely of air). Unless you can bleed your system in a moisture free clean room, as soon as the bottle of brake fluid is opened moisture absorption begins.
“Like all fluids in a high performance vehicle, brake fluid should be changed at the start of a new season and every couple of races. Brake fluid begins to deteriorate as soon as the seal is broken on the bottle exposing it to the moisture in the air. All those open bottles that you have in your garage that are older than 2 months or have been sitting without the top securely fastened: throw them out!” decrees Sollenskog.
“Changing of fluid color is never an accurate indicator of fluid freshness. Brake fluid “goes bad” when it absorbs even the smallest percentage of water. The vast majority of brake problems can be directly attributed to “bad” brake fluid. Change your brake fluid in regular intervals and when in doubt.”
Sollenskog continues, “The minimum brake fluid that should be used is DOT 5. All the DOTs are glycol-ether based except for DOT 5.1, which is silicon based. Dot 5.1 is problematic in high performance applications. Silicon has a much higher rate of expansion than the glycol-ether based fluids. Unless you are willing to run a wider brake pad air gap to compensate for the loss of gap due to DOT 5.1 expansion, stay away. Remember the larger the air gap the longer the pedal travel!
Since our full-contact brakes are designed with minimal pedal travel we found out in testing DOT 3, 4, and 5.1 wouldn’t work for us. After initial fluid problems and an extensive fluid study we chose to use synthetic NEO 610 with our brakes. The right fluid is worth it and that is why we supply a bottle of NEO 610 with all our brakes. The 600 series is above DOT-5 and removes all doubt.”
Paul Martin agrees, but offers a counterpoint concerning the advantages of using DOT-5 silicone based brake fluid: “Most American built karts using the MCP/Enginetics systems use DOT-5 silicone brake fluid. This fluid is non-hygroscopic, meaning it does not collect moisture. In its original form, the fluid is clear but a purple dye is added to give it color. If using nylon brake line, the sun’s rays will bleach this color out and the fluid returns to near clear. This has no effect at all on the properties of silicone brake fluid.”
“Standard DOT-3 and DOT-4 fluid are extremely hygroscopic,” continues Martin. “They are very prone to absorbing moisture, especially in a humid environment. When subjected to moisture, these brake fluids absorb this moisture and the consequence is a lowering of the boiling point of the fluid. This is why most racing brake systems suggest changing the brake fluid every race day. DOT-5.1 fluid is a synthetic fluid with properties more in line with glycol based DOT-3 and DOT-4 fluids. It has a high boiling point but not as high as silicone fluid. Every brake fluid has it good points and bad. Go with the brake fluid recommendation and you should be fine.”
Maintaining the Pad and Rotor Air-gap:
Along with faulty, or the wrong brake fluid, the air gap between the pad and rotor are another point of contention for karters to concern themselves with. For obvious reasons, karters don’t want the pads and rotor to make contact when they’re not applying the brakes (for example, while under load exiting a corner). This will rob the kart of horsepower as easily as choking the motor. However, there is a trade off; too much air gap will give an extra long pedal throw which isn’t optimal for performance.
“Air gap for the pads to rotor clearance is another of the many adjustments available to the user. If the driver likes a long brake pedal stroke, then he/she will use a fairly large pad gap,” explains Martin. “Obviously, the opposite is in effect if the driver likes a short pedal stroke. If you start out a race with a short stroke and the stroke increases noticeably during the race, your pad gap will be much larger and need attention. The only reason a “too big” air gap might cause the brakes to lock more easily will happen more because the pedal ratio will change as the stroke gets longer.”
Sollenskog elaborates on the changing pedal ratio: “Many karts have different linkage pick up points on the brake pedal and master cylinder to change the kinematics. In effect by changing the mechanical advantage in the linkages you can increase the volume of fluid relative to the pedal travel. Keep in mind however that when this is done, the effective line pressure to the brake pads is reduced. This creates less brake force. It is a tough balancing act and most racers just deal with a long pedal.”
“Brake rub as many racers have experienced happens in corners as the chassis flexes allowing the inside rear wheel to lift off the ground. As a rule of thumb the longer in overall length the brake pad is, the more air gap is needed to compensate for brake rub. The increased air gap leads to an increase in pedal travel to achieve a braking event.”
One rather new (at least to karting) concept to eliminate brake rub is the concept of the floating rotor. Used in formula cars for years, many of the top European manufacturers at least offer a rotor system with some degree of free-float. Some of the new full-contact systems, such as the Thor Brake, have over six degrees of free float, which according to the manufacturer allows for a rear wheel to be up to 3” off the ground under chassis load without any rubbing.
Karters do have a few options in reducing (or managing) air-gap. Most karting systems require a thin aluminum shim to reduce the air-gap. Some manufacturer’s systems, such as CRG, are self-adjusting via a free-floating pad that knocks back the caliper piston when the rotor flexes against it. MCP has a simple system that allows for adjusting the air-gap with an Allen wrench while in the pit/garage.
“The shims are generally thin sheets of aluminum that do not hold their flatness. These stacked together will act like a spring giving the driver a spongy feeling at the pedal,” states Solleskog.
Martin explains the use of shims in a bit more detail, “There are two ways to use shims. The American systems of the past used shims between the caliper halves to adjust pad gap. There was no reasonable limit to how many shims could be used in this manner. The European style of using shims puts the shims directly behind the pad between the brake pad and the caliper piston. There will be a limit to how many shims can be used in this manner because as shims are added, the amount of stroke is being limited. It would be possible to add so many shims that the return springs reach a coil bind state. As shims are added, the feel of the brake is effected somewhat because the return springs are being compressed more every time another shim is added. Although the springs don’t have a lot to do with pedal effort, you can feel how much is required just to push the springs as this is the pressure required up to the point the pads make rotor contact. This effort will increase as the springs are forced to start from a more compressed position.”
Rebuilding the Master Cylinder & Caliper:
As Paul Martin suggested earlier in this story, it is a good idea to rebuild your caliper and master cylinder annually. Brake fluid is a very harsh chemical and dries seals out, especially when left sitting for a prolonged period of time causing leakage and a loss of pressure. In the following paragraphs and photos, we’ll examine the disassembly and replacement of the brake seals.
Step one is to break the line and drain the old brake fluid –trying to keep it off the kart’s paint. Remove the brake line from each mounting point on the master cylinder and caliper, and blow out an excess fluid with a compressed air nozzle. The next, and obvious step, is to remove both the master cylinder and caliper from the kart.
For this example, we’ll assume the caliper is of the traditional type (this excludes the Birel “banana” caliper, which uses an external piston design). Assure that all the fluid is drained before; this might require not only remove the lines but venting the system by removing the bleeder screws. Next, remove the brake pads. As mentioned earlier, most kart calipers use a system of retainer bolts and return springs to; these will need to be removed for the pads to be removed and eventually the caliper pistons.
With the pads and brake fluid removed, it’s time to remove the caliper pistons. Over the years, many karters have tried twisting, pulling, and prying the pistons out. This is 100% the wrong thing to do! It is essential not to scratch the piston or cylinder wall; this could cause a leakage of fluid or failure to build enough pressure to stop the kart at all.
To properly remove the caliper pistons, the first step is to seal off the bleeder screws and all but one of the brake line fittings. If the caliper has two brake line fittings, it is likely they operate independently and you will not need to seal off the opposite one. Using a very low air pressure and increasing it slowly, place a rubber tipped air-nozzle in one of the brake line fittings and “blow” (slowly!) the piston out. You may even use a small piece of wood where the brake rotor would be to stop the piston if it shoots out quickly.
Behind the pistons or on them (o-ring) is the piston seal. Typically, anytime the caliper pistons are removed, the seals are replaced. Also, take a look at the caliper cylinder walls for scratches to assure no debris got by the piston and seals.
At this point, the pistons need to be inspected for brake dust and other buildup that has not allowed them to return fully into the cylinder. Often there is some residue from brake pads, grease, and “track dirt” that build up around the outer edge of the piston. To remove any buildup, wipe off with a brake parts cleaner soaked shop rag.
Clean all pieces thoroughly with brake parts cleaner, and begin to reassemble with new seals. When installing the new seals and pistons, lightly coat both with the manufacturer’s recommended brake fluid. This will help ease reassembly and assure a better seal.
At this point, it’s time to move to the master cylinder. Most kart master cylinders are very simple, and not that dissimilar to any other racecar’s master cylinder –with the exception of the volume of brake fluid. The lever that joins to the brake rod, and eventually the pedal, pushes a piston –which is usually backed by spring.
The master cylinder piston is usually held in by a snap-ring. By removing this snap-ring, the piston, seal, and spring can be removed. Both the seal and spring should be replaced (springs can become compressed and reduce pedal pressure). Clean out the master cylinder and assure no debris scratched the cylinder walls. Reassemble as it came apart with similar procedures used on the rear caliper. Most master cylinders have a dust shields at the lever to keep dust out of the piston area; assure these shields are in place and free of tears.
In this example, the piston & dust ring ride in front of the seal. As pressure forces fluid against the seal, it pushes the piston out. The piston is returned when the spring pressure from the pad retainer springs/bolts exceeds that of the hydraulic pressure coming from the opposite direction.
Reinstall the calipers, master cylinder, and brake lines. Finally, you will need to bleed the brakes using the manufacturer’s recommended fluid. What’s proven best to bleed kart brakes time and time again is a gravity fed bleeder that screws into the master cylinder. This style bleeder essentially pushes the air out the bleed screws, almost doing the work for you. Be sure all air bubbles/pockets are out and test the brakes before hitting the track full throttle!
With the exception of full-contact brakes such as Sollenkog’s Thor Brake line, the future of karting brakes appears to have reached an evolutionary period. Carbon fiber pads and rotors made a brief appearance around the turn of the millennium; however, cost issues legislated them out.
“Carbon/carbon systems normally require a fair amount of heat to work properly. This is hard to achieve in the short time karters go out for qualifying sessions or the first few corners of a race. Most carbon metallic systems work instantly regardless of temperature and therefore have prevailed. I don’t think exotic materials have a place in karting in my opinion. This doesn’t mean they won’t appear,” explains Martin. This being said, some karts are arriving from Italy with optional ceramic-based rotors.
Your kart’s brake system is as important a part of the overall performance as it is a safety item. Today’s kart brakes come in a variety of shapes, sizes, and pad compounds giving drivers the opportunity to tune their brakes to their personal preference. But no matter what combination is best for your personal driving style, it is well advised to follow the manufacturer’s recommended maintenance schedule and use only their recommended fluid to assure safe and consistent performance.
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