Brake System Design and Optimization: Troubleshooting Common Brake Issues
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Troubleshooting Common Brake Issues
14.39
| 00:00 | - As we come to the end of the course, you should now have a good understanding of the braking system as a whole and have your system designed correctly to put you on the right track for optimum performance. |
| 00:11 | The reality is though that you're probably not going to achieve the perfect braking system for your car without a lot of testing, tuning and maybe some significant changes to what you had originally designed. |
| 00:25 | There are some problems that may arise that just aren't possible to calculate for and come down more to the quality of components, maintenance of the system and supporting parts such as cooling and suspension hubs. |
| 00:39 | If we're aware of the potential problems we might face and their causes, we'll be able to avoid them altogether at best or quickly diagnose and fix them at worse. |
| 00:49 | We've discussed some of these already throughout the course but in this module we'll be bringing them all together in a comprehensive list. |
| 00:56 | Let's start at the brake discs and work our way inward with the first consideration being disc torque vibration. |
| 01:03 | This is felt as judder or vibration when the brakes are applied and can range from light to very pronounced where it upsets the dynamics of the vehicle, especially when in resonance with the suspension. |
| 01:17 | This is commonly caused by disc thickness variation and runout, being the variation in the distance from the disc surface to a reference plane like the wheel hub. |
| 01:27 | A potentially more dangerous issue that can also cause vibrations is cracked discs. |
| 01:33 | But to be clear, cracks in iron discs are normal and should be expected. |
| 01:38 | However, the discs need to be inspected regularly to ensure no cracks have propagated to the edge of the disc, at this point they need to be replaced immediately. |
| 01:48 | The general assumption is that vibrations are caused by warped discs where the friction surface is no longer flat. |
| 01:55 | But this is actually often not the case and the warpage is more likely seen as coning of the disc which is where the disc distorts so the friction surface is no longer parallel with the hub surface and therefore the pad surface. |
| 02:10 | This usually results in compliance and pad wear issues rather than vibrations. |
| 02:16 | These issues are usually the result of the disc experiencing high temperature gradients and there are a few different ways to avoid this. |
| 02:23 | For the driver, this means warming the brakes gradually before using them at full power as well as cooling the brakes gradually on an in lap and not coming to a complete stop with hot brakes. |
| 02:36 | If the brakes are getting too hot and the temperature is difficult to control, then the thermal mass or the cooling system needs to be improved. |
| 02:43 | Take another look at the correct methods of cooling in the tuning brake temperatures module. |
| 02:48 | More often than not, the cause of vibrations is actually pad deposits on the disc surface. |
| 02:55 | Recalling from our brake bedding module, the friction between the pads and the disc is a combination of abrasive and adherent friction, AKA molecular bonding. |
| 03:06 | More street orientated pads tend to function primarily on abrasive friction and typically have a higher wear rate and face at higher temperatures. |
| 03:17 | When they reach their effective temperature range, they'll transfer material onto the disc face in a random and unevan manner. |
| 03:25 | This pickup causes variations in thickness and therefore vibrations. |
| 03:30 | With adherent friction on the other hand, material diffuses between the pad and the disc in a more uniform manner but the effective temperature range is generally higher. |
| 03:40 | So with motorsport style pads, this tends to be less of an issue however abrasive friction is still required to keep the disc surface smooth and clean. |
| 03:49 | Regardless of the pad composition, the best way to prevent this is properly bedding the brakes and also managing the temperature as gradually as possible while avoiding stopping the vehicle with the brakes engaged when still hot. |
| 04:04 | Thankfully there is a cure for pad deposits if it does go wrong. |
| 04:08 | In the case where there's not too much buildup of the deposits on the disc surface, it can be possible to fit semi metallic pads and use them hard to clean the disc surface. |
| 04:18 | But realistically the best bet is to replace the discs or at least have them blanchard ground. |
| 04:25 | The next issue, this time felt predominantly through the brake pedal is compliance. |
| 04:30 | We've discussed this in detail throughout the course but compliance is essentially where not all of the pedal force from the driver is converted to clamping force at the callipers. |
| 04:40 | The result is a soft pedal feel and more pedal travel and effort from the driver to achieve the desired brake torque. |
| 04:47 | Humans are much more effective at modulating by force rather than displacement so a firm short pedal is more desirable than a long light pedal as long as the force to achieve the max brake torque is appropriate for the driver and application. |
| 05:02 | There is a lot of causes for this so we'll start with probably the most common and easy to fix, being swelling of the factory, fabric reinforced rubber flexible brake hoses with pressure. |
| 05:15 | The fix here is to replace these with stainless steel braid protected hoses of extruded teflon. |
| 05:20 | Which practically eliminates the swelling and also much more resistant to damage. |
| 05:26 | Just be sure to upgrade the front and the rear to avoid changing the bias and interfering with ABS if present. |
| 05:34 | Likewise, other components are also susceptible to flexing and introduced compliance. |
| 05:39 | Sliding callipers generally suffer from poor compliance so upgrading to a fixed calliper can offer a good increase in stiffness with additional increase coming with monoblock callipers and their bridges. |
| 05:52 | Equally as important is the calliper mount to the hub because the advantage of a high stiffness calliper can easily be lost if the support isn't up to the task. |
| 06:02 | Sticking with compliance, an often overlooked factor is firewall flex with strong pedal forces. |
| 06:10 | It can be possible to brace the master cylinder to other parts of the chassis like a strut tower to help with this or alternatively upgrading to a motorsport style pedal box will offer some improvement depending on the factory setup of course. |
| 06:24 | Another very common cause of compliance is boiling fluid. |
| 06:27 | Factory braking systems are quite capable of completing a high power stop in an emergency but are usually not designed to deal with repeated hard use like we see on the track. |
| 06:39 | Old fluid that has taken on moisture and has previously overheated has a significantly reduced boiling point and once the fluid boils, the gas in the system is compressible so the pedal becomes long and mushy. |
| 06:53 | The driver may be able to achieve adequate brake pressure with extra pedal effort but any chance of good modulation is gone. |
| 07:01 | Degraded fluid needs to be removed from the system completely and it should be rebled with quality motorsport grade fluid before it's driven again. |
| 07:11 | If this doesn't solve the issue, then the cooling system should be improved or the thermal capacity of the components and the insulation of the brake fluid needs to be upgraded. |
| 07:22 | The last culprit on the list when it comes to compliance is pad taper which as we know is when the pads wear unevenly and form a wedge shape. |
| 07:30 | This is usually in the form of the leading edge wearing faster than the trailing but the outer edge of the pad in the radial direction can also wear quicker if there's a calliper stiffness issue. |
| 07:42 | Pad taper not only leads to more compliance but also reduces the lifespan of the pad, can prevent the calliper pistons from sliding freely, increases heat generation and can cause uneven temperature distribution in the disc. |
| 07:57 | Unfortunately this naturally happens due to how the pad and the disc work together but there are ways to minimise the effect. |
| 08:05 | Firstly we want to make sure the calliper compliance is minimised and the calliper is square with the disc. |
| 08:11 | On top of this, quality callipers will often have staggered pistons so the force on the front of the pad is less than the rear, helping to correct this wear. |
| 08:21 | Next let's move onto pad fade. |
| 08:23 | To be clear, pad fade isn't felt as compliance, instead the pedal remains firm but the brake power decreases significantly for the same pedal effort, meaning much more effort is required to achieve the same brake force. |
| 08:37 | The primary cause in this instance is the temperature being too high for the pad compound and above its effective operating range. |
| 08:45 | If the pad compound isn't appropriate for the application, for example running a street pad in endurance races, then obviously the first step will need to be changing to a pad of a more suitable compound. |
| 08:58 | Alternatively if the pad is suitable but the driver is still experiencing fade then the temperatures are obviously too high and the cooling system needs to be improved. |
| 09:08 | Moving on, one of the most common problems we see are wear issues. |
| 09:12 | Starting with the simple and costly problem of the pads wearing too fast. |
| 09:16 | The same idea applies with pad fades here, it's important to make sure the pad compound is suitable for the application, though the first step might be changing to a motorsport style pad. |
| 09:29 | However, if we have race pads and they're still wearing faster than expected, then there's a good chance they're getting too hot so attention should be turned to controlling the brake temperatures and improving the cooling of the system and the thermal mass. |
| 09:43 | Less commonly, it can be possible to see a pad on one side of the calliper wearing faster than the other. |
| 09:50 | This could be from sliding callipers or pistons sticking, potentially meaning the callipers need some maintenance. |
| 09:56 | Or if the maintenance is up to date, this could suggest the brake disc temperature is likely not even. |
| 10:02 | If so, we should look at the design of our cooling system and see if we can improve it to achieve more uniform cooling. |
| 10:08 | On the other side of the interface, faster disc wear is to be expected with aggressive pads running hot. |
| 10:16 | If this is an issue, we may need to consider if we can use a less aggressive pad or again look to improve cooling. |
| 10:23 | With this and everything we've discussed up until this point in the module, if heat is an issue, I'd recommend increasing the brake fluid insulation, the thermal mass of components and the cooling using the disc with more efficient vanes before turning to ducting. |
| 10:38 | Obviously ducting is required to tune the temperature of the brakes for different conditions so this will depend on your application and competition level. |
| 10:48 | However ducting can be a headache to say the least. |
| 10:52 | It's usually difficult in terms of packaging and race cars with ducting often require the steering rack stops to limit the steering angle and avoid damaging the ducts, making them difficult to maneuver at low speeds. |
| 11:06 | So if practicality is important and this is your street car, it may be an issue. |
| 11:10 | Next up is brake knock off or knock back which is essentially the pads being knocked back by the brake disc as it doesn't run perfectly true through the calliper. |
| 11:20 | Meaning the driver needs to take up the extra piston travel before the brakes engage. |
| 11:25 | This issue will always exist to some extent but excessive knock back can be very disconcerting for the driver at high speeds. |
| 11:34 | So how can it be minimised? The first step would be to ensure that the wheels bearings and discs are properly maintained and run as true as possible. |
| 11:43 | Stiffer hubs and other suspension components can have an impact here as well. |
| 11:48 | Properly designed piston seals, anti knock off springs, which are small springs behind the pistons helping to prevent them from being knocked into the callipers, or residual pressure valves that essentially hold a small amount of hydraulic pressure in the calliper for the same reason, will help to prevent the pads being knocked back so easily. |
| 12:08 | The final issue we'll cover, and maybe the most critical of them all is brake locking. |
| 12:13 | Locked wheels will result in a significant decrease in longitudinal grip and therefore deceleration but will still slow a vehicle. |
| 12:21 | Lateral grip however is hugely reduced with a locked wheel, meaning the ability to control the direction of the car mostly goes out the window. |
| 12:31 | If the front wheels lock first, this means understeer, far from ideal but a stable condition that's relatively easy to correct. |
| 12:41 | Rear wheels locking first on the other hand, usually always results in oversteer which isn't stable and makes the car want to spin since the weight is also transferred off the rear wheels when braking. |
| 12:53 | The brake system should be designed so the front wheels lock just before the rears in the interest of stability. |
| 12:59 | Regardless of which end locks up first, we really want to avoid it altogether since it's clearly not ideal for performance and can damage expensive tyres quickly. |
| 13:10 | If we find either end is locking too easily and it's difficult to modulate the braking power then clearly we're producing too much braking torque for that axle for the grip available. |
| 13:22 | Either the grip needs to be increased which could involve the longitudinal load transfer, or the braking power needs to be reduced. |
| 13:30 | If the pedal effort is too light and long, this can also make modulation difficult in which case the pedal ratio can be decreased or the master cylinder size increased. |
| 13:41 | On this topic, the system really needs to be considered as a whole and it's best to use the knowledge found in the brake system design section to analyse the system and determine why it isn't balanced and what changes could be made to fix it. |
| 13:56 | We've covered a range of different issues, their effects, potential causes and solutions in this module. |
| 14:02 | These were just some of the common issues we felt were important to readdress, although there are a lot more problems that can arise when developing, tuning and testing the braking system. |
| 14:13 | When making changes to the braking system, it's important to keep in mind what good braking really is and that's not always going to involve an increase in braking power. |
| 14:23 | Good braking performance is first and foremost consistent, controllable and balanced. |
| 14:28 | Issues with reliability, modulation and bias should be addressed first when looking to improve the system. |
