Race Driving Fundamentals: Brake Bias
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Brake Bias
05.52
| 00:00 | - When applying the brakes in our car, we won't end up with the same hydraulic pressure at the front and rear brakes, instead the majority of the braking effort is done by the front wheels and hence more hydraulic pressure is required at the front brakes. |
| 00:14 | This is what we refer to as brake bias. |
| 00:16 | The reason for this comes back to weight transfer as we brake. |
| 00:20 | When the brakes are applied hard, weight is transferred towards the front of the car which increases the grip available on the front wheels. |
| 00:28 | Conversely, weight is transferred away from the rear wheels which decreases their grip. |
| 00:33 | If we had the same pressure being applied to the front and rear brakes, we'd find that we'd simply lock the rear wheels every time we tried to brake hard. |
| 00:41 | This is why we require brake bias and it's also why you'll see the size of the front brakes are typically much larger than the rear, they're doing more work and need to reject a lot more heat which is the byproduct of stopping your car. |
| 00:54 | The brake bias is something that's determined by the manufacturer in a standard road car and is generally controlled by the size of the front and rear brake master cylinders. |
| 01:04 | In a race car with a pedal box however you'll often find an adjustable brake bias knob which allows the driver to change the front to rear brake bias on the fly. |
| 01:14 | The goal here is to adjust the proportion of the braking forces between the front and the rear in order to maximise the overall braking performance. |
| 01:23 | Changing the brake bias doesn't just affect the car's stopping performance in a straight line, it also affects the way the car turns into a corner. |
| 01:31 | If we run too much rear brake bias for example, we risk the rears locking before the front. |
| 01:37 | The result, is less stability in a straight line but when we start turning into the corner, the car will also tend to oversteer. |
| 01:45 | This isn't going to be a slow easily controlled oversteer either. |
| 01:49 | The car will want to over rotate very quickly and you'll be facing the wrong way before you know what even happened. |
| 01:55 | The opposite occurs when we have too much front bias. |
| 01:58 | Firstly you'll end up under utilising the rear tyres and losing braking performance but the front end will be more likely to lock which can cause the car to understeer. |
| 02:07 | When the brake balance is not adjusted correctly it can affect the amount of weight transfer in the initial braking phase. |
| 02:14 | When set correctly the car will squat, loading the front and rear suspension to their optimum and allowing the car to brake more efficiently due to maximum braking performance occurring just before the tyres lock. |
| 02:26 | Although there's a lot of nuance to getting this exactly right, let's take a quick look at how to set up a basic starting point with our bias if we don't have a fancy dash display data readout. |
| 02:37 | First we need to put the car on stands with someone in the driver's seat. |
| 02:41 | Start by spinning a front wheel by hand. |
| 02:43 | The person at the controls can then increase the brake pressure gradually until we can no longer spin the wheel. |
| 02:49 | Make sure the driver keeps the same brake pressure on and move to a rear wheel. |
| 02:54 | If we're barely able to spin the rear wheel by hand, our brake bias should be roughly correct for a starting point. |
| 03:01 | If not, we can adjust the bias more towards the front until we get some movement. |
| 03:05 | While this will get the brake bias in the ballpark, we'll then need to test and optimise this on the track. |
| 03:11 | If you've got data that includes wheel speeds, you can use this to help you. |
| 03:16 | What you're going to want to do is brake hard in a straight line to the point that you've just achieved lock up. |
| 03:21 | You're aiming here to adjust the brake bias so that the front wheels lock just marginally before the rears. |
| 03:28 | If you don't have the benefit of data then you can use a spotter on the side of the track to help you instead. |
| 03:33 | Once you're a little more accustomed to driving on the track, you'll be able to feel when your bias is set correctly, as you move the bias towards the rear, you'll get to a point where the rears start to lock first and the car will feel unstable under brakes. |
| 03:46 | Brake bias is something that can be constantly adjusted throughout a race and for a number of reasons. |
| 03:52 | In the dry, one of the main factors is the amount of fuel in the vehicle. |
| 03:55 | As he fuel load burns off, the balance of the vehicle changes requiring a brake bias change to the front or rear depending on where the fuel tank is located. |
| 04:05 | The other main reason we'd want to adjust the bias is if the track is wet. |
| 04:09 | In this situation we'd generally want to run more rear brake bias. |
| 04:13 | This may sound counter intuitive however in the wet we won't be able to generate as much force under brakes which limits the amount of weight transfer towards the front of the car. |
| 04:23 | This means that compared to a dry track, there's less grip available under braking from the front tyres and more available from the rear. |
| 04:31 | Winding the bias towards the rear will optimise the braking performance from the rear tyres and reduce the likelihood of a lockup. |
| 04:38 | So what if your car doesn't have a bias adjuster? There's no need to rush out and get one installed straight away. |
| 04:45 | Standard road cars go through huge amounts of testing at the factory in order to get the best braking balance for all conditions. |
| 04:53 | Generally favouring a front bias by considerable margin. |
| 04:56 | This makes a road car's factory brake bias safe and predictable and this is more than sufficient for track use. |
| 05:03 | It's only when we're starting to look for small gains with slight balance changes on the fly that brake bias adjustments can be a worthwhile addition. |
| 05:10 | Summing up this module the key points here are that brake pressure isn't distributed to the front and rear equally, with a higher percentage going to the front where most of the stopping power is needed. |
| 05:22 | This percentage is set by the factory for a road car and for the vast majority of cases, will do a perfectly adequate job on the racetrack. |
| 05:29 | When a driver starts searching for those last 100ths of a second, this is where an aftermarket bias valve or controlled really comes in handy. |
| 05:37 | With these systems the bias can be controlled from the cabin on the fly and this is often used in endurance racing to rebalance the braking distribution to compensate for fuel load or wet track conditions. |
