Brake System Design and Optimization: Proportioning Valves
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Proportioning Valves
05.50
| 00:00 | - While we can use the hydraulic brake bias to set the optimum brake pressure distribution we need, this alone only applies at a single longitudinal G force. |
| 00:10 | This means if the maximum grip the tyres can produce in braking is 1G, the brake bias will usually be set to account for the load transfer from the rear to the front that will occur at 1G. |
| 00:23 | If the brake bias is optimised for this maximum braking event, this means that during less aggressive braking events, we'll have too much front bias. |
| 00:33 | This is because we have less longitudinal load transfer and therefore the rear tyres have more vertical load on them than at the 1G stop. |
| 00:43 | The result will either be a locked front wheel if too much front brake pressure is used or less retardation and less use of the rear brakes than is optimal for that situation. |
| 00:54 | This is where the ideal of a proportioning valve comes in. |
| 00:58 | This is a device that reduces the hydraulic brake pressure to the rear axle with increasing input pressure. |
| 01:06 | Before we go any further in this module, I just want to touch on the name proportioning valve. |
| 01:11 | Strictly speaking, this isn't a very helpful term to describe what this valve does. |
| 01:16 | A more appropriate name would be a pressure reducing valve. |
| 01:20 | Nonetheless this is the name that's widely used in the industry so we'll stick with it in this course. |
| 01:26 | When we're using lower pedal effort in moderate and light stops, a proportioning valve allows the full applied brake pressure through to reach the rear brakes. |
| 01:35 | This is appropriate because not much vertical load has been transferred off the rear tyres and they still have quite a bit of braking capacity. |
| 01:44 | In this case we want more rear bias to take advantage of it. |
| 01:48 | When we want to stop the car quickly in a high G stop, we'll be using high pedal effort and therefore high output pressure from the master cylinder. |
| 01:57 | In this case, with high longitudinal G, we're transferring more vertical load from the rear tyres to the front tyres. |
| 02:05 | This means we need to shift the brake bias forward to match the new vertical load distribution. |
| 02:12 | In this case, a proportioning valve would reduce the amount of pressure applied to the rear brakes relative to the front brakes. |
| 02:20 | Thereby, modifying the hydraulic brake bias. |
| 02:23 | The valve itself is sensitive to input pressure which as we know is related to the braking force demand of the driver and therefore the longitudinal G force. |
| 02:33 | Proportioning valves are found in both OE street cars and motorsport applications. |
| 02:39 | In OE applications, they can be integrated with the master cylinder direction or sit in line at some position between the output of the master cylinder and the rear brake calipers. |
| 02:50 | Proportioning valves are only ever fitted to the rear brake circuit, never the front. |
| 02:55 | In motorsport applications, proportioning valves often have some adjustability built into them. |
| 03:02 | This gives the driver a way to adjust the point at which the proportioning valve becomes active. |
| 03:09 | There are a few different styles but in most cases there's a rotary knob that the driver can turn to adjust. |
| 03:16 | This means the plumbing for the proportioning valve will need to run inside the cabin. |
| 03:22 | Looking at a plot of the rear brake pressure vs front brake pressure, we can start to understand more about how a porportioning valve works. |
| 03:31 | At lower brake pressures, the relationship between the front and rear brake pressure is at a constant ratio. |
| 03:37 | The slope of this line and therefore the ratio is set by our brake bias bar. |
| 03:42 | When we have a proportioning valve fitted, at some point as the brake pressure increases, the valve becomes active, referred to as the knee point of the plot. |
| 03:53 | When this happens, the relationship between the front and rear brake pressure changes. |
| 03:59 | We're now getting less additional rear brake pressure for a given increase in front brake pressure. |
| 04:05 | By the proportioning valve becoming active, we have effectively changed the rear brake bias. |
| 04:11 | When a proportioning valve is adjustable, the only thing it's adjusting is the point at which it becomes active. |
| 04:19 | This moves the knee point up and down the graph like this. |
| 04:22 | When the valve is being adjusted, it's changing the spring pre load that's holding the internal valve closed at low pressures. |
| 04:30 | The relationship between the input and output pressure isn't changed above or below the knee point. |
| 04:38 | By modifying the brake bias, via a bias bar and tuning the knee point of the proportioning valve, we can achieve the optimal brake bias needed in high G stops without having to have a more forward bias than we need in lower G stops. |
| 04:53 | Here we can see that the ideal relationship between the front and rear brake pressure isn't constant. |
| 04:59 | It's changing constantly with load transfer. |
| 05:02 | It's not a perfect system but by adding a proportioning valve, the ideal behaviour can be approximated. |
| 05:10 | A quick note here if you are adding an adjustable proportioning valve to your vehicle, be sure to never add this in line with an existing one. |
| 05:19 | The factory valve must be removed and replaced with the new valve so we're not doubling up on the effect. |
| 05:26 | So in summary, proportioning valves can be used in the rear brake circuit to modify the relationship between the front and rear brake pressure to account for front to rear load transfer. |
| 05:38 | This allows the front to rear brake pressure ratio to approximate what's ideal based on the amount of longitudinal G involved in the stop. |
