ABS Solenoid Valves Flow rate

[Sangram Patil]

Hi

I am working on an Anti-Lock Brakes setup and am looking for solenoid valves that are generally used in ABS hydraulic circuits. I need information on the standard flow rates that are used in ABS valves. I am using a normal bike caliper and since I need to control only the small amount of fluid flowing to the piston, my calculations show that the flow rate should be lower than 5ml/sec.

Either I am not getting how the ABS hydraulic circuit works or such low flow rate valves are actually used in ABS.

[Drew Price]

Surprised you didn't get a reply on this.

The way I understand it most OEM ABS systems work is you don't control the flow of fluid to the caliper, you have a valve / solenoid in the brake circuit that bleeds high pressure off and returns it to the low pressure reservoir of the system. It's basically just working as a pressure regulator, think of it as a blow off valve.

That's at least in a system designed to use an accumulator / high pressure supply pump, as a passive system.

5 ml/s sounds really low to me, but I don't actually know how much would need to be bled off, consider a very high speed panic stop going into a corner where ABS is active and bleeds a large part of the transmitted pressure out of an unloaded wheel. You could probably compute the flow you need.

Alternatively, if you're using bike calipers and -3 or -4 lines, any OEM ABS hydraulic block will probably flow waaaaaayyyyyy more fluid that you will actually need to move.



Drew

[Sangram Patil]

Thanks for the reply, Drew. It is true that the solenoid valves are used for pressure control. I wanted to know the process of “pressure bleeding” that you mentioned. Let me try and explain why the flow rate would be of critical importance in this application.

In normal ABS circuits, an inlet pump and relief valve maintains a high pressure at the inlet. Let us call this pressure Pmax. Now consider the following situation – There is an inlet line at Pmax, a valve in between and an outlet line (which is also filled with fluid but at a LOWER pressure). Now since the fluid is INCOMPRESSIBLE as soon as a small amount of fluid is transferred that brings the friction pads in contact with the brake disc, there will be an instantaneous rise in the output pressure to Pmax. So there is apparently NO PRESSURE CONTROL for very high flow rates.

Also consider the situation of panic braking that you mentioned. Suppose the wheels lock completely, then an ABS circuit will switch off the inlet valve and open the outlet valve. Since everything before the calliper is AIR TIGHT and filled with fluid, this situation is SIMILAR to holding a straw filled with fluid upside down with your finger on the top opening. No fluid will come out. In our situation the only reason why fluid will come out in this case is that the friction pads (the piston at the calliper) will move back, displacing that small amount of fluid.

So overall, I concluded that the ABS circuit is working only on the addition/removal of this small volume of fluid due to piston displacement. I calculated the piston displacement to be around 2.5ml to 3ml volume. That is why I am thinking of such low flow rates for pressure control.

Hope I made my thinking clear. I really need to verify that this is how the ABS hydraulic circuit works.

Thanks,
Sangram