When I built my toy car I had to design and fit a handbrake ( parking brake) mechanism to link up to the rear calipers which are 1988 C4 corvette items with the handbrake operating onto the main pads.
I sourced two cables, the lightest handbrake lever asembly I could find, and made a sheet steel box to hold the handle mounts and enclose my home made equaliser bar. It had adjustable reaction plate for the cable outers for slack setting. The car passed the registration (SVA) test handbrake standard of 18% efficiency so I though no more about such a boring subject as handbrakes.
However the car failed its annual inspection last week because the handbrake eficiency was only 14%. Very puzzled so I started thinking about mechanical handbrakes and realised they are , in some ways, harder than the main hydraulic brakes to make work reliably.
I should have realised that
- The cheaper OEM's always said " we kept rear drums so the handbrake would work , instead of fitting rear discs".
-The high end OEM's use little drum parking brakes inside the disc hubs
- There are good reasons why OEM's are going to electric handbrakes.
The Vette parking brake mechanism on th caliper is so clever it was subject to a patent
for Girlock in Australia who made the calipers . Basically its a version of the old VW beetle " jamming" car jack approach . UNFORTUNATELY it relies on very close tolerances and clerances to work at all. Shades of my favourite brake hate - sliding calipers over 5 years old.
The test effiiency of 16 to 18% is actually quite hard to meet as it is the percent of the fully loaded total vehicle weight applied to just one axle. So if laden weight is 1190kg the braking effort at the tyres mut be around 200kg to pass. If you reckon on a friction co eff. on the pads of 0.4 (?) then the clamping force per wheel must be 250kg or so I think i.e 200kg/2/0.4.= 250kg. Now the clever Girlock design rocks the pad clamping device against an adjustable stop so to get 250kg on the pad you have to push with about125kg on the little thrust pin ( see patent ). With two wheels you need 250kg total force.
The caliper mechanical advantage is 3.5/1 and the handle ratio is 6.3 to give an overall ratio of 22/1 . So , to get the get the 250kg force you need a brake handle pull force of 12kg and ( the hard part) not use up all the system slack before the lever runs out of ratchet notches. Remember the overall leverge rstio is 44:1 so any slack is critical.
The specified free play on th adjustable stop is 0.024" to 0.027" so it shows how critical some clearnces are!
I think the calculations are probably about right because the GM manual apparently species the parking brake should lock after 5 clicks of the ratchet with a 27kg pull force on the handle.
Basically the design is very clever but relies on almost no slack to work. I got it to pass (JUST) by lots of "tuning" but I had never realiased the kind of forces you need to generate to get 18% effiency . Put another way the UK pass for the hydrauic brkes is 50% but shared between two axles so its 25% per axle ( 0.5g roughly). So the handbake on th back has to be 70% as good as the hydarulic brake using just mechanical linkages and hand effort not power brake assisted foot power!
No wonder OEM's are going to electric parking brakes - just fit a decent motor and low gearing on the rear hub and power it up , until it all falls apart after a few years due to its vibration and dirt ridden location!
Edited by mariner, 03 May 2012 - 14:02.