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handbrakes is harder than you think


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#1 mariner

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Posted 03 May 2012 - 13:38

A bit of a rambling and OT post but anyway...
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

http://www.patentgen.../4499977-4.html

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.


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#2 Greg Locock

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Posted 04 May 2012 - 00:02

Yes using a hard washer to jack a soft shaft is about as cheap as you can get. For my sins I had the dubious pleasure of being the park brake design engineer for 12 months, I learned a lot about cables! The park brake handle at that time was an ugly umbrella type that you pulled. 5 enthusiastic pulls would take it from fully adjusted to out of spec. We used a loose nut on a coarse thread to get the ratcheting effect, which must have cost 10 times that idea you showed.

On the cars of my yoof the idea of a self adjusting park brake would have been a fantasy, luxury was being able to adjust from inside the car (Mini) whereas the Escort was done from underneath.

Late edit - I just realised the screw thread gave a large mechanical advantage as well, so it might have been a wise choice.


Edited by Greg Locock, 04 May 2012 - 00:16.


#3 bigleagueslider

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Posted 04 May 2012 - 02:05

mariner,

As far as the mechanical efficiency of pull cable systems goes, it can vary greatly depending upon factors such as the number of bends or the angle of the bends. Since the cable bears against the inside bend surface of the housing, each bend creates friction and the friction loss becomes greater as the included bend angle decreases. It is easy to imagine a pull cable system with lots of sharp bends having very low mechanical efficiency.

As for the input energy needed for drum brakes versus disc brakes, you need to remember that drum brakes tend to be self-energizing in one direction. My Chevy truck has rear disc brakes, but the manual cable parking brake system is a pair of small drum brakes located inside the disc brake rotor hats.

slider

#4 gruntguru

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Posted 04 May 2012 - 07:47

As for the input energy needed for drum brakes versus disc brakes, you need to remember that drum brakes tend to be self-energizing in one direction.

The function of a handbrake as an "emergency" brake is probably more relevent to the forwards direction, whereas the "parking" brake function needs to be effective in either direction. The statutory test mariner refers to is probably forwards only.

#5 munks

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Posted 04 May 2012 - 14:41

I'm just interested in the fact that your car has to undergo inspections, much less one for the parking brake. In the States, I guess I'm not really aware of any safety inspections for home-built cars (I see the Ohio DMV requires some sort of proof that you own all the parts, but other than that ... nothin'.)

#6 mariner

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Posted 04 May 2012 - 20:28

The Uk is actually one of the easier countries in Europe to register a home built car. Basically all EU cars should go through full homologation , as I think they must in the USA.

Howeve , the SVA , now IVA test , is designed to make the car comply with the intent of homologation wihout a zililon euro/pounds in compliance costs.

A PDF version of teh testing standard is here

http://www.minimag.c...al-May-09-1.pdf

if you are interested ( and have a lot of time!).

#7 rachael

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Posted 05 May 2012 - 06:35

So what is the 'toy'?

#8 Lee Nicolle

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Posted 05 May 2012 - 11:26

Here in Oz the ADRs state that the handbrake must hold the vehicle on a incline,A Tapley meter, effectivly a steep incline. It is not designed to be an emergency brake.Not since the late 60s.
In my experience most handbrakes really are pretty dodgey, older cars with rear drums will lock the wheels at 50kmh,, but you can drive off backwards quite easily.you have to pullit out another 2 or 3 notches to make it hold backwards.
Then you have the handbrake lever in the calliper type which are a pain in the backside, seldom ever work properly. And defenitly will not lock the wheels at any speed.And very often sticks on. The baby drum inside the disc is a bit better but still very iffy and again useless as an emergency brake.
Then the older style commercials with a large contracting band on the end of the gearbox. Actually work quite well but I once pulled it on hard as an emergency brake and pulled the tailshaft off!! Gently gently when doing that. 25 tonne of truck and load!
Our local Government Vehicle Inspection Station still I believe insists on using their brake dyno to test park brakes in direct contravention of the ADRs. It has been known to cause grief with the punters. Especially with the tailshaft handbrake style. An aquaintence took a semi hotrodded small Dodge truck through recently. And got a variation when testing the park brake, surprise surprise the differential action had a lot to do with that.
Oh and hydraulic handbrakes are not approved!
And I doubt an electric one will work any better than any of the above.

#9 mariner

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Posted 05 May 2012 - 11:38

My 'toy car' is her on this old post here

http://forums.autosp...w...=121264&hl=

BTW going back to handbrakes the self wrap properties of drums do not, theorectically help a parking brake as the car is ( usually) stationary when it is applied.

However little drums inside the disc hubs are still very popular. One big advantage is that there should be no wear on the shoes as no rotating friction. Therefore very tight clearance can be used to the benefit of mechanical leverage. The downside is that the small drum radius requires more force to stop the car moving .

The new trend in rear parking brkes is a mehanical system which pushes on the back of the single piston via a screwthread and captive nut. It allows very high machnical advantage whilst being largley inside the caliper so shoudn't rust. The pad wear is taken up by a racheting mechanism in the pushrod to the back of the piston.

The only downside is that you need a special tool to push the pads back as you have to hold a precise gap to keep the ratcheting mechnism free.

#10 Lee Nicolle

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Posted 05 May 2012 - 11:46

My 'toy car' is her on this old post here

http://forums.autosp...w...=121264&hl=

BTW going back to handbrakes the self wrap properties of drums do not, theorectically help a parking brake as the car is ( usually) stationary when it is applied.

However little drums inside the disc hubs are still very popular. One big advantage is that there should be no wear on the shoes as no rotating friction. Therefore very tight clearance can be used to the benefit of mechanical leverage. The downside is that the small drum radius requires more force to stop the car moving .

The new trend in rear parking brkes is a mehanical system which pushes on the back of the single piston via a screwthread and captive nut. It allows very high machnical advantage whilst being largley inside the caliper so shoudn't rust. The pad wear is taken up by a racheting mechanism in the pushrod to the back of the piston.

The only downside is that you need a special tool to push the pads back as you have to hold a precise gap to keep the ratcheting mechnism free.

hardly new, that has been around since the 70s and is an absolute pain in the backside system. And getting the piston back can be a real drama when installing new pads, pads that have worn out prematurely as the spring wont pull back against the screw.

#11 kikiturbo2

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Posted 06 May 2012 - 20:06

The new trend in rear parking brkes is a mehanical system which pushes on the back of the single piston via a screwthread and captive nut. It allows very high machnical advantage whilst being largley inside the caliper so shoudn't rust. The pad wear is taken up by a racheting mechanism in the pushrod to the back of the piston.

The only downside is that you need a special tool to push the pads back as you have to hold a precise gap to keep the ratcheting mechnism free.


yes, this is the new system,... .before that they had a system with a small "cam" inside the caliper that pushed onto the piston via a screw type mechanism that took up the slack when pads wore...

You do not have to use a special tool, you can screw in the piston using any flat piece of metal, 4mm thickness.. Of course, the cam used to wear and the bloody handbrake would stop working..

#12 GreenMachine

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Posted 07 May 2012 - 00:25

The only downside is that you need a special tool to push the pads back as you have to hold a precise gap to keep the ratcheting mechnism free.


Allen key in my case. I would not have called it a special tool, but thats just me ...  ;)

#13 gruntguru

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Posted 08 May 2012 - 23:30

BTW going back to handbrakes the self wrap properties of drums do not, theorectically help a parking brake as the car is ( usually) stationary when it is applied.

Actually you are right, but for a different reason. The "directional" effect of twin-leading-shoe drum brakes is due to rotation of the shoes being resisted by a fixed stop when travelling forwards and the hydraulic cylinders when travelling backwards. The handbrake levers OTOH are arranged to act symmetrically on the two shoes and so are not directional.

#14 RDV

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Posted 30 December 2012 - 20:46

This is serious braking. 125.2 MJ Brake application speed: 90.07 m/s