Herald Dual Circuit Brakes

[Adrian Girling]

I've previously posted about my project to use a Herald 13/60 chassis to build a replica of a 1936 BMW328 sports car.  I've got a good chassis and drivetrain, but the brake and clutch master cylinders are missing as well as the foot pedals.  So I'm thinking of converting to dual circuit, diagonal.  Has anyone done this?

From the drawing in the handbook it's clear from the pedal that it has a high mechanical advantage to allow the front disc calipers to work without a servo and the rear drum slave cylinders are are fairly small diameter to allow them to balance with the calipers in the single circuit.

I'm wondering, does anyone have the bore diameter of the brake master cylinder and the dimensions of the foot pedal (length and pivot point to work out mechanical advantage)

Grateful for any insights and/or advice!

 

[RobPearce]

I suggest you don't. The use of diagonal split requires a significant change to suspension geometry. Without that, failure of one brake circuit leaves you with a car that will not stop in a straight line, and is likely to spin when the brakes are applied. That's why Triumph used a front/rear split. Modern cars with diagonal split all have the suspension set so that the steering is horrid, and overcome that with PAS.

[Nick Jones]

Rob beat me to it.  You really don't want diagonal split.  Without the afore-mentioned suspension geometry changes the car will pull very hard to one side if a circuit fails.

The alternatives are

To do a front/rear split as done on some late/export Spitfires and GT6s (master cylinder not at all easy or cheap to find) This won't give you as much braking reserve braking in the event of the front circuit failing but better than nothing.

To use true split circuit brake calipers such as the Austin Princess ones which have 4 pistons each in 2 pairs operated by different circuits.

My view is you'll probably do just as well to stick with the factory single circuit, use best quality components and keep up on the maintenance.

Nick

[Adrian Girling]

2 hours ago, RobPearce said:

I suggest you don't.

Thanks Rob, fair point.  I'm not a fan of front/rear split either - if the front fails and the back locks...

As luck would having, trawling through the boxes that came with the car I discovered the attached.  So I'm back to sticking with the original single circuit.  Only serious bit missing is the rod with circlips which is the pivot point for the clutch pedal.

 

[Adrian Girling]

5 minutes ago, Nick Jones said:

My view is you'll probably do just as well to stick with the factory single circuit, use best quality components and keep up on the maintenance.

Thanks Nick, I was writing my reply as your post appeared - All good advice, hopefully useful to others at some time.

[Dale]

To finish answering the question and provide a little extra info; the OE brake master has a 0.625" bore which is fine for a std setup but if you want a bit more stopping power without having to go mad and change everything, you can fit a remote servo which improves things a lot. The problem then is increased pedal travel which is best countered by fitting the 0.70" master from some later cars (GT6 etc) or, if you can find one, a 0.75" one but don't go any bigger than that - even if you find something which looks like it will work - it won't.

There are 3 different bores of rear wheel cylinder and I would advise using the smallest (5/8" or 0.625" in old money) ones and also upgrading the front pads to the larger (Dolly Sprint pattern) ones which fit the later 13/60 calipers/discs fine and give a little more contact area than Herald/Spit. As you don't have a heavy car you can get away with Green Stuff or if you are deaf, have a loud stereo or habitually wear ear defenders, Mintex 1144s or equivalent, which stop incredibly well, don't seem to fade ever and will drown out the noise of a Russian artillery strike even when visiting the  Ukraine................

Motul RBF 600 fluid or similar will complete your braking upgrade to the good old British 'jolly impressive' level, if not quite 'I've just won the bragging war down the pub' grade which some 'experts' on other Forums might possibly, on a bad day, recommend by using other products, some from well known 'snake oil' distributors. 

I aren't no engineer (strictly speaking) but have built loads of these and prevented them from mingling with scenery over the years - you pays your money and you takes your choice.

[Nick Jones]

10 hours ago, Dale said:

Mintex 1144s or equivalent, which stop incredibly well, don't seem to fade ever and will drown out the noise of a Russian artillery strike even when visiting the  Ukraine................

Agree with most of this, except to say that they can be made to fade (though only if you are a complete animal), and I have no idea about the noise comment - I’ve been through quite a few sets of these on several cars and not had noise issues. For me they are the first choice, go-too pad.

I did try Greenstuff once. Wasn’t impressed. No bite at all.

Nick

 

[Adrian Girling]

10 hours ago, Dale said:

Motul RBF 600 fluid or similar will complete your braking upgrade

Dale, thanks for all the info - for now I'll probably stick with the standard set up and see how it goes but you've answered another a question I wanted to ask - what brake fluid to use.

When I was a graduate apprentice with AP/Lockheed fifty years ago I learned that the ingredients of brake fluid and margarine were virtually the same except for the water content.  I've always insisted on butter

My most likely need to make a change will be to have the smallest bore rear brake cylinders and larger front pads to compensate for the overall reduction in weight with the CG moving forward due to engine weight.

[Nick Jones]

6 minutes ago, Adrian Girling said:

graduate apprentice with AP/Lockheed

Sorry…. Can’t resist it any longer, did your surname cause any comment?

[Dale]

As in 'When using Girling brakes take heed in case they lock'?

The difference between a Herald and the heavier Vitesse, plus personal preference, will dictate the choice of pad material. I use 1144s on 2000s as they're heavier cars and Green Stuff can fade badly on those but I only ever had one issue on the Herald at Castle Combe when I set fire to the caliper seals although the brakes hardly faded at all.

[Adrian Girling]

On 22/05/2022 at 11:46, Nick Jones said:

Can’t resist it any longer, did your surname cause any comment?

Indeed it did, but just mild amusement 🙂