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Drum Brakes .. Tolerances & Machining
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whiteshadow



Joined: 24 Jan 2011
Posts: 78

PostPosted: Mon Aug 28, 2017 7:47 am    Post subject: Drum Brakes .. Tolerances & Machining Reply with quote

Hi,

I have all round rod pull drum brakes on my 1947 Singer Roadster, and I'm slowly improving them from shocking to slightly better. I think part of my adjustment issues may be that the drums are slightly out of round.

What sort of tolerance on the oval-ity is acceptable on a brake drum of this 'chunky' engineering era....

Second part...

If we acknowledge the whole 'you don't mess around with brakes!", "you should give them to a machine shop who with charge you for screwing them up", "It can only be machined using the tip of a unicorns horn in special machines only operated by high priests"

I have a suitable capacity lathe, tools, dial gauges and 4 jaw chuck and would make tooling to hold the drums securely......Is it realistic to skim the drums myself, and what sort of surface finish is required (Ground?)??

Please bear in mind we are talking 1940s tech here (rod pull - 1920s?), not the brakes off some 1000bhp hypercar.

Interested in your comments.
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ukdave2002



Joined: 23 Nov 2007
Posts: 4100
Location: South Cheshire

PostPosted: Mon Aug 28, 2017 8:50 am    Post subject: Reply with quote

Lockheed recommended a maximum skim of 60 thou (1.5mm) increase in diameter, for thier 8" drums.

I skimmed my own Morris 8 drums. To sit the drum in the lathe I first turned up a mandrel that sat in the drum hub aperture and used the 3 jaw chuck, it's important that the hub sits absolutely flush with chuck, so check its true before removing any metal.

Lockheed also sold shims that could be fitted between the lining and shoe to accommodate the slight increase in diameter when drums were skimmed..I have never found anyone that ever fitted them, but it makes engineering sense.

Dave
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Kenham



Joined: 12 Mar 2012
Posts: 209
Location: Kent

PostPosted: Mon Aug 28, 2017 9:23 am    Post subject: Reply with quote

Simple enough job to skim them , cast cuts nicely but sometimes you can find a hard bit. Use a tungsten tool and you will be fine, quite satisfying job I find and the finish will be good to go. I find it is sometimes easier to set the drum up in the lathe bolted to a front hub and run between two centres in the bearings , you know it is right then, I believe this was specified for CF bedfords in the manual. Old timers would put a little bit of weld on the end of the shoe to set it out a bit more if adjustment was not enough.
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47Jag



Joined: 26 Jun 2008
Posts: 1480
Location: Bothwell, Scotland

PostPosted: Mon Aug 28, 2017 12:13 pm    Post subject: Reply with quote

Don't forget that the linkage has to be set up correctly before any other adjustments are done. If it's the Girling system you should have reasonably good brakes. Ask the old Ford boys and Peter Scott.

Art
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whiteshadow



Joined: 24 Jan 2011
Posts: 78

PostPosted: Mon Aug 28, 2017 12:21 pm    Post subject: Reply with quote

That's really helpful....many thanks!

How much out of round would measure before you thought it was time to true up?? Or is it that fact I can feel patches of resistance when I rotate the wheels (when setting brake adjuster) enough of an indication?

Regards
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ukdave2002



Joined: 23 Nov 2007
Posts: 4100
Location: South Cheshire

PostPosted: Mon Aug 28, 2017 2:22 pm    Post subject: Reply with quote

10 thou (0.25mm) is what my Lockheed publication from the 50's recommends is the maximum ovality.

Dave
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peter scott



Joined: 18 Dec 2007
Posts: 7113
Location: Edinburgh

PostPosted: Mon Aug 28, 2017 4:42 pm    Post subject: Reply with quote

If you do skim the drums to any great extent then you will need to insert shims between the shoes and the linings to build them to the new diameter.

Peter
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PAUL BEAUMONT



Joined: 27 Nov 2007
Posts: 1281
Location: Barnsley S. Yorks

PostPosted: Tue Aug 29, 2017 12:50 pm    Post subject: Reply with quote

Before you set off skimming drums, you need to make sure that everything else is correct. Very small things wrong with rod brakes can make a world of difference. Is it a Girling system? If it is it is important that the brake expander units can slide on the back plates. On my Jowett this is the usual reason why the wheels turn with tight spots. The self locking nuts need to be tightened only sufficiently to ensure free movement.
It is vital that the lever arm on the compensators cannot go past 90 degrees. If they can then the more effort you put in the less braking you get. I presume that the system acts on all 4 wheels. You must ensure that the central linkage works properly in the event of a rod failure - ie so that you still have 2 wheel brakes. You may need to spend some time adjusting rod lengths to achieve this. Once the system is working properly and the individual wheels are adjusted correctly you should have excellent brakes - well better than a real mini anyway!
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whiteshadow



Joined: 24 Jan 2011
Posts: 78

PostPosted: Tue Aug 29, 2017 9:04 pm    Post subject: Reply with quote

After taking the car out and finding that the brakes had basically become non-existant, I have been on a journey of discovery!
After replacing the seals which were contaminating the rear shoes, I then adjusted the linkages to take up the considerable slack (with info from experts in the club). I don't want to adjust the linkages any further (it's a pain in the butt!) without the shoes bedding in and correct adjustment at the drums. Because of the run out on the drums it is difficult to adjust so that it isn't rubbing, but also is close enough for effective braking when they heat up. i also found that if they are adjusted to rub then there appears to be brake fade.
I will keep make small adjustments to the whole system untill they are up to scratch.
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jp928



Joined: 07 Jun 2016
Posts: 249
Location: Australia

PostPosted: Thu Aug 31, 2017 8:43 am    Post subject: Reply with quote

If you do skim your drums you should get your shoes ground to match - a brake shop can measure your drums and grind the same radius on your shoes . Unlikely you can justify buying the drum micrometer and radiusing machine. Note also that mechanical brakes usually need pretty soft linings (woven is good), and minimal slack every where!
jp 26 Rover 9
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Kleftiwallah



Joined: 27 Oct 2016
Posts: 222
Location: North Wiltshire

PostPosted: Sun Sep 03, 2017 4:09 pm    Post subject: Reply with quote

I have doupted the ovality of the front brake drum of my old Triumph Model 'P' motorcycle (sorry, only two wheels) but I have to get the front wheel out from the rigid front forks, remove tire, remove inner tube, take out all the spokes and THEN I can mounty it in the lathe for checking for ovality (or not).

You four wheelers have it easy!
Very Happy
Cheers, Tony.
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Miken



Joined: 24 Dec 2012
Posts: 544

PostPosted: Tue Sep 12, 2017 2:24 pm    Post subject: Reply with quote

At the risk of stating the blinking obvious, Boring brake drums will clearly make them round again but if they are not machined concentric to the locating bore (where it fits onto the hub) then its not making things much better.
If the centre locating bore is , say about 75mm diameter then I try to obtain a slice of aluminium bar about 100mm, chuck it on the lathe and turn a register for the drum to locate and run truly. a second slice of the bar with a centre drilled hole on one side becomes a pressure pad to transmit drive by applying pressure from the tail stock.
Cast iron drums (and discs) usually turn chatter free with a bit of experimentation with the speeds and feeds and tool angles.
Pressed or spun drums can often ring and chatter horribly and give more trouble. I have resorted to boring drums that ring like this by engaging the feed and revolving the chuck by hand. Yes it takes ages to complete the cut but you get there in the end. Motorbike brakes are best machined fixed to their hubs and set up to revolve on their own bearings via the spindle. This results in perfect concentricity.
Lastly get your shoes relined by one of the specialist companies like Saftek. They recently did some for me. Very quick turnaround. back via the post within 7 days.
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