BRAKES WHAT'S NORMAL

[JonE]

Bob, Would post-early cams actually fit in early systems?

Saying "it is clear the pre Girling rear cams differ front to rear and the rear side to side" is still unclear!

Are you saying that its possible the parts list has been simplified because they were "quite close" in spec and have been rationalised to merely two when there used to be 4 for pre-Girling?

[Mike Costigan]

Just had a quick look in a 1931 Parts List, and we have BL31 & BL32 numbers for the front brake cams, and BO66 & BO67 for the rear brake cams; so, yes, four different parts.

[JonE]

Fabulous!

By 1983, Platts catalogue confirms just two parts; 2 needed for offside and 2 for nearside.

Has anyone got a very original low mileage car they will strip the cams out of to enable us to work out what the actual difference is?!

[Bob Culver]

The RP spares book lists BL76/77 front and BO149/150 rear. Dunno how these differ from earlier. I was just musing whether the latter suited (some) pre Girling Rubies or if there is yet another pair, which would seem necessary if in fact the cams are further forward of the axle tube.
(Longer levers were used but on RP interfere with axle, so often seen upside down!) Has anyone got a Ruby 1 spares book?
Presumably later cams for wider shoes are longer but can they find way into earlier cars?

[Timothy P]

From the index cards, also on the magnificent archive site LINK, which provides part numbers as well as the 'reference number' found in the parts lists.

1926-1932
Offside cam front - Reference number: BL31 - Part Number 1A4032
Offside cam rear - Reference number: BO66 - Part Number 1A4032
Nearside cam front - Reference number: BL32 - Part Number 1A4033
Nearside cam rear - Reference number: BO67 - Part Number 1A4033

1932-1936 (change given is the introduction of the wider drum and shoes)
Offside cam front - Reference number: BL76 - Part Number 1A7194
Offside cam rear - Reference number: BO149 - Part Number 1A7194
Nearside cam front - Reference number: BL77 - Part Number 1A7195
Nearside cam rear - Reference number: BO150 - Part Number 1A7195

So front and rear on a given side should be the same for pre semi girling systems. What the change in the cam was between the 1" and 1 1/4" systems I don't know.
The early parts lists only give reference numbers, and as the front cams appear with the front axle parts which all have BL prefix, and the rears with the rear axle parts all with BO prefix it confuses the fact that they are actually the same front and rear. Other parts are similarly effected with the different 'reference number' despite being the same, drums shoes etc.

Bob these are the only 2 sets of cams given, before the semi girling system. The last parts list to cover the pre girling system, as far as I can tell, is 1406A available on the archive site referenced above. This one also includes part numbers, and these along with the reference numbers, match the second set above.

[Bob Culver]

Thanks Timothy.

My interest is only curiosity, prodded by Jons question.

It is surprising that the same cam suffices for front where it is directly below axle and at rear where is 20 deg or so rotated, although castor accounts for some of the difference and cotter direction is selectable. 

Am still curious whether all pre Girling rear cams are in the same position. Otherwise why are so many long rear levers fitted pointing up?

I think the original cams pre conversion on my car were home made from model T axle so no sure guide!

[Ruairidh Dunford]

Parts lists contained quite a bit of duff/misleading information over the years, I do consider them pretty and of mild interest however- well done Timothy for drilling down via the factory records.

[JonE]

So if 1 1/4" shoes and drums retro fit the earlier system, likely that the cams are just a revised part number perhaps?

What would be interesting is for people to photograph any early cam specimens which differ from those in the paired shots in the cherished supplier images. It's quite easy to tell if they have have been doctored/rewelded, but old worn out things will still show whether there was ever any difference, or perhaps a revision of angles as the 1 1/4" part number came in to better cope with the different front and rear positions?

Good sleuthing, Timothy.

[Timothy P]

Thanks all. It really highlights what a brilliant resource the online archive is. I for one am eternally grateful to everyone who made it happen and for the continued work digitising and adding new collections to the site.

My understanding of the cam angles is below, I've drawn it as if you are looking at the lever side of the backplates of each side. Hopefully shows that the lever is at the same angle to the centre line front and rear as dictated by having the same cam, and that the angle is mirrored about the centre line for the opposite side. Obviously the angles aren't exactly as they appear on the car, but hopefully it gives an idea of how the vertical front backplate and the angled rear backplate can both achieve the correct lever angle with same cam.

   

[JonE]

Well that explains it very well, thankyou. You should market a backplate app, Timothy.... may make you a fortune :-)
Bob - does that answer your musings?