Got to love it when reality comes to visit.
At 09:11 PM 6/30/02 +1000, Tim from Sydney wrote:
>.... the 1967 factory special tuning booklet .... has this to say ....:
>
><quote>
>Brakes
>
>After many consecutive applications of the brakes during competition driving
>some brake fade may be experienced with the standard linings.
No kidding, but such is life. For street use there is some compromise to
be chosen between ultimate braking force, brake fade characterists, and the
service life of the lining material (and possibly cost). The original
materials specified by the factory are generally the best known selection
to best satisfy all of the requirements at a reasonable cost.
> Competition
Of course competition use has a totally different set of functional values,
where reduced heat fade has much higher priority, cost per mile is much
less important, and even increase in pedal force requirement may be
tollerated to achieve the ultimate competition performance.
>front disc pads and rear brake?shoe linings or lined shoes are available
>(see list).
And today there is an even greater list of available materials.
>The rear linings are made to a thickness suitable for grinding
>to radius after fitment.
In past years it was common practice (even the norm) for a repair shop to
turn the brake drums, then measure the finished internal diameter of the
drums, and then grind the new brake shoes to match the finished radius of
the drums. I watched a shop do exactly this for my MGA in 1968 (as well as
all other cars that received new brake shoes). The replacement shoes then
generally had linings intentionally thicker than the originals to allow for
this process.
I have not seen this done in recent years, and it is my experience that
today's replacement brake shoes do not have overthickness linings. I'm
sure this is a cost compromise between the cost of parts and the cost of
labor in modern times. You could still grind them to match the radius of
the drums, and this would enhance the initial performance of the brakes and
reduce the break-in period to nearly zero. This would be a prefered
technique for any competition application, but it does reduce the life of
the brakes a little by reducing the amount of lining material (in the
absence of overthickness material to begin with).
>The front pads are of a suitable heavy?duty material. With fair
>competition driving these linings will be free from fade, but will give a
>harder pedal effort on application.
Such is the compromise required for improved ultimate braking. If you are
not racing, and you don't like the idea of increased pedal force
requirement, maybe you wouldn't want to do this to your street car. Or
maybe some of the other materials availabe today can improve on the
available combination of compromise.
>When the lining friction value is altered from that of the standard car
Aye, there's the rub. A change of material may change the pedal force
requirement, which may in turn affect the front/rear brake force
bias. Front disc brakes have higher braking force. Rear drum brakes have
higher surface area and lower surface pressure. Change of pedal force may
affect these two systems differently
>it may be found that changing the rear wheel cylinders to ones of smaller
>size (3/4 in. [19.05 mm.] dia.) will improve the front to rear brake ratio.
So in the case of the comtemporary vintage spec competition lining material
(at the time this manual was written), this change of material would likely
change the ratio of front to rear braking force such that the rear wheels
might lock up before the front wheels have reached full braking
capacity. That is to say the the change may give more improvement at the
rear than at the front, which may lead to premature rear wheel
lockup. Changing to smaller bore rear cylinders would reduce that problem.
Another meaningful way of looking at it is that the resulting increased
braking force may cause quicker decelleration, which causes more weight
transfer from back to front. In that case the front wheels with higher
weight loading can be made to do more of the braking work, while the rear
wheels with proportionately less weight might benefit from having less
braking capacity (to avoid lock-up). The same logic may be applied to
modern racing tires which have a greater grip on the pavement and can
tollerate greater braking forces. In a case of stronger braking it may be
beneficial to change the brake force ratio to have more at the front and
less at the rear. Installing smaller rear wheel cylinders is a cheap and
easy way to do that, and is also another reason that the pedal force
requirement may increase.
>As these wheel cylinders have a dowel on the fitting face, it will be
>necessary to drill a hole in the back plate to match this dowel.
Just a bit of advice that avoids having to purchase new parts. When labor
is cheap or parts are scarce you can modify the original parts. When labor
is expensive and replacement parts are available at a reasonable price, it
might be cheaper to replace the parts. Economics (and parts availability)
change over time.
>Braze a steel plug in the existing hole and face off level with the plate
>before drilling the new hole. The size of the hole is .170 to .175 in.
>(4.32 to 4.45 mm.) dia. and drilled .578 in. (14.68 mm.) above the centre
>of the cylinder mounting hole and .350 in. (8.89 mm.) offset from the
>radial centre?line of the cylinder mounting hole.
This may be a little cryptic, but in essence if the new hole is very close
to or overlaps the old hole it may be difficult to drill in the required
location unless the old hole is plugged first.
>When using your car on a racing circuit always remove the dust shields
>from the front disc brakes. This will enable the discs and brakes to run
>at a lower temperature and will decrease the possibility of brake fade.
><unquote>
This is another functional compromise for competition use. Removing the
dust shields will improve cooling, which is very important for
competition. But for street use removing the dust shields will likely
result in increased wear on the rotors and pads from the ingestion of more
road dirt.
>Make of that what you will.....but it seems the factory is recommending
>SMALLER rear wheel cylinders for uprated brakes, not larger...
And YMMV depending on the type of brake lining material used, the type of
tires used, the type of driving you will be doing, etc. In the end, making
any changes from original spec to brakes, tires, suspension features, or
the type of driving normally encountered can upset the braking system just
as likely as it may improve the system. If you are thinking about making
any such changes you should at least believe that you have some reason for
doing so, and you should have some reasonable expectation for the end
result. Also don't be terribly surprised if the change in wheel cylinder
size does not have the expected result, or if it may have other unpredicted
side effects (good, bad or indifferent). You might keep in mind a "plan B"
to revert back to the original parts if it doesn't work out as well as you
would like.
Class dismissed,
Barney Gaylord
1958 MGA with an attitude (and totally original 4-wheel drum brakes)
http://www.ntsource.com/~barneymg
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