Re: [T3] question on rear disc brakes continued...

["Dave Hall" <dave@hallvw.clara.co.uk>]

See below..

Dave.
UK VW Type 3 & 4 Club
http://www.hallvw.clara.co.uk/
------
----- Original Message ----- 
From: "Jim Adney" <jadney@vwtype3.org>
To: "type3.org" <type3@vwtype3.org>
Sent: Wednesday, April 28, 2004 4:22 AM
Subject: Re: [T3] question on rear disc brakes continued...


> On 28 Apr 2004 at 1:13, Dave Hall wrote:
>
> > I'm still worried by the suggestion Type 3 front and rear brakes differ a
lot in
> > their contribution to braking.  Every year my brakes are tested on a rolling
> > road, and the fronts go about 120 on the dial before locking up, while the
rears
> > are around 220.  There's no weight transfer operating, but in most normal
brake
> > use, I don't think there is much of that anyway.
>
> In normal casual braking I think you're right: The stops are low decel and the
> weight transfer is small. At this point we are also a long way from maximal
> braking. Since the only reason for installing rear disks would be improve
> maximal braking (push the envelope) the question becomes how does this all
work
> at the limits?
>
> The short answer is that I don't know, but things seem pretty well balanced,
> with no particular tendency for either the fronts or the rears to break loose
> first. You may be right.
>
In the '69 Auto Fastback Vehicle Analysis you mention next paragraph, they say
both rears locked at 100lb pedal pressure - no mention of the fronts.

> We could understand this better if we knew where our center of mass was.
Didn't
> you have an article that published this?
>
For a '69 auto Fastback, it was (1.68') (20.8") above ground lever with 150lb in
each front seat and 150 lb in rear seat; cg to rear axle is 3.06'.   Pitching
inertia is 1,125 slugs ft squared.  Get calculating Jim!

> > Also, I suspect the volume of lining material that wears off from the rear
shoes
> > is comparable to the front pads,
>
> Yes, this seems reasonable, but I wouldn't be surprised if pad material was
> tougher stuff that shoe material, so I don't know if this means anything. My
> experience is that rear shoes last MUCH longer than front pads: something like
> 30-40k miles for pads and 100k miles for shoes.
>
I wouldn't claim as much as that for shoes - obviously it depends mainly on
style of driving and road traffic conditions.

> > I don't see how consistent results on measuring equipment which is regularly
> > checked in accordance with the Ministry code can be ignored.
>
> I accept your numbers, but I don't know how to convert them to a useful
measure
> of dynamic braking force.
>
A bit of research shows they are reading kg.force.

(snip)

> > This has increased the number of runaway parked vehicles, as the discs
> > cool and contract, releasing the brakes!   Just a thought for those
> > putting rear discs on their Type 3s.
>
> Have you really seen evidence of this?
>
It's made the consumer programmes, and many manufacturers tell owners to apply
the handbrake firmly and put in gear as well, which I've always done on my cars
anyway, plus a bit of lock 'SF style' when parking on a hill.  It probably
wouldn't be as much of a problem in the USA with the higher proportion of
automatics.

> A question for the guys with the early cars: How are the brakes on the early
> cars which had front drum brakes with dual cylinders and dual leading shoes?
> It's just possible that those brakes were as good as the later disks.
>
The Analysis produced a stop of 0.9g with cold drums on the '61 Notchback and
65lbs on the pedal, while the disc braked '69 Fastback did 0.82g with 100lbs on
the pedal.  Food for thought there!   Maybe that's why the contemporary road
tests emphasised the powerful brakes.


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