Date:         Sun, 3 May 2009 13:06:56 -0700
Reply-To:     Jake de Villiers <crescentbeachguitar@GMAIL.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Jake de Villiers <crescentbeachguitar@GMAIL.COM>
Subject:      Re: Rear Brake Shoe Design Questions
Comments: To: colorworks@gci.net
In-Reply-To:  <5D79295FD16E4FC9961C826537384BBC@troyb5bff49d63>
Content-Type: text/plain; charset=ISO-8859-1

Troy, front lockup is what you want - it keeps you going forward.  The rears
are still doing some retarding of the forward motion while the front tire
are acting like skates.

When the rears lock up first, as in an unladen pickup truck, the fronts are
still retarding motion while the rears are not, so the rear of the vehicle
swings around to overtake the front and then you're going sideways and out
of control.

If you want to stop well on snow and ice, get a set of modern winter tires.
=)

On Sun, May 3, 2009 at 12:59 AM, Troy <colorworks@gci.net> wrote:

> I can say that one situation where this braking ratio is not very
> effective, and even downright dangerous is in the winter on very slick icy
> roads. I'm sure that other people have run into this and know exactly what
> I'm talking about. Let me explain: You're on a really slick surface, hit the
> brakes and the fronts locked up, but the rears do not--they keep pushing the
> van.  Instant vanagon sled! Here's a case where I think the split would be
> better if it was 60/40 or some other ratio to stop those rear wheels from
> moving or locking the front prematuely and causing a skid and an out of
> control van! This is actually a real-world situation that happens frequently
> in the winter.  I think I am still going to try some shims in mine to
> increase the braking contact surface for the rears. From what I can see, and
> from experience driving my own van, greater braking action in the rears
> would be welcome. I certainly do understand your point, but don't think I'm
> anywhere near throwing off the ratio or making the van dangerous at this
> point. Doing nothing makes the van dangerous IMO.  Just curious what happens
> when you go to "big brakes" in the front?? Do you then need disc brakes in
> the rear to balance things? I do have plans for the big brakes upgrade at
> some point.
>
> Troy
>
>
>    The part that many folks may not be aware of, is that the 'duplex' wheel
> cylinder/ single pivot drum brake arrangement has both a 'leading' shoe and
> a 'trailing' shoe.
>    Due to the rotating action of the drum, a leading shoe's lining
> (different on each side) gets a stronger 'bite' against the drum's braking
> surface, due to a 'wrapping' effect.  The trailing shoe has a lesser bite
> for the same reason.
>    So, your rear brakes will have an actual effective action that's
> dynamically better in motion, than while watching it operate while standing
> still.
>     BTW, the split window bus front drum brakes had two simplex wheel
> cylinders/ two shoe pivot points, so that both shoes were leading shoes.
>  Incidentally the rear drums were the same duplex arrangement that carried
> through the bay window bus, and onto our Vanagons.
>    As a side note, 356 Porsche front drums were aluminum with steel
> friction surfaces pressed inside, and dual simplex/ leading shoes.  The
> rears were still duplex.
>    This shows that the rear brakes on most vehicles are needed to balance
> the braking forces, but must not equal or exceed the front brakes
> effectiveness. The optimum ratio is normally 70/ 30 front-to-rear braking
> power.
>    The bottom line is that our 'hokey' rear drum brakes are plenty
> effective enough to do the job properly.  Any major improvement may actually
> cause unsafe front-to-rear balance during heavy braking, and could result in
> loss of control as the vehicle tries to swap ends with the rears locked up
> prematurely.
>
>  Mike B.
>



--
Jake

1984 Vanagon GL
1986 Westy Weekender "Dixie"

Crescent Beach, BC

www.crescentbeachguitar.com
http://subyjake.googlepages.com/mydixiedarlin%27