Date:         Sat, 2 May 2009 23:59:26 -0800
Reply-To:     colorworks@GCI.NET
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Troy <colorworks@GCI.NET>
Subject:      Re: Rear Brake Shoe Design Questions
Comments: To: Mike <mbucchino@charter.net>
Content-Type: text/plain; charset=Windows-1252

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.