Date: Sun, 3 May 2009 07:04:44 -0700
Reply-To: Don Hanson <dhanson928@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Don Hanson <dhanson928@GMAIL.COM>
Subject: Re: Rear Brake Shoe Design Questions
In-Reply-To: <vanagon%2009050304181191@GERRY.VANAGON.COM>
Content-Type: text/plain; charset=ISO-8859-1
Brakes, and how they are 'tuned', that's a fairly complex subject that most
drivers never need to know about, because their modern vehicles have some
sort of Antilock Braking System (ABS) These systems usually work with some
types of wheel speed sensors and a 'computer' that when it senses a wheel
may be locking up during braking, quickly diminishes the braking to that
wheel and increases the braking to the other, still gripping wheels. The
more modern of these ABS systems will 'cycle' the braking power off and on
to a wheel that's losing traction, much like we do when we 'pump the brakes'
during a hard stop at the traction limit. There's an odd 'buzzy, pulsating'
feeling when some ABS systems engage. Modern sporty cars are often 'driven
by the ABS' in that the drivers just ram them into a corner at over-speed
and let the ABS system control the direction of the car in the corner by
it's electric brain sensing which end of the car is skidding and which has
traction. This is closely related to one facet of the "traction control"
they always talk about in Formula One racing...
Below is a site with plenty of brake stuff, as well as all kinds of
other useful parts. I used this company extensively when I was racing my
Porsche and they are very good. Minn. USA, I think. The reference is to an
adjustable proportioning valve for brakes like I used in my racer. If you
are savvy enough to know how, you can adjust braking forces to match loads,
track conditions, tire wear. brake wear, etc. You can also easily get it
"wrong" if you aren't experienced at braking near the limits.
For the question about 'big brakes' in front...You could and probably
'should' put a little more rearward brake bias into your system if you
install better front brakes...But for a Vanagon, that may be a little over
the top. The knob-style brake proportioning valve, you install between the
front and rear system. At it's Zero setting, it is not changing anything,
but a few turns will alter front/rear braking balance..up to the limits of
traction..
http://www.pegasusautoracing.com/productdetails.asp?RecID=4889
Since our Vanagons are pretty heavy, short wheel based and with a high
center of gravity, I'd say almost all the braking is coming from the front
wheels, and I'd say any braking in a corner is a No No with the Vanagon near
traction limit. What happens is when you brake, the tires 'drag back' on
the direction of travel, causing the 'bottom' of the van to slow while the
top wants to keep going. This 'rotates' the weight of the van onto the
front wheels. Leaving almost no weight on the back (hence, no traction back
there either) So you could have Porsche Big Red Brembo 8 disc brakes back
there and you still wouldn't stop any quicker... The trick is to 'balance'
the system so that as the front wheels lock up, so do the rear...Of course,
this is a laughable simplification of something that engineers spend lots of
time to improve..
Anyhow, if you're a catalog junky like I sometimes am, check out the Pegasus
racing website...(no affiliation with them)
Don Hanson
On Sun, May 3, 2009 at 1:17 AM, Rob <becida@comcast.net> wrote:
> At 5/3/2009 12:59 AM, Troy 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.
>>
>
> Put in the clutch or put it in neutral, take the power away from the
> wheels. Ice sucks.
> Front disks do 60-75% of the braking.
>
>
>
> Rob
> becida@comcast.net
>
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