Date: Thu, 7 Nov 2002 02:12:37 -0000
Reply-To: Clive Smith <clive.harman-smith@NTLWORLD.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Clive Smith <clive.harman-smith@NTLWORLD.COM>
Subject: Re: Drilled Rotors vs. Big Brake Kit
Content-Type: text/plain; charset="iso-8859-1"
Slight correction..
>Braking force for this instance is the torque
applied to the axle by the brakes multiplied by the distance from the hub to
the center of the pad.<
Thats a torque multiplied by a distance
Ft-Lbs-Ft = Ft ^2 Lbs - wrong units for force, should be lbf only!
Braking force = braking force (presumably you mean at the centroid of the
pads)
Braking torque = braking force multiplied by the distance from the hub to
the center (centroid) of the pad(s)
But all you have to really do is use similarity and multiply the original
braking force by the ratio of the New wheel diameter by the Original one.
Easy!
Fnew = Fold * Dnew/Dold, so if the new calipers are the same distance from
the hub centre as the old, we have a 15/14 or 7% greater requirement, but as
the wheels themselves contribute to momentum (angular in this case), as
discussed exhaustively a month or so ago, the requirement is even greater
(when increasing wheel size).
Suffice to say that since I often carry a ton or more of earth/sand/ballast
in my Syncro, which has 15" wheels, regardless of the debate, I know a set
of solid ventilated SA discs and calipers I've ordered are going to make a
difference - if push comes to shove - which I hope it doesn't too often. I
also happen to think that the standard T25 brakes are pretty good and if
yours aren't, then take a good look at them - the usual problem is thinning
discs and non OEM cow-dung pad material - same goes for Italian cars, put
Lancia pads on a Lancia and it stops, even down a steep hill from 100 mph +;
put any others on and I wouldn't be too sure (speaking from a near death
experience). Don't mess with tyres and brakes, use the best - they're you're
interface with the road effectively. Money well spent. I suspect the drilled
discs with metallic pads are an effective economical replacement option, as
for one, they'll be new!
Clive
'88 Syncro Transporter
----- Original Message -----
From: "pensioner" <al_knoll@PACBELL.NET>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Wednesday, November 06, 2002 10:52 PM
Subject: Re: Drilled Rotors vs. Big Brake Kit
> The engineering reason for the SA large brakes with the 15" wheels and
> larger tires has to do with the "effective" braking force. Larger
diameter
> tires require a proportionately larger retarding torque on the wheel to
> generate the same stopping force. Putting 215x75-15 tires on a standard
> syncro westy produces a loss of stopping power as the braking force has to
> work through a larger "moment arm" (distance from axle center to road). A
> few definitions help here: Braking force for this instance is the torque
> applied to the axle by the brakes multiplied by the distance from the hub
to
> the center of the pad. Stopping force is the force generated at the road
to
> tire interface that actually declerates the van.
>
> For a given braking system increasing the tire diameter will decrease the
> stopping force.
>
> To compensate, VWSA added more braking force via a larger disk and caliper
> assembly.
>
> Drilling rotors actually has very little effect in terms of increasing
> braking force. It does allow slightly better cooling over the solid
rotors
> and slightly bettter wet performance. The SA vented rotors are far
superior
> in terms of heat dissipation characteristics than the smaller solid
rotors,
> drilled or not. Drilling can postpone brake fade but not really increase
> the braking force by any significant amount.
>
> The pad to disk interface describes a cylindrical swept area on the disk
> itself. This interface generates heat and the function of the pad is to
> continue to provide contact friction as the temperature of the interface
> rises. At some elevated temperature the pad material will break down and
> exhibit a "glazing" or "carbonizing" characteristic that has a poorer
> pressure to friction coefficient. For the same pad pressure the failed
> material produces significantly less friction and therefore less heat.
The
> heat being dissipated by the thermal mass of the disk and its
configuration
> is how the energy of motion of the van goes from kinetic to heat slowing
the
> van.
>
> Are you listening, Dr. Feynmann? How'd we do?
>
> "fizzix, it's always just fizzix"
>
> pensioner (start slow then taper off)
>
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