Date: Thu, 7 Nov 2002 21:02:22 +1300
Reply-To: Andrew Grebneff <andrew.grebneff@STONEBOW.OTAGO.AC.NZ>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Andrew Grebneff <andrew.grebneff@STONEBOW.OTAGO.AC.NZ>
Subject: Re: Drilled Rotors vs. Big Brake Kit (long)
In-Reply-To: <013601c285a8$4c760c80$392c6620@laptop>
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>If I were choosing between the ventilated brake kit and the Big Brake kit
>.... I'd go for Big Brake.
>
>If I were choosing between Ventilated and Stock .... I'd stay stock.
>
>In theory the Ventilated just doesn't seem to me to be that much of an
>improvement, if any.
The big South African brakes are the ventilated ones. Drilled discs
are NOT ventilated... unless someone drills a set of ventilated discs!
Ventilated discs are basically 2 discs with RADIAL vents running from
the hub to the edge... making the disc a fan, which draws cooling air
into the hub center and blows it out the edge slots.
DRILLED discs are merely discs which someone has used a drillpress or
mill to drill holes ACROSS the disc, from side to side. These holes
are useless for anything but causing accidents... when the holes hold
water and prevent the brakes working effectively, or when the disc
breaks, locking the wheel with a bang when the calipers pinch into
the missing bit (or breaking the caliper off and ripping the hose,
resulting in no brakes at all on that circuit. The holes will not
dissipate much heat, as there will be no real flow through them.
Taller wheel/tire combinations will have greater leverage on the
brakes, thereby decreasing the brakes' ability to work effectively
than smaller-diameter tires. When fitting tires/wheels of larger
overall diameter, just to keep the preexisting level of braking
larger-diameter discs must be fitted to gain mechanical advantage.
Fitting larger discs without increasing tire diameter will increasing
braking force available. Think of the thing as a lever. The axle is
the fulcrum point; the crowbar is the tire. With the hub held
stationary by the brake, push on the crowbar (which is levering
against the axle) at a point one foot out from the axle (about one
normal tire diameter). Then shift your grip outward 6" along the
crowbar, simulating a much larger tire (exagerrating here, but it
illustrates the point) and push again. You now have much greater
leverage against the brake, therefore to slow a tire of the same
diameter it will take a lot more braking force than for the
smaller-diameter tire. have I confused anyone?
>That's pretty much what I would expect. The stock brakes can lock the
>wheels, so the stopping power is being limited by the tires, not the
>brakes.
It would take quite an effort to lock the stock brakes with decent
tires on a hard dry surface. And to hold them just a flysh*t short of
lockup, where braking is at its maximum, it would be near-impossible
to modulate them to hold them at this point in an emergency stop.
Bigger brakes have more bite, and are in theory easier to modulate.
My old 1972 Honda CB740/900 Four K2 had an extra disc added. A stock
750 Four can lock its front wheel, but not controllably. On mine I
could, using 2 fingers, howl the front tire from 80mph all the way
down to zero. That is, more powerful brakes make for better control.
Now, these discs were stainless and had been drilled by a PO (OK,
they LOOKED good), and in the wet the holes would retain water, which
the pads would then sweep a film of from the holes across the disc
surface, thereby reducing braking to virtually zero.
>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 very opposite, in fact, as my bike showed so well.
> Any better cooling going on is offset by the mass
>lost to holes. It is mostly just a claim blown out of proportion to sell
the drillings.
Actually, drilling increases the surface area of the disc. As
cololing rate is a function of surface area to volume, the drilling
in theory would INCREASE cooling, but only in a passive way. Not
actively, as with venting.
> Does a Vanagon need cross drilled rotors? -
>NO. Not gonna feel any benefit unless you are a brake pad manufacturer.
>Eats pads.
My old 900 didn't eat pads. And I rode it hard... it would easily do 140mph.
>Slotted/vented
Ventilated. Slotted is a gimmick on street cars, like crossdrilling.
>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
Ventilated discs are NOT solid... the have chanels within them.
UNVENTILATED discs are solid eg early T3s.
>rotors are the highest performance rotor you should go with
>on a street vehicle IMHO. Quality brake pads like Mintex or Ferodo along
>with quality brake fluid like ATE or Pentosin will make a bigger difference
>on stock rotors than any big brake kit. Unless of course you are adding
>more pucks (pistons) to your calipers which I don't think is available for
>the Vanagon. I'm sure if one spent enough money though you could get a 4
>piston StopTech or Bembro system with 11" slotted/crossdrilled rotors fitted
to the Vanagon. But what's the point.
Well, I for one WILL be fitting Porsche 993 brakes all round. For
these I need at LEAST 17" wheels.
>the limitations on braking in a westy are pure physics...you still have
>to stop a massive amount of weight with an awesome kinetic energy
>storage for something that size. the bigger disk sets i have seen
>mounted just help lock the brakes up and the braking distance is still
>the same except you now have a worn out set of tires as well as loss
of control.
What proportion of Vanagons are Westies?
Go for ventilated discs. AVOID drilled discs.
Metallic pads such as Bendix Metal King are good for long pad life
and work very well when hot (so olong as the fluid doesn't boil,
thereby fading the brakes by another means!). All of our cars
(including taxis) are running Metal Kings.
>I don't know how you are getting that kind of stopping performance,
>but my '84 westy has *never* been able to do that (and i've been
>riding in it since day 1). Within the last several months, i've had
>my entire brake system overhauled (pads, all cylinders, etc.) and
>still can merely "slow down". I've checked if my brake booster is
>working (according to bently method, it is...), but still i come no
>where near locking up any tires... dry or wet (as wet as california
>gets, anyway).
>So maybe i should just get some cheap, *really* hard tires? :-)
Same for my 84 Caravelle, and that's empty and not a camper!
Another way to increase braking is to fit calipers with more swept
area; this often means a caliper with more and/or larger pistons, up
to 6 per caliper. Combine this with larger disc diameter...
--
Andrew Grebneff
165 Evans St, Dunedin 9001, New Zealand
<andrew.grebneff@stonebow.otago.ac.nz>
Seashell, Macintosh, VW/Toyota van nut