Date:         Mon, 21 Jun 2004 10:28:03 -0700
Reply-To:     Ryan Press <ryan@PRESSLAB.US>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Ryan Press <ryan@PRESSLAB.US>
Subject:      Re: Brakes (upgraded and standard)
In-Reply-To:  <E76F6359-C33E-11D8-8D2C-000A958E5A98@mac.com>
Content-Type: text/plain;charset=iso-8859-1

Kim Brennan said:
> I was giving some thought on a drive home late Saturday to the issue of
> brakes. The few folks that have actually upgraded to the "big brake"
> kits haven't had really definitive comments on whether they allow you
> to brake "better. This is not what this posting is about however.
>
> Braking is, in essence, a way to transfer motion energy into heat
> energy. it does this by the mechanical application of calipers (and
> brake shoes) to the front discs (primarly, but also to the rear brake
> drums). The resulting friction of the brake pads/shoes against the
> discs/drums results in the generation of heat. In time it also slows
> the vehicle down. However, now you have hot discs/drums.
>
> In order to improve your braking capability, then, you need to do at
> least one of the following, increase the clamping force, increase the
> friction factor, increase the transfer of energy (heat) to the
> destination (discs, shoes).
>
> The Big Brake kits, primarily attack the last two items. The friction
> factor is increased (marginally, in my opinion) by having a larger wear
> area (larger diameter disc). The transfer of energy is increased by
> adding more mass to the discs and adding ventilation to the discs
> (meaning they can accept more heat in, as they in turn can transfer it
> to the air faster. )
>
> Note that cross drilled discs (frequently available in high end sports
> and sporty cars) add increased ventilation with the cross drilling,
> with a slight loss of mass of the disc. I presume that they have done
> sufficient testing to notice a gain  in the heat transfer with the
> added ventilation despite the loss of mass. Since there is also,
> however the added benefit of removing unsprung mass (increasing
> acceleration performance) there may be a separate compromise decision
> impinging on cross drilling.
>
> Of course, this is all just 2am rambling. The type of discussions that
> geeks have way too late at night...
>


I've been giving thought to my 2wd Vanagon Westy's braking capabilities as
well.  Just this last weekend, towing my dirt bike down the Hopland Grade,
my brakes were overheating.

It takes a pretty substantial pedal force to lock the wheels up, and
obviously the brakes are becoming too hot for the friction material.

With the big brake conversion it seems like a lot of money for a marginal
increase in braking capability.

I am not convinced cross-drilled rotors are the best solution either.  If
the holes are truly drilled and not cast-in then they can become the
starting point for stress cracking.  I much prefer slotted rotors, with a
cast-in slot.  I have these on my MkIV GTI.

Also, I believe the benefit to cross-drilled rotors is to allow hot gases
to escape the brake pad surface, and that cross-drilled rotors do not
substantially cool the rotors down vs standard vented rotors.

I have in my possession a set of shoes and pads from Porterfield Brakes.
They have the R4-S Carbon-Kevlar compound.  They have an average
coefficient of friction of 0.4 mu, and advertise low dust and long
rotor/drum life.

I have not yet installed these pads and shoes and I'll post an update when
I've had a chance to give them a good test.

Ryan