Re: TOY: Sequoia Rear Brakes - more

["Erik Bibelheimer" <thebeebs@xxxxxxx>]

Interesting calculations & info, Thom, but how is it different for drums
vs. disks...?

Most of the goal of a disk brake swap is to be RID of the drums (or so I
thought...?).

So how hard would it be to rig up a drum style t-case e-brake? :-D
I know some TLC's had them but supposedly they were problematic? Or was
that only if the t-case leaked?  If its not that bad of a design, how hard
would it be to bolt one onto a mini? (I've never seen one).

I'd like to swap to rear disks, but I'd also like to have a strong
mechanical e-brake of some sort so that is the main thing holding me up.
I've heard of a few problems w/ the AP t-case brake and the price is more
than I'd like to pay anyway...

- Erik Bibelheimer, Auburn, CA
'86 44Runner, '97 T100
http://home.off-road.com/~bibelheimer

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> From: Thom Singer <ntsqd@yahoo.com>
> 
> If MISF's experience is anything to judge by, hydro-mechanical calipers
> should be avoided on 4x4's. Drums for parking brakes are the way to go.
> The mechanical or hydro-mechanical caliper works fine until you park on
> a steep hill. My mentor at work is fighting that battle right now.
> (Wonder if the OEM's use San Francisco's streets as a guideline for
> hill parking ?) I think the only reason AP's t/c mounted caliper & disc
> works at all is because of the diff gears.
> 
> Some rough numbers:
> Leverage ratio of a Mech. caliper - ~8:1 (typically varies from 6:1 to
> 12:1 depending on caliper & application)
> Pad Coef. of Friction (Cf) - ~.5
> Truck est. weight - 3500 lbs.
> Rotor radius (to pad centroid) - ~5.25"
> Slope - 20* (WAG)
> Assumed: Tire to ground Cf - Infinite (not real, but simplifies things)
> 
> I come up with needing 6285 lbs*in of braking torque required to hold
> the truck still on a 20* incline (3500*Sin 20=1197lbs;
> 1197*5.25"=6284.6). The .5 pad Cf means that it needs 2398 lbs clamping
> force (1197/.5)=2398). Which means there has to be 300 lbs tension in
> the cable (2398/8=299.75). Average upper design limit for an Ebrake
> handle pull is about 75 lbs, which means that lever hanging under the
> truck has to have about a 4:1 ratio (299.75/75=3.996), not likely. OR
> you could move the brake off the axle and take advantage of the diff
> gearing.