Re: [AMC-List] Rear disc brakes (was: pulling an axle)

[Tom Jennings <tomj@xxxxxxx>]

On Fri, 27 Oct 2006, Swygert, Francis G MSgt 436 CES/CECM wrote:

> "But on the clamp-on disk brake setup Mark points out, the
> brakes are trying to rotate that bracket on the axle tube;
> that's a lot of force. 

> I agree with ost of your comments, but look at your suspension. There
> are only a few 3/8" bolts holding the whole shebang in! Rear leaf
> springs have two in front, two in back on the shackles, that's it. A

There's a huge difference -- moment arm. The axle is about 3 -
4" diameter.  500 ft/lb torque (200 ft/lb engine, 3:1 axle) on
the axle driving the tire directly translates to the axle tube.

Stamped-brake-pedal forces on that little bolt holding the
caliper clamp would have HUGE forces on it. Leaf spring systems
never see those kind of forces.

Leaf springs go 24" or so out in each direction. That's a lot
of leverage.  The front spring bolt is large, 1/2" - 5/8",
is doing fore/aft load etc and is in shear.  The rear spring
bolts, 3/8", are also in shear; with the leverage ratio (1"
axle shaft vs. 24" spring half) that's a lot of leverage that
3/8" bolt has over the axle rotation.

Also, the rear spring support (shackle, etc) is failsafe --
should a part catastrophically fail, the spring clunks up to the
body, bangs around a bit, and that corner of the car drops 4"
-- but no crash or disabling failure. I wouldn't want my rear
brakes to depend on one single 3/8" or so pin bolt, no matter
how sufficiently strong it might be. It could loosen, corrode,
be flawed, overtorqued, etc etc etc.

Coil spring AMC rears (torque tube and Matador leading arm style)
the tube/arm takes the axle rotational force.

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