Posted by Ben on February 25, 01 at 10:56:25:
In Reply to: No, but here's why I asked about the retrofit... posted by JungleBoy on February 25, 01 at 08:33:01:
In order of importance:
1) Thermal expansion
2) Adjustment
3) Water trap in drum
1) Discs expand increasing the rotor thichness, beneficialt to the operation. Drums expand in dia, detrimental as the shoes now have to go farther out to contact. Can mitegate by initiating parking brake before hydraulics (can get a shoe "stuck" out there). With proper sized drums, lessens affect, but does not remove effect.
2) Disc's do not need any adjustment, they self adjust in the way they work (pistion does not retract but stays out there after friction material is consumed. Drum shoes retract (those springs) after hydraulic pressure is removed. Any friction material consumed is not accounted for. The spreader (star wheel assembly) accounts for and has an automatic adjustment mech whereby initiation is by braking action in reverse. That action must rotate the whole shoe assembly (both shoes) about 15deg, which pulls against a triangular metal pawl that strikes the star wheel, rotating it about 2-3 clicks (complete rotation is about 20 clicks). That pawl wears a spot at contact point and becomes rounded, not allowing good purchase on star wheel. Also the mating threaded parts of the star wheel assembly become corroided or siezed with brake dust/rust/etc and do not rotate freely enough. Plus the reverse braking must be of high enough force to actually rotate the shoe assembly. Manage by keeping the shoes adjusted tight either manually or taking note to brake hard in reverse MANY times.
3) Drum acts like a bowl holding water in there after wading through a puddle. That water acts like lub between shoe friction material and drum surface. Very little to no friction till the water is drained out. Only a temp thing, but if traveling through a large puddle as speed and need to brake on the other side, lessened brake capcity is issue.
Since most drivers do not speed the time to maintain drums, disc rears make more sense, BUT conversions do not have enough engineering and testing to give me confidence to switch. Also maintain my drums very well (Traveler is even farther out there in attention than my routine).
Have been following the threads on disc conversion and comment that those "grinding" noises a few have noted is due to the design of the rear axle & bearings, IMHO. Disc's require a very small axial tolerance, where drums don't need as tight of a tolerance (axle allowed to slide side to side). In order to stop that sliding the bearings will need to have thrust capabilities (Timpkin tappered is an example) and to switch from straight bearins to tapered is EXPENSIVE or impossible without remachining/tooling/etc.