Does anybody actually know

[bigginger]

and could (if they do) they explain WHAT exactly the crankshaft damper (yes, I am talking 1275s here) damps? Do the water pump or fan really damage the crank without it, as I can't see what else it could be?

[Packedup]

The damper damps the vibrations in the crank. The crank will vibrate and even whip around, which can be bad for the bearings and the crank itself. The damper will damped the vibrations at a given frequency range - I don't know what range the OE designed damper is effective at, but I imagine it's somewhere between 3 and 6k (as that's the useful range in a factory A series).

So i's nothing to do with the pump or alternator, and purely about reducing the distortion of the crank!

[MoggyTech]

The damper reduces the amplitude of vibration in the system. The vibration can be torsional or linear. A crankshaft will twist back and forth at a high frequency while it is rotating. This twisting can be reduced by using a crankshaft damper. The amplitude of twist can be something like ±0.4° without a damper and ±0.12° with a damper.

The actual forces are harmonic due to different cylinder pressures, with the main force on the crank at the firing cylinder, the least force on the exhaust cycle cylinder. As these forces can vary in location on the crank very quickly at high revs, the damper reduces force loadings and prevents metal fatigue.

[bigginger]

How does it damp them, then? I'm finding it hard to see how a bit of rubber spinning in the same plane as the crank,and not attached to anything except the crank, could do this

[bmcecosse]

The outer ring is rubber mounted to the inner hub - which is bolted to the crank. It's all designed to damp out the vibrations !! I know I broke two cranks on my 948 Rally Minor (many moons ago) then got hold of a damper from an 1100 in a scrap yard (crash car only a year or so old) and never broke another crank ! That was the earlier large diameter two-piece damper - also fitted to the Cooper S at the time. But the newer small diameter damper works well too. A secondary advantage is that it saves the timing chain from getting a beating, I guess by preventing the nose of the crank from whipping around quite so much.

[bigginger]

So the others said. I have one on each of my 1275s, and a couple of spares, but was wondering HOW it damps crnk vibrations

[bmcecosse]

Just by the rubber mounted outer ring moving in the opposite direction to the crank as it vibrates. All I know is - it's well worth having on ANY A series engine. Helps timing chain life too.

[bigginger]

I trust that the thing works, but I was idly wondering HOW it works. Guess I'll move this to 'do you know', and hope for an answer

[MoggyTech]

I trust that the thing works, but I was idly wondering HOW it works. Guess I'll move this to 'do you know', and hope for an answer

All to do with Newtons law. As the rubber damper is capable of a greater twisting motion than the crank, the dampers mass reduces rotational oscillations within the crank. Engineers design these things, by measuring the force frequency of oscillations (In Kilohertz). The damper is designed to oscillate in the opposite phase to a crank oscillation (180 degree)

Kind of hard to explain it fully without conducting a PHD course in Physics

[bigginger]

Nah, undestood. but hard to believe (while I'm quite happy so to do) that there's enough mass there to make a difference Thank you

[rayofleamington]

if you think that's hard to believe, have a look at a driveshaft on most modern FWD cars - There'll often be a floppy rubber ring sitting there for no apparent reason... (same reason - to change the vibrations)