Selecting a new Granny-ring
Copyright © Jonathan Levy, 2000. All rights reserved.
Granny-rings which resist wear well have :
Strong, Hard-wearing, and tough material such as a specially selected and heat treated steel (an appropriate type of alloyed steel or stainless steel, eg the old Shimano STX), or titanium. Avoid all aluminium for granny rings and also the die-pressed powder-alloy rings (eg the old Shimano LX which looks chunky and accurately made, but is far too weak). If the problem item is a larger chain-ring (say 30 or more teeth), only then are the higher-strength alloys of aluminium acceptable.
An initial tooth profile which is less likely to wear into a problematic shape. Pressure zones, at and above the tooth roots, with a more accurate match to the shape of the chain's rollers will spread their load more, with less stress concentration and less liklihood of metal failure. Flatter tooth profiles will suffer more stress concentration from the circular rollers, and more indentation failure.
Accurate machining by a process suitable for the material. Die-stamping tears the metal, often leaving only half the tooth width initially able to contact the chain's rollers and bear its loading ; although this will result in an earlier onset of chain-suck due to the resulting inaccuracies with stress concentrations causing indentations more easily, it is often not as important as the material strength/type and basic tooth shape.
In summary, special high-strength steels with well shaped and accurately machined teeth are best.
When assessing products, company literature should give evidence that wear and pressure-face failure of their granny-ring teeth, and chain-suck, are specifically alleviated. Some apparently suitable granny-rings (test results for wear and failure of pressure-faces not known to me), are made by the following companies :
- Stainless Steel : Action-Tec, Blackspire, Real, some Shimano, etc
- Titanium : Action-Tec, Cycle Dynamics, etc
Note: Not all products of the above companies are necessarily of suitable material; you must select and specify carefully what you want.
Re-using a worn granny-ring by Flipping it
This method can be used only if the existing granny-ring was completely symmetrical when new (left-vs-right sides, and pressure-vs-trailing tooth faces). Usually, this means that the ring must be a completely flat plate without sideways offsets in either the teeth or fastening holes.
Such a ring can be flipped so that the unworn trailing faces of the teeth become the pressure faces. Having done this, the undamaged "new" pressure faces will start wearing from this time onward
If not symmetrical, the ring should not be flipped, but replaced as described above.
If the ring is of really soft material, it is a waste of time flipping it ; once you've gone to the necessary trouble of removing the crank, rather install a strong ring.