A4 scale = 70%
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Cycle transmission notes page 2
The cheapest cranks are cast from a soft grade of aluminium alloy, where molten metal is simply poured into a mould. The strongest aluminium cranks are forged from a high grade alloy - they are hammered into shape in massive presses. Both types of crank can then be machined to increase their dimensional accuracy, as forged ones invariably are, but this doesn't alter their strength.
Unfortunately all aluminium cranks look similar to the untrained eye so it is worth taking advice when buying a pair. Don't be fooled by the deliberate misuse of the term 'forged' as we've even seen that label written on the backs of poor-quality cast cranks. Fortunately, however, many cast cranks are perfectly strong enough for normal use.
Other materials and processes can also used, and some are good whilst others are poor. The worst cranks we have ever seen were made of plastic with an expensive veneer of carbon-fibre stuck to them.
A lot of rubbish is written about chainsets. In order to transmit maximum power, you need accurately made, stiff cranks which don't flex under load, and accurately made, strong chainrings which run true - even if they do weigh a few grams more than other chainsets. Finally you may see references to esoteric terms like 'Q-factor' but they mean little in practice and can simply be ignored.
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PCD (pitch-circle diameter) and BCD (bolt circle diameter) are the same thing; they measure the diameter of an imaginary circle passing through the chainring fixing bolts and determine the smallest chainring that can be fitted to a crank. The pcd can be difficult to measure directly, so the bolt spacing (measured centre-to-centre) is often used instead. Most 'triple' cranks have 2 sets of chainring bolts - one pcd for the outer and middle chainrings, and a smaller one for the inner chainring.
Cranks and chainrings can have up to 6 arms and chainring bolts. Choose a common pattern so you can find replacement chainrings when you need to, for example:
| no of arms | outer/middle pcd (mm) | inner pcd (mm) | smallest chainring | comments | |
|---|---|---|---|---|---|
| Compact Drive 4 arm MTB triple | 5 4 5 | 94 104 110 | 58 64 74 | 20 t 21 t 24 t | best for low gears not all makes take standard chainrings still popular after 35 years |
| Road 'Compact' Shimano 'Road' Campag 'Road' | 5 5 5 | 110 130 135 | 33 t 38 t 39 t | no inner chainring chainrings too big for general use chainrings too big for general use |
'Road' cranks, as used by racing cyclists, only take large chainrings and are inappropriate for most cyclists. Smaller pcds allow smaller chainrings to be fitted, providing lower gears for climbing hills and carrying loads comfortably.
These charts will help you identify your crank's pcd and what chainrings can be fitted to it. Count the number of bolts holding your chainring and click on the appropriate link below:
| 1 bolt | 3 bolts | 4 bolts | 5 bolts | 6 bolts |
| © Chris Bell, 2006 | continued on the next page... |
| HIGHPATH ENGINEERING Cornant, Cribyn, Lampeter, Ceredigion, SA48 7QW, Wales, UK phone / fax: +44 (0)1570 470035 (UK office hours only) email: 
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