37 | P a g e
From a Solidworks CAD model, these parameters can be found
Wire Diameter :1.6mm Number of coils :10.5
Outer Diameter :12mm Material : Stainless Steel 17-7ASTM A313
With all this information, the spring constant for a selected spring is found to be 1.255N/mm [10]
Risk Analysis
This section, Risk Analysis, outlines the risks and failures that are associated with the rear gear derailleur.
It discusses potential ways in which the derailleur could fail as well as evaluating these risks and the
actions taken to combat them.
It will make use of Failure Mode and Effects Analysis (FMEA) system, which has two types; design and
process. Design type is undertaken before the part is produced and highlights its function, while process
type is analysing the assembly process of the product after its completion. [11] [12]
The failure most associated with the derailleur is actually its attachment to the bike frame. The derailleur is
attached by a bracket, known as the derailleur hanger. This hanger can snap but is more likely to bend.
The effect of the hanger snapping is that the derailleur is only partially attached, only through the chain, and
gears cannot be changed. The cause of this failure is due to the fact most derailleur hangers are made
from stamped aluminium [13] [14]. Stamped aluminium is relatively fragile [15]. However, this material is
chosen purposely so that the bracket will bend/bracket rather than the frame of the bike breaking. A new
bracket cost under €20 [16] [17] and therefore can be sacrificed to preserve the more expensive frame. The
cause of this failure can occur from simply heavy duty use such as long haul journeys, rugged terrain and
mountain biking. However, it can also occur from having the wrong gear combination for the type of terrain,
but this is rare. With these heavy uses mentioned the occurrence of this failure can be as little as every six
months.
A resulting risk of this failure is the fact that if the bracket bends by snagging on the cogs, it can bend in
such a way that the spokes become damaged. A much more severe outcome, is if the entire derailleur
interacts with the wheel. This can occur from the bracket snapping, or bending far too much, as well as the
limiting screw being incorrectly set. If the lower limiting screw is set incorrectly, set too loose, it allows the
derailleur to travel too far inwards –past the cogs and towards the wheel. This could entirely destroy the
derailleur, cable, housing and the wheel itself. More so, the human injury it could cause is quite dire; a
locked back wheel or the entanglement of the derailleur and chain could cause huge damage. If traveling at
any speed and suddenly stopping, the cyclist may be thrown from the bike. This then leads in to the
aspects of, if the cyclist is wearing a helmet and the surrounding environment for which they will be falling
on. The most severe case would result in death, from either a head or neck injury. Paralysis could also
occur.
However, it is rare for this to occur, with the more likely outcome being the derailleur partially interacting
with the wheel, but still allowing for the bike to move until the cyclist can slowly stop –resulting in little injury.
The precautions in place are, that if the derailleur hanger breaks, the metal snaps so that the derailleur
hangs on the chain but does not interact with the wheel. The position of the derailleur also makes it unlikely
for interaction with the wheel as well as the gear cable attaching to the derailleur.
The other failures can occur from ordinary metal fatigue and everyday cumulative damage. Such as the
wearing of the sprockets, which results in unsmooth gear change. These can be easily replaced when worn
too low. The other obvious failure that could occur, is if the bike itself was in a crash, where there is a side
impact which could smash and bend the derailleur.