As is painfully obvious I spend more time than is healthy thinking about this stuff... the most recent realisation is the potential for both chains and chainrings to wear heavily in two specific locations.
Watching a video on chainring wear last night reminded me of how the teeth on any chainring should wear unevenly about its circumference; with greatest wear occurring in two opposite locations. These areas of maximum wear should occur at the teeth transmitting load to the chain when the force input from the pedal stroke is at its greatest - i.e. when the lever of the crank arm allows the rider's leg to exert the greatest mechanical advantage about the crank axle. This should be around the point where the crank arm is horizonal / parallel to the ground on the downward stroke.
Further to this I recall that the the out-of-the-box two and six speed Bromptons have a 50T chainring and 100 link chain.. the fact that one is exactly divisible by the other meaning that as the crankset turns each link in the chain makes contact with the same tooth on the chainring, and each tooth only ever contacts the same two links.
This means that not only will the chainring wear more in (two) specific spots relative to the pedal positon, but so too will the chain (albeit in four places since it has double the amount of links than the chainring has teeth) . The result of this wear not being evenly-spaced across all parts of the components involved being reduced component life since this will be dictated by the areas of greatest wear.
Conversely this exact divisibility between the number of chain links and chainring teeth apparently doesn't hold true for any of the other Brompton chainring options on these bikes - the 54T chainring apparently requiring a 102 link chain and the 44T chainring running a 98 link chain.
Due to the lack of divisibility between the sprockets on the back (13T & 16T) and the number of links on the chain, with each complete rotation of the drivetrain the chain links contacted by the sprocket teeth will constantly change; evening out wear.
So... the upshot is that specifically the standard 50T ring / 100 link chain setup found on off-the-shelf two and six speed Bromptons will wear more unevently, and hence give reduced service life compared to either 54 and 44T alternatives.
This could potentially be mitigated in a couple of ways:
Firstly, since the bike only runs a single chainring it doesn't require shift ramps / pins to be indexed relatively to the pedal stroke; so the chainring could be periodically rotated relative to the crankset to even out the wear. Since the crank runs a five-bolt bolt pattern this gives the opportunity to move the two points of maximum wear to up to 10 different locations; unlike a four-bolt pattern which would only allow four.
Secondly the chain could be removed and refitted to even out the wear; as happens randomly if the chain is repeatedly removed and refitted - for cleaning or waxing for example.
Thirdly and ideally, this direct divisibility could be removed from the system by manipulating the chainring size - ideally going with an uneven number of teeth as this would maximise the constant variation in tooth / link interactions and, coupled with periodic rotation of the ring on the crank would give the greatest natural evening-out of wear across both parts.
It would be interesting to know just how much faster this situation causes 50T / 100L combinations to wear relative to others - next time I have my chain off I'll take a look at the ring to see if there are any obvious variations in tooth wear..
Watching a video on chainring wear last night reminded me of how the teeth on any chainring should wear unevenly about its circumference; with greatest wear occurring in two opposite locations. These areas of maximum wear should occur at the teeth transmitting load to the chain when the force input from the pedal stroke is at its greatest - i.e. when the lever of the crank arm allows the rider's leg to exert the greatest mechanical advantage about the crank axle. This should be around the point where the crank arm is horizonal / parallel to the ground on the downward stroke.
Further to this I recall that the the out-of-the-box two and six speed Bromptons have a 50T chainring and 100 link chain.. the fact that one is exactly divisible by the other meaning that as the crankset turns each link in the chain makes contact with the same tooth on the chainring, and each tooth only ever contacts the same two links.
This means that not only will the chainring wear more in (two) specific spots relative to the pedal positon, but so too will the chain (albeit in four places since it has double the amount of links than the chainring has teeth) . The result of this wear not being evenly-spaced across all parts of the components involved being reduced component life since this will be dictated by the areas of greatest wear.
Conversely this exact divisibility between the number of chain links and chainring teeth apparently doesn't hold true for any of the other Brompton chainring options on these bikes - the 54T chainring apparently requiring a 102 link chain and the 44T chainring running a 98 link chain.
Due to the lack of divisibility between the sprockets on the back (13T & 16T) and the number of links on the chain, with each complete rotation of the drivetrain the chain links contacted by the sprocket teeth will constantly change; evening out wear.
So... the upshot is that specifically the standard 50T ring / 100 link chain setup found on off-the-shelf two and six speed Bromptons will wear more unevently, and hence give reduced service life compared to either 54 and 44T alternatives.
This could potentially be mitigated in a couple of ways:
Firstly, since the bike only runs a single chainring it doesn't require shift ramps / pins to be indexed relatively to the pedal stroke; so the chainring could be periodically rotated relative to the crankset to even out the wear. Since the crank runs a five-bolt bolt pattern this gives the opportunity to move the two points of maximum wear to up to 10 different locations; unlike a four-bolt pattern which would only allow four.
Secondly the chain could be removed and refitted to even out the wear; as happens randomly if the chain is repeatedly removed and refitted - for cleaning or waxing for example.
Thirdly and ideally, this direct divisibility could be removed from the system by manipulating the chainring size - ideally going with an uneven number of teeth as this would maximise the constant variation in tooth / link interactions and, coupled with periodic rotation of the ring on the crank would give the greatest natural evening-out of wear across both parts.
It would be interesting to know just how much faster this situation causes 50T / 100L combinations to wear relative to others - next time I have my chain off I'll take a look at the ring to see if there are any obvious variations in tooth wear..
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