Hallett's Howlers
Posted: 4 Jun 2020, 4:31pm
I’m certainly biased, but I think Cycle needs a new Technical Editor, or even an old one! In the past six years I have often choked upon a half-baked piece of technical advice in Cycle magazine, swallowed hard and said nothing. But in the latest issue Mr Hallett takes the biscuit with duff answers to not one but two readers.
I can no longer remain silent because his advice to Steve Brown with vague shifting on the small end of his new 11-40 cassette, to remove two chain links, is actually dangerous. Sooner or later Steve will engage big & big inadvertently. Then the pedals suddenly jam and probably something will break – if he’s lucky just the chain, if not perhaps a bone! For safety’s sake, a chain must always be long enough to accommodate big & big. Shortening it isn’t even likely to improve the shifting.
For better shifting I would advise a 9-speed ‘Shadow’ mech, such as RD-M592, that doesn’t need an ‘Extender’ and gets low by virtue of a steeper parallelogram slant. Not quite as low as 40, but down to 36-teeth and also up close enough to the smaller sprockets that those shifts don’t become vague. With that I’d recommend the Shimano 12-36 cassette – because Shimano cassettes shift better, but 11-36 can also be had if you really want that 118in top!
So how does Steve still get his leg-saving bottom gear? His outer ring is 48, so I’ll wager the inner is 28 and that the chainset will accept as small as 24 teeth. A 27in wheel with 24/36 becomes like 18in, one inch lower than 28/40: job done!
It’s possible that in dropping four teeth (=8mm) further, the chain may sometimes miss the inner ring and end up wrapped around the bottom-bracket. So fit a chain watcher to be on the safe side. The total difference of this arrangement, being 48–24+36–12= 48 teeth, exceeds the mech’s 45T chain wrap capacity. But so long as the chain is just long enough to accommodate big & big, it doesn’t matter and does no harm if it hangs loose in small & the smallest two or three sprockets.
If even lower is wanted, fit a MTB chainset that takes rings down to 22T. The outer will have to be smaller, so ask yourself: where did top gear ever get, that you wouldn’t have got in about the same time just freewheeling?
Next up, Mark Duce is breaking rear wheel spokes and Mr Hallett cannot say why! Honestly, broken rear wheel spokes are the bane of touring and commuting. Someone who serves this organisation in a technical capacity should very well know why. Please let me explain, in easy steps that someone without an engineering degree might understand.
For a durable wheel it’s really important that the spokes are tightened uniformly to just the right tension, bearing in mind the expected use of the wheel and the type of spoke – and to a lesser extent the type of rim and hub. Any good wheelbuilder will tell you that. What they’ll not be so keen to discuss is how spokes in the rear wheel of a derailleur-geared bike cannot be a uniform tension. That’s because the wheel is dished to make space for all those cogs, obliging the chain-side spokes to be as much as twice as tight as spokes on the left. Given this situation, it takes all of a wheelbuilder’s skill to make the best of a bad job. With experience it is often possible nevertheless, to get the left spokes just tight enough and the right spokes not too tight, that neither side starts to fail before the rim wears out. But it’s still a bad job, at best a compromise.
The rear wheel does not have to be your bike’s Achilles heel. Some ‘factory wheels’ balance the dish with twice as many spokes on the rear right than the left, so all the tensions work out the same. Good job! But since most bikes don’t get ridden enough for ill-tensioned spokes to start failing, standard hubs and rims are made with the same number of holes on each side – for convenience.
The thinking wheelbuilder’s answer is to use different types of spokes in the different sides of a dished wheel. Thinner spokes – for which the ideal tension is lower – on the slack side and thicker spokes – that easily take a higher tension – on the side that has to be tighter. An ideal balance is achieved by specifying spokes that elongate similarly when brought to their necessarily different tensions. Thus as the wheel rolls against the road, spokes near the bottom relax in equal proportions on both sides of the wheel, keeping the rim central with no spoke going completely slack (or not before its opposite number).
Anything closer to that ideal is better than the usual same-old. So my simple recommendation for derailleur wheels is double-butted spokes (thinner in the middle so they elongate more for a given tension) on the left and single-butted (like plain spokes but even thicker at the hub end) on the right. And have a good craftsman build it of course.
In all my time advising CTC members thus, nobody ever complained – apart from the occasional “if it’s good enough for Eddy Mercx” wheelbuilder, reluctant to order those single-butted spokes only tourists seem to want!
Chris Juden, CTC Technical Officer 1983 to 2014
I can no longer remain silent because his advice to Steve Brown with vague shifting on the small end of his new 11-40 cassette, to remove two chain links, is actually dangerous. Sooner or later Steve will engage big & big inadvertently. Then the pedals suddenly jam and probably something will break – if he’s lucky just the chain, if not perhaps a bone! For safety’s sake, a chain must always be long enough to accommodate big & big. Shortening it isn’t even likely to improve the shifting.
For better shifting I would advise a 9-speed ‘Shadow’ mech, such as RD-M592, that doesn’t need an ‘Extender’ and gets low by virtue of a steeper parallelogram slant. Not quite as low as 40, but down to 36-teeth and also up close enough to the smaller sprockets that those shifts don’t become vague. With that I’d recommend the Shimano 12-36 cassette – because Shimano cassettes shift better, but 11-36 can also be had if you really want that 118in top!
So how does Steve still get his leg-saving bottom gear? His outer ring is 48, so I’ll wager the inner is 28 and that the chainset will accept as small as 24 teeth. A 27in wheel with 24/36 becomes like 18in, one inch lower than 28/40: job done!
It’s possible that in dropping four teeth (=8mm) further, the chain may sometimes miss the inner ring and end up wrapped around the bottom-bracket. So fit a chain watcher to be on the safe side. The total difference of this arrangement, being 48–24+36–12= 48 teeth, exceeds the mech’s 45T chain wrap capacity. But so long as the chain is just long enough to accommodate big & big, it doesn’t matter and does no harm if it hangs loose in small & the smallest two or three sprockets.
If even lower is wanted, fit a MTB chainset that takes rings down to 22T. The outer will have to be smaller, so ask yourself: where did top gear ever get, that you wouldn’t have got in about the same time just freewheeling?
Next up, Mark Duce is breaking rear wheel spokes and Mr Hallett cannot say why! Honestly, broken rear wheel spokes are the bane of touring and commuting. Someone who serves this organisation in a technical capacity should very well know why. Please let me explain, in easy steps that someone without an engineering degree might understand.
For a durable wheel it’s really important that the spokes are tightened uniformly to just the right tension, bearing in mind the expected use of the wheel and the type of spoke – and to a lesser extent the type of rim and hub. Any good wheelbuilder will tell you that. What they’ll not be so keen to discuss is how spokes in the rear wheel of a derailleur-geared bike cannot be a uniform tension. That’s because the wheel is dished to make space for all those cogs, obliging the chain-side spokes to be as much as twice as tight as spokes on the left. Given this situation, it takes all of a wheelbuilder’s skill to make the best of a bad job. With experience it is often possible nevertheless, to get the left spokes just tight enough and the right spokes not too tight, that neither side starts to fail before the rim wears out. But it’s still a bad job, at best a compromise.
The rear wheel does not have to be your bike’s Achilles heel. Some ‘factory wheels’ balance the dish with twice as many spokes on the rear right than the left, so all the tensions work out the same. Good job! But since most bikes don’t get ridden enough for ill-tensioned spokes to start failing, standard hubs and rims are made with the same number of holes on each side – for convenience.
The thinking wheelbuilder’s answer is to use different types of spokes in the different sides of a dished wheel. Thinner spokes – for which the ideal tension is lower – on the slack side and thicker spokes – that easily take a higher tension – on the side that has to be tighter. An ideal balance is achieved by specifying spokes that elongate similarly when brought to their necessarily different tensions. Thus as the wheel rolls against the road, spokes near the bottom relax in equal proportions on both sides of the wheel, keeping the rim central with no spoke going completely slack (or not before its opposite number).
Anything closer to that ideal is better than the usual same-old. So my simple recommendation for derailleur wheels is double-butted spokes (thinner in the middle so they elongate more for a given tension) on the left and single-butted (like plain spokes but even thicker at the hub end) on the right. And have a good craftsman build it of course.
In all my time advising CTC members thus, nobody ever complained – apart from the occasional “if it’s good enough for Eddy Mercx” wheelbuilder, reluctant to order those single-butted spokes only tourists seem to want!
Chris Juden, CTC Technical Officer 1983 to 2014