Thursday, 25 July 2013

Musings on multi-speed freewheels

One of the mechanical features of running older derailleur equipped lightweight bicycles (in the British sense, hand crafted from a lightweight tubeset such as Accles & Pollock or Reynolds) is multi-speed freewheels. Late 1940s or early 1950s machines were often equipped with a four speed block, which quickly changed to a 5 speed block during the 1950s. Derailleur gearing was favoured more by proponents of mass start road racing and some members of the Cyclist's Touring Club up until the late 1950s, as many UK clubmen still rode fixed gear. These older derailleur systems usually ran with 'half step' gearing if a double chainring was fitted. 'Half step' usually meant between 4 to 8 teeth difference between the outer and inner chainrings. The small difference was caused by two factors, first, the limited capacity of the front mech, which was often rod operated, e.g Simplex or Cyclo-Benelux and secondly, the limited capacity of the rear mech in handling the larger tooth difference. There have been various freewheel manufacturers over the years from the 1930s. The days of buying the appropriate sizes of sprockets from your local bike shop, to build up a custom multi-speed freewheel for the gearing you required, has long since passed. The modern offerings by Shimano, Sunrace or Falcon are sold with set sprocket sizes, i.e. 14~24T, or 14~28T and are available as either 5, 6, 7, or 8 multi-speed freewheel blocks. What is very noticeable is that the durability of the freewheel bodies has declined markedly from former years. (It is possible to recondition some freewheels which are over well over 40 years old for further use on a restoration project). Having examined the 5, 6, and 7 speed Chinese made Shimano offerings, the sprocket sizes were as per the table:-

5 speed
14, 17, 19, 21, 24
5 speed
14, 17, 20, 24, 28
6 speed
14, 16, 18, 20, 22, 24
6 speed
14, 16, 18, 21, 24, 28
7 speed
14, 16, 18, 20, 22, 24, 28

One of the problems (or joys?) with older derailleur mechs alluded to above, is the capacity of the mechs, ie the difference in teeth it can handle between the rear sprockets, in combination with the difference in chainring size. Some older rear mechs would not have the capacity to handle the teeth difference on a 14~28T block. I had been pondering this for a while and after studying the Shimano offerings, wondered if the sprockets were all the same fitting? If this proved to be the case, it would in theory, be possible to cannabalize several freewheels of different sizes to produce freewheel blocks of a smaller tooth range, ie 14~22T in both 5 and 6 speeds. This idea appealed to me, as these freewheels were readily available for free from local skips or from dumped bikes. 

The first job was to source some sample blocks for stripping down. Six speed blocks seemed to be the most common found locally. All the multi-speed freewheels I selected were clearly marked Shimano – China, however, I quickly noticed there was a difference in the style of sprockets between the 5 speed blocks and the 6 speed versions. The difference would only really become clear once the blocks were dis-assembled. 

The dis-assembly sequence requires that the freewheel is firmly held, so the outer threaded sprocket can be removed. 

Once the threaded sprocket has been taken off, the splined sprockets and plastic spacers have to be removed.

This should then leave the bare freewheel body, which is stepped for two sizes of splined sprockets.

This particular 6 speed donor freewheel was seized, so the next stage was to dismantle the freewheel body.  The first step was to unscrew the outer faceplate.

The 1/8 inch ball bearings can be seen in the top bearing cup along with dirt and corrosion.  The next stage is to separate the assembly for cleaning over a suitable container to stop the ball bearings cascading onto the floor.  Note there are shim washers under the faceplate.

This is useful as removal of one or a combination of these may allow the faceplate to be tightened to remove some or most of the excess play in worn bearings.  Now the dirty job to start cleaning.  I usually polish the bearing cups in the freewheel body before re-assembly.

The cleaned main freewheel body components are now ready for new grease and ball bearings.

To return to the subject of the sprockets.  I quickly discovered that the sprockets are not the same on Shimano 5 and 6 speed freewheels.  The 5 speed sprockets are splined but do not have a dedicated key unlike the 6 speed sprockets.  The key is marked like the sprockets used on Shimano cassettes.

In all cases the sprockets will only fit on the freewheel body the correct way, so there is no chance of getting it wrong.  I think this probably has more to do with using unskilled labour in the assembly plant, rather than a deliberate contrivance aimed at preventing the less experienced cycle mechanic from getting it wrong.
The upshot of all this tinkering, is that the idea, although good, is in this case, sadly impractical.  It does prove however that the working life of these cheap and at the moment, readily available freewheels can be extended by a suitable service or rebuild if the owner wishes.

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