Tricky to adjust ,at least if it's your first time
Recommend to potential buyers:
yes
Full review
Well, s*** in a bag and punch it, Chris has written a short(ish) one!
If ever there was an item that I’ve bought purely on price, it’s my Shimano 105 ‘rear mechanism’ for the pushbike I’m currently cobbling together.
The name ‘Shimano’ is now very nearly ubiquitous*, especially on new bikes bought from Halfords, the PC World of the biking fraternity! That of course is not necessarily good publicity, but since I’m only putting together a ‘hybrid’ bike, in more senses than one, I’m not into ‘Roll-Royce solutions’. A hybrid bike would I suppose have been called a ‘tourer’ the last time I was into buying a new one, it having fairly slender less knobbly tyres compared to a mountain bike, with similar gears but larger diameter wheels, (almost as large as a racing bike) and straight handlebars.
When I say ‘rear mechanism’, if you’re not sure of which part of the bike I’m talking about, I’ve supplied a photo of the finished article below.
*Is it possible to be ‘nearly ubiquitous’, or is that like being quite pregnant?
Let me explain what it does first. The rear mechanism serves two main functions on a bike. The primary one is to change the gear ratio on the rear wheel. Secondly, it takes up the slack in the chain caused by moving to ‘smaller gears’, these being the ‘higher gears’ really.
a) The primary function is achieved by exerting a sideways pressure on the chain until it finally takes the hint and de-rails itself jumping onto another gear in the process (hence the other name for rear mechanisms, dérailleur). Inward movement toward the wheel spokes gives a lower (i.e. larger gear) ratio, and conversely, movement towards to outer end of the axle gives the smaller (higher) gear ratios. Even those with no interest in bikes may have noticed how the gears on most bikes form a sort of toothy cone, getting larger towards the wheel itself.
b) Its secondary purpose, i.e. taking up the slack in the chain is achieved by passing the chain over a pair of toothed pulleys, which are on a sprung arm, exerting rearward pressure on the pulley wheels.
A strong but slender cable creates the sideways pull on the mechanism with its other end usually on the handlebars, although many ‘racers’ will still use a control lever lower down on the frame. In my own case, I’ve gone for matching Shimano ‘indexed’ levers, with a kind of sequential gears control, that is to say, you use your thumb for ‘up-changes’ and the index finger for down.
I bought the mechanism from the web site of Chain Reaction, www.chainreactioncycles.com for £24.99 including p/p.
THE TECHY/SPECCY BITS
To mis-quote Chain Reaction, its attributes are (with a few notes from me):-
Compatible with 9 or 8-Speed cassettes – this really means that it has enough sideways movement to range over up to 9 gears. Also can be used for 7-speed, I know, I’ve done it!
Long-Life Pulleys with 11 teeth made of Zytel ® (Dupont) to reduce chain noise.
Max. Permissible Rear Gear Sprocket 27 Teeth
Min. Permissible Rear Gear Sprocket 11 Teeth (therefore it needs to be able to cope with [27-11=16] 16 teeth difference in chain length)
Front Difference of 22 Teeth (Most bikes also have a front dérailleur to shift the chain across say three cogs on the chain wheel – these are allowed a 22 teeth difference, for example you may have three cogs ranging from 22 through 32 to 44 teeth each.)
Total Capacity 38 Tooth capacity – This is its overall ability to soak up 16+22 links worth of chain slack should you choose the smallest front and back cogs at the same time.
Weight 231g – This might not seem a lot to worry about, but when you’re trying to cannibalise what was a 39-pound mountain bike into something lighter, suddenly, weight counts for a lot.
FITTING
There’s not actually a lot to tell about the fitting process, it’s the adjusting it to work that takes time. The unit is designed to be bolted onto a special bracket, that most bike frames are already equipped with these days. The Ridgeback frame that I’m using for my new and ‘doned’ parts is no exception, so the mechanism can be physically attached to the bike with one bolt, which is supplied. Likewise, the control cable is easy to thread into place and loosely connect – there absolutely NO WAY that this will work without adjustment, so don’t bother to tighten anything yet, OR trim the cable to length.
ADJUSTMENT
You have one ally in this – the fact that the mechanism tries to ‘relax’ under its own spring tension into the ‘top gear’ position on the smallest cog.
In the best traditions of a Haynes Manual, ‘fit chain’.
With the chain in place, you should be able to tell if this relaxed setting is correct, i.e. does the chain run smoothly or does it sound like its trying to change gear? If the latter is true, you twiddle one of two small adjusting screws, the one marked ‘Hi’, until it all runs in a slick manner.
Now in my case, I’m only using seven gears with a mechanism that can work with nine of the little devils, so it will also be necessary for me to adjust the screw marked ‘Lo’ to make sure that my mechanism’s scope is limited, thereby preventing it from selecting gears that ‘aren’t there’ – getting a chain stuck down between the gears and wheel spokes is no joke. If you have a second pair of hands get someone to press the mechanism sideways to its full extent, whilst you turn the pedals. You adjust the ‘Lo’ screw until your last (largest) gear runs smoothly, without the mechanism being able to travel any further.
If all this sounds difficult, rest assured that I worked this all out for myself, having never done it before, and only with the barest of advice from others, so a novice CAN do it. You just need a logical mind.
With all this done, it’s time to connect the control cable. Make sure that the lever at the handlebar end is set for the highest gear (slackest cable), and that the rear mechanism has ‘relaxed’ into top gear. In theory all that remains is to tighten up the cable so that it is neither slack nor pulling on the mechanism. The indexation of the lever at the control end should take care of each down-shift from there. If it doesn’t, some fine tuning may be needed, either of the cable length, using the screw adjuster at either end of the cable, or your two ‘end stop’ (Hi/Lo) screws may also need a tweak.
Having somewhere to hang the bike while you pedal it is very useful. I’ve made a sort of gallows that fits in my B&D Workmate, but you can get proper bike stands with a clamp, if you’re really serious about bike maintenance. These are the cycling equivalent of being able to put the car up ramps, and they make working alone on a bike so much easier..
You also need to make sure that the chain runs without snagging or rubbing on something in all permutations of your gear settings (21 in my case). If when slackest, it rubs on itself coming the other way as it meanders through the various cogwheels, then you need to consider shortening it, using a link extractor.
After this, all that remains to do, is lubricate it and road test it making sure that you use all the gear permutations – my own personal preference would be to ride a circular route. By all means ride a long way in total, but stick close to home, just ‘in case’ a long walk back is needed!
MAINTENANCE
The main problem with dérailleur gears actually stems from their chief advantage. Whilst they are comparatively easy to understand the workings of, thanks to their open-to-the-air visible construction, this latter feature means that any lubrication will also be a magnet for road dirt and dust. It is therefore necessary to clean them fairly often, and start all over with the lubrication process. The Shimano’s tough plastic pulleys will resist a certain amount of wear and tear, but they do revolve rather more quickly that the bike’s road wheels. These are relatively easy to replace, thankfully. Other adjustments may be need – my existing experience with this gear type shows that ‘indexed’ gears, i.e. the ones that give you a numbered indicator at the handlebars, need fairly frequent tweaking to keep them stepping onto the right gear.
IN USE
So far, my use has been largely for testing but it all seems to be behaving itself, with nice almost silent down changes, and reasonably slick upchanges. Of course, the ‘proper’ cycling fraternity, the ones who get their bikes (and backs) coated in mud as a right of passage will probably tell me that this one’s not the best to get, but heh, it’s reasonably priced and as long as it lasts a few thousand miles, that means it will last me years in effect; it’s only one of my two bikes after all, and the least useful one at that. My other one folds up into the car boot to be taken anywhere.
This product has not yet been reviewed. 
