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Re: Need Help, I hate Cycling

01 May, 2012 11:05PM
The issue is the relatively crude way in which derailleur systems work.

You have two sets of cogs, connected by a chain.
The front set of cogs (typically 3 nowadays) is known as the chainset/chainrings.
The back set of cogs (anywhere from 7 to 11) is the freewheel or "sprockets".

The bicycle chain is pinned together to allow the links to move (so they can go round the sprockets and chainrings) and whilst there is a small amount of lateral movement allowed, the chain essentially wants to travel in a straight line.

Now consider that both the front chainrings and rear sprockets form straight lines but there are only a few combinations of those front and rear cogs that have a straight line joining them.

Unless things have changed dramatically in the past few years since I was last involved in sales, the front set of chainrings are offset slightly in favour of higher gears, ie the middlechainring does not form a direct staight line to the middle rear sprocket.

The "grinding" noises you are experiencing are the direct result of contact between the chain sides and one or other of: the inner faces of the front derailleur, the inner face of a larger chainring than the selected one or the chain riding over the jockey wheels in the rear derailleur (those two small platic toothed wheels in the rear gear machanism that take up the slack in the chain). Most likely are the first two.

The illustration I've added above hopefully demonstrates the lineups of the various gears (in a 3x7 system).

http://demo.i-bikeshop.com/smsimg/uploads/gear-crossovers.png

In the left hand illustration, the smallest chainring is selected. As you can see, it lines up very well with the third sprocket and pretty well with sprockets 2 and 4. Even 1 and 5 don't show too much deviation from a straight line, but once we start to look at sprockets 6 and 7, the deviation from straight is becoming quite extreme. Indeed, on 7th sprocket there is a fair chance that the chain will begin to drind on the middle chainring's inner face. Pretty much all shops/stores will tell you to avoid using the smallest chainring with the smallest sprocket simply because the off-line aspect is so great that it will cause premature wear of your chain.

In the middle illustration, the middle chainring is selected. As you can see, it lines up pretty well with all of the rear sprockets with the exception of the biggest (1) sprocket. The middle chainring to lagest sprocket is advisable not to be used though shouldn't cause excessive damage to the chain. The combination will, however, mean that the chain is almost definite to grind on the inner face of your front derailleur mechanism. It is also possible for the chain to grind on the inner face of the front derailleur in middle -> smallest. For this reason, many better quality front gear shifters used to have (I'm not sure if they still do) two settings for the front mech position, to alleviate this grinding. Back in the days before "indexed" gears, cyclists would regularly adjust the front derailleur position to alleviate any grinding, but with modern index systems, this isn't so easy.

In the right hand illustration, the largest chainring is selected. Again, you can see that this lines up pretty well with the smallest four sprockets, however the biggest three get progressively more out of line... infact moreso than the combinations on the smallest chainring... because the chainset so offset to favour higher gears. On the biggest chainring, it is never wise to use the two biggest rear sprockets and I also mention that the thrid biggest will, as above, also leave the chain grinding on the inner face of the front derailleur.

In short;
on the smallest chainring, do not use the smallest 2 sprockets.
on the middle chainring, you can use all rear sprockest but I'd advise avoiding the very biggest
on the biggest chainring, do not use the biggest 2 sprockets and I'd advise not using the third biggest aswell, but this depends upon your specific gear setup.

Using these poor combinations leads to excessive and premature wear of your chain, chainrings and sprockets AND means you are almost certain to experience the grinding noises you are hearing.

As Cotterpin has already mentioned, the "no-go" gears are duplicated elsewehere anyway...
counting sprockets left to right (biggest to smallest)...
smallest + 7th =approx= middle + 5th
smallest + 6th =approx= middle + 4th
biggest + 1st =approx= middle + 4th
biggest + 2nd =approx= middle + 5th



Edited 1 time(s). Last edit at 01/05/2012 11:08PM by WhyCycle? Admin.
Subject Author Posted

Need Help, I hate Cycling

Marty Robinson 01 May, 2012 05:29PM

Re: Need Help, I hate Cycling

COTTERPIN 01 May, 2012 06:08PM

Re: Need Help, I hate Cycling

Marty Robinson 01 May, 2012 09:04PM

Re: Need Help, I hate Cycling

DocB 01 May, 2012 10:43PM

Re: Need Help, I hate Cycling

siwatts 01 May, 2012 11:05PM

Re: Need Help, I hate Cycling

Murray R Langton 02 May, 2012 04:37PM

Re: Need Help, I hate Cycling

Marty Robinson 02 May, 2012 07:32PM

Re: Need Help, I hate Cycling

neilf 02 May, 2012 09:14PM

Re: Need Help, I hate Cycling

siwatts 02 May, 2012 10:52PM



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