Rear Mech question

[akafudge]

Dear members, I need some help!

I am quite new to mountain biking and know little about parts and compatibility of parts when upgrading. On that basis, my problem is that I am looking for a rear mech that is more precise with snappy downshifts. My Dawes MTB has Shimano Alivio 8 Speed Rear Mech (RDM410) which costs about £30. if I was to upgrade to a £100 rear mech, how would I know if it will simply fit straight on as a direct replacement? Secondly, would a £100 rear mech prove any better than a £30 rear mech that I have?

Cheers

Jonathan

 

[jig-sore]

shifting problems are usually down to the shifters, not the mechs

 

[akafudge]

Thanks for that but the shifters and the mech actually work fine. I am after a better/quicker downshift than the shifters/mech have the ability to perform. It could well be that I need to replace both shifters and mech I suppose.

 

[Cubist]

ALivio is SHimano 8 spd. To move onto a diferent gear system you're going to have to change up to 9 speed I guess.
First you need to find out if a 9 speed cassette will fit onto your freewheel hub. If it will you're well on your way to a decent setup. Shop around online and you'll get say a deore 9 spd cassette for 20 quid or less, a pair Deore shifters, slx mechs for about 60 quid, and a new chain for about 15. You don't need to spend a ton to get decent mechs.

 

[e-rider]

don't spend £100 on a rear mech - that would be an insane waste of money.

Shimano Deore works fine. The quality of the shift is down to well maintained and greased cables, chain, cassette and overal adjustment and set up of the system. Simply fitting a mega expensive rear mech will not help if the other problems are not resolved.

 

[lukesdad]

Youl pick up an xt rear mech for £30-35 on ebay 9 speed shifter for similar money and a 9 speed cassette again for same money and the cassette will fit your freewheel as long as its not screw in which I dont think it was for 8 speed

 

[RecordAceFromNew]

Firstly in case there is any confusion the number of "speeds" (sprockets on the cassette) is governed by and has to be consistent with the shifter, and for the current purpose independent of the rear mech. This is because all current Shimano rear mechs have the same cable pull ratio and the difference in width of 8 and 9 speed chains/cages is very small.

Therefore to answer the question, technically all current Shimano mtb rear mechs (Tourney, Alivio, Acera, Deore, lx, Saint, slx, xt, xtr) should work just fine as a direct replacement of the Alivio rear mech as long as the SGS (often called long cage) version is selected. Whether the GS or road ones will work or not will depend on the chain wrap and large sprocket size capacity required by the chainset and cassette.

As jig-sore and tundragumski have said shift sluggishness is usually due to something else rather than the rear mech. However if the cable/shifter/mech are clean and working properly, then the quest for easier or perhaps snappier downshift can be assisted by choosing a "rapid rise" (also known as "low normal") rear mech such as this. These mechs are designed to shift into low gears when cable tension is released by the shifter, while typical / high normal / top normal rear mechs such as this would shift into high gears.

 

[akafudge]

Thanks guys, I have learn't a lot from just this one thread. I had no idea you could get low normal and top normal; I suppose top normal is much more common. I also knew nothing about cage length so thats very good to know also!

Without much experience, I am wondering if my expectation that the the speed of a 'top normal' downshift is greter than it should be.

Thanks again.

 

[RecordAceFromNew]

Glad it helps!

But just to complete the picture and hopefully not to confuse you, rapid rise / low normal mechs are "easier" to shift down but will only be quicker if the shifter/cable/mech aren't gunged up by muck. You see with the typical top normal mechs even if the system is gunged up your hand will still be able to force a down shift, and not having to wait for the spring to do the job.

 

[akafudge]

That makes sense, I have just realised a big mistake in my original words .

I have confused downshifts and upshifts; my problem is the upshift (to a smaller rear cog). My downshift is fine but the upshift is reliant on the rear mech spring, it would seem, and therefore it is not possible to 'force it' as with a downshift.

Sorry about that

 

[lukesdad]

That will be the cables clogged most probably, no,it will be the cables clogged.

 

[Cubist]

If changing or lubibng your cables and the mech is not the answer, check these on ebay. You have less than 2 hours......looks like Deore
http://cgi.ebay.co.uk/shimano-gears...sure_cycling_bikeparts_SR&hash=item3cb17c8d78

 

[02GF74]

changing to smaller sized sprokect depends on the spring in the derialluer and the amount of tesnion in the cable - the latter is dependent on the amount oif friction in the cable and to some extent on the shifters.

cleaning and lubing the cable (some say not to lube but I found it does no harm) is the first step.

is the shifter old - the spring does loose its springiness and the shifter will eventually refuse to drop the chain on the vary small sprocket or two.

also it is worth checking the b-stop screw is adjsuted correctly and that the rear mech is adjsuted correctly too.

you can get rapid rise rear shifter that work in opposite way whcih in theory should solve you shift to smaller sprocket problem.

 

[akafudge]

Thanks for the replies guys, it has helped greatly