Need help with rear derailleur

[Mr888]

Hi Guys

I'm still in the market for a new rear derailleur for the MTB. Can someone please explain to me the differences between the differing lengths of cages (short, medium, long) and their applications?

Also what is this "rapid rise" term I keep coming across?

Are 9speed derailluers on MTBs compatible with 6/7/8 speeds?

Thanks

[europa]

When it comes to the number of gears, it's the SHIFTER that makes the decisions - the derailleur goes where it's told. So therefore, a 9 speed derailleur is compatible with 8/7/etc. If going the other way ie, more gears, you need to ensure that the derailleur has enough sideways movement to find the end gears.

Long, short and medium cages refer to the amount of chain the derailleur can hold and leads into the range of gears that the derailleur can handle. For example, the rear dr on my Black Beast has to handle a range of 11-32 - that's quite a range of cog size. Not only that, it has to account for enough chain to cover my three chainrings. So, you choose your rear dr to carry enough chain to cover the largest cog to largest chainring, while still keeping tension on the small to small (even though you don't ride in those configurations).

The derailleur does two things, it moves the chain over the new gear PLUS it maintains chain tension.

The easy way to buy a new one is to make sure it's got the same length leg as the one it replaces and is able to take at least as many cogs as the original did.

Actually, the easiest way is to ask your friendly shop assistant at your lbs, preferably with bike by your side

Richard

[Mr888]

Thanks Richard. I knew you'd have an answer.

The problem is I don't want the same type of derailleur. The current RD and shifters are from my K-mart special (15 speed set up). After many modifications, including an upgraded 8 speed rear cassette and front chainrings, I find that the RD and shifters are very inadequate.

As it is, the set up sort of works. On the largest chainring, everything works OK, but on the smallest chainring only the two largest cogs work and on the middle chainring only the 4 largest cogs work. The other cog combo will just freeze everthing including the ability to pedal sometimes.

Can I shorten the chain to make it all work? If so to what degree? Or will I need a long cage RD?

I don't want to take it to my LBSs as my BS metre always goes through the roof.

Cheers

[europa]

My feel is that you should be able to get it all to work. Unless you have dramatically changed the size of the large cog or the small chainring, the only issue you should have is range of side to side movement on the dr. Of course, the problem could be further up with the shifter or, what is more likely, a sticky cable. Good ol' Sheldon Brown maintains that most shifting problems are caused by cables.

Here's a go, squirt some very light oil such as RP7 (NOT WD40, you'll wind up corroding everything) or sewing machine oil into the cables to lubricate them and get them sliding as smoothly as they can. Then adjust the outer limits of your rear dr to get the cage to line up with the end cogs (can be done by eye). The trick then is to adjust the cable tension on the rear shifting system to get the rear dr to line up with the cogs. There is a mechanically fixed step from gear to gear so if your cable runs smoothly and the rear dr moves freely and the shifter isn't doing something odd, you should be able to adjust the rear dr to work (keep the chain on the middle chainring). After that, it's a matter of getting your front dr adjusted right.

BUT, the slightest problem with your rear adjustment will cause mayhem with your front dr - get the rear end working right first. I recently had an utterly feral front dr set up on the Black Beast, cured by a quarter turn adjustment on the rear dr cable I was not impressed.

Clean and lubricate everything you can - lots of kero, lots of light oil (chain oil works too). Get it all working smoothly, then it's just a matter of adjustment.

You need only buy a longer cage dr if you have dramatically changed the number of teeth on your largest and smallest cogs. All else is just cleanliness, lubrication and adjustment, bearing in mind that worn components can also cause trouble but realistically, on cheap systems, you can get away with horrendous filth and misalignment ... up to the point it all turns feral.

Trouble is, I suspect you're in for a bit of a learning curve and a lot of this stuff is intuitive, so keep fiddling, keep trying. Read what both Sheldon Brown and Parktools have to say on their websites (don't have the links to hand), and see where you go.

Worst case? You manually put the chain on a cog and a chainring and ride single speed until you get it sorted - none of this should keep you off the bike, though it might drive you quietly insane as you try to sort it out

Richard

[Mr888]

It's definately not a cable problem. The system works well when I put the 5 or 6 speed cassette on. It seems that the 8 speed has a significantly wider spectrum of cogs (11-36 as oppose to 13-32). It seem that with the 8 speed, the DR will swivel all the way back (so that the cage and jockey wheels are almost inverted) when the smallest cog is used with the smallest chainring. In fact the chain sort of loops back onto itselft so that chain touches both sides of one of the jockey wheels. This in effect causes chain to lock up (jockey wheel is pulled/pushed in both directions).

If I push the chain downwards (as if to make it shorter), the jockey wheels free up a bit. I've followed the advice Sheldon Brown suggests in regard to chain legnths (ie Largest chainring to largest cog technique), but it doesn't seem to have worked here. I'm thinking a long cage DR will do the trick

[Kalgrm]

The problem is I don't want the same type of derailleur. The current RD and shifters are from my K-mart special (15 speed set up). After many modifications, including an upgraded 8 speed rear cassette and front chainrings, I find that the RD and shifters are very inadequate.

Therein lies your problem. As Richard said earlier, the shifters tell the derailleur where to go. It does not matter what der you get now, you've changed from a 5 cog cluster on the rear to an 8 cog cluster. The shifter is not calibrated to work with 8 cogs, so you only get some of the gears working.

Do you really have an 11-36 cluster? I thought they only went to 11-34 .... Anyway, with either 11-34 or 11-36, you'll want a long cage rear derailleur. You'll also need to get a shifter which matches the number of gears you're running (ie 8 speed rear shifter).

Finally, "Rapid Rise" is Shimano's terminology for a "low normal" derailleur. On most derailleurs, when you release the tension in the cable, the derailleur shifts to a smaller cog (ie a higher gear, so you go faster for the same pedal speed). With Rapid Rise, the derailleur shifts to a bigger cog whe you release the tension in the cable. I've got one of these on each of my two bikes because, for me, it just makes sense for the gearing to work this way.

Cheers,
Graeme

[Kalgrm]

PS - you'll need a new chain too, if you haven't already replaced it. The chain used in 5 and 6 speed systems is wider than the chain in a 8 speed system, so it won't slip between the cogs properly on an 8 speed cluster.

Cheers,
Graeme

[Mr888]

thanks for the replies thus far.

I don't thnk that the shifters are the issue either. I'm running with standard grip shifters (non indexed) so I can manually adjust the tension and make sure each gear shifts correctly (a bit of playing around but you get use to it). I will be upgrading the shifters later on.

The thing is, on some of the gear combinations the derailleur makes it physically impossible to work. The chain and jockey wheels lock each other up. I'll try to take a picture of it tomorrow.

thanks Graeme for clearing up that rapid rise issue.

[LuckyPierre]

You won't find your old derailleur in the Shimano tech. info. pages, but you should be able to find out the 'magic cut-off point' for the transition between 'normal' and 'long cage' derailleurs. As other have said, different cages are required when the 'spead' and maximum number of teeth gets larger (eg. my road derailleur can only handle a spread 16 and a maximum of 27 - so it's OK for my 12-27 cassette, but wouldn't work with a typcial mtb cassette). This link shows the low-end Acera derailleur can handle a spread of 23 teath and a maximum of 34. I think that you will find it tricky getting a derailleur that handles 36 teeth (maybe there's a SRAM one - they're the guys who talk 'rapid rise' too).
ps. I'm off - the temperature's over 2 (or -18 depending on where you are) so the road beckons!

[mikesbytes]

I'd be inclined not to spend too much on a derailer as murphys law will probably mean that if you buy a good one you will probably wack it on a rock first time out.

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[Mr888]

I'd be inclined not to spend too much on a derailer as murphys law will probably mean that if you buy a good one you will probably wack it on a rock first time out.

Too true. I'll have to buy two. One cheap one to use as a sacraficial lamb and then put on the good one....but then again Murphy's Law will ensure that.....Dam you Murphy!

[Mulger bill]

G'Day Mr888, can you shift into the big/big combo? I'm guessing it'll be very difficult. Chances are it is the short cage mech that's the culprit but it will need looking at to be sure.

Shaun

[Mr888]

G'Day Mr888, can you shift into the big/big combo? I'm guessing it'll be very difficult. Chances are it is the short cage mech that's the culprit but it will need looking at to be sure.

Shaun

Big big combo is fine. In fact like I stated before, all the gears with the big wheelchain works fine.

I googled extensively last night and found a website that looks at adequate chain legnth working from the small small combo to the big big and determining the limits of the RD. The illustrations looks to be smilar to my scenario. I'll be giving this a go first.

BTW: i think I already have a long cage RD. Compared to my racer's RD, it's heaps longer

[seddo]

Great thread answered alot of my questions

cheers
Seddo

[Kalgrm]

Mr888,

You probably should not be using anything but the three biggest cogs on the rear when you've selected the small chain ring on the front in any case. The other gear combinations available on the small chain ring are duplicated by the middle chain ring gears.

You should aim to spend most of your riding time in the middle chain ring, with the small and large chain rings as "under-drive" and "over-drive" selections respectively.

My own experience is to use the middle ring for nearly everything, only moving to the small chain ring when I just can't spin the middle ring/largest rear cog combination any more. I only get to the big chain ring when I've run out of gears on a downhill run in the middle ring. In effect, I get two lower and two higher gears at each end of my "normal" range on my 27 speed set-up.

Sheldon Brown has a gear calculator on his web site. Count the teeth on the cluster and the chain rings and plug them in to his calculator. Note where you've got overlapping gears on your system and choose combinations which avoid the issues you have. This will save you some dosh in the short term and teach you some gear selection discipline in the long term.

Cheers,
Graeme

[Mr888]

Mr888,

You probably should not be using anything but the three biggest cogs on the rear when you've selected the small chain ring on the front in any case. The other gear combinations available on the small chain ring are duplicated by the middle chain ring gears.

You should aim to spend most of your riding time in the middle chain ring, with the small and large chain rings as "under-drive" and "over-drive" selections respectively.

My own experience is to use the middle ring for nearly everything, only moving to the small chain ring when I just can't spin the middle ring/largest rear cog combination any more. I only get to the big chain ring when I've run out of gears on a downhill run in the middle ring. In effect, I get two lower and two higher gears at each end of my "normal" range on my 27 speed set-up.

Sheldon Brown has a gear calculator on his web site. Count the teeth on the cluster and the chain rings and plug them in to his calculator. Note where you've got overlapping gears on your system and choose combinations which avoid the issues you have. This will save you some dosh in the short term and teach you some gear selection discipline in the long term.

Cheers,
Graeme

Thanks Graeme, I do do realise that there are double ups in the gear ratios, but do any of the gear combos on your bike cause the rear derailleur to freeze up i.e. locks the cranks from being able to pedal? I'm guessing not...especially when it's more than half your gear combination. There is something amiss with my set up and the more I research the more I'm thinking that it's the chain length. I'll let you know

In any case, I feel more comfortable riding with the biggest chainring as it's the most similar feel to my road bike. I've only used the middle chainring with the largest cog when tackling some of the hills on the Oaks trail (It was the only low gear combo I had that worked at the time ). I wish I had a larger big chain ring though because on some of those downhills pedalling on the Big Chainring Smallest Cog combo was useless (ie I was travelling soo fast that the gears could not lock. The speedo read 41km/h...a far cry from the roadie, but fun nevertheless)

[heavymetal]

There is something amiss with my set up and the more I research the more I'm thinking that it's the chain length. I'll let you know

You are probably right, but there is one other thing to check first as I've had problems. It's the movement of the rear derailleur. When I fitted my BOB trailer skewer the gear cable was in the way so I moved it down and out of the way.

The end result was that the outer cable couldn't flex enough and the rear derailleur couldn't move to take up the slack in the chain thus resulting in a drivetrain lock up.

Putting a longer gear cable outer on fixed the problem. It might pay to just check the travel of your rear derailleur to make sure it is moving in all directions freely.

Kev.

[Mulger bill]

If you can get the big/big OK but not the other way without jamming, and you have a long cage mech, then it does sound like the chain is too long. How do you measure yours? I almost use the Shimano method, probably because it was the first one I learned Wrap the chain around the big ring and the big cog, ignoring the mechs. Where the ends overlap is where you cut. I go a link shorter myself so I get less clatter on the chainstay, but be warned , shifting to the big/big combo on a short chain will break something.

Any chance of some piccies?

Shaun

PS Cross chaining is noisy, inefficient and wears things faster.

[Kalgrm]

<snip>Wrap the chain around the big ring and the big cog, ignoring the mechs. Where the ends overlap is where you cut. I go a link shorter myself so I get less clatter on the chainstay, but be warned , shifting to the big/big combo on a short chain will break something. </snip>

Shimano recommends adding two links to the overlap length.

Cheers,
Graeme

[Mulger bill]

Shows how long since I've read any chain makers literature.
The method stated works for me tho'

[Mr888]

Just re-did the chain calculations and removed FOUR links

Everything seems to be working OK now, although the big-big combo looks to be stretching the RD quite a bit....I think I'll stay well away of this combo.

All I have to do now is put it to the test under load and see how it fairs. Looks like I'll be saving my pennys for a little while longer yet

Thanks everyone

[Kalgrm]

Excellent news: A result without cost!

Cheers,
Graeme

[Mulger bill]

Gotta be happy with that