Shimano Derailleur hanger snapped twice - what's happening?

tallmat

Giant Rapid 2 -- 2011 model
Shimano Tiagra - 9x3
used extensively, London daily commute bike, but properly maintained and components renewed as required.

-- Four weeks ago, started skipping gears on rear cassette. Assumed indexing problem, checked and make minor adjustments. No Improvement.
-- Whilst riding home, shifted to lower gear, big snap, Derailleur hangar snapped and chain twisted.
1. Fitted new hangar and new chain. No improvement -- still skipping gears.
2. Fitted new rear cassette. No improvement -- still skipping gears.
3. Fitted new jockey wheels in derailleur cage. No improvement -- still skipping gears.
4. Finally installed new complete derailleur (cage looked slightly out of alignment) -- Shimano Sora (Tiagra no longer available in nine speed version).
5. Fitted new shifter cable.

Problem seem to be cured.

-- First proper Ride, shifted to lower gear, derailleur hangar snapped again.

Any ideas where I go from here?

Many thanks in advance for any suggestions.

Cheers

nicklouse

first off the hanger is part of the bike not the gears.

sounds like chain is wrong length. you say lower gear. which Combo and was it the same Combo each time.

apreading

Likely chain is wrong length or cage is too short for the range of gears.

When on smallest ring at front and smallest cog at back, does everything move freely or does the derailleur fold back on itself and bind up the chain movement? or is there too much slack chain? When on big/big is the chain too tight and derailleur cage flat our in front?

tallmat

Thanks guys.

Based on your comments then chain length could be the problem. When fitting the new chain I shortened it to the exact same length as the one twisted during the first hanger snap. That was probably short sighted as I when on either mid or largest cog at front, and larger cogs at the rear, the hanger was extended more than seemed right - it was almost at 4 o'clock position (i.e. pointing forward, if you see what I mean).

Can't remember if or why the first chain was shortened.

The cage length of the new derailleur is same length as the original one fitted by Giant.

When on smallest cogs front and back, then no, the derailleur didn't fold back on itself.

Thanks again.

apreading

tallmat wrote:

The cage length of the new derailleur is same length as the original one fitted by Giant.

That doesnt in itself mean that the derailleur is the RIGHT length... My Boardman has a medium cage when it should be a long one to use all the gears - I have to avoid using the highest few gears in the small cog or the derailleur binds up and throws the chain into the spokes...! One of these days I will replace it.

nicklouse

tallmat wrote:

Thanks guys.

Based on your comments then chain length could be the problem. When fitting the new chain I shortened it to the exact same length as the one twisted during the first hanger snap. That was probably short sighted as I when on either mid or largest cog at front, and larger cogs at the rear, the hanger was extended more than seemed right - it was almost at 4 o'clock position (i.e. pointing forward, if you see what I mean).

Can't remember if or why the first chain was shortened.

The cage length of the new derailleur is same length as the original one fitted by Giant.

When on smallest cogs front and back, then no, the derailleur didn't fold back on itself.

Thanks again.

the hanger does not move it is part of the Frame. it is where the mech attaches and is a (often) replacment part.

I guess you might be refering to the Cage.

hanger http://www.sheldonbrown.com/gloss_ha-i.html#hanger

Cage http://www.sheldonbrown.com/gloss_ca-g.html#cage

tallmat

nicklouse -- yes, I meant cage not hanger (and surprising the cost of replacement hangers - £20 for a little metal bracket...)

apreading -- your problem sounds very similar. I could only find either a short or medium derailleur, and the original factory fitment was a medium. Is it worth searching out a long cage derailleur ?

Would wrong chain length have caused the original gear skipping?

Also, is there a rule of thumb or some formula to calculate the correct chain length?

Thanks again
Cheers

keef66

Shimano road mechs only come in short or medium cage versions.

Chain length: wrap it around the largest chainring and largest sprocket but don't thread it through the rear mech, then add one whole link (inner plus outer)

Remember to thread it through the rear mech before joining

Alternatively size it so that the jockey wheels are vertically aligned when in the large chainring / smallest sprocket combo.

tallmat

Very helpful - many thanks.

laurentian

As an alternative, when sizing a chain, I always use the following equation (courtesy of Park Tools):

L = 2 (C) + (F/4 + R/4 + 1)


L = Chain length in inches. Round the final result to closest whole inch figure.
C = Chain stay length in inches (distance from centre of rear axle to centre of crank), measure to closest 1/8”. Use chart below to find decimal measurement.
F= Number of teeth on largest front chainring.
R= Number of teeth on largest rear cog.

All measurements in inches and you need to decimalise the inches for the "C" measurement as follows:

1/8” = 0.125”
1/4" = 0.25”
3/8” = 0.375”
1/2” = 0.5”
5/8” = 0.625”
3/4" = 0.75”
7/8” = 0.875”

Seems to work fine - good luck!

tallmat

I feel a spreadsheet coming... I'll give it a go.

Many thanks, from one Northants boy to another..
Cheers

laurentian

Much easier than it sounds! Whereabouts are you ??

tallmat

Near Towcester.

Ouija

tallmat wrote:

nicklouse -- yes, I meant cage not hanger (and surprising the cost of replacement hangers - £20 for a little metal bracket...)

apreading -- your problem sounds very similar. I could only find either a short or medium derailleur, and the original factory fitment was a medium. Is it worth searching out a long cage derailleur ?

Would wrong chain length have caused the original gear skipping?

Also, is there a rule of thumb or some formula to calculate the correct chain length?

Thanks again
Cheers

As a general rule of thumb you use long mechs on triple systems (to take up the huge difference in chain tension between the biggest gear and smallest), medium mechs for double systems with larger cassettes on the rear and short cage for singles or doubles with small cassettes on the rear (the amount on difference in chain tension between lightest gear and stiffest is small, so you don't need a longer arm on the mech to keep it pulled tight).

If you've got a triple front ring system but a small mech on the back setup in such a way that the mech doesn't completely fold back on itself when in the small/small combination (slackest chain), theres a good chance that when you go to the big ring at the front and the larger rings at the back that you'll be literally pulling the mech off the frame because the chains too short and tight.

Conversely, if you lengthen the chain so that it doesn't pull the mech off on the big/big combos the chain will go very slack on the small/small combo and the mech will fold up on itself (which is why one of the other posters asked, i imagine) as the mech doesn't have a long enough arm to deal with such a large range of gears and chain tensions. This is why long cage mechs exist in the first place (though quite rare on road bikes as doubles are far more common than triples).

tallmat

Great explanation - I'd never fully understood why different cage lengths, thank you.

I rarely see long derailleurs, even on triple front mech. I never use the smallest front chainring, not needed for urban use.

Thanks again