Damn rear mech

rhyko7

not sure if this is a question or a rant :roll: skip next two paragraphs to avoid rant and story. ;-)

i bought a new HT frame a few months ago and built it up for xc, i have only used it a dozen or so times and it seems to break every time. I have replaced most of the poor parts and it’s now got a half decent spec.
However in the last 6 rides I have broken 2 rear mechs: 1st an alivio, I thought it probably broke through wear and tear as it was old and battered.
I replaced that with another old and battered mech, a shimano LX, I have done about 4 or 5 rides with it on the bike, mostly on quite smooth ground.


Well today I decided to do a 3 hour ride from home (1 hour up to Cwmcarn and a lap and half around then 45 mins back home). Well I got to Cwmcarn ok and bike handled the 1st 3 decsents with no trouble, however I started climbing up an acsent and after about 30 seconds crunch, damn rear mech snapped and I fell off the bike like a falling tree due to being clipped in.

Im thinking two mechs very similar breaks is no coincidence; both broke whilst in the largest rear cog whilst riding up hill. Basically what happens is that the cassette drags the mech around with it.
I considered it got caught in a spoke and dragged around but surely a spoke would break before the mech

The only thing I can think of is that my chain may be too short, does this seem correct logic or does anyone have any other ideas?
:idea: :?

supersonic

What front cog are you in when this happens?

rhyko7

supersonic wrote:

What front cog are you in when this happens?

granny, you are thinking along chain tension too?

supersonic

No, that should be fine. If it was in the big fronnt ring, it may cause probs...

Next time check the top jockey wheel isn't contacting the largest rear cog - you use the B tension adjust screw to adjust the cog so it clears.

And of course ensure that there is no stiff links in the chain, the mech hanger is straight, gears adjusted correctly etc.

rhyko7

supersonic wrote:

No, that should be fine. If it was in the big fronnt ring, it may cause probs...

Next time check the top jockey wheel isn't contacting the largest rear cog - you use the B tension adjust screw to adjust the cog so it clears.

And of course ensure that there is no stiff links in the chain, the mech hanger is straight, gears adjusted correctly etc.

i think thats what has happened is the jockey wheel has come into contact with the cassette and been dragged around, the mech hanger is now very bent but was straight before it broke.
i think ile put another link in the chain and see if that helps, gonna have to be carrying around powerlinks now after pulling chain apart.

cheers for advice
p.s nice hat lol

supersonic

Extra link will make no difference though - if it was too short, you wouldn't be able to use the middle chainring ;-)

Chain length should be measured by wrapping the chain around the large front and rear cogs, bypassing the mechs, and adding two full links. Thi is to stop what happens when if you cahnge into that gear combo...

rhyko7

almost broke my XT mech on it today, it got pulled down again, i think the problem is something is twisted or bent, because the jockey wheels are running in lone, when i was looking down at it the chain was swaying loads too. Broke two damn spokes on it today, climbing in 1st gear again!
one day my HT will work, it has to cos i wanna race some enduro's on it this summer :roll:

im gonna get a new hanger as the one on it is bent and twisted too much!

bails87

Check the limit screws. Sounds like it's shifting too far and getting caught in the spokes.

77ric

supersonic wrote:

Next time check the top jockey wheel isn't contacting the largest rear cog - you use the B tension adjust screw to adjust the cog so it clears.

+1

where the teeth of top jockey wheel and the largest cog are at their closest you should be able to slip a pen or similar between them, basically a gap of around 6mm.

rhyko7

i finallly got round to having a good luck at it today, i am pretty sure now that the problem is the mech going into the spokes due to the hanger being bent in two directions and twisted, that D screw seemd screwed in enough and had good clearance, so i bent the hanger straight and mech is much farther from spokes, i should buy a new hanger as a spare now really, i wasnt to race enduros on this bike in the summer.
cheers for the advice

diy

supersonic wrote:

Chain length should be measured by wrapping the chain around the large front and rear cogs, bypassing the mechs, and adding two full links.

Any pics that show that - I have heard this suggested before, but can't visualise it

do you wrap them all the way around or or is it:

O===O

and is a link 3 studs worth or 2. (i.e. an inch or less than an inch)

rhyko7

it turns out all the problem i have had are due to the rear wheel.
its an xt rear hub that i built up myself (probably built it wrong)
i have been using a hope ii pro wheel off my other bike and havent had a single problem on it since, the last time i used the xt wheel i broke 5 spokes climbing singletrack due to rear mech.
i had the limiter screw fully in and the rear mech was still way to close to the spokes, anyone else had this problem with spokes snapping on xt rear mech?

im tempted to fork out for another hope ii pro hub now for my HT

supersonic

The hub should make no difference at all to the mech going into the spokes, but you do usually have to adjust the mech when changing between wheelsets.

ilovedirt

get a new mech hanger for a start, even if you bent it straight, it wont be entirely straight. They keep getting caught in the spokes because you haven't set up the limiter screws correctly. Simple.

rhyko7

ilovedirt wrote:

get a new mech hanger for a start, even if you bent it straight, it wont be entirely straight. They keep getting caught in the spokes because you haven't set up the limiter screws correctly. Simple.

the limiter screw is set up right, the limiter screw is almost fully in and if i screw it any more the chain wont go into 1st gear, so thats definately not the issue, although i wish it was that simple.
the rear mech hanger has been bent yes, i have put a new hanger on the bike now, it runs fine with the hope 2 hub and the mech is about 5-10 mm further away from the spokes with the hope in comparison with the xt hub. it doesnt make any sense. all i can say for sure is it works with the hope wheel and not with the xt hub.
im thinking either i buit the wheel up wrong or the hope 2 has a narrower flange than the xt and there is a larger space between the spoke entry flange and the cassette on the hope 2 pro.

ilovedirt

how strange.... i've not had any problems with my XT hub (actually my mech did go into the spokes once at low speed when i'd only just set the mech up, but it's alright now. Safe to say i shit bricks as my mech hanger on my alu scott frame isn't replacable!!!!)

bails87

Could it be that the XT hubbed wheel isn't dished correctly? Does it sit in the middle of the seat/chainstays? Or is it off to one side compared to the hope wheel?

benpinnick

Have you looked at the chainline? Might be that your chainset is too close to the bb so more prone to dropping the chain into the spokes.

supersonic

The dish won't make much of a difference - the cassette and freehub body will still sit in the same position relative to the spokes (the slight angle difference from a way out dish should not be a problem).

Any pics of this? Take a shot from directly behind when you are in the lowest gear.

rhyko7

yeah good idea i should have taken some pics, i have taken the cassette off the xt hub now so no point of tking photos.
i forgot to mention that the same thing happens on both my bikes, i may try a different cassette on the xt hub, it could be the cassette is somehow going on to far into the hub ( i have had problems with this cassette before as i lost one of the spacers and had to use a metal spacer)
ile try the cassette (Sram 980)off the hope hub on the xt hub and see what happens, this is my last effort before i throw this wheel out and buy another hope/DT swiss combo, just gotta order some spokes and rebuild it cos i snapped 5 on the last ride.
cheers for suggestions, ile take photos next time and im sure it will be much clearer as to what is going wrong.