Suddenly realised what was wrong!

[Mick F]

I have one of these:

because I use a braze-on front mech.

I've had it for five or six years, and I've always suffered from the chain coming off when going to the inner ring. It doesn't happen every time, just occasionally, and especially when I don't want it to happen!

I considered buying a Dog Fang, but always resisted it thinking that the adjustment of the front mech was the key. Sometimes I could go for months without the chain coming off, but since running a new chain, it's happening more often, but with fine tuning and gentle front changes, I can get by ok.

However, it suddenly occurred to me what was wrong for all these years!

The front mech is positioned parallel to the rings and as close as possible - 1mm above the big ring. The trouble is with front mechs, they have to have the correct profile - ie the curve of the mech has to match the curve of the big ring. Mine must do because it's a Campag Comp Triple mech with a Campag Chorus Triple chainset.

The front of the front mech is 1mm from the big ring ok, but the rear end of the mech was about 10mm far away from the ring making it have a huge gap to allow the chain to come off the inner ring. It was as if my seat tube is too vertical - if you see what I mean. It meant that I wanted to tip the mech anticlockwise to even out the 1mm gap all along the cage.

Then it hit me! The clamp needs to be rotated on the seat tube so the mech will move forward! Trouble is, the clamp wouldn't allow the mech to then get parallel again. So I took the clamp off and took a file to the slot to make it wider.

Now my front mech is parallel AND follows the curve of the big ring. All I need now is a road test and do some fast changes to the inner to test it out. I have experimented with the bike on the stand and observed the chain flying off and tried to work out what was wrong, but now I've "modified" the clamp I can't get the chain to come off at all!

I wonder if chains come off the inner because of the front mech's positioning fore and aft is the problem with many bikes? It always struck me that Dog Fangs are just a "sticking plaster" approach to get round a fundamental design problem.

Any comments?

[rjb]

You could also try a shim between the front mech and the mount below the attachment bolt which should bring the back of the cage closer to the rings. There's probably a whole list of other "bodges" available to try

[hubgearfreak]

It always struck me that Dog Fangs are just a "sticking plaster" approach to get round a fundamental design problem.

and bashing a roller chain from side to side isn't

[reohn2]

........... It always struck me that Dog Fangs are just a "sticking plaster" approach to get round a fundamental design problem.

Any comments?

No it isn't,its a chain catcher for when,for whatever reason,the chain overshoots on the down shift.
A Dog Fang will only stop the chain from going between the spider and BB cup by catching it.
To stop the chain coming off you need an N gear Jumpstop which has a plate to stop the chain overshooting:- http://www.gvtc.com/~ngear/whatis.html

[Ayesha]

"Dog fangs are fr**gin fantastic. Even when they are half of a Halfords Bike Hut replacement rear lamp bracket.

My problems were 1/ the inner ring is 22 tooth, and 2/ the down tube is elliptical.
I had to find something that had a 'neck' on it to get the business end over to the small ring.

BTW, when the chain goes over the small ring and lands on the BB, its called "shipping the chain".

[BigG]

No it isn't,its a chain catcher for when,for whatever reason,the chain overshoots on the down shift.
A Dog Fang will only stop the chain from going between the spider and BB cup by catching it.
To stop the chain coming off you need an N gear Jumpstop which has a plate to stop the chain overshooting:- http://www.gvtc.com/~ngear/whatis.html

I am a strong supporter of the Jumpstop and use one on all my three bikes. These have drops from middle to inner ring of 17 to 19 teeth. It completely avoids the probem of the chain derailling on the inside of the inner ring. Furthermore, it weighs next to nothing.

[reohn2]

BTW, when the chain goes over the small ring and lands on the BB, its called "shipping the chain".

Don't you mean unshipping?

I can see you're problem if the seat tube was eliptical but it appears to be round in the photos,I can't see why a D/Fang wouldn't work with you're S/tube.
The ex light fitting may work but due to its rounded end near to the granny ring it could under certain circumstances give the chain a lead in past it.
A DogFang is pointed for that reason so the fang bit will catch the chain as it unships/overshoots the inner ring.
BTW I use both D/F and Jumpstop on different bikes.

Edit:- Ah! I think I realise what you mean about the downtube,is it that the eliptical d/tube weld is stopping a D/Fang from being set low enough on the seat tube to be effective,ie;level with the granny ring's teeth? If so I cured it on a Cannondale frame(think really oversized tubes and smooth welding) by cutting some of the D/Fang band away allowing it to be set lower,that said it was a 26t granny.

[CREPELLO]

........... It always struck me that Dog Fangs are just a "sticking plaster" approach to get round a fundamental design problem.

Any comments?

No it isn't,its a chain catcher for when,for whatever reason,the chain overshoots on the down shift.
A Dog Fang will only stop the chain from going between the spider and BB cup by catching it.
To stop the chain coming off you need an N gear Jumpstop which has a plate to stop the chain overshooting:- http://www.gvtc.com/~ngear/whatis.html

Do you mean this device can't re-rail a chain back onto the small ring? Surely if it's set high enough, it'll keep the chain on most of the time.

I'd love to know what causes the chain to de-rail on some bikes but not others. For example, I've recently refitted a Deore chainset 107mm BB and front mech off an old Galaxy, which never had this problem. Fitted to the new frame (a 631 steel Galaxy look alike), the chain would consistently de-rail. I've made my own device to stop it happening.

[Ayesha]

BTW, when the chain goes over the small ring and lands on the BB, its called "shipping the chain".

Don't you mean unshipping?

I can see you're problem if the seat tube was eliptical but it appears to be round in the photos,I can't see why a D/Fang wouldn't work with you're S/tube.
The ex light fitting may work but due to its rounded end near to the granny ring it could under certain circumstances give the chain a lead in past it.
A DogFang is pointed for that reason so the fang bit will catch the chain as it unships/overshoots the inner ring.
BTW I use both D/F and Jumpstop on different bikes.

Edit:- Ah! I think I realise what you mean about the downtube,is it that the eliptical d/tube weld is stopping a D/Fang from being set low enough on the seat tube to be effective,ie;level with the granny ring's teeth? If so I cured it on a Cannondale frame(think really oversized tubes and smooth welding) by cutting some of the D/Fang band away allowing it to be set lower,that said it was a 26t granny.

You could be right. Its a while since I last used that term.

It took me about half an hour to get the rounded end of the lamp bracket into a position that stopped the chain 'unshipping' past the small ring. Then I moved it around to get a measure of the positional tolerance. It ended up to be quite a wide tolerance. The 'D-fang' device is above the ring's teeth and there is not enough room for the chain to get between the ring and the D-fang device.
It works impeccably.
I started experiencing 'unshipping' when the difference in ring teeth got to fourteen. A std 52/39 doesn't have a problem. A 32/22 pairing didn't present a problem. Then I changed to a 36/22 and the chain went straight over onto the BB shell. Remember the front mech is spring loaded these days rather than friction lever operated where one could move it fast or slow. A modern front mech 'Throws' the chain onto the smaller ring !

BTW, I don't like the term 'Granny ring'. My 36/22 double is an 'Alpine' set, specifically for climbing >25% gradients. Audax organisers are a sadistic bunch and they deliberately pick out the steep climbs

As the late & great Sheldon Brown stated, "if the gearing is low enough, climbing a hill will feel the same as the flat, but you will be riding VERY slowly".

[Mick F]

Planning a short ride this morning to test out my gear changes. Trouble is, the weather is unpredictable with heavy downpours interspersed with bright cloudless skies!

Dog Fangs or Jump Stops are a great idea, but I still maintain that they shouldn't be needed, however I do concede that some bikes suffer (from unshipping) more than others. Mine was intermittent and I could never diagnose why, and I'd have preferred it to have been a solid problem instead.

Maybe I've sorted it, maybe I've not.
If I have, I'll put it back the way it was and take photographs of "before" and "after".
If not, I'll be scratching my head again!

[groberts]

I am sure that getting the front mech right is an essential pre-requisite to ensure the chain is not dropped on changing down to the Granny. However, I am equally sure that the Granny Ring is intrinsically prone to this problem, probably through the extreme difference with the middle chainring, thus 'amplifying' the movement of the chain downwards. I had this problem on my Audax (52-42-32) chainring which has been completely solved by fitting a Jump Stop n-Stop - from that moment I have had no more problems. viewtopic.php?f=5&t=50126. It's a cheap, easy to fit item, that will ensure this problem never happens and / or at least acts as a failsafe just in case it might, even if your front mech is perfectly fitted; sods' law it always comes off when you least expect it and / or at a dangerous moment. Better safe than sorry.

I happen to think the Jump Stop looks better than the Dog Fang but I'm sure that they both work equally well. As a matter of safety I believe all bikes with a tripple chainring should be sold with either as a standard fitting and was pleased to find that my daughter's new Specialized Dolce Sport, bought a couple of months ago, was fitted with a Dog Fang, most are not.

[CJ]

Dog Fangs or Jump Stops are a great idea, but I still maintain that they shouldn't be needed, however I do concede that some bikes suffer (from unshipping) more than others. Mine was intermittent and I could never diagnose why, and I'd have preferred it to have been a solid problem instead.

It generally is intermittent. It happens especially with triples because the outer cage of the front mech, that does the middle to inner shift, is necessarily too far away from the teeth it's trying to derail the chain off, to make a clean job of it. That's the reason Frank Berto and I used to advocate half-step triples, because then the middle ring is near enough for a clean shift. But then Shimano did stuff to the shapes of triple mechs to improve shifting with bigger outer-middle differences that made smaller differences unworkable. And besides: who needs half-steps with 10 sprockets where there used to be 5!?

But I digress. Even half steps benefit from a Jump-Stop, due to the sheer distance the chain has to fall from, say, a 42T middle to a 24T inner. Having said that, my old Mercian that has those rings doesn't seem to need one, but another bike I set up like that (with TA rather than Ofmega chainset) does. And it's not just a triple thing. A double can do it too, just sometimes. It's enough of a risk that racers in the TdF often have a Jump-Stop or similar fitted to their bikes. Perhaps that makes it respectable enough for Mick to fit one too?

On some triples, where the middle ring is too much smaller than the outer and has teeth that really hang onto the chain, the mech has to overshift in order to get the chain off it, so the chain then falls to the left of the inner ring, but spacing the inner further to the left then makes a gap between middle and inner ito which the chain will instead sometimes jam. The reason this behaviour is so erratic and can even happen sometimes on a double, is that the chain spends a bit of time in mid-air, falling from one set of teeth onto another and guided not very closely by a cage that's some distance away from the teeth it's got to fall onto. During this flight, it's liable to shake and wiggle sideways. All it takes is the wrong kind of tarmac (with bumps in) passing under the rear wheel whilst you're shifting, for a chain-wave to travel forwards and through the mech so that those falling more or less unguided links miss the waiting teeth. A Jump-Stop is the obvious way to engineer this situation properly. It's a wonder that all front mechs don't have one already attached. Maybe one day they will.

P.S. You may have noticed that I never call the inner ring a "granny", that being an ageist and sexist term that roadies use to denigrate sensible gears, and I know an actual granny who told me she does not like to shift into it for that very reason!

[reohn2]

Dog Fangs or Jump Stops are a great idea, but I still maintain that they shouldn't be needed, however I do concede that some bikes suffer (from unshipping) more than others. Mine was intermittent and I could never diagnose why, and I'd have preferred it to have been a solid problem instead.

Theres a lot of things in life that shouldn't be needed but if you don't want spoiled paintwork and that frustrating problem of getting the chain from between c/set and BBcup,on a wet afternoon etc then fit a either a Jumpstop or Dogfang,its like insurance,its therre but you hope you never need it.

Maybe I've sorted it, maybe I've not.

See above

If I have, I'll put it back the way it was and take photographs of "before" and "after".
If not, I'll be scratching my head again!

See above

[[XAP]Bob]

P.S. You may have noticed that I never call the inner ring a "granny", that being an ageist and sexist term that roadies use to denigrate sensible gears, and I know an actual granny who told me she does not like to shift into it for that very reason!

I've always thought "not many teeth" rather than "gear that granny needs"
Surely granny could simply adjust her whole gear range down ??

[Ayesha]

CJ wrote,

P.S. You may have noticed that I never call the inner ring a "granny", that being an ageist and sexist term that roadies use to denigrate sensible gears, and I know an actual granny who told me she does not like to shift into it for that very reason!


As previously mentioned, they are called "ALPINE" chainsets where the rings are deliberately small to achieve sub 1:1 ratios ( less than 27" gears ).