Front derailleur position query

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Thornyone
Posts: 346
Joined: 7 Dec 2017, 11:15am

Front derailleur position query

Postby Thornyone » 17 May 2019, 4:54pm

I’m confused. I have done many thousands of miles without any problems relating to front shifting. I have just replaced the middle and the 44T outer chainrings of my 9 speed triple crankset and it has just struck me that there is a pretty big gap (maybe 8 mm) when the outer plate of the derailleur is positioned directly above the outer chainring. It has only just struck me that this is so, but since the old chainring was also 44T, it must have been the same before.
All the online advice I can see seems to suggest that this value shouldn’t exceed 2mm. But it must always have well exceeded this amount on my bike because the derailleur cannot be re-positioned lower due to the presence of a bottle cage boss in the crucial place, just where the clamp of the derailleur should ideally sit. So how critical is this derailleur to chainring gap?

Dave

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Cugel
Posts: 1709
Joined: 13 Nov 2017, 11:14am

Re: Front derailleur position query

Postby Cugel » 17 May 2019, 5:22pm

Thornyone wrote:I’m confused. I have done many thousands of miles without any problems relating to front shifting. I have just replaced the middle and the 44T outer chainrings of my 9 speed triple crankset and it has just struck me that there is a pretty big gap (maybe 8 mm) when the outer plate of the derailleur is positioned directly above the outer chainring. It has only just struck me that this is so, but since the old chainring was also 44T, it must have been the same before.
All the online advice I can see seems to suggest that this value shouldn’t exceed 2mm. But it must always have well exceeded this amount on my bike because the derailleur cannot be re-positioned lower due to the presence of a bottle cage boss in the crucial place, just where the clamp of the derailleur should ideally sit. So how critical is this derailleur to chainring gap?

Dave


It depends on your chainline. If you have short chainstays, for example, being in big ring and the smallest couple of cogs makes the chain leave the chainring at a more acute angle as it goes to the cog. If there's a big gap between the front derailleur cage and the chainring, the chain can easily escape over the top of the chainring on to your pedal when you hoist it up whilst in one of the teeny cogs at the back.

If you have the front derailleur well-set in terms of it's throw, you can minimise overshoots of the chain through your too-wide cage-to-chainring gap. On the other hand, it may become more difficult for the chainring to pick up the chain when the chain is on one of the larger cogs (to the left of the cassette) as the derailleur cage doesn't quite push the chain over enough towards the big ring.

One way and another, you can manage this situation by confining your small to big chainring changes to when the chain is fairly straight - i.e. somewhere in the middle section of the cassette. If you have long chainstays, this too will keep the chain a bit straighter, chainring-to-cog.

Cugel

Thornyone
Posts: 346
Joined: 7 Dec 2017, 11:15am

Re: Front derailleur position query

Postby Thornyone » 17 May 2019, 5:51pm

Thanks for your reply, Cugel. I certainly tend to do most of my middle to large and middle to small chainring shifts when somewhere around the 3rd to 4th and 5th to 6th rear cogs respectively (counting from smallest up). I almost never use the smallest cog anyway. Mostly I seem to use the same two cogs with either the middle or large chainring for most of my riding in this area, and when I need the “granny ring” I anticipate the change rather than shifting under load. So maybe my riding style has coincidentally just happened to fit the requirements of this less-than-ideal front derailleur position?

Dave

Brucey
Posts: 33546
Joined: 4 Jan 2012, 6:25pm

Re: Front derailleur position query

Postby Brucey » 17 May 2019, 6:28pm

with a triple chainset the critical clearance is usually the clearance between the middle ring and the inner cage plate.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

Thornyone
Posts: 346
Joined: 7 Dec 2017, 11:15am

Re: Front derailleur position query

Postby Thornyone » 17 May 2019, 7:53pm

Brucey wrote:with a triple chainset the critical clearance is usually the clearance between the middle ring and the inner cage plate.



Thanks,
Dave

gxaustin
Posts: 449
Joined: 23 Sep 2015, 12:07pm

Re: Front derailleur position query

Postby gxaustin » 17 May 2019, 8:46pm

I have a similar problem - my front mech would strike the chain stay, if I could get it lower. There's a gap of about 5mm. I'm careful to set up the limit screws carefully but after about 9,000 miles I've never dropped the chain outwards.
You could maybe get your mech a bit lower, if you are concerned. You'd have to buy a braze on mech and the type of band on adaptor which allows you to adjust the height of the mech by means of a slot.

Thornyone
Posts: 346
Joined: 7 Dec 2017, 11:15am

Re: Front derailleur position query

Postby Thornyone » 18 May 2019, 1:49pm

gxaustin wrote:I have a similar problem - my front mech would strike the chain stay, if I could get it lower. There's a gap of about 5mm. I'm careful to set up the limit screws carefully but after about 9,000 miles I've never dropped the chain outwards.
You could maybe get your mech a bit lower, if you are concerned. You'd have to buy a braze on mech and the type of band on adaptor which allows you to adjust the height of the mech by means of a slot.

Thanks. This sounds interesting but I think I’ll just stop worrying and stick with my current setup because thinking about it rationally I have always used 44/34/22 chainwheels on this bike and have not had one instance of the chain missing a chainwheel on changing, and that is in 40,000 miles.
It was when I was fitting new middle and outer rings yesterday that I was suddenly struck by how big the gap was compared to that on my old (which is the “official” 2mm).

Dave