Tandem triple front mech issues

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Dynamite_funk
Posts: 520
Joined: 2 Nov 2011, 9:10am

Tandem triple front mech issues

Postby Dynamite_funk » 9 Sep 2019, 7:24pm

Hi everyone. Haven't posted for sometime but I came to you with a problem I am hoping you may be able to help with things.

We have just come back from a 16 day tandem tour of Europe. Had a fantastic time but decided we could do with slightly lower gearing. I therefore ordered a Spa cycles XD2 triple with 46/34/24 to match up to our 9 speed 12-34 cassette.

Due to the frame (a Dawes galaxy twin I believe) having cable guide on the chain stay designed for a larger chain set for I am struggling to get the derailleur to match up as it hits the cable guide if I try to get the FD plate to be in the right position to shift the chain correctly. I have fiddled and messed around for some time bending another derailleur's plate, changing the angle and other tricks and I simply cannot get this set up to work. I can get either the middle-outer or middle-inner to work but not all three.

The current set up is...

Spa cycle XD2 triple 46/34/34
Shimano Deore XT flat bar 3x9 shifters
Ultegra triple FD (I realise this potentially isn't ideal)

My thoughts are I can use an eccentric shim to shift the derailleur forward so it doesnt foul the cable guide on the chainstay and also then either move it slightly closer or further out from the seat tube. But is the FD suitable? I have tried a mountain bike type I had but this also fouls on a cable guide which is on the downtube. An XT FD M781 looks like it may work but is 10, would this be adding more trouble to the situation!

All advice and guidance gladly received!!! :mrgreen:
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FD1.JPG

pwu
Posts: 20
Joined: 8 Jul 2019, 2:48pm

Re: Tandem triple front mech issues

Postby pwu » 9 Sep 2019, 9:46pm

Unless the front mech is flat bar specific its the wrong pull ratio, its also too long.

zenitb
Posts: 274
Joined: 7 Aug 2018, 9:59pm
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Re: Tandem triple front mech issues

Postby zenitb » 9 Sep 2019, 10:00pm

Why not by-pass the old end stop. You would need a small screw tap in the bb...and to check this configuration works with the other cable stops... see pic..
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Brucey
Posts: 35175
Joined: 4 Jan 2012, 6:25pm

Re: Tandem triple front mech issues

Postby Brucey » 9 Sep 2019, 10:04pm

fundamentally you have a problem in that you are trying to use a MTB shifter with a road chainline, suboptimal chainring sizes and a road derailleur. Won't work, for several reasons.

You will be able to find a shallower 'road' FD that won't foul on the chainstay braze-on BUT it

a) still won't have the right shift ratio for your shifter and
b) it will most likely not have the correct depth inner plate to provide good shifting from inner to middle chainring.

So it is worth finding a ( probably older) different FD. I'd suggest a MTB FD of the bottom pull, bottom swing type, meant for MTB chainsets as found in the early 1990s (which is basically what you are trying to use). These mechs will still work Ok even at a road chainline. However these mechs are often made for a 10T middle to outer interval. You may find that

- you need to increase the size of the big ring to get it to clear the chainstay/BO and
- you need to revise the size of the middle chainring to make the middle-outer interval more compatible with the FD.


IME if you have a random FD (with a shallow inner cage plate) it will only make the shift from inner to middle cleanly if that interval is at least as big as the middle to outer interval. So 24-36-46 is quite easy to make shift OK but 24-34-46 is more likely to benefit from the exact correct FD.

If you can't find a suitable FD with a MTB shift ratio, I'd suggest using a friction shifter with the FD instead of the indexed one.

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

Dynamite_funk
Posts: 520
Joined: 2 Nov 2011, 9:10am

Re: Tandem triple front mech issues

Postby Dynamite_funk » 9 Sep 2019, 10:34pm

As ever Brucey thank you for your help with this. Would a Shimano Deore XT FD-M751 for the bill? May end up going with thumbies if this doesn’t work, I don’t want to end up with an unhappy stoker!

NickJP
Posts: 113
Joined: 24 Sep 2018, 7:11pm

Re: Tandem triple front mech issues

Postby NickJP » 9 Sep 2019, 11:07pm

If you changed the 46t for a 48t chainring, would that allow the FD to shift OK with all three chainrings? Here's one of our tandems with 48/36/24 chainrings and an Ultegra triple FD. The FD doesn't clear the chainstay by much, but it's an aluminium frame with very deep chainstays.

Image

Brucey
Posts: 35175
Joined: 4 Jan 2012, 6:25pm

Re: Tandem triple front mech issues

Postby Brucey » 9 Sep 2019, 11:17pm

definitely worth a go. However the chainstay angle is probably at or below the recommended minimum, which eats into the chainstay clearance which is already liable to be marginal because of the RD cable run. IIRC the middle to big ring interval is 12T which is OK and the 46T ring is OK too.

Image

suck it and see?

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

Manc33
Posts: 1384
Joined: 25 Apr 2015, 9:37pm

Re: Tandem triple front mech issues

Postby Manc33 » 10 Sep 2019, 1:23am

The only way I can see that being fixed is to put a bigger outer chainring on, so the front mech can be put higher. I've run a 52-38-24t with no shifting problems (also from Spa), on a FD-R773 or FD-R443 front mech and either the R443 or R773 front shifter.
Only weird bikes are interesting anymore.

brynpoeth
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Joined: 30 Nov 2013, 11:26am

Re: Tandem triple front mech issues

Postby brynpoeth » 10 Sep 2019, 5:55am

I saw a bike, not a tandem, with four chainrings
Is that a thing?
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Dynamite_funk
Posts: 520
Joined: 2 Nov 2011, 9:10am

Re: Tandem triple front mech issues

Postby Dynamite_funk » 10 Sep 2019, 8:19am

Have ordered said derailleur and will give it a go. I really do not want to go any higher on the outside chainring. Tandems require both riders to be happy with the set up, we had a 48 on there and we hardly used it so I think a 46 would be much better.

I may try an eccentric shim as well to shift the derailleur forward a fraction to help it avoid the chainstay cable guide. Will report back!

NetworkMan
Posts: 635
Joined: 25 Aug 2014, 11:13am
Location: South Devon

Re: Tandem triple front mech issues

Postby NetworkMan » 10 Sep 2019, 10:08am

Brucey wrote:definitely worth a go. However the chainstay angle is probably at or below the recommended minimum, which eats into the chainstay clearance which is already liable to be marginal because of the RD cable run. IIRC the middle to big ring interval is 12T which is OK and the 46T ring is OK too.

Image

suck it and see?

cheers

I have a 1993 Shimano catalogue. The nearest FD listed is XT M735 bottom route intended for 48-36-26 or 46-36-26
That one must be a bit newer but hopefully things will not gave changed too much in the interim.
Historical note: If you had Biopace rings things would be a bit different!

NickJP
Posts: 113
Joined: 24 Sep 2018, 7:11pm

Re: Tandem triple front mech issues

Postby NickJP » 11 Sep 2019, 1:39pm

brynpoeth wrote:I saw a bike, not a tandem, with four chainrings. Is that a thing?

I setup one of our tandems with same side drive using four chainrings on the stoker's crank. I had to use a set of triple chainring bolts plus additional spacers for the timing chainring to fit the three 110BCD chainrings, and the bolts off another triple set plus more spacers to fit the 74BCD granny ring.

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Image

But the derailleur on our setup still only has to change across the three final drive chainrings. Don't know where you'd find an FD that had enough throw to cope with changing across four chainrings if you were trying to have four on a single bike. If I wanted a ridiculously wide gear range on a bike, rather than additional chainrings I'd use a Rohloff hub with the Rohloff chain tensioner and fit two front chainrings. Running 48/38 rings with a 16t cog on the Rohloff would give gears from 120" down to 18".

hamster
Posts: 3154
Joined: 2 Feb 2007, 12:42pm

Re: Tandem triple front mech issues

Postby hamster » 11 Sep 2019, 6:17pm

brynpoeth wrote:I saw a bike, not a tandem, with four chainrings
Is that a thing?


Dawes had some ghastly cheapo thing in the early 90s with 4 chainrings. It was a gimmick.

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Tigerbiten
Posts: 1876
Joined: 29 Jun 2009, 6:49am

Re: Tandem triple front mech issues

Postby Tigerbiten » 11 Sep 2019, 6:33pm

brynpoeth wrote:I saw a bike, not a tandem, with four chainrings
Is that a thing?

It was doable in the past.
You needed an adapter which could take a cassette sprocket as the fourth chainring.
And then modify the front derailleur so there was more depth to the cage to cope with the greater range.
You could more easily do it today if you modded the front cage due to the wider range of chainrings.
But there's less call for it due to a bigger range of cassette sprockets.

I've in effect got four chainrings, twin chainrings on a Schlumpf HSD.
My chainring range is 38/55/95/137.5.

Five effective chainrings is also fairly easy, a 30/40/53 triple and a SA CS-RF3 3 speed hub.
With the underdrive/overdrive from the hub you end up with 22.5/30/40/53/71 chainrings.

I don't think there's an easy way of doing 6 ....... :D

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Tigerbiten
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Re: Tandem triple front mech issues

Postby Tigerbiten » 11 Sep 2019, 6:54pm

NickJP wrote:If I wanted a ridiculously wide gear range on a bike, rather than additional chainrings I'd use a Rohloff hub with the Rohloff chain tensioner and fit two front chainrings. Running 48/38 rings with a 16t cog on the Rohloff would give gears from 120" down to 18".

A 56/38 double would give you another gear up and the Rohloff chain tensioner will still cope with that range.
I only run a 55/38 double on mine because the 56 chainring interferes with the chain guard on my recumbent trike ............ :(