Tandem Cycling - Derailleur choice problems - High Capacity?

I'm currently renovating a 1970's French Tandem (See http://gitaingaves.blogspot.co.uk/ and THIS (http://www.bikeforums.net/showthread.php/842637-1970-s-French-Tandem-Restoration-Project-Advice-!) forum thread, if you're interested), and am having a little bit of trouble picking derailleurs...

The proposed setup for the gearing is:
Front: 52-42-28 (Existing cranks)
Rear: 11-13-15-17-20-23-26-34 (This cassette (http://www.parkersofbolton.co.uk/p-3500-shimano-11-34-hg30-8-speed-cassette.aspx))

This presents an overall (rear DR) capacity of ((52-28)+(34-11))=47 As far as I'm aware, the highest capacity any rear can officially handle is 45? We are very unlikely to every use the small/small combo, so a bit of slack shouldn't matter too much, my really question is, due to budgetary constraints, do you think we can get away with a 43 capacity (excluding the smallest 2 at the rear, whilst on the smallest at the front) without any problem? This (http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=251143618630&ssPageName=ADME:X:AAQ:GB:1123#ht_874wt_1165) one seems very reasonably priced...
For the record, I will need a claw adapter (included in the linked one, but around £5 separately).

At the front, It's a similar situation; (52-28)=24 is a larger capacity than most are rated for, and 52 is quite a large outer chainring! Does anybody know of a system that would work? The Seat-tube diameter (for clamp_ is 28.6mm

Thanks in advance for your help, which will doubtless be fantastic : D

Will

Data point: We're running 9sp 52x39x26 with 11-34 in the rear. RD is a Shimano LX M581 rated at 45T total capacity. FD is a Microshift 9sp shimano compatible triple. So I'm out of spec on the front and rear but it works fine. Big/big and small/small function (but of course you never ride in those). Front shifts out of granny can take a little patience, but it works AOK.

Data point: ...... it works AOK.
Brilliant!
By my maths, you're running a 45T Derailleur with an actual capacity of 49T, meaning we should be able to get away with the 43T Tourney (linked originally), on our actual 47? You say that the small/small does function, I would expect it to function, just with some slack in the chain? I'm not intending to use it, but is this the case? - Just wondering how honest Shimano are with their marketing!

The difference between your granny is 13, whereas we have 14, so one extra... How close is the clearance for the chain when on the granny? The thing that worries me, is that the chain might rub on the bottom of the cage on the granny if I had a 'double' DR, or might not clear the middle chainring (when chain is on the top) with a 'triple cage'... Obviously your Top and Triple are further apart, so there may be more space. How much clearance is there between the bottom of the cage and the Middle chainring when in 'top gear'?

Thanks,
Will

When on small/small there is about 5mm of clearance between the bottom of the chain and bottom of the FD (a triple cage) -- no problems there. But the RD is fully wound up on its stops and there is a bit of slack in the chain and things are very noisy, so obviously I'm exceeding the RD capacity. I'd say the capacity numbers for my RD are pretty accurate. In practice I don't leave the four big cogs when in the granny.

As to FD adjustment, when in the middle chainring my outer plate just clears the big chainring -- so the FD is about as low as it can go. Other chainring/FD plate clearances don't seem to be a problem.

.....Other chainring/FD plate clearances don't seem to be a problem.
Lovely,
Thanks very much for your help!

Possibly worth mentioning, just in case you're interested, is that There is a third (proper) (and probably final, at least until Christmas) post on the *NEW LOOK BLOG* (http://gitaingaves.blogspot.co.uk/)
Check out that background...

Thank you for supporting me thus far,


Will

We run 52-39-26 in front and 12-34 in back. Our XTR RD has been trouble-free. You never run small-small so don't worry about it. We never run below the 5th cog in back when in the granny ring. FD probably won't permit that anyway, and why would you? Set the chain for big-big.

We run 52-39-26 in front and 12-34 in back. Our XTR RD has been trouble-free. You never run small-small so don't worry about it. We never run below the 5th cog in back when in the granny ring. FD probably won't permit that anyway, and why would you? Set the chain for big-big.
Thanks for this, there's nothing as good as some real-world feedback to lift your confidence just before you fork out a whole load of money on a project! It's really appreciated. : D

Set the chain for big-big.

+1.

We use the lower 6 cogs on a 9 speed cassette when in the granny ring. Beyond that, shift to the middle ring on the front.

On the derailer choice, most shimano derailers (with a few specific exceptions) have the same geometry, so you don't have to look for a "7-speed" derailer.

On the derailer choice, most shimano derailers (with a few specific exceptions) have the same geometry, so you don't have to look for a "7-speed" derailer.
I should hope I wouldn't have to be looking for a "7-speed", since we have 8 on the cassette! From what I can tell, the actuation ratio (cable pull:lateral movement) is what matters, and, as you say, is constant. Derailleurs marketed as '9-speed' seem to have larger capacities, due to the likelihood of having a wide gear range being higher with 9 sprockets, but, I'm sure it's not a hard and fast rule. We'll probably be getting the 43T tourney.