IanS

would like to replace the 170 mm rear crankset on my Cannondale 2007 road tandem with 175 mm cranks. I assumed that I would be able to swap the front LHS 175 mm crank with the rear LHS 170 mm crank, buy a standard RHS 175 mm crank to replace the 170 mm RHS rear crank and put the RHS rear crank on the front RHS. So I would end up with 170s on the front and 175s on the rear.

The spec for this tandem says the BB (I assume both front andrear are the same) is a TruVatiV ISIS GigaPipe SL which I had assumed used a similar setup to Shimano Octalink. However I believe that this is not the case and the axel is part of the RH rear crank and the same istrue for the LH front crank. In which case, I won't be able to do what I had hoped to do as described above.

Could anyone please confirm twhether the axel is indeed integral to the RH rear and LH front crank arms and if so, suggestions on what I could do?

Also, and more importantly, there doesn't look like there is a great deal of clearance between the LH rear crank arm and the rear stays and was wondering whether anyone had successfully fitted 175 cranks of some description to the rear of a J/L 2007 c'dale tandem or could tell me whether there is enough clearance. The rear stay is not straight.

If I can put 175 mm cranks on the rear, is it possible to remove the axel from the LH front (if is it part of the crank) so that I could fit it on the rear and what RH arms are compatible with this BB?

Apologies if any of the above are obvious but I'm blind and so can't see pictures online which would probably answer most of the above.

Thanks in advance.

PMK

In regards to ISIS, that design is similar to Octalink (all versions), however ISIS and Octalink will only work as a system, they will not mix. ISIS is a pair of individual crank arms and modular BB that contains the BB spindle.

As for dimensions and clearance, not certain if it will clear. However, if you are staying with same brand items and BB lengths, there is a good chance it will be fine. Not uncommon for the crank arms to be one size for all casting or forgings, then have the length set during machining.

PK

flyag1

I just picked up a new set of FSA SL-K Ttandem Carbon Cranks with BB on ebay for $120 shipped. Comes complete with everything you will need. look up seller bikewagon and make offer. asking price was 124.93 plus shipping... i offered 120 shipped and have the set installed on my new 2010 Cannondale

IanS

I took a closer look at the rear cranks the other day and as suggested above, it looks like the cranks are drilled for 170 mm but this leaves about 12 mm of crank beyond the pedal hole. The entire crank length (centre to end of crank) is about 193 mm. Comparing with the front 175s, these are about 190 mm (centre to end of crnk) and there's about 7 mm of crank beyond the pedal hole. So the rear could even be used for a 177.5? This is extremely frustrating as I have a J/L frame and so you might assume that the person on the back needs longer cranks. I'd of been happy if they hadn't been drilled at all!

fortunately, my father in lore is a tool engineer so we are looking into plugging the existing hole and re-drilling, although this does have it's own set of issues. If anyone has done this sort of thing or has any suggestions on how to approach this I'd be interested to hear them.

The FSA SL-K option would seem the most sensible though given the price quoted above but have read bad things about the MegaExo BB. It is unclear to me whether this is a fundamental design fault or whether they are just not easy to install correctly, which leads to issues and failure. but given that i am based in the UK and these parts are virtually impossible to find over here, it is a concern.

PMK

Don't plug the crank nor weld it. While your wifes dad may pull it off, it's not worth the effort and if it doesn't work, the failure moment may not be pretty.

In regards to the FSA SLk cranks, I have not seen the stoker set in 175mm, but maybe Bikewagon has them. Captain 175's yes.

If you wish to do this with off the shelf parts, Sugino offers individual crank arms for both front and rear, properly threaded for the pedals, in 175mm. I assume you currently have Truvatives, so if you go with Sugino, the chainrings may not swap over. The Sugino chainring pattern is 74 /110mm.

What would be more easily gotten in the UK, but a bit more money is a set of Middleburn Tandem MTB cranks. These are light and stylish.

PK

flyag1

Yes, if anyone is interested the SLK stoker cranks are 175 mm... complete with all rings and (steel) BB. Brand new in OEM box.
If not for trying to put together a budget build, I would have liked your Middleburn suggestion... but heck I couldn't pass on the price of the FSA.

PMK

Yes they do show them on the Bikewagon website. Oddly, no catalogs, nor FSA's own website show 175mm stoker cranks. At first I thought maybe they had an excess of captain lefts and single bike 175's, but being a triple with a big ring of 54 doesn't quite seem like a mix -n-match.

Part in hand, new in box does not lie.

Very good deal on them.

PK

IanS

OK, the FSA SL-K cranks mentioned above were just too good an offer to refuse given my need to go 175, even including duty and shipping to the UK. Hopefully receive them soon.

flyag1 - Did you have any issues installing them on the c'dale? How was the chain line assuming you left the front Truvativ Elita tandem chainset as is? Did you have to make any changes to either BB?

Cheers?

flyag1

Well I actually started with just a frame, so I had to setup both front and rear. I'm running a 9 speed Mtn Cassette which has a 52.5 mm offset, the front crank-set is the FSA Gossamer Mega Exo, and the rear is the SL-K Carbon triple.

Stoker crank-set is as follows: I have 1 shim between the BB cup and the frame on the drive side and have zero on the timing chain side. This put my middle chainring @ 50 mm offset.

So I have 2.5 mm difference in the center of the cassette and middle ring on the stoker crank-set. (IMO: 2.5 mm is good enough)

I then moved to the front and installed 2 spacers under the BB cup on the timing chain side of the BB which put me within 1 mm of having both of the 42 tooth drive chainrings on center. There are zero shims on the drive side of the captions crank. I could have added a third spacer under the front BB timing side but decided 1 mm is close enough.

I'm running X9 shifters, (Flat Bar) XT front derailleur and X9 long cage rear derailleur. I installed a new duraace 10 speed chain and XT 9 speed 12-34 cassette. Shifting is fast and crisp... no chain drops or suck...

My wife and I don't have to work so hard to climb these Mtns roads with the 34 tooth cassette and the brakes (BB7s with 203 rotors) work!