]The cranks and bottom bracket form the heart of the drivetrain of a bicycle, they connect the downward force exerted by the rider's legs into the spinning drivetrain which propels the machine and rider forward in unison. Thus, it is important that the bearings the cranks spin on spin smoothly and hold the cranks securely, and that the cranks run straight and true.
Bicycle cranks have had a slow but persistent evolution, from when cottered cranks were the norm to square taper cranks to the current press in bearing bottom bracket cranks popular today. The Spécialités T.A. Pro Vis cranks pictured above that were fitted to our tandem are a classic component. They use a tiny 50.4mm chainring bolt spacing which allows an enormous choice of chainring sizes and setups. They were the crankset to have on a touring bike in the old days because of this versatility.
gave it all a clean and lubrication. What I noticed was that the spindle on the stoker's cranks had some pitting on the bearing surface. Over time this would get progressively worse. I pulled off the crank and pulled out the spindle, my suspicions were confirmed, what originally started as a small pit turned into a large gash...
Since these spindles aren't manufactured anymore the only option was to find it used or NOS. I managed to find a replacement spindle NOS on eBay which was lucky. The taper on these spindles are Spécialités TA specific, you can technically fit them on JIS spindles but they will require trial and error to get them to achieve correct chainline. The spindles themselves are beautifully made, they are hollow to reduce weight and have very hard and very smooth bearing surfaces. I replaced both BB spindles as the captain's spindle had a pit in it as well. They take two different lengths, the captain is the T.A. 344 spindle meant for doubles and the rear is a T.A. 374 which is for a triple setup. Both these spindles are the old asymmetric style. I also fitted new Grade 10 balls as well while I was at it, these are a higher spec ball built to a tighter tolerance than usual.
Part of the reason I was servicing them was to fit a smaller inner ring (or granny ring) for climbing, the existing chainrings were 53/42/34t which whilst evenly spaced didn't have a low enough gear for steeper climbs more than about 6-8%. We went for the lowest possible inner ring at 26t, I thought 26t was perhaps a little too low, however after some decent hilly rides we've found it's perfect. In fact you could say it is a Game changer. I'm normally quite used to having a 34t on a solo bike being sufficient for almost anything, however on the tandem our average power to weight isn't as good which is why the 26t inner ring was the right choice.
These cranks (and most cranks with the 50.4mm bolt circle diameter) require removal to swap chainrings. The way they are assembled into a triple setup is somewhat unique these days, the crank is directly bolted to the outer chainring, the inner and middle are then bolted onto this outer chainring via another set of bolts and spacers. However the major flaw of the design is that these bolts are all quite undersized compared to modern cranksets being about 5mm in diameter. Whilst reassembling the cranks I managed to over torque and shear off the head of the bolts which connect the outer chainring to the other chainrings. A new set of bolts costs £35 and there is no way to substitute anything else...
In the end I decided to go the whole hog and bought a new set of these bolts as well as three sets of the crankset to chainring bolts for good measure. There cannot be a worse thing than to be stuck in the middle of nowhere, unable to pedal because of some obscure bolt failure. It's likely the one I sheared off was original so would be pushing 40 years old about now.
The above four photos show how the inner chainrings are attached to the outer chainring using spacers. It's a somewhat unique arrangement as almost all modern cranks employ what is called a Spider type of design where the crank arms branch outwards to connect directly to all the chainrings.
It's a shame that the old bolts broke as they appeared to be the originals with a T.A stamping on the bolts heads. Once the drive crankset was done I moved onto replacing the bolts on the timing cranksets. This proved somewhat difficult as the backside of the flange features a recess which the bolts fit into, they are difficult to reach with a socket and as you could guess round out very easily. This meant I had to drill off some of the heads to be able to remove the bolts. I replaced them with new ones but put a second washer underneath to allow more of a purchase on the head next time they needed to be removed.
Since we have fitted the new spindles and bearings the drivetrain is noticeably smoother and quieter, I could actually feel the unevenness of the spindle whilst pedalling before. The 26t chainring has proven to be extremely useful, allowing us to climb up to 12% gradients on the tandem which would not have been possible before.