52telecaster

I have two of these cranks with 52-42 rings and have repurposed 1 for use with a 3 speed using the single 42 up front. However I would like to try to use the second as a double and of course the 36 tooth rings don't grow on trees. So I am going to attempt to make a 36 tooth ring out of a t.a. inner/middle 36 tooth cyclotouriste ring. Any advice is welcome. I just ordered the 36 from Peter white. We'll see.

Original crank

40 tooth inner/middle that would need 116 mm bolt circle added to three of the arms.

steelbikeguy

so you'd have to drill and tap holes in the crank arms?
I seem to recall people discussing similar projects, adding a granny ring to a Campy Record crank. The best results were obtained with proper machining equipment, in order to really get the holes in precisely the correct locations. I'm not qualified to comment on whatever method you are thinking of, but hopefully someone who is qualified will comment.
edit: or are you cutting the arms short on the chainring and drilling and countersinking holes on it? This seems a bit demanding too.

On a bit of a tangent... is that a Gran Sport that I see leaning against the wall over to the right??

Steve in Peoria

bark_eater 

It looks like your going to have a little less metal left than a stock TA 116 ring, so I would try and get a very close fit where possible.

Steel cottered crank 116 chain rings use a smaller chain ring bolt than an aluminum ring, so you could use one of those as a drill guide to get the holes in the right spot, and then enlarge them with a drill press or a tapered reamer from both sides. That way you can sneek up on the proper diameter, and have a chance of correcting any variance.

Once you get the holes right, you can bolt the new ring to the old and scribe the curve where the the chain ring arm buts up against the crank spider. Hack saw and a chainsaw file should do it. You might want to add a washer on the the inner small chainring face to stiffen up the new ring.

I would also sand and polish every cut you make, particularly where you cut the extra 3 arms off. Having the ring "tear" in half while your out of the saddle could be not so good.

Velo Mule

That should work. Good thinking! Now it comes down to execution. I don't think you have a milling machine with a rotary table so getting the position of those three holes will be critical. I would spot them with the holes on the crank. You could make your own transfer punches out of a piece of rod that is the same diameter as the chainring bolt hole.

One trick that I have used is a CAD program like Solidworks, AutoCAM or SketchUP to draw out the bolt holes in the array and then print it 1:1 on the printer. Cut out the print as needed to see your details spray some adhesive spray on the paper and stick it on your chainring. It may help to include some details to line up to like the ring circle. This will ensure that your pattern is concentric.

If you end up being off a little bit, a file is your friend here. I got a three diameter package of chainsaw sharpening files for not much money that works great for holes. Or you perhaps you have a rat tale file that would fit.

Keep the pictures coming as this progresses.

repechage

My hunch is that the bolt hole diameter on the ring being purchased is too small and opening it up will not be smart.
maybe adding three separate additional fasteners between the Pro 3 might work.

I like the Pro 3 crank set and felt lucky to find a 40t and paid dearly for it. Smallest I have seen offered is a 38, I missed that.

cyclophilia 

Have you considered modification of the 36T chainring (cut tabs, drill holes for chainring bolts) to fit the existing crank arm mounting flats and holes, as an alternative to drilling the crank arm?

non-fixie 

Excellent! I have a couple of these very lovely TA Professionel crank sets as well, and have been thinking about a similar project. Will be following this closely.

Glad you're trying this first.

52telecaster

I am not modifying the crank. I will modify the chainring. T.a. will make more! Actually I do have a vertical mill and an indexing head at my disposal as well as files and sander. I'll post pics of the debacle as I proceed.

juvela

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one of the big plusses in selecting the T.A. Trois Attachees for these sorts of experiments is that the inner face of the spider arms is machined to accept a chainwheel

with so many of the 116mm BCD three-arm chainsets the inner c/w mounts to the outer (or an adaptor ring) and the inner face of the spider arms is not machined to accept a c/w


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52telecaster

Also my biggest issue going in is whether the chainring will have enough stock for the holes I am making.

52telecaster

I have two Gran sports. 1 good fork. I hadn't thought about drilling the crank at all but if I did I could maybe do a modified t.a. 26 tooth. It would probably not be rigid enough but if I get brave I may.

non-fixie 

Which is why I have been contemplating the use of a 116 BCD 34T. Something like this Miche Supertype:

52telecaster

I didn't know this was available!

non-fixie 

It isn't. Well, not currently, as far as I can see. Hollandbikeshop have them on their site, but do not have any in stock. Hopefully it is a temporary problem.

EDIT: this is a 110 BCD, so not as easy as it looks, although 34T should work on 116 BCD.

52telecaster

I see.., I thought it was 116. I guess I'll soldier on with the t.a.

repechage

Since you have access to decent machinery... at the three crank arm locations, drill and tap the new ring after trimming the ineer circle to accept the factory chainring fasteners directly.
at the intermediate webs locate a set of smaller chainring bolts... maybe from a TA form factor... VeloOrange? parts with spacers., that should stiffen it up and provide decent attachment.

Or... cut a complete new chainring...

non-fixie 

I did think so too - sorry about that - but the reason I would still be inclined to buy one and try is that the flesh is in the right place. And the difference in radius in only 2.5mm. As long as *we* can get those three bolts to have enough purchase on the ring we should be good.

52telecaster

It's a good life when trying to figure out a chainring for an obscure crank is your problem.

Velo Mule

Wow, that's great. I wouldn't have thought. Yea, I'm jealous. This is a clever project, taking advantage of the 6 spokes of a T.A. chainring to adapt to a 3 pin crank. And then you may be able to even fit a 26 tooth? Double great! It's getting fun.

oneclick 

See if you can cancel.

The TA inner arms are too narrow, a hole the same size will only leave about a mm of wall at the narrowest point. The only way out would be to drill a smaller hole and use a stepped-shoulder bolt, of which you'd have to make three.

52telecaster

I will use the chain ring one way or another but the fact that it is 36 tooth makes the diameter close enough that the holes might be in the ring instead of the arms.

Duke7777

This looked like an interesting idea, so I traced out where the bolt holes would be in 34, 35 and 36 t cyclotouriste rings.


The outer circle is the countersink for the 12mm head, and the inner circle is the 9mm bolt hole. The 36t arms are perhaps getting a bit narrow next to the hole, although you could probably do away with the countersink if you have the chain stay clearance. For the 34t ring, there is a decent width of material next to the bolt hole, but the head of the chainring nut would interfere with the chain. It could still work if you ground off part of the head, and aligned the nuts to provide chain clearance. The 35t ring looks like maybe the best compromise, in that there is a reasonable width of material next to the hole, and the head of the nut doesn't interfere with the chain.

52telecaster

Since I have ordered the 36 it'll have to do. I haven't seen an available 35 or I would have ordered it. I really appreciate the graphic!