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Drilling a crank spider for 3rd ring

Old 07-09-22, 07:34 AM
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Krov9
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Drilling a crank spider for 3rd ring

So I bought a humble Dotek crankset in order to get 155mm arms for my stubby legs. They way the spider has been cast/forged seems to suggest they also offered this same cranks as a triple, and in the double version I have, just left the spider undrilled?
I'm interested in "triplizing" the crank ie. drilling and tapping the crank spider for a third chainring. You are also welcome to advise against this operation

So what things should I consider besides finding out the right BCD? How wide and deep holes would I want to make? Any tips for getting them centered? I think a local bike kitchen has a drill press I could utilize, so I think I would clamp the crankarm between woodblocks and adjust it with a spirit level. The cast seems kinda crummy, so I'm not confident the underside of the spider is exactly level either... would this be difficult to correct? Please chime in with anything else that comes to mind



The Sugino 34t seen here seems to be the smallest 110 BCD chainring size available

The material thickness should be enough so I won't have to drill all the way trough the arm, right?
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Old 07-09-22, 07:50 AM
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First thing I would want is a proper bottoming tap for the blind holes. You could probably rig up a spare chainring as the drilling guide to get the BCD right. If it's 110mm BCD outer/middle it's 74mm for the inner ring.
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Old 07-09-22, 08:08 AM
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If you decide to go ahead, test your procedure on a piece of alu bar. In my experience threading alu can be way more challenging than threading steel, given how alu is fragile relative to the tap. There is very little room for correcting the threading process and you need to get it right 5 times here.
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Old 07-09-22, 08:27 AM
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I know at least one mfr makes 110 middle rings that have threaded tabs machined into them that accept a 74mm third ring. Specialties TA are very good quality and here's an example. Do a search for availability and pricing. Specialites TA Zephyr-K 110/74 D-D Converter chainring (pedalrevolution.co.uk)

Last edited by Crankycrank; 07-09-22 at 08:51 AM.
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Old 07-09-22, 08:47 AM
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My thought is to take a different route. I"d first look for long chainring bolts that could span both the spider and the chainring, then drill snug clean holes for them in the spider.

Or, barring that, drill clean holes for a very snug fit to to a shoulder bolt, head outside. Have machined a sleeve the OD of the chainring hole and id of the bolt threads. Washers to go over that sleeve and between spider and ring. Finding suitable nuts will be a bit of a challenge.

I did something like that for a fix gear triple where I ran the outside chainring outboard of the usual outer ring. Used nice SS hex head bolts and nuts that I think were machined custom that took an Allen wrench. (It just occurred to me that regular old carpenter's T-nuts would work just fine. Drill or sleeve for them, insert, tap with a hammer to mark the teeth locations and dremel the chainring at the teeth inserts.)

Key point in any approach - the sleeves or bolts should fit snugly in the holes. Nuts must fasten tight to prevent any play. (Many a single speed conversion crankset has died a death of slow hole elongaton from using double chainring bolts where the threads bottomed out before fully snugging the chainring, My first died such a death.)
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Old 07-09-22, 08:49 AM
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Tapping a crank for a triple chain ring

​​​​​​Ever modified a crank to add a granny?

​​​​​​sakae cr crankset: adding a granny ring

​​​​​​My compact double crank has undrilled inner pads at 74 bcd---why?

​​​​​​campy NR triple conversion

​​​​​​https://www.flickr.com/photos/8379107@N03/8392813249/in/photostream
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Old 07-09-22, 08:52 AM
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Originally Posted by Krov9 View Post
So I bought a humble Dotek crankset in order to get 155mm arms for my stubby legs. They way the spider has been cast/forged seems to suggest they also offered this same cranks as a triple, and in the double version I have, just left the spider undrilled?
I'm interested in "triplizing" the crank ie. drilling and tapping the crank spider for a third chainring. You are also welcome to advise against this operation

So what things should I consider besides finding out the right BCD? How wide and deep holes would I want to make? Any tips for getting them centered? I think a local bike kitchen has a drill press I could utilize, so I think I would clamp the crankarm between woodblocks and adjust it with a spirit level. The cast seems kinda crummy, so I'm not confident the underside of the spider is exactly level either... would this be difficult to correct? Please chime in with anything else that comes to mind



The Sugino 34t seen here seems to be the smallest 110 BCD chainring size available

The material thickness should be enough so I won't have to drill all the way trough the arm, right?
...Avocet made and sold a crank that was drilled (all the way through) on the arms in a couple of spots to accept two different BCD small rings for the triple. You don't see many of them around in the used parts stream, because they all eventually cracked at the holes. If it already seems like a crummy casting to you, I would advise against it, even if you're not drilling all the way through on your holes. Because then, as has been mentioned, (even with a bottoming tap), the threading process becomes kind of iffy.

I do think that if you can get the ring you plan on using to use as a pattern, you should be able to tape it on there adequately to at least get the holes in the right places, and more or less centered.

But if it were me, and I needed shorter crank arms, I think I'd explore the possibility of finding a used triple of good quality, that would already accept the rings I wanted to use. Then I would go ahead and carefully measure where I wanted the pedals to insert on the arms, and drill and tap them to accept pedals at that shorter distance. The advantages here are you only need to drill two holes, and the threading process should be easy. Then, you can either leave the longer ends with the unused original pedal holes, or shorten and finish per your preference. It should be a cold forged alloy crank, not a cast one.

There used to be a guy offering this service by mail (shorter crank arm drilling and finishing). But I do not recall the reference, nor do I know if he is still in business. Good luck.
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Old 07-09-22, 08:55 AM
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Originally Posted by 3alarmer View Post
There used to be a guy offering this service by mail (shorter crank arm drilling and finishing). But I do not recall the reference, nor do I know if he is still in business. Good luck.

​​​​​​https://bikesmithdesign.com/Short_Cranks/faqs.html
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Old 07-09-22, 09:19 AM
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I wouldn't bother, get a different set, not worth the time effort and money, unless you have access to the machines and taps.
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Old 07-09-22, 03:54 PM
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Crankycrank has the right answer. The TA Zephyr-K "triplizer" is engineered and manufactured to be your solution, without drilling or tapping anything. Trouble is, they seem to be out of stock, even at TA.

IRD does one too. they too show it out of stock, but least it gives you another option to search for.
https://www.interlocracing.com/shop/...arch=chainring
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Old 07-09-22, 04:47 PM
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Originally Posted by 3alarmer View Post
...Avocet made and sold a crank that was drilled (all the way through) on the arms in a couple of spots to accept two different BCD small rings for the triple. You don't see many of them around in the used parts stream, because they all eventually cracked at the holes. ...
Avocet stuff broke. Mine broke at the pedal threads (and had the inner drillings). Early on, the bolts for the inner ring broke. So did both seatposts. My seat did just fine. (Thank you, Selle Italia.)
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Old 07-09-22, 05:41 PM
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Originally Posted by dedhed View Post
This is the right answer. Get a crankset with the right ring mounts at your local bike kitchen and move the pedal holes down like this. If you donít mind the shipping, send to this guy.

Getting 5 holes in position, perpendicular, properly threaded, and not too deep, is going to be a bit of a challenge. Not impossible, but not something easily done on a drill press unless you have enough skill to not need to ask this question. If you do go forward, at least use a transfer punch fit to the ring to mark the holes. Drill two, secure the ring, mark the next one with the correct transfer punch, etc.

Really though getting two through holes put in square for the pedals is a much more realistically achievable goal, and is much more plausible on a drill press. Additionally the local bike kitchen is likely to have pedal taps, unlike the other thread size. Do be sure to remove the old pedal holes, and remember which side is left hand thread.
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Old 07-09-22, 06:48 PM
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Did this many years ago to two Campy NR cranks, still have them but after getting even older I decided I needed a smaller middle ring than 41T. So they entered into the shelves of old parts and odd bike project stock.

The first time I used a ring of the same tooth count as the one already mounted than used transfer punches to mark the hole centers. I followed up with dividing calipers checking C-C pock markes and slightly shifting the pocks before a solid center punching. The drilling was done on a drill press with the arm supported on the ring taps and bolted through the square hole to the press's table. That one ended up fairly good with only one hole off enough to notice. The holes were through drilled (as the OEM Campy triple arm had) and Campy ting bolts/spacers were used.

The second time a friend made me a jig that fitted on the Campy ring bolt holes and had 5 drill bushings for the granny's pattern. This one had better results.

I would not bother to do this again. Andy
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Old 07-09-22, 10:00 PM
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Originally Posted by oldschoolbike View Post
Crankycrank has the right answer. The TA Zephyr-K "triplizer" is engineered and manufactured to be your solution, without drilling or tapping anything. Trouble is, they seem to be out of stock, even at TA.

IRD does one too. they too show it out of stock, but least it gives you another option to search for.
https://www.interlocracing.com/shop/...arch=chainring
One more triplizer brand is Willow, not in there in the pipeline either. The lack of availability suggests that the demand dropped so much that there was no point persisting with production. An issue with triplizers is that you need another one once the teeth wear out. A different crank-length, albeit expensive, option are da Vinci Designs cranks.
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Old 07-10-22, 11:29 AM
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Originally Posted by oldschoolbike View Post
The TA Zephyr-K "triplizer" is engineered and manufactured to be your solution, without drilling or tapping anything. Trouble is, they seem to be out of stock, even at TA.
Redclover also offers triplizer rings, for either Campagnolo (144mm) or Stronglight 93 (122mm) base BCDs.

https://www.redclovercomponents.com/...riplizers.html
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Old 07-10-22, 11:35 AM
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Originally Posted by 3alarmer View Post
Avocet made and sold a crank that was drilled (all the way through) on the arms in a couple of spots to accept two different BCD small rings for the triple.
Ofmega made the Avocet-branded cranks. The two bolt circle diameters were 102mm and 74mm. 102mm BCD rings are scarce, but 74mm BCD rings are widely available. You do need special stand-offs and bolts to mount the rings, though.

Fun fact: the Avocet stand-offs can also be used to mount inner rings on Campagnolo Record triple cranks.
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Old 07-10-22, 02:49 PM
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My guess is the IRD ones come from Taiwan in batches, It might be a long time before a batch of triplizers is a priority, but eventually, when the supply chain for more normal things is filled better than now, they may do another. Specialities TA make all their own stuff, presumably in more flexible batch sizes. They may get the triplizer back into the pipe line sooner. In the meantime, scour eBay and the bike mail order sites (particularly in Europe for TA) for the odd triplizer still available.

Maybe Red Clover could be persuaded to do some 110 and 130 mm BCD triplizers.
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Old 07-10-22, 06:03 PM
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I wouldn’t even try it without access to a vertical mill. Even then it would be to much work for me.

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