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122, 128, 130 BCD - all with 38T smallest ring?

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122, 128, 130 BCD - all with 38T smallest ring?

Old 05-18-22, 01:17 PM
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smontanaro 
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122, 128, 130 BCD - all with 38T smallest ring?

I'm working on my son's bike (Raleigh Competition) which has a later Nervar Star crank. According to Velobase it has a 128 BCD. Looking at Sheldon Brown's five-arm crank crib sheet, I see that the smallest chainring is given as 38 teeth. That's what's on the crank now. The table lists 38T as the smallest chainring for 122 and 130 BCD as well. I assume that must not be the theoretical minimum, but the practical minimum (the smallest produced by the manufacturer). Or are the chainring bolt diameters sufficiently different that it affects the smallest possible chainring?
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Old 05-18-22, 02:02 PM
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-----

Sutherland lists:

38T as smallest for 122

38T as smallest for NERVAR 128

39T as smallest for all 130's


-----
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Old 05-18-22, 02:12 PM
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Originally Posted by juvela View Post
-----

Sutherland lists:

38T as smallest for 122

38T as smallest for NERVAR 128

39T as smallest for all 130's


-----
I have a 38 on a 130 so there's that.
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Old 05-18-22, 02:12 PM
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38T is definitely possible for 130mm BCD. I have one, but I see evidence that the chain plates are able to contact the mounting tabs on the crank spider. I wonder if that is why most production chainrings are a tooth or two bigger than their theoretical minimum.

@jonwvara makes a 37T 122mm BCD ring, of course.
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Old 05-18-22, 02:14 PM
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Originally Posted by ThermionicScott View Post
38T is definitely possible for 130mm BCD. I have one, but I see evidence that the chain plates are able contact the mounting tabs on the crank spider. I wonder if that is what is why most production chainrings are a tooth or two bigger than their theoretical minimum.

@jonwvara makes a 37T 122mm BCD ring, of course.
That is an issue on some cranks. Of course I've used a 33 on a 110bcd. I just gotta go small when I can.
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Old 05-18-22, 02:50 PM
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-----

the 39T limit on 130 BCD sets listed on me Sutherland is likely due to it being a fairly early edition

Howard revises the manual every few years

-----
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Old 05-18-22, 06:58 PM
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Smallest Chainrings

Originally Posted by juvela View Post
-----

Sutherland lists:

38T as smallest for 122

38T as smallest for NERVAR 128

39T as smallest for all 130's
-----
Originally Posted by juvela View Post
-----

the 39T limit on 130 BCD sets listed on me Sutherland is likely due to it being a fairly early edition

Howard revises the manual every few years

-----
Gorbachev: TRUST BUT VERIFY!

Over the years I've found that at least 5-15% of the specs in Sutherland's Handbooks are wrong or . I have a second edition the we used back in the 70's (error factor 15%) and a 5th edition that covers the C&V era that I'm interested in (5-10% error factor).

37T were the smallest 122mm BCD chainrings that Stronglight produced. This 1980 catalog page shows the 105 Bis "drillium" chainrings but they made 37T standard chainrings too. They don't show up in any catalog prior to 1980 but... I have at least 15 bikes with Stronglight 49, 93, 104 and 105 cranks that I've mounted what I thought to be 38T chainrings on. When doing a tooth count, I found that over 1/3rd of them are actually 37T rather than 38T! Several date back to the late 1970's.



The 3 silver chainrings are 38T - the black one is 37T.



Shimanos are the only 130mm BCD cranks that I have experience with. I found that some of them are limited to a 39T small chainring without a modification.

Shimano 600 Crank with a 39T chainring.



A 38T chainring on the same crank. The chain will drag when going over the top of the spider arm.



I'm running 48T or 49T outer chainrings with 38T inners on most of my Shimano cranks. File about 1mm to 1.5mm off of the spider above the chainring bolt hole. This minor modification solves the clearance problem.



In defense of Sutherland's, during the bike boom, many bikes were sold or serviced by shops with little or no experience working on derailleur bikes; e.g. lawn mower shops, ski shops and so on. Sutherland's was a good resource for those kinds of businesses when for example they needed to replace a BB and so on.

Some of the dimensional specs were off when it came to little nuances like components with wide open manufacturing tolerances like BBs, hubs, FWs, and headsets. It was better than nothing because we had to learn ourselves by trial and error. We focused on better quality bikes so it became an issue for us.

juvela I'm not picking on you... just pointing out & sharing weird exceptions learned the hard way....

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Old 05-18-22, 07:33 PM
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Stronglight 93 et al. at 122mm should be able to take a 36t with a little trimming of the spider arm tabs. I plan to make one, have the holes marked out, needs drilling counterboring and trimming, just haven't got a round tooit.

CAD drawing is from here.

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Old 05-18-22, 08:14 PM
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Technically not true for the 130. There are some 130 cranks out there drilled for an 86 3rd ring. Takagi made triples in both 130/86 and 110/74. I'm guessing some other brands did as well.
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Old 05-18-22, 08:26 PM
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Doubles Not Triples

Originally Posted by seypat View Post
Technically not true for the 130. There are some 130 cranks out there drilled for an 86 3rd ring. Takagi made triples in both 130/86 and 110/74. I'm guessing some other brands did as well.
The discussion was about the smallest chainrings for 3 different brands of doubles and the specs that juvela posted are generally correct. To go to a triple would require replacing the crank or getting a 130mm BCD triplizer which are getting rare.

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Old 05-19-22, 06:28 AM
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Originally Posted by verktyg View Post
The chain will drag when going over the top of the spider arm.

You can file a little material off the end of the spider arm to prevent this. Ofmega actually machined their 144mm BCD cranks to provide clearance for a 41T ring, which was otherwise subject to the same issue:
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Old 05-19-22, 03:26 PM
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While I appreciate the discussion of all the possibilities in the surrounding BCDs, is there any chance a 36T or 37T 128BCD chainring for Nervar Star cranks was ever produced?
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Old 05-19-22, 10:41 PM
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Originally Posted by smontanaro View Post
While I appreciate the discussion of all the possibilities in the surrounding BCDs, is there any chance a 36T or 37T 128BCD chainring for Nervar Star cranks was ever produced?
Impractical at 37t, pretty much not possible unless you're willing to trim the spider tabs all the way down until you break through into the hole.
Probably completely impossible at 36t, unless there's some hack I haven't considered.

The website I use, that does CAD simulations based on BCD and teeth, doesn't have Nervar as a choice in its drop-down menu.

But if you put in 37 and 130 mm, you see the "valley" between two teeth actually touches the bolt hole. Nervar has a bolt circle radius just one mm smaller, so you could get that silly millimeter of clearance if you were desperate enough. Maybe with a smaller bolt diameter, smaller hole?
Do the same thing at 36t and I think you'll agree ain't happenin':
That's not even the worst place, a couple of the other holes overlap the chain roller even more maybe I shouldn't have cropped the screenshot that way.

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Old 05-20-22, 05:09 AM
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Thanks, Mark. Seems if I want to go lower I'd be better off searching for a 122BCD or 86BCD crank. Jake's young. I think he can handle it.
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Old 05-20-22, 06:30 AM
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Originally Posted by smontanaro View Post
Thanks, Mark. Seems if I want to go lower I'd be better off searching for a 122BCD or 86BCD crank. Jake's young. I think he can handle it.
There's also triplizing the Nervar, if a triple would fit the bill. Have you considered it already? I see four ways and there may bbe others.
  1. A factory-made triplizer middle ring for 130 BCD (TA, Stronglight, Shimano, Mavic, Willow, FSA or Thorn) can be adapted to 128. You only need to ovalize the holes by 1 mm each, like with a file or dremel. Looks a little wonky, but only if you look real close, and it works fine.
  2. Or a new triplizer can be made with an actual 128 mm BCD (no ovalized holes) by starting out with a 38t ring with a 74 mm bolt pattern. Rare but they're out there, I know where to get one right now. Then you add 5 new holes at 128 mm, and the granny bolts to the original 74 mm holes. I have done this, works a treat. Oh yeah you have to cut the inner "shelves" off the spider arms, like making it into a Track crank. The "shelfectomy" is a 15 minute job if you have a vise, hacksaw and file, and you are handy.
  3. Or a new triplizer can be made with an actual 128 mm BCD (no ovalized holes) by starting out with a 38t ring with a 86 mm bolt pattern, that is, a Stronglight 99 ring, not quite as rare. Then proceed as in item #2 including the shelfectomy.
  4. Finally there's the Campy old-Record (aka "NR") way, where the 5 spider arms are drilled and tapped for stand-off bolts that the granny attaches to. Here's one I made that way to take an 86 mm Stronglight 99 granny ring:


That's time-consuming ($$), and even just the 5 Campy #818 standoff bolts often sell for well over $100. Though there are cheaper alternatives to the Campy bolts shown here, like generic industrial bolts from McMaster, this is only really an option for someone (like me) who's just crazy about their Nervar Star and nothing else will do. For most sane individuals, there are easier ways to get the needed gearing. If you're one of the crazy ones, let's talk.

Mark B

Last edited by bulgie; 05-20-22 at 07:15 PM.
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