Designing a 104bcd dual chainring setup

Discuss light weight issues concerning mountain bikes & parts.

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alcatraz
Posts: 724
Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

Hi WWs

I put this topic in MTB even though I'm a road bike rider because I'm looking to convert from 110bcd to 104/64bcd on my climbing road bike.

I was wondering how to choose chainrings. Lets say I have a Cannondale hollowgram spider 120/80 or 104/64.

I'd like 46-44 as a big ring and a 30-26 as a small ring. Problem is it seems most big rings that large are designed to be the third ring and be on the outside of the spider.

For dual setups is the larger ring always on the inside? That means I have to find a 44t designed to be used on the inside of the spider, yes?

For dual setups, it seems my chainring choices are limited with 120/80 so naturally I'm leaning towards 104/64.

Which is lighter?

I was thinking of using extralite 44t as the larger and a 28-26t as the smaller. My concern is the spacing between and if the chain will catch on the chainring bolts or not.

Thanks for reading and commenting /a

by Weenie


Attermann
Posts: 219
Joined: Sun Jan 05, 2014 11:04 pm
Location: Denmark

by Attermann

Take a look at praxis works offerings, they already have what you seek


alcatraz
Posts: 724
Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

Thank you!

I'm not looking for a crank because I'd like to play with different chainring sizes. Also I have a left crank power meter that I like to keep.

This means I have the ability to choose a spider and set it up.

For extreme climbing rides I might go extreme 26/44 rings but for medium climbs rides maybe 30/46 for better shifting performance.

I'm thinking to just put the outer ring on the inside of the spider and just space the 64bcd ring correctly with appropriate spacers. What do you think? Bit ugly but functional and chainline shouldn't be garbage.

I'd need facetted nearly flush bolt heads on the inside of the big ring to make it work I guess. Anyone know any?

/a

NickJHP
Posts: 35
Joined: Wed Feb 14, 2018 2:22 am

by NickJHP

For a road bike, you should have a look at the Sugino OX cranks. They're designed from the ground up as a 110/74 BCD two chainring setup, and have the correct chainline for road bikes (unlike MTB cranks). They come at two quality levels - OX901 and OX601. Hubjub in the UK have both at a fairly good price, but 165mm only: https://www.hubjub.co.uk/extra-urban-27-c.asp. Alexscycle in Japan have the OX901 in lengths from 160mm to 175mm: https://alexscycle.com/collections/tour ... s-crankset.

Here's an OX crank that I fitted to one of my wife's bikes. It originally came with 44/30 chainrings, but when they wore out I fitted slightly smaller 39/26 rings.

Image

alcatraz
Posts: 724
Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

I have a cannondale sisl2 crank with a road axle. The crank is the same for road and mtb except for the axle length. I can slap on a 120/80bcd spider or a 104/64bcd spider. (Which will come out lighter I wonder?)

Seems 46-44t rings designed to be big+double or middle+triple are a rare thing to find.

Any other choices out there?

Seems 42t is my only choice.

/a

Pynchonite
Posts: 9
Joined: Mon Aug 07, 2017 7:15 am

by Pynchonite

You could always just get the outside ring for a triple setup, most of which are in the 42-46t range. Probably would negate any weight savings from the spider, though. If you could find a way to get an XTR triple outside ring on there, then you'd be golden, but they have a weird BCD.

alcatraz
Posts: 724
Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

Thanks Pynchonite. I was thinking of this before but they'd have to be fairly thin chainrings. Some are thick around the bolt holes which would throw off the chainline a lot.

Also I was worried about the bolt nuts sticking out and have the chain catch on them. These outer triple rings don't have nut/bolt grooves on the inside to let the nuts sit more flush. Hmm.. Maybe it won't be a problem.

So far I'm leaning towards 42/24 or 42/26.. and simply swap between 50/34 or this depending on the type of ride. Most of the time would be the 42/24 though because I have a flat road bike as well as this lighter one I'm trying to adjust gears on.

/a

by Weenie


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