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Discussion Starter #1
So the spider that converts our Raceface Ride (cinch) crankset to 2x so we can use it on our tandem shattered yesterday on a climb. The outer holes on 104 BCD ring--the main drivetrain ring--all broke.

I know 2x has fallen out of favor and our drivetrain is 1x, but we need the inner position 22t for the timing chain that feeds power from the front pedals to the rear.

Weight doesn't matter, we really just want/need the strongest possible setup.

100mm regular English threaded BB, 197 rear. The boost spider we were running has an 80mm chainline, and there was plenty of clearance with the largest 5.05" tires.

I'm really just at a loss as to what to do. The bike was designed at least partially around this crankset, but I really think if I replace the parts, it's just going to die again.
 

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Surly OD cranks would be perfect. Tough to find right now since the geniuses at Surly stopped makin em. Maybe Ebay.
 

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Hollowgrams are use on everything from road bikes to fat bikes. Plenty of options for spindles and spiders. Plus they work with cinch spindles too.
 

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What comes to mind is that SRAM cranks have replaceable spiders that are pretty burly, and adaptable many chainring configs.

I don't know the exact chainline tho.
 

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Discussion Starter #5
Thanks everyone. Here's what I've learned so far:

1. There are square taper BBs for fat bikes! I haven't seen them from a major brand, but they are available from China. Square taper was an excellent standard for many generations, and for us fat bike guys, it means we can use almost any crankset. So that's one option I'm considering, especially because cheap cranksets are available that are tough as nails. People looking for a $40 crankset aren't worried about weight like people looking for a $200 one.

2. SRAM has or had a dub 2x10 fat crankset that is potentially an option. I haven't been able to find information on chainline or find one for sale, but there is one made for a 5" tire on a 100mm threaded bottom bracket, so that's an option.

3. There's a chinese crankset on Amazon. I'm not sure it's stronger than the Raceface stuff I'm running, but it exists. It only comes in 175 mm length.

Raceface is sending me a warranty part so I'm going to install it with their chainring and their bolts rather than a generic one and hope it works for the time being. If that doesn't work, I think I'm going to install a square taper BB in the rear and install a cheap triple with a 32t narrow wide in the middle position, a bash guard on the outside, and the timing ring on the innermost.
 

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The outer holes on 104 BCD ring--the main drivetrain ring--all broke.
Wow, that's wild.

Any chance the chainring bolts had loosened up?

Edit: I also wonder if mounting a sturdy bash ring on a 3X spider would strengthen things up a bit? Something like the old Salsa Toothfairy?
 

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Surly OD cranks would be perfect. Tough to find right now since the geniuses at Surly stopped makin em. Maybe Ebay.
FWIW,

I have a nice used set of Surly OD 100 on the shelf. 175MM arms. If you want em, I'd let em go.
 

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Discussion Starter #8
Wow, that's wild.

Any chance the chainring bolts had loosened up?

Edit: I also wonder if mounting a sturdy bash ring on a 3X spider would strengthen things up a bit? Something like the old Salsa Toothfairy?
Not only were they not loose, they are still tight after it broke!

The 3x spider for this crank is not currently available that I can find, but I'm willing to try that. I just don't want this to happen again.
1912348
 

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Not only were they not loose, they are still tight after it broke!

The 3x spider for this crank is not currently available that I can find, but I'm willing to try that. I just don't want this to happen again.
Impressive :oops:

Nothing like a tandem to break $hit

edit: your spider doesn’t look like there was a pocket for the head of the chainring bolt like the ones I see online. I wonder if that caused some kind of stress riser?

edit2: does the standard have a pocket, but the boost not? Maybe the as-cast surface of the boost version isn’t perfectly flat and your steel bolt heads made a groove causing a stress riser. Sounds counterintuitive, alloy chainring bolts might be better. Or find a 3x and run a bashring or tab spacers

 

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What's up with the wear marks on the tabs of that chain ring? Definitely some flexing going on and it looks like maybe the two didn't match up correctly.
 

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Discussion Starter #11
Impressive :oops:

Nothing like a tandem to break $hit

edit: your spider doesn’t look like there was a pocket for the head of the chainring bolt like the ones I see online. I wonder if that caused some kind of stress riser?

edit2: does the standard have a pocket, but the boost not? Maybe the as-cast surface of the boost version isn’t perfectly flat and your steel bolt heads made a groove causing a stress riser. Sounds counterintuitive, alloy chainring bolts might be better. Or find a 3x and run a bashring or tab spacers

That's an interesting thought. There isn't a machined surface and the head sits against the spider. If the pictures online are true, the boost spider does not have that, but the non-boost one does. The 3mm chainline difference is likely not important to me.

They're sending me another 2x boost spider but when they're available again, I think I'm going to get the 3x one and install a bashguard. We do have a bashguard on the front right now. I'm also switching to their chainring and their bolts so there is zero doubt the parts work together.

What's up with the wear marks on the tabs of that chain ring? Definitely some flexing going on and it looks like maybe the two didn't match up correctly.
I had to remove some material from the chainring to get it to fit on the spider. Apparently not all chainrings fit every spider. I didn't really think it'd matter, but here's to hoping the raceface brand chainring fits their spider better than the generic one it's replacing.
 

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Also check with bike manufacturer. Some bikes need the wider RF cranks and SRAM doesn't clear the chainstays. Like the RSD Mayor.
I wouldn't use square taper. For both weight and durability. Just get a new crank with new external BB.
 

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Discussion Starter #16
Also check with bike manufacturer. Some bikes need the wider RF cranks and SRAM doesn't clear the chainstays. Like the RSD Mayor.
I wouldn't use square taper. For both weight and durability. Just get a new crank with new external BB.
This is a full custom bike that I designed. I am the bike manufacturer.

Square taper worked for 100 years. If you haven't used it, you're missing out. Instead of needing to be tandem, boost, or fat-bike specific, square taper allows the same crank to be used on all with a change of bottom bracket. Now that we rely on the crank spindle to provide the width and chainline adjustment instead of the bottom bracket, manufacturers have to make a ton of different products instead of the mostly-standard stuff they were doing before. If I'd known square taper was an option for q factors this wide, I might have never messed with an external cup bb on this bike.

Weight is absolutely never a consideration for me because nobody thinks about how great their bike is being a few grams lighter when they're pushing it back from the backcountry (sure wish I had a few more grams of material on the broken spider right about now). But even if weight was a consideration, Jan Heine's blog explains why square taper setups are not necessarily heavier when you take the crank into account and have substantial advantages, including durability when the crank has to be removed and reinstalled:



With a 100 mm BB shell, these will give you a Q factor of 210mm
If you can convince my stoker to do all the pedaling for two people, your 1x crank you suggest will be great. Otherwise I need a way to couple the captain's crank to the stoker's because this is a tandem!
 

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If I'd known square taper was an option for q factors this wide, I might have never messed with an external cup bb on this bike.
Lot's of bent square taper stories out there on fat bikes, so I still think you made the right call.

The engineer in me says something was wonky with the mismatched parts and grinding which caused rapid fatigue.

I'll bet a bag of chips that you'll be totally fine with the new parts.
 
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Discussion Starter #18 (Edited)
Lot's of bent square taper stories out there on fat bikes, so I still think you made the right call.

The engineer in me says something was wonky with the mismatched parts and grinding which caused rapid fatigue.

I'll bet a bag of chips that you'll be totally fine with the new parts.
I finally got a response from Race Face's engineering people today:

"All Race Face spiders are designed for anticipated loads generated by a single rider riding a conventional mountain bike. In this tandem application, the spider tabs are supporting loads from 2 riders. We have not designed this spider with this application in mind. I am not surprised that the tabs have failed in the way shown in the photos - it is not the intended use of our product. I would not anticipate any different result with the replacement spider either. We can not recommend that customers use our MTB spiders in this way. We should suggest that the customer use components designed specifically for tandem bicycle use. Unfortunately we do not have any products like that in our line."

I think it's square taper time. I wish there was a better answer but if there is, I don't know what it is. I'm going to install a narrow wide chainring and bash guard on a more conventional triple crankset and hope for the best. Newer ain't always better.

I do wonder somewhat about your point if there was a stress riser between the chainring and the spider. I'm not sure I'm curious enough to want to put it back together and have to walk back to our car again. It really ruined our ride and it is one of my favorite trails where we broke. I've not had a lot of time to ride lately and was really looking forward to some tandem time.
 

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If you can convince my stoker to do all the pedaling for two people, your 1x crank you suggest will be great. Otherwise I need a way to couple the captain's crank to the stoker's because this is a tandem!
Oh, sry. I just skimmed the thread. It is however a triple crank. (I just made it 1X)
 

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This is a full custom bike that I designed. I am the bike manufacturer.

Square taper worked for 100 years. If you haven't used it, you're missing out. Instead of needing to be tandem, boost, or fat-bike specific, square taper allows the same crank to be used on all with a change of bottom bracket. Now that we rely on the crank spindle to provide the width and chainline adjustment instead of the bottom bracket, manufacturers have to make a ton of different products instead of the mostly-standard stuff they were doing before. If I'd known square taper was an option for q factors this wide, I might have never messed with an external cup bb on this bike.

Weight is absolutely never a consideration for me because nobody thinks about how great their bike is being a few grams lighter when they're pushing it back from the backcountry (sure wish I had a few more grams of material on the broken spider right about now). But even if weight was a consideration, Jan Heine's blog explains why square taper setups are not necessarily heavier when you take the crank into account and have substantial advantages, including durability when the crank has to be removed and reinstalled:





If you can convince my stoker to do all the pedaling for two people, your 1x crank you suggest will be great. Otherwise I need a way to couple the captain's crank to the stoker's because this is a tandem!
You neglected to mention a lot of necessary information, like the fact it is a tandem.

The Rene Herse article you linked says they don't recommend their square taper cranks for MOUNTAINBIKES.
 
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