I have failed these 4 times. SRAM warrantied them 2-3 times. I currently have the last warrantied set NIB and waiting to be installed on my wife's bike. I'm using GX Eagle cranks now.

SRAM states these all failed due to pedal strikes. That could be but the actual "fail" occurs during a JRA. One legged riding back to the TH verily doth sucketh.

 

View attachment 2076311

I have failed these 4 times. SRAM warrantied them 2-3 times. I currently have the last warrantied set NIB and waiting to be installed on my wife's bike. I'm using GX Eagle cranks now.

SRAM states these all failed due to pedal strikes. That could be but the actual "fail" occurs during a JRA. One legged riding back to the TH verily doth sucketh.

Yeah, definitely not super interested in any crank snapping action, but the X01s have been faultless on the singlespeed so far. Interested in trying the NSB cranks when I get a chance, but all these fantastic plastic cranks have come to me through trades and bundled with other stuff, so I suppose I can’t complain. Very few rocks to strike here, too.

 

Damn.

I have a some Sram X1 Carbon Cranks and RaceFace Next R carbon cranks...Have had some pedal strikes but that never happened.

 

As pictured above, I snapped a set of X01 cranks 2 years ago as well. Small jump, then face plant as I totally lost it when my foot hit the ground. Never again....

 

I can't speak to today's cranks but in the first bunch of years XO-1 had an aluminum skeleton from the spindle to the pedal insert and XX1 did not... At least that is my understanding.

I have an older pair of XO-1's that have been to hell and back and are still in one piece. Given post #2 I likely wouldn't buy another pair

 

What was wrong with Al cranks? Did they fail?

Just because you can, doesn't mean you should.

130g difference between X01 and GX. That's 4.5oz. Is that really that important?

 

No, it certainly isn’t. If these all hadn’t come to me at very little cost I wouldn’t even bother.

 

Multiple SRAM & Raceface carbon cranks over the past 12 years and 20K miles with zero issues. Lots of pedal and crank strikes. YMMV

 

Despite light cranks not being anywhere near the top of my bike priority list, I've somehow ended up in possession of a set of X01 and XX1 cranks, and I'm scratching my head as far as practical differences are concerned. If I understand it correctly, the X01 are more or less the same construction as XX1, but filled with ~50 grams of additional foam, which may make it more durable somehow? I'm the furthest thing possible from a materials engineer, so I'm confused as to how that foam makes the X01 more durable in the real world, as the carbon is just as exposed on both cranksets, and how not including the foam begets a nearly $200 premium for XX1. There may not be a legitimate answer here, but since it's winter and all, I'm interested to hear thoughts.
View attachment 2076294

I think the foam is simply to aid in manufacturing. I'm actually not quite sure how they get the void in the XX1 - they may pressurize the interior(typically this is done with a removable bladder, but I don't quite see how they do that on these)* and use a vacuum on the outside, which would be a more costly process.

I'm sure the foam will not add structure. But apparently that 50g means something to someone.


*this maybe a dual cure process where they lay up the interior with a bladder or plug, cure it, then remove the plug, plug the hole and lay up the exterior structure, and do a second cure.

 

Most likely they use different foams. XX1 can be heated, melted, and poured out, while the X01 is likely a more rigid & higher melt temp foam that adds stiffness but does not improve impact durability.

XX1 DUB SL also have a lighter, co-molded spindle that likely adds a good chunk of that $200 added MSRP. X01 has a bonded heavier spindle (bonding requires a thicker wall where bonded). XX1 DUB Wide, XX1 FAT, and XX T-Type do not use this light spindle, likely for cost savings not passed along to the customer.

I have XX1 on both my bikes. Lots of heavy pedal/rock strikes (clipless) with no issues yet. They are my favorite cranks. Breakage may be more common with flats because you can move your foot pressure further away from the pedal insert and apply significantly more leverage.

 

I can't speak for cranks, but aircraft manufactures commonly use dense foam filled control surfaces , panels, skins, floors, doors and a multitude of items in a structural capacity. I would have assumed these cranks to be the same.

 

Does the foam provide structure, or rather a filling to separate the two layers composite to provide a boxed, and stiffer structure?

I'm thinking the instance of canoe bottoms. Often time manufacturers will use foam in the bottom to increase the stiffness but the foam has nothing to do with the structure - it's just in there to hold the fabric and resin during molding and is impractical to remove. It's actually quite detrimental if damaged as it can become contaminated with water or debris.

 

Good point on acting as a stiffener. While I can't answer that definitively, Flight controls surfaces can come under high aerodynamic and input loads, particularly flaps, spoilers and speed brakes. These are frequently foam filled between thin sheets of aluminum or carbon with (sometimes) minimal interior structure beyond skin and foam.. At least these I would assume to be structural in nature, but my structural colleagues could provide an answer. Panels I can see foam as a stiffening component. Load bearing flooring perhaps not?

 

Right - would depend on loads - if really low loads, then perhaps it does add a bit of structure. Foam has some stiffness, not a lot - I would imagine most of the stresses are being carried in the outer skins - that's the nature of beams anyway.

For cranks, I think it's not. Some frames have foam in them like this - that's just the process they use and I don't know if they can remove it without melting - and that may damage the resin. It's quite common to use "lost foam" for molding metal parts. You use it to build your mold and burn it out of the void.

The foam may provide damping - I know on frames it can certainly do that and keep them from being too noisy. I don't know if it improves ride quality in a noticeable way.

As far as load on a bike crank, foam isn't going to provide any structure. And all the loads and stresses are concentrated in the outer CF layer anyway. I'm sure this is just a manufacturing artifact.

If anyone knows the process they use to get a closed cavity like that, I'd be interested to know. It's not obvious, to me at least.

 

I'm confused as to how not including the foam begets a nearly $200 premium for XX1.

Dude the logos are gold though.

 

Foam is typically nothing more than a mold. The carbon fibger/fiberglass/kevlar cloth is then laid up on it, the matrix (resin, epoxy, or whatever) is then introduced, and the whole system is vacuum bagged. I would speculate that the foam is removed on the XX1 via some sort of solvent (e.g. acetone).

 

Not to totally crush your speculation, but rather to promote discourse, wouldn't that still require a hole?

Maybe the extra cost is complete BS, wouldn't surprise me, but they could also be using a much different and more advanced process... why? I haven't the foggiest. 50g of foam seems pretty inconsequential to me. Perhaps there are other differences we don't know about (besides the gold lettering)? We only know what the sell us on (unless someone does some expensive reverse engineering)...

 

When I first moved to carbon cranks from aluminum the difference in stiffness was palpable and the weight not insignificant. That was 2012 (I think) so not sure if that still holds true or not.

 

I ride XO1 because my bike came with them. I haven't been nice to them, and at this point they have a number of chunks missing due to airborne rock strikes. No issues so far, but I will be completely unsurprised if/when they let go and I eat stem.

Carbon cranks make no sense to me. They are a solution looking around for a problem and creating a few new ones in the process. I'm in the camp of, buy a set of RF Turbines and never ever think about your cranks again.

 

I ride XO1 because my bike came with them. I haven't been nice to them, and at this point they have a number of chunks missing due to airborne rock strikes. No issues so far, but I will be completely unsurprised if/when they let go and I eat stem.

Carbon cranks make no sense to me. They are a solution looking around for a problem and creating a few new ones in the process. I'm in the camp of, buy a set of RF Turbines and never ever think about your cranks again.

I've been whacking Shimano Deore/XT cranks on whatever I can for the past 8 years, and I don't see any degradation. I plan on running them for another 8 years.

Maybe if you're an XC racer and this bit of weight really matters, but for the majority of MTBers, I don't see the necessity.

 

I've squished the pedal insert in at least 2 pairs of XT cranks. I'm sure from hard strikes but possibly from hard landings also. I can count carbon cranks along with frames and rims as parts that have proven far stronger than their aluminum counterparts for me.

I've seen many pics of exploded carbon cranks here and elsewhere though and have friends where the spindle bonding came loose on RF SL's but until that happens to me I'ma stick with carbon.

 

Vacuum bagging also removes voids and improves surface finish.

 

Vacuum bagging also removes voids and improves surface finish.

The reinforcement (fiber) is what gives it its strength. The matrix (resin, epoxy, etc) doesn't do anything other than hold them in place. So vacuum impregnation only serves to eliminate excess matrix material, which reduces weight. But honestly, in a production environment, doing it any other way makes no sense.

I only know this fight from canoes, when small builders who did hand layup were being overtaken by more advanced builders using vacuum bagging. The argument was that you wind up both with a lighter, and stronger boat - the argument was lack of voids and thus a better bond between fibers with no internal stress concentrations.

Also I don't buy that the matrix doesn't do anything, because fiber has only tensile strength and shear strength perpendicular to the plane of the fiber. Ideally we want to load CF in tension because there it is very stiff and very strong, but that's impractical as functionally it needs a to take all sorts of combined loads, including shear and compression. I actually tend to see most failures of CF in the matrix itself, so in shear or compression. But there are certainly cases where both fail - the so called "catastrophic" failures - although, to me, complete matrix failure and inability to fully support compressive and shear forces seems like a total failure. In other words, you crank isn't going to crank very well if it's made of wool, or effectively gets reduced to that in an area.

 

Right, but that's why it is a tube, as one side will always be in tension.

Modulus of epoxy is 0.4GPA. Modulus of C fiber is about 150. Modulus of composite, assuming 50/50, is 0.5*(0.4)+0.5*(150) = 75.2GPA. Of that, the C provides 75 of it. That is to say 99.7% of its load bearing capability.

You are right about the voids, but that's really more a factor of it being stress related, which is force/area, and the area effectively is increased for the same fiber, meaning that the load bearing capability is going to be the same, but the theoretical measured stress is lower. My introduction to composites was messing around with homebuilt airplanes in the late 80's, and that was all vacuum bagged due to weight. Studied it in college, too, all my research was actually ceramic matrix. But that was a long time ago. Ended up going into engine development professionally, so didn't really use it after college! Well.....other than in hobbies.

 

I love when people start talking about how aluminum cranks are so durable. Did everyone just forget that Shimano had to recall like a billion aluminum cranks because they self destruct and that class action suits are coming left and right?

 

The discussion about vacuum bags also does not apply to the cranks in question. The exterior surface finish is too consistent to be externally vacuum bagged.

The cranks (both) are likely produced with the thermally triggered expanding foam that applies pressure internally to the laminate against an external hard (steel or aluminum) mold.

The difference between the two is likely that the xx1 foam is of a type that can be heated/melted and poured out (or dissolved), while XO1 isn’t, or is of a type that cannot be. If SRAM did their homework and it’s not compressible (significantly), it would add stiffness and potentially strength. The difference in price could also be driven by the different types of foam (if so), or the added process step to remove the foam (for sure).

 

The difference being that the xx1 foam is of a type that can be heated/melted and poured out (or dissolved), while XO1 isn’t, or is of a type that cannot be.

Common statement
Is this a fact or an assumption? Is there something produced by SRAM that validates this?

 

Absolutely. In fact, it's a key component of many composite systems. There's a lot of misinformation in this thread about how composites structures work, but the basic idea is that the overall stiffness of a beam/panel/etc is related to the thickness, other factors being equal. A flat, thin carbon skin is actually pretty flexible on its own, but if you sandwich a suitable core (foam, honeycomb, etc) between two of those flimsy skins it is now a thicker and therefore much stiffer structure, with very little added weight. Shape also matters - clearly a tube of thin carbon is much stiffer than if you were to unroll it into a flat sheet. I'm sure there are some other mech E nerds that know what I'm talking about and can probably explain it better, but most sane people's eyes glaze over pretty quick when you start talking about this stuff

I'm not saying this is all directly applicable to carbon bicycle cranks or explains why some do/don't have foam. Not all composites use foam as a structural component. But yeah, foam definitely CAN be structural.

What you say really is not making sense, and I'd like you to specifically point out the misinformation. Separating two thin sheets by a distance of course increases the second moment of area and thus the stiffness of the member. This is basic mechanics of materials. But you're not really considering vast difference in modulus between foam and carbon fiber. Take the foam piece by itself. It has very low stiffness even though it may have have a large second moment. It's only through the addition a of a high modulus material far away from the neutral axis do you create a stiff structure. The foam is simply there to hold the other high modulus material in place. And if one melts or dissolves it, then the structure can be just as stiff.

The article I linked did make a couple points about wrinkling during compression (panel stiffness) for very thin skins as well as impact resistance. But other than that, I still fail to see the foam as any major structure. I'm willing to bet the stiffness between the cranks without foam and with are with 1% or so, assuming the layup is the same.

The discussion about vacuum bags also does not apply to the cranks in question.

The cranks (both) are likely produced with the thermally triggered expanding foam that applies pressure internally to the laminate against an external hard (steel or aluminum) mold.

The difference being that the xx1 foam is of a type that can be heated/melted and poured out (or dissolved), while XO1 isn’t, or is of a type that cannot be. If SRAM did their homework and it’s not compressible, it would add stiffness and potentially strength. The difference in price could also be driven by the different types of foam (if so), or the added process step to remove the foam (for sure).

They are likely cured in an external mold but vacuum can be used with those as well. I have seen processes that use expanding bladders, and that's what I mentioned earlier.

EPS, which you mention, seems to be the state of the art in that it causes less shift of the fabric in the molding process. But you fail to acknowledge the issue with the hollow crank. Where did the foam go? How did it get out? There must have been a hole in the part, and unfortunately that's going to require some other secondary process to plug and be sure that it's structurally sound. Perhaps it's along the neutral axis and it's just plugged with some epoxy without fiber. Maybe that's acceptable?

A non-compressible foam, whatever that means, would only add as much strength and stiffness as the foam itself. Refer to my comments above. It's not like you magically gain something having a weak, soft material sandwiched in what could be a void. You only gain the superposition of its stiffness plus the stiffness of the shell, which to me must be much, much stiffer, and much, much stronger.

See here:

https://www.diabgroup.com/media/q5yldbe4/diab-guideline-to-core-and-sandwich.pdf

Faces:
The faces carry the tensile and compressive stresses in the sandwich. The local flexural rigidity is so small it can often be ignored. Conventional materials such as steel, stainless steel and aluminum are often used for face material. In many cases, it is also suitable to choose fiber or glass-reinforced plastics as face materials. These materials are easy to apply. Reinforced plastics can be tailored to fulfill a range of demands like anisotropic mechanical properties, freedom of design, excellent surface finish, etc. Faces also carry local pressure. When the local pressure is high, the faces should be dimensioned for the shear forces connected to it.

Core:
The core’s function is to support the thin skins so that they do not buckle (deform) inwardly or outwardly and to keep them in relative position to each other. To accomplish this, the core must have several important characteristics. It has to be stiff enough to keep the distance between the faces constant. It must also be so rigid in shear that the faces do not slide over each other. The shear rigidity forces the faces to cooperate with each other. If the core is weak in shear, the faces do not cooperate and the sandwich will lose its stiffness. It is the sandwich structure as a whole that gives the positive effects. However, it should be mentioned that the core has to fulfill the most complex demands. Strength in different directions and low density are not the only properties that the core has to have. Often there are special demands for buckling, insulation, absorption of moisture, ageing resistance, etc. The core can be made of a variety of materials, such as wood, aluminum, and a variety of foams.

 

What you say really is not making sense, and I'd like you to specifically point out the misinformation.

oh boy. I didn't really want to get sucked into this but here we go:

Separating two thin sheets by a distance of course increases the second moment of area and thus the stiffness of the member. This is basic mechanics of materials. But you're not really considering vast difference in modulus between foam and carbon fiber. Take the foam piece by itself. It has very low stiffness even though it may have have a large second moment. It's only through the addition a of a high modulus material far away from the neutral axis do you create a stiff structure. The foam is simply there to hold the other high modulus material in place.

This is true, and the basic principle of sandwich construction (which is explained well in the section 1.2 of the article you linked). In some sense the foam is in fact simply there to hold the "high modulus" in place, but an important part of that is being able to transmit shear stress through the cross section to put the other side of the sandwich in tension.

And if one melts or dissolves it, then the structure can be just as stiff.

No, because now there is no mechanism to maintain separation between the faces or transmit shear stresses from one "skin" to the other and they no longer work together to increase stiffness of the overall structure.

The article I linked did make a couple points about wrinkling during compression (panel stiffness) for very thin skins as well as impact resistance. But other than that, I still fail to see the foam as any major structure.

Yes, a foam can serve several purposes in sandwich construction but primarily it is the separation of the faces that makes it work (again, see the article you linked). Not all composite structures use this principle, but for those that do the foam is key element, it won't work without it.

A non-compressible foam, whatever that means, would only add as much strength and stiffness as the foam itself. Refer to my comments above.

No, refer to the explanation above and in the Diab article.

It's not like you magically gain something having a weak, soft material sandwiched in what could be a void.

Science, not magic, and it still has to be the right kind of relatively "weak, soft material", but yes you gain something - exponentially increased stiffness. That is actually exactly the point of sandwich construction.

You only gain the superposition of its stiffness plus the stiffness of the shell, which to me must be much, much stiffer, and much, much stronger.

sigh...

but again, I can only speculate on whether or not foam filled cranks are designed with that foam serving any structural purpose. There are a lot of sneaky manufacturing techniques out there, and some are pretty closely guarded.

 

oh boy. I didn't really want to get sucked into this but here we go:


This is true, and the basic principle of sandwich construction (which is explained well in the section 1.2 of the article you linked). In some sense the foam is in fact simply there to hold the "high modulus" in place, but an important part of that is being able to transmit shear stress through the cross section to put the other side of the sandwich in tension.


No, because now there is no mechanism to maintain separation between the faces or transmit shear stresses from one "skin" to the other and they no longer work together to increase stiffness of the overall structure.


Yes, a foam can serve several purposes in sandwich construction but primarily it is the separation of the faces that makes it work (again, see the article you linked). Not all composite structures use this principle, but for those that do the foam is key element, it won't work without it.


No, refer to the explanation above and in the Diab article.


Science, not magic, and it still has to be the right kind of relatively "weak, soft material", but yes you gain something - exponentially increased stiffness. That is actually exactly the point of sandwich construction.


sigh...

but again, I can only speculate on whether or not foam filled cranks are designed with that foam serving any structural purpose. There are a lot of sneaky manufacturing techniques out there, and some are pretty closely guarded.

You act like I'm some kind of layman dolt here - I'm a mechanical engineer. Perhaps read the whole thread. I'm not a composites expert but I spent many years doing structural design of engine components and I understand the mechanics pretty well.

The crank has a box structure - it doesn't need foam to transmit shear. It's not a panel. I understand this whole principle quite well.

Please provide the mathematics to show that the stiffness of the sandwich is any more than the superposition of the two individual materials - because if you look at any mechanics of materials books that shows reinforced concrete, that's exactly the way it will be calculated. And while you're doing that, let's compare the properties of both the sections and the properties of the materials and see what % of the load each is carrying. Referring again to reinforced concrete, it's so weak in tension that you often just neglect it in the sections that carry only tension.

 

Only crank I've jacked up was a Truvativ carbon MTB one that ended up with loose threads after a mega pedal strike. I've got 4 sets of newer fancier XO and XX SRAM ones that have never had problems (nor have any of my 20+ year old aluminum cranks of any sort).

 

SRAM’s response to my email is as follows:
“As I understand it, the foam core fills up the interior of the crank arms, reducing flex and increasing stiffness by providing resistance to flexing motion. The exact details of the foam are likely proprietary and not something that we could share, but having seen it myself, I would say it is a stronger and stiffer material than you would expect when you hear "foam." The XX SL and XX1 are both very solid cranks even without this, they are the lightest that we offer for MTB and are primarily XC focused, so if you are going that direction they would be a good choice. X01 and XX are geared towards more what you could call "all mountain." That being said, I have had a pair of XX1 cranks on my gravel/touring bike for 4 years and haven't ever felt like I needed more stiffness or anything like that. I am a larger rider and have ridden this bike hard and fully loaded on singletrack, sand, forest roads, etc and no complaints at all! It's hard to go wrong with either!”

I am still curious about their claims that the foam increases strength, but the email and some of the responses here seem to point towards something like aluminum foam or other extremely rigid structural foams that I have learned about from this thread and in my own research over the past couple of days.

At the very least, it points towards X01 having more overall stiffness and resistance to pedal pull out, which means I’ll keep it on the singlespeed that I’m cranking hard on pretty much all the time. I have followed up to the email to ask if they are using “strong” and “stiff” as synonyms, or if they claim the internal foam makes X01/XX more impact resistant somehow.

 

I was very much focused on bending stiffness and strength in this application, but I will now concede, if the core material has a high shear stiffness and strength, it could certainly be above that of hollow crank.

Given SRAM's track record of crank failures, I'm not sure I really trust what they say though. That could be entirely manufacturing related though, and perhaps they have sorted it out and made the designs more robust along the way.

 

As a follow up, I am trying to keep up with you engineer type folk as well as possible. I am but a layman, and am gleaning what I can

 

Not sure if it helps you, but even many engineers not familiar with sandwich theory or composites may not have seen this.

For sandwich beams it is necessary to account for transverse shear deformations. In classical engineering beam theory these are rightfully neglected as they add only marginally to the total deformation of beams with relatively high shear stiffness, such as beams with homogeneous cross-sections.

My education and experience was in that of classic beam theory with high shear stiffness. This quote is from a text I'm reading about this subject.

https://www.diva-portal.org/smash/get/diva2:1366182/FULLTEXT01.pdf

It gets pretty hairy, would not recommend unless you're a sicko like me.

Cranks are not really sandwich panels - again the high shear stiffness of boxed shell may be quite a bit more than that of the core. But, as I'm reading, some of the core foams can have have a very high shear stiffness. They also have the highest density, so it would make sense for them to not use them in their lighter cranks.

 

completely agree, I doubt a crank arm gains much from a foam core unless they are really skirting the edge of what's possible with the thickness of the outer skins. But yeah, not impossible.

SRAM’s response to my email is as follows:
“As I understand it, the foam core fills up the interior of the crank arms, reducing flex and increasing stiffness by providing resistance to flexing motion. The exact details of the foam are likely proprietary and not something that we could share, but having seen it myself, I would say it is a stronger and stiffer material than you would expect when you hear "foam." The XX SL and XX1 are both very solid cranks even without this, they are the lightest that we offer for MTB and are primarily XC focused, so if you are going that direction they would be a good choice. X01 and XX are geared towards more what you could call "all mountain." That being said, I have had a pair of XX1 cranks on my gravel/touring bike for 4 years and haven't ever felt like I needed more stiffness or anything like that. I am a larger rider and have ridden this bike hard and fully loaded on singletrack, sand, forest roads, etc and no complaints at all! It's hard to go wrong with either!”

Definitely a response from a salesman, not an engineer lol. Given the amount of time and money devoted by other industries (aerospace, marine, defense, etc) to composite R&D I highly doubt SRAM has developed a "proprietary" foam, but that doesn't mean they'll tell us what it is. And they might have figured out a clever proprietary manufacturing process.

He's not wrong about how these materials don't feel like foam though. High density polyurethane foams are most common, although there are tons of others out there. At least AFAIK based on experience w/marine products, it usually spec'ed by density in lb/ft3, from 2 lb that feels "foamy" and is basically just for flotation to 16 lb which feels like a chunk of hard plastic.

Also seems suspect that the volume of foam inside the crank arms could entirely account for a 2 oz weight difference. Assuming the void inside the crank arm is about 15 cm x 1 cm x 2 cm, filling it with "16 lb" PU foam (about 0.26 g/cc) would still only account for about 7-8 g per arm. But it is possible that it would still enhance stiffness and impact resistance.

I have followed up to the email to ask if they are using “strong” and “stiff” as synonyms

This is actually a great point. Not the same thing at all.

 

Q: for incompressible foam insert to spread the load around stiff box section skin effectively,
do they have to ensure they fill it 100% full then make it air tight so that any inward force against any side of the skin increase internal pressure which then resisted by other side of the stiff wall?

If this is how it works. They could also fill it with any incompressible liquid, right?
It's just that foam is lighter?

 

Q: for incompressible foam insert to spread the load around stiff box section skin effectively,
do they have to ensure they fill it 100% full then make it air tight so that any inward force against any side of the skin increase internal pressure which then resisted by other side of the stiff wall?

If this is how it works. They could also fill it with any incompressible liquid, right?
It's just that foam is lighter?

the foam would have to be bonded to the skins and fill the cavity, but in this case it's not the outward pressure that creates the stiffness, it's the way the foam transmits forces from one side to the other. A liquid (or gas) won't work - it has to be something rigid.