Can Donor Bike be Carbon

[mmarlin]

I have a Trek 7.9 FX carbon bike I want to add a Bafang BBS02. Bafang site says the motor mount may damage the frame. Is anyone familiar with what Bafang is actually stating. Should I not convert my carbon bike or go with a different bike?

 

[amberwolf]

There's a number of threads discussing carbon frame/etc stuff in more detail (even some for this bike frame, I think), but the problem with it is that it's failure mode tends to be sudden and dramatic without any warning. If the frame wasnt' designed for the specific stresses that whatever you're adding to it creates on it, it may not be able to resist those stresses, and may break at those points.

Also, carbon frames tend to be thicker around the BB shell than the mounting spaces on middrives allow for, so it might not fit even if you wanted to try it. You'd have to check the required clearances on the drive you want to use, and see if your frame fits in them.

 

[mmarlin]

Thanks, I've read the threads, I'm not going to use my carbon bike, not worth the "sudden and dramatic failure".

 

[raylo32]

I would be OK with using a carbon frame with a mild motor like TSDZ2. But you'd need to be careful not to over torque the mounting hardware. After just installing a Photon on one of my AL bikes I would not recommend that be used with carbon since it has no real solid mounting hardware and needs to be rotated into hard contact with the downtube. That would not be good with carbon.

 

[Chalo]

For me, it would depend on whether the frame has a substantial metal sleeve lining the bottom bracket shell (which would be good) and how/where the motor will make contact with the frame should the unit loosen up.

My experience with BBS02 and BBSHD has showed me that the motor housing will sag down under gravity, then swing up under load and make hard point contact with the downtube if the mounting hardware loosens. That's not a big deal for most steel frames, but no good at all for plastic ones.

 

[calab]

sudden and dramatic failure".

Sounds familiar on the news with the sub near Titanic.
Its a wise move to not use Carbon.

 

[richj8990]

Carbon fiber is 5 times as strong as steel (per gram). Not quite 5 times stronger than aluminum for some odd reason.

"The myth that carbon is fragile again rears its ugly head, but it is just a myth. While both frame materials are susceptible to catastrophic failure, carbon fiber frames tend to be stronger per pound than aluminum equivalents."


I think 'the myth' came about maybe 15 years ago when the Chinese and other offshore countries began producing lower grade Toray (below T700) carbon fiber frames. They would crack. T700 came around, then T800, now T1000. I just bought a T1000 frame in February (normal usage, not for an e-bike) after riding T700 for 3 years and T800 for one year, and wow can I feel the difference on an XC track. Much stiffer and significantly better acceleration. That frame feels bulletproof.

If you put a high-output mid-drive on a cheap and very thin carbon frame, sure maybe it will crack. If you put a 250-750W mid-drive on a thick T800-1000 carbon frame, I really doubt anything bad will happen. It's just a question of fitting the motor and axle in the thicker frame. Now a rear hub drive w/o torque arms on a carbon frame...that's a whole different story. I don't know why people think mid-drives are out to destroy frames. The chain will probably snap long before the frame would.

 

[amberwolf]

There are multiple ways of measuring the strength of a material (tension, compression, deformation, shear, yield strength, etc), and they can depend on the shape of the material for how they apply, as well as how the system they are used in is designed overall. You can look up the charts for specific alloys of varying materials to see which ways each one is stronger than other materials. Like anything else, CF is strong in particular ways, and not as much so in others. As with any other material, this is true not just as a raw material, but even when it is assembled correctly for the design intent.

With CF it also depends not only on the CF itself, but on the bonding material and molding and curing process used to make the fibers into a "solid" material, and the methods used to weave or otherwise make the fibers into a mat, etc. Similarly the alloying and casting and drawing processes for metals affect their actual strength in a particular usage.

Frames (of any kind, but bicycle ones specifically in this discussion) are designed in general to take certain kinds of stresses in certain directions, and any strength they have against other kinds of stresses in other directions is coincidental and may be insufficient.

Whether or not a particular frame handles a particular non-designed-for load would have to be tested under the conditions it would need to function in to be sure it couldn't fail from that load.

 

[nicobie]

From a practical point of view. carbon fiber stuff is only good for racing and breaking records, where if you are not on the bleeding edge you lose, and for looks. Nothing wrong if you want it to look trendy, just be aware...

 

[lnanek]

Reminds me of that carbon fiber submarine full of rich people that imploded recently. Steel doesn't get weaker each dive. Carbon fiber did.

 

[raylo32]

A good carbon frame is actually very tough and resilient. And these mid motors really don't make any more power than a super strong human cyclist. And there is less torquing of the BB shell because the force is applied at the chainring and not at the end of the cranks. And another data point... I had a carbon seatpost get frozen in a frame once upon a time and it took me a couple of hours to extract it with saws and other tools. That thing was brutally tough.

 

[amberwolf]

The middrive problems that usually happen on the CF frames as reported here on ES and other places are where the drives or their mounting brackets meet / clamp to the frame and apply pressure where the frame wasn't specifically designed to have such pressure; the frame failures or damage varies but usually is some form of deformation or cracking at that point.

Similarly, pressure from downtube/etc battery clamps or mounts can cause similar damage, and bottle-mount battery racks can rip out the nutserts embedded in the tube if they were not designed for the weight and lateral torque variations they can see from batteries that are heavier than the water bottles or other accessories they were designed for. (the latter is true of any frame material).

Not all frames have such problems as they may already be reinforced or designed in a way that coincidentally happens to prevent this problem, but unless they were designed for a specific middrive or otherwise specifically designed and built around the idea of having one there that applies pressure or torque to the areas it does, then there's no guarantee it cant' have a problem because of it.

Even the very thin lightweight aluminum frames (like an old Scott frame I had once) can be damaged or fail the same way for the same reasons, except you may be able to see the spreading crack before it separates the tube sections if you are doing inspections. I dont' know if CF cracks are visible until they fail; I haven't had one to test this way and haven't directly examined evidence from ones that did.

The very thin cromoly steel frames don't usually fail because of it, but they can still have deformations from the pressure/torque in such cases (though they usually don't).

 

[raylo32]

Right. I mentioned that in my first post on this subject. Need to really take care on the mounting. Fasteners, clamps and with motors like a Photon where the motor case comes into hard contact with the frame. Lots of ways to mitigate all that stuff.

The middrive problems that usually happen on the CF frames as reported here on ES and other places are where the drives or their mounting brackets meet / clamp to the frame and apply pressure where the frame wasn't specifically designed to have such pressure; the frame failures or damage varies but usually is some form of deformation or cracking at that point.