Am I asking for trouble? BBS02 on carbon road frame

Hi Guys,

I know there's no black and white when building bikes to use in a way they weren't designed to be used, but I am just interested in peoples opinion on this build.

I have a 3 year old carbon road frame which I wasn't using as often as I should have been, so I built the bafang on to it to use as a commuter. It was a well speced lightweight bike, 6.3 kg before I weighed it down with all the electric stuff. I've changed the wheels with stronger heavier Easton alloy wheels, though I imagine there is a lot more load on the fork and steerer than it was manufactered to deal with.

I've broken two frames already with heavy stuff, one a columbus aluminium frame cracked at the front of the head tube. A really unusual place for a frame to crack, so I could only put it down to carrying a heavy load. Me at 80kg, and potentialy 20more kg with bag and battery etc. The other, another aluminium frame, cracked at the chain stays, where the TSDZ2 bracket was mounted. I may have slightly overtightened the mount, though looking back, its a bit of a silly place for the designers of tsdz2 to apply more stress on the frame.

Anyway, I've started using it and love the way it handles, just curious if anybody has any insights as to whether I am doing something very foolish or don't have anything to worry about.

Cheers,
Biggsy

Opinion only here.
If you keep the weight down and don’t crash you should be fine. A steel frame will survive most crashes to some extent. Cracks from crashes and stress can be welded. All other frames types may give you one or maybe two crashes and your frame is unsafe or just broken beyond use. If you are a careful road biker and never crash then don’t worry about the frame.

Skaiwerd said:

Opinion only here.
If you keep the weight down and don’t crash you should be fine. A steel frame will survive most crashes to some extent. Cracks from crashes and stress can be welded. All other frames types may give you one or maybe two crashes and your frame is unsafe or just broken beyond use. If you are a careful road biker and never crash then don’t worry about the frame.

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Add to Skaiwerd's opinion that steel will typically start making some kind of new noise, such as a squeak before the a complete failure...That reminds me to identify the source of the new squeak on my superfast build that I ride most of the time right now.

While I appreciate the great feel of a very light non-motorized bike, once you add a motor into the equation a few pounds really doesn't make a difference.

Some individuals have situations where the BBS02 "loosened" in the BB area (if the nut and "jam" nut weren't tight enough). Make sure you check occasionally since it could cause a problem on a carbon frame.

Thanks for your ideas guys.
I've used the bike for a month now and love it. It handles really well, doesn't feel like the frame is under too much stress.
You were right 2old

2old said:

Some individuals have situations where the BBS02 "loosened" in the BB area (if the nut and "jam" nut weren't tight enough). Make sure you check occasionally since it could cause a problem on a carbon frame.

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It came loose after the first few rides, but now tight has stayed tight.

Skaiwerd said:

If you keep the weight down and don’t crash you should be fine. A steel frame will survive most crashes to some extent. Cracks from crashes and stress can be welded. All other frames types may give you one or maybe two crashes and your frame is unsafe or just broken beyond use. If you are a careful road biker and never crash then don’t worry about the frame.

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I expect that even more than a regular crash on a carbon bike that this frame will be totally destroyed. The extra weight of it hitting the ground in an accident makes me cringe, but if it happens I'll be binning the frame for sure.

I'd encourage others to choose a bike for conversion which is really comfortable and handles well at the speeds they plan to ride.
Building this bike wasn't a question of weight at all, of course it's still really heavy. On the other hand I've done thousands of km on it before converting and will now do a lot more. Gears shift crisply and the geometry is comfortable at higher speeds. My work commute has now even taken me home over Col Aubisque (A famous tour de france climb) for a bit of variety and longer rides I wouldn't normally be able to do after work.

So much about risk management is considering failure modes.

The failure modes with any metal frames are fairly gentle compared to carbon fiber. When carbon fiber fails, it shatters in a spectacular fashion.

Carbon fiber frames are laid out with the strands going in specific directions to handle stresses. This is unlike metal tubular frames where, for the most part (sans butting), the tube is simply the same length the entire time and then joined at the ends.

The carbon fiber frame is was not designed to oppose the torsional loading that the mid drive motor puts on it. If you decide to keep going this way, I would regularly inspect for cracks. I wouldn't be willing to take the risk.

I built up high-end titanium and aluminum frames spec'd with top-tier components, so I sympathize with the desire for something that feels good and performs well. It looks like what you have rides nicely!

Waste of a nice bike. Get a used steel frame and have two bikes. Under 100.00usd

mystryda said:

The carbon fiber frame is was not designed to oppose the torsional loading that the mid drive motor puts on it. If you decide to keep going this way, I would regularly inspect for cracks. I wouldn't be willing to take the risk.

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Thanks mystryda, I appreciate your specific feedback. Where on the frame would the stress be most concentrated? I assume based on the idea you give of torsional load that it would be highest on the chain stays. They look very solid at the bottom bracket but a bit thinner at the hub end.

999zip999 said:

Waste of a nice bike. Get a used steel frame and have two bikes. Under 100.00usd

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999zip999 you've missed the point a bit. I already have a much nicer bike which I use on weekend rides, this was my rainy day bike. It is the same dimensions to the mm as my road racing bike. My point is really that I think too many people get a dirty old bike and put a bafang on it. If you are going to spend a lot of time on a bike, why ride an old clunker? Subtle differences in frame and fork angles make huge differences to how a bike handles, particularly at speed.

Biggsy said:

mystryda said:

The carbon fiber frame is was not designed to oppose the torsional loading that the mid drive motor puts on it. If you decide to keep going this way, I would regularly inspect for cracks. I wouldn't be willing to take the risk.

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Thanks mystryda, I appreciate your specific feedback. Where on the frame would the stress be most concentrated? I assume based on the idea you give of torsional load that it would be highest on the chain stays. They look very solid at the bottom bracket but a bit thinner at the hub end.

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Background to frame my thoughts:

A rider primarily delivers force down (even as much as we attempt to pedal in circles and pull up on the backside) in a way that rocks the BB side-to-side (from the perspective of the rider); a strong rider can feel this flex in the rear end and may need to trim their front derailleur to avoid rubbing and accommodate the side-to-side movement of the chainring. The riders "equal and opposite" forces are distributed fairly widely through the points of contact with the frame: handlebars/stem/headtube, seat/seatpost/seattube, and pedals/cranks/BB.=

The "equal and opposite" force of the motor is going to be relatively focused, since it only touches the frame in a couple places. I haven't installed a mid drive, so I'm going to address the two scenarios I can imagine. I suspect that "reality" has some aspect of both:

1) The middrive installs "tightly" into the BB such that it delivers an equal amount of torque counterclockwise (from the perspective of the drive side) as it does in turning the pedals clockwise. Imagine having a giant lever like a extra-long crank that's locked into place in the BB and can't spin. Say it's four feet long in our thought experiment and sticking straight up (and there's no seat in the way). Now straddle the rear of the bike, grab the level, and pull backwards. Think about how the downtube will respond at the juncture with the BB: there will be compression on the bottom of the joint and tension at the top. Likewise with the seattube, the will be compression on the CW side towards the downtube and tension on the CCW side as it opens up towards the rear wheel. Same for the stays: compression the CW side/top, tension on the CCW side/bottom. I don't know how to mitigate this.

2) The middrive installs snugly in the BB but can rotate slightly CW/CCW. As the motor turns the cranks CW the body of the motor as a whole rotates CCW. Now the motor body will be pushing up in front, and where it touches the downtube it will apply pressure. In the same way that the front of the motor is pushing up, the back of the motor, fixed in the bottom bracket, will be pushing down. The BB is built to resist down forces, so that's all fine, but I'd be concerned about the upwards force where the motor body touches on the downtube. I think this could be mitigated successfully by spreading the force over a wider area. You could, perhaps, take a six-inch length of larger diameter pipe (PVC?), slit it twice lengthwise so that you only keep a quarter (looking at the round end, like a quarter of a pizza), and silicone that in place along the downtube so that the motor presses against the tube and the tube presses against the downtube. At least then you wouldn't have a point-load on the pushing into the tube.

From my couple middrive motor install videos I watched, I think that the scenario is far more like 1) than 2).

I recommend you visit a couple of the frame building forums online, link to the documentation for your motor and an install video when you ask your question, and get their take. You're clearly and justifiably concerned enough to ask the question, so if you really want to pursue this then ask folks who have more specialized knowledge. The cost of failure could be very high.

Best of luck. Let us know what they say!

mystryda said:

1) The middrive installs "tightly" into the BB such that it delivers an equal amount of torque counterclockwise (from the perspective of the drive side) as it does in turning the pedals clockwise. Imagine having a giant lever like a extra-long crank that's locked into place in the BB and can't spin. Say it's four feet long in our thought experiment and sticking straight up (and there's no seat in the way). Now straddle the rear of the bike, grab the level, and pull backwards. Think about how the downtube will respond at the juncture with the BB: there will be compression on the bottom of the joint and tension at the top. Likewise with the seattube, the will be compression on the CW side towards the downtube and tension on the CCW side as it opens up towards the rear wheel. Same for the stays: compression the CW side/top, tension on the CCW side/bottom. I don't know how to mitigate this.

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Thanks mystryda,
Your insights have given me an answer that I am almost certain is correct. I'd never really considered the concept of the bottom bracket spinout, but it must be the case.The frame almost certainly WILL fail at some point, though I wouldn't expect the failure would be a dangerous one. The aluminum of the BB will detach from the surrounding carbon. I've had this type of failure before on higher grade carbon frames, resulting in an annoying wobble of movement between BB aluminum which is not bonded to the carbon properly.
I'll take your advice and ask frame builders, though I am sure none of them will agree with what I've done. That said, the bike is such a blast to ride that I'll continue until it does break. I've limited the acceleration of the motor in order to minimize the sudden rotation, in the hope that this increeases the frame lifespan. I'll let you know what I find out and post an update here when the outcome is another unusable bike frame hanging on the garage wall.

Biggsy said:

The aluminum of the BB will detach from the surrounding carbon

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Maybe not, I was just looking at this thread about people inserting an alloy spacer into frames with a BB30 bottom bracket.
https://endless-sphere.com/forums/viewtopic.php?t=80319

The spacer is simply pressed in and glued in with locktite. If the Bafang was really applying equal force in the opposite direction to pedaling then surely the BB30 spacer wouldn't stay in place very long?

Has anybody here got experience in using a pressfit spacer with mid drive?

I don't think it's guaranteed that the bb insert will loosen up. They are in there pretty securely. Carbon will cause corrosion in aluminum if the builder isn't careful, and that would cause a loose insert. Like I said on bikeforums, I'm more worried about the battery.

Has anybody here got experience in using a pressfit spacer with mid drive?

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I bought an SRAM BB30 to BSA Bottom Bracket Adaptor but in the end didn't use it. I didn't like the internal BSA threads when I saw it and being concentric would make things harder for me.

I have a Cannondale CaadX Ultegra 2019 (68mm BB30) that I was fitting with a Tongsheng TSDZ2 mid drive.
I ended up making my own sleeve. Found a piece of pipe (happened to be stainless steel - heavy but strong) 42.5mm OD and 37mm ID. Turned it down to slightly under 42mm. Couldn't push it in by hand but didn't need to press it in either. Was able to just gently tap it in with a soft hammer and didn't glue it in. I don't want to work too hard if it ever needs to come back out. There's very little room under the bottom bracket so I moved the rear hydraulic brake line over the BB, removed the front derailleur and cable completely, and have run the rear derailleur cable in a full length sheath along the frame away from the BB.
I was able to mount the TSDZ2 off center (low) in the BB sleeve and packed the space left at the top with softish aluminium craft wire. By the time the rear clamp, side securing plate, and lock ring on the TSDZ2 are secured the motor isn't moving anywhere.

I'm using the Open Source FOC firmware and it's very smooth and natural (throttle not fitted).

Just a quick update on this thread for those who commented.

I never got any real conclusive answer and that's probably because there isn't one.
I've used this bike daily and am now at just over 7000km
Its awesome to ride on the daily commute as well as in the mountains.I am really pleased with the build.

I did recently manage to improve range, ride quality and reduce stress on the frame by reprogramming the motor.
I reduced startup power, set speed to 100% in all pas modes and set assist current in 10% incriments.

Thanks to everyone who gave me some input to the original question.

Cheers,
Biggsy