Using a damaged bike on the trainer

Now that the weather has gotten a little nicer I’ve been splitting my rides between indoor and out. Some days I end up not deciding which route to go until shortly before I ride based on the wind and weather. Anyway, it’s gotten to be a bit annoying taking it on and off each time so I am looking at buying a cheap bike that matches my current setup to just stay on the trainer 100% of the time.

This brings me to my question. Is there any issue with using a cracked (it would be carbon) frame on the trainer? Every so often I’ll see a bike for crazy cheap that has crack and wonder if this would be a ok route to go down. I’ve tried searching around for opinions, but most google searches lead to threads about if using a bike on a trainer can cause it to crack. I imagine the location of the damage is probably key. Something in the seatstays and/or chainstays would be more problematic. The one I saw recently was a small (<1 inch long) crack in the top tube. I’m using a direct drive trainer and rocker plate if that matters.

Impossible to say.

Someone else’s anecdote has no bearing on whether you will or won’t get injured using the damaged frame. This is a personal decision with regards to risk. IOW, it’s 100% your call.

Would I use a cracked frame on a trainer? Maybe, if I found some classic frame that has some nostalgic/emotional meaning to me, and the I deemed the crack to be safe for indoor use. As for my tolerance on crack size and location, couldn’t tell you until I’m standing in front of the frame, making a decision whether or not to buy.


If you do you have to promise to put the ‘afterwards’ photo on here.

In the mean time, please don’t add to your country’s probably overworked health service :slight_smile:

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First, I’ve had my bikes on trainers for 30+ years and have never had a problem. When I ride a trainer I use common sense. I’m not trying to do high torque out of the saddle efforts that would highly stress my frame.

But to your question - I would say it depends. If you find a screaming deal on a broken frame, I’d say repair it. Carbon fiber doesn’t seem that hard to repair especially if it’s a clean, easy to repair break. You can watch all sorts of youtube videos on carbon bike repair.

That said, the other option is to get something bulletproof like an old Litespeed or some other well built trainer bike that isn’t carbon and doesn’t need repairing.

Yeah, that is kind of where I was sitting at on it. It’s a name brand frame with ultegra 6800 on it. Price is $500 which is about how much the groupset and parts (stem, bars, seatpost, seat) would cost to pick up used on ebay. So if the crack did seem worse than it appears, I could just move the parts to a cheap frame. The horizontal crack in the red is on the top of the top tube and about an inch long

Interestingly enough, I work at a hospital. On the ward where the COVID an ‘rule-out’ COVID patients are placed.

I feel pretty confident that a fall at 0 mph, indoors, in a room with a carpet, from bike height is less likely to add the my hospital/health systems overworked service than people sitting around watching netflix all day during quarantine. Mostly just asking about thoughts perhaps the lack of stress (or increase in stress) on certain points of the bike might be when riding indoors versus outdoors.


Careful on that assumption. A local rider got bucked off his bike when a simple saddle mount bolt failed. He flipped over the rear of the bike onto his head and proceeded to get wedged between the rear wall and trainer in a nasty heap. No hospital visit, but we was out of commission for over a week off the bike, and took weeks to get back to speed.

Not saying it’s typical, but the “no speed = no risk” is not a safe assumption. If the right part fails at the wrong time, you can be in a world of hurt. There’s enough potential for “good” part to fail on a bike, I don’t see introducing a known “bad” part as a good idea.

If you want to save some cash, shop around the used marketplaces and find a nice, cheap , lower level or older road bike that is in what at least appears to be “good” condition.

Firstly, thank you for all you are doing to support your country’s health system.

Secondly, purely from a point of interest to me, I would be interested to see if the crack in the top tube, or wherever, did eventually fail. Weirdly, I think a gallery of before and after photos, might help other riders determine whether their ‘chip/rub/crack’ is safe to ride. And it may well be that a crack on the top tube is absolutely fine on a trainer, but that a crack on a seat stay isn’t fine at all. And it will depend on the size etc.

Thirdly, I want everybody to stay as well as possible. I appreciate that there will always be a risk of injury from everyday life, and I would want everybody to minimise that risk (particularly for you now that I know how important your job is).

Fourthly, echoing Chad’s point. I have come off my bike in much the same way. The saddle bolt sheared off when I returned from an out of saddle burst. The top of my leg scraped down the seat post. I pulled hard on handlebars as I sank lower and did a quite impressive wheelie. My other leg proved to be a great braking system for the high speed spinning back wheel. My feet were still clipped in and it was quite difficult to twist from a position where the bike is up in the air locked into a trainer and I’m sat on the floor! I didn’t need to visit A&E but I was quite sore and didn’t train for a week or so. Totally my fault (I check my saddle every few rides and it’s never come loose since) and I would feel bad at adding to the strain of the NHS.

Lastly, in the UK, I know that some hospitals are getting items donated to them. Even bikes. I wonder if you put a request out for an old road bike that you’d get a donation without any cracks?

Thanks again for everything you are doing. Stay safe and well.


Alternatively you can repair that for peace of mind. It would not have to be a nice repair with a proper cosmetic finish for road use, if it is only on the trainer. Resin and fiber glass cloth are available in every auto store. It is a work of minutes to sand back the paint finish and wrap it with resined GF tape. If really worried about the aesthetics paint it after. It will look butt fugly but wont fail without announcing itself (you will see the cracking spread out from under the GF patch). It should not do that as the loading on the top tube is in compression anyway.


Honestly, this could be patched well enough for a trainer. 2 pieces of metal duct taped to support it (one on top/ one on bottom) and I would ride it without any concerns. Or get the home carbon repair kits that are out there (think I saw one at Lowe’s) and patch it.

Would I ride it on the road? NOPE.

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Buy one, send it to a reputable carbon repair shop, and forget about it.

I am lucky enough to have two shops near me. A bike of my wifes has been to one and a bike of mine has been to another. When they repair the frames they come back stronger than new. Now, this isn’t always a positive thing as it can alter the frame flex depending on where the repair is. But, for the right repair or for a trainer bike who cares. It can save you thousands of dollars over replacement.

@mcneese.chad be honest. That “local rider” was you, wasn’t it? :rofl::rofl::rofl:

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Agree, the price seems good, so why not get it repaired by someone who knows what they’re doing? It’s not always as simple as sanding back to carbon and laying some new cloth. Also some damage can’t be repaired - and a reputable repairer would know when not to even attempt it and jeopardise someone’s safety.

Ha, luckily not. He is a team member though. But trust me. I’ve gotten hurt doing plenty of dumb stuff :stuck_out_tongue:

Aero engineer here…

For a top tube to be used in a trainer. It is that simple. Anywhere else… no. The majority of the effort in repairing a CF frame is in the cosmetics. Cutting and patching is relatively simple. So simple in fact amateurs are routinely certified to do this for homebuilt Aircraft. A somewhat riskier proposition than a bike frame for a turbo trainer.

The only caveats I would offer are the you need to ensure that you follow the resin guidelines to the letter and scrupulously prepare the surfaces and clean em. Most repairs fail through a lack of bonding usually due to contaminants like dust or oils from your skin.

And to check for other damaged areas. Coin check for delamination. If there is damage on any of the other tubes and the fail the coin check then get a different frame.

Good luck either way.


Just remember that composites have a habit of failing suddenly and catastrophically. It’s not like metal where you get deformation as a warning. This can happen from hidden and invisible damage too so I’d check any obviously damaged frame out very carefully for any other damage in a higher stress area

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What is the coin check?

Tap it with the edge of a coin. Damaged carbon sounds dull and buzzy, healthy stuff is sharp and percusive. It ain’t foolproof and even the more engineery ultrasound version can miss damage but it is good enough for military use as the saying goes and it will be good enough for this application.


This would be the approach I would take if I got the frame for cheap enough. Sand down the paint to the fiber to see the extent of the crack then patch a couple of layers of CF cloth and epoxy resin extending it a couple of cm beyond the ends of the crack. Aesthetics would not be a consideration and I also would not use it anywhere but on the trainer (preferably on rocker plates to lessen the overall stress)

That being said, I am setup to do non-structural CF work with all of the supplies and equipment. To get a strong patch, you really need to compress the resin and cloth in a vacuum to remove any air bubbles (voids). To do it properly you would need some release cloth, vacuum bag, absorbent, a vacuum that could do around 20psi of vacuum continuously for several hours while it cures. Supplies are easy enough to source online, the vacuum pump can get expensive unless, like me, you can find a substitute like a medical surplus. In the end, fir a one-off fix the cost of getting kitted up to do it may not be worth it (unless the frame was practically free)


That’s called gaining wisdom