Every dent is unique so I think it is ok to ask this question once more. I have a Stevens Superflight 2020 which has a dent on a frame: dentpic1 dentpic2 enter image description here

My plan is to create some carbon fiber sleeve around the dent, just to make it a bit stronger. Is it a reasonable solution?

UPD: Impact occured around a year ago. I rode about 100 km on this frame from the time of impact.

UPD: I ride my bike mainly on asphalt roads but also on country roads with some sand, dirt, bumps, cracks etc.

UPD: It happened when I secured my bike on top of my car with Thule ProRide 598 car rack. And then I successfully hit my bike into the cieling of an underground parking lot. So the arm of a Thule rack was grabbing down tube around the bottle holder. It bent the bottleholder and and forced it into the down tube. So you can see those two bent marks - it is where the alluminium frame of bottle holder impacted the bike tube. I have two exact bottle holders and the second one you can see on the photo. The main impact occured to the front wheel which hit the parking roof. So there were two main points of contact: Wheel iself and Thule arm. Wheel bent a bit and was repaired by a mechanic on a machine. Actually right after the accident I rode 10 km home and didn't notice any damage at all. I noticed damage to the wheel only when decided to change front wheel dynamo hub to other model. Thule arm pushed bottle holder into frame and

UPD: My insurance doesn't cover this case.

Conclusion for now 26.05.2023: I will try to create a carbon fiber sleeve. I will fill the dent with some Bondo putty, just to recreate the frame shape. Than I will use solder acid to prepare aluminium for a glue contact. Than I will apply glass fiber as a first layer to prevent aluminium from galvanic oxidation with carbon. Than I will apply around 3 mm of carbon fiber and cure it with a heating lamp.

As far as we investigated that down tube is a subject not only of tensional but also of torsion forces (that is why this model has sort of rectangular shape I guess) I think it will be a reasonable solution to create some kind of support in the place of a dent. It also should prevent any kind of aluminium wear or corrosion in that area.

I will try to post updates on my progress here or in other post.

Also, I will try to do this because I have time and effort. In other case I would just let it be and ride normally with periodical "health check ups" of the down tube.

  • 4
    I couldn't find an graphic scheme of the forces applied on a downtube, but I think it is mostly tension (longitudinal stress), so it's not really weakened by a dent as long as it has no cracks and needs no enforcement sleeve. However, I'm no mechanical engineer, hence I'm not posting this as an answer.
    – DoNuT
    May 24, 2023 at 11:25
  • 2
    That is a good one regarding graphic representation. Found this 46 minutes video with some graphic material: youtu.be/Uk9Nu3Wuh3s From what I could understand, it seems that this lower tube is affected by comparably low forces with horizontal vector of application. You can see some good picture on 25:05 May 24, 2023 at 12:52
  • 1
    +1 DoNut. There was the “Slingshot” mountain bike which had a steel cable instead of the down tube. Cables can only take tension forces. I think down tubes usually fail along the headtube weld. cdn.shopify.com/s/files/1/2318/5263/articles/…
    – Michael
    May 24, 2023 at 13:51
  • 3
    Due the cause of the damage, it is possible (likely?) not just the frame is damaged. Did the front wheel or handle bars hit? If the handle bars, strongly suggest a new stem and bars and strip down and inspect the head tube area and fork. If front wheel, check the head tube, fork and axle. Failure at these points likely causes a serious accident so is far more concern than the dent.
    – mattnz
    May 25, 2023 at 21:31
  • 2
    @AndrewMorton Do note that this part of the bike frame is loaded in tension and torsion, whereas your coke can example is purely in compression. Not a perfect analogy.
    – MaplePanda
    May 26, 2023 at 1:58

7 Answers 7


In general, it is definitely possible to repair some types of aluminum damage with carbon. Raoul Luescher discusses one such repair on his YouTube channel. That said, I would consider finding someone with expertise in the aerospace industry who has done this type of dissimilar materials repair. The issues with a DIY repair might include:

  1. You need to bond (i.e. super high strength glue) any carbon reinforcement to the metal. You would need to prepare the surface. Do you know how? I suspect that means removing the paint and meticulously prepping the surfaces - this isn't an elementary school arts class, this is something that could be under a great deal of stress. I don't know how they prepare the surfaces in the industry.

  2. You don't know exactly where is optimum to place the reinforcement, and how the fibers should be oriented.

As an alternative, you could just leave the frame be, but inspect the dent periodically. You'd especially want to look for signs of cracking. In medicine, this would correspond to watchful waiting (aka active surveillance), which is used with some types of slow-growing cancer (e.g. many prostate cancers). There are some factors that I think weigh in favor of watchful waiting:

  1. I think you have a hybrid bike used for short commutes. You aren't taking it over rocky trails. The loads on the frame are going to be smaller than a performance oriented bike would see.

  2. The aluminum tubing will be relatively thick.

  3. If the downtube fails during a ride, you will probably notice the symptoms first (e.g. major changes in how the bike rides, noise), and a failure shouldn't result in loss of control, i.e. you should be able to come to a stop in time. If we were talking about damage to the fork steerer or the stem, this would be very different.

Basically, I don't think the frame is necessarily unsafe to ride. It has been compromised. There ought to be enough of a safety margin to keep riding it in city conditions. Naturally, anyone in this situation should consider their risk tolerance. For example, I'd be confident of coming to a controlled stop if the downtube failed. Thus, I might be more risk tolerant than average.

  • 2
    I'd be confident of coming to a controlled stop if the downtube failed. I'm not so sure about that. If the downtube fails, the top tube will immediately be the only thing holding the bike up. And all the static weight and dynamic loads will concentrate the forces right where the top tube meets the seat tube, and that load will be trying to bend the top tube upwards compared to the seat tube, with an effective lever arm between the top tube/seat tube joint and the front tire contact patch of well over a meter. I'd think it's more likely the bike instantly collapses and the cranks hit ground. May 25, 2023 at 12:12
  • 2
    (cont) I don't think a total failure like that is likely as the downtube is usually under tension and the dent probably won't weaken the downtube's strength in tension much, but it'd almost certainly be catastrophic. A failure from a bump/impact that pushes the front wheel/fork rearwards and causes the downtube to bend is much more likely IMO. That would likely result in steering being compromised by the front tire hitting some part of the frame and might even push the front wheel far enough back that it winds up with negative trail. So again, I'd think losing control is likely May 25, 2023 at 12:31
  • Great information on the topic. I already ordered some carbon and glue. Now I'll order glass fiber and will use some heatlamp as well. I need to think how to apply carbon properly. What shape to use. Fill the dent with carbon, or fill the dent with some mold first, and than warp it with carbon. So carbon will replicate the beginning form of down tube. May 25, 2023 at 14:15
  • 1
    At University, a friend’s bike had a crack in the down tube just above the bottom bracket. As it failed, the bike got lower and lower until she couldn’t turn the pedals, over several minutes. (This is an anecdote, not an answer, and yours may of course fail differently) May 25, 2023 at 17:52
  • @user7761803 what material was that bike? If I were to guess from the description, I'd suspect steel. Aluminium has a tendency to fail more violently. Still, I agree with the answer: for me I'd definitely take the risk. It's true that a down tube failure could be sudden, but it would still not be the uncontrollable guaranteed-crash as with a head tube. In fact, the gear- and brake cables should be enough to prevent the bike from completely collapsing from a failure in that spot. May 26, 2023 at 16:45

Based on Simon's observation that this is a rare and uncommon 64cm frame, so personally I would do as much as possible to extend its life.

I've had a lot of difficulty finding anything affordable over about 58 cm.

  1. If you wrap this area in anything permanent like carbon fibre, then you loose the ability to inspect it.

  2. If you try and "un-bend" anything, there's a very high risk of causing a crack.

So my approach would be to strip off all the paint from this area of the frame, and then apply several layers of clearcoat.

That allows me to do ~weekly inspections looking for changes using a strong localised light at different angles. If nothing changes for a number of months then you could decrease the frequency of checks to about monthly.

It can also help to take photos of the area periodically so you can tell if something is changing. You can also write on the frame with sharpie, circling or otherwise noting the start/end of any features.

Another possibility might be to make a "cast" from carbon fibre or fibreglass, such that there is a pipe-shape. Cut that twice to make two long semicircular half-pipes, and put one on either side of the downtube. Use at least four suitably-sized hose clamps or jubilee clamps to hold it all together.

Downside, this makes inspection harder so you're more likely to skip it. Also, dirt and water will get in and sit in any gaps. Finally this will also pressurise the areas of downtube that are distorted out and could interfere with your pedal cranks or feet.

  • Good suggestion. I would feel a lot safer being able to see that the frame is fine through the clear coat. With any DIY "repair" I would worry all the time if the repair worked or not.
    – Robert
    May 25, 2023 at 21:46
  • Unless an engineer does some kind of stress analysis simulation for this very exact shape, i.e this particular dent, and concludes that it is repairable and adding material around it in certain ways will help prevent a fatigue crack, I would not trust my repair. I would think of my repair as something to hold the two ends a little bit together AFTER the failure, not something that prevents it.
    – Robert
    May 25, 2023 at 21:57
  • @Robert is your car insured ? Why would you not claim on insurance for this accident? Is it a large excess? Given the bike is only a few years old and an expensive belt drive model, I'd suspect that a multi-hundred dollar excess is still far less than the brand new replacement of both bike and roofrack. Insurance is a scam, but its times like this you need to use it for your own piece of mind. They will have to pay for an identical bike of the size you need.
    – Criggie
    May 25, 2023 at 22:10
  • 1
    I used 1st person verbs to write the comments above, but I'm not the OP. I wanted to describe what I would do if I was in that situation. Sorry, english is my second language. Good point with the ensurance though.
    – Robert
    May 25, 2023 at 22:17
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    Good answer for the first part. The half-pipes idea however is unlikely to accomplish anything useful, not without adhesives. May 26, 2023 at 16:38

To be perfectly honest I wouldn't consider that bike safe to ride. Aluminium isn't as stretchy and flexy as steel. A big dent like that causes a pretty substantial weakening of the downtube and if it starts deforming more under heavy load it can bend pretty much without warning.

A carbon fiber sleeve could help, but it's not clear to me how you intend to fix it in place. Keep in mind that aluminium expands and contracts more with temperature changes than carbon fiber does, so any kind of glue between the two parts will likely get worn out pretty quickly. At that point you just have a carbon sleeve sliding around the downtube. Keep in mind that the downtube is the part of the bike frame that takes the biggest load. This dent will reduce its strength.

You should consider getting a new frame.

  • Isn't "not safe to ride" a bit exaggerated? Aluminum is considered a soft material and as far as I know, a down tube mostly takes tension forces and is not the highest stressed component. Plus, it is a city bike that probably doesn't have to take serious MTB or trail action that could make the frame collapse, but that's just a gut feeling. I don't think the frame will suddenly collapse due to that dent, to be honest ...
    – DoNuT
    May 24, 2023 at 14:46
  • 6
    @DoNuT no really - aluminium is prone to cracking rather than bending, and it likes to do that quickly.. By comparison, steel will let-go slower.
    – Criggie
    May 24, 2023 at 19:29
  • 3
    @Criggie OK, but the beefy downtube on a commuter bike isn't a such risk as if you had the same problem on the forks or stays. I'm pretty sure plenty of beat-up commuter bikes are running just fine. I'd say the answer from Weiwen above is spot-on. Easy riding, observering if the dent further deforms or cracks are building without too much risk of a fatality. Would be a different story on a MTB or some road bike that gets thrown over alpine climbs and technical descents ...
    – DoNuT
    May 24, 2023 at 19:36
  • 4
    @DoNuT the downtube is also subject to torsion forces from pedaling. The main thing I’d be worries about is this thing getting worse so quickly you don’t notice before it’s too late. Also, it’s a size 64 frame so it’s a fair bet the rider weighs more than the average cyclist (by virtue of being very tall). If this was a MTB, it’d go straight in the bin. For commuting… maybe not. But personally, I wouldn’t trust it.
    – SimonL
    May 24, 2023 at 20:49
  • 1
    @SimonLundberg Great spotting - I missed that little detail. As a fellow tall rider, knowing how hard it is to find decent frames over ~58cm, I'd definitely consider go the extra distance to keep this bike useable.
    – Criggie
    May 24, 2023 at 22:28

This does not answer the question, but concerned my comment will get lost in the noise and this is a serious consideration.

Call your insurance company, the bike is likely a write off.

It is not just the tube dent that is the problem, mechanism of damage - being on a roof rack and hitting ceiling of car park (see comments) - has me concerned about the 'front-end'. The impact has possibly damaged one or more of the stem, handlebars, head tube, steerer, fork, dropouts and front axle. Failure at most of these locations can cause a serious crash, and the parts may be cracked, which could lead to sudden failure.

At a minimum, I would identify the contact point(s). If the handlebars I would be replacing the bars and stem, removing the fork and headset and inspecting the head tube for damage and checking the steerer is still straight. If the front wheel, check the wheel, axle, fork dropouts and inspect the fork for cracks and check it is not bent.

  • This totally answers the question - by replacing the bike because of the car accident.
    – Criggie
    May 25, 2023 at 22:07
  • I would go as far as to say because of how the damage has been caused, this is the ONLY answer. The force required to do that would be anything but insignificant so the question should be what other damage is there visible or not
    – Hursey
    May 26, 2023 at 4:19
  • 1
    THanks for the answer. Unfotunately my insurance doesn't cover such cases. The impact occured to the front wheel. So there where two main contact points: the front wheel and the down tube, where Thule rack arm holds the tube. So the wheel was bent a bit: mechanic easily repaired it on the machine. May 26, 2023 at 8:23

Can you fix this with carbon fibre composite plastic? Yes.

Should you? Maybe, but can't hurt.

How would you fix it? Just do a wet layup of unidirectional carbon fibres aligned parallel to the tube, using two component epoxy. Around 20cm long, centered on the damage. Either calculate how thick a carbon fibre epoxy composite would need to be be strong enough as a downtube if the aluminium wasn't there, or just go with 3mm or so which should be plenty (for a one-off, the cost of the fibres isn't really that high so no need to save thickness). Do not push the fibres into the dent, rather fill the dent with the same epoxy that you are using with the fibres, at the same time as you are putting the fibres on. After it's cured, put some paint on top.

No need to overthink this. Here is why:

How strong is an epoxy-carbon fibre-aluminium composite tube of the suggested thickness? Ridiculuously strong. 3mm wall thickness for the carbon fibre epoxy composite is complete overkill plus the aluminium inside, even if you disregard its structural strength, is a great filler that will prevent buckling of the carbon fibre epoxy composite (while the cf-epoxy composite prevents buckling of the aluminium at the same time).

How would this tube fail? It would bend. Consider tensional, torsional and compression loads. Under tension, the tube won't fail anyway (and by the way that's pretty much the ideal case if the bond or the tube is loaded: uniform and in the direction of the fibers). Under a torsional or compression load the tube would fail, if it fails, by buckling/bending. Buckling at the spot of the damage enables the tube as a whole to bend. Because it can't bend within the composite tube, the ways through which it could buckle are greatly reduced. Also to buckle in the composite tube, it would need to separate from the composite tube and deform the composite tube, which isn't going to happen.

Why can't the tube bend? Because the cf-epoxy composite sleeve is at its strongest against a bending force, if you align the fibers like I suggested.

How is the interface between the carbon sleeve and the aluminium stressed? Barely. Basically it just needs to stick on.

Does epoxy bond ok with alumium? Yes. Just removing the paint, lightly sanding the aluminium and thoroughly cleaning it with alcohol should be fine. A quick online search reveals that this could be improved by etching the aluminium with a strong acid solution like citric acid. But the acid needs to be cleaned/removed thoroughly, otherwise the bond is going to be way worse than if you didn't try with the acid. We don't need to concern ourselves with intricacies further than that, like the fact that there is an aluminium oxide layer on top of the aluminium which reduces the strength of the bond, because this isn't an application that requires a high performance bond.

Have you done this before? Not exactly like this, but I have fixed stuff before in a similar manner.

  • 1
    Why would the tube bend?
    – ojs
    May 25, 2023 at 19:28
  • 1
    Because that's what ductile metals do. How do you fix this? +1. See also, should you 'fix' this? no, the accepted answer says, "I don't think the frame is necessarily unsafe to ride." And somewhere in the middle here it says "dirt and water will get in and sit in any gaps" if you do a botched job.
    – Mazura
    May 27, 2023 at 5:21
  • @ojs In the spirit of my initial version of the answer, "Because that's what ductile metals do" is a great answer to your question. But I resigned to overthinking it anyway and explaining it in the answer.
    – Nobody
    May 27, 2023 at 8:34
  • @Mazura Personally, because I have the required materials around and have done similar stuff before, I would probably strengthen it. But no, I agree it's not necessarily required. I didn't want to go into that much detail for the answer, but regarding dirt/water/gaps: if you use some non-stick heat-shrinking foil to compress the thing while curing you should get a smooth finish great for painting.
    – Nobody
    May 27, 2023 at 8:38
  • @Nobody ductile metals do not bend on their own, but when there are forces to bend them. The main forces on the down tube are tension from supporting the rider's weight and torsion from pedaling. How do these bend the down tube?
    – ojs
    May 27, 2023 at 10:37

My progress on 03.06.2023.

  1. I removed paint from the area where I supposed the carbon would be. removingpaint1 First I tried to use grinder, but it was too slow and dusty. So I used special paint remover based on acids: removingpaint2 enter image description here It worked really fast. But make sure that you use protection, because it is really toxic and will burn your skin really hard. The result after removing the paint: enter image description here
  2. I applied some universal car putty with glass fiber: putty The result is next: putty2 putty3
  3. I started to apply carbon fiber. Carbon fiber 193 g/m2. Epoxy 619 and hardener 285. Glass fiber. I applied glass fiber as a first layer. Than I applied carbon fiber. Tried to make thinner layer on sides and thicker layer in the middle. After that I wraped everything with cooking sleeve and than wraped tightly with a duct tape. My goal here was to stick every layer together. It is not the best solution, because epoxy dripped from side on the bike frame, on the ground and etc. It was really messy. Unfortunately I have only photos after carbon hardened. carbon1 To cure everything faster I used infrared lamps: carbon2
  4. Next I started the hardest part. Grinding carbon to the form. I found out that carbon dust can damage your skin, so make sure to use protection. I really messed up this part, and will need to repaint my frame after that. Definitely it went sideways l, far from as I imagined. One advice I can give here: make sure to cut carbon just right in the size of what you try to cover. If you cover more it will be a real pain to grind the excessive carbon and not damage the paint. I covered a lot more. I used sanding circled of 80+ grid. So here are some images: enter image description here enter image description here enter image description here enter image description here enter image description here enter image description here enter image description here enter image description here enter image description here

So as a conclusion for now, I can suggest to really consider your ability to perform such "repair". It might be messy, dusty and needs considerable amount of free space and amount of tools, such as sander, screwdriver, sandpaper etc. Also, the quality of this "repair" is far from ideal. I'm sure, though, that it is better than leaving the dent alone. As far as I don't really care about messing up the paintjob.

Final result as of 06.05.2023: enter image description here enter image description here

  • 1
    Nice and detailed update! It looks ok in the end. After a little bit of paint it will look even better. Maybe you don't need to repaint the whole frame. Could use masking tape and paint just the repaired area. Maybe without even disassembling the bicycle, using newspapers to cover the larger components, like the crankset. It will look good even if the paint doesn't exactly match the old one. Or you can just use clear coat. Leaving the repair exposed. It will be an interesting conversation starter with your friends. A cool battle-scar!
    – Robert
    Jun 5, 2023 at 7:20

Whatever you did to that aluminum frame was strictly cosmetic and will not make the bike perform better. Carbon fiber is more flexible than aluminum and the aluminum will completely destroy the carbon fiber if the aluminum breaks anymore. The fibers need to be aligned correctly even on carbon fiber bikes and could buckle if not done correctly. Aluminum and carbon do not attach to each other. You would have needed two anchor points for the carbon fiber to attach to the aluminum. You just put a very ugly cuff on your bike. Good for clickbait tho!!

  • You say "carbon fiber is more flexible than aluminum;" doesn't it depend on the thickness of each, the orientation of the fibres, and the extent that the aluminium is compromised by the damage? It seems an overly categorical statement.
    – DavidW
    Dec 23, 2023 at 21:47

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