Is it possible to repair a dent on an aluminum frame using carbon fiber?

Question

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:

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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.

Answer 6

My progress on 03.06.2023.

1. I removed paint from the area where I supposed the carbon would be. First I tried to use grinder, but it was too slow and dusty. So I used special paint remover based on acids: 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:
2. I applied some universal car putty with glass fiber: The result is next:
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. To cure everything faster I used infrared lamps:
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:

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:

---

Answer 7

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!!