The Ins And Outs Of Damaged Aluminum Bicycle Frames


Has your Aluminum bicycle frame been damaged? Did you have a crash and it became cracked or dented? Let’s explore how you can manage this situation.

Aluminum has become very popular material for bicycle frames as it is light, rigid (when wide tubes are used) and relatively cheap. However, Aluminium is not as ductile as steel, the other common material for commuter bicycle frames. That means that Aluminium cannot bend as easily and is more prone to cracking in general use, and dents can happen after heavy blows such as if you have a crash or hit something hard.

Can you continue riding a bike with a damaged frame?

No. A damaged frame, either with a dent or crack will become weakened. The tube might last for a short time, but there is a huge risk the entire tube will buckle, and this can cause you to crash in an uncontrolled manner. Immediately review the damage with either a bicycle shop or an Aluminum welding expert.

Can a cracked aluminum bike frame be welded / repaired?

In some cases a cracked aluminum bike frame can be welded, but it is complicated, it will take a lot of your time (1-2 weeks of running around) and is expensive to do well, plus the frame will become heavier. Also, some cracks may just be too difficult to be safely repaired. If it is looking too difficult, unfortunately you may need to simply buy a new frame or bike.

Of course, the first thing you want to do is fix the frame. It should be easy right? Not quite.

WARNING: Welding thin wall Aluminum tubes as used on bicycle frames is very difficult and should only be done by trained and experienced professionals. Welding a bicycle frame will void any warranty on the bicycle and of course, I can take no responsibility for the safety of the weld.

Points to ensure before welding:

  • Review the location of the crack:
    • If an existing weld has cracked, be careful – welding on top of another weld is very risky as the original weld will have already affected the surrounding material
    • If the crack is near the wheel drop-outs do not repair it, as the area is thin, the loads are high and the positioning needs to be accurate to attach the wheels, and sometimes gears, brakes or cables in a tight space
    • If the crack is near the bottom bracket area (pedal cranks), this area has many welds due to the 5 tubes welded together in a small space, so this will be impossible to repair.
    • If the crack is on the head tube this cannot be repaired easily. The trouble is that the two bearing sets that hold the forks in place could be affected by the heat from repair welding. If the tube is distorted in any way and the bearings cannot turn, then you cannot turn your front wheel!
  • A filler or patch material will need to be added. This needs to be welded across the crack. You will need to make this, or have a piece made by the welding expert.
  • Talk about the options with a welding expert. If they say something is not possible, or is a large risk, listen to them! I personally talk with a car radiator repair shop, as they repair thin sheet Aluminum all the time. Bicycle shops typically do not repair Aluminum bike frames, but they may be able to recommend someone who does in your area.
  • When preparing the area for welding, all paint, coatings and nearby clip features will be stripped off the Aluminum. It is worth considering if you want bare Aluminum around the weld after, or if you want to spray paint it afterwards.

Example of my welded Aluminum mountain bike frame that was cracked on the seat tube just above the join with the top tube. Note that the patch material was wrapped from one side of the top tube, around the back of the seat post and to the other side of the top tube, with welds right around it’s edges.

The local area was then painted black to best cover the weld marks.

Can a dented frame be fixed?

A dented frame can probably be fixed if the dent is on the top tube, seat tube or down tube and the tube is round, but it is complicated, and will take some time, and unless you have access to a lot of equipment it will cost some money too. It might be easier and safer, and cost around the same, to just buy a new frame.

Steps to repair a dented Aluminium frame:

  • Make or buy a wooden “smoothing” tool – this is two blocks with a semi-circle cut out of each that when put together make a circle which is the diameter of the tube you want to remove the dent from
  • Remove the paint from the local area
  • Clamp the smoothing tool to the frame, and rotate the frame/tube in the tool. The harder you can clamp, the more of the dent you will remove
  • Fill the remained dented area by braising, and sand the surface flat
  • Paint the frame close to the original color

For more detailed instructions this video is a great reference.

Can a bent Aluminum frame be straightened?

A bent aluminum frame can’t be straightened. Aluminum, unlike steel, does not bend well. So if the tube bends one way, it will likely crack if bent back, and it will already be cracked to some degree anyway, even if you cannot see it. So, if the frame tube is bent, unfortunately you will need to replace the frame or bike. 

Are there dents/cracks you don’t need to worry about?

The only If the damage you should not worry about are cosmetic scratches – that means only the paint and undercoat have been removed – then this does not need to be repaired. Aluminium does not rust, so this is only a cosmetic problem. If you don’t mind this, then it will be OK to leave the scratch.

Lifespan of an aluminum bike frame?

There is not a specific lifetime for the frame, but its life is determined by a combination of the number of heavy loads on the frame and the temperature. Design guidelines for bicycles require 100,000 cycles with a high load applied on the pedals. Depending on your size and riding style this may mean 5 heavy hits per ride or none! Therefore, if the bike frame is older than five years, and it seems to feel or sound strange, get it checked by a trusted local bike shop.

Signs that your bike is coming to its end of life?

Be alert – the first sign of a weakening Aluminum frame is a creaking sound. If you hear creaks when pedaling hard, check the area for small cracks. Then decide on whether you want to take the repair adventure or simple replace the frame or bike.

In the example of my Mountain Bike, there were some creaks in the frame when I pedaled hard, and after some inspections I saw the crack on the seat tube.

It can be confusing, as in the same area the bottom bracket bearings can creak (my bike currently has this issue) and the chain can creak. But If you see hairline cracks like in this picture, you have a problem.

Example of a hairline crack – if you see this on your bike stop riding it and replace the frame!

As the crack is near other welds in the bottom bracket area it will be impossible to fix this one.

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In conclusion

As we’ve seen, most damages to an aluminum bike frame are just too difficult to and not economical to fix. If you have a dearly beloved bike with much sentimental value attached to it that you want to salvage, you may give it a try, but in most cases you’re probably better off buying a new frame altogether.

Happy pedaling!

Simon Faneco

Simon is a bicycle engineer and entrepreneur from Australia currently living in Switzerland. He's the inventor of the first true center mounted CVT for bicycles. Follow his invention at ratiox.ch.

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