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I had a serious accident, my fork suddenly snapped when riding. I sent my bike for repair (to get a new fork installed) and the guys at the shop hammered the frame in place, as apparently the frame bent from the crash. Now after the very scary crash I had, I am wondering if I should even ride the bike in that condition? I know it is impossible to predict how much time will it take for it to fail, but other than checking very often if the area next to the bumps changes, what else can I do? I know that getting new pieces welded is not an option as it is hard to weld aluminum and it is probably less expensive or equally expensive just to get a new frame.

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    @David Richerby, in the case Al frame & bending it back in shape it's clear cut. There is no way one can safely do that.
    – gschenk
    Commented Jun 3, 2019 at 12:50
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    @gschenk I did carefully say "I don't think anyone can say 'that bike is safe to ride'" rather than "I don't think anyone can say whether that bike is safe to ride."
    – David Richerby
    Commented Jun 3, 2019 at 12:54
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    Honestly, with a serious crash, you're unlikely to ever feel safe riding the same bike again. Just get a new bike.
    – Ian MacDonald
    Commented Jun 3, 2019 at 20:39
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    You have two choices if you want to enjoy riding the bike without worry. Replace the frame, or go to a different bike shop and get a second opinion (then replace the frame).
    – mattnz
    Commented Jun 3, 2019 at 23:02
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    Would be good to know exactly what the hammering LBS did. I cannot understand how a crash caused damage at the lower end of both seat stays. I'm guessing two spokes were replaced or do the red ones mark the valve stem ?
    – Criggie ♦
    Commented Jun 4, 2019 at 6:55

5 Answers 5

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If an aluminium frame has to be bent back into alignment, it's trashed. Aluminium cannot be deformed without causing weakness in the material.

If the bicycle repair shop literally hit the seat stays with a hammer near the dropouts to straighten the frame is probably weakened in those areas and is very likely unsafe. I personally would not ride that frame.

The bicycle repair shop should have told you the frame was un-repairable and not replaced the fork. Maybe they wanted to make some money of a repair job but I can't believe they passed up an opportunity to sell you a new bike.

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  • Or maybe hammering a frame that wasn't bent in the first place seemed like a good way to generate a new bike sale.
    – Andy P
    Commented Jun 3, 2019 at 12:04
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    Better not assume malicious intent when incompetence explains behaviour sufficiently.
    – gschenk
    Commented Jun 3, 2019 at 12:58
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    As always, in case of doubt abstain. And I'd keep out of that 'shop'.
    – Carel
    Commented Jun 3, 2019 at 13:41
  • @gschenk Unfortunately life has taught me that if you assume the worst then you usually aren't far wrong :(
    – Andy P
    Commented Jun 4, 2019 at 8:38
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For the long term, that repair should not be trusted and the point of weakened aluminum is correct. However, if this is your only transportation and you can't get anything else immediately, I would say it can be ridden until an alternative is found.

The front fork is two pieces, and if either side fails (or the center), the front wheel comes off and that normally results in an immediate, and often pretty bad crash.

If one of those weakened points on the rear wheel breaks, there is still another connection holding the wheel on (4 posts, two on each side). Depending on how it breaks, the wheel may twist, but it's probably not going to immediately fall off. In other words you will have some warning and the ability to stop (still have your front braking power).

So, again, long term - get a new bike, or argue with the shop that "repaired" it, but if its essential, I would ride that bike in the short term.

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    @Mast Uh, why do you say so? In real world, it is just a risk of it breaking, and the longer you use the larger the risk becomes. No bike is 100% safe to ride.
    – jpa
    Commented Jun 4, 2019 at 7:03
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    To me, saying that the bike needs to be replaced "in the long term" suggests some time in the next few years. I'd be OK with an answer that gave a message of "You should replace it as soon as you can but you're probably OK to ride it until your next paycheck or something, if you have to; be aware that this carries risks", but you seem to be suggesting "Yeah, you should plan to replace that eventually" which seems way too slow, to me.
    – David Richerby
    Commented Jun 4, 2019 at 9:40
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    I didn't mean to imply that long term. "Until an alternative is found" was meant to mean you should be actively looking for a replacement because the "repair" shouldn't be trusted for long.
    – JPhi1618
    Commented Jun 4, 2019 at 14:30
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    @DavidRicherby probably even more people, worldwide-speaking, drive wreck bicycles that are about to fall apart through rural areas where there are barely any cars. Others like to push their downhill MTBs through as dangerous terrain as possible. The point is, stop making assumptions about who does what and wants what, just point out what's likely to happen with bike X in situation Y. This answer is exemplary.
    – leftaroundabout
    Commented Jun 5, 2019 at 8:49
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    @leftaroundabout More than tens or hundreds of millions of people? I'd like to see any evidence at all for that claim. And downhill mountain-biking is just a blip on the radar; there are half a billion bicycles in China, the majority of which are in urban areas.
    – David Richerby
    Commented Jun 5, 2019 at 9:47
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Don't take it to a bike shop, take it to a reputable frame builder particularly one who works in aluminum.

He'll have the jigs and the experience to tell you what's wrong with the frame and if it's reparable. He may be able to remove the entire rear triangle and replace it for less than it would cost to get an equivalent new frame, or he may tell you that it's simply not worth it.

I once had a huge wreck on a steel frame which misaligned the whole bike. A builder was able to adjust everything back into alignment and told me that the little wrinkle in the down tube was something to keep an eye on, but should be OK for a while. I put many thousand more miles on that frame. Of course, that was steel and this is aluminum, so your mileage may vary, but a frame builder will be able to be the best judge.

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Without knowing the specific aluminum alloy and level of heat treat (if any) it's impossible to estimate what portion of the design safety margin has been compromised. Fortunately that's not important. The real question is how long to you want to ride a bike that may come apart unexpectedly? You're the only one able to make that evaluation.

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Bending aluminium does not cause sudden failure. Take a piece of aluminium and try it your self. Like any metal, repeatedly bending aluminium will gradually make it weaker. (This is a common failure mechanism for very old steel bikes). Unlike steel, aluminium has no "absolute minimum" value for softness (which enables you to specify intrinsically safe steel components). For this reason, aluminium parts have to be over-specified, because they always just get gradually softer and softer in use.

Bicycle frames (especially expensive bicycle frames) are designed to be as light as possible. For this reason they are shaped into the most efficient shape possible. If you dent a tube out of shape, it will not be as strong as it would be. If you dent a tube on any bicycle, it will materially weaken the tube. This is not because it is Aluminium or Steel: it is because it is a tube. On a bicycle some tubes are critical (the front forks, the head piece), some tubes are not critical (the cross bars: you can just keep on riding with broken cross bars), but will critical in traffic or when making a sharp turn on a fast downhill. And some tubes barely matter at all (the rear wheel posts), as you know if you've ever had one fail at speed in traffic.

The pictures you've posted show damage that hardly matters at all, at a location that matters very little. The effect of softening is marginal (bicycle alloys are selected for handling 1000's of repeated stress cycles), and it's a location where the change in shape is less than critical. The bike shop (which doubtless see lots of damaged bikes discarded), didn't even think it was worth trying to sell you a new bike.

If you ride a frame long enough and hard enough, it will fail. This frame may fail at the rear wheel, or, the way you're riding it, you may do the front forks again first.

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  • If you use any object "long enough and hard enough", it will fail -- that's just what "enough" means. The question is, pretty much, how long and how hard is "long enough and hard enough".
    – David Richerby
    Commented Jun 4, 2019 at 13:46
  • It is correct that "bending aluminum does not cause sudden failure". I don't see anywhere in this conversation that anyone asserts this as true.
    – David D
    Commented Jun 5, 2019 at 17:48
  • @David Richerby If you haven't had bicycle frames fail from fatigue, you haven't ridden long enough and hard enough. Many people haven't ridden a frame that long and that hard, and damage of this sort won't change that.
    – david
    Commented Jun 7, 2019 at 5:42
  • @David D Since nobody has asserted sudden failure, we can agree that sudden death, as questioned in the original post, is not going to happen.
    – david
    Commented Jun 7, 2019 at 5:42

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