18

I had an unplanned collision with a car yesterday (everyone is fine, thankfully) but my bike – a Giant Revolt 1 – now has a lovely new pair of dents.

I cycled it home from the collision (~6km) and it seemed “fine” although I was probably still in shock.

Looking at it today, and having looked at several other questions regarding dented aluminium frames, I’m trying to ascertain what the most likely failure is?

Is this frame likely to continue degrading/cracking over time (i.e. I might get several more months from it)?

Or is it more likely to fail catastrophically and fold in on itself?

I exclusively ride on paved roads although there are often small potholes.enter image description here

13
  • 2
    Most likely the top tube will fail, the front wheel will fold under you, and you will be thrown on your face. If I absolutely had to use the bike I'd try to straighten the top tube, then find a piece of something like steel angle-iron to clamp to the tube to re-enforce it. And stay alert for growing cracks. – Daniel R Hicks Aug 23 '20 at 16:35
  • 10
    +1 for realising you were in shock. That's hard to spot in one's self. Time to involve your insurance company to chase their insurer. Upshot - if it was driver's fault, you get a new bike out of it. – Criggie Aug 23 '20 at 19:35
  • 2
    Car hit me side on (my left hand side). I’m guessing the bike got tossed forward into something... or something. I was busy being flipped like a pancake over the hood and windscreen. Little sore today but very lucky and grateful not to have anything worse than a few bumps and bruises. – Rob Burke Aug 23 '20 at 19:53
  • 5
    I always plan my collisions, specifically to avoid this kind of problem. It would be foolhardy in the extreme to continue to ride this bicycle. – Strawberry Aug 24 '20 at 10:04
  • 6
    Just to chime in, I can assure you assuming the accident was the driver's fault, their insurance almost certainly will buy you a new bike. I would certainly suggest filing a police report if you did not yet do so and filing a claim against their insurance. – Vality Aug 24 '20 at 16:43
63

How is this damaged aluminium frame likely to fail?

Sorry. It's not going to fail, it has failed. It's no longer ridable. The head tube is near vertical making the steering very unstable and the front wheel has been pushed back and will overlap with your toes. The frame tubes are not dented they are buckled and severely weakened. If you ride it the frame will catastrophically fail (either when you hit a bump or brake hard) resulting in a nasty crash.

I hope that if the car driver was a fault you got their details.

4
  • 7
    I would say there isn't much point carefully checking over a piece of scrap. – Turksarama Aug 24 '20 at 3:43
  • 3
    OP, good on you for accepting this answer despite the bad news, FWIW, I'm another upvoter. Chiming in here to suggest that also investigate the front wheel, fork and handlebars, for the frame to sustain that deformation means the force has to be transferred somehow, the same force would have been felt by other areas (thankfully, not you :) – Lamar Latrell Aug 24 '20 at 20:15
  • @Michael there is a lot going on in the geometry of a bicycle. The steering axis as it intersects the road, the point of contact between tire and road, and the angle all inter-relate to provide stability, caster, and the tendency of the wheel to steer just the right amount into the lean (aids turning and stability). The accident clearly rotated the steering axis, moving its intersection with the road back, altering the overall geometry. It will affect stability and handling. – Anthony X Aug 25 '20 at 22:28
  • 1
    Personally, I would avoid using the fork and front wheel too, unless told otherwise by experienced mechanic after careful examination. – Mołot Aug 25 '20 at 23:32
17

Oh, that frame is done. It’s not worth saving a few hundred bucks now and spending thousands in the hospital later.

Make sure to check over the rest of the bike, especially the fork. It might have cracked from the impact.

8
  • 2
    The fork looks like a carbon fiber fork. If this observation is true, unfortunately, it is impossible to "check" a carbon fiber fork if you don't have access to a $100000 X-ray machine. Carbon fiber can weaken in invisible ways internally, and then if you're riding a carbon fiber component after collision or some other accident, it is possible the carbon fiber component catastrophically fails suddenly when just riding, without any warning. That's one of the reasons why I don't prefer carbon fiber. – juhist Aug 23 '20 at 15:51
  • 17
    @juhist There is no reason to spread this all over the site. You cannot check for hidden metal defects without expensive defectoscopy either. Carbon fiber will normally crack from impacts while aluminium alloys can weaken from fatigue and fail unexpectedly as well. Airplanes from alloys have failed even with rigorous regular checks in the industry. – Vladimir F Aug 23 '20 at 19:42
  • I would not any safety relevant parts of this bike-frame, like the frame, the fork or the bar as they may have suffered from the impact. For the wheels it's a different matter. If the FW is tacoed it means that it has absorbed part of the impact and should be written off as well. – Carel Aug 24 '20 at 12:26
  • 4
    I wouldn't assume hospital trips cost money - OP might not be in the US. – thosphor Aug 25 '20 at 10:16
  • 1
    @thosphor Very good point. I hope my sentiment is clear though: it’s not worth the pain, time off work, bodily harm.... – MaplePanda Aug 25 '20 at 18:34
13

The problem with aluminum is, that it tends to break abruptly. Aluminum can bend under pressure, but it does no stretch significantly under tension before being ripped apart. You get no warning, you simply get a tube or two breaking apart. In this, aluminum behaves much worse than steel which is known for plastic deformation under tension, long before breaking.

With your frame, the failure mode would be that the down tube suddenly breaks off. Riding your bike puts this tube under tension, and tries to unbend the already weakened area. The vibrations from riding are likely to quickly trigger cracks to form, and soon the downtube breaks apart at the buckle area. This will suddenly place the entire load on the bent top tube alone, which will likely immediately break apart itself. This may happen in the fraction of a second at any time. Effectively, your bike would vanish from beneath your butt, leaving you to fall onto the road at speed. Face down.

If you value the look of your face, don't ride this bike a single meter.

10
  • 3
    Hmm, I probably could do with some facial updates... but maybe not via this method. Thanks for the firm advice! – Rob Burke Aug 23 '20 at 19:54
  • 9
    This is not correct. The situation at hand makes clear that aluminum will bend and plasticly deform before it breaks. – Adam Rice Aug 23 '20 at 22:29
  • 4
    The general intent of this answer is helpful, but aluminium certainly does not fail abruptly, it is highly ductile (at least at normal temperatures). But given that failure has already happened here, not much has to move before it collapses catastrophically – Phill Aug 24 '20 at 6:02
  • 6
    @AdamRice I have bent both aluminum and steel. And while thin aluminum can be bent rather easily, it breaks even more easily. You can bend 1mm aluminum to a 90° angle, but when you try to bend it back, it will break. You can bend thicker aluminum under enough pressure (which stops cracks from forming, but you cannot bent it far under tension. The accident has deformed the downtube under lots of pressure, but normal riding puts it under tension, and the vibrations will quickly trigger the formation of cracks. It will break without warning. – cmaster - reinstate monica Aug 24 '20 at 7:13
  • 2
    @cmaster-reinstatemonica I would guess that the "~6km ride home" has already started the process of crack formation and it is already weaker than it was immediately after the collision. Aluminum is notorious for fatigue cracking and the points where buckling has occurred have become stress concentrations most vulnerable to the process which will accelerate as cracks form and further concentrate stresses. – Anthony X Aug 25 '20 at 22:36
4

You don't really have a "frame" anymore. I am sure you can deform it visibly with your bare hands just by pulling the two wheels together. You don't have to believe me, just try.

The same happens as you ride. The bent metal is not rigid anymore, it "works" and deforms near the dents.

But the material was never meant to work by deformation, it is not a spring (and even springs sometimes break). It will develop cracks, more cracks, deeper cracks and will ultimately break.

Chalk it as "shit happens" and send it for recycling. And count these 6km as "lucky". If it was for 6km, I would walk them.

1

Two things not mentioned in the other answers:

  • you might have another accident and or the frame just breaks while riding and then it could hurt your legs or stomach, depending how you fall on it.

  • read on whipslash injury, can also happen at small speeds, you never know. so you are probably not okay, even if you think you are and might feel the consequences a few years later. go to the doctors, document any tiny health problem like stiff neck, any pain, even if small.

also, don't trust people on the internets, none of them will come and take care of you later. :-)

1
  • 1
    A sound advice by itself, but not really ontopic. – fraxinus Aug 26 '20 at 12:19
-5

Safety wise, the entire bike is now a write off, The pro's would say that other parts of the frame can now crack unexpectedly after such a shock.

Personally I would do a 1 day repair using carbon fiber. you'll have to cut the top tube to bend the whole thing straightish.

Use more carbon fiber than this image, about 5mm CF wall thickness. enter image description here

The advantage of carbon fiber is that it doesn't undo the crystals in the aluminium frame, because the entire frame is heated to 400 degrees and left to cool in high-spec ASTM standards.

Carbon fiber also costs 25 dollars, and it is stronger than a weld and than steel, but it will add 500 grams to the bike.

I have fixed a cracked aluminium frame using carbon fiber, it held up fine for 3 years until the suspension bearings wore out (centurion, german bike)

The problem with epoxy-carbon bonds is that they can be unstick, you have to research the best epoxy for aluminium, and perhaps treat it with acid first to take off the oxide, even after brusing it, and straight away after the acid is removed, put on some epoxy resin, and afterwards wrap it in carbon, then when it's all wrapped, heat it over a radiator for a few hours to waterproof the epoxy. Total work time is about 3 hours, one hour to prepare the frame, one hour to wrap the epoxy and carbon.

You have to make some dents and grooves in the frame to ensure the epoxy grips the frame like steel. enter image description here

15
  • 3
    I would not repair that bike. And if OP is not at fault, the cost of making good is on the driver, which means a new bike frame at a bare minimum. I applaud your creative thinking though. – Criggie Aug 24 '20 at 12:18
  • 8
    Even bicycle manufacturers (think Specialized) have had significant trouble bonding Aluminum and CF. You seem to think that a casual application of CF with some "dents and grooves" will suffice. I think you are giving bad advice and I recommend that you delete this answer. – jwh20 Aug 24 '20 at 14:00
  • 3
    Hey people, there's no need for “lynch mob” downvoting. The other answers (and this one itself) have already made it clear that this is dangerous and/or silly, and of course bike companies don't do this kind of stuff on a new bike. But danger is relative; roadies tend to think of crashes as a big catastrophy but in MTB they happen all the time. Practice to get your reflexes up! – leftaroundabout Aug 25 '20 at 8:12
  • 5
    This answer does explain the “right” way to fix it, if one is determined to just go ahead with it. I'd only remark that, because carbon is so strong, the aluminium at the edges of the carbon sleeve will become much more of a problem zone (small flex movements will concentrate there, instead of spreading over the entire top/down tube). – leftaroundabout Aug 25 '20 at 8:13
  • 3
    @jwh20 the difficult thing is bonding CF and Al with only a small contact surface in a way that will withstand high tensile loads. That's not needed here: you have a big contact surface area, and because the tube is entirely wrapped, the only force the bonding epoxy needs to withstand is some shear. That's not an issue. The issue is that it's redundant material, which for a new bike would just be completely unnecessary weight and complexity. (Plus, bike manufacturers are happy to spread any information suggesting that something is impossible to fix...) – leftaroundabout Aug 25 '20 at 8:20

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.