Dented top tube; DIY repair

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Brucey
Posts: 42208
Joined: 4 Jan 2012, 6:25pm

Dented top tube; DIY repair

Postby Brucey » 11 Jan 2021, 6:54pm

If you ride bikes built in nice quality steel tubing, denting the top tube seems to be an occupational hazard. Making the centre section of the top tube very thin-walled appears to be something which has an irresistible attraction for those who design tubesets. Making the top tube oversize also seems appealing, but of course is often also accompanied by a further reduction in the tube wall thickness.

The result of this is that in a 'standard' DB tubeset (with a ~1" top tube) the wall thickness is often about 0.5mm in the centre and about 0.8mm at the ends. If the tubeset has lightweight and/or oversize pretensions, it can be 0.4mm or even 0.3mm. Such tubes dent easily.

In practice dents in the top tube often look terrible but need to be pretty bad before they are likely to actually be a real structural problem; the service stresses in the middle of the top tube are not usually particularly severe. For example I rode one frame (complete with a nasty-looking dent in the top tube) for nearly twenty years before I finally conceded that the frame probably wasn't going to crack after all (having been substantially retracked etc) and only then did I bother to do anything about the top tube dent. I only bothered to do anything about it at all because the dent just looked awful; I could see it when I was riding the bike too, so the thing was never quite out of sight and out of mind.

Of course as soon as I had fixed it (you genuinely can't tell it was ever dented now) I wondered why I hadn't fixed it before then, and the answer is of course that I was worried I might waste my time or perhaps even make it worse. I wouldn't be the first person who had started with a fairly small problem and wound up with a much larger one;

Image
Father Ted started with 'just a small dent' too....

You will read of 'rolling dents out', using 'tube blocks' and such like. Having fixed about half a dozen frames with dents in the top tube now, I don't think you need fancy equipment; once you have leapt the mental hurdle so that you actually start, you then mostly need simple equipment, combined with a little skill and patience. The standard I work to is to get the tube round and straight to within a tolerance that is comparable to the tube wall thickness. The damaged area is then of practically no concern (structurally speaking) and susceptible to being painted with a few more coats of primer than normal, then rubbed down until flat. If the frame is being professionally refinished they may do likewise, or (with small but deep dents in particular) they may offer to fill the dent with braze metal instead/as well. Filling with braze metal is rarely costly and I guess the worst case is that the top tube is so damaged it needs to be replaced, which is usually about a £100 job.

So the 'cost of repair' might (if the frame is painted in a colour you can touch up) vary from practically zero (your time and some paint locally) to at worst, a tube replacement, and a professional respray. If you are going for a respray anyway, filling might cost £15-20 extra and worst case a tube replacement is going to add about £100 to the bill. In any event if you are going to repaint the frame anyway, the consequences of 'having a go at fixing it' vary from 'costing nothing' (if it works as well as hoped) through 'filling with braze' if the dent is still there but large, to 'new tube required' which is the worst case. So you don't have 'nothing to lose by trying' but if the dent is bad enough you probably won't enjoy riding the bike in its dented state either; if so, you may as well try and do something...?

I have occasionally 'taken a punt' on a dented frame, in anticipation that I would probably be able to fix a dent. A few months ago I bought a bike which had been recently dented, with this in mind.

Below there are some before and after photos;

DSCN1657.JPG
Ow!


The top tube on this frame is a non-standard 29.0mm dia Nivachrom tube. As measured the tube is around 29.1 to 29.2mm dia (with the paint on) but where it was dented it was effectively 'D' shaped and it measured (at worst) +2.0mm and -2.3mm on diameter. The dent was sufficiently bad that the tube was no longer quite straight when viewed from above, either.

After about an hour's worth of the usual treatment (during which more paint came off, obviously) I got this result;

Image01994.jpg
thin 'guide coat' shows surface profile without the distraction of local paint damage.


The tube diameter now measures (at worst) ~+0.2mm and ~-0.7mm. This is (for me) 'within tolerance', so I'll probably quit while I'm ahead. The frame probably needed a respray anyway, but I may just do a partial respray for now ( eg on the main tubes in a contrasting colour or something).

If I'd bought tube blocks before now I'd have wasted my money twice over; not only were they not necessary but they wouldn't have fitted this size of tube anyway.

The dent shown above is probably the worst one I have tackled to date. Needless to say I heaved a sigh of relief when I realised I might get away with it.

Anyone else 'had a go'...?

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

hamster
Posts: 3718
Joined: 2 Feb 2007, 12:42pm

Re: Dented top tube; DIY repair

Postby hamster » 11 Jan 2021, 7:10pm

Stop teasing and tell us how you did it!!!

Brucey
Posts: 42208
Joined: 4 Jan 2012, 6:25pm

Re: Dented top tube; DIY repair

Postby Brucey » 11 Jan 2021, 7:26pm

hamster wrote:Stop teasing and tell us how you did it!!!


I just used the methods I have previously described in this thread;

https://forum.cyclinguk.org/viewtopic.php?f=1&t=85344

The tools I used were a 4oz hammer with a reasonably smooth (and radiused) end, an offcut of 3mm thickness aluminium, and the kind of small vice which is designed to clamp to a tabletop. I used the vice as a freehand clamp, to squeeze the tube. The vice had smooth jaws so I could use it directly on the tube where necessary.

FWIW I doubt I would have made the repair much better had I bothered with fancier (external) tooling. Obviously if you get something up the inside of the tube you can (in theory) make the tube 'perfect' again, but this isn't easy to do, not with a top tube.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

Greystoke
Posts: 377
Joined: 8 May 2018, 7:41am
Location: Lincolnshire

Re: Dented top tube; DIY repair

Postby Greystoke » 11 Jan 2021, 8:42pm

I've repaired quite a few cars that looked a lot like father Ted's above :shock:

PDQ Mobile
Posts: 3976
Joined: 2 Aug 2015, 4:40pm

Re: Dented top tube; DIY repair

Postby PDQ Mobile » 11 Jan 2021, 9:40pm

Brucey.
As a serious enquiry, do you not think the strength would be more intact and the final finish much better if one used an automotive body filler?

Brucey
Posts: 42208
Joined: 4 Jan 2012, 6:25pm

Re: Dented top tube; DIY repair

Postby Brucey » 11 Jan 2021, 9:52pm

automotive body filler has a pretty low modulus; it sticks to cars because when the panel moves, so does the filler, at a stress which isn't enough to start it peeling off. So its ability to 'help carry load' is somewhat suspect IMHO.

In the case of the kinds of dents seen in top tubes, unless they are very shallow, filler alone isn't really an option; the tube is normally so far out of round that just slapping filler on can't give a smooth finish unless the tube obviously 'grows' (by filler addition) too.

There is also the issue that dented tubes are often no longer perfectly straight, either. Filler won't fix that.

Bottom line is that if the dents are shallow enough it doesn't really matter what you do. However if the dents are more severe, just using filler isn't even an option.

IME suitable primers work well enough at thicknesses of ~0.5 or 0.8mm (several coats; they have to be in order to work on cars); they also flat off nicely (unlike a lot of fillers) and there is no blending problem because there is no blending. Using fillers (with DIY paints especially) just adds another material (which may react with overlying paint layers, or leave an obvious step when sanded) to the mix, and is best avoided if possible.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

PDQ Mobile
Posts: 3976
Joined: 2 Aug 2015, 4:40pm

Re: Dented top tube; DIY repair

Postby PDQ Mobile » 12 Jan 2021, 1:42pm

I wasn't really thinking that the filler carried any load but rather that the tube was perhaps stronger without straightening. Because of the fatigue involved in re-straightening?
Certainly the filler could, with time and effort, give a practically undetectable finish to such a repair.
I suppose a little fiberglass and resin on roughened tube surface would add a little strength - before the filler was applied?

Brucey
Posts: 42208
Joined: 4 Jan 2012, 6:25pm

Re: Dented top tube; DIY repair

Postby Brucey » 12 Jan 2021, 2:26pm

if you overdo it, it is of course possible for the steel to crack because it has finite ductility. However against that, the closer the tube is to round and straight, the lower the overall amount of residual stress in the tube is likely to be, and more importantly the lower the service stresses are likely to be too; dented tubes pose a massive geometric stress concentration by comparison, and are chock full of residual stresses too.

Most steel tubing is pretty ductile, as received. Brazing can make it less so in many cases, so dents near brazed joints are a different kettle of fish. If you get the chance, cut a strip of steel from a scrap frame and see what amount of bending back and forth it takes to actually break it; you might be surprised.

If the material had limited ductility then you would expect to see more dented frames split in the dented region as the worst dents are made. This is not impossible (esp if something small diameter clouts a frame) but it is pretty rare IME.

Note that the vast majority of lugged and brazed steel frames are 'cold set', which means they see yield magnitude stresses at a late stage in manufacture. If you are going to worry about frames breaking, that is perhaps the sort of thing that ought to concern you, since it leaves high levels of residual stress in places where (by definition) some service stresses will also be concentrated.

FWIW I have seen a couple of top tubes break and this happened because the tube corroded on the inside. There was basically nothing left of the tube wall; even so the crack took a fair while to work its way round the tube and of course was very easily seen/felt in the meantime.

My plans for this frame are to paint it and ride it. I possibly won't ever part with it (the last Roberts I bought I still have, 35 years later....) but in any event if there is going to be a problem (which I think highly unlikely TBH) then I'd expect it to manifest itself sooner rather than later. On the basis of what I've seen before now, I don't think I have much to worry about.

BTW my fallback plan (if the dent didn't come out without the frame cracking) would be to perform a local weld repair. A warty great poultice of glass fibre might be something I'd try if I was stuck mid-tour, but as workshop repair for a steel frame it doesn't even enter into my thinking.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

bgnukem
Posts: 565
Joined: 20 Dec 2010, 5:21pm

Re: Dented top tube; DIY repair

Postby bgnukem » 12 Jan 2021, 3:33pm

I remember bending a chain stay of an aluminium Giant Rapid frame once (I reversed into a lamp post in France with the bike on the back of a car) and the bike was basically unrideable afterwards, there was so much flex when trying to pedal, so got scrapped. It was a bend in the entire tube section rather than just a dent though, and aluminium is a lot less stiff than steel.

I wondered if those car dent removal kits which involve spot welding a rod or series of rods onto the surface then using a slide hammer to pull out the dents would work? I guess the higher strength of the steel used in a bike frame vs. car panels, plus the curved profile of the tube might make this more difficult though, plus would the spot welding create an untempered hardened area in the tube wall?

Don't like the sound of the current incredibly thin-walled steel tube sets such as Reynolds 931 and 953 at 0.3mm thickness, must be very easy to dent.

Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: Dented top tube; DIY repair

Postby Brucey » 12 Jan 2021, 4:30pm

the spot welding of 'pullers' onto the surface risks the worst kind of thermal cycle in the steel. OK for car bodywork I suppose. Probably not very good for bicycle frame steel at all, (else more than only the very cheapest frames would have spot-welded and projection-welded fittings on them).

Thin-walled frame tubes do indeed dent very easily!

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

PDQ Mobile
Posts: 3976
Joined: 2 Aug 2015, 4:40pm

Re: Dented top tube; DIY repair

Postby PDQ Mobile » 12 Jan 2021, 5:34pm

Actually I was thinking of a discrete bit of fibre glassing then concealed by a thin layer of filler. Done by someone with time to spend it xould e very neat indeed. I simply do not know if it would be equal in strength to a "pulled" dent.
Though intuition tells me it might be- the steel having only been bent once during the initial disaster.

Regarding pulling the dent out with something welded on, there are some very strong and sticky adhesives that have remarkable shear strengths. (And nasty chemicals). Tiger Seal type.
They might just conceivably work in a similar way - without the risk of welding.
Take at least 24 hours to "cure".
Just a thought.

Brucey
Posts: 42208
Joined: 4 Jan 2012, 6:25pm

Re: Dented top tube; DIY repair

Postby Brucey » 12 Jan 2021, 9:31pm

Glass fibre is not an efficient solution; it is another low modulus material which means that any patch (which will do any good, that is) will be comparatively thick. Hence my earlier comment about a 'warty great poultice'. You would be as well off bonding a piece of tin can to the tube. Not that I'd do any such thing, not while there is any alternative; any such 'repair' would be completely obvious to the eye, and therefore a waste of time.

Remember that (despite nasty geometric stress concentrations and high levels of residual stress) most top tube dents don't go on to cause problems; arguably the main reason for fixing them (as I explained upthread) is that they look dreadful. Slapping a load of rubbish on top is not a good step forwards! It isn't even really practical; typical top tube dents comprise an obvious place where the tube is pushed inwards but this is also invariably accompanied by parts of the tube which are pushed outwards too. This means that if you slap anything on top of the dented area and want to make the tube remotely round, it ends up being much larger in diameter.

So in the case of the frame here, the tube would have been +2mm larger diameter if simply filled to be circular (and not concentric with the original tube), plus whatever thickness of glass fibre is laid on top. There's no way of 'concealing' that.... 'ugly' doesn't even begin to describe the likely end result. 'Ugly and pointless' might be a good description?

Remember that (as is commonly the case) the whole tube wasn't perfectly straight on the undamaged side either; this has two consequences; first there is no way of hiding that and second it betrays that the whole tube is under considerable residual stress all the time; most of the deformation that you can see will be elastic, and the amount of plastically deformed material may be relatively small.

Note that when a framebuilder 'fills a dent' with brass, the heating of the dented area mitigates the residual stresses, and they will also knock any high spots down before adding the spelter. The modulus of the spelter is pretty close to that of the steel, too. Thus this repair is actually a pretty reasonable repair, and if it is done well you can't easily tell it has been done, not without taking all the paint off the frame.

Trying to slap low modulus rubbish over a dent is at best a bit a bit like expecting a wheel that is built with a bent rim, (where it is only approximately straight because of highly unbalanced spoke tensions), to be as good a wheel as one where the rim is straight to start with. It is obvious that you would be better off straightening the rim before building the wheel. It is equally obvious that a frame will only ever be repaired properly if the geometric stress concentrations are reduced, the residual stresses mitigated, and of course the end result looks acceptable. A 'sticking plaster'/ 'poultice' type repair properly addresses exactly none of these issues.

A final comment is that if you are contemplating a 'poultice' type repair, you literally have nothing to lose by trying to correct the dent. If the (unlikely IMHO) event that the dent correction doesn't work you can always slap a poultice on top of it, and if the poultice is of any practical use whatsoever it shouldn't matter what exactly lies beneath it.

I may post a photo of one of the frames that I have repaired previously; it will probably will have done many miles with a dent in and many more since being repaired. The thing is that there is arguably no point at all in such photo, in that it just looks like a perfectly normal bike frame; you would need to use measuring calipers to detect any deformation in the tube.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

rjb
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Location: Somerset (originally 60/70's Plymouth)

Re: Dented top tube; DIY repair

Postby rjb » 13 Jan 2021, 11:32am

What are the pros and cons of drilling a small hole and using a screw to pull it out, then use a small amount of body filler to disguise it.
At the last count:- Peugeot 531 pro, Dawes Discovery Tandem, Dawes Kingpin, Raleigh 20, Falcon K2 MTB dropped bar tourer, Longstaff trike conversion on a Falcon corsa. :D

Brucey
Posts: 42208
Joined: 4 Jan 2012, 6:25pm

Re: Dented top tube; DIY repair

Postby Brucey » 13 Jan 2021, 12:21pm

drilling holes without reinforcing them in some way can be a real stress-raiser.

In theory as soon as you drill a hole, stresses are increased by at least a factor of three (*). In practice it is usually a lot worse than that; the burrs on the inside of a hole drilled in a thin-walled tube can easily turn into the initiation site of a crack. For example I have seen several frames break because there was a drilled hole (eg for a dynamo wire) near the lower head lug.

(*) x3 is the result in consideration of 2D stresses in an infinite flat plate with a round hole in it. Barring substantial reinforcement of the hole edge, almost anything you do to vary that situation tends to make things worse, so x3 stresses is a 'best effort' for a drilled hole, not 'worst case'.

Its worth comparing what happens when you carry out a weld repair; this alters/damages the material in and around the weld, and may introduce (and will certainly change) whatever residual stresses are present. In addition there will always be stress-raisers vs a plain/smooth tube. So hardly perfect either. However I would unhesitatingly choose to weld a drilled hole in a thin-walled tube up rather than leave it or cover it with filler.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

MikeDee
Posts: 690
Joined: 11 Dec 2014, 8:36pm

Re: Dented top tube; DIY repair

Postby MikeDee » 13 Jan 2021, 3:27pm

I've seen where people put a short leather top tube protector over the dent.