Rolling out dents in titanium frame?
 

I am restoring a vintage titanium Ron Kit/Speedwell Gear Case bike. The frame has a couple dents in it--one on the top tube (shown, presumably from an impact) and one on the down tube (not shown, perhaps from a too-small or overly-tightened FD clamp). I have read about dents getting rolled out of steel frames. Can the same be done with titanium frames? Any tips or suggestions? Keep in mind this is a '73 titanium frame, so it's not the same grade of titanium that's used now.

FWIW, I'm not worried about what this may do to the finish. Since taking the photo below, I have been going over the frame with scuff pads and fine sandpaper to bring back the original polished look (it's turning out really nice). So I'll be able to match the finish again if I attempt to roll out the dents.

http://i850.photobucket.com/albums/a...m/IMG_3891.jpg

http://i850.photobucket.com/albums/a...m/IMG_3876.jpg

Thanks in advance for your input.

Randy, I'll take a stab at answering your question.

Ti alloys have an ultimate tensile strength and yield strength similar to 4130 steel, but a Young's modulus significantly less than steel (105-120 GPa compared to 200 GPa for steels), so to provide the same strength and stiffness as a steel tube with the same diameter, a Ti alloy tube has to have thicker walls (as much as twice as thick). Since the density of Ti alloys is about half that of steel, the thicker walled tubing will still weigh slightly less than the steel, but because the walls are thicker rolling out dents in a Ti alloy tube would be more difficult than rolling out dents in a steel tube. Compounding the thicker walls problem, Ti is less ductile than steel.

Dents can never be rolled out completely, so after rolling a steel tube the remaining deformation can be filled with brazing material, lead, or bondo, faired so that the deformation is not visible, and then painted. While dents in Ti can be filled with bondo, it would have to be painted to cover up the repair.

Other than a tube of given OD and doubled wall thickness actually weighing 30% more than a steel one in most titanium alloys, and the UTS of titanium alloys an even just plain 4130 being widely varied dependant on previous cold work and heat treatment, you're right ;-)

Steels are essentially all cubic metals, which provide a healthy number of fully active slip systems at room temperature and a well-behaved texture when rolled. Alpha titanium alloys are hexagonal. Almost all you'll come across in tube form are either near alpha, alpha-beta or completely alpha and as a result are less well behaved under rolling loads. It's not impossible to roll out defects in titanium, but you must understand that the tube started life with a preferential grain and crystal alignment, which that big, fat dent just ruined, and now you're trying to return it to previous form in a disobedient metal. Without care and experience in deforming titanium sheets and tubes, it's far easier to make the problem worse in Ti than steels. Just my two penn'orth.

And, it looks like a fairly uniform, smooth geometry dent. You're better off letting it lie.

it probably was commercially pure Ti like the Teledynes were. These frames were notorious for their lack of stiffness. I would try to roll it, and then hang it on a wall :)

Which is probably more a case of misunderstanding how to get the best out of the material, not just because it was one of the soft alloys :-)

Thanks, everyone, for your comments. Hmmm, interesting feedback to consider. I certainly don't want to make the dents worse. I suppose I could hide it behind a cable guide to some extent.

yes, the fact that they went out of their way to make standard size tubes was really a waste. Any advantage that Ti has is that the low density allows you to make bigger tubes and thus make up for the low modulus. Teledyne had the right idea, but the crimped downtube was a little ridiculous. Of course, that was the least of their problems, the low strength of the CP Ti really did them in.

But the strange thing is that even then, the industry was awash with low(ish) cost alloyed titanium with hugely superior mechanical properties.

Are you sure of that? IIRC the tubing selections early on (for Ti) were motivated by other industries. Yes, there was a marketing splash re Ti early on. But I always thought that was much of the "it's different and expensive, we have pros riding it, so it must be better" **** that every "new" material has had. Andy.

What other industries? Aerospace? Been using 6/4 predominantly since the early sixties, both domestically and shell-company-acquired from Russia. Chemical? possible, although they prefer theirs to have palladium in it.

There was some reason why Teledyne used CP, but I don't remember why or if it was even a good reason

No, I should hope so. There must have been.

Anyways, I wouldn't say that attempting to remove that dent would be worth the effort. It's shallow, smooth, uniform..

Probable: there is some metal stretching in making the dent..
it wont compress back from the outside...