Never ridden a ti bike, but I hear they're quite flexy, although mostly in a "good way".

So can you make a stiff, but light ti race bike? Or would you be better off with aluminium/carbon?

you can make a stiff Ti frame. I've heard it said about the cove hummer.

I have a Merlin Metalworks (USA) circa 2001, now hanging on the wall, light, strong and its stiff!! Climbs like a scolded cat but not really enough give for 'modern' riding/trails. The current Merlin XLM is a 'stiff, but light ti race bike'.

Rode a Soda once and found it too flexy – especially as the Soul is so right, the Cotic team rider rides one but I reckon its OK because its a small?. Hummer is stiff (but still v.good in the right way) but not really a race machine.

My Ti frame feels really stiff to me raleigh Ti sp 100 or something.

The chainstays and seat stays are straight which leads to a stiff frame I believe

The stiffness (Youngs Modulus) to weight ratio of steel, titanium and aluminium are roughly the same.

I thought the young modulus (not youngs) was a measure of stiffness and strain, independent of the size and length of the material?

And anyway, its about how you build it, surely.

What do you mean by 'stiff'? Do you mean does it bend when you pedal or some mythical property that some riders talk about?

Rode a Soda once and found it too flexy – especially as the Soul is so right, the Cotic team rider rides one but I reckon its OK because its a small?

not the only one to come to this conclusion. dunno if the new ones are any different.

Very approximately a heavier frame will be stiffer than a lighter frame, regardless of whether it is made of aluminium, titanium, steel.

If the Young's Modulus of a material is taken as the stiffness of a given material then it is possible to factor in the density of the given material to get an idea of the stiffness to weight ratio.
http://reynoldstechnology.biz/assets/pdf/rtl_steel_alloys_extract.pdf

Titanium is stiffer than aluminium, but titanium is more dense.

There is the possibility that the idea of springy titanium comes from the difficulty of cold working titanium, where there can be spring-back.
http://www.timet.com/fab-p24.htm

If I recall my chemistry studies from many years ago, most metals are pretty soft in their pure form, they way to make them stiff is to alloy them with small quantities of other metals like vanadium and chromium. I believe this can be fine tuned to get the properties you want. I would imagine that the stiffness of a frame depends on the composition of the metal as well as the geometry, tube dimensions etc.

I expect someone who knows more about it will be along soon.

BTW wasn't there a magnesium alloy framed MTB some years back, horrendously ugly thing, the frame was basically a girder and they kept snapping. Another project that seemed like a good idea at the time. Scrap ones must have burnt nicely though 😆

There were some decent looking magnesium alloy frames around a while back, but I don't think they got a toehold in the market. Sort of a shame – on the one hand it's sort of solving a problem that doesn't exist, on the other it's cool to experiment with new alloys to see if they open up different design possibilities.

Kirk made a cast magnesium frame that looked like Black and Decker Workmate.

I think them were kirk? Were black with purple stripes I think, were also very heavy. Especially compared to my zaskar I had back then, mid 90's I think

There is the possibility that the idea of springy titanium comes from the difficulty of cold working titanium, where there can be spring-back.

because people can design with Ti's unlimited fatigue potential (below a certain level of movement) i.e design in flex in certain planes to provide comfort etc to exploit the feature, not something you would want to do with aluminium alloy frames as those alloys have a finite fatigue life.

as for stiffness generally, tube diameter and shape are the biggest influences on stiffness, bigger the diameter stiffer the tube

http://aluminium.matter.org.uk/content/html/eng/default.asp?catid=89&pageid=2144417037

There was a very good article on the net a few years ago, covering the merits / drawbacks of different frame materials.

Can't find it now – thought it was on Sheldon Brown, but it isn't…

http://www.ibiscycles.com/tech/materials_101/4/

http://www.sevencycles.com/buildingbike/techsupplement/ti.php