boots

Just curious. What, if any, are the disadvantages of titanium as compared to steel, in a frame? (Other than cost, of course.) Is titanium essentially a lighter, corrosion resistant steel, or a completely different metal?

biff

Here's a good primer.

yonderboy

Titanium is a totally different metal, which is usually an alloy of titanium and vanadium. It is lighter, at the cost of being more brittle, than steel. It doesn't rust, but its chemical makeup makes it susceptible to seizing, so you need to use an anti-seize compound or lots of grease when using titanium in any friction joint.

For all the gritty numbers, visit Sheldon Brown:
https://www.sheldonbrown.com/frame-materials.html

bostontrevor

I've seen people claim Ti delivers the ride that steel promises.

Is it true? Hell if I know, I've never actually ridden it. It definitely builds lighter but equally durable frames as steel and doesn't corrode.

One disadvantage I've heard about Ti is that due to it's non-participation in galvanic reactions, those things attached to the frame which do (basically every aluminum and non-stainless steel part) tend to just melt away as they become ionic sinks for the Ti.

I expect Reynolds 953 to dramatically change the Ti/steel debate.

LóFarkas

Ti and brittle? I've never heard that... Ti can take anything you can throw at it, and not even "age" like Al does, let alone crack.
From what I gathered, Ti is the best frame material all-round. Light and damping as carbon, but it has as long life as steel. Needs no paint, can take crashes, doesn't change its properties over time, doesn't rust. The only downsides besides price are the corrosion where it contacts other metals and, some say, too much flex. But the corrosion is easy to keep in check, I guess, and a slightly beefier frame will not be too flexy and still weigh half as much as steel...

the_shogster

i have a Ti fork on my SS MTB, its hella springy and uber light. its so springy its like a poor mans expensive suspension.

humancongereel

i've never heard of it as more brittle. though the corrosion where other metals touch...what is this about?

zonatandem

Have ridden/owned single bikes and tandems of steel, alu and titanium.
Our money, and our butts, are on carbon fiber.

bostontrevor

I don't pretend to understand the exact chemistry/physics involved, but galvanic corrosion requires that one metal act as the anode and one as the cathode. The anode will undergo accelerated corrossion while the cathode will experience slower corrosion. The nobler (less reactive) metal will become the cathode and Ti, being among the least reactive metals, is an excellent cathode. Consequently it turns the metals around it into very strong anodes and they corrode much more quickly.

That's the theory, anyhow.

edit: Here's something interesting on the subject. https://www.stainless-steel-world.net...connecting.asp

edit edit: this is why you use Ti prep. It's specifically designed to avoid bringing the Ti into contact with the dissimilar metal thus forming a galvanic cell.

humancongereel

hmmm...would this contact be with parts (are there parts you COULDN'T make of Ti, too?), or would it be, say, locking it up with a non-ti lock on a non-ti pole someplace?

linux_author

- this is a good question, and yet another reason there needs to be a BikeForums FAQ section!

:-)

bostontrevor

It could be either.

A galvanic reaction requires three things: an anode, a cathode, and an electrolyte to tie them together. Broadly speaking, the saltier the water, the better it acts as an electrolyte.

The kicker is that Ti is one of the most noble metals available (just below silver, gold, and platinum) while Al is one of the most active, near the opposite end of the spectrum with Beryllium and Magnesium. This means bad things for your Al parts unless you take care to insulate them from the evil Ti.

Steel (like your lock or the pole you're locking too) is substantially less anodic. I don't know whether it's enough less active to not concern us.

humancongereel

huh. i wish i remembered my high school chemistry better.

yonderboy

That's the name of the game when you're talking about weight vs. strength. The yield point on titanium is about 20% lower than steel, meaning it will fail first. So yes, it is more brittle than steel. Not as much compared to aluminum, but whatever. There's a reason the aviation industry moved away from titanium. For bike parts, not so much of a big deal, but a part fails if a part fails.

boots

which frame builders will make me a platinum track bike?

vobopl

They are two general commercial versions of titanium, both alloyed with Al and vanadium. One has these at 3.5 and 2% while the other is 6 and 4%. The latter is much better frame material, however it is not ductile so drawing the tubes is difficult. It is also difficult to weld togheter different titanium alloys, so mixed tubes framesets are rare.

My road Pinarello frame is titanium - it is not the lightest (1500g frame only) but very nice ride. Sitffer than my steel frames so it climbs and accelerates ok, yet, in combination with light carbon fork dampens road vibration nicely. The only thing I do not like about is whippy front der hanger - you can bend it with one finger using the leverage provided by the deralilleur cage.

ImOnCrank

Call ¢50, he'd know.

Blakey

Woah there. Lower yield point =/= more brittle.

Hold onto your hats folks, math ahead, apologies if there are any numerical or unit errors, but the examples and conclusions are still valid. Also, it's all in SI units, Imperial belongs in the dark ages.

4130 steel (standard Cr-Mo) quenched and tempered has a yield strength (starts to stay bent instead of springing back) of 951 MPa and UTS (highest stress possible) of 1110 MPa. It stretches plastically by 14.7% before fracturing. Its modulus (resistance to bending) is 205 GPA. Density is 7.85 g/cc.

Ti-6Al-4V, annealed has a yield of 880 MPa, UTS of 950 MPa, elongates to 14% at fracture and has a Young's modulus of 113.8 GPa. Density is 4.43 g/cc.

7005 series aluminium (T6 heat treated) has yield 290 MPa, UTS 350 MPa, 13% elongation, modulus 72 GPa and density 2.78 g/cc.

And the new kid on the block, 953, 1500-1900 yield, 1750-2050 UTS, modulus & density similar to other steels.

So whilst it's true that 4130 has a higher yield point and UTS, that's like saying that the stiffness of Al is 1/3 of steel, so Al frames must be really flexy. Sure, if you used tubes of exactly the same size and wall thickness. In addition, gold has a far lower yield point compared to steel, and LDPE even lower, yet these are orders of magnitude more ductile, not more brittle because they deform plastically at a lower load. Pure yield point data has nothing to do with a bike made of one material breaking before another, it depends on how much material is bearing the load.

Comparing strength to weight and stiffness to weight as a basic performance index you get the following:
str/weight stiff/weight
Steel 121.1 26.1
Ti 198.6 25.5
Al 104.3 25.9

(note little variation in stiffness to weight between materials. Stiffness is purely a measure of atomic bond strength)

So from the above you can see that for a given mass Ti wins, or for bars of the same strength, Ti will be the lightest.

But lets use some real numbers and try shaping the material into something useful, like tubes for bikes:

Given a 4130 tube of 25mm OD and 1mm wall thickness, (75.4mm^2) something like a classic lugged steel bike, the max force it can support before yielding (in tension, ignoring bending, torsion, compression and buckling instability for now) is 71705N (stress = force/area). Assuming it's 500mm long it'll weigh 296g.

For the same weight, a 500mm Ti-6Al-4V tube can have an area of 134mm^2, which is pretty close to an OD of 43mm, still with 1mm walls. (Going to .5mm walls has an OD of ~87mm.) Likewise, for Al7005, with 1mm walls, for the same weight the tube can have an area of 214mm^2 which is an OD of 69mm.

The Ti tube will be able to support 116113N and Al 61952N. So for the same weight tube, (varying the cross sectional area to keep the mass constant), the Ti tube can bear the highest load before plastically deforming, then 4130, then Al7005. This also means that the Ti tube can be lighter that the steel it replaces whilst bearing the same load.

But the important part is the stiffness, as it takes a major crash to plastically deform (pretzel) a frame of any material but the stiffness is felt all the time. The numbers a couple of paragraphs above showed that stiffness/weight doesn't change much, so how can we improve it? By shaping to increase the second moment of area. This is why I beams are used for construction, you can bend your ruler easily in one direction but not the other and Deep V's are so much stiffer than shallow box section rims (also, deep dish pie is stiffer than shallow crust).

If we were to just use solid rods of the same areas as the above tubes, the loads borne in tension would be identical but the moments of area for them would be 3.8, 5.1 & 6.5 mm^4. So you can see that larger area = stiffer. Going back to the 1mm wall thickness tubes it now becomes 5438.1, 29110.8 & 123503.9 mm^4 for 4130, Ti-6Al-4V & Al7005 respectively! See how the equal weight Al7005 tube has a moment of area 22 times higher.

And just for kicks, if it were a Ti tube with 0.5mm walls, (85mm OD for equal mass) the moment of area would be 118471mm^4.

But what does that mean? Converting this into real deflection under say, a 50kg load (*9.81m/s^2 = 490.5N), considering the tubes in 3-point bending from a centred point load with one end pinned;
Deflection = Fl^3/C1EI,
F = force, l = length, C1 = 48 for this example, E = modulus and I = moment of section.

Deflection of the tubes is 1.15, 0.39 & 0.14mm for 4130, Ti-6Al-4V & Al7005 respectively.

So, for equal weight tubes they're ranked Ti, Fe, Al in terms of strength and Al, Ti, Fe in terms of stiffness.

The 0.5mm thick Ti tube would deflect 0.09mm under the same load, but buckling instability becomes a real problem when you push the OD out and thin the walls. Think how much easier it is to crush an aluminium beer can on your forehead now compared to 10 years ago.

This doesn't consider the myriad of other factors, like fatigue life (Fe/Ti have fatigue limits which can lead to infinite life if the stress is below it), cost, environmental impact, corrosion resistance...

Another important factor in ride feel is the damping coefficient. Brass doesn't damp well, that's why bells are made of it. Cast iron and Mg do damp well, so you can make good engine test beds out of them.

I will just touch on corrosion briefly though. For two metals coupled in a seawater environment (galvanic corrosion) the most noble (cathode) won't corrode at all, whilst the other (anode) will experience accelerated attack. The series goes something like: Pt, Au, Ag, C, Ti, Ag, 18-8 stainless, nickel, silver solder, bronze, brass, Sn, Pb, Pb-Sn solder, 13%Cr stainless, steel, Al alloys, Cd, pure Al, Zn, Mg.

So Ti coupled with anything below it will accelerate corrosion of the other part and the bigger the distance apart, the worse it is. Woe betide the man who puts an Al BB cup into a Ti frame without proper prep, as a large cathode and a small anode will destroy that anode in short order. Large anode and small cathode (eg iron roofing with copper nails) will be much slower.

And once again, this ignores all the other forms of corrosion, such as crevice, stress corrosion cracking...

Questions?

bostontrevor

Ooooh! Materials science dis!

helvetica

This is true, Ive seen it happen when people use Ti valve retainers in race motors. always nice when the valve spring binds up drops a valve and you have a hole in the valve cover.

12XU

Blakey just owned us. Yikes!

Rev.Chuck

Kind of like saying baloney is more brittle than a cracker because it will yield first?

I have always wondered about galvanic reaction and Ti. Half my family fished for a living and they had zinc sinks on the motors to keep the alluminum from corroding away. And I never see a Ti frame that does not have a half disintegrated front der clamp on it.

Blakey

Well, I'm not entirely sure, as matweb and other texts don't publish data for baloney and crackers. But if I had to hazard a guess, I'd say the cracker is slightly stronger, but has no ductility, and the baloney isn't that ductile either, unlike say, hmm, this is tough, dried fruit? It seems strange to a metallurgist like me, but journals that deal with food science and processing go heavily into properties like shear resistance, texture, heat treatment etc, but it's just as important to them as those properties are to us.

I realise my earlier post was somewhat verbose, but better to put out some real data and comparisons that to have people read 'hurr titanium is bad, it's really flexy and weaker than steel, aluminium frames will break in a year and feel dead'. That is, as long as someone actually reads it.

I also didn't qualify before, stress = force/area. So something with a higher yield stress can either take a higher force on it, or use less of it at the same force.

dabern

I think the galvanic issue is more theory than real world problematic, at least in bike applications. I've had a ti Paramount for 6 years now and have had no probs whatsoever...I live in FL; not on the beach but I am within 7 or 8 miles so there is plenty of salt in the air which would supposedly hasten galvanic reation. I did, of course, grease every thread just as I would on any bike and I use Finish Line Ti Prep on BB & headset cups.

Ti generally rides great but I suppose there are exceptions depending on builder's choice of tubing and skill. Anyway, if you are interested in ti don't worry about anything seizing up or corroding as long as you take reasonable steps.

darkmother

Wierd. I have never seen that. I've got a 13 year old ti MTB frame, and I've never had a problem with galvanic corrosion due aluminum parts in contact with it. At least not in such an extreme fashion, anyway. What does happen, more or less once a season, is the BB cups will start to squeak in the frame. Then I take it apart, grease up the threads and reassemble. No big deal really. I've never seen anything like what you are describing. Are you saying that the FD clamp is actually corroded away? Is there a lot of salt in the air where you live-like are you near the ocean or something?