Titanium or Carbon for new road bike ?

[Tim Bennet.]

That is a different test in effect, with an impact like that I would expect Ti, steel and Aluminium Alloy to all dent, carbon would probably crack

So your saying the breaking strength of carbon fibre is less than the yield strength of aluminium, steel and titanium?

That's odd. I can't find any figures to support this.

Yield Strengths:
Aluminium alloy (2014-T6) 400 MPa
Steel, (high strength alloy - ASTM A514) 690 MPa
Titanium alloy (6% Al, 4% V) 830 MPa

Breaking Strength:
Carbon Fibre (common grade) 5650 MPa

As the units are MPas, these are forces per unit area of the cross section. But as high end carbon and high end titanium frames are fairly similar in weight (they're percentages of each other not orders of magnitude), and that carbon is under half the density of titanium, we can assume that the carbon tube wall thickness will be thicker and thereby accentuating the strength differences.

By the time a carbon frame has received enough force to crack (5650 MPa) a titanium of similar weight, has not only elastically deformed (dented) at 830 MPa but also split at 900 MPa. (the figures are both lower for steel and aluminium)

These figures do have to be reduced because of manufacturing constraints. But it applies to all materials; you have to assume the lay up of carbon composite is not perfect and that the welds in titanium are also less than perfect.

But if you are pursuing frame lightness as a goal, you have a greater margin of error with carbon fibre than you do with titanium. If 'light enough' is okay, then both are well inside their comfortable working limits. Just don't confuse their mechanism of failure with any comparison of their relative strengths. They make the safety pods of racing cars of carbon fibre because weight for weight there is nothing to touch it. If it broke at anywhere near the forces that titanium dented, it would never be used. The fact is it stays intact, not only at levels that titanium would dent and crush the driver, but also way above the forces at which titanium would be ripped apart.

 

[Paul_Smith SRCC]

I was implying that a test with a car running over a tube is not the same as it receiving an impact with a spanner.

Carbon frames can be designed to be very strong, as part of their R & D some desingers use testing jigs to simulate forces that a rider will put through the bikes and then more, to the point of destruction, as such as I mentioned in an earlier post they should last a very long time. However many will often consider Ti/steel and Aluminium Alloy when the want a bike frame that is more robust than a comparable Carbon bike frame, especially a road bike frame, after all the header was "Titanium or Carbon for new road bike" and included in the initial post “I can't rule out the possibility of maybe a week's touring as well, although that's probably stretching the capabilities of a road bike” so I felt the comparisons I made were valid.

Carbon is a very versatile material, you correctly mention that carbon safety pods are used in race cars to withstand forces in the event of a high speed race car crash, but they are designed for that, where as carbon bike frames of course are naturally designed for something completely different. A carbon bicycle frame if designed and built correctly should be strong, although that is not quite the same thing as robust. An egg is after all extremely strong, almost impossible to crush in the palm of your hand, yet it can be cracked very easily; strong can still be delicate.

That is not to say that all carbon bike frames are the same, carbon ATB frames for example are designed much more with robustness in mind when compared to a carbon framed road bike. No bike, carbon or otherwise should fail during the use for which it has been designed.

Paul_Smith
www.bikeplus.co.uk

So your saying the breaking strength of carbon fibre is less than the yield strength of aluminium, steel and titanium?

That's odd. I can't find any figures to support this.

Yield Strengths:
Aluminium alloy (2014-T6) 400 MPa
Steel, (high strength alloy - ASTM A514) 690 MPa
Titanium alloy (6% Al, 4% V) 830 MPa

Breaking Strength:
Carbon Fibre (common grade) 5650 MPa

As the units are MPas, these are forces per unit area of the cross section. But as high end carbon and high end titanium frames are fairly similar in weight (they're percentages of each other not orders of magnitude), and that carbon is under half the density of titanium, we can assume that the carbon tube wall thickness will be thicker and thereby accentuating the strength differences.

By the time a carbon frame has received enough force to crack (5650 MPa) a titanium of similar weight, has not only elastically deformed (dented) at 830 MPa but also split at 900 MPa. (the figures are both lower for steel and aluminium)

These figures do have to be reduced because of manufacturing constraints. But it applies to all materials; you have to assume the lay up of carbon composite is not perfect and that the welds in titanium are also less than perfect.

But if you are pursuing frame lightness as a goal, you have a greater margin of error with carbon fibre than you do with titanium. If 'light enough' is okay, then both are well inside their comfortable working limits. Just don't confuse their mechanism of failure with any comparison of their relative strengths. They make the safety pods of racing cars of carbon fibre because weight for weight there is nothing to touch it. If it broke at anywhere near the forces that titanium dented, it would never be used. The fact is it stays intact, not only at levels that titanium would dent and crush the driver, but also way above the forces at which titanium would be ripped apart.

 

[andy_wrx]

I'd hope that if I simply dropped a spanner on my bike it would bounce off, simply having scratched the paint...

 

[papercorn2000]

I've used both C and Ti in the past and I prefer the "feel" of Ti. Neither of my Ti bikes (my roadie and my cross bikes) are as light as equivalent C bikes but they just feel nicer and will probably put up with more abuse! As asn aside, both have C forks!
My roadie is an Airborne Zepp and my cross bike came from these guys: http://www.xacd.com.cn/ £400 - odd for a frame made to my own specs!

 

[Blue]

Was he up a ladder?

He knocked the spanner off a shelf a couple of feet above the bike.