Flex, titanium versus steel!

[Anthony]

Yes, but the tube diameter and thickness are not the same.
Titanium is far less dense than steel, so even though the Young's Modulus of steel is more than 1.5 times that of titanium, that's measuring stiffness in relation to volume. If the volume of material in a titanium tube is more than 1.5 times that of a steel tube (as it might well be in a bike frame), their stiffness will be about the same.

I agree - tube size is a very significant factor. It's not the volume per se though. For example, a large diameter thin tube may contain less material than a small thick one, but will be stiffer due to the greater moment of inertia.

I said the volume of material, not the volume of the tube including the air! The volume of material in a 31.8 x 1.0mm tube is greater than the volume of material in a 31.8 x 0.9mmm tube, even though the volume of each tube is the same.

Actually, this is all starting to sound a bit boring . Riding bikes is much better than discussing the theory

Bike websites don't exist because discussing bikes is more interesting than riding them. They exist because discussing bikes is more interesting than work.

 

[danson67]

I think there's two aspects to look at:

Firstly, the material itself. Ti isn't magic, just good. If you look at stiffness for a given mass (mass specific modulus), steel, Ti and aluminium are pretty close. (25+/-0.5, 25+/-2, 26) So for a given stiffness, shape and size the frame weight should be very similar. Different alloys of each material do not vary the stiffness.

Secondly, the form of the frames needs to be taken into account:

:A tube's stiffness will increase in direct proportion to wall thickness: 20% thicker walls, 20% stiffer, 20% heavier.

:A tube's stiffness will decrease in proportion to the square of its length: shorten a tube by 20%, stiffness will increase by 44%, 20% lighter.

:A tube's stiffness will increase in proportion to the cube of its diameter: Increase the diameter by 20%, stiffness will increase by 73%. 20% heavier.

So make the tubes 20% fatter, with 73% thinner walls and still have the same stiffness :shock: But make the tubes too fat and thin walled and they will buckle (Euler limit) It's not as simple as which material...

The differences between materials are predominantly in the fatigue strength and yield strength (permanent bending) characteristics, where Ti and steel win over alu due to their fatigue limit, whereas alu continues fatiguing until failure. Here different alloys plays a huge part in the lifespan and strength of the frame. Commonly used steels can vary in tensile strength by 300% (Reynolds 501=771, 953=2000+), yield strength by 200%. Ti alloys do not vary quite as much in their properties, 3/2.5 and 6/4 are within 10% of each other in these properties.

So, if your two frames are the same shape, tube diameter and weight, they'll be roughly the same stiffness, whether they are made of any alloy of steel, ti or aluminium. How long they last and how tough they are is a different matter.
The differences in the material properties themselves are far less significant than the differences produced by tube profile and spec.

All the best,

 

[highlandsflyer]

The only thing you test on that way is how stiff or soft your wheels are, sorry to say it but you have a lot to learn about frames and how to compare them.

Its very common that people test that way but that don't make it right, when talking stiffens in a frame its not sideways you are loosing energy its on the long side of the bike.

If everything else is the same, then differences can be felt. Especially if you are very accustomed to jumping off one onto another.

Most of the energy is recovered regardless of flex.

Good article here.

http://www.sheldonbrown.com/frame-materials.html

 

[danson67]

I alway like the quote from Mike Burrows:

Different (frame) materials do have different qualities, responsive steel, damping aluminium, shock absorbing carbon etc, but the differences are only measurable with instruments, not your bum.

Not sure I entirely agree, but who cares...

All the best,

 

[jonnyboy666]

while i realise my initial flext test is far from scientific, there are so many similarities in the design and components etc i still feel it's an obvious example of the difference in the flex, yes it does flex the wheels/tyres but then it flexes the wheels and tyres on both bikes so if alot of the variables (tyres/wheels, frame design etc) are the same it can show a difference between the frames, not scientific but still a difference.


am i the only person who is able to try a test like this with 2 similar brand and designed bikes but with differing materials? on this site with all the bikes we have and the people who have many bikes from one brand i can't be the only one to beable to compare!

i do regularly use all my bikes, some more than others but the Vit T2 is so wildly different and flexy it's so obvious during the ride, to the point that i have to adapt my riding style to suit the bike.


brought some discussion though

 

[Blackstar]

Dan Towle writes a good article about frame materials and how they affect rider comfort:

http://www.rodbikes.com/articles/material-world.html

For me, it's a frame by frame thing.