Flex, titanium versus steel!

[jonnyboy666]

so, on the brighton cake ride yesterday me and Kampos (dave) were talking about the fact he was considering getting a kona hei hei but he's quite tall and would need a large frame, there is currently one on ebay. i suggested that titanium may be pretty flexy especially the older stuff so it set me thinking.

i have a 1995 vitamin t2 and a 1995 p7 nickel aswel so i have videoed the b/b flex with my phone when pushing sideways on the pedal. the only major frame difference other than material is the lack of chainstay bridge on the P7

http://www.youtube.com/watch?v=d25DbP43r9g

i knew the bike was flexy as i have ridden it a fair bit since owning it and i find that i have to sit down when climbing on it where as the p7 is happy in or out of the saddle.

don't get me wrong here, i like the ride of both bikes and i am not concerned about the flex in the Vit t2 but i just thought it was something worth chatting about, it's almost a shame i sold my 1995 e3 as apart from being aluminium it was again a very similar design so would have been good to compare aswel

 

[oldave]

I rather imagine that the "stiffness" is more a factor of design, including the tubes' properties and the area/shape of the joints, than the material per se.

In the only "scientific" comparison of frame stiffness (deflection at the bottom bracket) that I recall seeing, a titanium "won". A Raleigh Timet road frame - and having ridden one it didn't surprise me - like a brick!

 

[integerspin]

Isn't the Youngs modulus of titanium about half that of steel?

 

[jonnyboy666]

i don't claim to be an engineer or a scientist or anything, this stems from a conversation we had but regarding shaping of the tubes etc, they both have 31.8mm seat tubes which are both round, the b/b's are both 68mm and i think the down tubes are similar size aswel, both have 1" headtubes and very similar design rear ends with the exception of the chainstay bridge or lack of it.

just think it shows the difference in the properties of the tubing which is quite interesting

 

[oldave]

Given such design similarities, then in this specific case yes it probably does!

 

[boxxer]

Wise man once told me that if you push on the pedal / cranks to test how flexy a frame is all you end up testing is the quality of the wheel build....

I've never noticed any real flex on my frames and I've had skinny steel, fat Ali, suspension and Ti

 

[Anthony]

i don't claim to be an engineer or a scientist or anything, this stems from a conversation we had but regarding shaping of the tubes etc, they both have 31.8mm seat tubes which are both round, the b/b's are both 68mm and i think the down tubes are similar size aswel, both have 1" headtubes and very similar design rear ends with the exception of the chainstay bridge or lack of it.
just think it shows the difference in the properties of the tubing which is quite interesting

Those aren't the only factors to take into account. How thick are the walls of the tubes? What do the two frames weigh? The P7 was never a lightweight and was fairly stiff for a steel frame, but if the Vit weighed the same it would be way stiffer. OK, I'm sure it doesn't weigh the same, even though I've never seen the weight of a Vit frame quoted, but the point is that whether it's anywhere near as stiff as the P7 will mainly depend on its relative weight.

 

[jonnyboy666]

yeah the wall thickness obviously affects the things aswel, but the T2 is defo way flexy by comparison, it's so noticeable during a ride! never any point standing on the pedals!!

 

[Anthony]

I can't see any point in standing on the pedals anyway, unless it's a very short climb! It uses up too much energy. I just spin the pedals and I usually find that if somebody is 10% faster than me at the foot of a climb, I can usually overtake half way up.

My KHS (Litespeed) weighs 3.1lbs, which is quite light for a ti frame. People say they can see the rear axle moving up and down on climbs in a way they can't see with a steel frame. Nevertheless, I find it very fast uphill. Bear in mind that the flexing is lateral, not in the plane of the chainstay - I can't believe there is any longitudinal compression whatever. So no power is going to waste and if anything the improved traction is making me faster.

A 97 Hei Hei of the same size weighs 3.5lbs though, and a King Kahuna slightly more again, so they are going to be stiffer than the KHS. I don't know where the Vit slots into that range, but I think it was designed to be a light race bike, whereas the P7 was an early development in the enduro/freeride direction. So I think you're comparing apples and pears there really.

 

[Rampage]

I agree that you are testing the flex of the wheels / tyres not the frame!