Steel frames - too flexy?

jaynesmith

Currently riding one of these modern 100-130mm HT 853 steel frames. Had a go on a alu bike today and was blown away over how much better I could put the the power down - even though my bike has way better/lighter transmission. I was expecting a bit of difference but was huge.

Is this the typical tradeoff between steel and alu?

dhxcme

Steel does flex, which is why it is a good impact resistor. Alu doesn't flex very much and is more likely to snap but is alot lighter. If the frame flexes alot then it maybe faulty as my one was preatty tight till it came to landing then it flexed ever so slightly.

supersonic

You an still get stiff steel frames - and flexy alu ones. Depends on how is constructed. Steel is 3x stiffer than alu, but 3x denser.

dhxcme

Due to it's grain structure alu will not flex to the extent steel will. The geometry of the frame will make difference but, again steel will flex more before it will snap and so a clever frame builder will take advantage of that.

rob cole

Due to it's grain structure alu will not flex to the extent steel will

nothing to do with grain structure, cromoly steel is actually a much "stiffer" (more resistance to flexure) material than the aluminium alloys used in bike frame construction

Cromoly steel also has a much higher tensile strength than aluminium alloy, and has infinite fatigue life as long as it is not overstressed (yield strength)


with aluminium alloys the bike designer has to use a larger diameter tube to provide adequate strength and frame stiffness to meet the demands of mountain biking, and this is why aluminium alloy mountain bike frames end up feeling "stiffer" under power

with cromoly steel the higher strength means you can use smaller diameter tubing, and design some flex into the tube itself, to provide more comfort

with both steel and aluminium alloy it is getting a balance between tube diameter, tube shape and especially wall thickness, bearing in mind the extra density of steel compare to aluminium - you can have a large diameter tube with a thinner wall when using aluminium

as tube diameter doubles, stiffness increase 16X, but attention has to be paid to wall thickness to avoid buckling failure (beer car effect)

it all comes down to understanding the materials, and how to get the best out of them to suit the frame application (ie.. cross country steel frame is different to freeride steel frame)

hamboman

jaynesmith wrote:

Currently riding one of these modern 100-130mm HT 853 steel frames. Had a go on a alu bike today and was blown away over how much better I could put the the power down - even though my bike has way better/lighter transmission. I was expecting a bit of difference but was huge.

Is this the typical tradeoff between steel and alu?

Something to bear in mind, new bikes always seem stiffer because over time everything, from the headset to the bb, becomes a little loose unless immaculately maintained.

jaynesmith

Hmm.. my 853 has seen better days - maybe thats counts for a bit.

supersonic

The frames dont get any more or less flexy. But saddles bed in, forks loosen up. Loose parts feel different to flex.

dave_hill

jaynesmith wrote:

Hmm.. my 853 has seen better days - maybe thats counts for a bit.

Highly unlikely. A good 853 frame (what make is it by the way?) should last years and years - I know someone who bought a Dave Yates frame built from Columbus Max OR in 1989, and he still rides it. Unless it's actually snapped somewhere it shouldn't be affecting the ride that badly.

Given the choice of a decent steel hardtail over an aluminium hardtail for an all-day trail bike, I'd choose the steel frame every time simply for the comfort and responsiveness, something which is very difficult to emulate with aluminium.

rob cole

Highly unlikely. A good 853 frame (what make is it by the way?) should last years and years

totally agree - steel frames do not suffer the fatigue loading that aluminium alloy frames do - this is why alot of old aluminium alloy frames that have been ridden hard end up cracking somewhere

with aluminium alloy - every single knock and bump on the trails adds a fatigue load to the metal - this is something that doesn't happen with cromoly steel alloys - this is one reason that alot of manufacturers have dropped lifetime warranty in favour of 2 year warranty on their aluminium frames - its only the largest companies like Specialized who have the financial resources to absorb a lifetime warranty for aluminium alloy frames

this is also why it is wise to replace aluminium alloy handlebars about every 2 years, or perhaps every year if you are doing lots of dirt jumping or freeride stunts


however, you have to watch for internal corrosion with steel frames, this can eventually kill them....


given the choice, i would never ride another aluminium alloy hardtail - I have owned many hardtails in both steel and aluminium, and find the comfort and ride "feel" (zing) of a good quality steel frame is much nicer than even a high end aluminium alloy frame

aluminium alloy is perfect for suspension frames so that there is little flexure in the frame itself (to allow the suspension at both end to work more efficiently) without the weight penalty of trying to do the same design in steel

for a hardtail, i would go for high quality reynolds or true temper steel