How Stiff Do You Want It?

[Phatmike]

That right there make me instantly not want to buy one.

The idea might not even make it to market Al. So you probably won't be able to even if you wanted to! It's all purely conceptual at the moment.

[dirt jumper jake]

Like i said to you before, if its anything like your pads, It'll be a stonker

[Matt Vandart]

I would like to question this statement. a metal will have the same breaking point irrespective of it's rigidity, for example if you were to take the same amount of alloy and make a solid rod rather than a hollow tube it would have the same tensile strength yet the tube would be much stiffer because of it's overall diameter. If anything flexy frames are less likely to snap because they will reduce the momentum caused by impact rather than stopping it in a single go, thus there will be a slower acceleration and less force actually put through the frame. 

In my opinion the best frames are sufficiently rigid not to feel flexy yet have enough movement to make the ride more comfortable, that is why steel frames feel great because they are stiff yet allow a certain amount of flex

Not necessarily, see below,

Flex does usually play a part in the strength of a material, but the main issue with frames is fatigue. Aluminium is very poor compared to steel, which is worse than Ti.

To put numbers on it you could bend a piece of alu once, bend it back and it would snap. Steel would last 20 bends and Ti around 1000! That's what makes Steel & Ti such tough materials. Have you seen the dents put in Stedman's steel bikes over the years? I remember when we'd throw massive rocks at each others bikes trying to dent them! I wouldn't imagine a Sky being so tough, even if it was much heavier.

Stress is a measure of internal stresses in a deformable body (thanks wiki), "internal forces are distributed continuously within the volume of the material body". That is to say that an optimum design would really distribute stress evenly throughout a frame rather than let stress areas build up. Even if we ignore stress risers such as welds or design flaws (which just make an area more likely to break) then flex won't always lead to snappage, even repeated, cyclic stress.

Providing the stresses that are cyclically applied don't exceed the endurance limit then theoretically the part will never break. The endurance limit is the level of stress that can be applied cyclically, indefinitely before a part breaks - essentially a type of elastic limit. A factor of safety could even be calculated and built in for big moves and crashes - as in the graph above the uts is 4 times the endurance limit.

If you wanted to be really technical you could measure the forces put on a bike and their frequency. Over a set time the stresses could be used to calculate exactly how long a part would last given an individual rider's usual riding! Seriously geeky stuff

The bottom of the curve would be close to the endurance limit. (In this case around 70Mpa for an alu of 320Mpa Ultimate Tensile Strength)

So if you want flex and spring and you can calculate it, where would it be? Up and down in the back end, but not torsional?

Edit: Hmm, this all sounds very german. I'll try and keep the visuals a bit more organic at least!

Well no actually.

Aluminium alloys WILL break even if they do not exceed their elastic limit, it's due to work hardening.

Steel will to all intents and purposes NOT break if it never exceeds it's elastic limit, I say to all intents...........because the number of cycles is way up in the millions region, a figure one would never reach in the life of a frame.

[PaRtZ]

I was under the impression work hardening was when a material is subjected to stresses past its elastic limit, and then the stresses are relased. This creates a 'longer' line of plastic deformation before it is work hardened further

I reckon youre right about failure before elastic limit, but it cant be work hardening as you have to go past the elastic limit to work harden

[oakley]

I hate the feeling of anything flexy pretty much anywhere on my bike. Ever since the old Slinger/T-Master proto frame I rode for Onza yeeeeears ago, I've always loved really stiff frames/setups. I usually overtighten my headset for that reason, and run really high spoke tension too (as well as using paper-thin grips and thin pedals). Flexy wheels are the absolute worst - it's amazing how much 5 minutes with a spoke key can transform how your bike rides...

Sticking with frames though, I'm much more of a fan of having them as stiff as possible. I like to be able to feel what's going on when I'm riding, and feel what my actions are doing to my bike, so having a frame that flexes too much just kills that totally. I like the feeling of control you have when you know that the input you give is going to have the reaction you'd expect, in much the same way I 100% prefer riding hardtails compared to full sus bikes because you can use pumping and movement of your body much more.

I am completely and absolutely with mark here!!

[casualjoe]

Stiff as possible please, I want to be able to feel what the tyres are doing.

I thought flex in a frame is mainly beneficial when tracking fast over bumps, not something a trials bike does too often.

[Matt Vandart]

I was under the impression work hardening was when a material is subjected to stresses past its elastic limit, and then the stresses are relased. This creates a 'longer' line of plastic deformation before it is work hardened further

I reckon youre right about failure before elastic limit, but it cant be work hardening as you have to go past the elastic limit to work harden

Sorry not concentrating properly, well sort of but with the following points to take in mind.

The failure will be from fatigue, as you are probably aware aluminum has a (relatively) low cyclic failure figure.

This means aluminium tubes will fail below their ultimate tensile stress limit if enough cycles of this low load occur.

This is due to fatigue which is initialized usually by localized dislocations within the material crystaline structure.

Where do these dislocations come form? From localized work hardening where there is a wide stress range, i.e where it is flexing.

All within the materials elastic limit.

Or something like that I'm still not concentrating cos Bruce almighty is on and its funny.

Edited December 11, 2010 by Matt Vandart

[sstein]

I had a long response written out but my computer crashed and I lost it. All I really need to say is that it depends on how well a frame is designed as to how flex will affect it. If a frame is designed around a certain level of flex such that the frame dissipates said flexing forces through a large area of the frame and not a single point then the flex can indeed make the frame more resistant to failure. If however the frame is designed to be ultrastiff and flex is caused by the natural deterioration of the material after a certain amount of use then it will cause the frame to break. 

I never said that the frame will not break, only that it is less likely 

[Rusevelt]

The rear frame triangle on TRA's signature Caelifera, has got to be one of the stiffest frame designs currently on the market.

Edited December 11, 2010 by Rusevelt

[PaRtZ]

Sorry not concentrating properly, well sort of but with the following points to take in mind.

The failure will be from fatigue, as you are probably aware aluminum has a (relatively) low cyclic failure figure.

This means aluminium tubes will fail below their ultimate tensile stress limit if enough cycles of this low load occur.

This is due to fatigue which is initialized usually by localized dislocations within the material crystaline structure.

Where do these dislocations come form? From localized work hardening where there is a wide stress range, i.e where it is flexing.

All within the materials elastic limit.

Or something like that I'm still not concentrating cos Bruce almighty is on and its funny.

lol ok that makes more sense. Yeah I agree there

[Ali C]

I wouldn't class myself as a great example of someone who can tell if a frame is stiff or not. I have had one flexy frame (the Global) and you could feel that flex side to side when pedaling. Ever since that frame I have never though "oh wow, this frame is stiff!" or "oh wow this frame is flexy", they have all done the job and I havn't noticed if they are flexy or not.

I have ridden other peoples bikes that are famous for being stiff (koxx boxx) and yes, the rear brake was possible the stiffest I have felt without a booster, but it didnt "feel" any different when riding, the same goes for flexy frames such as the Kot MS2, that "felt" normal to me too.

I will agree that stiffer is better, even if I cant feel it I understand the benifits from having a stiff frame over a flexy one. Talking about steel or ti having a good "feel" and stuff only really works for xc frames from talking to people, the longer tubes makes all the difference, you don't hear bmx riders going on about the "feel" of steel.

[Kieran Morrison]

Are all these threads requesting pics and opinions leading to something Mike? Your not planning on bringing out a Phatframe are you???

That would be coool! being able to say you can ride a Phat frame haha. (or PhatPhrame )

[indianatrials]

I think on a mod its good to be just about as stiff as you can get it, mostly because you get more tire squish but it just generally feels better. On a stock though, the most comfy stock i ever rode was a kortz II which is IMO extremely flexy. Right now i have because trial 24" bike and its just medium as far as stiffness goes and it rides great.,

[N.Wood]

I wouldn't class myself as a great example of someone who can tell if a frame is stiff or not. I have had one flexy frame (the Global) and you could feel that flex side to side when pedaling. Ever since that frame I have never though "oh wow, this frame is stiff!" or "oh wow this frame is flexy", they have all done the job and I havn't noticed if they are flexy or not.

I have ridden other peoples bikes that are famous for being stiff (koxx boxx) and yes, the rear brake was possible the stiffest I have felt without a booster, but it didnt "feel" any different when riding, the same goes for flexy frames such as the Kot MS2, that "felt" normal to me too.

I will agree that stiffer is better, even if I cant feel it I understand the benifits from having a stiff frame over a flexy one. Talking about steel or ti having a good "feel" and stuff only really works for xc frames from talking to people, the longer tubes makes all the difference, you don't hear bmx riders going on about the "feel" of steel.

I agree... The only areas I feel in regards to flex are the brake being stiff (or not) and the rear end flexing doing hard short uphill runups or whatever.

Never felt a frame was flexy when hopping about as there are lots of variables (tyre pressure, tyre type, tube type, depth of tread, temperature etc) that IMO would make more of a difference than the frames stiffness.

[sstein]

you don't hear bmx riders going on about the "feel" of steel.

You also dont see many BMXs made out of anything apart from steel though for that matter. I agree that most people will notice very difference between steel and ally for a trials frame and that once again it does make a huge difference how the frame is designed but personally I do notice a difference between steel and ally frames, not so much from a comfort point of view but a confidence point of view. The more forgiving feel of steel frames makes me feel less like my frame is about to snap if something is not pulled off perfectly (though that mat just be me)