what does “STIFF” mean?

as above, stiff frame, stiff stem, stiff cranks, stiff………..does it mean less flex so more power transfer? it is talked about as if very important so can anyone tell what a stiff stem feels like/tell the difference between riding a stiff stem bike and a not so stiff? or stiff frame etc? if it is so important then surely a Ti frame is crap because it is too flexable? answers on a postcard please

so can anyone tell what a stiff stem feels like ?

just ask the ladies ?

or gayers, obviously…

In terms of fork’s lateral stiffness, it means more accurate tracking and less sideways deflection. Stiff stems twist less, so again, more accurate feeling. Same goes for cranks.

But I think I know where you’re going with this – what’s the point of having stiff ‘everything’ and then spending 1.5 grand on a sus frame. I concur entirely.

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😉

“what’s the point of having stiff ‘everything’ and then spending 1.5 grand on a sus frame”

Because the stiff sus frame will have been designed to allow only movement in the suspension direction, not laterally (Well thats the idea)

Stiff is good, dead is not. Aluminum is described as a dead material as when made into a frame produces something that dosent absorb shocks, it reverberates them. This is why Aluminum fatigues alot quicker than Steel.

Titanium absorbs vibration very well, lending itself to a comfortable ride. It dosent have much flex though, so it can be classed as being laterally stiff. Generally, stiff frames and components are good. Flexible components, are not.

Happily, as with many such things, while the differences involved are noticeable and measurable, they affect the enjoyment of the amateur sportsman very little.

😉

stiff cranks feel stiff (hard to notice unless you back to back test some cheep truvative ones on a square taper BB against some newer HT11 ones)

ditto frames, full sussers tend to wag their swing arm’s from side to side, so a stiff one generaly* inproves things.

a ti/steel frame will be less stiff than a alu one, but the idea is to shape the tubes so that its flexy verticaly (giving a few mm of suspension) and not horisontaly or in torsion.

Theres a video somewhere of a frame being tested on a machine, a big rigid pole thriough the headtube and a ram bending it back as if the forks were slaming into rocks repeatedly. You’d be supprised how much frames flex!

How much felx is a personal thing, some people like easton RAD cove stiffee’s, other like Ti jones’.

*some felx is needed, if it were entirely rigid then it would snap, flex allows forces to be distibuted through the structure, hence why headtubes are gussetted, as the frame flex’s the gusset helps transfer that load to the down/top tube. Ditto the cannondale propet, in its first MBR review it was slated for beign “flexy” in a recent one they couldt praise it enough!

Oh I do love to see the amateur material scientists come out and get it totally wrong. 😆 at dooge – where did you get that lot from?

but the idea is to shape the tubes so that its flexy verticaly (giving a few mm of suspension) and not horisontaly or in torsion.

I’m curious – how exactly do you do that?

Theres a video somewhere of a frame being tested on a machine, a big rigid pole thriough the headtube and a ram bending it back as if the forks were slaming into rocks repeatedly. You’d be supprised how much frames flex!

Well all thats showing is how much the frame flexes on that test rig. Not on a ride.

a ti/steel frame will be less stiff than a alu one,

why?

but the idea is to shape the tubes so that its flexy verticaly (giving a few mm of suspension) and not horisontaly or in torsion.

Really?

I’m no metallurgist but what doodge said sounds like rubbish 🙁

I doubt its possible to feel the differnce in flex between one 4 bolt stem and another… maybe a real long, cheap 2 bolt stem you might.

Think cooked and dry spaghetti.

tinsy –
I doubt its possible to feel the differnce in flex between one 4 bolt stem and another… maybe a real long, cheap 2 bolt stem you might.

The difference is quite alarming actually. Once you fit a “stiff” stem to your bike and go ride you’ll wonder how you managed without it.

You’d be supprised how much frames flex!

No I wouldn’t. I’ve been watching aerial shots of sprint finishes in the TDF for many years.

I could make you a VERY stiff steel frame, just give me a welding rig and some scaffold bar.

I could also make you a very flexible ally one. Where did I put my bacofoil?

“Well all thats showing is how much the frame flexes on that test rig. Not on a ride.”
presumably the test rig is designed to replicate a reality? otherwise its a pointless test?

“why”
higher tensile/compressive strength, so less material used, so mroe flexible (modulous tends to vary less than absolute strength). Yes could could make the alu more compliant with thinner tubing and spending more moeny, but equaly you could spend more money on the steel one as well.

“Really? “
well a steel frame flex’s more, torsional and lateral flex are generaly bad things (ever ridden a shopper bike, they hate corners as the frames cant keep the wheels lined up!) that leaves a bit of vertical compliance to remove some fo the vibraion formt he rear wheel.

Ol’ Sheldon says:

“Much of the commonplace B.S. that is talked about different frame materials relates to imagined differences in vertical stiffness. … Virtually all of these “differences” are either the imaginary result of the placebo effect, or are caused by something other than the frame material choice.

rigid metal is rubbish at absorbing shock and vibration. Lead might work.

gnar, I must be shite then, have ridden 4 different 4 bolt stems, a Specialized, Hope, Thompson Elite and Thompson x4 not noticed any stem flex on any of them.. Like I said maybe if I had a 1ft long quill stem I would have noticed, but between 4 different 4 bolts I cant tell..

thisisnotaspoon, I believe the test rig was to test frames to destruction, to show up any weaknesses in the design or manufacture.

Stiffness is good when it is in the place you need it and bad when it is in the place you don’t.

Forks, cranks, rear triangle, do you want them all to be the same level of stiffness? I don’t think you’d get me on that bike!!!

Modern racing motorcycles are built with “controlled flex” as with no flex the suspension does not work went leant right over – sus forces work vertically but the sus movement is at 60 degrees from the vertical. without some flex they get chatter.

On the tandem when braking hard you can see the front fork flex back towards the frame by an inch or two. One day the headtube will snap off

tinsy

gnar, I must be shite then, have ridden 4 different 4 bolt stems, a Specialized, Hope, Thompson Elite and Thompson x4 not noticed any stem flex on any of them.. Like I said maybe if I had a 1ft long quill stem I would have noticed, but between 4 different 4 bolts I cant tell..

I’m sure it’s not because you’re shite.

Maybe you’re just a buttery smooth rider, or maybe you are very light, or perhaps you have arms like a six year old girl, or perhaps your frame and fork have a relatively large amount of flex meaning you cannot appreciate the relative stiffness of a given stem, maybe you dont stray off of fire roads. Or perhaps all the stems you tried were similarly stiff.

Anyway I went from an FSA dh 4 bolt stem to a Diabolous. The difference was immense, despite the FSA being 30mm and the diabolous being 65, the diabolous was much much stiffer. Just sitting on the bike without moving you could appreciate the difference.

On the tandem when braking hard you can see the front fork flex back towards the frame by an inch or two.

Yes, but almost all that flex will be in the fork which is cantilevered, not the frame which is triangulated.

well a steel frame flex’s more, torsional and lateral flex are generaly bad things (ever ridden a shopper bike, they hate corners as the frames cant keep the wheels lined up!) that leaves a bit of vertical compliance to remove some fo the vibraion formt he rear wheel.

So as I asked before, how exactly do you get rid of the bad torsional and lateral flex whilst leaving the good vertical flex in?

TJ, excactly my point, some felx good, some flex bad.

I had some manitou minutes, they flexed so bad that the steerer rubbed on the inside fo the headset cup’s! Thats bad flex 🙂

imagined or not, my current DMR feels much less harsh than my old carrerra aka merrida aka specialized hardtail.

It’s the amount something deforms due to given loading.

http://en.wikipedia.org/wiki/Stiffness

Can o’ worms.

As regards stem – I have a 120mm bonty stem (light two bolt) and a 75mm thomson – with them installed on the bike there was zero flex – the frame was held rigid – I leaned my full weight on one end of the bar and the other end of the bar did not move at all – so under my full weight on a long lever no flex.

You do notice stiffer components, if you are paying attention.

On my Pace bought in 2001, I initially had XTR cranks on it (from 2000 I suppose) but I went to a Middleburn XC duo when it arrived. The difference was quite surprising as up til that point I didn’t believe crank flex would make any difference.

As for stem and bar flex – if you sprint hard heaving on the bars, you’ll notice it flex. You can clearly tell on my XC race bike.

Fork flex affects how stable your bike feels on big rocky bits – not visually noticeable but you can really feel it when you move to something stiffer.

Wheels can flex too – I’ve got Mavic 317s on Sapim super thin spokes on my 5 and they are too flexy – the spokes are too thin I think. You can really tell when hitting the rocks as the frame and forks are stiff but you get pinged off line very easily.

Frame flex – well, tricky one this. As I understand it (and I know there are framebuilders on here of which I am not one) some materials absorb vibration at different frequencies due to their density and young’s modulus etc etc, and how much of the energy going into the deformation of the material is stored as potential energy and how much is released as heat through friction…. or something.. anyway materials differ in how they transmit vibrations I think. There’s also other considerations for instance alu is not as strong as steel but lighter, so you can make frames with larger diameter tubes which will flex less. I’m guessing the fabled ‘spring’ of ti frames comes from the fact that the tubes are thinner, like a steel frame, so that they flex – but the thing is lighter which makes the whole bike feel more sprightly…

Although some ti bikes are quite stiff and dead, and some alu bikes are springy and lithe. My old Pace for instance was very flexy (so much so in the BB area that I couldnt’ honk out of the saddle cos I’d lose the chain off the big ring). The rear triangle on the other hand was reasonably stiff so it rode nicely.

This is all armchair theorising tho, but it IS certainly the case that stiffness in certain areas of the frame changes the way it rides. For other components, stiffness is pretty much universally good. Apart from bars and seatposts maybe 🙂

Hmm I sound slightly drunk in that last post, don’t I?

As regards stem – I have a 120mm bonty stem (light two bolt) and a 75mm thomson – with them installed on the bike there was zero flex – the frame was held rigid – I leaned my full weight on one end of the bar and the other end of the bar did not move at all – so under my full weight on a long lever no flex.

This is impossible.

You mean “I could not detect or see any flex”.

If you loaded the front of the bike, the front tyre wold compress. So the bar appearing to “not move at all” would suggest the other end has rotated slightly to counteract the compression on the front tyre.

Stems twist. They just do.

some materials absorb vibration at different frequencies due to their density and young’s modulus etc etc, and how much of the energy going into the deformation of the material is stored as potential energy and how much is released as heat through friction.

nah.

Also beware of using words like thinner. A tube is a structure with an outside diameter and a wall thickness. How you play with those makes frame rides how they do.

Also beware of using words like thinner

I meant smaller external diameter when I said thinner…

Also with regards vibration absorbtion, I reckon it does happen but I don’t know if it makes a difference with metals etc. Certainly happens with rubber 🙂

some materials absorb vibration at different frequencies due to their density and young’s modulus etc etc, and how much of the energy going into the deformation of the material is stored as potential energy and how much is released as heat through friction.

no, the word is ‘hysteresis’, which is low in many metals and high in rubber, explaining their mutually exclusive use for frames and tyres, for instance.

Hysteresis? With respect to materials and loading isn’t that just another name for plastic deformation?

With respect to materials and loading isn’t that just another name for plastic deformation?

no – even in nonplastic deformation some work is done

Stiff: 75-150 CuKN/m^2, an approximate N value of 16-33

or for field observations “cannot be moulded by fingers, can be indented by thumb”

and that, is a Geotechnical Fact.

no, the word is ‘hysteresis’, which is low in many metals and high in rubber, explaining their mutually exclusive use for frames and tyres, for instance.

I was going to type hysteresis, but then I read the wikipedia entry and realised it wasn’t quite what I meant.

How does Carbon fibre compare to metals on that score tho? Everyone in the road community knows the value of carbon for reducing ‘buzz’.

Hysteresis? With respect to materials and loading isn’t that just another name for plastic deformation?

No – hysteresis is energy loss due to elastic deformation.

As sfb says, but to elaborate, all common engineering metals used in bicycles have such a low hysteresis that they absorb vanishingly small amounts of energy compared to the amount absorbed by the tyres/saddle/your bum. The springiness of a ti frame is all in the BB deflection, not in the vertical direction, hence what people think of as a comfortable frame is just noodly and not actually any more comfortable at all.

splay too