Any (mechanical / bike) ENGINEERS in the house - opinions needed

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Any (mechanical / bike) ENGINEERS in the house - opinions needed

55 posts & 39 voices | Started 2 years ago by Aus | Latest reply from jond

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mick_r - Member

I'm apprentice trained + degree + Chartered Mechanical Engineer working as Principal Engineer in a fatigue test laboratory - and I haven't a clue when it will break!

It will break eventually just like any other lightweight bike component (1 year - 50 years - who knows without testing a statistically representative sample?). But then very few companies in the bike industry test a statistically representative sample as it costs too much and takes too long. Calcs and FEA are fine but a fabricated Ti structure is going to be pretty variable (especially where the point of maximum bending is the weld).

Just ride it and enjoy the bike. It is unlikely that both chainstays will fracture at the same moment and throw you to the ground (and is very easy to visually inspect bare Ti for cracks once in a while)

Posted 2 years ago #
ampthill - Member

Hmm I'll look in bycycle science tonight

Brant has undermined my post I thought bikes were within fatigue limit

Posted 2 years ago #
MidlandTrailquestsGraham - Member

...especially where the point of maximum bending is the weld

That was my point about the tapered chainstays.
No doubt the maximum force is at the point where the chainstays are welded to the BB, but by butting and tapering the tubes the elastic deformation can be made to take place more or less evenly along their length.

Posted 2 years ago #
thisisnotaspoon - Member

The Garry fisher sugar and I think some trek fuel/liquids were prrof that you can build an aluminium frame that has flexible stays rather than pivots.

Was it kooga who built an XC bike about 6 years back that had a crbon fiber plate under the downtube which the swingarm bolted to?

Posted 2 years ago #
futon river crossing - Member

Slasa make the El Mariachi - a Scandium softail and the Big Mama uses pivotless chainstays also in Scandium - so they must be happy with its longevity.

Spesh still has a lifetime warranty on its frames.

Posted 2 years ago #
mick_r - Member

Graham - the chainstays on the Global don't look very tapered to me so probably isn't any clever adjustment of the hinge point going on down there. Not sure about butting without cutting the tube open or asking Craig at Ride-On. As far as I'm aware they haven't had big problems with them breaking and they are nice guys if it does go wrong.

If it does ever break then could just weld it up and make a rigid strut to give new life as a hardtail

Posted 2 years ago #
cookeaa - Member

The question is an open ended one really, but you could simplify it down to a simple beam bending calc as it stands though it is meaningless as the following has not really been defined:

1- The specific material (your frame will be made of a Titanium Alloy of some sort the exact composition of which will affect it’s material properties – modulus of elasticity, ductility etc)

2- The crodimensions of the chainstay (effectively the “beam” in question) a lot depends on this really, cross sectional area shape and wall thicknesses as well as length as these would be used to define the second moment of area and point at which the load is applied, you probably won’t be able to get these without a detailed spec sheet and/or a hacksaw…

Working in a materials elastic range requires a bit more thought and calculation but it’s far from impossible…

It’s worth noting of course that it hasn’t actually failed yet, and I assume it hasn’t gone past it’s yield point as your stays still return to their original position after each compression (they do right?), it’s also worth noting that Titanium springs aren’t exactly unheard of, which probably cycle through far bigger deflections, more often and under greater loads than your soft tail.

So in short your mates Dad appears to be an Expert armchair engineer and not much more…

Posted 2 years ago #
cookeaa - Member

The question is an open ended one really, but you could simplify it down to a simple beam bending calc as it stands though it is meaningless as the following has not really been defined:

1- The specific material (your frame will be made of a Titanium Alloy of some sort the exact composition of which will affect it’s material properties – modulus of elasticity, ductility etc)

2- The crodimensions of the chainstay (effectively the “beam” in question) a lot depends on this really, cross sectional area shape and wall thicknesses as well as length as these would be used to define the second moment of area and point at which the load is applied, you probably won’t be able to get these without a detailed spec sheet and/or a hacksaw…

Working in a materials elastic range requires a bit more thought and calculation but it’s far from impossible…

It’s worth noting of course that it hasn’t actually failed yet, and I assume it hasn’t gone past it’s yield point as your stays still return to their original position after each compression (they do right?), it’s also worth noting that Titanium springs aren’t exactly unheard of, which probably cycle through far bigger deflections, more often and under greater loads than your soft tail.

So in short your mates Dad appears to be an Expert armchair engineer and not much more…

Posted 2 years ago #
juan - Member

Titanium presumably has behaves in a similar way

You'll find that all metal behave in a similar way. Some are just more plastic than others...

Posted 2 years ago #
aracer - Member

it’s also worth noting that Titanium springs aren’t exactly unheard of, which probably cycle through far bigger deflections, more often and under greater loads than your soft tail.

Not necessarily. Remember that a spring is simply a coiled torsion bar, so the loads in it aren't very similar to those in a softail's chainstays. Bigger deflections maybe, but that doesn't necessarily mean higher strain (using strain as a technical term there).

Posted 2 years ago #
monkeyp - Member

Titanium alloys have an endurance limit - that is, a stress level below which fatigue failure will not occur. This is the same for ferrous alloys (steels). Aluminium alloys do not have this, so even if the stress level is within the elastic limit, failure will occur at some point.

I would guess/hope that the design of the frame is such that any stresses are easily within the endurance limit of the material so failure will not occur. I say easily, because there would be a great deal of uncertainty of the stress levels at any welds.

Effectively you have a cantilever beam with a load at one end (at the drop-out) and fixed at the other (at the BB). The highest bending moment occurs at the BB (beam length x load) and the stress at this point (ignoring stress raisers such as the welds) is given by (bending moment x deflection)/second moment of area. So knowing the section properties at the BB you can calculate a stress level - allow extra for stress raisers and Bob's your mothers brother.

so if:
l = 0.4m
f = 2000N (assumes 200kg to bottom out shock)
then m = 800Nm

assuming 2 stays and that shock supports half the load = 200Nm/chain stay

estimating second moment of area for an elipse (I have assumed solid for simplicity but it shouldn't be too far off):
major dia = 0.025m
minor dia = 0.015m

I = 1/4 x pi x minor x major^3
I = 1.84E-07

Deflection = 0.05m

Therefore the stress = 54MPa

Which is quite easily within the range of an endurance limit for a titanium alloy even if you consider a 5 times factor for stress raisers and the other simplifications.

Suprising how far you can get with simple calcs

Posted 2 years ago #
monkeyp - Member

oops - I assumed 2" of travel not 1", so even better

Posted 2 years ago #
aracer - Member

Aluminium alloys do not have this, so even if the stress level is within the elastic limit, failure will occur at some point.

Even if the stress level is within the elastic limit, fatigue failure can occur with ti and steel. That's inherently the case unless those materials never fatigue (fairly sure all metals do to some extent or another), since as soon as you go beyond the elastic limit we're into plastic failure, not fatigue. As Brant points out (he'd know better than me, or most on here), steel components are designed so that they stay within the elastic limit, not necessarily within the endurance limit.

Suprising how far you can get with simple calcs

Yes, but GIGO. Not convinced assuming a solid is a very good approximation for example given how thin the walls are typically on a bicycle frame tube.

Posted 2 years ago #
monkeyp - Member

Ti and Steel DO have a stress level below which failure will not occur - termed the endurance limit. This will be within the accepted elastic range but depends on the specific material. Failure at loads above the elastic limit (i.e. plastic) are still fatigue - just at lower life at which point LCF strain-life becomes dominant along with many other considerations.

I would be concerned if components are not designed with the endurance limit in mind. The elastic limit describes the point at which the onset of yield becomes significant (typically at 0.2% strain) which could lead to low cycle fatigue - materials are NEVER completely elastic UNLESS they have an endurance limit and are below that limit. I do hope that someone with at least a bit of structural analysis capability designed my kit!

And tbh I couldn't remember or be bothered to find the formula for second moment of area for a hollow ellipse!

Posted 2 years ago #
stevomcd - Member

And tbh I couldn't remember or be bothered to find the formula for second moment of area for a hollow ellipse!

You already used it... just subtract the "internal" ellipse from the external one...

Posted 2 years ago #
R979 - Member

Materials scientist here. Although not going to claim to be a failure analysis or fracture mechanics expert.

I'm going to avoid getting drawn on some specifics of materials properties being thrown around in this thread. I would agree with the others who posted regarding the endurance limit (i.e. below a certain stress the fatigue life is practically infinite) of certain titanium and ferrous alloys, this was also my understanding.

Short answer to the OP - I wouldn't worry. This isn't based on any sort of professional judgement - it's just that plenty of other people have done it: Ibis, Moots to name a couple.
When I first saw this thread it reminded me of an Ibis I read about years ago - turns out it was the Ibis Bow Ti - which it also turns out was developed by the same guys Brant mentioned earlier in the thread:

http://www.castellanodesigns.com/diff.html

Moots are still making the YBB by the looks of things

http://www.moots.com/#/product/bicycles/mtb_26/ybb/

I've not come across pivot-less softails made out of any other materials other than Ti (EDIT: with the exception of the Fango already referenced). Maybe that's a fatigue related thing, maybe it's easier to get an appropriate spring-rate vs stiffness balance - dun know? Lahar did a pretty random carbon fibre thing though (though that's more of a soft(hard)tail I suppose):

I would assume these designs aren't that popular because people think (rightly or wrongly) they are going to break. That was my first thought when I saw that Ibis years ago. No plans on owning one, but I personally think they are a pretty interesting design.

Posted 2 years ago #
danflowbiker - Member

the man is jealous!

Posted 2 years ago #
igm - Member

[suck teeth mode]It's the poor quality welding that'll do for it, son.[/suck teeth mode]

Posted 2 years ago #
Jon Taylor - Member

Brant - both point valid but

1. the natty aluminium chainstay bridge is only necessary if the simple oval/circular section tubes don't work with alu, no? I hypothesise anyway, maybe he just wanted an expensive gimmick to make people believe it would work anyway.

2. I wouldn't expect any frames to be designed for infinite fatigue life, maybe only a judgement of useage and cycles. As you know there are empirical (crap) rules for adding your high/low cycle fatigue and given the fluffiness of any estimates of useage, riding and major impacts I'd be surprised if many even did that.

Anyway, I'm not a bike designer so toodle pip

Posted 2 years ago #
jond - Member

>I've not come across pivot-less softails made out of any other materials other than Ti

Cannondale Scalpel ?

Posted 2 years ago #