Bike porn

where has all the porn gone?

+1

Pea-rost.

A very good one though ;-)

Why would you want to make this out of CFRP?

Why would you want to use a skinny axle like that in the first place? A BB30 axle can be over 3 times stiffer at less than half the weight.

that really, really makes me laugh..

de we have more pics of the rest of the bike??

what are the Dura-ace thingys on top of teh fork arms, something special or just BB screw holders?

just BB screw holders.

^

They look like crank dustcaps. I love that photo.

It's from Kris @ Julie Racing Design. Old (road) 30mm Cannondale fork with Marzocchi fork crown and custom "legs".

http://storage.canalblog.com/12/73/116602/53850051.jpg

Why would you want to use a skinny axle like that in the first place? A BB30 axle can be over 3 times stiffer at less than half the weight.

Not quite, to go to a BB30 for the same material you will maybe see a doubling of specific stiffness. Not 3 times.

You would then be able to have a lighter axle to have the same stiffness as you did with a standard 25mm axle.

Not quite, to go to a BB30 for the same material you will maybe see a doubling of specific stiffness. Not 3 times.

Show me your working.

In practice, 30mm axles are not made half the weight of 16mm solid square taper ones as the wall thickness would be impractically thin, so they end up more than 3 times as stiff.

that really, really makes me laugh..

and how it was made... http://www.oilycog.com/twist.htm
check that guy :)

Why would you want to use a skinny axle like that in the first place? A BB30 axle can be over 3 times stiffer at less than half the weight.

Show me your working.

In practice, 30mm axles are not made half the weight of 16mm solid square taper ones as the wall thickness would be impractically thin, so they end up more than 3 times as stiff.

Ok;

Assume four point bend deflection (but works for all small scale deflection cases), non-dynamic loading case. Where major span is 100mm, and minor span is 50mm. Again, as this is constant this does not need to be exactly the same as real BB's.

Assume square axle of width (a) 17.68 mm, (b) 21.2mm. Where (a) is the maximum width for a square axle on a standard 25mm BB, and the same for (b) where 21.2mm is the maximum for the BB30 standard.

Then assume an exposed axle length of 25mm (not that this is used in the calculation, but needs to be non-zero to allow for over roller rotation). Apply symmetric loading on the central loading rollers.

As in;

     .......↓.........          ↓.........

..↑...................... ↑..

Then calculate area moment of inertia, I. Where;
I=bh^3/12 where b=width, h; height

From above we say that b=h, thus I=h^4/3

Thus for,
BB25 I=(0.01768)^4/12=8.14e-9m^4
BB30 I=(0.02122)^4/12=1.68e-8m^4

Let stiffness be, say.... 163GPa, (medium stiffness UD CFRP - In this case thats Hex IM7/8552)

Then using;

E[flex]=F(L^3)/4Id;

where d; deflection. Re-arrange for d

d=F(l^3)/E4I

Letting F (quasi again!) =1kN

Thus;

d|(a)=(1000)(0.1^3)/(163,000,000,000)(4)(8.14e-9)
d|(b)=(1000)(0.1^3)/(163,000,000,000)(4)(1.68e-8)

Thus;
Deflection for
(a)[25mmBB]=1/5308=1.88e-4m, or 0.188mm
(b)[BB30}=1/11016=9.08e--5m, or 0.0908mm

Which is pretty much exactly 2. So for the same load, the deflection is half. Not a third.

I think that will do. And there was me thinking that I finished work at 5pm :-)

They are reissuing the laser. Finally Cinelli is doing something nice.. (though i believe they've made some "stupid" changes like ahead sets and shit like that..)

Cinelli Lasers are almost always pure porn in my opinion. Whatever happened to Cinelli...

Anyhoo rest of bike

Nice. Dude... no luck?

Assume square axle of width (a) 17.68 mm, (b) 21.2mm.

Where do you get these numbers? BB30 axles can be treated as a tube of 30mm OD, square taper axles typically use 6903 bearings, so can be treated as a solid rod of 17mm diameter (although some are hollow*). I was concerned with torsional stiffness, i.e. how much the axle winds up as the left crank is pressed down, since this seems to be a bigger issue than bending, certainly it dominates among BB factors when measuring total pedal deflection under load. BB30 axles are supported much further apart than typical square taper units (i.e. less overhang), so bending deflection is further reduced.

Now calculate the relative torsional stiffness of a 30mm OD 25mm ID tube versus a solid 17mm OD rod, since these are of roughly equal mass per unit length (the tube is 5% lighter). This is proportional to the polar moment of inertia, d^4 for the rod or (OD^4-ID^4) for the tube, so the ratio of torsional stiffness comes to 5:1 in favour of the tube.

The same rationale makes ISIS/Octalink axles (22mm OD 14mm ID) almost precisely the same weight as square taper and 2.3 times as stiff, which seemed like a good idea except that stuffing 22mm ID bearings inside a standard BB shell doesn't leave much room.

*If you bore through a 17mm rod with the 6.5mm pilot for the axle bolts, you need to increase the ID of a BB30 spindle to 25.5 to match the weight, and it ends up 4.7 times as stiff.

Where do you get these numbers? BB30 axles can be treated as a tube of 30mm OD, square taper axles typically use 6903 bearings, so can be treated as a solid rod of 17mm diameter (although some are hollow*). I was concerned with torsional stiffness, i.e. how much the axle winds up as the left crank is pressed down, since this seems to be a bigger issue than bending, certainly it dominates among BB factors when measuring total pedal deflection under load. BB30 axles are supported much further apart than typical square taper units (i.e. less overhang), so bending deflection is further reduced.

Now calculate the relative torsional stiffness of a 30mm OD 25mm ID tube versus a solid 17mm OD rod, since these are of roughly equal mass per unit length (the tube is 5% lighter). This is proportional to the polar moment of inertia, d^4 for the rod or (OD^4-ID^4) for the tube, so the ratio of torsional stiffness comes to 5:1 in favour of the tube.

The same rationale makes ISIS/Octalink axles (22mm OD 14mm ID) almost precisely the same weight as square taper and 2.3 times as stiff, which seemed like a good idea except that stuffing 22mm ID bearings inside a standard BB shell doesn't leave much room.

*If you bore through a 17mm rod with the 6.5mm pilot for the axle bolts, you need to increase the ID of a BB30 spindle to 25.5 to match the weight, and it ends up 4.7 times as stiff.

To be honest... this is going to be too much like work for me to finish it. Feel free to hire me(££) and I will be happy to!

But, 25mm standard BB, maximum circular axle is 25mm, maximum square axle to fit in this 25mm hole is... 25^2+25^2=a^2, thus a=17.68 mm. Same idea for BB30.

You should have mentioned torsion earlier then!

For flexure, BB30, will not increase stiffness by 3. See above. For torsional stiffness, I dont know, you can work that one out!

Anyway, its a wank idea. Something like a BB axle should not be composite. End of!

For flexure, BB30, will not increase stiffness by 3

Agreed

For torsional stiffness, I dont know, you can work that one out!

I have

Something like a BB axle should not be composite

Also agreed. Any time you're constrained by external dimension, your first port of call is likely to be the material with the highest stiffness by area, not by weight. I think this might break down with BB30, since my calculation indicates that making a 30mm axle which is as stiff as a square taper axle (presumed stiff enough, given that they are still used by track sprinters) yields a wall thickness so thin that buckling could become an issue. Moving to a low density material such as aluminium or CFRP could start to look favourable in that case.

You guys can take this technical discussion elsewhere, this is the pron thread.

It's necessary to demonstrate with mathematics why a carbon square taper spindle isn't porn.