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PostPosted: Thu Apr 17, 2014 1:16 am 
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Joined: Tue Feb 07, 2012 7:17 pm
Posts: 277
desperado, you are coming off the regular ti ones. I am interested in getting a pair of these but all this alu failure talk has me thinking of sticking with ti.

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PostPosted: Thu Apr 17, 2014 10:06 am 
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Joined: Wed Mar 28, 2007 12:49 pm
Posts: 1581
Location: Near Horgen, Switzerland
It doesn't take much common sense to work out that an aluminium pedal splindle is not a free lunch:
1) Nobody else in the industry currently offers an aluminium spindle
2) It was tried before and the pedals frequently broke
3) The material used here is a standard engineering aluminium alloy. It does not offer magical new properties which increase young's modulus, yield strength and fatigue limits
4) A quick look around the bicycle shows how big aluminium components need to be in order to resist pedalling loads. While the load cases are somewhat different, cranks and bb30 bottom bracket spindles are reliable and are much larger than pedal spindles.

That said, with a proper Engineering approach, it could be that an aluminium spindle might work in this application. Some thoughts:
1) The pedal uses a plain bearing, so the spindle can be somewhat larger diameter than a pedal using a ball or roller bearing. Since stress in the surface of the spindle decreases with the cube of shaft diameter, a small increase in diameter may enable a decent life
2) Use of a plain bearing means that the shaft is subject to wear. Imagine the case where a piece of quartz somehow enters the bearing area. This will notch the shaft and add a stress concentrations which may negate the benefits of (1).
3) The geometry of the shaft could be improved over the steel or titanium part. A quick look at their website shows the steel shaft is a constant diameter cylinder with a few flanges to locate the crank and pedal body. However, an aluminium shaft could be conical with a larger diameter at the crank. This would move the point of maximum bending stress in the shaft from the crank to the edge of the pedal body.
4) Static stress calculations (such as those mentioned by the manufacturer) are not a complete approach to analyse shaft stresses. For example ASTM standards suggest safety factors of 2-3 where rotating shafts experience heavy shock loading.

So in summary it might work, but only if the shaft diameter and geometry are changed to enable use of the different material. A straight material swap will not offer the same reliability.


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PostPosted: Thu Apr 17, 2014 4:52 pm 
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Joined: Wed Apr 08, 2009 7:49 pm
Posts: 46
As I said, I am definitely not a metallurgist as I said. All I know is that the pedal feels every bit as stiff as the ti. I won't say stiffer because I never felt the ti flex at all either. (I am currently about 190 pounds). I will take your comments and send them to Aerolite and see if I can get them to register for Weight Weenies and weigh in. I am told that the switch to the aerospace aluminum alloy with the special coating was recommended by an aerospace metallurgist, perhaps he can shed some light on his thinking. I believe the hard black coating on the simply protects the alloy spindle from gouging and provides a super slick relatively friction-free surface for the turcite sleeve to rotate over. The timetrial model should be out now and be slightly shorter (and lighter) and even less prone to stress. Their test showed that the ti pedal bent at 3,000 pounds and the aerospace alloy at 6,000....but they did not break. I still suspect that to be the case if fatigue is really a factor.


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PostPosted: Thu Apr 17, 2014 5:11 pm 
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Location: Luxembourg / Sweden
7068-T6511 would be the only commercially available alloy that is stronger than 7075. I guess you could call it a rocketsurgery alloy.
https://online.kaiseraluminum.com/depot ... ochure.pdf

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PostPosted: Thu Apr 17, 2014 5:41 pm 
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There are other aluminum alloys stronger than 7075-T6. As it happens, 7068 is extremely expensive and not that much stronger. 6Al/4V titanium is a cheaper and stronger option. Not my choice if I were making a reliable pedal axle.

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