Pedal Axle length and force on crank arms

[DaveQB]

Hi,

After my most recent bike fit, a numb lateral part of my foot issue I've had since taking up cycling finally had a solution, I need longer pedal axles due to the way I am built and thus pedal.

LONG STORY
I run Speedplay and the standard 53mm long spindles. I found they had multiple lengths, up to 65mm (12mm longer) but for $180+ a pair. I found a few aftermarket options for longer Speedplay spindles, but I was on the edge of the weight limit for them so I was scared off. I then found Knee Savers (http://www.kneesaver.net/). These give me 20mm extra length and are a lot cheaper. I bought a set to test that it fixes my issue, it does. In talking with Randy, the maker and seller of Knee Savers, he said he has only had 1 crank failure with someone using these extensions in his 20+ years of selling them and they were on a carbon crankset although he said his typical customer is not racing and sprinting etc. In talking about my weight and power numbers, he said I probably shouldn't run them (and do any sprinting) on my Sram Force 22 crankset. I think carbon is stronger than alu and contacting Sram on Twitter asking if they have a pedal axle length limit on their cranksets, they replied they did not. I have since hit 900 watts a few times using these while commuting and taking off from traffic lights etc. So not a real hard go, but a little effort and nothing apparent has happened.

SHORT STORY
So, is 20mm extra length on my pedal axles anything to be concerned about? Do Speedplay's have +12mm as their maximum axle length because they think it as safe as you can go or just based on consumer demand? I am sure there is science that can work out the differences in forces a 73mm axle causes on the crankarm vs a 53 long axle. Am I worrying about nothing or should I stick to the 65mm Official Speedplay Spindles and be done with it? It is seeming like the optimal cleat position is at about the 67mm axle length for me.

Thanks for any advice.