Saturday, July 11, 2009

Crank Brothers Eggbeater Pedals

If the little bearing on end of the pedal fails the pedal body can come off the spindle. The rest of the ride will be one-legged (if your lucky, if un-lucky a bad crash and ambulance ride) . Now i have had this happen to me, but not with an eggbeater, and I was lucky and didn't crash.

To help prevent this from happening an extra bearing can be added. In addition to another bearing (686 sealed, vxb sells cheap), Will need a longer 4 mm bolt (local hardware) and a washer overlapping the inner seal lip, helps shield from the elements (since rubber end cap now can't be used..). Place 2nd bearing in after clip. Note will not work with all eggbeater spindles. (some use nuts rather than bolts to hold bearing in / hold pedal body unto spindle. )

Also will need to use thread lock or have the bolt bottom out perfectly without putting too much preload on outer bearing. Also should probably have a washer in between the two bearings the same or slightly greate thickness than the snap ring clip. Ok possibly if you get the spacing correct on the in-between-washer (not shown below), you might not need thread lock, etc.

2014 comments :  Hmmmm.  I havent' done this in years..  I don't even know if its possible . I am still riding Candy's though. 

Hauling Steel

Been taking steel to Metro Metals. They have a pretty nice and fast process and helpful and friendly people. Weigh on the way in, dump steel, weigh on the way out, they print a check a give to you.

This was $50 worth of steel,(I added a couple more items, and had 700 lbs of steel !)

Disassembling the metal shaper, still need to get the body hauled. Maybe next weekend if my little truck can handle the weight.

Building Tandem Wheel

Some pictures of working on tandem wheel.Here are the spokes, the nipples, the washers, nut driver. The nut driver bit had to make myself, with a file, brazing and then turning on the lathe. It is square to fit over the end of the nipples. The nipples took a long time to prep. I had to lathe-turn each one to get rid of the slot and to put a slight curve to them.

Here is a picture of the Wheel half-laced.

And here is a picture of the nipple (flipped) and washer inside the rim, to help spread out the load. Higher spokes forces are possible with lower peak stress onto the rim and nipple. Fatigue life should be improved with a stronger wheel. Also improved aerodynamics with hidden nipple. Fortunately with these rims, the rim bed was nearly flat for 9 mm od of the washers, So the washers are only slightly taco shape. The ideal is to have enough surface area in between the under force components that everything stays elastic.

2014 comments. since been sold.  New tandem wheels are only 24 spoked, but built using the same principles... 

Spoked Wheel - Part 4 - Elbow Strength

From previous post , the tensile strength and cross section areas are
233 ksi (2.34 mm) 0.0066658 in*in
240 ksi (2.0 mm) 0.0048695 in*in
245 ksi (1.8 mm) 0.0039443 in*in

Then it follows that:
The ultimate tensile load of 2.34mm spoke is 1553 lbf
The ultimate tensile load of 2.0 mm spoke is 1169 lbf
The ultimate tensile load of 1.8 mm spoke is 966 lbf

The spoke is predicted to break in tension at those forces. But for infinite spoke life, what we are interested in is is fatigue strength. For ductile steel, fatigue strength (or endurance limit) is approximately 0.5 of tensile.
Taking the elbow as the limiting factor, which we assume to be in shear we want the shear fatigue strength. Using distortion energy (von mises) failure theory, this is 0.58*tensile fatigue strength.
To summarize the max force that should be put into a spoke:
2.34mm: 233*0.5*0.58*0.0066658 = 450 lbf (2004 N, 204 kgf)
2.0 mm : 240*0.5*0.58*0.0048695 = 339 lbf (1508 N, 154 kgf)
1.8 mm : 245*0.5*0.58*0.0039443 = 280 lbf (1247 N, 127 kgf)
What this means is that for a spoke that has 1.8mm elbows, if the force in the spoke is kept under 280 lbf, the spoke should have infinite life.
In the elbow, the force transitions from an axial force into a shear force . Someday I may model this with FEA to see the stress contours. If there is bending stress in the elbow, which is definitely possible from not adequately stress relieving the spokes, then the stress in the elbow could be above the fatigue strength, and thus infinite spoke life will not be achieved.

Also note that just because this much force can be put into the spoke, doesn't mean it should be. The rim and nipple strength might not match. In the days of box section rims, the amount of spoke tension was determined by tightening the wheel's spokes to within a half turn of the onset of rim buckle. With today's deeper profile and/or sturdier rims, buckling is not the limiting factor. I have only experienced it with a very light box section rim, long ago...

Also of course, in the future, will look into the nipples and rim. The goal is to have as highly tensioned spokes as possible with infinite fatigue life of the spokes, nipples, and rim.
2014 comments:  Ahh this is good stuff. I still use this, my own blog post,  as reference :) 

Spoked Wheel - Part 3 - Fatigue Cycles

How many revolutions should a wheel last (need for fatique calcs)? Infinite is great of course, but what is reasonable? 10-20 years ? Rim-braked rims wear thin of course, so... For now I will go with15 years. Say 25 miles per day. 9125 mile per year. 210 cm (0.00130488 mile) circumference wheel (700x23c). 9125 miles/0.001305 miles per revolution = 6,992,882 revs per year or ~ 7 million wheel revolutions per year.
In 15 years, 105 million revolutions.

For a racing wheelset: 2 or 3 races a month, maybe 200 miles for 9 months a year. 1800 miles a year. Keep only 5 years. 6-7 million wheel revolutions.

However, typically the knee for fatigue life or endurance limit for steel is right around 1 million cycles. In other words, if the steel component being dynamically stressed doesn't break after 1 million stress cycles, it probably will have infinite life.

One issue is that Aluminum does not have an endurance limit.
Summary: Wheels see enough stress cycles that they should be built for infinite life.