SPIDER SIMULATION/TECH ARTICLE

 

 The video images shown above are 10x true scale deflection of the spiders under a 90 N-m moment load. The 90 N-m moment load represents a 125 lb force on the pedals of 160 mm long cranks. The maximum stress seen in the 4-bolt spider at worst case conditions is ~260 megapascals and in the 5-bolt spider just shy of 300 megapascals! The yield stress of AL7075 is 503 megapascals, so these spiders have a nice factor of safety! Please keep reading for a comprehensive breakdown of this analysis.. 

A moment load in engineering terms is defined as the perpendicular force times the distance of that perpendicular force where the moment load is being calculated... so what the hell does that mean? Think of it this way, a force at a certain distance from a pivot point will want to rotate something, the most common example is a door. Referencing the image below, if you push on a door all the way out by the door knob, or at distance d2, it is pretty easy to close or rotate the door because the moment load at the hinge of the door is relatively high. Now, if you push on the door with the exact same force by the hinge, or at d1, it becomes somewhat difficult to close or rotate the door because the moment load at the hinge of the door is relatively very low because the distance between the force and the hinge of the door has been significantly decreased. Let's put some numbers to this. Say F1 = 10 lbs and d2 = 3 feet, the moment load at the hinge of the door is 10 lbs * 3 ft = 30 ft-lbs. So what is the moment load at d1 if d1 = 0.5 ft? Well, it would just be 10 lbs * 0.5 ft = 5 ft-lbs or 1/6 of the moment load at d2 even though we are pushing on the door with the same force, F1. The higher the moment load, the more likely an object will bend or rotate around a pivot point. Just one more ponit to drive home, the third F1 in the image below, the one pushing on the end of the door provides 0 moment load at the hinge, why?  Because remember, the moment load only considers the perpendicular force and this illustrates why that is. 

 

So why go through all of that explanation about what a moment load is? This is exactly how this simulation was established. A moment load is replacing the force of 125 lbs on a pedal at a distance of 160 mm away (by the way, the max weight limit and the longest Identify BMX cranks available). The force from the rider transfers into a moment load on the bottom bracket which then transfers through the spider, through the chain ring bolts, through the sprocket teeth and finally the chain.

To simulate the stresses in the spider due to a rider hammering on their pedals, we've modeled a solid disc that the spider mates to and applied a moment load to the blue highlighted surface shown in the left view. Since a moment load is the exact same thing as a force times a distance, it can replace the crank length times the force a rider is pushing on their pedal. For example, a 125 lb force on the pedal on a 160 mm crank is the same thing as putting 90 N-m moment load on the modeled disc below!!! In our simulation, we constrained the center of the spider to prevent it from moving and applied the moment on the disc and then watched what happened! To take it one step further, the forces will actually transfer through the bolts that hold the spider on the crank, which would lower these stresses even more in real life applications. The simulation is truly in worse case conditions and the spider holds up.