Or, Don’t Try This At Home!

If you’ve checked out the Speedhound Dropout System, you probably know that the interchangeable dropouts are attached to the frame receivers with standard steel chainring bolts.  These fasteners have the benefit of a large surface area to contact the chainrings and crank spider, or in the case of the SDS, the dropouts and the receivers.  The threads on the bolts don’t come into contact with any of the mating surfaces, and the bolts do a good job of handling shear forces.

Think of a crank spider and chainring as the blades of a pair of scissors, trying to cut through the bolts.  As you jump on the pedals, your power is transmitted from the crank spider to the chainring, which pulls the chain forward.  A large, powerful rider could generate 500 pounds of pulling force at the chain, especially on a small chainring and long cranks.

Once in a while I like to do some sprints to rev up my heart and keep my legs sharp.  I’ll do hill sprints, on the bike or on foot.  Other weeks I’ll just make sure to include some focused accelerations during my rides.  During the winter, I get it done indoors on a trainer, rotating through five spinning CDs.  I aim to average 10 minutes of actual sprinting every week (not counting warm-ups and rest periods).

I rarely get on the “nowhere bike” during the summer, but last week a heat wave hit, and I was feeling short on sprints, so I went down to the cool of the basement to catch up.  I’ve got a fixed-gear Speedhound that I use on a variable-resistance fluid trainer.  It’s got a Gates Carbon Drive belt and 50X20 sprockets and it’s very smooth.

We’re finishing up the development of the production version of a slider-style dropout for the SDS (more on that in a future post).  I needed three chainring bolts for a meeting at the machine shop that’s helping us with the project.  I didn’t have time to go to Speedhound HQ, so I pulled the bolts from the Sugino RD-2 cranks on my Speedhound fixie.

The other day, a panicked thought crossed my mind – had I done my last 40 minute spinning session with only two chainring bolts in the crankset?   Yikes.  I ran down to the basement to look.  Here’s what I saw:

Missing Chain Ring Bolts

The two bolts were next to each other, leaving 288 degrees of the sprocket unsupported. But the sprocket ran true and there were no clicks or any other signs that parts were missing, other than three empty holes.  Nothing bad happened, no sprocket tacos, no noises, nothing different at all.  Why did it work?  Spot-on alignment between the front and rear sprockets, the lateral stiffness of the Gates Carbon Drive belt, the meshing of the teeth and sprockets, and the location of the sprocket centerline to the inside of the shoulders on the spider.  Oh, and the ability of those two chainring bolts to resist the shearing forces as I pedaled.  I’m back to five bolts now.  And please, don’t try this at home!