Ken- I'm a lowly bike wrench with frame building experience. So both mechanical and dimensional aspects of bikes interests me. My understanding of bearing load is from many sources including my own. This is, actually, one of my pet peeves with modern design. The trade off of smaller balls for "cleaner" design. But if the execution is not spot on the smaller balls have less compliance to miss adjustment or alignment, let alone the greater brinnelling potential. Not a problem if you replace your bike every 3 years.
So while I understand the contact point load/deformation concerns between small and large balls, my preference has been for roller bearings for a long time (WRT head sets). I have no dog in this fight.
I guess I just take issue with the confusion between hype and mechanical principles. ----- Original Message ----- From: Ken Freeman To: Andrew R Stewart Cc: Dean Nixon ; classicrendezvous@bikelist.org Sent: Sunday, January 10, 2010 7:53 PM Subject: Re: [CR] Identifying Campagnolo Pista vs. Strada Headsets
You sound like a mechanical engineer, and I'm just an electrical and systems engineer, so I guess I basically defer and would like to learn. Are you talking about the force at the (deformed, I guess) contact between a single ball and the race, or the total load carried by the entire bearing. For two bikes of similar weight, load, geametry, travel speed, and current road condition, the load carried by each bearing system (top headset system and bottom headset system) should be about the same.If there are more contact points, how can the per ball load be greater. I could see the peak load in a given ball/race contact patch being greater in a smaller ball perhaps due to the smaller radius.
I guess you're saying the load capacity per ball is greater for larger balls because the radius at the point of peak contact pressure is larger that for the smaller ball. Greater durability in the bigger ball headset versus the track headset.
On Sun, Jan 10, 2010 at 6:19 PM, Andrew R Stewart <onetenth@earthlink.net> wrote:
Ken- Your take on bearing load capacity is the opposite that I understand. I've always heard (and see in my work) that the load a bearing is a geometric factor of it's diameter. So half the balls of twice the diameter will result in the unit having twice the load. If the ball count remained the same (and the overall size of the unit then grew) the unit would have 4 times the load capacity.
My take on the smaller balls in the Pista unit was two fold. First the smooth track conditions didn't require maximum load capacity and the overall stack height could be lower. this allowed a lower bar/stem position.
Any one out there have more definitive information?
Andrew R Stewart
Rochester, NY