So I posted this the comment below on Dan's preview of the new Specialized and he responded that he disagreed with the Cp rationale, so we're moving it here to discuss further:
"...As for fork leg placement, we proved from a number of angles that you need the center or pressure for the front wheel in front of the steering axis and not behind it. This is due the way that countersteer affects lean angle..if the Cp is behind the steering axis, a transient gust of wind will steer the wheel into the wind which makes the contact patch go in that direction initiating a lean in the opposite direction (lean with the wind) which then requires a large steering correction to stabilize. It seems counterintuitive, but having the Cp in front of the steering axis means that a gust of wind will steer the contact patch away from the wind which initiates a lean into the wind, so the effect is naturally stabilizing.
Sadly, Zipp doesn't show much CFD or data anymore, we used to have lots of interesting stuff on this when I was there, you can Google AIAA Matt Godo Zipp CFD or my name and find a lot of it, but the great example we used in the original Firecrest stability white paper was that a front disc has a Cp well behind the steering axis which is why a front disc outdoors can be stable in stable wind, but can be downright terrifying in gusty or variable wind. Similarly most 3 spoke wheels have Cp in front of steering axis at low yaw and behind it at high yaw which explains some of the challenging handling behavior of those designs in high yaw or variable wind."
The backstory here is that when we were developing Firecrest using Matt Godo's CFD protocol we realized that we could predict handling using side force mapping and Center of Pressure (Cp) calculations that were already beautifully built into the FieldView software.. the problem them became where to move it? Initially we wanted it positioned aft to give the wheel a rudder effect, but early prototypes were NOT stable. Benchmarking wheels at that time we realized that wheels people thought were stable, namely shallow wheels all had Cp that was forward of hub and of course low side force and wheels that people thought were unstable, front discs, the 1080, 3 spoke/5spoke wheels all had Cp behind the steering axis or Cp that migrated from in front to behind during the yaw sweep. The only other player who has really made a claim in this area is ENVE who along with Simon Smart targeted a Cp in front of the steering axis that resulting in a linear torque rise with wind angle, while the Zipp philosophy was to try and keep the Cp in front of but as close to the axis as possible... the difference is that the Zipps had lower torque that was non-linear to yaw angle while the ENVE's had higher torque that behaved more linearly.
Having said all that, this was roughly 10 years ago now and I've been out of the game for a few years so I'm free to talk and really interested to hear Slowman's input as he's much closer to that side of the industry at this point than I am..
Josh
http://www.SILCA.cc
Check out my podcast, inside stories from more than 20 years of product and tech innovation from inside the Pro Peloton and Pro Triathlon worlds!
http://www.marginalgainspodcast.cc
"...As for fork leg placement, we proved from a number of angles that you need the center or pressure for the front wheel in front of the steering axis and not behind it. This is due the way that countersteer affects lean angle..if the Cp is behind the steering axis, a transient gust of wind will steer the wheel into the wind which makes the contact patch go in that direction initiating a lean in the opposite direction (lean with the wind) which then requires a large steering correction to stabilize. It seems counterintuitive, but having the Cp in front of the steering axis means that a gust of wind will steer the contact patch away from the wind which initiates a lean into the wind, so the effect is naturally stabilizing.
Sadly, Zipp doesn't show much CFD or data anymore, we used to have lots of interesting stuff on this when I was there, you can Google AIAA Matt Godo Zipp CFD or my name and find a lot of it, but the great example we used in the original Firecrest stability white paper was that a front disc has a Cp well behind the steering axis which is why a front disc outdoors can be stable in stable wind, but can be downright terrifying in gusty or variable wind. Similarly most 3 spoke wheels have Cp in front of steering axis at low yaw and behind it at high yaw which explains some of the challenging handling behavior of those designs in high yaw or variable wind."
The backstory here is that when we were developing Firecrest using Matt Godo's CFD protocol we realized that we could predict handling using side force mapping and Center of Pressure (Cp) calculations that were already beautifully built into the FieldView software.. the problem them became where to move it? Initially we wanted it positioned aft to give the wheel a rudder effect, but early prototypes were NOT stable. Benchmarking wheels at that time we realized that wheels people thought were stable, namely shallow wheels all had Cp that was forward of hub and of course low side force and wheels that people thought were unstable, front discs, the 1080, 3 spoke/5spoke wheels all had Cp behind the steering axis or Cp that migrated from in front to behind during the yaw sweep. The only other player who has really made a claim in this area is ENVE who along with Simon Smart targeted a Cp in front of the steering axis that resulting in a linear torque rise with wind angle, while the Zipp philosophy was to try and keep the Cp in front of but as close to the axis as possible... the difference is that the Zipps had lower torque that was non-linear to yaw angle while the ENVE's had higher torque that behaved more linearly.
Having said all that, this was roughly 10 years ago now and I've been out of the game for a few years so I'm free to talk and really interested to hear Slowman's input as he's much closer to that side of the industry at this point than I am..
Josh
http://www.SILCA.cc
Check out my podcast, inside stories from more than 20 years of product and tech innovation from inside the Pro Peloton and Pro Triathlon worlds!
http://www.marginalgainspodcast.cc