Skating Force - A Video Demonstration

You learned about the primary driver of skating force in Skating & Anti-Skating and you also know we have been whining about the fact that too many people - including some tonearm manufacturers! - claim that offset angle is a primary cause of skating force. Some manufacturers even use this belief to make changes in their pivoted tonearm designs which they erroneously claim allows their product to avoid skating force. FACT: It doesn't matter whether the design involves a pivoted headshell, an ultra-long armwand, a thales circle pivoted linear tracker or whatever: if the arm is a pivoted arm, it WILL have skating force that must be dealt with to preserve the stylus and the grooves.

PRACTICAL DEMONSTRATION - OFFSET ANGLE AND SKATING FORCE

We figured that publishing the mathematics behind our assertion would leave most people out of the conversation so we devised a simple practical demonstration to prove offset angle has nothing directly to do with skating force - though there is an INDIRECT relationship we'll discuss later.

With a grooveless record on the platter and a perfectly aligned cartridge with a spherical tip profile, we set about our little demonstration. The fact that the stylus tip profile is spherical is VERY important and will be discussed later as it is related to the relatively minor and indirect role that offset angle plays in generating skating force.

Here is the first video:

For the second step, we twisted the cartridge in the shell so that the cantilever would be aiming directly at the pivot point (on the horizontal plane, of course). This approach completely eliminates the offset angle of the arm/cartridge unit. We re-checked tracking force to ensure it hadn't drifted and tried it again.

Note that with such an extreme change in the offset angle - from normal (about 21 degrees) to zero degrees, the skating force was nearly identical, as demonstrated by the velocity of the arm movement toward the spindle. We found that VERY slight variations in tracking force influenced this velocity. Of course, this should be expected since one of the two major components of skating force is the friction of the stylus against the record - which gets higher as the VTF is increased. As tracking force increases, skating force increases.

This is the very principle behind the WallySkater method of anti-skating measurement, by the way!

But those two tests weren't enough for us. We decided to see if any variation of the skating force would be observable if we skewed the cartridge past the zero offset angle position to allow for as much "negative" offset angle as the headshell slots would allow. This was important since - if the others were right and we were wrong - the skating force should reverse direction toward the outer edge of the platter. 

We found what we expected to find: no observable difference in skating force direction or velocity. 

HOW DOES OFFSET ANGLE INDIRECTLY AFFECT SKATING FORCE?

The answer is in the stylus' tip contact profile; i.e., the shape of the part of the stylus that touches the record surface.

We originally performed the above experiment with a fine-contact stylus profile. (Some examples of a fine-contact stylus profile include Gieger, microridge, line contact, Shibata and, to a lesser extent, elliptical.) We found that the inward velocity of the arm generated by the skating force was faster when the cartridge was mounted properly versus when it was mounted to eliminate the offset angle. This would have seemed to be a demonstration in favor of the offset angle-causes-skating crowd, no? NO! It does not support that argument as the observation does not take into consideration the impact the stylus contact profile had on the inward velocity.

When the cartridge is mounted properly (with an offset angle, of course), the contact surface of the non-spherical stylus (in our case, a microridge stylus) is parallel to the record radius; i.e., the direction of travel of the skating force. Therefore, there is little resistance for the skating force to shoot the arm toward the spindle quickly because the stylus contact profile is "aiming" towards the spindle. However, when the stylus is mounted to eliminate the offset angle it results in its contact surface to be at an angle to the radius of the record (the direction of travel). With significantly more of the stylus' contact surface now at an angle to the direction of travel, a resistance to the skating force is introduced, slowing its velocity. 

The demonstration clearly showed that offset angle can, at most, only be a counter force to skating force and certainly cannot be a cause of it.


What follows is the Skating & Overhang portion of this study. It's entirely in video format, so enjoy!

This is where it all comes together:

Want to learn more details? See the attached presentation. Beware! Geometry is involved!

WAM Engineering WallyTools Skating Force
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