Yes, I really doubt that sharp leading edges form except in extreme. Rather, I prefer the notion of compound curved contact areas with lower curvature (due to wear) that blend into original curvature at extremities. Probably.
I remain thoroughly confused about what's reported on this thread. I re-read carefully to try to piece it together and this is what I summarise as what is posted.
(Please point out where I'm mistaken)
1. The only Jico public info cited is a single sentence in the link that snead kindly posted, referring to a scant statement that Jico anticipate 500 hours wear life for the SAS stylus, and that this is 3-4 times more than typical other stylii, according to Jico.
2. It is reported on this thread that Jico have a test which examines distortion versus hours use. Conditions of this test are not posted here, as to frequency, level, rpm, spindle radius. There is suggestion that the test is at 15kHz, except for sphericals where it is at 7kHz.
3. Test results for new stylii of all types are reportedly c 3% or less distortion. Failure threshold for this test is c 5% or more distortion.
4. On this test, sphericals reportedly transition from 3% to 5% in 133 hours use. Elipticals c 200 hours use. Jico SAS c 500 hours. VdH1 800-1000 hours use.
I find this whole lot impossible to reconcile !
Harmonic distortion correlates with stylus contact curvature in a well known, established way. For example, a 4um minor radius stylus at 15kHz, 33rpm, -10dB (ref 5.6cm/s@1kHz), 15cm spindle radius exhibits c 3% harmonic distortion. 6.5um minor radius produces 5% distortion.
So supposition might be that 500 hours of use causes stylus contact curvature to wear from 4um to 6.5um. That seems plausible. After 500 hours use the SAS stylus still has superior minor radius geometery to, say, a new 7um eliptical.
But similar models for sphericals and elipticals aren't plausible. For posted spherical figures @7kHz on this thread, transition from 3% distortion to 5% distortion means migration from 18um to 29um in 133 hours ! At 29um, the stylus would be seriously degraded. Obviously this does not happen.
For Elipticals, a migration from 7um to 12um in 200 hours also implausible, and would be readily observable with common microscopes. But it isn't.
So I reckon quite a bit here makes little sense, because I can readily construct numerical examples that just don't support the reports.
The devil must lie in the detail of Jico's test definitions. Without knowing that, I think we're just left with a single fairly scant sentence on a Jico website that, on the face of it, is contradicted by common experience and established analysis. I'm not convinced at all, and prefer to ignore it until proven. Frankly !
I'd be surprised if stylus wear rate was not at least a first order approximation to volume of diamond eroded per hour. Then higher curvature radius stlii might well wear (change curvature) faster than more bulky lower original radius types.
I should say that I fully accept the principle of measuring harmonic distortion as an indicator of stylus wear. I would love to see a standard method by which that could be repeated domestically. These days, sw spectrum/distortion analysers are common place, and it seems a likely very useful test to be able to do at home. Desktop referred to his own measurements which would also be great to see posted.
I'd love to be proved wrong, and/or for someone to come up with Jico (or any other) measurement standard for stylus wear. But it has to be test method first. Rather than conclusion/results first and work backwards, which seems to be where we are here !