a) They are not 'bringing back' anything yet - they're only looking for feedback from the riders at this point.
Again, how do you know this without being privy to the inner sanctum of Clermont Ferrand what is being planned for COTA or beyond? Michelin have already strongly alluded to both constructions being available at Austin. I would suggest that if the compound is amenable, the option to race the #70 will be there. However, this thread is increasingly moot - particularly given that Valentino's team have 'found something' and 'turned a corner' (without the #6 tyre de-forming). That being the case, it will be interesting to see the extent of any future valuation of the #70 carcass at the request of say, just Cal Crutchlow and Andrea Iannone.
b) All four Yamaha bikes had problems with chatter on the stiffer front, including Rossi's. Which was one of the reasons for introducing the current (softer) tyre.
I did mention that chatter was a major factor in its overwhelming rejection. Earlier, you said this, remember?...
Right. Overwhelming in terms of numbers preferring it, yes - '90%' of the field liked it acc. to Goubert (I thought you meant overwhelming no. of riders faced chatter.)
And in spite of this chatter...
"According to a Michelin spokesman, Rossi was the only member of the 23-rider MotoGP contingent to express preference for the harder casing at the close of testing."
Rossi wanted/wants to try out a stiffer front again (old or new; unspecified) and so do several other riders. Bikes evolve, development stabilizes - bike-tyre interaction today is not necessarily identical to what was observed at Valencia.
And in terms of both practicality and expense, this is usually addressed through access to and development of different compounds. Once more I will refer you to the original Michelin objectives for 2017 which were to target only compound changes from circuit to circuit. Again the statement from Taramasso
“the aim for 2017 was to provide one casing and one profile for the season, and only change compounds track by track.”
Can I suggest that you find the Dorna produced Márquez documentary 'From Cervera to Tokyo'. If you genuinely love motorcycle racing, this is wonderful viewing - impossible not to warm to the guy. (I would also recommend this to anyone that contends that the Repsol Honda is always the most 'favourable bike on the grid'.) A fascinating contrast between the nuances of Spanish and Japanese working culture with participation from Shinichi Kokubu, Takeo Yokoyama, Santi Hernández Carlo Liuzzi, Jordi Castella, Carlos Liñán and of course Shuhei Nakamoto. In discussing tyres Nakamoto observes that Marc will invariably request the hardest option - "Marc always says harder, harder, harder". Of course he is referring to compound, but as you have quite correctly pointed out, why wouldn't Marc also be interested in at least evaluating a stiffer construction in the future? However following the race at Qatar, he made it clear that he made a preference for a harder compound. Regarding the #70 construction meanwhile:
"We cannot have more [than three], or less," Marquez said.
"
For some reason Michelin brings this extra tyre in the front, and then we speak a bit in the Safety Commission, 'why do we have this tyre'.
"Even race direction didn't know, they didn't have any information.
"
There was no need for safety to use this new tyre with a different character.
"For that reason it's better to try it in a proper test, after Jerez or something like this."
Thank you. This should hopefully clarify matters sufficiently.
Indeed it does. Entirely my own conjecture but as I have consistently suggested, had Valentino not requested the renewed evaluation of the #70 construction I'm not convinced that it would have happened so quickly - if at all.
"Based on rider feedback, led by Valentino Rossi who believes a revised profile makes the casing feel softer, Michelin brought a fourth front tyre type to Argentina with the stiffer casing used at Valencia last November."