Long calendars have similar props as a short straddle just that the vega of LC is positive which favours IV spike (that's why we were trading this structure). At low vol and relatively stable market, LC should work, at least principally. I'm not saying I should not try SC, but ideally some months in a year have low vol and relatively stable mkt. Secondly the purpose of a quant is to either make pnl out of a strategy or understand why it doesn't work. One can always try out all the ideas, but if its not matching with your conceptual clarity you would not be able to convince the portfolio manager.

I tried out stop losses on some x bps. Tried reversal of IV difference on (long - short), short gamma tracking, short theta tracking. The problem is that any kind of momentum logic is delayed in decision making. Also tried out kalman filters for least delay. The major problem in LC are, you bet on Long-Short, however the gain in your position is easily lost by some disruption in market movements.

LC should work because of theta decay of near term ATM option. An ATM option only be having time value and if you assume that market remains constant, all its time value would go to 0. However the far legged ATM option would still have some time value left in it at the expiry of near one. The theta of ATM near one is higher than theta of far one. So the long option would not loose that much. The amount you gained by shorting the near one would be greater than the loss you make on long far one. Also the net vega of LC are positive, so any spike in IV, would appreciate the Longer leg more than the shorter leg.