What is the most critical takeoff shear margin condition for a Type IV anti-ice fluid?
Under the following conditions, we can't take off:


Temperature below -10 degrees C, and
Small Hail or ice pellets (i.e. METAR "PL" reported), and
Rotation speed below 115 knots


To operate with PL reported, we have to have Type IV anti-ice applied, and under those conditions of temperature + precip, they (yeah, "they") have determined that you need at least 115 knots to be sure the fluid adequately shears off.  There are various ways to drive an increase to your VR speed; it takes a pretty light 737 to rotate at 115 knots.  Other aircraft, of course, might normally rotate well below that speed.

I suspect that this limitation is probably applicable across the US airline industry, but the charts I have are specific to one carrier, so I won't generalize beyond that.

This (PL, -10C) is the only place in all the de-ice/anti-ice charts that we have this 115 knot limit.  With all other forms of precipitation, we have a holdover time and the requirement to visually check the fluid for failure; if it has failed, then we don't take off but rather go back &amp; get it washed off &amp; re-applied.  In the case of PL, there is neither a visual check nor a holdover time, but an "allowance time" after which you have to assume fluid failure &amp; go back to start over.  (With PL, you can't visually identify fluid failure -- so you have a conservative time, and after that you're just done.) Speculating, I'm assuming that the nature of the ice pellets is such that they can, in sufficiently cold temperatures, generate the conditions where the fluid needs a particular speed to shear off that some of our operations might not reach -- thus the 115 knot limit.  Whatever can happen with the other precip, presumably can't keep the fluid from shearing off at whatever our lowest possible VR might be -- which I don't know offhand.  Again, for aircraft with a slower VR, you might get those sorts of situations with combinations of precip &amp; temperature that we wouldn't worry about. 

Not a complete answer to your question, but hopefully this data point may be helpful.