I've been thinking about the responses here and doing some more math.
Pretty clearly, manufacturers limit the largest socket sizes offered for a given drive as torque requirements for larger fasteners could potentially exceed the capabilities of the drive. But, according to my math, the load that needs to be applied to the handle to break it is beyond practical. This is due to the limited length of the ratchet handles (which is what I was suggesting in my last post). Here are some numbers:
A T72 ratchet handle requires a 270lb load, applied to it's end, before it will break. Apply the load an inch in from the end and the required load goes up nearly 100lbs (360). Most of us could stand or jump on one of these ratchets and couldn't break it. This leads me to believe that 1/4" drive is more than strong enough to handle any socket made for it. But I shouldn’t stray too far from my caution about long handles. A 9” TLL72 needs only a 120 lb force to break.
What I missed in my earlier comments was the load required to produce a given torque. This may explain why so many respondents either didn’t favor ¼” drive or chose the cut-off socket size well below the largest size offered for that drive.
For example: A 3/8” bolt requires a 9/16” socket, which Snap On supplies in ¼” drive. While the torque required for this socket is indeed roughly half the capability of the ratchet (30-50ftlbs vs 90ftlbs of ratchet capability), the handle load required to achieve this torque is excessive. A high grade 3/8” bolt may require a 150lb load to be applied to the T72 handle. That’s likely double what any of us would be comfortable with.
The obvious solution is to provide longer ratchet handles with more comfortable grips to reduce user fatigue. But, as I pointed out earlier, there are limits to the safe length of a ratchet. A TLL72 would require roughly half that much force (75lbs) to break free the same 3/8" bolt.
The other part of this that I missed is the smallest socket for a given drive. While the load required would be high, it is conceivable that a long 1/2” breaker, or even my much appreciated SHLF80A ratchet, could shear the head off a 5/16” bolt (using the available 1/2” socket). The torque rating for this drive exceeds the shear strength of many fasteners at the low end of the available socket sizes. This was my caution about long handles.
In conclusion, though I haven’t 100% given up on the idea that there can be a right or wrong answer to the question; "which drive size is appropriate?" you all have convinced me that this is far more complicated a question than I initially thought.
Having given the subject more thought, here is what I can conclude:
1) Mechanics likely have normal range of handle loads they are comfortable applying. My guess is that number varies from 20-100lbs. This may explain the responses here and why some folks' ½” and 9/16” ¼” drive sockets see very little use. Those fasteners need more than 100lbs of handle force for most small ratchets.
2) Tool manufacturers limit socket sizes available. It appears they have a 2 : 1 margin of recommended fastener torque to drive capability.
3) Tool manufacturers offer a range of different handle lengths and grip styles to allow mechanics to loosen and tighten things without having to apply excessive hand force. But there are clearly upper limits to handle lengths, likely based on the capabilities of the drive. We all know what increasing a ratchet handle's length with a section of pipe can do.
4) The capabilities of the drives exceeds the force we are generally willing to apply. This means many of us might be more satisfied with 1/4” drive if we had longer and more comfortable ratchet handles. Those of you who don’t prefer 1/4" drive may find a new longer more comfortable ratchet provides a great deal of added utility. But the wide range of sizes available in 1/4” drive means we need to be careful when using long handles on small sockets. This is generally true for every drive size.
For the OP, my sense is that the limits for a given drive are defined by the range of sockets offered, not the strength of the drive itself. 1/4” drive is fully capable of handling every normal fastener that a 1/4" drive socket will fit on. No rule of thumb is necessary.
But, respondents offering rules of thumb here raise good points. Practical limitations on drive size (for a given socket) are probably based on comfortable hand pressure required to actually use the tool to loosen properly torqued fasteners, and reinstall them to the recommended torque. People who prefer the smaller drives may be the people with the longer ratchet handles or those with premium grips.
For me, my take away is that I will probably buy a THLF72. I now have more confidence in the strength of 1/4" drive.
I hope this wasn't a waste of anyone's time.
