How can towtrucks tow a heavy truck on icy roads? (This isn't hardcore physics, but bear with me. I've been watching too much Highway Thru Hell and I'm curious about this)
A common problem on the Highway Thru Hell TV-show is trucks stuck on an icy road that cannot make it up a hill. So the towing company sends a towtruck to pull the truck up the hill. What I'm wondering about is how exactly a towtruck can pull a truck up the hill. I somehow doubt the towtrucks simply have better engines or tires/tirechains to prevent wheels from spinning out, and with the additional equipment they carry they are pretty heavy in and of themselves. And even if it was due to better engines, why don't manufacturers build trucks with these better engines so they can keep driving on icy roads.
So how exactly are towtrucks able to pull trucks up an icy hill?  
 A: The trucks stuck on the smasher usually never chained up at the chain up area at the bottom of the hill. It is much harder to chain up after you are on the hill stopped  and get moving. Most truck you find stopped on the hill don't have locking differentials or power dividers although that doesn't mean you wont chain up just better chance of making it to the top without chains. If you pay attention the tow truck always has chains on even if only on the front drive axle, plus they probably have full lockers. I have seen a lot of guys chain up there rear drive axle and still not make it up the hill because they have no differential lock or only a power divider. So the axle they chained up wont get any power to it because it will go to the axle that spinning any way and they need a tow. I personally feel the guys need to throw more sand down more frequently as a few truck pass over the sand lane and it wipes most of the sand away. That how you get guys stuck on the smasher, clear road and no sand. It makes for good tv and is great gig for the tow operator.       
