Road Cycling - Harsh ride on MTB compared to Roadie!

I have a MTB with slicks that can be pumped up to 100psi and I find the ride harsher than my roadie! Why is this so? Talking about sealed roads here.

I presume that we're talking about 26" MTB wheels compared with 700c road wheels. It could be a combination of things The spoke length of a 26" wheel is less than that of a 700c resulting in a stiffer wheel due to the reduced lateral flex. Bear in mind that a MTB is designed with off road strength in mind while using a larger volume tire run at a lower pressure. Compared to your road bike which takes into account the reduced volume of the typical road tire and has to have a bit more "give" tuned into it in order to compensate for the lack of tire cushion.

A wide tire at a given pressure will have less flex/give/deformation than a skinny tire and will thus have less rolling resistance(given the same materials) and a harsher ride. They will have the same size contact patch in square inches/cm but one is a long skinny patch and the other is short and wide. You must flex a tire more to acheive a long contact patch than for a short contact patch. The biggest part of rolling losses comes from the tire materials damping(converting mechanical energy into heat energy) their own flexing. Other factors come in like wind resistance, weight, and the pressure a tire is rated for. A tire half as wide will have half the surface area(1/2surface=1/2weight) and with the same thickness and type of construction withstand twice the pressure. and BTW tire usually have ratings at about half of their actual blowoff pressure due in small part to deformation and manufacturing tolerances but more to lawers and sudden shockloads.

20 square inches times 100lbs per square inch= 2000 pounds of stress, 10 square inches at 200 pounds per square inch=2000lbs but the higher pressure will reducing the contact patch by half but the tire is half as wide so the tire flex is similer but there is half as much material flexing so less damping=less energy loss. Also the flex and damping go up much faster than the size of the contact patch for a given width tire.
For a given tread compound traction is greater with more lightly loaded area than less area with more weight so 10 sq in gives more traction than 5 sq in even though 10sq in has half the pounds per sq in, this is in referance to paved surfaces by the way. with thin walled tires the area of the contact patch is always equal to the load divided by the tire pressure. The pressure of the contact patch on the road(and thus the tread) is equal to the air pressure in the tire, stiff tires like truck or motorcycle tires will affect this a little but only noticebly at very low pressure.
Want more? Maybe some formulas or clarification on a single point? I got it, do ya want it? Come on ask I dare ya. See if I keep it to my selfish self or bury you in physics.:D

Also, a roadbike geometry tends to put you in a position where your weight is more evenly distributed across the bike. An MTB puts you in a more upright position which can result in much of the shock being transferred directly to the spine which may cause more fatigue in the long run.