's are typically sensitive to tire pressure. The little pads "squirm", instead of rolling over the road, if under inflated. This causes "cupping" and shortens the tires useful life. In a modern tire/compound the tire pressure supports the motorcycle.
If you look back at owner's manuals over the past 50 years you will find recommended pressures as low as 15 pounds (Vincent Black Shadow). In those days the carcass of the tire did a lot, if not all, of the work.
As a rule of thumb the tire should not increase in pressure more than 2 pounds when being used. If you set your tire at 30 pounds and after a decent ride the pressure increases to 33 pounds, or more, the tire is under inflated. The tire is flexing. Flexing creates heat which increases tire pressure. Flexing will also decrease fuel mileage.
Required tire pressure varies with load. A modern K70
is rated for 617 pounds @ a max. pressure of 36 pounds. If you and your "main squeeze" are full figured and choose to carry a bit of kit on the back you will be pushing the 600 pound limit. Failing to adjust the tire pressure from solo pressures for the new load the least you can expect rapid tire wear. There are other important considerations as well. Not all deflated tire incidents end up with you fixing the tube on the side of the road.
As far as running the tire at what it is rated at, when riding solo, the ride would be quite stiff. Not particularly unsafe, but uncomfortable especially on the kidneys. For sure the contact patch on the road would be smaller than if the tire was properly inflated. At 10% of the rating, or 32 pounds for a K70
, you would be in the ball park. Again take pressures cold and then after a ride when the tire has had a chance to warm up. If the pressure hasn't raised a bit, and the ride is still a bit stiff, you could lower the pressure a bit. You want the tire to flex a bit so you get a decent size contact patch without the tire flexing so much as to create a lot of heat.
If I was riding two up a lot I would set the pressure for two up riding and leave it there and not lower it for when I rode solo. All you have to do is forget raise the pressure for two up riding once. This is especially important for a K70
, because very little riding with an under inflated K70
will start it cupping. Once cupping starts it cannot be reversed by raising the pressure after the event.