I think you have the tire size wrong. The Frontier is a mountain bike and will not have 26 x 1 3/8 tires on it. That size is the old Raleigh 3 speed and department store 10 speed size.

I would bet it is actually 26x1.75 -- It will be a decimal size and not a fractional size. What size is the other tires that is still on the bike?

Just realized it could be a 650b size -- . There were a few models and few years where Schwinn brought in bikes (I had a Mirada model) with that size tire to avoid the tariff on 'mountain bikes' which was defined by the tire size.