With roller coaster interest, banks starting variable-rate small loans

''People can see a can of beans costing more, and they can see the price of cars going up; but they don't seem to understand that money gets more expensive, too.''

But that comparison, says Howard Maddock, a loan officer at the State Street Bank in Boston, helps explain the reason his bank and a few others around the United States have started doing something with personal loans that mortgage lenders have been doing for some time: writing variable-rate loans for things like cars, home improvements, and simple personal loans. With these loans the interest rates change as often as every month during the life of the loan.

The high cost of money and volatile changes in rates banks pay for money they borrow to lend others, bankers say, have led them to adjust interest on loans with terms as short as two years.

So far, only a dozen or so banks in the US are known to be writing variable-rate personal loans. But if the requests for information these banks have received from other financial institutions are any indication, many more banks will be doing this in the near future.

''We've had an interminable number of requests'' for information since the program started in September of 1980,said James Virostek, vice-president of retail administration at the Union Planters National Bank in Memphis.

''We've had inquiries about it every week, sometimes on a daily basis,'' said Vincent South, a loan officer at Huntington National Bank in Columbus, Ohio.

One thing inqiring bankers and customers have been told is that, unlike variable or adjustable mortgages, where the monthly payments can change every few years, monthly payments on variable personal and auto loans never change. And while the rates won't change, the payment schedule will.

Every month or every quarter, the banks will use a variety of indicators, including six-month Treasury Bill rates, to figure how much their cost of money has changed. At State Street, the bank uses the T-bill average rate and adds four or five percentage points to figure the new rate for a particular month. If the rate changes, the borrower is notified that the length of time they will be paying off the loan has changed.

''You could start with a 24-month contract,'' explained Richard Merryman of the installment loan department at Huntington National. ''If the rates go down, you might have a 20-month contract. Or it could go to 26 months if rates go up. If the borrower is paying $142.12 a month, he always knows it's going to be $142 .12 a month.'' The only time the payment might change is in the final month, when it would be smaller, to pay off an odd balance.

At some banks, like State Street, customers are offered a choice of a fixed or variable rate in certain cases. On an auto loan, for instance, ''we will offer both fixed and variable loans,'' Mr. Maddock said.

That the bankers are doing this now, says Jonathan Gray, a banking analyst with Sanford C. Bernstein Inc., shows the ''quandary'' they are in these days. With interest rates apparently coming down, the banks ''are switching to variable rates just when fixed rates may be more profitable.'' If a bank can lock in loans at current higher interest rates, he said, it may show more profits as rates come down.

But the banks haven't been writing many loans at all in the present interest rate environment, Mr. South of Huntington National points out. Now, at least, if a person needs a car loan because the jalopy won't wait for rates to drop, he has some hope of relief.

Just a few years ago, Mr. Virostek of Union Planters Bank says, using adjustable rates for loans with terms of less than four years would have been totally unnecessary. Then, they could depend on their ''core deposits'' of passbook and checking accounts to support fixed loan portfolios.

''We can't depend on the price of our core deposits to be stable anymore,'' he said. ''Everything is more volatile.'' Now, the banks also have to pay interest on 6-month certificates, 30-month certificates, and All Savers certificates - all with interest rates that change monthly and leave bankers not knowing what the next month will bring.

''When it comes to predicting interest rates,'' Virostek admits, ''we're not much smarter than anybody else.'