" what is the connection between the savings held in banks and the supply of capital goods? Because JJ is talking about the pre-existing supply of goods not the goods that might come into existence depending on the interest rate."
There is no connection as such, the problem is that there is a money illusion. If the government could, with a flip of a switch change the consumption V. investment ratio to create a interest rate of whatever percentage that they are trying to reach then there would be no business cycle, furthermore if the government could engage upon credit expansion, dictate that projects A-Z would be completed, and then ended its credit expansion, ensuring that no other projects were embarked upon or were being embarked upon at the point where the interest rate rose and prices adjusted, then there would be no business cycle. What happens is that the government injects credit into banks that didn’t exist there before, they haven’t changed the consumption V. investment ratio, they have merely “enlarged” the whole pie in favor of investment, and because money isn’t neutral it takes prices to adjust and for the ratio to fall back to its original state.
What happens then is that firms start behaving as if the rate of interest is whatever the new rate is. This means that they bid up prices of existing capital goods. Now this may lead to scarcity in the higher orders of production, but successful and hopefully wise entrepreneurs will be able to recognize the fact that there will be this bidding war. If the interest rate remained where it was at that point then this would be fine, entrepreneurs would be able to wait, or plan accordingly, until there is a proper capital buildup. This does not happen, however, because the entire savings v. investment ratio is attempting to shift back to its “real” spot.
If credit expansion ended then inflation would occur throughout the entire economy and entrepreneurs would bid the interest rate up to its initial level. For this reason entrepreneurs are going to be continually engaging in projects, and when the interest rate comes back down or when inflation becomes to volatile, the entire house of cards crashes down, and the projects which these entrepreneurs were engaging upon could not be completed because they no longer had the funds necessary to complete the project at the adjusted price level. This is why Rothbard acknowledged that on the free market a “sudden and large change in time preference” would lead to a small crash and period of business difficulty. And, as Mises stated, some malinvestment will always occur so long as men aren’t infallible, but the volatility and false signals given out by significant credit expansion in the investment sphere make large malinvestments much more probable and at least small business cycles almost certain. The connection between the “lack of complementary goods” and the business cycle, is that when the interest rate readjusts the capital structure in existence cannot complete the projects at hand, and so price reflects this, causing many projects partly in development to fail.
So then, to answer your question as concisely as possible: There is no direct relation, it merely leads to expectations about future capital goods which changes the way that entrepreneurs use existing capital and factors of production, but this cannot last indefinitely because, due to the fact that the money is newly created credit, the economy is constantly trying to drift radically away from the current picture of prices and resource availability that the economy is currently using as a map to guide future projects, as it were.
Once again, hope that answers your question.