If, for example, you thought small caps were under valued, and you wanted to tilt small cap, you could keep doing exactly what youre doing, and just add a small investment into small caps on the side. So at Schwab, if you hold Schwab Target 2060 Index Fund (SWYNX) you can tilt by making sure Schwab U.S. Small-Cap ETF (SCHA) is 10% of your portfolio. At the moment, SCHA is 6.8% of SWYNX, so you would need an additional 3.2%. I dont know if you even need autobalancing for something so simple, just fix your allocation every time you make a contribution.

You’re trading one issue for another. By holding x% of a small cap fund, where x does not equal the actual market proportion for small cap funds, you’re choosing to over or under weight that asset class.