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There is a great site where you can play around with different risk levels and the TSP funds. It is called financialengines.com and was created by Nobel laureate William Sharpe. I have often punched in the F fund for the fixed component of a long-term buy and hold allocation and invariably, it recommends the G fund instead. For example, I just told it that my allocation is:
40C, 25S, 25I, and 10F and it recommended 35C, 30S, 25I, 10G. In other words, lower C fund, increase S fund, and get rid of F. The returns project out higher at the same risk level. But it seems to be a pattern where any time I indicate tha I am using F, they remove it and recommend G. Usually, financialengines is a fee service but since TSP has a limited number of funds, they are currently waiving the fee. Hopefully, this will not change.