To improve the communication performance in IEEE802.11-based wireless mesh networks (WMNs), several dynamic metrics have been proposed. However, all of them have a severe risk of generating temporary routing loops which may cause severe congestion and disruption of communications. Although the routing loop is an essential problem that degrades network performance, no essential solution is provided so far for wireless multihop networks. In this paper, we propose a mechanism called Loop-free Metric Range (LMR) to make existing dynamic metrics loop-free by restricting the range of metric values to change. LMR is applicable to a major part of existing metrics including ETX, ETT, MIC, etc. without any message overhead. We first provide theoretical results that shows LMR guarantees loop-freedom if no message loss takes place. We next show that LMR is also practically effective in practical scenarios where message loss may take place; we show through simulation and actual evaluations that LMR works effectively as a limiter on dynamic metrics to reduce routing loops and to improve network performance through similation and real evaluation.