Hi,

I just started to work with single end reads, which are already trimmed for adapter sequences and quality. 
Do I have to trimm the reads now to the same length of e.g. 100nt for mapping them with STAR?
Is there a negative effect, if I don't?
If the qualities are ok and there are no adapters you can proceed with mapping.
There is  a recent paper about trimming of RNAseq data and its possible consequence on downstream analysis - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4766705/