The collision took place at Hodge Brow.
Now Chorley Police have launched an appeal for witnesses.
A spokesman said: "Did you witness a collision between a cyclist and an off-road biker on Hodge Brow, Rivington at 4pm on 29th August?
"We’re trying to establish what happened and trace two off-road riders."
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Email [email protected] quoting log number 915 of 29 August.