Security is being stepped-up for a mountain bike competition near Port Talbot due to police concerns it may be targeted by an organised crime gang.
Hundreds of riders are expected to take part in the Kona Mash-Up at Glyncorrwg Mountain Bike Centre.
Bikes worth £30,000 were stolen from an event in nearby Margam Park in May.
South Wales Police said it was "absolutely essential" visitors to Saturday's event take every precaution to keep their belongings safe.
As well as the theft from the National Mountain Bike Championships two weeks ago a further eight bikes worth around £10,000 were taken from a secure lock-up at the Afan Forest Park Visitor Centre on Tuesday.
Insp Karl Eenmaa of the Port Talbot Neighbourhood Policing Team said: "This weekend will see hundreds of mountain bikers come to Glyncorrwg to enjoy the fantastic trails and facilities on offer.
"It is absolutely essential that all the visitors take every precaution available to them in order to keep their belongings safe.
"We also want to ensure that there is adequate security at the event.
"I have no doubt that there is an organised crime group targeting this area and these events to steal the expensive mountain bikes.
"Investigations into the previous incidents are ongoing and we are working with the event organisers to ensure that security is in place."