Shanna Moakler -- I'm Riding With the Hells Angels ... Travis, Beware

Travis Barker has good reason to be worried about a run-in with Shanna Moakler's biker boyfriend ... turns out he's a Hells Angel.

TMZ has learned the boyfriend Moakler allegedly threatened to let loose on Barker ... is in the club's San Fernando Valley Chapter. Shanna and Travis were both arrested a couple of weeks ago for slinging death threats.

Sources close to Shanna believe Barker will try to use her bf's Angel ties against her in their custody battle, but we're told Shanna's insisting he's a nonviolent pussy cat with no rap sheet.

Despite her concerns ... Shanna posts pics of her man on the reg, but always blurs the Hells Angel patches on his cut. We're told Shanna's reminding friends the Angels do a lot of good charity rides and fundraisers.

Just don't ever touch his hog ... unless you're Shanna.