That's because Harley motorcycles fall into one model families - Touring, Softail, Dyna, Sportster, Vrod and Street - and there are different types within these.
"You need to know what's what," says Medlyn, who got a Honda 175 dirt bike when she was 18 because her then boyfriend had one. "I had an 'off' near Western Springs and my father found out about it and had a hernia! He said I wasn't riding again."
So she didn't - at least until she was in her forties and bought a Honda Rebel but she always longed for a Harley. Medlyn called the Auckland chapter of the Harley Davidson Owners Group to see if she could join and was told she needed a bike to become a fully- fledged member.
She spent five years attending chapter meetings and rides as a guest while honing her biking skills on a Yamaha 230 and, later, a Yamaha 650. Ironically, it was her dad who made it possible for Medlyn to get a Harley.
Just before he died, he told her he was leaving her some money and she wasn't to spend it on bills, but on something that brought her happiness. Po has now been a beloved part of Medlyn's life for three years, allowing her to ride solo round the South Island twice and explore New Zealand as she's never seen it before.
"I love to take photos and this country - oh, man! You think it's beautiful? Wait till you see it from a bike! It has allowed me to enjoy this country even more than I ever have and, at 59 years old, I'm still discovering the beauty of this place."
Her favourite ride is from Auckland to Wellsford, including a stop to buy a pie for lunch.
She says riding Po gives her the kind of rush she experiences on stage.
"When I'm performing on stage, it's a lot like riding. There is this amazing feeling of freedom and creativity and openness and vulnerability. When I get on the bike, I open myself up to nature; on stage, I open myself up to an audience."
And she believes Po has brought benefits which flow through to the stage.
"I'd always been the kind of person who would get frustrated if I did not get something straight away but for two years, I was on a learner's and restricted licence riding smaller bikes. I had to put in the graft - just like you have to do with singing - and the hard yards. I think it's taught me fortitude."
She says Po, who she talks about like a much-loved friend, has given her more confidence.
"Like a lot of performers, I've always felt a bit unsure in the world - except when I'm on stage - but he's taught me to be more confident about what I can do. It helps that he's big and he's noisy, a bit like a big brother. I think that's helped me grow as a person and performer."
While she'll don a nun's habit for Sister Act, the Musical - based on the film starring Whoopie Goldberg - Medlyn is unlikely to ride Po on to the Waterfront Theatre stage. Not that she hasn't done that before.
In Blenheim for a fundraiser for the Alzheimers Society Marlborough, she dressed in full leathers and rode Po on stage where she stripped down to a red satin jumpsuit while singing songs from Carmen.
As Mother Superior, Medlyn plays a pivotal role in the story of Deloris Van Cartier - a wannabe diva whose life takes a surprising turn when she goes into a witness protection programme in a convent. New Zealand Idol finalist Keisha Tunks plays Deloris.
Sister Act, The Musical is on from November 25 - December 16 at the ASB Waterfront Theatre.
