Jody Blatchley (34) and several of his adventurous friends, including Aaron Ewen (19), who was injured about two years ago, have tested what he calls the ''gravity quad'' on various terrain, including the Cardrona Alpine Resort access road and the mountain bike tracks and jumps at Wanaka's Sticky Forest and Lismore Park.
The bike was ''amazingly stable'', with no crashes to date, and had got up to 65kmh, Mr Blatchley, a snowboarding coach and judge, said.
''My friend Aaron, he got in it and thought it was awesome.
''He's going to be able to get back to doing his stuff with his friends.
''Getting him in it - it was like I've won ... I've accomplished what I wanted to do.''
The idea came to him as he mountain biked down the ski trails at Cardrona last summer, when the field opened for the first time during the off season.
''I thought, you've got a lift to get back up to the top, so why can't people who are in a wheelchair do this, too?''His research revealed the only other downhill mountain bikes for people with a disability were made overseas, were ''absolutely ridiculously'' expensive, and there were long waiting lists for them.
With help from his Tauranga based father, Kim Blatchley, he started to develop the gravity quad using a steel frame built by the pair.
It has four mountain bike wheels, each with more than 20cm of suspension, mountain bike shocks on the rear wheels and disc brakes, also from a mountain bike. The rider sits on a seat from a sit ski.
''We've spent hundreds of hours getting it to a point where we feel it's good.''
The gravity quad is 1m wide and 1.5m long and weighs about 40kg, around the weight of two downhill mountain bikes.
It can be transported on a towball bike rack or in the boot of a sizeable car. Riders wear the same safety gear as downhill mountain bikers, including a helmet and padded clothing.
At this stage, the gravity quad needs to be pushed when on the flat or going uphill, but Mr Blatchley said he might consider making it electric in the future.
One of the differences between his gravity quad and similar concepts built overseas was that the parts he used were mainly ''off the shelf'', rather than specifically manufactured and therefore expensive.
It was also aimed at both disabled and able bodied riders.
''It's about anybody - anybody can go on it. You can be a person paralysed waist down or fully functional and be on a level playing field.
''I like to see things fair in life,'' Mr Blatchley, who has also suffered snowboarding injuries, said.
''There's quite a few people in this area who sit ski - they love gravity sports. They're my friends - it's going to be good when we get some more built so we can all go down the trails that we go down in winter.
''It's awesome - the idea is becoming a reality.''
Mr Blatchley recently completed the second version of the gravity quad, which he has entered in the Red Bull Trolley Grand Prix in Auckland on November 22.
Organisers are calling the course in the Auckland Domain a ''new and challenging track ... [for] fearless drivers with nerves of steel''.
Longer term, Mr Blatchley, a Snow Sports New Zealand Adaptive Sport Committee member, wants to build enough gravity quads to either sell or rent out at, for example, Cardrona Alpine Resort over the summer.
The gravity quad was designed for use on a gravel or dirt surface, as it might reach unsafe speeds on snow, he said.
By Jessica Maddock.
