PU tyres go the distance

Solid polyurethane tyres helped one team’s vehicle go a long way on very little fuel in this year’s Shell Eco-marathon Americas. Sarah Houlton reports.

Every year, Shell’s Eco-marathon competitions pits teams of high school and university students against each other. Their goal: to design and build ultra-energy-efficient cars, and then compete against each other on the track. There are various classes for cars powered by different fuels.

In April, a team from Mater Dei High School in Evansville, Indiana won the urban concept battery electric class in the Americas competition with the help of polyurethane wheels. More than 1000 students representing 88 teams from the Americas took part in the competition. It was was held at the Sonoma Raceway in California.

The competition has its roots in a bet between a couple of Shell scientists at the Wood River, Illinois facility back in 1939, about who could drive furthest on a gallon of gasoline, according to Shell spokeswoman Pamela Rosen. The competition grew over time, but came to a halt during the 1973 oil embargo. It was resurrected in Europe as a student competition in 1985, and now there are annual Eco-marathons in Europe, the Americas and Asia.

Smaller tyres, smaller wheels

As Bob Neisen, an advisor to the Mater Dei team, explains, the competition rules stipulate that the car’s wheels have to be 15 inches (38 cm) in diameter. ‘Any time you can reduce the diameter of the wheel, even if it weighs the same, it has less flywheel effect,’ he said. ‘If we could make it smaller and lighter, it would accelerate faster with less energy – and cut energy consumption.’

This specification is for the wheel itself: the tyre adds to the diameter. A traditional tyre would be at least 2–3 inches thick, so they wondered whether they might be able to take a leaf out of the skateboarding book, and use cast polyurethane instead. It worked. ‘We put about three-eighths of an inch of polyurethane around our home-crafted aluminium wheel,’ Neisen said. ‘This allowed us to reduce the overall diameter from 22 inches to about 16.’

Being able to reduce the wheel size had other knock-on advantages, too, not least that it allowed the car to be made smaller, reducing its air drag. ‘There is also the benefit of no flat tyres!’ Neisen said. ‘Skateboards are really fast going downhill, and we know they are quiet and smooth. There are a lot of different aspects that made us want to reduce the size and weight of the wheel, and polyurethane was the best option to do that.’

Better on bumps

He added that PU has characteristics that mean it performs better than rubber when rolling over bumps. It pushes back off the surface, returning some of the energy that would be otherwise lost.

Working with polyurethane was a real learning curve, he said. ‘We are still learning, but we know a lot more than we did six months ago!’ he said. ‘We know you can mix PU to almost any density that you desire. We worked with experts who cast PU onto truck wheels and castors to come up with a hardness we thought would be good.’  The result was 60 Shore D polyurethane.

The PU tyres were cast for them by Sunray, based in Rutherfordton, North Carolina. Neisen has also been speaking to Aend Industries in Huntington Beach, California. ‘You can now buy polyurethane bicycle tyres that are airless, and I believe they are also starting to get into the car industry in spare tyres,’ he said. ‘And in the construction industry, where I work, you need good, tough, strong tyres that do not go flat.’

Between now and their July entry in the World Drivers’ Championship at the European Eco-marathon in the UK, Neisen said that they plan to build a second set of wheels that are slightly softer. By having two different sets of wheels, they will be able to better match the tyre to the road conditions.

‘Every [road] surface has an optimum durometer, and you are not sure what surface you are going to be driving on,’ he said. ‘The smoother the surface, the harder the tyre can be while still being efficient. But if the surface gets a little bumpy and your hardness is too great, then your wheel starts bouncing a little on the bumps.’  This reduces energy efficiency.

Breaking records

The PU tyres helped the team win their class in the Americas and, their Supermileage 2 car broke the local record. ‘We raised it from 63 miles/kWh to 68,’ he said. ‘And we qualified to go to the drivers’ world championship in London in July.’

In the drivers’ world championship, qualifying cars are given the same amount of energy, equalised across different energy sources.  They then drive a fixed number of laps around the track. The winner is the team that crosses the finish line first. As their energy supply is limited, they have to go fast enough to win, but not run out of fuel. The prize is to drive their car on the Ferrari test track at Maranello, Italy.

‘When you look at the numbers in Europe, we will have to make big improvements to be competitive. Their top car is at about 115 miles/kWh!’ Neisen said. The softer tyres might help, but he thinks there is a balance. ‘If we went too soft, maybe the rolling resistance would go up, as happens when you run tyres too low on air. This was our first attempt, so there’s a lot of room for improvement, maybe even in our polyurethane tread design. We are optimistic that we can improve this and hopefully post a better number in Europe.’

Shell’s Rosen believes the competition is a great way to get students interested in science and technology careers. ‘These are people who are going to lead us into a cleaner, better energy future,’ she said. ‘From the Shell perspective, it’s an incredible experience.’