The recently released VY series Holden Commodore has some significant aerodynamic changes over its predecessor VX and VT shapes. A new nose and tail, revised exterior mirrors and an alteration of treatment around the wipers were all aimed at reducing wind noise and improving the drag co-efficient to achieve better open-road fuel economy.
That's the sort of stuff you'll read everywhere - but what about assessing how well the changes worked? Like, does the SS's rear spoiler make any positive difference? What about the smaller spoiler on the S model? And those sharper front-end angles - do they cause aero-problems in getting a smooth, attached airflow over the front of the car?
For this exclusive series of stories, we talked to Holden aero engineers, pored over in-house company technical data and were given access to never before seen images. For tech-heads this is brilliant stuff - the significance of the aero changes are assessed and measured. The aero mods and their outcome also give a very good indication of the sort of results that will be able to be seen on similar shape cars...
Bootlids and Spoilers
As with most car model cycles, preparation for the VY model started at the time of release of the pictured VT: yes, the new model was only just out but the engineers were already deeply involved in the body shape of the next major model. The VT Commodore went on to become one of Holden's best-ever sellers, but in the opinion of the aero engineers it was a far from ideal shape. Primary cause of concern was the trailing edge of the boot-lid - rather than a sharp change of angle that promotes a clean separation of the airflow off the back of the car, the VT's rearmost contours were rounded.
"The VT was mainly a styling exercise - some aerodynamic work was done on the car but the reality is that there was very little in comparison to some of [GM's] European product," says Senior Engineer Christian Peric. "We looked at the car and the obvious realisation is that you want a sharp trailing edge. For us it was a major point to try to control the separation on the back of the car - the separation line does wander on a rounded edge like the VT. This can vary the drag and make it a lot higher than it ought to be."
In addition to the drag variations created by the lack of a clean separation point, rear lift can also alter. And what about actual on-road conditions - where there's other traffic around and the wind can swirl and eddy past roadside obstructions?
Supervising Engineer Dean Niclasen is based at the Lang Lang Holden proving ground. "The average level of turbulence on the road is around 5 per cent, which is fairly extreme when compared with wind tunnels that normally generate fractions of a per cent. The variation in the wind conditions that you see on the road will make a big difference to the way the car behaves."
So gaining a consistent separation point at the rear of the car is a vital factor in designing a vehicle that will give low drag and lift results in realistic, on-road conditions. A primary reason for gaining consistent separation is to keep the wake (the area of disturbed air being dragged along behind the car) as small as possible.
"We try to get the wake as small as we can," says Peric. "Boat-tailing, a slight angle change of the decklid at the rear (you want to have a slight downward angle); things of this nature all help in making the wake as small as you can get it - and making sure that it stays that way."
Dean Niclasen: "Basically, you have got turbulence occurring in the wake of the car. It takes energy to create that turbulence and that energy must come from the car. So therefore, the smaller the wake, the less energy that the car has to provide - and so the more efficient the car becomes."
The VX model Commodore (effectively, a VT with some minor cosmetic tweaks) has a measured drag co-efficient of 0.329. Rounded up to two significant digits, a 0.33 Cd is nothing fantastic these days (remembering that the Cd figure needs to be multiplied by the cross-sectional frontal area of the car to gain a total drag figure - and the Commodore's a big car!). And what Cd number does the pictured VY turn in? Primarily because of that alteration in the separation point, it's 3 per cent better at 0.319.
However, it gets more interesting when lift figures are examined. The overall co-efficient of lift of the VX is 0.212, versus the new VY model's 0.113. That's a huge 47 per cent lower. But it can be split up further, as the following table shows.
As can be seen, the VY model actually develops a downforce component at the front of the car (a negative lift number means there's a push downwards!), whereas the VX developed front lift. At the rear of the car, lift has been decreased considerably. But it gets more complex - much more complex! The data presented so far is all for a zero yaw angle - in other words, no crosswind component. But driving on days when there is no crosswind is quite rare - so what happens to drag and lift when there is a yaw angle of 15 degrees in the airflow?
Looking at the VY, it can be seen that total lift goes up very fast with a crosswind - in fact it rises by more than three times! The front changes from having a minor downforce to have demonstrable lift, while at the back the lift nearly doubles. Those figures don't look good, but they are in fact better than was achieved on the previous VX model.
But when you start talking about lift and drag, what's the first idea to pop into your head? Spoilers? Glad you asked... Both the 'S' and 'SS' models have specific rear spoilers. And if you think that the spoilers were an afterthought add-on, think again. Both models had extensive wind tunnel testing and both spoilers have a positive outcome in reducing lift.
First the bad news. The presence of the rear spoilers standing above the boot-lid can only increase the size of the wake, resulting in an increase in drag. The smaller 'S' spoiler lifts drag by 2 per cent, while the 'SS' spoiler lifts drag by 5 per cent. To put it another way, the VY 'S' model has about the same Cd as an unadorned VX... so if you're chasing the fuel consumption gains that apparently resulted from the better drag co-efficient of the VY, you'll throw it away by getting the small spoiler fitted... (And the previous model VX SS? Try a Cd of 0.354... getting seriously bad in this day and age!)
But looking at rear lift you can see that both spoiler types are effective. The 'S' spoiler drops rear lift by 86 per cent while the 'SS' spoiler actually turns a rear lift situation into one developing positive downforce. But pushing down on the back will have a see-saw effect on the front of the car, and it can be seen that with either rear spoiler fitted the front lift increases - although to be fair, it still remains very low indeed.
The VY has a re-styled nose in addition to its new rear.
"What we ended up doing with the front was to really make sure that it didn't degrade over VT," said Christian Peric. "The VT was a nice round shape and the flow's very attached and clean. Because of the VY's more sharp edges, we wanted to make sure that we didn't start screwing up the flow."
The clash between styling and aero is an unspoken - the aero engineers would have been happy to keep the rounded VT front with the squared-off VY hindquarters, while the stylist wanted to retain a visually cohesive car. Peter Vawdrey, Engineering Manager and the senior engineer present, feels the need to present the overall company line.
"It's not always a conscious decision - do we ignore aerodynamics or do we take notice of aerodynamics?" he says in measured tones. "Certainly in earlier days - probably VS - maybe it wasn't a significant priority. We [also] didn't give our styling people any technical guidance at the beginning of the VT program, as to 'here are the aspects of the style of the car that you can tweak in this direction to get a better aerodynamic outcome'. [But] this is what we did with VY - give them guidelines which in many cases they were able to fall in line with.
"Aerodynamics is an engineering aspect of the car rather than a styling aspect - and they've got some pretty good ideas as to what generates a better aerodynamic result. But they're not engineers, they're stylists."
He adds a significant market realist view, "We all know that recently in the last Falcon, the AU, [Ford had] a better aerodynamic car than VT - but it wasn't very attractive to customers."
So how do you go about giving a sharp-edged nose attached airflow - optimising a shape to generate a minimum of separation and turbulence?
Christian Peric: "You basically look at angles in side view and angles in plan, and you're looking at radii. There is a certain guideline range of angles that we have, based on what GM uses globally for aerodynamics. We say, 'OK, that angle is not within that spec' and then we try to get it within that spec and see how looks - and then we physically test it. If we get the chance, we'll also do an analysis beforehand on the computer."
But - as with most things aerodynamic - there's a lot more to it.
"You can't take too much of a cookbook approach to it," adds Dean Niclasen. "The nature of aerodynamics is that everything affects everything else. So when you make changes to the rear of the car it does have an affect on the front of the car and vice versa. So if it was as simple as saying 'you have to have X radius' everyone would have brilliantly aerodynamic vehicles. But it's not - it's a whole package..."
Next: you'd never believe that making exterior mirrors quiet could be so much drama...