New aero

Posted on May 17th, 2015 in Aerodynamics,Driving Emotion by Julian Edgar

Automotive aerodynamics keep changing.

Recently I read a paper on the development of some late model Audis. Much of the rationale behind the aero development was as you’d expect – minimising flow separation, keeping the area of the wake low, using a smooth underfloor – stuff like that.

However, a significant amount of effort was directed at reducing the size of the vortices being shed from the angled vertical surfaces – the C pillars in sedans and the D pillars in hatchback (or wagon) body styles. After all, if the wake size has been minimised, any further reduction in pressure drag comes from controlling these vortices – the whirling ribbons of air being dragged along behind the car.

Interestingly, much of the technique in reducing these vortices occurred under the car – the shape of the rear diffuser influencing the size of the vortices that were being shed. In fact, the differences between the body shapes (eg sedan and hatch) was such that the underfloor attachments needed to be tailored to suit.

And you can see another, more visible change, in aero occurring as well.

For a long time, the trailing edge of the upper car surfaces has been sharply cut off to promote better flow separation. So think of the roof extension spoilers used on the trailing edges of hatchbacks, for example. Or boot lid extension spoilers on sedans.

However, now the focus has clearly moved to additionally promoting clean separation on the side panels of cars. Take a look at the rear three-quarters of newly-released cars and you’ll often see a vertical crease on the corner of the car. This crease causes flow separation to then occur cleanly at this point, rather than the airflow wrapping around the rear edges. In fact, one Honda Civic I saw had a small vertical spoiler mounted at this location. I thought this factory attachment looked pretty trick – and doubly so when you think of its function.

If you do a lot of driving in slow-moving traffic, looking at the different styling approaches that manufacturers are using to achieve this clean side separation can keep you entertained for hours!

And finally, the way in which front and rear coefficients of lift are regarded is changing. The traditional wisdom has been that a low coefficient of lift (eg at the rear) promotes a stable car, and that a car with a higher coefficient of lift will consequently be less stable.

However, Mazda research indicates that this may not be as valid as first thought.

The trouble is that coefficients of lift tend to be averages – rather than taking into account fast changes in lift values that may occur due to transient changes in local airspeeds. What sort of transient changes, then? Well, the wind does not blow constantly as a steady stream: it contains gusts and other fast speed changes caused by roadside obstacles (and other vehicles) creating turbulence.

It is suggested that the reason that some cars are more aerodynamically stable than others – despite both cars having ostensibly the same lift figures – is due to differing behaviour under these rapidly changing conditions.

Car aero is a fascinating subject….

Sprung and unsprung weight natural frequencies

Posted on May 10th, 2015 in Suspension,testing by Julian Edgar

My major job – training people in business and government writing skills – takes me all over the country. Usually that involves lots of flights, but recently I chose to take the Greyhound bus between Coffs Harbour and Port Macquarie.

The bus travel was actually very pleasant – though I didn’t envy the driver threading his way through the dusk traffic on narrow roads constrained by constant roadworks.

When I was sitting in the bus, I started analysing its ride quality over the often poor road surfaces.

To cope with the large variation in possible load while still giving the best ride quality, long-distance buses typically use air suspension. (This also lets the bus ‘kneel’ as people get on and off.)

The air suspension stiffness is set to give a natural frequency of about 1Hz – the best frequency for ride quality.

And, in the bus, the ride felt about right for a 1Hz natural frequency – the absorption of large bumps was superb.

However, sitting back and admiring the flowing scenery outside the window, I thought I could feel another ride quality characteristic – and this one was not so pleasant.

Superimposed on the soft suspension movements was a higher frequency judder. It was like riding in a conventional car travelling on a road that had long wavelength bumps – but a corrugated surface.

Rather than guess any longer, I whipped out my iPhone and, using the ‘Vibration’ app, recorded the ride accelerations being experienced by the bus body. The seat next to me was empty and so I put the phone down on the cushion and gently held it in place.

Ten seconds later I had a record, and a moment after that I used the software to perform a Fourier analysis, giving the dominant frequencies in the waveform.

This showed a peak at 1Hz (the air springs) but also another peak at about 10Hz. The latter was the juddering “corrugations” I could also feel.

But what was causing this higher frequency of vibration?

The higher speed juddering was caused by the natural frequency of the unsprung mass – the weight of the suspension acting on the “springs” that comprise the tyres.

But it gets more complex. How do the 10Hz unsprung weight vibrations get through the 1Hz air spring isolation? With the forcing frequency (10Hz) so far from the natural frequency (1Hz), wouldn’t the transmission be almost zero?

I am not completely sure, but I think it has to do with the massiveness of the unsprung weight. Was that rapid shaking of the huge tyres and suspension arms feeding a vibration through the suspension mounts that I could feel?

Reflecting on this, I realised that I’d felt all this before – but to a lesser degree. In 4WD passenger cars using solid front and rear axles (ie a high unsprung:sprung mass ratio) you can feel something similar… it’s a bit like the car is being shaken by the suspension. So the soft main springing was being subverted in ride quality by the high unsprung weight bouncing on the tyres.

Here’s another point: dampers need to control suspension movement at both the suspension and tyre natural frequencies…. but the requirements for controlling each mode are quite different. One requires damping of large amplitude, low frequencies (the movement on the body springs) – and the other damping of high frequency, low amplitudes (the movement on the tyre springs).

It would be interesting to talk to a damper manufacturer about the decisions in damper design that they must be making.