Last week we introduced the idea of undercar aerodynamics on road cars. This week we modify the flows and assess the results.
Obviously, in any story like this, the modifications and their outcome will be specific to the car – you can’t expect to do the same mods on anything from a Mini to a Mack and get identical results. That said, the techniques that are used and the way the results are evaluated are common to nearly all cars.
The car in question is a NHW10 Japanese-delivered 1999 Toyota Prius. A hybrid petrol/electric car, it has excellent aerodynamics straight out of the factory – a claimed drag coefficient (Cd) of 0.29, which for a small car is excellent. Lift coefficients are not available.
Looking over the body shows a high tail and gently sloping rear window (resulting almost certainly in attached flow right to the trailing edge of the boot, so reducing the size of the wake) and smooth transitions from the lower edge of the bumper over the bonnet and headlights through to the windscreen and then the roof.
However, peer under the car and the picture dramatically changes. Especially under the front, it’s an aerodynamic dog’s breakfast. A plastic moulding (1) covers the front of the lower engine and power split device (ie gearbox), and short deflectors (2) are positioned ahead of each front wheel. But there’s no continuity in line backwards from the lower edge of the bumper and bits and pieces are hanging down into the flow everywhere.
About the only clear evidence of underfloor aero treatment is this infill panel positioned near the rear of the car.
So with the front underside looking as if it could most easily be improved upon, efforts were concentrated there.
As was covered last week, in a typical aero-slippery car, about one-third of the total drag is caused by undercar flows, with most of that from the front wheels. It’s for this reason that it’s now common for manufacturers to place small, rectangular deflectors ahead of the front wheels (and in some cases ahead of the rear wheels as well.)
However, SAE paper 2004-01-1307 – authored by Volvo’s Dr Simone Sebben - shows that while these deflectors can reduce drag, if they are too large, drag can actually be increased. Further, in nearly all cases, the flat deflectors cause an increase in front lift.
Let’s take a quick look at why these outcomes occur. In the engineering paper – which is based around validated computer modelling of flows – six different front defector configurations were tried. The first was of the base model car without deflectors, while the following five configurations trialled front wheel deflectors of different sizes and shapes.
(With all these numbers, the higher the number, the greater the effect. So a higher front lift number indicates greater front lift, a higher drag number indicates greater drag, etc.)
This graph makes it clearer.
First, let’s take a look at #5 – that’s where a large deflector was placed well ahead of the wheel. This design results in low front lift (in fact, as indicated by the negative number, a little downforce), as but also has the highest drag – more than standard.
On the other hand, when a large deflector was placed just ahead of the tyre, drag dropped but front lift increased – as shown by #1 and #4. But if the deflector was too large (as in configuration #2), drag and lift were both high.
So what’s going on? Some simple rules can be applied:
However, missing from this is something very simple: why do the deflectors have to be flat plates? Apart from the fact that they are likely to scrape on the ground at full suspension bounce – and so need to be easily replaceable – there doesn’t seem to be any reason why flat plates need to be used. Instead, wouldn’t curving the undertray downwards in front of the tyres shield them to the same degree without stalling the airflow or deflecting it sideways, creating so much drag?
We wrote to the author of the SAE paper to ask her but didn’t receive a reply.
Back to the Prius
So looking at the front underside of the Prius there appeared to be a few options.
We decided to do the last of the three options, firstly building a quick and simple prototype undertray to see if this approach would work.
Some thin plastic that originally formed a sign was sourced. It was cut to the right shape and the rear edge reinforced with aluminium extrusion (arrowed). Two pieces of sign needed to be used and these and the aluminium strengthening piece were held together with high quality plastic adhesive tape.
More tape was used to hold the trial undertray in place.
Here’s what it looks like taped into place. Note that more than 300 kilometres of freeway driving was completed with the undertray held in place like this.
The curve of the undertray over the standard deflector can be seen here.
Testing can be carried out in a number of ways – these will be covered in more detail later. At this stage, with the trial plastic undertray in place, two preliminary factors were assessed:
On the multi-lane freeway – always a good test of car stability with its relatively high speeds, air disturbance from other vehicles, and open space for crosswinds to impact – the Prius felt just the same. Or at least, this driver couldn’t tell any difference. If he had to guess, he would say that the car felt a fraction more stable – but in the real world, increasing the weight in the power steering (a modification previously undertaken) had improved things much more dramatically than the effect of the undertray.
However, indicative of lower drag, there was a clear and measurable improvement in fuel economy.
The Prius uses a colour LCD to show average fuel economy in 5-minute intervals. That is, each 5 minutes the screen updates to show by means of a bar graph the fuel economy over the last 5 minutes of driving. (The same thing can be achieved on other cars by reading the figure just before resetting the average fuel economy display every 5 minutes. It’s a test technique that's highly recommended.) This Japanese domestic market car shows the fuel economy in kilometres per litre. (100 divided by km/l = litres/100 km)
On a flat road at 100 km/h, in standard form the Prius invariably turned in a 5-minute average fuel economy reading of 18 kilometres/litre (5.6 litres/100 km). That figure was achieved in cruise conditions on flat roads over literally thousands of kilometres of testing. Just occasionally, 20 km/litre was achieved – but rarely.
However, with the trial undertray in place, 20 km/l became the normal 100 km/h cruise fuel consumption. In this configuration, it was 22 km/l that was the occasional best result.
In other words, 100 km/h cruise economy improved from 5.6 litres/100 km to 5 litres/100 km – and economy in the Fours was now occasionally occurring.
(We have readers all over the world: in English mpg, the improvement was from ~ 50 to 57 mpg, and in US mpg from ~ 42 to 47 mpg.)
In a highly developed car like the Prius, to reduce drag sufficiently that open road cruise fuel economy improved by about 10 per cent is a startling result - far better than we had hoped for. Especially with no noticeable downside in stability.
Making the New Undertray
While the prototype undertray appeared to work very well, it had a downside – being made of thin plastic and adhesive tape, it wasn’t going to last very well... or look too good either! A new one was needed. As we did with the undertray fitted in the ‘Undertrays, Spoilers & Bonnet Vents’ series (starts here at Part 1), we decided to use ABS plastic to form the Prius undertray.
ABS is tough (it can be bent, hit with a hammer, etc, without shattering) and can be cut, filed and sanded with normal tools. Additionally, after it has been heated, it can be bent into shape. For AUD$66 we bought from a plastics supplier two-thirds of a sheet of black, 4mm grained ABS. Note that it can be rolled into a cylinder for easy transport home. Either 3mm or 4mm is suitable.
After cutting it to approximate size, the flat piece of plastic was held against the underside of the car – in this case, by using some crates and metal drawers.
Taking this approach allows you to easily see what needs to be cut off to obtain the required shape. But don’t do any cutting yet...
Before being marked for the cut, the sheet has to be held firmly in place in the correct orientation. Here this was achieved by inserting two bolts into existing holes in the underside panelwork. Once this was done, a line could be marked on the sheet around the front edge of the bumper...
...and then that line highlighted with tape. But don’t cut along the line yet...
The undertray was to be held in place with screws inserted around the leading edge, screwing into the existing bumper underside. Very coarse thread, short woodscrews were used. The heads of the screws were sanded back to provide a flat-head fastener. At this stage only the front screws were put into place.
Each end of the undertray needed to be bent upwards, allowing it to fit around the existing deflectors (the deflectors were retained but completely covered by the new undertray). The bending was achieved by marking a bend line with tape and then using a heat-gun to heat the plastic along that line. When pliable, the undertray could be bent into position.
This view from the side shows the gap that needed to be covered by bending each end of the undertray.
With the sides bent into approximate position, the undertray could be removed and the front cut to shape.
Cut-outs were also made on the trailing edge to give the standard tyre clearance – ie the rear cut-outs were within the wheel-arches. The two yellow stripes aren’t go-fast stripes but instead show where further bends need to be made. Bends made along these lines give the undertray a slight curve from front to rear. In the final iteration, only the rear bend was made.
The heat was applied along the bend lines then the undertray formed into the right shape. It is easier if a piece of timber is clamped along the bend line and the plastic bent around the timber. Depending on the power of the heat-gun, the bend might also need to be made in small sections along its length.
Near to the final tray, with the shape and bends correct.
When fitted, the rear edge of the undertray didn’t have the stiffness that it was thought the bend would give it. To provide this rigidity, a length of aluminium angle was bolted to the upper surface of the trailing edge.
Note that the upward-projecting lip of the aluminium angle is very short. In addition to the front mounting screws, each end of the rear of the tray bolted to an existing plastic moulding, with the bolt passing through both the angle and the plastic tray.
The finished appearance. This can be compared to...
...the standard undercar appearance.
Even up on ramps, the new undertray is nearly invisible. With the car’s front wheels back on the roads, you have to crane down to see anything different.
The final undertray varied from the prototype plastic quickie in four respects:
The first task on the road was to assess whether the fuel economy had continued to be better than standard. This proved to be the case.
Secondly, was there now any noticeable aero instability at speed? Certainly, the final version of the undertray is no worse than the plastic quickie – and may well be s-l-i-g-t-l-y better. It’s very similar to what was said above – if this writer had to guess, he’s say it might be a fraction better again than standard. Indications being used are that the car feels less unstable when being passed by trucks and the steering might be a fraction heavier in bends taken at speed. However, no particular claims are being made in this area, other than the car is definitely not noticeably worse in stability than standard.
Finally, an unexpected outcome of the undertray is the car is quieter. Whether that’s a reduction in undercar aerodynamic noise, or simply the blocking of a path for engine noise transmission, we’re not sure. But what is now noticeable is that tyre whine and A-pillar aero rustles can be heard – previously they were drowned out by the noise that overall has been decreased.
And the downsides of the undertray? There are a few, but we think they’re minor. Firstly, the undertray will need to be removed each time an oil filter change is carried out. Secondly, the front tow/tie-down hooks are not accessible with the undertray in place.
Making and fitting a custom cut and bent ABS undertray isn’t a 5-minute job... but making and fitting a quick plastic sheet prototype is! So have a good look under the front of your car and think about what you see. If the passage of air from the front bumper lip back at least as far as the front tyre line isn’t smooth and controlled, it’s quite likely that you’ll be able to make an improvement. It won’t take you long to find out!
And in the case of the Prius? To achieve a marked improvement in fuel economy in a car whose raison d’etre is good fuel economy is a stunning outcome...