Aero on road cars

This question's been on the tip of my tongue for ages. I'm not a technical type but I suspect I know the answer.

I was washing my car the other day when this elderly Camry pulled up a few doors away and an even more elderly gent emerged from it and stumped slowly across the road. 

It was the mundanity of the car and the age of its driver that triggered the thought.

Do the vestigial wings that sprung up on family shopping trollies some years ago have ANY effect? At ANY speed? At least, at any legal speed? I can think of a couple of cars which sported wings that popped up above 80km/h; Audi TT and Porsche something or other. Did they have any effect at 80 km/h or thereabouts? Or were even they an example of corporate onanism?

Edited by Gary Davies, 08 December 2018 - 13:04.

The boot lid wings were mostly styling. The annoying air dams that you smash when parking are somewhat useful at normal speeds as they do reduce drag a bit, but as you will have noticed, aero at normal speeds is not actually all that important for mpg.

The main handling reason for the air dams is to reduce front end lift, and the boot lid wings certainly can provide downforce, but I think you'd have to be driving rather faster than 70 mph to see much difference. I am currently textbook free at the moment so i can't work the answer out.

Aero is important at normal speeds, for things like crosswind stability, so perhaps I'm being a little cynical. 

And when i worked at a sports car manufacturer the rear wing was set to provide downforce so long as it didn't reduce top speed, so in practice it was not doing anything much.

The first rear spoiler to achieve notice appeared on a 1961 Ferrari sports prototype racer. The car had a sloping rear end with a cut off "Kamm tail". According to different histories, a spoiler was added to increase rear downforce, reduce drag and/or to prevent exhaust fumes entering the cockpit. With the benefit of 50 odd years of aerodynamic study, we know that all of the potential reasons apart from styling are related in one way or another.

For a three box saloon such as the Camry (non-Kamm tail), we might assume that a spoiler is functional in one aspect (creating a wake which disperses dirt from the rear of the car) if cars which do not have a spoiler are dirtier. 

However we can assume that BMW's 3.0 CSL wing kit (supplied with homologation specials but not fitted for road use) made a difference...

Here's some papers

https://www.matec-co...v2017_01027.pdf

https://www.research...c_Drag/download

https://scholarcommo...ntext=mech_mstr

They find quite significant effects are possible, but the Nascar one emphasises that if you want downforce you are going to see a lot of drag.

Of those the Cakir one finds unbelievable benefits from a boot lid spoiler. "It provided 6% drag reduction (dropped the drag

The Cheng Mansor paper gives a rather more typical view, looking at a roof mounted spoiler on a hatchback. Figure 8 is the important one, a plot of Cl vs Cd. Cd increases with negative lift, ie downforce. That's a  pretty typical result with modern car shapes as I know from my own work, where I'd like to minimise lift and balance, but the mpg boys always need low Cd.

Having compared Cakir with the other two, ignore it.

Many years ago I saw a Mitsubish Starion road car with an aluminium rear wing that would definitely have produced downforce. Mainly through weight!

The thing was massive, looked like it was almost half the size of the car.

I only have a piece of anecdata, and it relates to a proper high mount wing.  The wing was an s1223 profile, about 1.8m span and 300mm chord, mounted at the rear of an MX5 racecar behind the boot at roof level.  The car was trailered to the circuit, no ill effects noted.  On the return trip, the combination was plain evil, wandering around at 110-115kmh virtually unmanageably.  Checking tyres, load distribution no problems identified, on resuming behaviour unchanged (unsurprisingly), and speed reduced to more moderate levels to rein in the instability.

The penny dropped when we realised that the tow was into a strong headwind (tailwind on the way down), and the downforce on the rear of the car was enough to decrease the towball weight enough to affect stability.  After that the wing was removed for towing.

Many years ago I saw a Mitsubish Starion road car with an aluminium rear wing that would definitely have produced downforce. Mainly through weight!

 

The thing was massive, looked like it was almost half the size of the car.

Static downforce

 

Of those the Cakir one finds unbelievable benefits from a boot lid spoiler. "It provided 6% drag reduction (dropped the drag

coefficient from 0.232 to 0.217) but the negative lift force has been increased by 17%

(dropped the lift coefficient from -0.222 to -0.268)."

 

In fact the basic model used was already unbelievable.

 

However at 30 m/s (67 mph) that enormous change is only worth about 74N, or 17 lbf in Imperial units. Even at 140 mph that's only worth half a tank of fuel.

 

Those number did seem pretty spectacular, especially since the spoiler in question has next to no AoA. Having said that, I've seen relatively small spoilers (50mm at steep AoA) completely transform cars.

I only have a piece of anecdata, and it relates to a proper high mount wing.  The wing was an s1223 profile, about 1.8m span and 300mm chord, mounted at the rear of an MX5 racecar behind the boot at roof level.  The car was trailered to the circuit, no ill effects noted.  On the return trip, the combination was plain evil, wandering around at 110-115kmh virtually unmanageably.  Checking tyres, load distribution no problems identified, on resuming behaviour unchanged (unsurprisingly), and speed reduced to more moderate levels to rein in the instability.

 

The penny dropped when we realised that the tow was into a strong headwind (tailwind on the way down), and the downforce on the rear of the car was enough to decrease the towball weight enough to affect stability.  After that the wing was removed for towing.

A friend towing a Sprintcar with a big wing on an open trailer [30+ years ago] used to have the l/r tyres occasionally scrubbing on the mudguards. And the poor old F truck used LOTS of fuel to tow it.  Though it does prove the wing was giving inside weight,,, it was a 2 ton trailer!!

Towing my now 50 y/0 Supermodified around with the wing off is a noticeable improvement, even with the wing flat it is giving downforce and lots of drag!! On occasion I will move the car forward about 100mm to stop the downforce effecting  the trailer.

It is amazing that on a 2.5 load rocker sprung tandem trailer with about a ton of racecar and spares on board.

And when I added a wing on my Sports Sedan I learnt how loose the car was without it. What I gained in cornering I lost half of in extra drag. But the car was so much more stable, especially over 130mph. Never really noticed much at towing speeds, if any thing it seemed better with the wing at roof level instead of about a foot lower. Higher it was better at race speeds however

As for boot handles on road cars,,, well it looks faster!! And you need heavier boot struts to hold the boot  lid up!

As Greg has said modern cars have a bit less lift meaning a bit more stability at the front. The sporty ones often have less as they have knocked them all off. A friend with a modern V8 Commodore with all the wings and splitters etc says he really does notice it missing at 110k and has had it flapping at over 100mph. It has been replaced at mega bucks about 5 times now. He lives in the hills with spoon drains and steep driveways.

Though even many mundane family hacks drag the front in that scenario. VT on Commodores especially though I have done similar in Falcons and Mitsis and even Oh what a snooze cars

Edited by Lee Nicolle, 14 December 2018 - 12:57.

This makes a _huge_ difference on the front of a Porsche Cup car (not the GT3, just the Cup car). They have a reasonably sized rear wing that's at roof height, but this little bit of plastic clipped on the nose balances it.

There is a well researched aerodynamics book out now which discusses modifications to road cars here; https://www.linkedin...ar-willem-toet/

Willem Toet was also a major contributor to my upcoming book, and as you would expect from such a legendary F1 Aero man his insight is exceptional.

Edited by Ibsey, 16 December 2018 - 06:57.

Good old Julian, still flailing around! His most famous article was in Silicon Chip entitled "How to Hold a Garage Sale", complete with photos.

Good old Julian, still flailing around! His most famous article was in Silicon Chip entitled "How to Hold a Garage Sale", complete with photos.

Good old Julian, still flailing around! His most famous article was in Silicon Chip entitled "How to Hold a Garage Sale", complete with photos.

Ouch!

Road car mods?

Step 1: Roll up windows

Step 2: Heli-tape all body seams

Step 3: Draft

Beyond that, each car is going to be it's own project. Test.