Diffusing the myths (Diffuser discussion)

[TLicense]

Over the last few years diffusers have become more and more popular aftermarket components on our cars. They are mostly fitted for aesthetic reasons, but occasionally are fitted with the prime goal of improving car performance by increasing downforce.

But how and why do the work?

 

The best way to think of it is from the back to the front of the car. As the car moves through the air, it disturbs the air and leaves a ‘hole’ or area of low pressure behind the car. As nature abhors a vacuum, air from around the low pressure region rushes in to fill it. This flow will come from around the sides of the car, but most importantly (and usefully!) from under the car.

 

As most people will know, an aeroplane wing works due to the fact that the air moving over the top surface is moving faster and has lower pressure than the air under the wing. So if we use this same principle on a road car, ie we can get the flow travelling under the car to travel faster, then we will generate lower pressure all over the floor and basically ‘suck’ the entire car downwards.

 

So the faster the flow under the car, the more downforce there will be.

 

The diffuser helps here as it exposes the flow under the car to a larger area of low pressure. Think of it like this, as the car is travelling it creates a region of area that is 5000 pascals lower than the ambient pressure. This means you’re pulling the flow under the car with 5000 Newtons per metre^2. If your diffuser is 1 metre wide and 10 cm tall, this means you will be pulling the flow under the car with 5000 N/m^2 * 0.1m = 500 Newtons or approximately 50Kg’s. If you either reduce the pressure in the region behind the car, or increase the area then the force pulling the air under the car will increase.

 

So why isn’t the diffuser the entire width and height of the back of the car? Well unfortunately as with most things, there is a limit.

 

Taking a cross section along the length of a car with a diffuser. If the ride height of the car is 20cm from the ground to the bottom of the car, and our diffuser trailing edge is 50cm’s tall, we obviously have to go from 20cm’s to 50cm’s at some point. There is a theoretical limit as to how quickly we can do this. I won’t go into it here, but suffice to say in theory the ramp from the bottom of the floor to the trailing edge of the diffuser should be no bigger than 7 degrees. If it’s any more than 7 degrees the flow is likely to separate. Once the flow separates it becomes turbulent and generates a ‘boundary layer’ of slow non-laminar flow that increases in thickness along the length of the surface of the diffuser. This thickened boundary layer effectively reduces the amount of exposure that the flow under the car has to the low pressure region at the back of the car, resulting in less ‘pull’ for the flow. So the flow under the car travels slower and the car starts to lose downforce all over the floor.

 

So we want to increase the height of the diffuser as much as possible, but the ramp from the bottom of the floor can be no more than 7 degrees from the road. Now that’s an important point to note. The 7 degrees is a maximum angle to the road. So if the diffuser is 7 degrees to the road with the car stationary, it’s quite easy to see when driving, if you brake the nose of the car dips, the rear raises and the angle of the diffuser to the road is increased beyond 7 degrees and stalls.

 

Anyway, back to the limiting factors. So as high as possible, but at a rate no more than 7 degrees when the car is at the attitude where you want it to generate the most downforce. The problem then becomes one of actually fitting the thing.

 

On a Supra, the limiting factor at this point is the fuel tank. But what if the fuel tank wasn’t there? Or what if in an imaginary world we could take make the diffuser absolutely massive? Well the problem is we need something for the flow under the car to pull on. If it were all diffuser and no floor then the fast acting flow under the car would have nothing to pull on. But wouldn’t it be pulling on the diffuser itself? Well it would yes, but by the very nature of the fact that the diffuser is expanding, causes the airflow to slow down. Slow flow = lower dynamic pressure which = less suction.

 

So the ideal diffuser is a careful balance. Maximum exposure to the low pressure region, a ramp no greater than 7 degrees and maximum floor area for the high speed flow to pull on.

 

The main point to understand is that the diffuser doesn’t really generate any downforce. It’s a means for the rest of the floor to generate more downforce. Once that’s understood, it’s possible to predict what the effect of the additional downforce will be on a Supra. Mainly it is unlikely to generate a rearward balance shift due to increased rear downforce, but more likely a neutral increase in downforce or a slightly forward balance shift as the increase in airflow is acting on the relatively smooth engine undertray and the floor area under the cabin. As the flow gets closer to the rear it will be obstructed by the exhaust, diff and depending on where the diffuser starts, the fuel tank. These obstructions are likely to cause the flow to become turbulent. This makes it much harder for the diffuser to do its job and once again can cause the flow to stall before it even gets to the diffuser. If the diffuser is stalled then the connection between the low pressure region at the back of the car and the flow under the car is lost, so you can lose downforce from the entire floor.

 

I’ve tried to explain this very much in layman’s terms, and it’s vastly simplified but hopefully explains a little about what the diffuser does and does not do.

Edited April 19, 2010 by TLicense (see edit history)

[Pixelfill]

Great post Tony

 

makes the whole concept clearer to me and I "sucked" at aerodynamics

 

Mike

[Wez]

Nice write up matey

[Abz]

Extremely good write up! Well detailed but not over baffling, liked the title too

[Paul Whiffin]

Nice bit of info there. Fancy designing me a flat floor and diffuser for my car?!

[tbourner]

Didn't F1 used to have contoured undertrays in the 80s? How is that different?

[SupraStar 3000]

Nice read. thank you Tony.

[dangerous brain]

Crikey that brings hours of Johnny Monotone the instructors hours of aerodynamics flooding back. I think the hours spent in wind tunnels were a tad wasted on me as I spent my entire career on massive transport helicopters that are anything but aerodynamic. I had aspirations of making a sheet aluminium undertray for the do-Luck aristo to go from the front splitter to feed the diffuser style ramp on the rear bumper. There's no undertray on the engine compartment so with that and the trans tunnel, diff, and double exhaust there must be huge amounts of turbulence under there causing massive drag. I should imagine any cars trying for high top speed figures would hugely benefit from reducing the drag under there.

[TLicense]

Nice bit of info there. Fancy designing me a flat floor and diffuser for my car?!

 

Sure, but it would need to be tested at somewhere like Mira to find out what it is doing.

 

Didn't F1 used to have contoured undertrays in the 80s? How is that different?

 

They did yes. The difference being that they massively increased the amount of 'suck' by effectively creating a venturi under the car which increased the speed of flow under the car even more. They also didn't need a 'diffuser' as such as they just extended the trailing edge of the aerofoil shape to expose the flow to the low pressure area behind the car.

[Paul Whiffin]

Sure, but it would need to be tested at somewhere like Mira to find out what it is doing.

 

 

Bit out of my budget that one!

[Swampy442]

Bit out of my budget that one!

 

Budget? Sure a man who tunes and builds race cars can have no such problems

 

Good write up Tony

[hoff]

Thats an excellant write up, and one that even I can understand.

 

I have only had the chance to drive mine a couple of times since fitting my diffuser. In that brief time on the road I dont think I've notice any change in the behaviour of the car (wasn't really expecting to) but certainly feel aware of some drag (maybe due to noise and the petrol gauge;)).

 

I assume for drag racing I am best to drop it off to achieve a few extra 100th's? I have only really fitted it for look more than anything as I dont go on track just up the strip every now and again.

[TLicense]

Bit out of my budget that one!

 

Honestly I'm not trying to be obtuse on that, it's just that anyone who recons they can design a diffuser for any car straight off and make it work without having to measure what it's doing or iterate the design is full of it.

 

I assume for drag racing I am best to drop it off to achieve a few extra 100th's? I have only really fitted it for look more than anything as I dont go on track just up the strip every now and again.

 

Yeah definitely get rid if you're loking for the nth degree in drag racing.

Pretty much any device that makes downforce will make drag. In ten years, I've seen maybe only a couple of bits put onto a car that have been both a downforce and drag benefit.

Even if you managed to find something that made little drag, the downforce would effectively be increasing the vertical load on the wheels so your rolling resistance would increase.

Edited April 26, 2010 by TLicense (see edit history)

[Miguel]

Honestly I'm not trying to be obtuse on that, it's just that anyone who recons they can design a diffuser for any car straight off and make it work without having to measure what it's doing or iterate the design is full of it.

 

 

 

I think that would also apply to 90% of the aftermarket body parts that claim to offer aerodynamic benefits. How many of them actually hit the tunnel before hitting the market.

 

Good write up Tony. Enjoyed the read.

[hoff]

Yeah definitely get rid if you're loking for the nth degree in drag racing.

Pretty much any device that makes downforce will make drag. In ten years, I've seen maybe only a couple of bits put onto a car that have been both a downforce and drag benefit.

Even if you managed to find something that made little drag, the downforce would effectively be increasing the vertical load on the wheels so your rolling resistance would increase.

 

Makes perfect sence. Would there be a massive differance on the 1/4 mile with a bpu supra with the shine diffuser? Half seconds and the like. Obviously without seeing it on the car e.t.c. you cant say for certain, but theorectically over that distance.

 

I'm sure at suprapod I'll try both on and off. Just dont want to be mid 13s when I can be high 12s without it.

[Terminator]

I think that would also apply to 90% of the aftermarket body parts that claim to offer aerodynamic benefits. How many of them actually hit the tunnel before hitting the market.

 

Good write up Tony. Enjoyed the read.

 

Totally agree. Toyota will have done all they can with in the limits of their design budget. If a company thinks that with a fraction of that budget, it could design a body part that would make a significant positive difference to the car's performance, it is having a laugh. Go faster stripes anyone.

 

The original post should be a tech reference document. Well written Tony.

[Paul Whiffin]

Honestly I'm not trying to be obtuse on that, it's just that anyone who recons they can design a diffuser for any car straight off and make it work without having to measure what it's doing or iterate the design is full of it.

 

 

How much does Mira cost these days?

[Chris Wilson]

Sealing the whole of the underfloor to feed the diffuser will also seal in the rad air exit, so it will overheat. These things need designing with the cooling system in mind, too. Getting a road car low enough to effectively work a diffuser will be "interesting", although from the looks of some of the MKIV's these days people are trying hard

[Ryan.G]

How much does Mira cost these days?

 

£6k = 1/2 day.

 

You would need a couple of days to get it right im sure

[supramkivcork]

Great post very informative, if you don't mind I've copy/pasted the post as a thread on the independent forum I'm a member on here in Ireland.