Race Car Drag Reduction

[andylaurence]

Sorry for the somewhat off-topic post. I run a car for short sprint races (<3 miles) around race circuits in the UK. I have a small bike engine in the back of my car, which means reducing drag is important for going quickly. Previously, I had a rear wing with a 300mm chord and a 10mm Gurney on the trailing edge, which I've replaced with a 200mm chord rear wing and no Gurney. I'd appreciate any ideas from the knowledge-base here on how I can reduce drag to go that bit quicker.

Three photos attached show the front and the rear of the car, albeit with the old rear wing. The two intakes above the front splitter don't appear to do anything. Am I best off blocking these up?

[ChrstphrR]

What restrictions do you have for the class you are racing in?

Changing the shape here and there to reduce drag might not matter if you contravene one of the rules you have to abide by, for weight, dimensions, shape, functional bits that must be present, etc., etc.

I see there are what appear to be NACA ducts on the front face of the car's bonnet, in addition to the two lower ducts near the splitter. From the pictures, I can't tell if they're attached, or if the top ones are even functional.

That and, the obvious spot to me would be the front of the rear fenders, looks like a potential area to reshape so it's a smoother transition from the body -- if it's allowed by the rules you race under.

Another two big areas, which again, rule-dependant, would be potentially the short, blunt back end of the car past the rear wheels, and the underside which isn't shown.

My suggestion would be, first, dig up the rules, see what restrictions you have with modifying the bodywork -- and if any such improvements to reduce drag are out and out allowed, are not mentioned, or not explicitly forbidden, but are part of a grey area in the regulations.

[euromodder]

Originally Posted by andylaurence

Three photos attached show the front and the rear of the car, albeit with the old rear wing. The two intakes above the front splitter don't appear to do anything. Am I best off blocking these up?

They appear to be venting into the NACA outlets above them.
First thing to find out is why they are there at all, if they are functional.

If the rules allow for it :

* a boat-tail fairing behind the drivers head / helmet
(could maybe be integrated with the next point)

* behind the roll bar, there's a nice streamlined fairing, but with a square box mounted on top of that - give it a boat tail

* aerodynamic race mirrors (with clear extensions on the rear) can reduce some of the mirror drag

* close the gap aside the driver - obviously with something that won't hurt him in a mishap.

* if you don't need to change tyres in a hurry during the race, cover the rear wheel openings

[andylaurence]

Thanks guys! First of all, to answer the rules question, I'm pretty much free on what I can do with bodywork. The sport is goverened by the MSA and the regulations are in the Blue Book, specifically the sections on sports libre cars in sprints and hillclimbs and vehicles. That boils down to:

Maximum width - Measurement between the outer tyre walls of the front or rear (whichever is the wider) + 20cm
Maximum rear - 150cm behind overhang rear wheel axis
Maximum height - 90 cm from the ground (open cars)
Maximum front - Not to exceed the top aerofoil height of the front wheel rim
Maximum rear aerofoil - 90cm from the ground height (closed cars)

The NACA ducts on the front are not functional. I believe they are used for brake cooling in endurance racing. I'm not sure of the function of the holes at the front. My splitter is not standard and the original car may have a front diffuser that uses these holes.

Boat-tailing behind the rear wheels sounds like an interesting idea I'd not thought of.

The underside of the car is completely flat (aside from a few rivets) from the front of the splitter to the start of the little diffuser that starts at the rear axle. I've considered making the diffuser bigger as that will bring it closer to meeting the top bodywork, bring more downforce and start to work with the rear wing.

A boat-tail behind the driver's head sounds a good bet, along with smoothing off the box on the rear engine cover.

The mirrors are a quick win - I don't need them as cars run against the clock, not head-to-head and the regulations do not require them.

The cockpit opening must be at least 85cm wide, so I could close it a little.

How would I securely fair in the wheels? I guess a sheet of perspex and some cable ties would be a good start. What about wheel balance? At 130mph, wheel balance is quite important! Brakes don't get very hot in sprinting as the longest event is 3.3 miles, including just 9 presses of the brake pedal from cold.

I'm impressed at the detail you guys have picked up so quickly. Some stuff I'd just never noticed. Thanks for the suggestions. Time to get some foam...

[XJguy]

Visit this site: mulsannescorner.com much knowledge to be had for protype racing.

At a quick glance I see a lack of wheel well ventilaton, the driver shield or lack therof could be improved to promote air flow over you. Exhaust can be altered to aid rear air flow, at the very least be centered and positioned to pressurize a low pressure area. The front intakes might be contributing down force and or feeding the undertray to generate downforce at the rear. A proper diffuser with strakes will allow you to diminish your wing further and not lose downforce.

[JasonG]

Definitely late to the party on this one.
All above sounds good.
If they restrict your mods beyond the written, the deflector in front of you and the "headrest" difuser would be your best bets.
Careful on enclosing the cockpit. It would be safer to use lips to direct the airflow.

Look at the current F1 cars for more ideas.

[JasonG]

Definitely late to the party on this one.
All above sounds good.
If they restrict your mods beyond the written, the deflector in front of you and the "headrest" difuser would be your best bets.
Careful on enclosing the cockpit. It would be safer to use lips to direct the airflow.

Look at the current F1 cars for more ideas.

[davidgrey50]

Anything which produces downforce will increase drag - that is always going to be the trade-off. Lose your wing and you'll go faster in a straight line, but around the twisty bits, not so good. F1 cars are designed for maximum downforce - period. Their drag is astronomical. Sir Stirling Moss commented recently that simply lifting off in a modern F1 car produces more braking effect than standing on the brakes of the cars of his era.

[andylaurence]

Thanks for the ideas guys. I've spent a bit of time at mulsannescorner.com and some time looking at similar cars at various events. I also keep my ear on the ground with F1 (Scarbs is good for that).

The wheel well ventilation is something I've been thinking of as it's an option on the car and there are sections to cut out on the body to make it nice and easy. I assume that the wheel wells are high pressure, hence venting out the top reduces the pressure, generating negative lift. I guess it probably helps drag too as the pressure must be venting somewhere and that must be to the side, increasing effective frontal area.

The exhaust is something I'd like to change to blow into the rear diffuser. I'm worried about overheating fibreglass and effects of a new exhaust length on power. It'll go hand-in-hand with a new diffuser and would assist in reducing exhaust noise, which can only help me in scrutineering! That brings me onto strakes. How do I know the right number? I understand that it's the vortices created next to the strakes that energise the flow and assist in creating the downforce, but more strakes won't always mean more downforce.

The intakes at the front don't appear to do anything - they just vent straight into the wheel wells. The whole underside of the front panel from a few inches ahead of the dash is just a big open cavity that my feet, the battery and allsorts sit in. I would assume that closing it up will reduce drag, forcing air over the clean body, rather than through the car, just like a grille block.

On enclosing the cockpit, I'd use a flexible material, rather than fibreglass as I want to retain the ability to take passengers.

I really appreciate all the assistance you guys give me - there's a lot of clever people in this forum and efficiency is as important in racing as it is in fuel economy.

[some_other_dave]

You might get some small gains by making larger-radius fillets where the vertical wheel housings join the horizontal top of the body. This isn't a common concern on most cars, but I know that fillet shape is something that gets a good deal of attention paid to it on aircraft, particularly where the wings join the fuselage. And since there's some sharp-ish corners where the cockpit surround sticks up from the top of the body, you might gain a little from larger-radius corners there too.

Smooth hub-caps on the front are worth looking into, as well as the complete skirting of the rear wheels. You may run afoul of regs with the caps; most track-day outfits that I know of want hub caps and "beauty rings" removed before the car hits the track.

-soD