I've been concerned for a little while over my brake temperatures on track days, the increased wear of the inner brake pads and the fast wear after the pads get to 1/3 thickness. I therefore had a look at ways that I could get brake cooling without spending a fortune. The more I looked at what other people had done on other cars, eg here and here, the more I wanted to use my experience and knowledge of aerodynamics rather than just cobble something together. They are both easier and cheaper than my project, but they are too inefficient and agricultural for me (I was spoilt by doing an aeronautically-biased engineering degree, doing an Aeronautical Systems Masters and working with Germans for 7 years in which everything I did had to be spot on).
Why bother with aerodynamics? Apart from my finding the subject fascinating, a poor system design can increase drag significantly. Cooling systems and front wheels account for some 35% and 13% respectively of total drag; taking clean air from the bumper, throwing it or fitting big tubes into the front wheel wells can make this situation far worse.
I’m doing this in the first instance on my R3 then on my S1 project car. The main differences between the 2 that affect this are the fog lights and how the radiator inlets are configured.
I began by breaking the problem into 2 parts: where to get cooling air and how to take it to the brakes. The second question has 2 possible answers: to duct it to the centre of the brake discs directly or to angle air across the wheel wells towards the brake discs. Whilst large hoses in the wheel wells can cause some drag (think of all that rotating air hitting the hoses), throwing air into the wheel wells will almost certainly cause much more drag.
The air flows and pressures inside and adjacent to a wheel well are complicated and dependent on such factors as the shape of the wheel. You can get some unexpected results; for example, Vauxhall found that when they extended the bumper of the Calibra (class-leading CD of 0.29) downwards the air exiting the wheels interfered with the air coming off the air dam produced more drag rather than less as had been expected.
I looked at 4 ways of getting cooling air: a hole in the bumper or radiator air inlet, a scoop under the undertray, a scoop attached to the wheel hub and a hole just forward of the front wheel air dams (the short blades that come down from the undertray just in front of the front wheels). I don’t like the idea of taking air from the bumper that would otherwise pass down the side of the car with relatively low drag, but it is the simplest and somewhat obvious. I ruled out scoops under the undertray (too much drag, reduced ground clearance, risk of damage) and on the hub (hard to get cooling air without restricting steering movement), so that left only taking air from in front of the air dams.
What are the air dams for, you may ask, and how can they get cooling air? They’re there to trip underbody air before the front wheels and so reduce interference with the air from the front wheels; when designed correctly they reduce drag by around 4%, albeit the increased pressure they generate causes lift. If I place the inlet just in front of the air dams then the increased pressure will drive air through a hose to the brake discs, thus reducing the air left to cause drag at the wheels and further along the underbody.
To measure the static air pressure for both these choices I carried out a DIY pressure check with some differential pressure gauges connected to plastic tubing run to 3 places: the number plate (for stagnation pressure), the air dams and the centre of the brake disc. The pictures show the gauges and the tubing going to the brake disc.
Compared to atmospheric, the pressure in the brake disc was slightly lower below 50 mph and slightly higher above, and the pressure difference at the air dams was 70-75% of the pressure difference at the number plate (approximately stagnation pressure).
x
Calculating the size of the hose is so complicated that I took the easy route of seeing what others had claimed for their projects and scaling accordingly. This gave me a diameter of 40-50 mm and the available space at the brake discs made 50 mm (2”) the best size to start with.
When I look to route the hoses I’ll look to leave room for modding the oil cooler exhausts later on. The oil coolers vent into the wheel wells immediately ahead of the tyres, with Mazda making an attempt to angle the outflow downwards to minimize drag with the air rotating around the tyres. Mazda's design shows all the compromises for space, complexity and cost inherent in any road car and, IMO, have done a reasonable job, notably having inlet ducting. That said, the design is far from the ideal (radiators are most efficient when they have inlet and exhaust ducting with an inlet/outlet 1/4 of the radiator width at 1 width ahead/behind the radiator) and I have a project on my list to assess making cooling more efficient by fitting exhaust ducting to my project car (IMO the design is perfectly adequate for standard cars, to the extent that blanking is needed for cold weather). For completeness, I’ll mention that I’ve considered a few other improvements around the wheels: vents in the top of the wings (too flash); vents in the rear of the well (too much work?); aerodynamic fairings in front of the wheels as in Merc SLRs (too much trial-and-error effort to avoid stability problems); and fairings on the wheels (looks a bit silly). Sucking air out of the rear wheel wells and venting it out the rear bumper using an electric fan will significantly reduce drag, say by 8%, mainly by filling in the low pressure area at the car rear.
I’ve now ordered the bits I need and will update this thread when I’ve got more to show.
Why bother with aerodynamics? Apart from my finding the subject fascinating, a poor system design can increase drag significantly. Cooling systems and front wheels account for some 35% and 13% respectively of total drag; taking clean air from the bumper, throwing it or fitting big tubes into the front wheel wells can make this situation far worse.
I’m doing this in the first instance on my R3 then on my S1 project car. The main differences between the 2 that affect this are the fog lights and how the radiator inlets are configured.
I began by breaking the problem into 2 parts: where to get cooling air and how to take it to the brakes. The second question has 2 possible answers: to duct it to the centre of the brake discs directly or to angle air across the wheel wells towards the brake discs. Whilst large hoses in the wheel wells can cause some drag (think of all that rotating air hitting the hoses), throwing air into the wheel wells will almost certainly cause much more drag.
The air flows and pressures inside and adjacent to a wheel well are complicated and dependent on such factors as the shape of the wheel. You can get some unexpected results; for example, Vauxhall found that when they extended the bumper of the Calibra (class-leading CD of 0.29) downwards the air exiting the wheels interfered with the air coming off the air dam produced more drag rather than less as had been expected.
I looked at 4 ways of getting cooling air: a hole in the bumper or radiator air inlet, a scoop under the undertray, a scoop attached to the wheel hub and a hole just forward of the front wheel air dams (the short blades that come down from the undertray just in front of the front wheels). I don’t like the idea of taking air from the bumper that would otherwise pass down the side of the car with relatively low drag, but it is the simplest and somewhat obvious. I ruled out scoops under the undertray (too much drag, reduced ground clearance, risk of damage) and on the hub (hard to get cooling air without restricting steering movement), so that left only taking air from in front of the air dams.
What are the air dams for, you may ask, and how can they get cooling air? They’re there to trip underbody air before the front wheels and so reduce interference with the air from the front wheels; when designed correctly they reduce drag by around 4%, albeit the increased pressure they generate causes lift. If I place the inlet just in front of the air dams then the increased pressure will drive air through a hose to the brake discs, thus reducing the air left to cause drag at the wheels and further along the underbody.
To measure the static air pressure for both these choices I carried out a DIY pressure check with some differential pressure gauges connected to plastic tubing run to 3 places: the number plate (for stagnation pressure), the air dams and the centre of the brake disc. The pictures show the gauges and the tubing going to the brake disc.
Compared to atmospheric, the pressure in the brake disc was slightly lower below 50 mph and slightly higher above, and the pressure difference at the air dams was 70-75% of the pressure difference at the number plate (approximately stagnation pressure).
x
Calculating the size of the hose is so complicated that I took the easy route of seeing what others had claimed for their projects and scaling accordingly. This gave me a diameter of 40-50 mm and the available space at the brake discs made 50 mm (2”) the best size to start with.
When I look to route the hoses I’ll look to leave room for modding the oil cooler exhausts later on. The oil coolers vent into the wheel wells immediately ahead of the tyres, with Mazda making an attempt to angle the outflow downwards to minimize drag with the air rotating around the tyres. Mazda's design shows all the compromises for space, complexity and cost inherent in any road car and, IMO, have done a reasonable job, notably having inlet ducting. That said, the design is far from the ideal (radiators are most efficient when they have inlet and exhaust ducting with an inlet/outlet 1/4 of the radiator width at 1 width ahead/behind the radiator) and I have a project on my list to assess making cooling more efficient by fitting exhaust ducting to my project car (IMO the design is perfectly adequate for standard cars, to the extent that blanking is needed for cold weather). For completeness, I’ll mention that I’ve considered a few other improvements around the wheels: vents in the top of the wings (too flash); vents in the rear of the well (too much work?); aerodynamic fairings in front of the wheels as in Merc SLRs (too much trial-and-error effort to avoid stability problems); and fairings on the wheels (looks a bit silly). Sucking air out of the rear wheel wells and venting it out the rear bumper using an electric fan will significantly reduce drag, say by 8%, mainly by filling in the low pressure area at the car rear.
I’ve now ordered the bits I need and will update this thread when I’ve got more to show.
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