Downforce, Drag and Their Effects
Discussion
The lower the cd, the better the aerodynamics and thus the fuel economy of a car.
Add a spoiler and you increase downforce thus increasing drag and lowering fuel economy?
Lowering a car (suspension tweaks) increases drag and thus reduces fuel economy?
I'm having difficulty obtaining a clear opinion on the above. Anyone care to chip in?
Add a spoiler and you increase downforce thus increasing drag and lowering fuel economy?
Lowering a car (suspension tweaks) increases drag and thus reduces fuel economy?
I'm having difficulty obtaining a clear opinion on the above. Anyone care to chip in?
You are comparing lots of different things, which are related but not necessarily causal.
First:
You want to look at CdA (i.e. cd, drag due to the shape, x the area of the car). What does this mean? Well it means a shape which can slip through the air very well might still have lots of drag if it's massive. But yes, all else being equal, better Cd (as I said its the CdA that's important) = less energy required to move the object through the air = better fuel economy and higher top speed and better acceleration at higher speeds.
Just saying 'adding a spoiler increase downforce thus increasing drag' is very simplistic. Some spoilers decrease cd (e.g. the spoiler on the back of a prius) as they smooth out the air flow beghind the car. generally a spoiler designed to increase downforce will increase drag but it's much more complicated then that. Is it possible for a spoiler to increase downforce and decrease drag? I'm not sure but I would suggest that yes, it is. It is certainly possible to decrease downforce (i.e. produce lift) and increase drag...this is what most spoilers on cars from the 80's/90's do.
Lowering a car will have lots of different effects but generally lowering a car will decrease the Cd and so increase fuel economy.
This could be a very interesting discussion, looking forward to learning.
First:
You want to look at CdA (i.e. cd, drag due to the shape, x the area of the car). What does this mean? Well it means a shape which can slip through the air very well might still have lots of drag if it's massive. But yes, all else being equal, better Cd (as I said its the CdA that's important) = less energy required to move the object through the air = better fuel economy and higher top speed and better acceleration at higher speeds.
Just saying 'adding a spoiler increase downforce thus increasing drag' is very simplistic. Some spoilers decrease cd (e.g. the spoiler on the back of a prius) as they smooth out the air flow beghind the car. generally a spoiler designed to increase downforce will increase drag but it's much more complicated then that. Is it possible for a spoiler to increase downforce and decrease drag? I'm not sure but I would suggest that yes, it is. It is certainly possible to decrease downforce (i.e. produce lift) and increase drag...this is what most spoilers on cars from the 80's/90's do.
Lowering a car will have lots of different effects but generally lowering a car will decrease the Cd and so increase fuel economy.
This could be a very interesting discussion, looking forward to learning.
Edited by varsas on Friday 4th May 13:33
Yes - Lower Cd better fuel economy.
Some spoilers actually improve the airflow at the rear of the car and so can improve Cd, but if set for downforce (not very much at legal speeds in UK) they can improve the "Feel" of the car when cornering.
Lowering may also reduce drag as less air going under the car getting churned up.
It is a very complex subject though and I'm sure there will be more expert opinions soon!
Some spoilers actually improve the airflow at the rear of the car and so can improve Cd, but if set for downforce (not very much at legal speeds in UK) they can improve the "Feel" of the car when cornering.
Lowering may also reduce drag as less air going under the car getting churned up.
It is a very complex subject though and I'm sure there will be more expert opinions soon!
Spoilers are called spoilers becuase they spoil the generation of large trailing vortices behind the car.
These vortices intensity is the square of there size and delaying their reattachment to the car will reduce the pressure drop behind the car.
The pressure difference between the front of the car and rear is part of the combined drag on the car. Hence you can see lower Cd from a spoiler, its whats its for, a wing is to manage lift/downforce. The vortices also explain road dirt on bluff rear end cars when the low pressure region lift road dirt towards the back end of the car where it sticks.
These vortices intensity is the square of there size and delaying their reattachment to the car will reduce the pressure drop behind the car.
The pressure difference between the front of the car and rear is part of the combined drag on the car. Hence you can see lower Cd from a spoiler, its whats its for, a wing is to manage lift/downforce. The vortices also explain road dirt on bluff rear end cars when the low pressure region lift road dirt towards the back end of the car where it sticks.
JonnyVTEC said:
Spoilers are called spoilers becuase they spoil the generation of large trailing vortices behind the car.
These vortices intensity is the square of there size and delaying their reattachment to the car will reduce the pressure drop behind the car.
The pressure difference between the front of the car and rear is part of the combined drag on the car. Hence you can see lower Cd from a spoiler, its whats its for, a wing is to manage lift/downforce. The vortices also explain road dirt on bluff rear end cars when the low pressure region lift road dirt towards the back end of the car where it sticks.
All very true - but spoilers aren't necessarily just for cleaning up low pressure wakes or trying to negate too much lift over an axle (you can have a front spoiler too). An important part of road vehicle aerodynamics is cross-wind / yaw stability - i.e making sure the car doesn't suddenly get all light at the back when there's a sudden gust of wind across the motorway. The early Ford Sierra suffered a lot with this problem. So a spoiler can ensure that the separation point of the flow is consistent over a range of wind conditions, giving consistent/predictable, and more importantly, smaller changes in the location of the centre of pressure, which gives the driver a feeling of stability. Road car aerodynamics is a very complex subject (motor racing even more so, and I should know These vortices intensity is the square of there size and delaying their reattachment to the car will reduce the pressure drop behind the car.
The pressure difference between the front of the car and rear is part of the combined drag on the car. Hence you can see lower Cd from a spoiler, its whats its for, a wing is to manage lift/downforce. The vortices also explain road dirt on bluff rear end cars when the low pressure region lift road dirt towards the back end of the car where it sticks.
). If you are interested I can recommend getting a hold of the book 'Road Vehicle Aerodynamic Design' by Barnard.Lowering a car to a point may actually decrease drag - you can generate lower pressure under the car by accelerating the air through a smaller gap, especially with the right underbody treatment. A lot of the new efficient-eco-dynamics cars sit lower for this reason.
Edited by RenesisEvo on Friday 4th May 13:59
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