Lowering a car for aerodynamic purposes

[deveng]

Remember reading a while ago someone had lowered their car in an attempt to reduce Cd and increase downforce.

It makes sense to me, but wonder if anyone else has heard of/done this.

Say you spend £300 on lowering springs and get a 3mpg increase from 30 to 33 mpg it would take about 15,000 miles (fag packet maths) to be back in the black, so 1-2 years motoring.

Tempted....

[WeirdNeville]

It's the same as buying a new set of tyres for better efficiency - you'd need big increases and huge milage to justify it.

[Superhoop]

Well VW clearly think it's a good plan - All of their Bluemotion range run lower suspension than the standrad models

[ewenm]

Superhoop said:

Well VW clearly think it's a good plan - All of their Bluemotion range run lower suspension than the standrad models

As do Volvo on their DRIVe models.

[Theoldfm]

deveng said:

Remember reading a while ago someone had lowered their car in an attempt to reduce Cd and increase downforce.

It makes sense to me, but wonder if anyone else has heard of/done this.

Say you spend £300 on lowering springs and get a 3mpg increase from 30 to 33 mpg it would take about 15,000 miles (fag packet maths) to be back in the black, so 1-2 years motoring.

Tempted....

Better still, sell your expensive, low profile alloys and buy some cheap, narrow wheels instead. More cash in the bank and even better MPG.... you could always sell one of the wing mirrors too for even better efficiency gains!!!

[stemll]

And, based on some of the comments on the aftermarket DRL thread, remember to turn your headlights off

[kambites]

I'd imagine it can make a measurable difference. Drag at a given speed is proportional to the frontal area of the car which in practice includes the bit under the car since there's so much turbulence under a road car. Knock 5cm off the height of a 150cm high road car and you'll drop the drag by 3%.

There are other things I'd do first though, if I was trying to make my car as efficient as possible.

[alephnull]

On any decent car you would spoil the handling by changing the springs. You would really need a very talented expert to set it up to ensure the spring/damper settings are suited to your specific car. What is good on a BMW M3 would quite likely be less good on a lighter car or something with a different weight distribution. Companies spend a LONG time test driving to find good suspension settings, and only Lotus (and a few other cars) get it right despite all the development money they all spend.

That being said, if you lowered a diesel snotter and did 25k miles/yr, it might make a small difference. Over inflating your tyres would make a bigger difference, as would driving like a fairy.

[RemainAllHoof]

stemll said:

And, based on some of the comments on the aftermarket DRL thread, remember to turn your headlights off

Well, indeed, any electrics being used will cost in petrol; so turn off lights, aircon, fans, heaters, indicators, radio... sod that, I'll suck up the petrol cost.

[davepoth]

Lowering is done because it's easier than the correct option - a flat floor. By stopping air flowing under the car you reduce the drag there, but that means a big airdam at the front which isn't great. Smoothing the floor will help a lot more, and the airflow can be diffused at the back to make "free" downforce to keep the car on the road.

[Ian974]

I'm not sure, is it possible to use a front spoiler, rear diffuser and flat floor to just reduce drag rather than for downforce/ground effect?

[kambites]

With today's obsession with fuel economy, I've never really understood why they don't build all cars with flat under-bodies. It can't be that hard, surely?

[deveng]

Ian974 said:

I'm not sure, is it possible to use a front spoiler, rear diffuser and flat floor to just reduce drag rather than for downforce/ground effect?

Well the flat floor does both, reducing drag and increasing downforce. Rear diffuser will do the same, allowing airflow from above and beneath the car to rejoin more smoothley, reducing the low pressure wake, reducing drag, and producing downforce by accelerating the airflow along the body of the vehicle (air's viscosity will help pull the diffuser toward the air under the car.).

Thanks for the replies, but I think you've missed the point a bit, I wasn't after an aerodynamics lesson (my final year project at university was on aero) I was just after peoples opinions of whether they see it is a good way to save fuel.

I've also thought since, on the plus side, there is no additional mass to haul around!

[PKLD]

Actually.... Why aren't there retro-fit flat floor fitters? In this day and age with the pressure to reduce fuel surely that could be a money spinner

(goes off to patent the idea before you bds steal it )

[Ian974]

deveng said:

Well the flat floor does both, reducing drag and increasing downforce. Rear diffuser will do the same, allowing airflow from above and beneath the car to rejoin more smoothley, reducing the low pressure wake, reducing drag, and producing downforce by accelerating the airflow along the body of the vehicle (air's viscosity will help pull the diffuser toward the air under the car.).

Thanks for the replies, but I think you've missed the point a bit, I wasn't after an aerodynamics lesson (my final year project at university was on aero) I was just after peoples opinions of whether they see it is a good way to save fuel.

I've also thought since, on the plus side, there is no additional mass to haul around!

I think it's the idea of changing suspension to adjust the aero of the car which is sidetracking things. (plus I wasn't sure if a diffuser actually reduces overall drag, or is just lower drag than a wing)
It may help to a degree, but with most of these things, you'd probably be best doing a heap of things at once (stating the obvious eh!). Stripping out a car, lowering it on narrower, higher pressure tyres with a flat undertray would probably make a noticable difference.
Chop the roof too and go for an eco hot rod!

[deveng]

I like the idea of a flat floor, but honestly wouldn't know how to go about it, as in what to use and how to keep it attached to the car, and thats all without adding too much weight to the car and reducing around town fuel consumption for the sake of long trip fuel consumption.

[PKLD]

deveng said:

I like the idea of a flat floor, but honestly wouldn't know how to go about it, as in what to use and how to keep it attached to the car, and thats all without adding too much weight to the car and reducing around town fuel consumption for the sake of long trip fuel consumption.

You can be my first customer for my flat flooring fitting fully flush fuel reducing company then

[TheEnd]

Flat floors can make parts overheat, gearboxes and diffs were planned to have cooling air around them.

I'm not sure how big the difference is, but it is a consideration.

[kambites]

My car already has a flat floor, but bugger knows if it makes any practicable difference to anything.

[PKLD]

TheEnd said:

Flat floors can make parts overheat, gearboxes and diffs were planned to have cooling air around them.

I'm not sure how big the difference is, but it is a consideration.

Mere minor details! Remember most of my customers don't think straight! Need a more efficient car, I know, I'll sell my perfectly serviceable car and spend/finance a brand new supermini that does 700mpg in tests but only does 10mpg better in the real world than the 5 year old fiesta it is replacing!?! And they'll only be covering 3000 miles a year so they'll get a diesel...

With that logic in mind I'm going to sell and fit a flat floor to you for £800 - in secret strictly controlled tests it'll show a 5mpg improvement so it'll only take 10 years to pay for itself.

I'm onto a winner here