h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

i was driving behind a car the other day which appeared to have some kind of wing on the roof. not like a rear spoiler, but an actual wing, even tho it was square.

now i can't say that's what it was, it looked home made, but it got me wondering.

if you had a wing providing lift on a car, would this reduce the weight bearing on the wheels and thus reduce the energy required to move it?

Unlikely as this upward lift is driven by forward motion and therefore originates from the engine.

To go to the extreme of your scenario --> what is an airoplane other than this? They use lots of fuel don't they?

Also, it reduces grip and would probably be dangerous as you would have an interesting time trying to go around corners with uplift.

gliders don't use fuel.

"if you had a wing providing lift on a car, would this reduce the weight bearing on the wheels and thus reduce the energy required to move it?"

I think I may have missed the point here, but why should less weight on the wheels make a difference? The mass that's being accelerated remains the same, so the car still has to push backward on the ground to go forward.

Perhaps on a hill it might make a difference, but surely less than the additional drag.

gliders fall and hit the ground.

And yes, they do use fuel. How do they get up in the air in the first place?

Racing cars have wings that do the opposite. They press the car downwards, increasing grip between the tyres and the road. That's probably what this homemade one was for too.

I had this exact debate with a chap at uni. The answer is no - the mass of the car remains the same; weight is irrelevant. It would take as much fuel to accelerate the car on the moon (allowing for air resistance and friction of course).

gnomon. it definitely wasn't a performance enhancement. it was an older car driven by an even older chap.

dave. *chuckle* yes, the mass doesn't change. nor does newton's first law. but that's cool. my wife thinks i'm an idiot too.

my question relates to whether the reduction in downward forces can attain an advantage over friction greater than the energy cost taken from forward momentum, at least in theory. this is certainly the case with hydrofoils.

A hard wheel does not lose any energy by friction between the wheel and the road surface, because the wheel is not moving relative to the road at the point where it touches. The friction all happens between the wheel and car, that is, at the axle.

Soft tyres are slightly different, in that energy is used to deform the tyre from a circle into a circle with a flat bit. Try cycling a mountain bike with soft tyres to see what I mean. A huge amount of energy is needed just to keep the bike going.

When a car is travelling at a constant speed, the only things that are using up energy are friction with the air and this energy used to deform and heat up the tyres.

thank you for that explanation gnomon. so the downforce exerted by a spoiler is expressed where exactly? my understanding is that f1 tyres will not create enough friction with the road surface to move the vehicle forward without spoilers. what am i missing here?

if the downforce created by a spoiler increases friction only between the wheel and the car...how does that improve traction? and how is traction not friction?

indulge me.

AG - apologies if I misunderstood the question

Point of pedantry - spoilers and wings do very different things. A spoiler is so called because it disrupts the airlow at the back of the car, causing turbulence and reducing drag. A wing produces downforce.

>> my understanding is that f1 tyres will not create enough friction with the road surface to move the vehicle forward without spoilers <<

That can't be right, or they'd never get off the starting line. I can believe they'd suffer from excessive wheelspin at speed without the wings though. Downforce increases the pressure between tyre and road which increases friction. I guess it also causes the tyre to deform more, giving a greater surface area, giving greater traction, which is probably why F1 tyres are inflated to very low pressures.

the trick with wings for downforce is that they provided additional downforce *at speed*. At low speed they don't really do much, other than perhaps provide a bit of extra weight, although this is minimal.

As the car goes faster so the effect of the wing is enhanced. This helps because at higher speed the forces on the car, and therefore theforces that the tyres are fighting against, are higher. More downforce means more traction at high speeds which means greater high speed stability and higher cornering speeds.

Also worth remembering that a car 'wing' is upside down compared to an aeroplane wing since the desired force is in the other direction.

You may also have heard of ground effect, whereby the underneath of the car is modified such that, again at speed, it sucks the car down onto the road. Effectively (as far as my limited understanding goes) this has the effect of turning the entire car into a wing. So good they banned it in F1! But now seen on lots of top end and some other sports cars on the road (if you look under a ferrari with it you'll see spaced vertical 'runners' along the length which create channels for the air which causes the effect).

thx dave. i meant, as you suggest, that they won't move forward without excessive wheel spin and that the wing needs to be moving thru the air at a sufficient speed to create the needed downforce.

as an aside, isn't it a wing when it produces uplift, hence the use of the term spoiler when it is the reverse?

i am only enquiring about downforce in order to establish whether uplift reduces friction. from what i have read, and i don't pretend to understand it, is that subject to the configuration of the vehicle etc, shape of the wing, there is an equilibrium point where the uplift reduces road friction and will yield efficiencies. i understand also that the point where aerodynamic forces exceed road friction is typically just above 40 mph. if however the wing were computer controlled and subject to real time shape and position modification, this might cover a range of highway driving conditions.

"isn't it a wing when it produces uplift, hence the use of the term spoiler when it is the reverse?"
Yep, supposedly, although as usage defines meaning for English I suppose it is as correct to call it a wing or a spoiler when on the back of a car.

As for less friction = more economy - that was the premise behind some make of tyre not so long ago. They claimed that the tyre had less friction and thus increased fuel economy by 5% (I think it was) - although over what sort of route this would be I am not sure.

Not seen them for a while. Whether it was just cobblers to sell the tyre or whether people thought 'actually, I'd like my tyres to keep me on the road' I'm not sure. But I guess there is a fair bit of difference between what the average (ie not cheapo bakelite type) road tyre can withstand and what the average user needs it to withstand. Depends on the car and driving conditions, atmospheric conditions; even driving style.

Also the rear wing, to provide lift and reduce friction, would be great if you are rear wheel drive, but if front wheel drive you'd end up with more friction due to the weight bearing down more on the front than it would under normal conditions (if thinking about the car/wing as a type 2 lever.

the 'wing' i saw was on top of the car. now that i think about it, it was probably to improve air flow over the car when towing a caravan or horse float. the consensus seems to be that a wing on a car would likely give little lift and if it did, the loss of stability would be unacceptable.

what about wings on trains? or am i pushing this too far....

"it was probably to improve air flow over the car when towing a caravan "
Ah, yes, think I've seen those. Like the fiberglass tops you get on HGV cabs to smooth airflow to the trailer type thing?

You haven't addressed the crucial problem: how does it look? Would someone want one on their nippy little French hatchback?

icotan. yep. same as with truck cabs. tho not nearly as stylish.


which leads me to

fluffy. i think you'd look very good with one.

>> as an aside, isn't it a wing when it produces uplift, hence the use of the term spoiler when it is the reverse?<<

No: a wing is a wing - something that produces a force perpendicular to direction of travel. A spoiler is something else entirely, as I said. Well technically anyway. Your average boy racer probably couldn't tell the difference.

>> what about wings on trains? or am i pushing this too far.... <<

Yes, I think so. Train wheels don't have a lot of friction to begin with, on account of being solid.

dave. i shall remember to call f1 wings, wings in future.

can't agree with the 'perpendicular to direction of travel' tho. it is inaccurate when describing sails and propellors. sailboats can sail into the wind and propellors can have variable pitch.

the force is applied to the reverse surface of the wing regardless of which direction the wing is travelling, provided the leading edge of the wing has sufficient air moving across it and is able to produce a difference in pressure between its upper and lower surfaces.

thus the force is applied at right angles to the leading edge of the wing. this is usually the same thing as perpendicular to direction of travel, but not always.