Is it worth chasing downforce in a human powered vehicle?

[coln72]

G'day all,
I'm running a school team that compete in a 24 hour race for human powered vehicles. We average around 30km/h for the 24hrs so I was wondering if it would be worth redesigning our vehicle to try to generate some downforce. Theory is that if we can corner faster then less effort would be required to accelerate on the straights.

Was looking at some form of 'vortex tunnels (?)' if I can incorperate it into the design.

Understandabley I need to keep drag to a minimum.

Cheers
Colin

[Notso Swift]

Personal opinion
No
At that speed the wings would need to be pretty extreme to have any benefit.

Focus on drag, and weight to ease accel.
Q how much do you need to slow down? If you have more thab 2 wheels try and maintain speed by geometry, bit of camber goes a long way. Don't over pressurise the tyres, I think you need about 12% drop to get maximum grip on a clincher type tyre. (Little more resistance though)

[coln72]

Its a three wheeler and this year I am going to enclose the front wheels to try to reduce drag. Camber is about right as tyre wear is reasonably flat.

Unfortunately there isn't a lot I can do to reduce weight that we can afford. The vehicle has a CrMo and aluminium frame with a coreflute body. Weight is 27kg. It is only a kg or 2 heavier than some of the carbon fibre bikes. Other school teams have tried to build light bikes but they break during the event so there is no advantage as they loose more time fixing them than what they gain on track.


This is why Im looking at another area to gain an advantage.

[ensign14]

Smoke bombs.

[JohnD]

Colin,
I think it all depends on the fact that drag and lift vary as powers of velocity.
As an example, in his book on "Downforce" Simon McBeath uses figures from a 1996 McClaren F1 car of 1820kgs of downforce at 200mph. If you could achieve the same Cl at the 15mph your vehicle averages, then you could achieve 1820x15^2/200^2 = 10kgs of downforce. But as you will corner at less than your average speed, say, 7mph?, then 2kgs where it matters. Where it doesn't matter, on the straight where you might achieve 40mph, the downforce could be 70kgs.
So is that worth the drag penalty? That will be in top speed, and a cyclist's Cd is about 1 which can be reduced to a half (?) by enclosure, but must get worse if you add aero for downforce. A good production car Cd is 0.3, and an F1 is about 0.75, so assume a doubling for best downforce from a least drag.

McBeath quotes for top speed, (BHPx1225)/(CdxFrontal area)=Velocity cubed [metric]. So if your vehicle has minimal drag now and the Cd doubles, then V^3 halves. If you top speed now is about 40mph, then it will fall to about 30mph. You need to look at the course you run to determine if that is worth while.

Hope that helps!
The McBeath book is worthwhile, if you haven't read it.

John

[MagnetON]

It's been scientifically proven (by me saying so) that having tentenths.com stickers makes any vehicle more aerodynamic and increases style. Stickers are available from.....

[1200Datto27]

I'd definatly give the wings a miss, the minimal cornering benefits would be outweighed by the gains in weight/drag. Spend some time going over things like the wheel bearings, chain drive brakes, etc. and reduce as much drag as is possible. You may have to break a few things in testing, but try a lighter weight grease or even oil in the bearings, if you can reduce the wheel drag by 10% then that is a massive gain in acceleration potential, as well as reducing the strain on your riders.

Off topic, Magneton, the Attack Hamster rocks.

[avsfan733]

your going to end up with more problems with drag than a benefit from the downforce. liko datto said lighter oil and other thigns to reduce rollling. Make sure your suspension geometry is all setup to allow you to corner as fast as possible from a mechanical standpoint

[Notso Swift]

Originally Posted by JohnD

and a cyclist's Cd is about 1

I thought a CD of 1 was a solid barrier that went for infinity in both the x and y axis? IE something you can't get around
The scale is from 0: no resistance, to 1: infinite resistance

[Seoigh]

don't do it!...downforce is unnecessary at under 60kph (and if you DID achieve such speeds, would only present more weight to overcome). Back when I cycled seriously, I used to spend $100 extra to save 20 grams of weight on a derailleur....can't imagine adding weight and expense to a peddle-powered vehicle.

Aerodynamics are vital, but you want to slip through the air...not stick to the ground

[JohnD]

Notso Swift,
Drag is proportional to half the air density(or rho) times frontal area(A) times velocity squared.

So you can write an equation: Drag = 1/2(rho x A x V^2) x a constant.
Or more usually Constant = Drag/ 1/2(rho x A x V^2)

This is the drag constant, or 'coefficient of drag'
For an object with infinite drag, no air flow could occur, so V will be zero and so will V squared. Divide anything by zero and the answer is infinity, so the Cd will be infinite. Except we have said that drag is infinite, so the Cd is 1. That's why infinities in an equation are a problem - they lead to other infinities, no useful result and madness. Don't go there!

A flat plate at right angles to the air flow has a Cd of about 1.2
A long teardrop has a Cd of 0.04

A cyclist has a lot of drag, with a high Cd, not because he resembles a flat plate, but because of all the turbulence that his complex shape and moving parts exert. That's why velodrome cyclists resort to funny ways of sitting on the bike, even funnier hats, very expensive cycle frames with tubes that are flattened in the direction of flow and wheels that are discs, not spoked.

As I said above, a well designed production car can approach a Cd of 0.3
An F1 car, however swoopy it's appearance, will have a much larger Cd, like 0.8 because like the cyclist, the air is pushed about by all that downforce aero on the car. The penalty for downforce is drag.

John

[Mad-Dog-mk2]

Downforce for a HPV / Pedalcar really is a waste of time, cutting down on drag and general aero efficiency however isn't.
Things like fairing the wheels - discs and a fairing structure infront and around the wheels,
reducing rolling resistance - using the narrowest tires you can find and running at a high pressure (we run at just over 100psi),
getting good airflow around the car - no openings etc,
reducing the front cross sectional area as much as possible,
Are all what you should be aiming for.

Two websites that might be good to look at for ideas etc are:
www.bhpc.org.uk
www.pedalcars.info

HPV's being the most aero efficient vehicles, although they won't think too much of a 3 wheeled design since it has more rolling resistance then 2 wheels.
And Pedalcars being the sort of racing that you'd be doing, although in the UK (France, Italy, Hong Kong, Holland) you have to have a 4 wheeled car.

[TEAM78]

its best to do some simulation and experimental work rather than sourcing opinions

[coln72]

Wasn't thinking about wings as such, too easily damaged when (not if) the vehicle is rolled (some dirty tactics occured at the end of last years race.....)

Have to run a three wheeler due to the regs. Already running narrow tyres at high pressure. Can not run without some sort of ventilation into the vehicle as it is normally 30+ C during the day.

Was thinking about tunnels like they run on indy cars (were banned from F1) as vey little extra material would be required to make them. I agree about creating too much drag but we are at the point where we cannot reduce the CSA of the vehicle any further, again due the the regs (hell we are right on the ragged edge of being legal ) and I need something for the team to bull**** about at the presentations we are required to do.

For the event see

http://www.racvenergybreakthrough.net/Default.asp

For some crappy photos of our bike



(ours is the blue one, it is narrower than it looks in this photo - 630mm wide)

[JohnD]

Coiln,
What a great contest! Lead-on to Formula SAE?

How about a splitter. Your vehicle has a nose that may encourage air to pass underneath, though nothing like as much as some others that look as if they are sponsored by Outspan oranges. You also have a flat undertray as far back as the wheels, so if you added a flat plate to that, projecting as far forward as the nose and dropped a flat surface from the nose down to the plate, you would have:
reduced air flow beneath the vehicle (not downforce, I think, but less turbulence)
A pressure source from the null zone in front for as much ventilation as you wish/feel wise. Duct drag is very real.
And are you not allowed to enclose the wheels?

But please tell me - how does you driver see out? His legs have a wonderful view of the road, but his head is enclosed!

John

[coln72]

The driver can see, just..... They can actually see above their knees.

The nose is one area we altered on the current bike. Previously it was alot higher. We can enclose the wheels - which I am currently doing if I can get out of the classroom long enough to fire up the welder.

Good idea about trying a front spitter. Simple, light and if (when!) it is damaged just rip it off. Looks like some testing coming up

[kevinthenerd]

I was once playing with a bicycle I got out of the garbage (a typical road bike / "ten speed" bike with the skinny tires), and I had a strange idea that worked very well for me. After fixing it up, I wanted to cut out rolling resistance, so I filled the tires with mostly water with a little bit of air left (to properly adjust the pressure). Acceleration wasn't all that great, but once I got going, the lack of rolling resistance allowed me to achieve 30mph (48 kph) averaged over a mile distance, and I'm certainly not athletic by any standard.

One thing that helped the bike perform well before filling the tires, in my opinion, was the fact that I insisted on synthetic lube for the bearings and chain. For the tiny amount you'll use of it, it's worth the advantage if you're using exotic materials elsewhere.

If you want cornering, go with a stickier tire compound. I wouldn't even recommend putting less air in the tires for handling since it'll hurt you so much in top speed, but your results may vary. (I would recommend an experiment to discover your ideal pressure.) The reduction in straight-line performance due to downforce will not be worth your trouble. Plus, keep in mind that downforce in a two-wheel setup may actually HURT you when the bike leans. (Part of the force will be pushing you OUT of the turn.)

[JohnD]

Kevin,
Water filled tyres is a very interesting idea! How did they affect your cornering? The additional weight at the rim of the wheel would surely have increased the gyroscopic effect, so that turning the handlbars required more effort? Possibly none at speed, as you lean more than turn then, but manoevering? And a speed wobble would be unrecoverable, I fear.

I have a bike with solid tyres. These are the bike eqivalent of run-flats, in that they are puncture proof. They are thin, race-type tyres, not chunky off-roaders. They are little heavier than conventional tyres, and made no difference to cornering, or low speed manoevering.

John