I've decided to modify my performance calculator to include downforce. I don't know how to.

I have collected data for a couple of cars where manufacturers have posted it up. For example:
"Our car produces 100kgs of downforce at 186mph"

What I would like to do is calculate how much downforce that means at all other speeds. Any ideas?

You need to find some data that shows downforce at different speeds - you can then reverse-calculate to work out how downforce increases with speed

I'm going to guess that it correlates with drag (ie it squares with speed). However, I know that some cars (especially racing cars) can create downforce without creating drag (eg by flat floors), so I doubt its as simple as it looks

This is surprisingly hard to find. The only car I ave found this for is the Koenigsegg One: 1 and the numbers... don't make sense... unless there is an element of diminishing returns with downforce?

610kgs @ 260kph
880kgs @ 440kph




I've got drag and frontal area sorted using CdA correctly, but I didn't want to get caught up mixing it with downforce. For the purpose of straight line acceleration, I'm going to treat it as an increase in the cars overall weight as it gets faster. Ideally, I want to predict downforce at any speed based off the one given force X at speed X.

Predicting performance of really fast cars is difficult because a lot of them produce downforce. Past 120mph they come out too fast...

OK - so if you plot enough cars, you should be able to generate a delta of your calculator's error against the known performance stats - this should give you a starting point of the effects of downforce

Personally, I think you're opening a can of worms, as some cars (Coupe included, IIRC) have negative downforce at speed

You can't change your calculator to accommodate extra (or less) weight, as this isn't how downforce works. The actual weight of the car isn't changing, its only the rolling resistance, due the downforce.

If you try to fudge the calc by adding weight, you're going to screw the rate of acceleration.

Similarly, if you try to adapt drag, you're going down the wrong route, because drag doesn't increase with downforce (in fact potentially, drag could decrease with downforce, as the air under the car speeds up as the ground clearance decreases)

I think the answer is to incorporate the ability to include rolling resistance from tyres, which will increase as the contact patch area increases due to downforce - does the calculator have the ability to use different tyre pressures, or does it always assume a constant?

To be honest, I doubt it... lift (which is what downforce is, only in the other direction) is proportional to the square of speed but is sensitive to wind direction, rain, air density, parasitic drag, form drag, and no doubt a handful of other things. Its position also moves around on a wing chord depending on speed and angle of attack.

On top of that, the same wing on two different cars will produce different amounts of downforce depending how and where it's fixed and the shape of the vehicle you're attaching it to.

And the car is generally a *lifting* body to some extent, too...

This is why car manufacturers spend time on supercomputers and build models in wind-tunnels - Mr Stokes and Mr Navier did not leave equations behind which were in any way easy to solve.

As a first and very rough approximation: you might try increasing the weight of the car over the rear axle scaled by the square root of the speed at which the downforce is specified.

Yes, but increasing the weight of the car (over either or both axles) will screw the calculations by affecting the rate of acceleration - the car isn't actually getting heavier, but the tyres (and suspension, obviously) are effected as though it was

The issue I have with this approach, is that cars that actually produce positive downforce (at least where the manufacturer has published it) are so fast that road test magazines always don't measure performance at he top end. Stats up to 150mph for a car that gets there in 20 seconds but then does another 40-50mph don't really show the full picture. This is why I want to calculate downforce - it's this final part of the story I want to know. It they published the numbers I'd need to work it out, I wouldn't need to work it out!



Yeah, well aware of that but, my thinking is that once I have the downforce thing worked out, I should be able to use data for cars I have 0-top speed data for to work out how much lift they produce!



This is one of those rules that I struggle with. If a force is pushing a car down, then it must be harder to accelerate.

If I had to choose between two cars that were identical in every respect, save for downforce (i.e. drag was the same too - a more efficient system was in place), I'd have my money on the car producing less. If the answer is that it is faster, then where else but weight would this go in the calculator?



The thing is, I've applied the rule to two cars so far. It's terrible and unrealistic I know, but I quickly chucked in a fixed rate of increase e.g. 100kgs @ 186mph = 50 @ 93mph

However for the two cars, it corrected the performance data almost perfectly, both cars by 2 seconds to the speeds below.

Giulia Quadrifoglio
20s to 150mph vs actual 19.9
190.2mph top speed rather than 190.8 (some would call me pedantic...)

Koenigess Regera
20.5s to 249mph actual 20.9s

CD won't change much really though - if the springs moved that much from 450kgs at 250mph (spread over four wheels) imagine how much it would dive with four times as much weight on the front axle under hard braking!!



It doesn't include rolling resistance. Perhaps this has been offset by the natural lift of most cars so I've gotten away it to date?

And no, it doesn't factor in tyre pressure either. There are a few variables that I chose to keep out. It's more about road cars than anything else. Having read up on why a drag car can pull of the line faster than a 4WD car, I made the decision that that part can FRO! Apparently, it allows the wheel to essentially push backwards against what may as well be a wall. Even my obsession doesn't go that far

Correct, but only because there is more rolling resistance and not because the car actually weighs any more (which obviously can't happen, as nothing is adding weight to it while it is moving)

The true weight of the car HAS to be static and therefore the effect of weight on the acceleration will be constant. If the car was actually gaining weight during acceleration, it would become more difficult to accelerate.

A heavier object takes more energy to accelerate than a lighter object, given the same force. In the case of a car's acceleration, the force (engine power) is the same, so an increase in weight means the car will accelerate less quickly.

As downforce broadly correlates with drag, I think you may be able to incorporate a variable drag value to account for the effect of downforce (ie allow Cd to increase slightly as speed increases)

There is much scope for confusion.

There is no physical weight change, but there is a *significant* drag change; you *can't* get lift (in either direction) without drag. And the more lift, the more drag.

And it's that drag that needs modelling. I offer it merely as a first-level approximation since the drag isn't published, but the lift is and it tracks the drag.

Consider an F1 car with the flap open: it's suddenly 20 or 30kph faster, not because there is less weight on the back wheels but because there's suddenly less drag. They're not equal, but they're equivalent.