6 replies to this topic

[NRoshier]

Formula Libre hillclimb cars have notable aero freedom, but really are optimised for low speed. One of the freedoms is that they can mount their wings much further back than say a F3 car and obviously the leverage considerably assists the amount of downforce at the contact patch. However it has been suggested to me that this leverage will mean that the aero balance will be 'upset' at higher speeds (one sprint in my area is at the Phillip Island GP circuit where the Gould GR55b was on 200-225kph in the final sweeper - the Gould is not the car in question but is simply used as a recognisable indicator).
It seems to me that this is more a factor of experience and ability to engineer the car to suit the track, rather than a specific leverage effect...can someone provide insight into aero balance/leverage effects?

[murpia]

Originally posted by NRoshier
One of the freedoms is that they can mount their wings much further back than say a F3 car and obviously the leverage considerably assists the amount of downforce at the contact patch. However it has been suggested to me that this leverage will mean that the aero balance will be 'upset' at higher speeds (one sprint in my area is at the Phillip Island GP circuit where the Gould GR55b was on 200-225kph in the final sweeper - the Gould is not the car in question but is simply used as a recognisable indicator).

Total downforce won't be influenced by increasing rear wing leverage, unless the new wing position offers better flow to the wing, or influences diffuser flow in a beneficial manner.

Instead, the effect of increased leverage is to shift the aero balance more rearward. I would not expect this effect to 'upset' the aero balance at higher speeds, unless some other flow effect is at work such as ride-height sensitivity.

One way to take advantage is to increase front flap angle to compensate for increased rear wing leverage, thus increasing total downforce for the same aero balance as the no-leverage case. However that pre-supposes that increasing front flap angle has no detrimental effects on rear downforce, something that sadly isn't the case for most cars.

I recently did some work in this area, referenced somewhat here and also published in the November 2008 RaceTech magazine. To cut a long story short, most results were not 'obvious' in as much as we rarely confirmed our 'intuitive' predictions but instead found many solutions that either did not work or did something unexpected. The latest round of testing has shown up a clear aero balance shift with increasing speed, which we hope to address.

So, there's no substitute for hard data. Do you have datalogging on the car in question?

Regards, Ian

[NRoshier]

Thank you for sharing the link. I'm not trying to be obtuse, but I would like to clarify your first sentence if I may - just to be sure I understand you correctly. If we have a wing in free stream directly above the axle line giving 100kg of downforce at 200kph, then we move it 1000mm to the rear of the axle line, the contact patch will not see any additional force via the 1000mm lever?
Unfortunately the car in question does not have any datalogging at all...it is a very unusual vehicle that defies normal description!
How ironic...I was in the first discussion...it's late, I'm tired and I just damn well forgot!

[murpia]

Originally posted by NRoshier
I would like to clarify your first sentence if I may - just to be sure I understand you correctly. If we have a wing in free stream directly above the axle line giving 100kg of downforce at 200kph, then we move it 1000mm to the rear of the axle line, the contact patch will not see any additional force via the 1000mm lever?

I think either you or I have misunderstood:

By total townforce I mean the sum of front and rear downforce (at the contact patches). This is not affected by leverage (all else being equal).

If you increase rear wing leverage you increase rear downforce and decrease front downforce by an equal amount. So, it's a rearward aero balance shift.

I.e. adding rear wing leverage can't increase total downforce (which is how I interpreted your initial statement of 'the leverage considerably assists the amount of downforce at the contact patch'). Did you just mean the rear contact patch here?

If by 'the aero balance will be 'upset' at higher speeds' you just mean the aero balance will shift rearward at all speeds, affecting the high speed balance of the car, then you could be right. The distinction here is that a constant aero balance does not necessarily mean a constant car balance through the speed range. A car with too much front wing might be fine at low speeds and have too much oversteer at high speeds.

If you have already found the correct aero balance for your car, then shifting it rearward through increased leverage is probably not a good idea. Unless, as I said before, you can add front aero to get back to your ideal balance.

Hope that helps. In any case a simple analysis using moments about either axle will tell you all you need to know.

Regards, Ian

[Fat Boy]

Originally posted by murpia

If you have already found the correct aero balance for your car, then shifting it rearward through increased leverage is probably not a good idea. Unless, as I said before, you can add front aero to get back to your ideal balance.

This is true, but often, front downforce is the easiest to create. You've got a wing in relatively clean air with all sorts of ground effect help. It's often as simple as cranking in angle. Open wheelers almost always have an excess of front wing (available) and GT cars almost always have a lack of it.

[NRoshier]

Yes, I can see how I have mixed up my terminology and not given enough information. I did mean downforce at the rear contact patch only re leverage. Thinking through some of the permutations I am just getting an idea of the complexities of the balance issue. the forces are not likely to be linear and rear wing eff'cy would also be dependant on levels of aero masking at different speeds as well plus a few other things...

[murpia]

Originally posted by Fat Boy
...often, front downforce is the easiest to create. You've got a wing in relatively clean air with all sorts of ground effect help. It's often as simple as cranking in angle. Open wheelers almost always have an excess of front wing (available) and GT cars almost always have a lack of it.

I agree, front downforce is relatively easy to produce and it's usually possible in an open wheel racecar to generate more than enough front downforce to go past the correct aero balance to too much front.

However, front downforce does not usually add to total downforce 1:1. Beyond a certain front flap angle, or front flap spanwise trim, or when fitting 'dive planes' etc. the corresponding downforce loss at the rear is usually so significant for overall grip that other chassis balancing methods are more appropriate. This is especially relevent in situations where rear tyre degradation through a stint is pushing you to move weight forward and balancing it with front aero.

The reasons for this are as complex as the airflow over the car, in some cases it's upwash from the front wing affecting the rear wing airflow, or it could be the effect of less underfloor flow through the diffuser. Whatever the specifics of an individual car, it's a real effect and a real challenge to get hard data on.

Often a customer racecar will be provided with a graph of aero balance vs. front wing angle (for a given rear wing configuration) and another graph of total downforce vs. rear wing angle (for a given aero balance). What you also need is to plot total downforce vs. front wing angle (for a given rear wing angle), and you should see the effect at work.

If I recall correctly, this effect has shown up in some of the data presented in the 'aerobytes' articles in Racecar engineering.

Regards, Ian