28 replies to this topic

[Froilan_G]

With that Jordan-chief-designer-thing going on, I have read in an article that Eghbal Hamidy designed a Jordan last year for the Hungarian GP (the EJ11B) which was very pitch sensitive. It was so bad that Alesi reverted to the old car for the race, which proved to be much better.

Now what exactly is pitch sensitivity? I know it’s aero-related, and it has to do with the underbody of the car… I even know it was a big problem before the stepped-bottom-rule, but what exactly does it do with the car?

[Marco94]

You've guessed right. Pitch sensitivity is a measure for the changes in downforce resulting from a change in pitch angle. In particular the change in the front/rear distribution of the downforce. This distribution is directly related to the cornering behaviour of a vehicle. Under- and oversteer behaviour.

[Yelnats]

Pitch sensitivity in a F1 car is more related to the effects that pitch changes have on ride height that the actual pitch changes of the car to the approaching airstream. This is because of the massive influences that small ride height changes have on the rear diffuser anong with the proximity (boundary) effect of the front wing riding near to the road surface. Pitch sensitivity was one of the reasons that the F!A raised the front wing height last year, the other being to reduce downforce overall. The theory being that the loss of front downforce had to be balanced by a designed in reductuction of rear downforce to produce a balanced car.

[Jezztor]

Yelnats is right. Pitch sensitivity is also partially determined by ride stiffness / damper travel. You're correct in saying that it is related to the area under the car, with different angles of the undertray under braking, acceleration, cornering, rather than above the car, with the conventional fr & rear wings. It marginally has to do with the area underneath the wing, but not really the wing in correlation to the airflow, as such.

jezz

[carlos.maza]

This is why modern F1 cars "must" have an almost constant ride heigh, resulting in a very very short suspension travel and a very stiff suspension. This is also the problem that drived Colin Chapman crazy with the Lotus 80.

Correct me if I'm wrong. Since F1 cars have a flat underbody, they should all be by nature very Pitch sensitive. Right?

Or, can the rear diffuser design alone compensate for this sensitivity?
Does anybody know where (aprox) is the Center of pressure of the underbody? Does the resultant downforce pass through the CG of the car?
I assume the relationship between this point and CG is relevant. Given that the longitudinal position of the CG can be altered moving ballast, can the C of P be changed replacing the rear diffuser? Do F1 teams do this?


Carlos

[Marco94]

Whoops, thinking to quick and typing to slow. I did indeed mean the effect that the pitch angle has on the front and rear ride height.

The centre of pressure does in general not have the same position as the centre of mass. This cases problems in setting up the car. The downforce distribution between front and rear will depend on the speed of travel.

[RDV]

Carlos.Maza Re pitch sensitivity, here is some current data on flat bottom single seater ( not F1..) but very similar I tried sending graph , but failed miserably...if you have the patience here is data you can plug into spreadsheet and do your own aeromap....

Units
Model Scale 40 % ModelFrontalArea 0.2200 m*m
Reference Speed 150 MPH Reference Barometer 1013.25 mbar
Reference Temp 20.0 Deg C Scale Factor From Coefficients to FS xxxlbs.




Ride Heights Totals
Run Ride Map Front Rear Total Lift Front Lift Rear Lift
627 10.000 10.000 1411 559 853
627 5.000 5.000 1419 582 837
627 5.000 10.000 1469 611 858
627 5.000 15.000 1506 636 870
627 5.000 20.000 1539 662 878
627 5.000 25.000 1571 687 883
627 5.000 30.000 1593 711 882
627 10.000 5.000 1358 530 828
627 10.000 10.000 1411 556 855
627 10.000 15.000 1443 580 864
627 10.000 20.000 1472 602 870
627 10.000 25.000 1499 625 875
627 10.000 30.000 1521 646 875
627 10.000 35.000 1539 668 871
627 15.000 5.000 1284 479 806
627 15.000 10.000 1352 505 847
627 15.000 15.000 1382 526 856
627 15.000 20.000 1413 550 863
627 15.000 25.000 1444 574 870
627 10.000 10.000 1402 554 848

Yes , very pitch sensitive, and yes rear difuser can correct, but is never linear, CP generaly @36 to 38% ft , as for CP/CG match , only at one position , if you are lucky or work very hard....

on looking at preview also found not very clear , if anyone wants zipped copy of excel file mail me separately...... replies might be long in coming depending on my workload, am already skyving and should really be preparing next race......

[palmas]

Originally posted by Jezztor
Yelnats is right. Pitch sensitivity is also partially determined by ride stiffness / damper travel. You're correct in saying that it is related to the area under the car, with different angles of the undertray under braking, acceleration, cornering, rather than above the car, with the conventional fr & rear wings. It marginally has to do with the area underneath the wing, but not really the wing in correlation to the airflow, as such.

jezz

well, I thought pitch sensitivity was only determined by aero. The pitch (pitch variation) itself is determined mostly by the suspention. But I could be wrong!

[Jezztor]

Pitch sensitivity was one of the reasons that the F!A raised the front wing height last year, the other being to reduce downforce overall. The theory being that the loss of front downforce had to be balanced by a designed in reductuction of rear downforce to produce a balanced car.

Yelnats is right.

I was referring to that.

Jezz

[paulb]

Originally posted by RDV
Carlos.Maza Re pitch sensitivity, here is some current data on flat bottom single seater ( not F1..) but very similar I tried sending graph , but failed miserably...if you have the patience here is data you can plug into spreadsheet and do your own aeromap....

I just finished a plot, should the curves look like sawtooths?

You can post the graph by converting it to an image file, posting on the web, then linking to that URL from this forum. I can post mine if there is interest.

[Jezztor]

Go ahead, please..!

Jezz

[Melbourne Park]

I would like to introduce minor speed bumps to race track corners; having a number of corners with raised sections in the corners would result in less downforce and more suspension travel. Less downforce would allow more overtaking, which is good for racing spectators. More suspension travel would be good for mechanical grip, which is relevant to road cars. It would also be good for drivers' backs.

[desmo]

Didn't Brasil have those last year?

[Jezztor]

apparently so, desmo!

[silver]

Also 1999 McLaren was supposed to be very pitch sensitive. It was very fast car but extremely unforgiving and therefore difficult to drive

[MclarenF1]

Does anyone know if the Brabham F1 car in the early 80's (82?) without a front wing was as pitch sensitive? Would the ground effects um effect the pitch sensitivity of the car? And as a side-note in a Motorsport magazine article from last winter on the turbo era cars, Gordon Murray spoke about how the center of pressure had moved so far forward (and so the CG) that when ground effects were banned most teams did not compensate for this change in the center of pressure. (and the resulting loss of traction at the rear) So when the monster BMW turbo engine was used in 83 he moved the CG as far to the rear to give as much traction as possible. While most teams used the same weight distribution as before.

[carlos.maza]

RDV:
Thank you for the data. Very interesting...

MclarenF1:
AFAIK all the wing cars were "very" pitch sensitive, because it is impossible to have a fixed CP of the downforce generated by the venturis with the car running at variable pitch angles. Maybe even the magnitude of the downforce could vary due to the pitch angle (obviously it changes with air velocity). Well, this is true for ALL cars.

For most of the wing cars it was possible to work without front wings because they had to move weight forward, to compensate for the high downforce generated by the venturi tunnels. This "new" downforce moved forward the CP and made it neccessary to move forward the CG because it is known that in order to have a stable car, the CP must be a given length behind the CG.

When venturi tunnels were banned, suddenly the CP went backwards, due to the use of massive rear wings, so it was neccesary to bring the CG backwards too, to keep the same CP - CG relationship. This is why Brabham put so many weight in rear (65-35% ???)

All teams moved the CG bacwards, but maybe not as radical as Brabham. Most of them had short sidepods and very backward-mounted lateral radiators. Also the driving position was changed.



Another question:
In the 60's and 70's it was very exciting to see the cars "bury the nose in the ground" under braking and seeing the anti-roll bars working under cornering. Weren't these cars pitch sensitive? What happened? Was it only that nobody knew or talked about it?

Thanks

Carlos

[Melbourne Park]

Originally posted by desmo
Didn't Brasil have those last year?

Don't forget also that SPA has those dips at Au Rouge or whatever its called; the dips add to the driver skill and also forces the teams to change the car setup higher; the higher setup makes for a better race; so everyone loves SPA. But if there were bumps in the corners, all tracks would be like a race at SPA.

[RDV]

...Saludos Carlo
In the 60's and 70's it was very exciting to see the cars "bury the nose in the ground" under braking and seeing the anti-roll bars working under cornering. Weren't these cars pitch sensitive? What happened? Was it only that nobody knew or talked about it?


No not very pitch sensitive ,pre ground effects most chassis had a very rounded form , transverse flow took care of any big pressure variation under chassi , also planforms on flatsection under car were quite small, any wings used on late 60@s /early 70`s were quite high, even nose wings.

We used also to use quite soft springs , running frequencies @ 90to 100 CPM.... pitch sensitivity started being an issue as soon as we started using wide noses ( like on tyrrel or Chevron f2`s), and using the resulting surface to generate downforce


Does anyone know if the Brabham F1 car in the early 80's (82?) without a front wing was as pitch sensitive? Would the ground effects um effect the pitch sensitivity of the car ?

As Carlos has mentioned the reason for the removal of the front wing was with sidepods reaching to just behind the front wheel , the "venturi" (sic) throat gave a CP fairly forward , ( highest speed is in smallest section of thoat , thus maximum depression) so front wings were not really necessary, also without front wings flow into "venturi" was cleaner , giving more overall downforce and better L/D ratios. The change in weight distribuition was more a result of placing fuel tanks behind drivers to enable bigger venturis , (incidentaly favoring honeycomb as a structural material) , this pushed drivers forward on chassi , also because of bigger loads on front of car ft tyre carcasses were getting stiffer , and eventually started growing in diameter , so from our average weight distribuition of 32 to 35% on front ( to have enough weigth to enable traction) in the mid 70`s % ft was heading for 45 to 46% on wing cars.

Balancing the car was a bit more difficult because no adjustable item on front aero , at first several teams used to cut sliding skirts shifting the CP slightly backwards , on some Brabham photos you will see round holes cut in front part of skirts, this had same effect. eventually there was so much downforce, trimming with rear flaps or ft rideheight became more common......

At the beggining of venturi cars there was a slightly confusing time when to cure high speed oversteer , you gave more angle to rear wing, as in normal conventional winged cars, but result was MORE oversteer, as increasing wing angle gave a higher depression under rear wing, which in turn would extract more air from sidepods , which would speed it up in the throat , which would shift CP forwards...

[desmo]

Excellent post there RDV, thanks. I always wondered about those holes in the Brabham skirts, that makes sense.

I assume by springs having frequencies of 900-1000 cps, you mean undamped- that's a nice bass note! It'd be interesting to sample the ring from modern F1 torsion bars after they're struck and check those on my guitar tuner. Wright lists some modern forcing frequencies in his book: Suspension ~5-7Hz, Wheel hop ~15Hz, Engine >250Hz, axle to axle torsion: >30Hz.

[RDV]

desmo

----frequencies of 900-1000 cps, you mean undamped--

Undamped yes , I should have said natural frequency , but 90 cpMinute , not per second , this is for the spring , as resonant frequency for any sprung mass is
Resonant frequency = 1/(2xPi) ) x SQRT(spring rate k / mass m) (in Hz) so 90 cpm is 1.5 cps or Herz

to put things in perspective a tyre is an undamped spring , with average values in F1 of 300 N/mm ( roughly 1800 lbs/in for the metricaly challenged..) todays values for Cart tyres are @ 10hz for front and @7 for rear , for ovals circuits , and 7ft and 5 rr for ovals , F1 are @ 5.5 ft and 4.7 rr for most tracks , LMP same range

as tyres are springs in themselvers , total ride rate is =1/ ((1/spring rate(tyre ) + (1/spring rate ))(springs in series) and frequency is related to amplitude the higher a spring resonant frequency , the lower the amplitude......

so chassi spring rates in themselves relate to tyre stiffness , after a given point stiffening a spring will not make ride rates go up , as only the tyre carcass will move , higher aero loads and pitch sensitiveness required higher ride rates , so tyres evolved in that direction ( we digress again..)

translated = to keep suspension movement down(amplitude) for less aerodynamic disturbance , run higher spring rates (forced frequency) , but mechanical grip goes down, as car would tend to bounce more over surface irregularities, ( prime example is different tyre rates from different manufacturers giving different spring requirements, think of mercedes lemans LMP taking off due to the ultimate in pitch sensitivity induced disturbances, and bounce frequencies due to ft tyre rate and springing , but this is another story that would fill many pages!!)

coming back to original discussion ride rates today are about 3.5 to 5 times stiffer than 60 /70`s pre wing era cars , hence less chassi movement, and aero forces damp cars considerably themselves

running stiffer springs and bars in itself requires stiffer chassis ( consider chassi as a big torsion bar , as the engine and gearbox are if transmitting beam and twist loads , in fact any component which is in the load path is considered as a spring; think of 747 wings as a huge leaf spring!)

[Yelnats]

Very informative RDV! Have you any info on the Dual Chassis Lotus (88?) I understand it as an effort to decouple the chassis areodynamics from chassis movement so that their suspensions could be optimised individualy. i.e. downforce transfered directly to the tires without passing through the chassis suspension.

But it in a sense it wasn't the first of these efforts, for the original Chapparel (and many later F1 copies) appled the wing downforce directly to the suspension uprights, avoiding the loading of the chassis suspension components (until this was banned). The Lotus 88 of course advanced this considerably by doing the same with chassis generated down force and in theory removing all chassis pitch movement from the underbody downforce generating devices thus eliminating the pitch sensitivity problem.

[MclarenF1]

So with a tire spring rate of 1800 lbs./in and a spring rate of 6000 lbs. /in (from Racecar Suspension Design) which (depending on the motion ratio of the bellcrank) essentially makes the suspension solid. So what is the point of having springs at all? Poor drivers

Were there any efforts to figure a way to dampen the tire effectively. With the tire being an un-damped, un-sprung, rotating mass I would think that keeping it under control would be very important (or at the very minimum knowing what will happen under certain conditions).

Could someone explain the effects of adjusting the roll stiffness distribution via A/R bars vs. the springs. I know that the springs will effect the wheel rate (and therefor-natural freq.) but in a steady state corner would the A/R bar and spring act the same? Previous experience with stiff A/R bars and low natural frequencies ~1.5 cps produced great results in our FSAE cars. Regardless of the fact that stiff A/R bars decrease the independence of an independent suspension. I will shut up now....

[RDV]

------Have you any info on the Dual Chassis Lotus (88?)

no , after my time there , but Peter Wright covers it adequately in his book.

Generaly it was trying to get to grips with the problem of ...that word again ...pitch sensitivity , and also trying to avoid lifting of side skirts off the ground thus loosing total downforce... as mentioned before , ride rates were beginning to excede drivers abilities to cope with it

, having the body attached directly to wheels (as such) would keep basic aero part static relative to ground, and allow lower suspension rates to cope with bumps and ripples , and also give drivers a more forgiving environment

I understand it as an effort to decouple the chassis areodynamics from chassis movement so that their suspensions could be optimised individualy. i.e. downforce transfered directly to the tires without passing through the chassis suspension

.

no , as mentioned above , more to maintain body( venturi/skirts ) at fixed incidence and height relative to ground, aero effects would swamp everything else, different orders of magnitude..... second part of assumption correct , also to pass loads directly to tyres...

[Jezztor]

Alan Henry's 'FORMULA ONE TECHNOLOGY THROUGH THE 80'S' also covers the dual chassis Lotus.

Jezz

[Yelnats]

Thanks RDV, we agree on the dual chassis Lotus but have used different terms to describe the same effect

[imaginesix]

I have just gotten around to graphing the ride height / downforce data that RDV posted. Interesting stuff.

Looking at the graph one can see that ride height changes, both front and rear, seem to primarily affect the downforce at the front.

The angle of the lines for 'Front downforce' indicate the front end's sensitivity to rear ride height, while the separation between each of them indicates it's sensitivity to front ride height. The 'Rear downforce' data plots show neither of these characteristics. Thus lowering the front of the car while also raising the rear will most severly increase front downforce. Ergo 'pitch sensitivity'.

So while the downforce levels change as a direct result ground clearance (ground effects), it is in fact the pitch angle of the car that will move the CofP fore and aft. So the term 'pitch sensitivity' is in fact far more accurate than had previously been thought (by myself anyways).

In heave (cresting a hill or dipping through a valley), the downforce can be kept reasonably consistent despite the consequent change in ride height. Looking at the data plots for the 'Front downforce' again, a car with 10mm/15mm F/R static ride height will produce nearly equivalent downforce at 5mm/5mm (in compression) and 15mm/25mm (in extension). With rear wheel rates being roughly half the front wheel rates to the best of my knowledge, it is reasonable to expect that the rear will compress and extend twice as much as the front in these instances.

Thanks RDV, that info is golden!

[Melbourne Park]

Originally posted by imaginesix

Looking at the graph one can see that ride height changes, both front and rear, seem to primarily affect the downforce at the front.

I guess that's why with hotted road cars the front is dropped to improve turn in ... sorry not relevent

[BRIAN GLOVER]

Its all relevent, MP, Its about cars isnt it?

When F1 cars lifted the front wing out of ground effects, by 50mm, they lost 2 1/2 to 3 seconds per lap. "Cataclysmic", according to Mark Handford. With longer wheel basis and multi element wings, and end plates and fences, etc, this loss has largely been regained, at the cost of much drag. Pitch sensitivity is less pronounced but still a huge problem. I remember Mika's 2000 McLaren riding the curbs with ease compared to the Ferrari's hard suspension. Ferrari had awful problems with pitch sensitivity and lengthening of the wheel base and stiff suspension, didnt help much. I bet the rule change was on Ferraris behalf. The cheating bastards.

The front wing is closer to the ground than the back wing and therefore its performance is affected by track temp on top of ambient temp. A cloud could go over the track and the rear wing will suddenly get a lift gain and overall C of P moves back. The ground effect wing is more sensitive to height, whereas the higher wing is more sensitive to density.

( The pitot tube is essential for air density measurements and I dont know why they dont have one mounted infront of both the front and back wings instead of where it is. Height measurements are crucial also, so thru real time telemetry, accurate adjustments to incidence can be made in the pits for current track conditions. )
The ground effect wing in pitch forward(dive) increases the venturi effect(Closer to ground) and therefore downforce and if this same section was used out of GE in a dive, it would lose lift. A different section must be used which has little curvature where the lift is induced further by increased angle of attack. The airfoil section used in GE would also move the centre of pressure forward on the airfoil with increase angle of attack and therefore, moment goes forward also. In a high speed turn, the higher wing will be less affected by roll than the GE airfoil. In acccelleration out of a turn, the balance can be upset by aft pitch or squat. The GE airfoil will suffer the most here. Maybe the engineers would not want to go back to the GE wing???.The suspension geometry has broader design parameters now.
The regs require only the 200mm centre section can remain at the 2000 levels. If there were just one wing above the driver, like in the sprint cars of the Race of outlaws' series, the effects would be more predictible, but the wings must fit into boxes with specific dimensions at each end of the car. One hell of an engineering challenge to keep lift constant in pitch changes.

Do you remember when the Voyager took off in California, for its round the world flight? With full fuel, the struts had no travell , so someone added some more pressure to them out of spec. On the take off roll, the wingtips were forced onto the runway because the angle of attack went negative. Only when there was sufficient airspeed, could Rutan change the pitch with elevator deflection. The wings suddenly bent the other way to a dramatic dihedral and the plane lept ito the air. Upon his return, the first thing he inspected, was the wing tips.