Why wouldn't you put the Roll Center below ground?

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Belatti
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Re: Why wouldn't you put the Roll Center below ground?

Post by Belatti » Tue May 10, 2011 2:05 pm

Just finished all the geometrical measrements of a Crespi XXV Formula Renault chassis. Front RCh -25mm (thats underground friends!) Rear RCh -13mm (that, too).

Aero reasons?

Damper position sensors shows me that all 4 dampers are compressed even when the car rolls 0.6º at 1.8 Gs.
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Jersey Tom
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Re: Why wouldn't you put the Roll Center below ground?

Post by Jersey Tom » Tue May 10, 2011 3:11 pm

I'd imagine so.
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Caito
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Re: Why wouldn't you put the Roll Center below ground?

Post by Caito » Tue May 10, 2011 5:20 pm

Belatti wrote:Just finished all the geometrical measrements of a Crespi XXV Formula Renault chassis. Front RCh -25mm (thats underground friends!) Rear RCh -13mm (that, too).

Aero reasons?

Damper position sensors shows me that all 4 dampers are compressed even when the car rolls 0.6º at 1.8 Gs.

The suspension is the same for all cars, or can it be adjusted?


Basically both RC below ground would mean that when you turn you lower the car, which would be benefical if there's a min height rule.

EDIT: I realized the following is plain wrong
If the wheel gains(negative) camber when the wheel goes up. And all wheels go up, that would be bad "mecanical traction" speaking. Right? But the aero advantages would outweight the disadvantages.
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Belatti
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Re: Why wouldn't you put the Roll Center below ground?

Post by Belatti » Wed May 11, 2011 1:11 pm

The min ride height rule does not exist in the Formula Renault (or FR metro or FR plus series).

It does exist in TC and the norm is 70mm min, I think.

I recall the Ford Falcons used to have the front roll center below ground, not because of a choice, but because thats where it lays when you lower the car with the wishbones pick up points that the rules stated. That was later "corrected" and now they are all above the ground (I think).

Anyways, these cars have horrible aero :lol:
"You need great passion, because everything you do with great pleasure, you do well." -Juan Manuel Fangio

"I have no idols. I admire work, dedication and competence." -Ayrton Senna

olefud
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Re: Why wouldn't you put the Roll Center below ground?

Post by olefud » Thu May 12, 2011 11:45 pm

Jacking was a parameter of little concern to me when setting up much lesser racecars, i.e., without much aero. Over-simplified, springs were to set ride height and frequency with rates that wouldn't bottom out. Roll centers were roughly set to produce just a bit too much roll during cornering so that anti-roll bars trimmed both total roll and adjusted the roll couple resistance at the wheel pairs to set over/understeer. Depending on the CG, this often set the front roll center near ground level and the rear about a foot above. The point being that the roll centers tend to be dependent on the springs,etc. which are independently set.

With the aero of a F-1 to contend with, spring rates are very high, more like a kart suspension. Also, the low CG of a F-1 would produce rather small roll couples if the roll centers are above ground. In order to produce an effective roll couple relative to the springs it may well be necessary to have the roll centers below ground level.

DaveW
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Re: Why wouldn't you put the Roll Center below ground?

Post by DaveW » Fri May 13, 2011 12:29 pm

olefud: I like the logic of your first paragraph, but I'm not sure that I agree with your second.

From what I have seen, the front vertical stiffness of an F1 car is certainly very large, but not the roll stiffness. That would suggest that the front roll centre height is high, something that might be guessed from an inspection of the suspension geometry, perhaps. A logical conclusion would be that the front vertical stiffness must be large in order to hold the nose DOWN during cornering. Allegedly, the improved airflow under the front of the car leads to aero benefits that outweigh the mechanical disadvantages of the layout. I don't suppose anybody has tried a true back-back track evaluation, so we'll probably never know.

olefud
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Re: Why wouldn't you put the Roll Center below ground?

Post by olefud » Fri May 13, 2011 11:53 pm

DaveW: My point was intended to show theoretically that the ground is pretty much immaterial relative to the location of roll centers and, in my opinion, jacking is down the list in the hierarchy of design considerations. When it come to actual cars, you have to realize that I have a deep and profound ignorance. The last time I poked around F-1 cars was at Watkins Glen fifty years ago. I did look at a few front photos. It appears that the front roll center could be a tad below ground though the front wing rather obscures the lower arm.

That said, it doesn't appear that spring stiffness can be decoupled in the vertical and roll modes. Both are manifested by wheel movement -assuming a wheel isn't being lifted- with concurrent spring deflection. I agree that a fairly constant ride height maintained by stiff springs enhances aero. Maybe with enough spring deflection to pick up a bit of ground effect.

DaveW
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Re: Why wouldn't you put the Roll Center below ground?

Post by DaveW » Sun May 15, 2011 3:44 pm

olefud wrote:...That said, it doesn't appear that spring stiffness can be decoupled in the vertical and roll modes...
Apologies olefud, but the suspension of a push/pull rod race vehicle can have springs attached between the rockers & the vehicle ("corner" springs) & a spring attached across the rockers to allow symmetrical movement, but to resist differential rocker movement (anti-roll bar). It is also possible to attach a spring between the rockers to allow differential movement, but to resist symmetrical movement (heave/pitch spring).

Many F1 vehicles use, or have the potential to use, all three types of spring at both axles. If only "corner" springs are fitted, then the heave/pitch and the roll stiffnesses are perfectly coupled. However, if corner springs were to be omitted (and the are sometimes at the rear axle), then heave/pitch stiffness will be completely decoupled from roll stiffness. It follows, hopefully, that soft "corner" springs, a soft arb, but a very stiff heave/pitch spring will yield the characteristic I described.

olefud
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Re: Why wouldn't you put the Roll Center below ground?

Post by olefud » Sun May 15, 2011 11:05 pm

DaveW; I've followed the pitch control springs but seem to have missed "floating" the corner springs to decouple jounce from roll. I'll get up to speed on this. Thanks.

hardingfv32
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Re: Why wouldn't you put the Roll Center below ground?

Post by hardingfv32 » Sun May 15, 2011 11:26 pm

This is one form of the subject: Spring-Less Rear Suspension

http://scarbsf1.wordpress.com/2010/12/0 ... evolution/

Brian

ubrben
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Re: Why wouldn't you put the Roll Center below ground?

Post by ubrben » Mon May 16, 2011 9:39 am

Caito wrote:
Jersey Tom wrote:The total load of the tires is the same, you're just moving the sprung mass up and down.
Oops, I was clearly missing this point.

One axis car just for simplification. If I have a negative jacking force pulling my car down, hence, compressing the springs. How can it be that the load of the tires is not increasing?I'm still missing a part.

Wouldn't a negative jacking force pull the car down enough for it to have a significant effect in CoG height? Which would be an improvement, but I think the downsides are much bigger.
The jacking force is a component of the vertical load. As JT has pointed out mass and aero dictate the total load. If you have a jacking force of a certain amount the spring carries proportionately less load therefore deflects less - if you want soft springs for grip, but need to maintain ride height for aero reasons, I'm sure a little bit of jacking force could be useful.

Most of the racecars I've seen have front RCs within 25 mm of ground and the rear RC slightly higher - this is mainly to get front and rear roll in phase - basically making the rear load transfer happen a little faster than the front to compensate for the later development of slip angle on the rear axle vs. the front.

Ben