Pitch Centers

[dancin stu]

where does everyone have their puitch centers located? about figured out where its gonna be longitudinally, but vertically im still a bit unsure, what sort ofpercentage heights ofyour cg have people got?

[dancin stu]

where does everyone have their puitch centers located? about figured out where its gonna be longitudinally, but vertically im still a bit unsure, what sort ofpercentage heights ofyour cg have people got?

[clausen]

Hi,

I havent got that far yet, but I'm thinking that it should be at least high enough to make sure that it never goes through ground level, because that is when it would start moving forward and backward radically (just the same as a RC going though the ground does), which can't be a good thing for consistency.

By the way - have a look on the newest Race car engineering (V13 #12) page 39. Does the top rear wishbone look like its on a massive dive/squat angle to you?

Regards

Paul Clausen
Uni of Adelaide

[Frank]

we just used a tiny bit of anti-dive to "stick" it.. it still moves around a bit though

[ben]

We run 0 anti-dive and about 35% anti-squat. The anti-squat is acheived simply by rotating the plane on the lower rear wishbine up by a few degrees.

Ben

University of Birmingham
www.ubracing.co.uk (http://www.ubracing.co.uk)

[dancin stu]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by ben:
We run 0 anti-dive and about 35% anti-squat. The anti-squat is acheived simply by rotating the plane on the lower rear wishbine up by a few degrees.

Ben

University of Birmingham
http://www.ubracing.co.uk<HR></BLOCKQUOTE>

interesting that both yourselves and Toronto run 0% anti dive on the front, with you being perhaps some of the quickest teams at formula student...........

is your pitch center on the gorund then, or are your front wishbones completely parallel with the ground at all ride heights and rake angleshttp://fsae.com/groupee_common/emoticons/icon_wink.gif

do you experience much dive under braking or are you running quite high spring rates?

[ben]

Don't know about all the transients, so I suspect we may be getting some by default as you are inferring.

More correctly then - the wishbones are both parallel to the ground plan at the design ride height :-)

Ben

University of Birmingham
www.ubracing.co.uk (http://www.ubracing.co.uk)

[Sten]

I'm new to FSAE, but shouldn't the pitch centre be at the driver's hip?

[dancin stu]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Tonto_Sten:
I'm new to FSAE, but shouldn't the pitch centre be at the driver's hip?<HR></BLOCKQUOTE>

the pith center affects the amount of weight transfer under a longitudinal acceleration. it can be anywhere, think ofit like a roll centre but in side view.

whats the reason you think it should be at the drivers hips?

[Garbo]

I think I instigated the 'driver's hip' comment.

I have discussed the pitch centre location with a few people... having the pitch centre 'in' the driver... this makes pitch easier to feel for the driver.

The thought was that a change of angle is easier to sense than a change of height so, having the pitch centre as close longitudinally to the driver's centre of mass as possible helps the driver sense dive and squat.

Obviously, the height of the PC is tied in with long'l weight transfer and all the rest of that good stuff.

Garbo

[PatClarke]

I spoke at length with Ron Tauranac on this subject at FSAEA. RT is adamant you DON'T want the pitch centre at the drivers hip area! I was of the opinion that the hip solution was about right until I listened to what he had to say.
His suggestion is that the p/c should be about two wheelbase lengths BEHIND the car (In the case of a RWD car). When the p/c is between the wheels the car 'hobby horses' around, and is less stable and more difficult to drive.
When you look at the suspension axes necessary to get the p/c back there, some things become both obvious and interesting. I now tend to agree with RT!
Hopefully I have sent you all off on a new quest http://fsae.com/groupee_common/emoticons/icon_cool.gif
PDR

The older I get, the faster I was.

[Mick_P]

Very Interesting indeed with the idea of p/c behind the car.... got me thinkin.. http://fsae.com/groupee_common/emoticons/icon_razz.gif

Mick Partridge
Swinburne University of Technology

[PatClarke]

Mick,
The general idea WAS to get you thinkin' http://fsae.com/groupee_common/emoticons/icon_wink.gif
PDR

The older I get, the faster I was.

[ben]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Suddenlee:
I spoke at length with Ron Tauranac on this subject at FSAEA. RT is adamant you DON'T want the pitch centre at the drivers hip area! I was of the opinion that the hip solution was about right until I listened to what he had to say.
His suggestion is that the p/c should be about two wheelbase lengths BEHIND the car (In the case of a RWD car). When the p/c is between the wheels the car 'hobby horses' around, and is less stable and more difficult to drive.
When you look at the suspension axes necessary to get the p/c back there, some things become both obvious and interesting. I now tend to agree with RT!
Hopefully I have sent you all off on a new quest http://fsae.com/groupee_common/emoticons/icon_cool.gif
PDR

The older I get, the faster I was.<HR></BLOCKQUOTE>

Did he have an opinion on it's height? Or does he think anywhere as long as it's off the back of the car?

Ben

University of Birmingham
www.ubracing.co.uk (http://www.ubracing.co.uk)

[PatClarke]

Ben,
He didnt seem to think the height was that significant as long as reasonable ie not 5 meters high or something silly.
When I drew up a few layouts to get a grip on what happens, I arbitrarily put the pitch axis at axle height when static. It was somewhere to start I guess.
PDR

The older I get, the faster I was.

[PatClarke]

Hi Brian,
Your question should be directed to RT, but I don't see pitch and bounce being the same thing.
Pitch being an angular change of attitude, whereas bounce is a linear motion. Sure, they could be the same thing, or overlap somewhat. I understand what you are calling a 'bounce centre' and I am now wondering if the geometrical pitch centre in the middle of the car has any relevance at all?
What I do know is that RT built a lot of Brabhams and Ralts with that policy on pitch axes, and they won a lot of races, so who am I to argue.
Still, whatever the subtleties of the argument, it has started a delightful debate http://fsae.com/groupee_common/emoticons/icon_smile.gif
PDR

The older I get, the faster I was.

[This message was edited by Suddenlee on January 04, 2004 at 03:37 AM.]

[This message was edited by Suddenlee on January 04, 2004 at 03:44 AM.]

[spayce]

Pat,

This talk about pitch centres got me thinking... it may be an interesting solution to the pitch sensitivity problems of ground effects as well as a neat transform of angular velocity to linear.

Is the latter point the sole purpose? Has RT applied this theory to Ground effects vehicles before.

jayce.
Monash uni

[PatClarke]

Jayce, a very significant number of the Ralts RT built were GE cars, the Ralt brand spanned the entire GE era.
Yours is an excellent point, one I had not considered. Seeing as all openwheelers racing today want to keep their underside's attitude to the ground constant, this may well be the grounds for RT's design protocol.
PDR

The older I get, the faster I was.

[ben]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by bwalby:

Are they talking about the same things, or 2 different things with the same name? I tend to think the latter is the case.

<HR></BLOCKQUOTE>

I think the latter is correct.

Wong's talking about the two mode shapes of a 2dof half car model.

RT and Rouelle are talking about the kinematics of how forces are feed into the chassis.

Ben

University of Birmingham
www.ubracing.co.uk (http://www.ubracing.co.uk)

[spayce]

to reference earlier posts then,

the height and longitudinal distance of the pitch centre may have something to do with how much attitude gain (and rate of gain) you want your GE (primarily diffuser rake gain) to undergo.

that is providing it's a significant factor??

You'd certainly want data...(or it mounted unsprung...) I'd imagineer that the above factors may impact on 'geometric' pitch stiffness (which is undamped for rouelle aficionado's).

once again the significance is debatable, forum style.

jm

[STRETCH]

It is my understanding that the pitch centre cannot fall outside the wheelbase of the vehicle! If you could have the pitch centre behind the vehicle, both front and rear suspension would squat under braking, which is not possible, as weight must transfer from rear to front, make sense? Only the bounce centre can exist outside the wheelbase, and not within it, and both centres are functions of front and rear ride rates, not geometry interestingly.
See 'Fundamentals of Vehicle Dynamics' - T.D. Gillespie, page 175 onwards...

[C.Zinke]

Hi,

Even if its sounds strange it is possible to build a vehicle that lifts the nose under braking. We did so on our new car to gain braking performance due to lower camber at the front. I would suggest that you look at the longitudinal instant centres. That helped me understanding all effects of pitch and i never used the pitch center again.

BTW & OT: We posted some pics of our frame in the FSAE.com gallery: http://fsae.com/eve/forums/a/tpc/f/8356059423/m/71210560441

[JHarshbarger]

I too can attest to this strange behavior. One of our Bajas a few years ago squated in the rear under braking. Similarly, there have also been cars with roll centers above the CG that rolled into corners. The pitch and roll of the car is not neccesarily an indication of weight transfer.

[STRETCH]

Thanks for the input guys... although the case of pitching and rolling the opposite to the norm by placing the PC/RC above CG is a slightly different argument, it did get me thinking, and I cracked it!
My reference to the book refers to external effects, such as traversing a bumpy road or having sum1 pogo-sticking on the roof, so the centres are derived from ride rates. Driving and braking forces act through the linkages, so the chassis movement is dictated by geometry. With a bit of effort, you could indeed get the pitch centre outside the chassis as PAT mentions earlier in this thread; it would not compress any springs under forward accn, it would be just the weight of the car that stops it from flipping over backwards! Similarly the whole vehicle squats under braking. Perhaps beneficial for GE cars, but I would rather concentrate on implementing anti-squat into the rear geometry than a rearward facing PC.