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Quote:
Originally posted by Eagledriver
Come to think of it...if you have the roll center below the ground I think you would have the CG of the car move to the outside of the turn more than if you have the roll center above the CG. If you constructed a car with the roll center co-located with the CG then the weight transfer would be all due to lateral acceleration. You could never have a car transfer weight to the inside wheel on a corner unless the CG was underground (a difficult engineering problem).

-Andy
All you need to do is build an car with the CG below the roll center. Imagine a tall wagon with weights slung way low below it.

Jerry Kroeger

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Old 08-06-2004, 11:32 AM
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All you need to do is build an car with the CG below the roll center. Imagine a tall wagon with weights slung way low below it.
Or rather a Tesla!

(bumping the thread just for the fun of it!) - J
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Old 12-15-2020, 12:38 PM
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Its cool to lift the inner tire, just remember to tap the brakes so the wheel stops spinning mid air!

Me and a friend took turns in a boring 180 corner to do that during a track day session, he was a little better at it, stiffer body and softer torsion bars... My targa is more... road compliant...
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Old 12-17-2020, 07:34 AM
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Quote:
Originally Posted by jluetjen View Post
PS: BTW, if you were to redesign the suspension such that the roll centers are below ground, the result will be a car that leans into a corner rather then away from the corner. But loading up the inside tire has a number of wierd knock-on affect like jacking a car down while cornering. I think that is why I'm not aware of any cars designed with that geometry.
Okay, I just read this old old thread with some recents posts and found the above text.

If the roll centers are below ground there is no way the car could lean into the corner. For a car to lean into the corner, the roll centers need to be above the cg. The cg is always above ground.

When we lower the 911, the roll centers are lowered as well. The front roll center is not too hard to get under ground. The more you lower the roll centers, the more the car wants to lean in corners and this is because the cg is getting farther away from the roll centers.

The reason we raise the spindles in the front when we lower the front is to get the front roll center back closer to stock. Raising the roll center at the rear is a more complicated proposition as you have both inner and out pickup points that would need to be raised. Luckily, the rear roll center is higher than at the front. However, it would be really good to keep the rear roll center higher than the front and to keep it as close to stock as possible.
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Old 02-01-2021, 10:19 PM
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Ok, since I don't agree with the subject of this thread, who has the photo with their front wheel up the highest?

This was a nasty mid-corner bump:


This might actually be one front and one rear in the air:
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Old 02-04-2021, 04:38 AM
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Well, if you could keep the balance of your car right AND keep the inside front tire on the ground handling some of the lateral load, you should be able to go faster than with the inside tire up in the air.....so the premise of the thread title is reasonable but not always practical.

This is all I got:



And I am trying to eliminate that!
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Old 02-04-2021, 08:12 AM
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Quote:
Originally Posted by 304065 View Post
Has anyone actually plotted the suspension pickup points of the 911 to determine, statically anyway, the location of the roll center? What is the calculation for CG, I have found a spreadsheet that does it but want to code one myself. (I know how to figure it in an airplane. . .)
Yes. I've measured lots of things on my 88. (blue is Front, yellow is Rear)



CG needs one end of the car pretty far up in the air and some big blocks under the scales. I tried it (though not on my 911) with the result that the 1lb dither in the scales was sufficient to limit CG height accuracy to - I actually don't remember, but it was +-30 or 40mm. A reasonable guess gets you right in the same ballpark with less work.

NHTSA measured a boatload of vehicles (a 911 not among them AFAIK), and the data is public domain, though SAE will charge you for their paper covering it. But it will provide a good guesstimate starting point.

Here are some free links:

https://kktse.github.io/jekyll/update/2020/10/25/estimating-vehicle-inertia-properties-with-nhtsa-database.html

https://pdfs.semanticscholar.org/ea35/f7875912b31021c8bdc42df10264a462f86e.pdf
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Old 02-05-2021, 07:34 AM
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Quote:
Originally Posted by burgermeister View Post
Yes. I've measured lots of things on my 88. (blue is Front, yellow is Rear)
Thanks for sharing. On the RC height graph, is it vehicle ride height on the vertical axis? If that's the case, RC height is increasing substantially (especially in the rear) with decreasing ride height. That would be surprising.

I need to dig out my susprog files.
Old 02-06-2021, 07:48 AM
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Scott's correct.

But having the FRC below ground level isn't that big of a deal. And F1 cars have been known to run FRC's below ground level. It's not like you're going to overcome the laws of physics by doing it.

Having personally tried all kinds of roll centers on circle track cars, I've concluded that it's only one small element in handling.

The reason that cars lift a front wheel is because the front roll couple has reached 100%. Could it go faster with 4 tires planted on the ground? Sure. But how much faster, given the work to achieve that? And if you do the math on dynamic corner weights at say, 1.5G, would having 200, or 100, or 2 lbs on the inside front tire make the car that much faster?
Old 02-06-2021, 09:15 AM
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Quote:
Originally Posted by stownsen914 View Post
On the RC height graph, is it vehicle ride height on the vertical axis? If that's the case, RC height is increasing substantially (especially in the rear) with decreasing ride height.
Sorry for the eye chart graphs ... they used to be scaled better, but LibreOffice rescaled the axes from what MS-Excel had, and I was too lazy to go in and fix it. Alas, the price for LibreOffice is right!

Ride height is indeed on the vertical axis. + = jounce, -=rebound made sense to me and was standard at my former employer. Around 50mm of jounce travel from factory ride height (as measured by torsion bar & wheel centers), the front roll center goes negative (ie, below ground).
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Old 02-06-2021, 09:23 AM
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Originally Posted by dannobee View Post
But having the FRC below ground level isn't that big of a deal. And F1 cars have been known to run FRC's below ground level. It's not like you're going to overcome the laws of physics by doing it.
A 911 is not an F1 car. Roll is much greater on a 911 than it is on an F1 car as the cg is much higher on a 911 than an F1 car so doing what we can to keep the front roll center near Factory height (a couple of inches above ground) is a good thing. There are a few reasons we raise the spindle height on a 911 when it is lowered and roll center height is a big one. When we build race cars, we raise the spindles as much as our wheel package allows. Well, with 18" wheels maybe not as much as possible....but that depends on tire diameter and how low you go.

You raise the spindles so the physics don't make the car roll in front any more than it has to AND to stay in a proper relationship to roll at the rear. It's all about the right balance.

So yeah, it is kind of a big deal. Let me put it this way: I would be real happy if my racing competition left their spindles at stock height!
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Old 02-06-2021, 11:47 AM
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Would a raised spindle affect the front RCH more than the 20 or 30mm that the spindle is raised?

If not, given the CG location 60% towards the rear, wouldn't a 30mm raised spindle change the roll moment by 0.4 * 30 = 12mm, or about 3%? Which, while definitely something, isn't all that big a change.
Would the effect of camber compensation caused by a raised spindle be more significant than the effect of the raised spindle on vehicle roll? They are both changes for the better, just wondering which would make a bigger difference.
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Old 02-06-2021, 01:43 PM
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It all depends on how much you lower the front of the car. If I lower the front 20mm and raise the spindle 20mm, the front roll center height does not change.

I have never calculated the roll moment changes but I do know that keeping the front roll center slightly above ground with the rear roll center a bit more above ground than that is important for how the car handling balances. Of course the camber curve is better when the a-arms are less angled....but the front suspension doesn't compress a lot anyway so I would say that the camber curve is not as a big a problem as the roll center. Radial tires like the camber gain.
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Last edited by winders; 02-06-2021 at 03:38 PM..
Old 02-06-2021, 02:42 PM
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3 wheel racing, lol.


My old 911 would lift a wheel, part of the problem was the diagonal outside rear spring collapsing allowing the inside front to leave the ground.


Only time I got a wheel up in the Imsa GTO Camaro was in a drift over a bump.
Old 02-08-2021, 05:41 AM
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Turn 9 @ the Roval in Fontana. Hot Slicks are a workout.

BTW; Instant G racing has a lot of mass, spring rate & frequency rate data calculations for 911 rear swing arms on their website. It was done in conjunction with the Society of Auto Engineers mentorship program in the school lab.

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Old 02-08-2021, 11:28 AM
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Winders has it right
reducing weight transfer is good because a pair of tires can create more lateral grip than a single tire can
when a car corners the inertial force act through the center of Mass, trying to rotate the car about the roll axis which is determined by the front and rear roll centers, this transfers weight from the inside wheel to the outside wheel, max lateral grip is generated by 2 equally laded tires due to the tires load sensitivity curve

weight transfer can be reduced by lowering the CoM and/or increasing track width and/or sway bars

the roll centers move around a lot with suspension motion, when the roll center goes down it increases the lever arm of the roll couple, when the roll center goes up it shortens the lever arm of the roll couple

In practice the longer the lever arm the more roll, the shorter the less roll, Lowering the CoM has the mirror effect though it's harder to accomplish and once set doesn't move far.

When a car is lowered it changes the suspension geometry in our cars moving the roll center down unless counteracting steps are made to restore the geometry, counter steps are raising the steering rack, o/s tie rod ends and raised spindles

Since the lateral inertial forces act through the fixed CoM and roll the car about the roll axis anything done to shorten the distance between them will reduce roll
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Old 02-25-2021, 12:35 PM
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Yeah but my car launching off a bump looks cool
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Old 02-26-2021, 05:45 PM
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Old 03-08-2021, 02:12 AM
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Rookies....


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Old 03-09-2021, 06:09 AM
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Old 03-15-2021, 04:42 PM
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