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Quote:
Originally Posted by stownsen914 View Post
It doesn't work out that way on 914 at least. Roll center location depends entirely on pickup point geometry. In the case of a 914 the roll center is relatively high. Plotting it tells the story.
i believe you are missing a critical factor in determining the RC...

tire diameter and its relationship to the ground plane is needed to determine a the lateral RC on a car

what plot are you using btw? a side view plot?

Last edited by panzerfaust; 06-03-2017 at 07:20 PM..
Old 06-02-2017, 11:38 AM
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Quote:
Originally Posted by stownsen914 View Post
It doesn't work out that way on 914 at least. Roll center location depends entirely on pickup point geometry. In the case of a 914 the roll center is relatively high. Plotting it tells the story.
Show me.

It's all about how much the tire moves laterally while it moves up. If it's a semi-trailing arm that is nearly a plain trailing arm (even closer than a 911) it won't move much left or right so the FVSA length will be very long. Even if the IC ends up high when you draw the lines from the contact patch to the IC that is way far away to the opposite side of the car they will intersect near the ground.

Really the height of that intersection isn't what matters, it is the slope of the line from the contact patch to the IC on the heavily loaded side.
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Old 06-02-2017, 08:26 PM
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Originally Posted by Flieger View Post
Show me.

It's all about how much the tire moves laterally while it moves up. If it's a semi-trailing arm that is nearly a plain trailing arm (even closer than a 911) it won't move much left or right so the FVSA length will be very long. Even if the IC ends up high when you draw the lines from the contact patch to the IC that is way far away to the opposite side of the car they will intersect near the ground.

Really the height of that intersection isn't what matters, it is the slope of the line from the contact patch to the IC on the heavily loaded side.

I used susprog, a suspension analysis program. It was a few years ago, so I don't think I even have it installed on my current computer. Let me see what I can dig up.
Old 06-03-2017, 01:59 AM
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If you have the pivot point geometry (including a ride height) and some tire/wheel geometry (at least a radius and a centerplane location relative to a pivot point) I think I could do something in excel.
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Old 06-03-2017, 09:38 AM
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All good points here (after semantics and errors were addressed). As I understand it, the bottom line is to reduce the CG/RC lever arm length to a reasonable degree, which would then require less roll stiffness, resulting in better mechanical grip of all four tires.

To a slightly separate point, I read somewhere that in terms of roll stiffness (not squat and dive), there is no difference between sway bars and springs/torsion bars. That seems somewhat counter-intuitive to me. What say you suspension gurus?
Old 07-05-2017, 03:50 PM
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To a slightly separate point, I read somewhere that in terms of roll stiffness (not squat and dive), there is no difference between sway bars and springs/torsion bars. That seems somewhat counter-intuitive to me. What say you suspension gurus?
Of course there is a difference. The stiffer the anti-roll bar setting, the less independent the suspension becomes at that end off the car and that is not a good thing. The outside tire is being pushed up by the suspension compression during roll. The inside tire is being pulled up by the anti-roll bar and it's spring is resisting that. If we are talking about the rear of the car, do you really want the inside tire being pulled off the ground? Nope.....
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Old 07-05-2017, 04:34 PM
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There is no difference if you are only looking at a pure lateral acceleration/force and there are no preloads going on. There is a difference in how fast a one-wheel bump disturbance is transferred to the opposite wheel, though. Sway bar is faster since it transfers it's force to the other side at the speed of sound vs. having to work through the chassis inertia/displacement. This means grip should be better with the equivalent all-spring setup, though the force transmitted to the chassis will be higher in that case. The magnitude of those points will vary based on how stiff the sway bar is relative to the springs.
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Old 07-07-2017, 06:19 PM
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Quote:
Originally Posted by Flieger View Post
There is no difference if you are only looking at a pure lateral acceleration/force and there are no preloads going on.
But who looks only at that? Someone in academia since people in the real world worry about how the entire system is affected.
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Old 07-07-2017, 08:59 PM
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[QUOTE=winders;9654307]But who looks only at that? Someone in academia since people in the real world worry about how the entire system is affected.[/QUOTE]

System in action:
Here is what T-bars get you; driving around corners on 2 wheels.

In theory, if coil overs were in use, those wheels in the air would be on the ground due to the higher response rate of the coil overs, correct?
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Old 07-11-2017, 12:57 PM
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Quote:
Originally Posted by 3rd_gear_Ted View Post
Quote:
Originally Posted by winders View Post
But who looks only at that? Someone in academia since people in the real world worry about how the entire system is affected.[/QUOTE]

System in action:
Here is what T-bars get you; driving around corners on 2 wheels.

In theory, if coil overs were in use, those wheels in the air would be on the ground due to the higher response rate of the coil overs, correct?
I take it that by "response rate" you mean spring rate?

The wheels in the air would still be in the air, but not so high, if you simply increased the spring rate.

The wheels might be on the ground if you had a super progressive rate, but they wouldn't have any real weight on them besides their own, so they wouldn't really be doing to any good. If you didn't have tender springs then they could droop all the way out with an effective spring rate of 0 but again they wouldn't do you any good.

The reason the wheels are in the air is because of weight transfer. To get them on the ground you need to lower the cg and/or increase track/wheelbase.
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Old 07-11-2017, 07:34 PM
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??? my 2cents: Swaybars, the stiffer they are, the more you'll have wheel in the air. Disconnect and every wheel stays on ground until travel end is reached.
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Old 07-12-2017, 02:54 AM
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The front wheel in the air is a combination of the anti-roll bar and the fact that the front is much more roll stiff than the rear. As I stated previously, the anti-roll bar causes the inside tire in the turn to be pull up.

Coil overs don't mitigate this at all.....unless you are using more spring rate and less anti-roll bar rate or the chassis balance is changed.
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Old 07-12-2017, 10:25 AM
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Quote:
Originally Posted by winders View Post
The front wheel in the air is a combination of the anti-roll bar and the fact that the front is much more roll stiff than the rear. As I stated previously, the anti-roll bar causes the inside tire in the turn to be pull up.

Coil overs don't mitigate this at all.....unless you are using more spring rate and less anti-roll bar rate or the chassis balance is changed.
+1... both axle can dictate how high the front raises
Old 07-18-2017, 04:48 PM
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Thanks for the insights all.

Getting used to full Slicks - 305 rear, 255 front, car is around 2450 lbs
Front Tarret ARB was tightened by about an inch prior to event, gonna back that off, I'm learning

If dropped front spindles were in play, what da ya suppose then?

BTW, thanks to Cali- Photos for getting me the shot I asked for. Will get same photo again in Sept to compare.
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Old 07-20-2017, 10:19 AM
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Thanks for the insights all.

Getting used to full Slicks - 305 rear, 255 front, car is around 2450 lbs
Front Tarret ARB was tightened by about an inch prior to event, gonna back that off, I'm learning

If dropped front spindles were in play, what da ya suppose then?

BTW, thanks to Cali- Photos for getting me the shot I asked for. Will get same photo again in Sept to compare.
Only back off the front sway bar if that is what you need to get better chassis balance. In other words, if you desire less understeer/more oversteer then go ahead and soften the front or stiffen the rear.

Raised spindles raise the front roll center which is a very good thing. But I don't think you would notice much if any difference in chassis attitude. Lowering the CG or widening the track is what you need to do to reduce roll with the grip you have.
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Old 07-21-2017, 11:14 AM
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Is there any consensus for a rough starting point for street/track height??

This is a bit nieve, I know, given the infinite variable combinations and range of desired results.
If we aim for a "more sporting street useage" mid year chassis set up, that is starting from stock height/alignment how much can we lower the front/rear without adversely impacting linear high g cornering?
I. Do not want ultimate grip, at the cost of twitchy, sensitive,narrow range balance.
The good Dr. Verberg once opined that the front could be lowered a bit, reducing the roll moment. But what about the rear roll moment.
I am Not at all clear about how the front/rear roll couple affects predictable, linear response. Clearly, camber curves and toe steer complicate the quest for linear response.
Am I. Correct in assuming that we don't want any "self correcting" suspension dynamics??

Still a little uncertain 😳
Chris

Last edited by chrismorse; 05-13-2018 at 03:27 PM..
Old 05-13-2018, 12:16 PM
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Originally Posted by chrismorse View Post
This is a bit nieve, I know, given the infinite variable combinations and range of desired results.
If we aim for a "more sporting street useage" mid year chassis set up, that is starting from stock height/alignment how much can we lower the front/rear without adversely impacting linear high g cornering?
I. Do not want ultimate grip, at the cost of twitchy, sensitive,narrow range balance.
The good Dr. Verberg once opined that the front could be lowered a bit, reducing the roll moment. But what about the rear roll moment.
I am Not at all clear about how the front/rear roll couple affects predictable, linear response. Clearly, camber curves and toe steer complicate the quest for linear response.
Am I. Correct in assuming that we don't want any "self correcting" suspension dynamics??

Still a little uncertain 😳
Chris
probably a tad lower than euro spec. stock ride height the 911 strut suspension already exhibits more bumpsteer than a double a arm agreements. lowering it much beyond stock creates massive bump steer especially noticeably since there isnt a power steering to damping the steering kickback in your hands. on the rear you will loose some anit squat tendency in which the 930's have in raising the pick up points i reckon.

im not sure what Dr. Verberg meant but if you lower the front excessively your rollcenter goes subterranean in effect it making your suspension softer due to larger roll moment from the cg to the rc. your camber camber curve would be reduced even more....early 911 had very little to begin with. this is the reason folks use raised spindles so you dont alter the rc much while u lower it. still you will have bumpsteer twichiness to deal with separately with raised spindles.

roll coupling is the side axis of the two axles. nearly all road cars engineers this for the front to loose traction first for safety reasons. rear rc are typically much higher than fronts... at times 2, 3 or even 4x higher. i dont know the roll coupling of 911's, its a strange and interesting platform. :-) anyone have mech drawing of this?

you absolutely can benefit from "self correction suspension dynamics" which is what most modern complicated 5 link suspension are. they tailor the toe through the jounce and rebound travel for the desire effect. i believe the 993 rear end has some attributes of it. the 928 also did with deflecting suspension bushings in creating toe vs toe out of the semi trailing arm under braking to tame the rear end.

for practical purposes there are enough worms out of the can, so dont tread there with "dynamic suspension" and lose the unique quirky feel that many of use love. anyone here besides me prefer the feel of the 964 over the 993 besides me?

my 2 cents
Old 05-14-2018, 05:52 AM
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Quote:
Originally Posted by chrismorse View Post
If we aim for a "more sporting street useage" mid year chassis set up, that is starting from stock height/alignment how much can we lower the front/rear without adversely impacting linear high g cornering?
First you have to toss out is the notion that there's a magical point where things switch from "good" to "bad." Everything is a gradient.

So there's an ideal point where the RC to CG point is a good distance, and the roll axis created by the front and rear roll centers is good, etc. Raising or lowering the car slightly from these points is likely to make an indiscernible impact on the car to most drivers in most situations.

Now it's not necessarily linear, either. You can't for sure say "2cm didn't change it that much, so 4cm won't either" as the fat end of the curve is likely centered near euro height and things start going south real fast the further you stray from that.

IMO, you're more likely to run into handling problems caused by running out of suspension travel and smashing bumpstops from excessive lowering before you notice handling differences from roll center changes.
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Old 05-14-2018, 10:00 AM
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In addition to Driven's comments above, there are other things to consider when you take a suspension to the limits of its designed working range. For example when you lower a car more extremely, and especially if you put larger diameter wheels/tires on it. Among the obvious things is a tendency to bottom out, but then there is bump steer. And one people don't think about as much - scrub. Not scrub radius, but scrub (they are different). Scrub is the side to side movement of the wheel/tire as the suspension goes through its travel range. Scrub is usually more at the extremes of the suspension travel range (e.g. a very lowered car), and the larger it is, the more adversely it affects handling.

Raising the spindles to lower the car helps avoid all of this.

Scott
Old 05-17-2018, 04:18 AM
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The OP asked the question as an “either torsion bars or coil overs”. The reality is that Porsche always used torsion bars on air cooled 911’s even with coil overs. This introduces another factor when determining suspension dynamics. Also, the standard raised spindle was 19mm as a compromise attempt at keeping balance. The more extreme changes like 30mm created a whole new set of challenges typically only used for racing.
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Old 05-17-2018, 06:28 AM
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