Roll centers - Page 2 - LotusTalk - The Lotus Cars Community
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post #21 of 49 (permalink) Old 07-17-2009, 10:46 AM
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No, if one control arm in completely horizontal the lines should still cross. They would only not cross if both control arms are horizontal which would not happen.

IN this formula, if either control arm is horizontal it gives an error, so something seems wrong.

In your diagram, the lower arm is completely horizontal and all the lines cross and the roll center is not zero.
Sorry I misunderstood, I thought you meant both horizontal. let me work on that, it shouldn't be hard to fix
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post #22 of 49 (permalink) Old 07-17-2009, 11:02 AM
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Ok try it now

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post #23 of 49 (permalink) Old 07-24-2009, 08:02 PM Thread Starter
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I'm taking my measurements of this. Where would be the proper place to measure the knuckle ball joint height? The point where the knuckle bolts to the knuckle or the center of the balljoint on the a-arm?

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post #24 of 49 (permalink) Old 07-25-2009, 05:23 AM
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I'm taking my measurements of this. Where would be the proper place to measure the knuckle ball joint height? The point where the knuckle bolts to the knuckle or the center of the balljoint on the a-arm?
The pivot point is what's important, which is probably about the center of the joint.

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post #25 of 49 (permalink) Old 07-26-2009, 07:45 AM
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I made a mistake in the logic try this update. I'm going to run the calculation manually

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post #26 of 49 (permalink) Old 07-27-2009, 02:00 PM Thread Starter
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I believe there is still an error in the formula. In this formula, the roll center is higher off the ground as the car gets lower (distance to the control arm pivots shortens).

So, something does not seem right.

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post #27 of 49 (permalink) Old 07-27-2009, 02:33 PM
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Didn't someone say this is an extremely simple calculation?



It's not. Especially if it needs to work with any suspension, symmetrical or not. It took us thousands of lines of C++ code and many months to perfect it in Speed-Wiz.


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post #28 of 49 (permalink) Old 07-27-2009, 06:43 PM
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OK so the spreadsheet did have a small error. I fixed it and even graphed several examples to check and it looks fine with the new rev. Even still as you lower the car the roll center gets higher, and that’s because the lower control arm is already pointing up so as you lower the car more it points even higher by the time the line projected intersects that the intersection point gets higher. It's counter intuitive, but looks correct.
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post #29 of 49 (permalink) Old 07-27-2009, 06:49 PM
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Didn't someone say this is an extremely simple calculation?



It's not. Especially if it needs to work with any suspension, symmetrical or not. It took us thousands of lines of C++ code and many months to perfect it in Speed-Wiz.

I personally believe just the roll center it straight forward. It's a couple of lines intersecting. Although I have had to revise it like 6 times.

I do believe its functional now
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post #30 of 49 (permalink) Old 07-27-2009, 09:33 PM Thread Starter
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OK, so I did accurate measurements on the location of my control arms.

Based on the formula, my front roll center is 13.3mm off the ground.

I am at 110mm ride height and have shorter than stock tires (225/45-15 Hoosier A6)

However, if I was to raise my ride height to LSS spec (120mm), my roll center would be lower = 10.5mm above ground. And at the base car ride height of 130mm, the roll center would be 7.3mm above ground.

This of course seems illogical. Lowering the car makes the roll center higher in this formula?? I don't see how that should be possible.

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post #31 of 49 (permalink) Old 07-28-2009, 04:28 AM
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Originally Posted by trompazo View Post
OK, so I did accurate measurements on the location of my control arms.

Based on the formula, my front roll center is 13.3mm off the ground.

I am at 110mm ride height and have shorter than stock tires (225/45-15 Hoosier A6)

However, if I was to raise my ride height to LSS spec (120mm), my roll center would be lower = 10.5mm above ground. And at the base car ride height of 130mm, the roll center would be 7.3mm above ground.

This of course seems illogical. Lowering the car makes the roll center higher in this formula?? I don't see how that should be possible.

Yea I did not expect that result either. That was why I did some double checking but here is a bad (really bad) pic that might help. The left is the car at stock height and since the LCA is already point up towards the wheel as you lower it more it point way up so the green lines are the control arm projections and the yellow is the line you draw to the opposite wheel. so as its slope increases it raises the height of that intersect point and the roll center.
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post #32 of 49 (permalink) Old 07-28-2009, 04:35 AM
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Also if you look in the spreadsheet m1 is the slop of the lower control arm m2 the slope of the upper y2 is the y intercept of the upper (the lower I defined to intersect thru the origin. And I think the rest you can pick off the diagram.
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post #33 of 49 (permalink) Old 07-28-2009, 12:29 PM Thread Starter
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Since the lower control arm is higher at the balljoint than the chassis pivot, and the upper control is essentially flat, the lines intersect outside the plane of the car, correct?

So a line drawn from that intersection point to the middle of the tire doesn't pass through the middle of the car. So I am not sure how it calculates the roll center in the middle.

The control arm angles seem odd.

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post #34 of 49 (permalink) Old 07-28-2009, 01:41 PM
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Originally Posted by trompazo View Post
Since the lower control arm is higher at the balljoint than the chassis pivot, and the upper control is essentially flat, the lines intersect outside the plane of the car, correct?

So a line drawn from that intersection point to the middle of the tire doesn't pass through the middle of the car. So I am not sure how it calculates the roll center in the middle.

The control arm angles seem odd.
The first part you said is correct,
but that projected line can go thru the car. The roll center which is just a point in space could be on the inside of the car. If you look at some of the variables on the sheet like h, see how high it gets, and L how far away you have to project before the upper and lower control arm lines intersect. If those lines are intersecting really high, like higher than the height of the car (because they both pointed up) you will defiantly have to draw a line thru the car to the tire on the opposite side. Usually I don't think this would happen but our cars so low to the ground I guess it's possible.
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post #35 of 49 (permalink) Old 07-28-2009, 02:02 PM Thread Starter
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Yes, but isn't the roll center located at the point where the control planes converge with the line that goes to the tire on the same side of the car?

You can't just draw it to the tire on the opposite side, since technically that would not be the roll center.

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post #36 of 49 (permalink) Old 07-28-2009, 02:12 PM
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I don't quite understand the picture showing how roll center moves when the car is lowered. Roll center rises in the picture if the wheel is lower, but if the car is lower then the wheel must be higher? Right? Am I reading to much into a hastily drawn picture? It seems to me that as the car is lowered the lower control arm becomes flatter, lowering the roll center.
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post #37 of 49 (permalink) Old 07-28-2009, 03:09 PM
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I don't quite understand the picture showing how roll center moves when the car is lowered. Roll center rises in the picture if the wheel is lower, but if the car is lower then the wheel must be higher? Right? Am I reading to much into a hastily drawn picture? It seems to me that as the car is lowered the lower control arm becomes flatter, lowering the roll center.
Typically yes but not if the arm is already pointing up, then the arm points even higher as the car is lowered. The picture is pretty poor it's actually backwards, the box is meant to be the body of the car and the wheel is not drawn.
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post #38 of 49 (permalink) Old 07-28-2009, 03:17 PM
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Yes, but isn't the roll center located at the point where the control planes converge with the line that goes to the tire on the same side of the car?

You can't just draw it to the tire on the opposite side, since technically that would not be the roll center.
Yes same side of the car I think I messed that up, so what would you do as the LCA goes from our previous picture to this scenario? (The picture is over exaggerated)

Would the roll center be below the ground?
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post #39 of 49 (permalink) Old 07-28-2009, 03:30 PM
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the box is the car not the wheel! now it makes sense to me
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post #40 of 49 (permalink) Old 07-28-2009, 03:43 PM
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the box is the car not the wheel! now it makes sense to me
Yea really really really bad picture.
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