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PostPosted: Sun May 13, 2012 12:53 am 
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I have played around with some of the free “wishbone” design programs but find them hard to use. I want to have a go at designing it in solid works.

I can get the SAL, camber change and roll centre when working in bump and droop but I’m not sure how to set it up for body roll.

What I have done so far is draw the chassis off a centre line. I can then change the angle of the centre line between 89-91 deg to give 1 deg of roll.

My question is where do I pivot this centre line from?

My first attempt is to pivot it from my static/natural state roll centre point. i.e. 25mm above the ground and keep the distance from the chassis to the RC the same.

My car is a sports prototype.
Target 1 deg of roll 25mm bump.
RC approx 25mm
Uprights are 200mm rod end to rod end (13’ rim) bottom wishbone is 350mm long.
Track is 1480mm


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PostPosted: Sun May 13, 2012 1:09 am 
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Below are 2 pics of how I have done it so far by pivoting around the RC point when the chassis is at normal height and with no roll. Is this correct?
25mm RC then 150mm from RC to chassis/ pivot height 175mm
therefore centre of the chassis/pivot height will be 25+150cos(roll angle) mm above the ground
(for my examples i have only added in the pivot height/width not the full subframe)


Attachments:
File comment: Close up of pivoting around the natural RC
design 2 25mm rc2.jpg
design 2 25mm rc2.jpg [ 21.96 KiB | Viewed 875 times ]
design 2 25mm rc.jpg
design 2 25mm rc.jpg [ 38.69 KiB | Viewed 875 times ]
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PostPosted: Sun May 13, 2012 3:28 am 
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I did most of my suspension stuff in CATIA, SolidWorks' big cousin, but it's pretty similar for stuff like this.

For my 2D sketches, I had the midpoint of the bottom line of the chassis constrained at whatever ride height I demanded (127mm for design, 75mm for ~2" bump, etc), and then I had that bottom line constrained at an angle to the fixed ground line. That gave me control over the roll and bump of the chassis. I then constrained the centerpoint of the tire contact patch to the ground line to provide all the changes in geometry, etc.

From page 6 of my build log. This is my static setup, where I input track width, static camber, etc to get control arm numbers; there's a "disturbed" sketch below it that takes those control arm numbers and locations, and then shows how camber, roll center, track width and stuff changes with roll and bump:

Image


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PostPosted: Thu May 24, 2012 5:44 am 
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With my design im trying to minimise camber change as it goes through bump. But what should I do with the roll centre? Should I target it to stay in the same height above the ground or should I try and target it to move at the same distance as bump? i.e. the roll centre drops 1 closer to the ground for 1 inch of bump?


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PostPosted: Thu May 24, 2012 9:30 am 
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explode64 wrote:
With my design im trying to minimise camber change as it goes through bump. But what should I do with the roll centre? Should I target it to stay in the same height above the ground or should I try and target it to move at the same distance as bump? i.e. the roll centre drops 1 closer to the ground for 1 inch of bump?

That's easy, make both arms parallel and equal length. However, I don't think you really want to do that because in roll you'll then get zero camber compensation. In general, you want the roll center to stay at a fixed location relative to the chassis, though some designers have altered that rule to say it should remain at the same height (relative to the chassis) but don't worry about it moving sideways.

You have to face the fact that this type of suspension is a compromise and nothing you can do will ever result in it being perfect for every situation.

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PostPosted: Thu May 24, 2012 10:53 am 
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KB58 wrote:
You have to face the fact that THIS TYPE OF SUSPENSION is a compromise and nothing you can do will ever result in it being perfect for every situation.



Is there another type that is better?

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I can explain it to you,
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PostPosted: Thu May 24, 2012 11:19 am 
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Short of active suspension, no.

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Kimini book: Designing mid-engine cars using FWD drivetrains, http://www.kimini.com/book_info/


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PostPosted: Thu May 24, 2012 11:50 am 
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Location: SW West Consin
Even active needs a little camber gain, minimal scrub and a roll center that doesn't jump all over the place. You still have to attach the wheels :lol:


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PostPosted: Thu May 24, 2012 1:25 pm 
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Location: Mountain View, CA
vroom wrote:
Even active needs a little camber gain, minimal scrub and a roll center that doesn't jump all over the place. You still have to attach the wheels :lol:


With active suspension, you could design for very low, stable RC with long parallel A arms (maybe tweaked to minimize scrub in bump), and get camber gain by tilting the chassis into the turn slightly.

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