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  #16 (permalink)  
Old 09-25-2009, 04:54 PM
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This is becoming VERY interesting! Doug referred me to Olley's roll steer discussion, beginning on page 169 of the Millikens' Chassis Design. He explains roll steer on the basis of roll axis slope AND on the basis of instant center relationship to the axle. If you have the book, I'm going to consider the 4link picture on page 179, but with the lower links parallel in plan view. This would provide roll oversteer, according to Olley. But, I'm going to consider the instant center well below the axle centerline, which...according to Olley's torque arm explanation...would cause roll UNDERsteer. Then, I'm going to attempt to add the effects, using, if necessary, CAD to validate my conclusions.
http://www.racetec.cc/shope
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  #17 (permalink)  
Old 09-28-2009, 07:57 AM
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There are two effects (causes). It would appear that the roll axis effect would be the larger, though the instant center effect is certainly significant. The linkage programs used by today's suspension design engineer would catch it, but, for those of us who must use cruder tools, it's nice to know. Here's an example:

The car has a rear roll axis that is inclined 0.349 radians up from the rear (roll oversteer). The roll angle is 0.0333 radians. This would mean that, in a left hand turn, the axle would rotate clockwise 0.349 times 0.0333 or 0.0115 radians

The instant center is 60 inches from the axle centerline and is 10 inches below the axle centerline. The angle of the equivalent torque arm would be 0.1675 radians angled down from the rear (roll understeer). The spacing between left and right torque arms is 45 inches. The change in angle of the torque arm would be determined by the product of the roll angle and the ratio of half the arm spacing to the arm length.

axle rotation = P{cos (B - S(phi)/(2P)) - cos (B + S(phi)/(2P))}
---------------------------------------------------------------
S

where "P" is the torque arm length, "B" is the radian measure of the orignal torque arm position, "S" is the lateral spacing of the torque arms, and "phi" is the roll angle. The "S" and the fraction line should be shifted to the right of the equals sign, of course.

This comes out to a counter-clockwise axle rotation of 0.00555 radians.
http://www.racetec.cc/shope
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Old 10-02-2009, 01:58 AM
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So would the results from both equations apply and be additive? I really have wanted to get Olley's but haven't yet. Maybe this will prompt me to get it.

The problem is the equations only work with small angles of roll. I am working on something right now that can get 25.5deg. of pure roll and a max axle to chassis angle of 27.9deg. (it has a little more droop travel than bump). At 25.5deg. of roll it has 0.14deg. of axle steer in the roll oversteer direction. At ride height it has a roll axis very slightly on the side of roll understeer but does have the IC higher than the tire radius with slightly triangulated lower links. Next I need to design an anti-roll bar that will stand up to that much roll angle and not break not to mention handle the 24+ inches of wheel travel.

Then sometime I need to figure out where your anti-squat equation and mine don't mesh... I don't see it.
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Old 10-02-2009, 05:13 AM
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Quote:
Originally Posted by Triaged
So would the results from both equations apply and be additive?
Yes. When I was in school, I was taught the instant center effect, with no mention of the roll axis effect. Olley and RCVD talk of the roll axis effect, with no mention of the instant center effect. But, yes, they both exist and the effects are additive.
Quote:
Originally Posted by Triaged
The problem is the equations only work with small angles of roll.
Is this true? When you go from sine to angle in radians, a small angle assumption is made, but, when you start and continue with radians, I'm not certain you're limited. Still, with your application, I would think development of a linkage program...and abandonment of these simple equations...would be justified.

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Originally Posted by Triaged
Then sometime I need to figure out where your anti-squat equation and mine don't mesh.
I mentioned that I verified with CAD. Have you used your CAD to verify my understanding of your numbers?
http://www.racetec.cc/shope
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Old 10-02-2009, 09:05 AM
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Quote:
Originally Posted by BillyShope
...But, yes, they both exist and the effects are additive.
Thanks for that. I had never even heard of the IC effect till now.

Quote:
Originally Posted by BillyShope
Is this true?
Wouldn't it have to be?

Quote:
Originally Posted by BillyShope
Have you used your CAD to verify my understanding of your numbers?
I haven't. I have been trying to do it by hand.
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  #21 (permalink)  
Old 10-03-2009, 08:13 AM
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Quote:
Originally Posted by Triaged

Wouldn't it have to be?
Yes, it probably would. The 4link is moving the IC around and, with the instant center effect, the rate of change of axle rotation to roll angle is changing drastically.

You definitely need a linkage program. Sounds like a fun project, but I've too many others in progress. Plus, I suffered a TIA a week ago, which adds urgency to my work.

My ancient version of SolidWorks might be able to handle a 3link assembly. If I get a chance, I might give it a try.
http://www.racetec.cc/shope
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Old 10-03-2009, 10:00 AM
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I love reading all this. Makes me miss engineering a little. Maybe just a little. haha I too have SolidWorks and Cosmos

It must be raining hard for you guys to be sitting and pondering all this stuff. It's good however.

Been raining here too but the sky is getting lighter. There is stretch of pavement not far from the garage that needs some rubber repair that I'll quickly take care when it is dry. I have a 4 link that needs some adjustment so I'll toss the jack, stands and some tools in the trunk for an afternoon of test and tune.

Billy...I too had a soapbox but I was a bit too heavy so it became a streetracer. We had a steep hill so we could get some good speed ..for us. Fast enough to toss the skinny orings off the soapbox wheels. I convinced my dad that I needed the big tires for my weight. Got them after a number of mowed lawns. I'm probably the only guy that has ever rolled a soapbox. There wasn't AAA back then so a couple of us carried the soapbox home. Among scrapes and bruises I got a rear end tanned and grounded for a week.. haha Pretty funny today.

anyway good discussion, keep it going.
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