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> What about Roll Center?, where is it, how do you guys handle it?
Chi-town
post Sep 13 2018, 08:50 PM
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I think I've ready about a dozen threads relating to bump steer.

I see the issue and it's easily remedied by the aftermarket parts available.

Rear bump steer or "passive rear steer" as Honda called it in the CRX doesn't really worry me.

What I'm noticing in all the images I see of lowered front suspensions is the angle of the lower control arm and the comments about lack of ride quality.

If you look at a pic of a stock height suspension, the arm sits at about a 20 degree angle downwards (estimated).

If you look at a lowered car the angle is usually less than 10 degrees or even parallel with the ground. (eek!)

I know we have a few racers / engineers here on the forum, has anyone calculated the roll center of the car stock / lowered?

Has anyone tried an extended lower ball joint? I've seen de-cambered but not extended.


I'm curious to see what you guys think.
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dr.tim
post Sep 14 2018, 06:21 AM
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The Roll Center (RC) is sensitive to suspension type. For the McPherson strut, the Instant Center is at the intersection of a line extending from the lower control arm and a line 90deg. to the strut axis (Kingpin angle). The RC is then the intersection of the lines for each side (L and R) which connect each instant center with its contact patch (where the strut axis intersects the ground).

As the control arm angle decreases, this extends and lowers the instant center, lowering the RC. Extending the BJ would also have this effect by increasing the kingpin angle.



The utility of Roll Centers (RC) are in the impacts to the Roll Axis (RA) and the Roll Couple.

The RA is the line through the front and rear RCs. As per its name, the RA defines the axis the car will 'roll' around in pure lateral loading (steady state cornering--no acceleration or braking). In most cases the RA is lower at the front of the car than the rear. This allows more weight transfer at the front than the rear, increasing the front grip--desirable because the front tires are the ones doing the turning. The aggressiveness of this effect is dependent on how steep the RA is. (The EVO 7 has a wicked-steep RA to allow the AWD car to turn.)

The Roll Couple is the interaction of the Center of Gravity (CoG) with the RC. A moment (torque) is applied by the CoG on the RC in cornering. In this condition, the CoG is being accelerated laterally, the force it applies is M*a (Newton's 2nd law). It acts to cause the vehicle to 'roll' about the RC. Therefore, and increase in distance between the CoG and the RC increases this torque, increasing roll and weight transfer. As lowering the car increases this distance, it effectively makes the springs softer.

Low or flat control arm angles also have negative impacts to the camber curve and scrub.



Some RC management solutions on lowered 914s include raising the front spindles to lower without changing geometry, and building (cut and weld) camber into the rear trailing arm.
http://www.tangerineracing.com/suspensionservices.htm


The best resource I have on my shelf for this is the Milliken book:
Race Car Vehicle Dynamics




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