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Click here to view this topic in its original format _ 914World Garage _ What about Roll Center?

Posted by: Chi-town Sep 13 2018, 08:50 PM

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.

Posted by: dr.tim Sep 14 2018, 06:21 AM

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.

The best resource I have on my shelf for this is the Milliken book:

Posted by: Chi-town Sep 14 2018, 08:17 AM

Thank you!

Sorry I always get the terms mixed up.

I will look into the spindle and trailing arm mods.

Posted by: 914forme Sep 14 2018, 01:18 PM

agree.gif With Dr. Tim,

If your going low you will also need to look at raising the rear suspension pickup points and upper shock mounts.

Again Tangerine Racing is your stop for these items also.

Changing the camber on the arms will work to a degree but really you need to move pickup points to make the most of the suspension.

My EG Conversion
Raised front spindles, adjustable for bump steer, camber boxes
Raised rear pickup points, Raised rear shock towers, and redone rear arm cambered for the tires I will be running. Chris knows this stuff.

Don't forget semi solid non binding bushings, none binding sway bars and all the other items that make for a proper suspension and setup.

Posted by: jd74914 Sep 14 2018, 01:47 PM

QUOTE(Chi-town @ Sep 13 2018, 09:50 PM) *

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've modeled the whole suspension and extracted roll centers, etc. as have a few other people. Have to look into my notes/model later to get you some numbers.

Extended lower ball joints are scary since you're really increasing the moment arm. I know they are popular among the VW crowd, but they really are a poor engineering solution.

You can see some of the suspension mods to my car (driven by the desire to drastically lower the car) in my extremely-slowly progressing build thread:

IPB Image

Posted by: larryM Sep 14 2018, 09:07 PM

there is an exhaustive technical discussion of this in several articles in Circle Track magazine of a few yrs ago

it is worth finding & reading

or just google "roll center"

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