914 Suspension motion ratios, Comparing notes Options

[groot]

I measured my front and rear motion ratios yesterday and wanted to compare notes with any that have done the same.

For the rear suspension I took ~20 data points and did a best fit analysis to come up with 1.00 to 1.00 damper travel to wheel travel with strong statistical correlation.

My front geometry is a bit different than a standard 914, so the motion ratio should be different, too. I measure ~10 points and found 0.83 damper travel to wheel travel, again with strong statistical correlation.

I'm about to order my rear shocks and this is one of the bits of data they need to build the shocks.

[groot]

I'm going with Koni 30s in the rear. They have an incredible range of adjustment.

I know we've beat the RC discussion to death a few times, but let me add a few statements from what I've learned.

The front roll center on a lowered 914 front stock suspension is below the ground. Bad, very bad. There are several approaches to fix this. Eric mentioned one method.

The rear roll center is higher than the front...that's okay, it's supposed to be that way. The rear roll center, from what I've read, is determined by the angle of the mounting points to the chassis in the horizontal plane, in our case 12 degrees. Another line is drawn from the intersection of the 12 degree line with the wheel centerline to the center of the tire patch. The roll center is where this line intersects the center line of the car. So, the rear roll center is between the ground and the center of the wheels and it's height is determined by the angle of the trailing arm points relative to the wheel. BTW.... thanks to Brett W for helping me figure this stuff out. We traded many e-mails working through this discussion.

So, raising the points in the chassis does nothing for the rear roll center. But, raising the points does change the angle of the trailing arm relative to the body, which is where there can be a benefit.

Gillespie says that if a semi-trailing arm is angled up front the rear wheel, it contributes to roll oversteer, if it's level, neutral steer, and angled down from the rear wheel, roll understeer. And this is due to the track change at the outside wheel. However, Gillespie does not quantify this.

Now, we disagree about the severity of this effect.