I finished off my last post part way through showing how my modified suspension behaved as compared to stock, at least theoretically. This post is about the rest of the parameters starting with caster.
CASTER VS BUMP
For the front, it's very important so I'll expand on it briefly. Caster is the inclination of the steering axis from the side view. In other words if you were to draw a line between the upper and lower ball joints in the side view, the angle the line forms away from vertical is the caster angle: I didn't get into the details of what caster is in my posts about the rear suspension since it really doesn't have much effect in non-steerable wheels.
If you were to extend the line to the ground, caster is said to be positive if the point falls ahead of the center of the tire contact patch. Positive caster has a stabilizing effect by making it progressively harder to turn the steering wheel as the steering angle increases (when in motion). It's also what causes the steering wheel to return to center after making a turn. Negative caster would do the opposite and clearly wouldn't be a safe suspension characteristic. The stock car is set up during alignment to have 5 degrees of positive caster at ride height so that's why this next graph shows 5 degrees at 0mm of bump travel.
Ideally, the caster angle shouldn't change as the suspension is bumped, so the actual performance of the stock suspension isn't perfect. This non-ideal characteristic is a by-product of having tilted the upper control arm mount as seen from the side view. It's one area where a compromise must be made to gain some anti-dive at the expense of caster change.
From the graph above, the modified suspension clearly follows exactly the same caster properties as stock, so no harm done.
ANTI-DIVE VS BUMP
Anti-dive is a front suspension characteristic similar to anti-squat in the rear, which I covered in post #24. Anti-dive reduces the amount the car will pitch forward under braking and is achieved by tilting the upper front control arm so the rear mount is lower than the front mount. Anti-dive is calculated using the following formula:
%Anti-Dive = Front Brake Bias (Tan (A) (Wheelbase Length / CofG Height) X 100
Where "A" is the angle between the ground and a line drawn from the centre of the tire contact patch up to the longitudinal instant center. Here's a comparison between the calculations for anti-dive between the stock and modified front suspension.
Street cars typically are designed to have no more than 20-40% anti-dive. Any more tends to bind up the suspension under bump since the greater the difference in tilt between the upper and lower control arms in the side view, the less both arms track together, so they begin to bind.
From the formula, anti-dive in bump decreases with a decrease in the roll center height or the wheelbase, or an increase in the CofG height. The raw data from the Lotus software (not shown) indicates that as the wheel moves up in jounce all of these variables change simultaneously. The roll center and CofG lower at different rates, and the wheelbase elongates despite the center of the front wheels moving backwards. This is because the rear wheels actually move backwards even further. What I'm getting at is that the magnitude of change in anti-dive isn't particularly intuitive. Here's what the Lotus software said:
The stock '88 Fiero has relatively constant anti-dive between 22.5% to 23% through the entire range of bump travel, whereas the modified suspension loses about 0.8% over stock. From where I sit, that's close enough to be insignificant.
ROLL CENTER VS ROLL ANGLE
Finally, the last parameter I'll cover is how much the roll centre migrates as the chassis rolls. (For a refresher on what roll centre migration is, backtrack to post #28 in the "Rear End" section.) A picture, or in this case a graph, says a thousand words so I'll begin with that:
As always, the blue line represents how the stock car's roll centre moves about when the entire chassis is rolled from 6 degrees right to 6 degrees left. GM engineers did a remarkably good job at controlling the front end roll centre since it moves no more than 2.5 mm vertically, and 19 mm on either side of the centreline of the car. The roll centre is located higher in the front at 105 mm above ground level, than it is at the rear suspension which sits only 73 mm above the ground. More on this later.
The modified suspension (shown in green) tracks an even tighter roll centre movement that shifts no more than 1.5 mm vertically and 9.5 mm on either side of the centre line. More significantly though, it drops the roll centre almost 33 mm over stock, down to 72 mm above the ground. Here's a scale drawing showing just how little the roll centres move through 6 degrees of roll... you'll have to look hard for the plots!
While the reduction in roll centre movement is hardly significant, the drop in roll centre from stock to modified suspensions is noteworthy. The reason is because the slope of an imaginary line drawn between the front and rear roll centres, called the roll axis inclination, has an influence on whether the car oversteers or understeers. The stock front roll centre sits 105 mm above the ground at ride height, whereas the rear roll centre (from post #28) sits only 73 mm above ground. That results in a stock roll axis inclination of 0.77 degrees, sloped downward to the rear and contributes understeer. While understeer is a desirable trait in a rear weight-biased car like the Fiero, many believe (as I do) that GM built-in too much understeer.
The modified suspension on the other hand reverses the inclination so that it's higher in the rear at 192 mm and lower in the front at 72. The resulting angle is 2.80 degrees sloping upward to the rear. The change, while small, produces more oversteer than stock, however many other factors also have an impact on whether the car oversteers or understeers overall.
There are many other suspension parameters that could be considered, however in the name of keeping this blog rolling, I'll leave it at that. What lies next are the trials and tribulations of embodying all of these modifications into a physical front suspension system (read: more pictures!). Stay tuned.