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| Is the Ackerman angle on the 88 really bad? It looks it. (Page 4/5) |
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lou_dias
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JUN 30, 11:12 AM
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Yellow-88
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JUN 30, 07:45 PM
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Hi
It’s called suspension geometry because it’s all angles and arcs. It's theoretical lines drawn in two dimensions. It’s all about figuring out how to keep the tire contact patches in complete contact with the road while the suspended part of the chassis is being affected by dynamic outside forces. Tell you what... You young CAD guys have no idea how beautiful the modeling software has become. Chevy Chevette suspension was probably drawn with T squares and triangles, on paper, figured out with slide rules. I give them credit. (Does anyone still have your first slide rule?)
Google suspension geometry / images and you’ll see that there are a lot of ways to design a double A- arm suspension, all with the same goal. Compensate for camber change and bump steer caused by roll angle. It’s in the geometry design drawings were you see terms like steering axes inclination, instant center and roll center. In a nut shell some designs are better than others but, all designs are specific to a purpose. The 88 front end was designed specifically for a sports car.
The roll center is a point in space about which the sprung mass of the chassis rotates as presented in two dimensional geometry. Its location is determined by the designer’s choice of control arm configuration. The roll center is dynamic in that it moves relative to the degree of longitudinal roll. Where it’s located and how dynamic it is, is part of what separates one design from another. There is a front and a rear roll center so connecting them establishes the roll axes for that particular suspension geometry front rear combination.
Up to this point, this chassis is still rolling in a straight line. The “forces that cause roll” are not from cornering forces, they're from rolling on a tilted surface.
I think we can call it "simulated" Lateral G force. ( ?? )
Before we add steering forces into our virtual chassis, there’s one more important concept that involves “steering axis inclination” geometry. That is Scrub Radius.
If you draw a line between the upper and lower ball joints and project it through the road surface plane, you have defined the “steering axis”. Ideally, its “inclination” will pass directly through the center of the tire contact patch. That would be zero scrub radius. It "feels" really, really nice, if you can do it. We'll visit it later.
So …. It’s able to roll perfectly straight on bumpy undulated surfaces that are sometimes tilted to pretty steep angles that are still bumpy and undulated and still continue to roll straight. OK. It’s time to see how it corners. We need another force.
If you’re waiting to hear about how to build the ultimate 88 Fiero, stay tuned. I think it’s extremely important to understand why you do something, way before you do it.
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Yellow-88
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JUN 30, 07:50 PM
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IMSA GT
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JUN 30, 08:59 PM
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So back to the very first post. What was your goal here? Retrofit Solstice knuckles?
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ricreatr
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JUL 03, 11:57 PM
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Well, the goal was to use better hubs. Yes, i am toying with solstice knuckles. Also c5 knuckles. My solstice knuckles had been machined for c5 hubs. Maybe 5x120 wheels are a good choice?
Anyway, The steering geometry refresh is just what i needed. I keep trying to get greedy with camber gain. Raising the upper ball joints too high.
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lou_dias
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JUL 05, 03:15 PM
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I heard WRX 2004ish front knuckles are similar to 88 rear knuckles...
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Yellow-88
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JUL 05, 06:22 PM
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| quote | Originally posted by ricreatr:
Well, the goal was to use better hubs. Yes, i am toying with solstice knuckles. Also c5 knuckles. My solstice knuckles had been machined for c5 hubs. Maybe 5x120 wheels are a good choice?
Anyway, The steering geometry refresh is just what i needed. I keep trying to get greedy with camber gain. Raising the upper ball joints too high. |
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I imagine that you mean the sealed wheel bearing / hub unit that bolts into the cast iron "knuckle". Better meaning more meaty? If I may ask, why?
You aren't the only one looking for more camber gain, meaning negative camber. That may be a hang over from the days of tall floppy tires. It's also a natural thing to do as bipedal creatures. We tend to angle our legs against the outside of a turn, or brace against steep down will. In a car, the goal is keeping your footprint as flat on the ground as possible .... always.
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Yellow-88
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JUL 05, 10:17 PM
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So our suspended chassis is able to roll perfectly straight on bumpy undulated surfaces that are sometimes tilted and are still bumpy and undulated and still continue to roll straight. OK. It’s time to see how it corners. To do that, we need a new term. Slip Angle.
Our test track is a straight line with a section that is banked, like a banked corner but straight. It is mildly undulated with an occasional bump as its surface.
As our chassis travels onto the banked section, its roll angle increases, transferring weight. A well designed suspension system adjusts camber angle to compensate for camber change. Zero camber always. That is tire radial plane perpendicular to the road surface plane.
As our chassis travels into steeper and steeper banking along the straight line, at some point the tire contact patches begin to “slip”. They are responding to (“G-Force”) pushing them beyond their limit of “absolute” adhesion. If we hold the bank angle (roll angle) steady, keep traveling and trace the line that the contact patches make relative to the straight line, we’ll see an angle forming. The higher the chassis longitudinal roll angle, the higher the contact patch Slip Angle. The chassis is still following the straight line because its roll axis is parallel to the straight line. It’s slipping sideways, away from the straight line.
So …. How do we get this thing off of a straight line?
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fieroguru
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JUL 06, 08:18 AM
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| quote | Originally posted by lou_dias:
I heard WRX 2004ish front knuckles are similar to 88 rear knuckles... |
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It would be the rear, not the front. The 88 rear uses a tri-link suspension with 2 lateral and 1 trailing links per side. There is no lower ball joint, which is what is used in 99% of front knuckles to enable turning. The rear WRX knuckle is also a tri-link design, but offsets and angles are different, which will make retrofitting one to a Fiero more of a challenging than most are willing to undertake.
Here is what the WRX rear one looks like.
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BillS
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JUL 06, 03:02 PM
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I owned an 87 and 88 Fiero GT at the same time. Easy win for the 88. Sold the 87.
I also owned the 88, which had the suspension modified (Konis + sway bars front and rear) and had a 300 bhp turbo engine at the same time as I owned (and still own) my 09 Solstice GXP coupe with 375 bhp (yes, also somewhat modified) and with nonstock sway bars and shock/springs) so I can confirm that the Solstice is a better handling car but the Fiero done the way I had it was no slouch either, and was head and shoulders ahead of the pre-88 Fieros, and was possibly the equal of the GXP in dead stock form before I improved it.
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