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I've always been leery of the roll center adjusters. It seems like they would make the instant center move more vertically due to the greater difference between upper/lower arm angles.
Any suspension engineers on board? There is much discussion from armchair quarterbacks stating the obvious that lowering a car on springs or coilovers changes suspension geometry. Now that we have some pics to work with can somebody explain the actual impact? From my assessment of the top photo:
Segment S (shock length) shortens along its axis
Segment CS (ball joint to upper shock mount) shortens to meet the new lower location of the upper shock mount.
Segment C (Upper A-arm to ball joint) shortens due to lowering the body and thus the upper mount location.
The included angls between S/CS (caster?) and U/C become more obtuse? Does it simulate a squat like compressing the spring? What is the impact?
Bill
I've done some suspension design work for some race cars, and currently work for a tire company doing development work. The question you asked really opens up a huge can of worms, but I will try to make it as brief as possible.
1. The biggest thing you are going to do is lower your center of gravity. (Duh.) This benefit is HUGE. Do not underestimate this.
2. Lowering the car will increase the static camber. Remember that if uninhibited, a control arm will rotate in a circle about the inboard pivot point. Shorter A-arms will rotate around a tighter circle, so think about the two and see how camber will change as the chassis travels up and down.
3. As far as knowing Instant center/roll center movement, you have to have fairly accurate points for each corner in space. Just by looking at the pictures, your front IC will drop and shorten up, and roll center will drop as well. The rear looks like the instant center will move in even more, but have less drop in rear roll center. Collectively what this will do is act as an increase in roll stiffness in the rear. When selecting spring rates/arb rates, you would probably want to go with a softer combination of the two in the rear relative to the front to make up for this in order to keep the balance on the car correct. You should also adjust the static camber settings on the car to compensate for the movements of the geometry. This will take playing with a good bit, but I would start by making the rear a tad more positive than stock to make of for the increase in camber gain (like 0.5 degrees or less), and go with the fronts at the stock setting.
5. This is THE most ghetto suspension analysis I've ever done. In no way do I know that this will work. If someone wants to measure the points in space relative to the center line of the car at the ground plane, I could do a lot more. The more accurate the points the better. Unfortunately, my S is my daily driver and cant take it apart to measure everything (although I would love to..) Also, each driver likes a car set up differently. What works for you may be too responsive or too stable for the next guy. The biggest thing is try it! Lowering the car has huge benefits in terms of performance, but you must be willing to play with the set up on the car in order to get the full benefits out of it.
I know this is extremely crude, but an explanation would take days of talking about this, and I can ramble on forever. If you have any questions, feel free to ask. Also, please feel free to correct me in anyway shape or form.
So one of the racing guys mentioned in a recent post that the car responded well to lowering as much as they could. The above drawings show that it is, in conventional wisdom of level arms, a 1.5 cm drop. In practical terms, dropping too far exposes the car to road debris even on the track. Is there a standard drop that keeps decent geometry but minimizes risk?
what do you mean by conventional wisdom of level arms? One of the (many) factors in designing the suspension geometry is determining a level of "scrub" the tire will see in vertical motion. You can also think of this as a change in track width. Alot of this is dependent on the tire, and the characteristics you are looking for. Usually, you have a wear consideration to take into account. The scrub can also contribute to heat generation, so consideration has to be taken that whether or not this will improve or reduce the performance of the tire.
The angle of the control arms also have an effect on the car's camber gain in roll, bump, and pitch. Since the upper arm is shorter, (as it is in almost every double wishbone set up) the upper ball joint will move towards the chassis at a faster rate compared to the lower ball joint. This means more camber gain. However, depending on the angles, this could be progressive, digressive, or mostly linear. In a pure race car, you would tune these gains to meet the characteristics of the tire.
Finally, as mentioned previously, your roll centers/instant centers will change. Instant centers really deal with your camber change and scrub (along with a few other things) which was just discussed. The Instant centers also define the roll center, but we will refer to as a separate thing just for simplicity. The Roll center (and no the car does not really roll about the roll center) serves as a good approximation on how much weight transfer will be moved through the suspension links, and how much will be moved through the springs. If the roll center is at the center of gravity height, roughly all of the weight transfer will be transmitted through the suspension links, and none through the springs/arb. If the roll center is on the ground, roughly all of the weight transfer will be through the springs and none through the suspension links. While you can play with the height of the roll centers to improve the handling, the most important thing is that the roll centers stay stable as the car pitches/rolls/heaves so the balance of the car stays relatively the same at all points in time. If people have slammed the car and raced it, and not complained of rapid transitions of balance characteristics from turn in to mid corner/steady state, then i would assume its fine for most drivers. Alot of the time, too much emphasis is placed on roll centers. While the roll center can shed alot of light on several areas of suspension design, it is still just a single part of the puzzle.
As far as what is a good compromise on ride height, it all depends on where you drive. Having just moved out of downtown atlanta to greenville, sc, I would be willing to lower my car at least a half inch lower in greenville than in atlanta (the roads suck there). Same goes true for a racing. Sebring will require a different ride height than VIR, and that exact ride height will be dependent on your spring and damping rates. There is no exact answer on what is a standard drop. If you want an easy way, see if you can find somewhere a standard speed bump height on your states highway regulations or the federal one. That might be a good starting point.
In short, lowering your CG (by means of a ride height reduction) will 100% of the time always reduce your weight transfer, which will thereby increase grip on both axles, as long as you don't destroy the camber set up on the car. Camber is always adjustable/tunable (as long as you have the kit), so once you lower the car you can play with your camber to find the optimum setting for your ride height choice.
I could not give a crap about theory, or state bumper requirements. I have been assembling SCCA suspensions for 2 decades now. I have seen subtle changes make vast differences, often counter intuitively (just look at the F;R spring rate debate for natural frequency bump response).
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The problem, as several people identified above, is that measurements needed to be accurate and precise require a whole bunch of things I don't have access to any more. However for a rough spring rate (within 50-100 lbs) for the same 2000 chassis, you can make assertions about ride height. This is what I'm looking for.
01-14-2008, 01:32 PM
