Understanding the complexities of aftermarket suspension modifications for a street car.
Suspension modifications for a street Spyder
I am by no means an expert on the subject but I made myself a student of building what I consider the best handling car that im satisfied with for safety and fun on the roads I travel. Much of it was completed around 2008 with some trial and error and I would like to share what I know that is important for food for thought.
I have found in the course of building my car with the choices I made that there is a lot of misinformation on internet forums. People tend to overcomplicated the subject because they really don't know what they are doing but you wouldn't know this because you might be ignorant on the subject and dazzled that someone knows more than you with terminology and complicated math. What people do on the track with radical set ups does not necessarily translate for the street and most often not for a win either. Some of the more conservative set ups have done exceptionally well for both street and track. I ended up reading suspension tuning books and I suggest that you do the same as these books are written by real experts that can make sense of it all so you can grasp sold concepts.
The key to building a great handling car for the street is not to mess with it too much and if you plan to make changes spend money on quality researched parts and understand how they effect the car globally or just leave it alone. Suspension tuning is 5D chess and if you only look at one modification or extreme adjustment you will end up lost in the woods. The more you mess with it the worse it can get but you will not know this because you will equate a stiffer ride with more responsive handling but what you might have done is lower the limits of your car and made compromises to your safety by the unpredictable nature of the car. Any suspension change has some advantages and consequences. It will never be perfect and you might have to give up something to gain something so you need to strike a balance of compromises.
For most street drivers the first thing that most want to do is lower their car for looks and for handling. It can improve the car but it comes with issues that have to be addressed in the alignment. Most lowering springs are a mild way of doing this without having to give up too many compromises as they are usually researched well so you can get a mild performance benefit that is generally safe. The problem comes when we go all in and lower the car further as we are now changing the geometry of the car often for the worst and with it a lot of issues that can negatively effect handling.
The best way I can explain it is to use articles that are to the point, well written and easy to understand. It may not tell you want to do but it will make you understand the consequences of suspension tuning and how difficult it can be so you can make better choices.
First we must understand the roll center. I found this video to the point and it explains it well enough for a person who wants to know the basics. There is a lot more to it but at least it will give you some reference. It demonstrates this for a double wishbone but the fundamentals are the same with our McPherson struts.
Next are two articles I found that address various components regarding weight transfer and body roll. Read the part 2 of the article first as its more to the point.
Now that we have some idea of what is going on its time to talk about suspension for the Spyder as a street car.
In my opinion from what I have done and having driven several set ups of other Spyders my advise is don't mess with it too much. Don't go crazy with camber, extremely wide tires or a set of cheap coilovers. All of these things will not make your car handle better just because you can throw money at it. Make your changes gradual so you can see what they do and if its something undesired you know how to go back so you don't get lost. If you plan to buy coilovers buy ones that are made for the street that have been researched for the street for the application. This doesn't mean some shop that put a set together and drove around the block. It is best to get a set from a reputable company that has used suspension engineers and sophisticated computers to determine the proper damper and spring rates. This suspension will generally be soft enough so it can maintain grip to keep the tires on the road because the road is not the track. Often the cheap stuff is not matched with the springs or the car, therefore has no give, will lose traction and will be a big mess not to mention it will be a rough ride. If someone has a radical track set up and they advise you to do the same think about what you are getting into. The best set ups for the street are the most conservative to match street tires. Where people make mistakes is the foundation of their build and what ends up happening is a downwards spiral of adding wider and wider tires, using extreme alignment setting and removing the rear sway bar to gain a desired result. You don't need to do any of that if you take your time and plan ahead with good knowledge of what you are doing so you can make your changes gradually. A lot of times these modifications are built around consumerism and not all of them are beneficial without context.
What are the goals of a proper street car in terms of handling.
For a street application you are not trying to win any races but at the same time you want to be satisfied with the handling of the car which means it should be challenging but not in a bad way if it makes you feel unsafe. The car should be communicative and with that it should inspire confidence that you can take it to the limits if you have to safely but you don't and will pull back to 8/10 and can still enjoy the excitement of the drive. You don't need to do any changes and can also enjoy this with a stock Spyder but raising the bar can give you more of the fun but upto a certain point where the car will be unsafe for the road due to the compromises so always keep that in check when modifying.
Here is a pretty good link with lots of references related to suspension tuning -> Suspension Tuning
I did my due diligence before liking your post by reading the article three times. It has excellent information.
Here are few points I picked up regarding the Spyder.
At very large values of static camber, the amount of grip in a straight line will start to be reduced, as the contact patch is not loaded evenly across its surface. This is generally not a big deal under braking, but it can be a very big deal under acceleration for the drive wheels, particularly for nose-heavy FWD cars. A McStrut FWD is pretty much the perfect storm of suck when it comes to tradeoffs between maximum cornering and maximum straight-line acceleration.
I had traction issues when I had induced more negative camber. The rear tires lost grip in a straight line acceleration and the front tires lost braking performance. For the street the increase in camber was no benefit when it came to handling because im not pushing it that hard like on a tight autoX course.
People want to do all sorts of things to cram all the camber they can which is the wrong way of going about it when the issues might be other settings or cheap coilovers or sway bar settings. Camber by its self is not a fix for a poor handling car. Camber should be optimized. There is a benefit but when people go extreme it means something else is wrong they are trying to correct for or a case of doing it because its a thing everyone else does.
A soft-sidewall, "mushy" tire (like a tall sidewall bias ply) will typically support a much higher slip angle than will a stiff sidewall, low aspect ratio radial. The direct consequence is that the radial will transition much more crisply as it takes less steer angle to build to the slip angle/grip peak.
This could be the reason why I love stiff sidewalls. For the street its about feel and stiff sidewall tires feel great even when not pushing the limits. With soft sidewalls you have to push the tires hard to feel the load. This in my opinion keeps you safe because you don't have to push those limits to enjoy the tightness of the drive. We do not have better choices with tires that have stiff sidewalls these days because the industry is trying to accommodate larger wheels on factory cars. Most people will not tolerate stiff sidewalls. We are basically dealt a bad hand by those that want bigger wheels and a smooth ride.
Rear toe works the same way, but on most cars (that don't have in-phase rear wheel steering) you don't get the benefit of the steer angle change to mitigate the outside tire's wrong-way slip angle direction. This means that running more than a little bit of rear toe is probably going to induce some degree of a turn-wash-grip cycle on the rear tires. This, however, can make the rear end very lively and can help rotate the car, especially in slaloms.
I like 0 toe in the front tires because it makes the car quick to turn however for the rear having 0 toe or toe out is not good in my opinion for a street Spyder because it can make the rear unsure especially around high speed corners. I like the rear to have a slight degree of toe in because this helps with high speed stability. The road is not full of cones and sharp angles to gain any benefit and the turns I take can be in excess of 60mph where I need stability.
In my opinion, there is a lot of good system information in the link and it needs to be read and understood when trying to balance any suspension dynamics.
As engineers, we like to state that there are no free lunches when it comes to making any changes within any system - every change has an effect on the desired outcome as well as some undesired outcomes.
With that said, a simple change like tire pressure can change an otherwise stable suspension dynamic into an unstable one by changing any number of the suspension boundary conditions or improve the dynamic by expanding its original boundary conditions.
The fun part is trying to figure out what is best for "your" particular set-up even though it may appear to be contrary to what others tell you is perfect.
Mono Craft GT-300 with a few upgrades...
Here it is in a nut shell.
Lowering the center of gravity is good for handling however as a consequence it lowers the roll center.
Lowering the roll center alters the geometry of the suspension arms and as a consequence creates more roll. Roll causes alignment changes under compression and that causes a lost of grip.
The solution is to arrest roll as much as possible by using stiff suspension by employing springs, struts and sway bars. Going too stiff creates a new problem with a loss of grip due to a loss of traction with the road and quick elastic weight transfer that can reduce stability.
The solution is roll center adjusters. There is only a solution for the front not the rear.
The best available compromise is researched suspension that is stiff enough to resist body roll to a degree but soft enough to make contact with the road so it doesn't lose traction. For additional stiffness use sway bars not higher spring rates.
Wouldn't it be nice if a company made a kit to restore our roll centers. I notice they have them for MK2 MR2 and a lot of other cars. From the reviews they do wonders for the way they improve handling. I think just this one modification could dramatically change the handling of our Spyders for the better. It would require adaptors on the spindle to raise the mounting points.
The McStrut means that as the suspension travel gets used up, the camber changes to the positive side. Obviously a lowered suspension needs reallignment but as lowering means less wheel travel/stiffer springs, the camber change range gets smaller too. Thus to have say almost 0 camber at fully compressed suspension a lowered car needs léss static negative camber than one with OEM wheel travel. On our Spyder this applies equally front and rear.
Second concerns the roll center. As mentioned lowering the car means lowering the roll center. Because the bálance between front and roll center also is ´a thing´ and no solutions for putting the rear roll center up exist for the Spyder, only mounting roll center ´adjusters´ at the front will create quite a different balance from stock. The TRD Sportivo kit lowers the front more than the rear* and to restore the roll center bálance TRD included spacers for the rear subframe, lowering the rear roll center a bit too.
* This gives positive rake. The results are less airfoil effect (from the car shape) and less pressure under the car, both reducing lift, leading to more stabilty and more traction.