Tuesday, July 31, 2018

Suspension Omnibus

Suspension Omnibus
Suspension Tuning, tips, and more
by: Matthew Eddy, 2018-07-31
You have a car and you plan to race it or track it and you want to make some adjustments to the suspension so you can go faster. What can you do? Well first, let us define what I mean by the suspension so everyone has a clear working terminology for this article. “Suspension” for this article refers to all springs, shocks, control arms, suspension links, sway bars, tie-rods, strut mounts, wheels, and tires.  It’s a lot of stuff you can tweak for good or bad. Some adjustments may be good for one track or condition and bad for others. Also, a change that you do now that makes you faster may be bad as you become a better driver or when you actually start collecting data such as lap times.

Part one will deal with essentially stock suspension adjustments and then part two will go over common modifications. It is important to keep in mind that the tuning mentioned in Part 1 is still relevant to any modifications that you may eventually or have already made.

Part 1 - Stock Suspension:
If you just got the car great – DON’T DO ANYTHING DRASTIC! Assuming the car is already a sporty car of some sort, the manufacturer already had a bunch of engineers drive the car around the road and track for thousands of hours tweaking the suspension so you have a pretty good base to start from. Also, if you are reading this you are new to suspension setup and tuning and probably new to the track. The last thing you want to do is make a bunch of changes to the car before you even know how to drive.

Now as you get more familiar with the car and you start getting a feel for it you should start making some changes.

#1 - Tire Pressure
If you are new to motorsports this may seem pretty minor but you would be surprised how drastic of a change the tire pressure can have not only on the feel of the car but your finished lap times. Ideally you will make changes based on quantitative analysis of some kind of data – like lap times. But even without that data you can adjust pressures to tune the feel of the car.

First, it should be clear that tires are not rigid and their shape will change depending on the weight of the vehicle, the shape of the rim, and their pressure. That’s why vehicle manufacturers tell you what pressure the tires should be set at and not the tire manufacturer. As a driver, you want to optimize this shape by adjusting the pressure so the tire sits as flat as possible against the ground. Over inflate the tire and the center of the tire bulges out and you reduce traction. Under inflated tires the center sucks in, you ride on the shoulders, and not only reduces traction but it feels less planted. So ignoring the manufacturers recommendations the ideal way to determine the correct pressure is to use a tire pyrometer and measure the temperature across the width of the tire (outside shoulder, center, and inside shoulder) on all 4 tires and it should be relatively even. If you don’t have a pyrometer – which is fine because they are kind of expensive – you start by lowering the pressure slightly and see how it feels. You can mark the side walls of the tires with a paint marker or chalk to see if its starting to roll over. Work your way down slowly until it feels like they are too soft or you start rolling the tires.

#2 - Alignment
Some alignment changes will help improve the feel of the car. Often car manufactures set the alignment to optimize tire wear which doesn’t necessarily optimize handling. This is something you could do yourself at the track but it takes some patience and a little practice.

Quick rundown of terminology:
Toe - if looking at the car from above, toe is how parallel each set of wheels are to each other. Toe-in means the (again when viewed from above) the tires are pointing toward each other. Toe out is the opposite. See images below for examples.

Camber – If viewing the wheel from front or rear of the car, this is a measure of how perpendicular the wheel sits to the ground. Negative camber means the tire leans in toward the car. Extreme negative camber is called, I believe “stance” which is something that has become popular with all the kids these days.

Caster – Not really mentioned but because it’s part of the alignment it is only on the steering wheels and is a measure of the steering angle. Sometimes this is adjusted for racing applications but I am not going to touch on that in this article.

People have probably already figured out the best alignment for your particular car so try and find that information but here are some general suggestions:
  •       Toe out the front tires. This can make the car feel more agile in regards to steering. This is only like 1-2° of toe
  •       For RWD cars that are prone to oversteer, you can help tame that a little by adjusting the rear toe in 1-3° 
  •       For FWD cars (or any car but more typically FWD) that suffer from understeer toe-out the rear to try and make the back end a little looser.
  •       Negative camber is good. For any stock vehicle you can adjust as much negative camber as you can get front and back. This will help with cornering. People will often get camber plates to add even more negative camber. There is a point at which you can add to much camber because the car is riding on the inside shoulder too much and you will lose braking distance

Part 2 - Suspension Modifications:
I will preface this section and say that likely any changes you make (new sway bars, springs, struts, coilovers etc) are likely to improve the handling of your vehicle. While that may be the case, the changes may not optimize performance or may have some other negative effects even if there is an improvement overall. Any of the changes below are starting to get into the range of moderate to advanced suspension tuning and if you are truly looking to optimize handling of your vehicle that you use this a stepping off point to do more research on suspension setups specific to your vehicle. It is also good to have a pretty good feel for the car in its stock configuration so you can make an educated evaluation of the changes you have made, be familiar with the terminology, collect data – lap times, tire temperatures, suspension travel, etc.  

#1 - Wheel and Tires:
One improvement you can make that is pretty straight forward is to go with wider tires. More tire = more rubber on the track = more traction = more speed in the corners. That’s a pretty simple formula. Just like there is no free lunch, there are some trade-offs that people should be aware of and some “tuning” that can be accomplished with this. You can choose to have the same size tire on all four corners of the vehicle this is called a square setup. Versus having larger tires on the drive wheels which is called a staggered setup. I suppose you may choose to put larger tires on the non-driving tires but I never heard of it but that doesn’t mean there isn’t good reason to do so for certain applications. The most typical of the staggered wheel setups is on RWD cars and they have wider tires in the back but I have also seen FWD cars with big honkin tires up front. This makes sense for FWD cars since the rear wheels are only there to keep the back end from dragging on the ground. By changing tire sizes you can increase grip but also compensate for under or oversteer to some degree. If your car currently has a square setup, and you get a lot of oversteer, then consider leaving the front tires the same size and increasing the rear tires. Or increase the size of all the tires but increase the rears more than the front. If you experience a lot of understeer, adding more tires to the front of the car may or may not help. This is something you will have to try experiment with since understeer can be caused by a few factors including bad driving line. There are some downsides to going big. One is that your top speed will drop. More tires means more traction. Another word for traction is friction. Even going in a straight line the tires are going to increase traction. This may sound bad but in general the car will make up for this loss of speed in a straight line by higher speeds in the corners. Second is that wider rims are heavier. Weight, and especially unsprung weight is not good but again the downsides are relatively minor compared to the improved handling. So this is a balancing act. How much tire is to much? That is something you will have to determine yourself with trial and error and  don’t forget - data.

*** A word of warning – changing wheel and tire sizes can lead to fitment issues. You might start to getting rubbing on the strut, the wheel well, etc. This may only occur while turning or hard turn. A good resource are forums or FB groups and ask what tire sizes people go with. You may have to get wheel spacers or roll your fenders.

#2 - Sway Bars (aka anti-roll bars):
The purpose of the sway bars are to reduce body roll in a turn. How sway bars help increase traction is a bit involved for this article but in short by reducing body roll, more weight is kept on the inside tires during a turn. What is important from a suspension tuning perspective is that adding a sway bar can have a huge effect on the feel and performance of the car. You can also add or reduce oversteer and understeer by changing the relative stiffness between the front and rear sway bars. Now it’s very important that you understand that the below guidelines are RELATIVE stiffness between the front sway bar and rear sway bar.
  • Stiffer front sway bar increases understeer
  • Stiffer rear sway bar increases oversteer

I will mention this a third time, the difference is relative. For instance, you can increase the relative stiffness of the front by reducing the stiffness in the rear and vice versa. Why this is important is that you may want to change the balance of the car (reduce understeer or oversteer) and adding a stiffer sway bar may not be an option because there just isn’t one available or you have maxed out the ability of your suspension to handle a larger sway bar. So instead you can reduce the stiffness of the other one to have the same effect. It’s a good idea, when possible, to get an adjustable sway bar so you can tune the car a little bit while you are at the track.

Here is an example. I have 91 MR2 and the car is known for its snap oversteer. A few years ago I installed a V6 in it and I wasn’t sure how it would handle on the track. So for my first outing at the track I decided to disconnect the rear sway bar to increase understeer and reduce oversteer. Once I was out there it didn’t feel necessary and unfortunately I didn’t get a chance to try it with the rear sway bar connected back up because I spun a rod bearing on the very first session. The important part of this example is that I was able to change the relative stiffness of the front sway bar by reducing the stiffness of the rear which I did by disconnecting it altogether. I was sad. Anyways, some real life application for you.

To tickle your brain a little – some people don’t like sway bars and would rather eliminate them. This gets into deep suspension theory, and this is a very small minority, but some people would prefer to just increase the spring stiffness substantially. The reason being is that sway bars link the pair of wheels together which is a lot like having a solid axle which is bad. The stiffer the sway bar, the less the wheels can act independently. My suggestion would be to still use a sway bar and educate yourself a bit more before doing anything drastic.

#3 - Lowering Springs & Adjustable Shocks:
Luckily there are a lot of options for lowering springs and adjustable shocks so will likely be a benefit if you go with a reputable brand. Lowering springs usually do two things, first is to lower the vehicle which lowers your center of gravity making the car feel more planted, and second is the springs are usually stiffer. If you go this route I suggest getting either adjustable shocks or shocks that were intended to be used with the lowering springs. I say this because the purpose of the shock is the dampen the tendency of the spring to want to oscillate (or bounce). If the shock is not strong enough the spring will over power the shock. If the shock is to strong it does not allow the car to settle in a reasonable amount of time. Meaning the suspension is still going back to equilibrium as soon as it can. Ideally you have a shock that is perfectly tuned to the spring. A stock shock is likely going to be to weak to dampen a stiffer spring so I would not suggest replacing only the springs. REpalcing just the shocks without replacing the spring is only going to give a small improvement to overall handling.

Like the sway bars mentioned above, you can adjust the understeer/oversteer balance of the car by changing the relative stiffness of the front springs versus the rear springs of the car. 
  • Stiffer front springs increase understeer
  • Stiffer rear springs increase oversteer 

One downside of lowering springs is that they usually don’t have a range of stiffness available so there is very little customization to change the understeer/oversteer balance of the car so you can compensate this by changing sway bars, wheels and tires and even the tire pressure. Another downside is the shocks are usually the same length and with the car sitting lower the shocks are compressed a bit more than usual. This means there is less travel available before hitting the bump stops.

#4 - Coilovers –
A coil-over comes from the term coil over shock which I think it’s funny because strut is a coil over right?  I believe this terminology developed before struts were invented and most vehicles had springs and shocks separate from each other. The main difference between a strut and a coil-over is that you can adjust ride height with the coilover.  Another difference/advantage with coilovers is the spring diameter is pretty standard so you can easily change out the spring for a different length and/or stiffness (aka spring rate). A lot actually goes into a good coil-over system and you get what you pay for because what you are relying on is that the manufacturer had done a good job tuning the whole system for your particular vehicle. There is a lot to tune in a system like this such as choosing the proper spring rate, the socks are paired well with springs, and this includes different springs/shocks for the front and the rear. If the shocks are adjustable, a good shock will have a very smooth consistent adjustment curve. So I am saying cheap coilover kits are not going to be as good as expensive ones but it may not matter because regardless you will likely see an improvement in handling.  

One benefit of being able to adjust the height of the car on each “corner” of the vehicle, is that this also changes the weight balance of the car. If you haven’t heard of 4 corner scales you will want to. They look like 4 bathroom scales hooked together and it tells you how the wright of the car is distributed across all 4 wheels. So if you get coilovers and you don’t do this – even a little bit – then you are missing out on one of the whole reasons to get them. If your plan was to just lower the car X inches and forget about it then you can buy lowering springs for that.

My suggestion is not to get coil overs until you have some track time under your belt. You have done a good deal of research about suspension setups in general and for your car specifically and are prepared to get the full benefit from the system.

#5 - Suspension Geometries
All the above is well and good but another step that is intermixed with the modification listed above is to actually start changing suspension geometries by lengthening/shortening arms, moving and/or modifying their locations, and changing mounts. There is too much variation between vehicles to get into specific changes so I will list some of the more common.

-           Camber/Caster Plates – Typically this is for camber but some plates also allow you to adjust caster. The will replace the top strut mount which will allow you to get more camber adjustment. Stance is life.

-          Control Arms – This one may be harder to explain but on many cars when you lower the vehicle you are actually changing the angle of the control arms. Imagine how a control arm moves. In the most simple example it’s a arm that attaches to the car and pivots. The other end attaches to the wheel. When that arm moves up and now, if you were to trace the motion of the end of the arm it makes an arc. Now imagine a wheel is attached to that arc and as the arm swings up and down the angle of the wheel in relation to the ground is going to change. That means the camber is going to change. The camber changing while the suspension moves means you are changing the feel of the car as the suspension travels. Some cars people make camber adjustable control arms or longer control arms so that the camber always stays negative.

#6 - Aerodynamics
I am not going to get into to much detail here but there are a few comments on aerodynamics and how that works with suspension.

The three main aero mods are
1           Rear Wing – This helps to push on the back of the car and keep the back end planted in a turn. The down side is that it creates a lot of drag and will slow you down in a straight line.
2.                 Front Splitter/Air Dam – Kind of the same but slightly different I’ll explain the difference briefly. The air dam projects vertically down from the front bumper and all it does is prevent air from getting under the car. By preventing this it helps push the car down. The front splitter essentially does the same thing but better because it also adds a lip that projects out in front of the car and will actually help to push the front end down similar to how a rear wing works. This also increases the drag on the car so will reduce top speed.
3.               Rear Diffuser – A rear diffuser is this magical aerodynamics thing that gives you free downforce without any drag loss. The only issue is that it’s very difficult to make is effective. The car needs to have a perfectly flat bottom, and then it has to curl up the back of the car a bit with some vanes to help keep the airflow going in the proper direction. A lot of cars have fake rear diffusers because they look racy but getting one to function properly is tricky.

Aerodynamics makes the car grip harder by physically pushing the car into the ground. By pushing the car down means the tires are being pressed into the road even harder which means they have more traction. A formula 1 car’s aero can produce three times its own weight in downforce.

A couple things make aerodynamics tricky. One is that the aero mods are literally pushing the car down and compresses the suspension a bit which makes the car lower. If you have adjustable height suspension such as coilovers, you will need to alter your ride height to account for this additional force which can be even trickier since a rear wing and a front splitter will likely be applying different forces at the same speeds. Second is that different tracks are going to have different mix of straights, fast corners and slow corners and the car is going to perform very differently in each and will require a fair amount of tweaking to optimize your setup.

The main points I want to make with aerodynamics that it adds a lot more variables to an already tricky equation. Ideally you will be getting real data on how much downforce is being generated at various speeds and use that to help make educated decisions on the proper settings.

I should also stress that a lot of people, especially if they are just doing track days for fun, don’t do any of this stuff. They buy it, bolt it on, and it will usually improve performance. They set it and forget it and then have fun. However, if you are competing then it would benefit you to consider all these effects to learn how to optimize your setup for each track, weather, and temperature.

The main purpose of this article is to give you some pointers for some simple suspension tuning and also a background in suspension modification and tuning in general. The thought is you take this foundation of knowledge and build on it for your own particular vehicle and goals. Look for future articles that go into more detail.  If you have questions or article ideas please feel free to leave them in the comments section below. I will try and get to them when I can.

Thank you.

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