Benefits of Square Setup STR Class
 

Hi Guys,

I have searched for a while, and couldn't find anything specific to my question.

I'm trying to understand the reasoning for using a square setup in the STR class. It seems that in order to counter act the oversteer caused by a square setup, you need to install a significantly stiffer front sway bar. The result of this is a car that tends toward understeer due to loss of front grip relative to the rear. Where does this grip go? Is the front grip lost, or is it just transferred/loaded to the rear tires.

Couldn't the same be achieved by running a smaller stagger differential front to rear, (235s front/255s rear for ex) and utilizing a more moderate fsb. I don’t see the need for the extra front tire width if the car is just going to understeer.

Can a stagger setup in STR be competitive on a regional/national level?

Hopefully I'm not too far off base. I appreciate any feedback.

Thanks

First off, I don't mean to insult you but it sounds like you might need a better understanding of some fundamental chassis tuning concepts.

1. You have 4 tires and each tire is capable of producing a certain amount of grip. That grip is dependent on the size of the contact patch and the load on the tire.

2. You want the highest potential grip possible so you want the largest tire that will fit your car or is allowed by the rules in this case.

3. To maximize the grip available you have to tune the chassis to distribute it's load on the tires properly while in a turn. This is done with springs and swaybars. You have to fine tune these items. You don't eliminate grip with a larger front sway bar. You are simply increasing the load on the outside front tire. If you do it the right amount with the proper thickness of bar or spring, you could restore the proper balance thus achieving a higher grip level than with a staggered set up.

Read this book, it's short and sweet but jam packed with a lot of good fundamental concepts: http://www.google.com/products/catal...ed=0CEwQ8gIwAA

Yup, I'll admit that I'm new to suspension tuning. Thanks for the info.

Sure you could run a balanced setup with staggered tires and it may even be fast. But if the guy next to you is running more tire, he is going to win (or at least he has more potential to) Glagola pretty much summed it up, run the most tire per the rules then tune it to make it work.

I would add one of the actual "benefits" is that you can rotate your tires on a square setup to get the most out of them.

This is interesting, I've been confused about sway bars reducing grip or not. I've read so many times on the internet, that stiffening one end of the car reduces roll but also grip, and softening increases roll but also grip. In general, I've also experienced both ways, a disconnected rear bar can put down power much better than connected in limited traction conditions, and a disconnected front bar can also have much less grip than connected.

I've also noticed more rear end rotation with stiffer rear rebound damping, and less with softer rear rebound damping. So does stiffening one end really not reduce grip at that end?

In regards to the damping comment, I believe the reason you get more rear end rotation with stiffer/faster rebound damping is because the stiffer/faster rebound damping causes the shock on the inside to extend faster when its coming off its compression stroke. So if you are in a left hand turn for example, the inside drivers side suspension (in the rear) is becoming "un-coiled" so to speak faster and thus throws that un-loaded side of the vehicle towards the outside of the turn. Basically it is causing the weight of the vehicle to be transferred much faster towards the outside.

I believe that is accurate, but if not someone please chime in.

You have four tires that when you set up the car to maximize grip, should all be working in harmony. Consider the two handling traits a car can have: oversteer and understeer. Each of these traits is a byproduct of the amount of grip that a particular axle makes. The axle for purposes of discussion is the pair of wheels in either the front of the car or the back of the car.

Our car is easy to work with due to a nearly 50/50 weight distribution. Let's consider sway bars and springs as one entity which equals the resistance to body roll. Let's also forget the concept of too much spring for the tire for this discussion as well. Consider only the amount of roll resistance an axle has and how it compares to the amount of roll resistance the other axle has.... it's the relationship of the two that determines whether the car understeers or oversteers.

Let's also only consider mid-corner phase and ignore entry and exit at this time. Mid-corner assumes there is no acceleration or deceleration happening. This is important because it relates to a longitudinal weight or load transfer.

OK, so our 50/50 car has 255s on all four corners. If the front of the car and the rear of the car are sprung the same, the car will be a sweeper machine! Both axles will have equal weight on the out side tires and equal weight on the inside tires. You cannot beat that set up for sweeping. Now, let's say you increase the stiffness of the front of the car only. What this does is puts more load on the outside front tire.

There is an important relationship between load and grip. As the load increases so does the grip but this relationship is not linear. As the load increases, less and less grip is made. So, you have two tires on an axle. If the load on the outside tire goes up, the inside tire does nothing if the rear axle is allowed to stay softer than the front. Now the load on the front of the car is mostly carried by one tire while the rear load is more spread between the two rear tires. What that means is that the rear is producing more grip because there is the same total load (mass doesn't change) but it's shared by a larger contact patch (two tires vs. the front which is just one tire).

So technically, yes, you have reduced grip overall by making the front swaybar stiffer but that's only in a sweeper and doesn't consider the specific demands that a rear wheel drive car puts on the 4 tires.

Ugh, the more I write, the more I need to write to explain this stuff.

READ THAT BOOK! ;)

This stuff is pretty complex, but that was a great post and explanation, thank you. I will get that book if talks to me like your post did.

It's way simpler than I make it sound. It's just hard to describe without diagrams and some simple equations.

It's important to keep in mind there is a sweet spot for pretty much all suspension tuning. You can't say softer = more grip or we'd all be running soft springs and no sway bars but when you're running close to an optimal setup then stiffer = faster weight transition and it's easier to break that tire loose, and softer = slower weight transition and more grip. It's not easy to determine which suspension adjustment will add grip and lead to balanced handling versus removing some grip to balance the car. Suspension tuning is something of a Black Art.