I agree, too much wheelgap. The only way to fix it with the 15's is taller tires or coilovers.

This is the stance you want for 15's...

 

here is your picture meb :

 

Awesome thread! In for the info...

Ok meb, explain your artwork to the ignorant crowd...coughMEcough...

mb

 

Well, I'm currently attending my Xmas party at work and I'm about 3/4 of the way thru a bottle of Barbera D'Alba...so

Couple of disclaimers:

* This is not to scale
* This does not depict the Mini's actual geometry
* I used only one side of the car to locate roll center, instant center etc as
is typical when locating static positions. If you want to see how these
migrate as the suspension moves, you ned to duplicate the instant center
on the other side.

First, the offset question since that was the original poster's question. If you look carefully, you will see that the roll center is located based upon the intersection of the angle of the strut tower ( or camber angle ) and the angle of the LCA - lower control arm. Where these two points converge is the instantaneous center. DRaw a line from this point to the tire's center line at the contact patch and the roll center is located at the car's center line.

Tire two depicts an exagerated offset. The point was to indicate what offset does to static roll center - it raises it - i did not complete the dotted line from tire two because it would have made the drawing harder to read.

So raising the roll center while maintaining CofG hieght decreases roll couple - a good thing. All forces act on the roll center first and the migrate to the CofG and then to the rest of the car. If you draw a stright line from the roll center up to the CofG (not shown, and should be some distance above the roll center on the car's axis in a perfect world) you will be drawing the invisible lever arm that connects the roll center with the CofG. This is a critical dimension since the length of this arm controls the force of roll. When we lower a Ma Strut, the roll center usually drops farther than the CofG because the lever arm is longer. In theory the car will roll with more force. I write in theory because when most of lower our cars, we do so using heavier springs and these often offset the increase in roll couple.

Increasing offset helps to retain some of the original roll center to CofG relationships. There are a few down-sides to increasing track with spacers but I'm not discussing those here.

You will aslo note that changing camber angles will change the location of the instant center and therefore the roll center. Just play with the geometry.

now for the complicated part This is a static diagram. As the car rolls, the instant centers, roll centers and CofG all move around. This is compounded by the fact that the car's body is more or less moving around with these three points...and that the tires are deflecting under load as well. So a car is a compromised piece of machinery in motion. The best way to make the car work is to...well, not allow any motion. Hence an F1 car. This is precisely why setting up a car for the street, and, track is SO difficult!

One more really important point. Look closely at the instant center. This is the fulcrum from which the wheel moves - or written another way, the instant center scribes the arc the wheel moves thru. The fulcrum is always opposite the wheel it controls. you can use a compass and place the point on the instant center and the lead at the center of the contact patch. NOW, this arc determines camber compensation. Many of us become so concerned with static camber angles when we SHOULD be looking at camber compensation angles because these determine what the tire does as the car leans. The rate at which the car leans is of course determined by springs and dampers. So you can at some level keep the car within an ideal geometry 'range' if it is stiff enough - F1 cars for example. ie - not allowed to move outside of ideal geometry. This is but one of the goals behind a stiffer suspension. And key to much of the info here, is if you can control roll couple, you can use lighter spring rates...if you can control roll couple and you have really good geometry, you can use even lighter spring rates! The location of the instant center(s) is also responsible for many other aspects of good handling because these are the fulcrums from which the suspension actually move. The hieght above ground or in relation the the wheel's center line affect the literal path the tire follows while moving up and down - I wrote about this more than two years ago.

I'm convinced that the mini is a great handler for many reasons. But one is its short travel; it still runs out of camber at extreme angles of roll but for most of its range, its a damned good car.

Oh, we can talk about side view roll centers at another time - these control dive, squat, and, yaw when combined with all the other geometry.

Getting geomerty correct is absolutely critical. But, really good damping rates and sticky tires can also perform magic! I'm suggesting that lowering a car for performance - not looks - can cause more problems than lowering attempts to fix. It's not as easy as it looks. I had my geomerty about perfect, but had two sets of poorly designed coilovers and the car ran like crap. Ya gotta get it all correct - within some window.

The other interesting geometry here is scrub. If you look at the angle of the strut/spring, it determines SAI or King Pin axis - same thing. If you follow with an imaginary line this angle down the ground, you will see that it intersects the ground at some point under the tire. Again, this is not a scaled drawing so just play along. Typically front wheel drive cars have positive scrub - the point where the SAI intersects the ground is on the outside of the tire's center line. Sports cars - non-front wheel drive - typically have this SAI intersect the ground on the inside of the tire's center line. And center point steering intersects and the tire's exact center - not good.

The SAI represents the leverage point for the steering, or the axis about which the tire steers. The tire is literally steering around this axis. When the SAI falls outside the tire's center line, it is not exactly in sympatico with the other forces acting on the tire - more on this in a moment. But, when one tire blows out or if one brake is lost, this leverage postion allows a reasonable amount of stability for the front DRIVEN wheels - remember that power is moving thru these wheels. This si so because this leverage position allows the tire to point towards the car's center line, an inherently more stable direction. When the SAI intersects the ground on the inside of the car's center line, this point is in sympatico with the tire's natural contact patch shape, especially if we consider some neg camber.

The important point here is that altering offset will change the point of intersection and you do not want center point steering! Having the leverage point fall outside or inside the tire's center line causes the tread the distort on either side of this point. This distortion is feed back! It is resistance and it is responsible for sending tingly little messages to the steering wheel. Negative scrub does this better - in simple terms - because it is working in an area of the tire that is performing most of the work. Positive offset works too, but when the leverage point is outside of the contact patch area or outside the slip angle range, it cannot telegraph info as well. Caster, pneumatic trial and a host of other things like seering gear and steering boost also affect how much information reaches your finger tips.

BIG THANKS TO UKCoopeR for posting this image!!!

 

Mep
Thanks for the detailed analysis. This is much deeper than I thought this thread would get but great information.
Thanks, Steve

 

For those interested in knowing more, try reading up on the subject. There are a few good books out there.
http://store.cdoc.com/products.asp?s...119&name=Books
http://www.amazon.com/How-Make-Your-...995537&sr=1-11
http://motorbooks.com/

And my favorite new book on getting started racing:
Go Ahead Take the Wheel by David Gran
http://www.goaheadtakethewheel.com/Home.html

 

http://www.sportcompactcarweb.com/te...uidelines.html

 

Great explanation Meb. Setting up a suspension is the toughest thing to do on a car. It's even harder when you have a road car. If you want to do it right, you have to have tire data. Believe it or not, tires will have highest grip at a certain slip angle. You must design the suspension and all your angles in order to maximize tire traction.

For our Formula SAE car, our suspension guy made a 2D model of the car in Solidworks and pinned certain points down so he could rotate the car like it was going through a corner. He was able to change camber, ride height, etc and watch it's effects on camber gain, roll center, instantaneous center, etc. It's pretty sweet to watch it in action.

 

lol, i did the same thing for a steering system i desgined

solidworks motion sim is pretty useful

 

I missed some of the finer points - the Barbera was good!

But I think everyone gets the point here, as kapps wrote, setup is really hard. If you begin to understand what all of these elements mean to one another, and, you can model the suspension, then you can come up with a pretty good setup quickly - from a geometry perspective. springs dampers are still an animal we have little control over since these are produced by folks who've made decisions and compromises about a market as a whole, not necessarily about a car in particular.

Thanks for posting book references too! These are valuable assets in any garage.

 

.... another great thread with the help of meb. .... and others.

(also enjoyed Kapps link to his Europa! )

 

All of this makes me feel like a complete idiot about this stuff. I mean, I know I want a coilover system, and I know I run a +40et wheel with 5mm spacers at the track, but do I really know what that does or doesn't do? NOPE! Just the other day I was considering using a 10mm spacer on the track to give my brakes more room and to widen the front track. I think I need the cliff's notes version of this thread.

And I think I'll pick up some books and start reading...

mb

 

mbcoops,

If I explained the drawing to you in person, you would understand it immediately. It's not hard really, it's rather comforting at some level to actually know that one thing influences the next and so on. Once you see this, all of your choices become deliberate, not arbitrary!

Of course, it helps if you can drive too...meaning, once you set your car up, you know what to expect from it when pushing it at track speeds. But then, I've had my share of problems!!!

Greg!!! Where have you been??? You don't call, you don't write??? give my a jingel when you have a free moment.