https://www.evolutionm.net/forums/mo...k-my-math.html

Send me some numbers and Ill plug them into my spread sheet

 

Some of us like to show off, doing the math in secret and reporting the results as if we're magicians, but, in truth, we all have spreadsheets to do it for us. Here's one: www.rastta.com/Files/SuspensionCalc.xls‎ Just plug in the values and prepare for the nerd-loving babes to start crawling all over you!

ps. if one of said babes is my wife, please send her home

pps. (edit) my post crossed with the above, but I'll leave it up even if he's offended....

ppps. savvy folks might wonder why an ex-DSM guy wouldn't post a link to the Far North Racing site; to you I say Ha! and wonder if the hidden text at the bottom of that page is still there

 

I measure in the rear and the AST's with helper springs have 1.5 inches more droop than the JIC's. But a big part of the not lifting the rear wheel is the added roll control with the spring rates. I was bottoming out on the bumpstops on the front outside suspension running the Hoosiers. 9k/10k is no match for A6's.


Here I am on the JIC's turning into T1 at Road Atl:






I know from taking the springs off the JIC's and checking stroke, that my LF is on the bumpstops here and my RR is hanging loose. I can tell you the car feels very edgy in this situation, somewhere around 85 mph on that entry. With the new setup, the car is no longer tripoding here and feels miles better. I can pick up maintenance throttle much sooner and as soon as I do the car really levels out and grooves. I got so used to driving my old setup I actually found myself coming out of T1 thinking 'Well crap, I could have went way faster through there'. I am simply relating this as my take on why minimizing the tripod action is a good thing.


Of course all my experience is on road courses and I know autox brings it's own demands and setup issues.

 

Hey now, my spreadsheets really are "my" spread sheets. I took the time to break apart the equations to understand them, and in the case of swaybars prove the equation.

Exam1: Where does the 3.13 come from in the NF equation :P.

I wanted to find out the % roll stiffness of the stock swaybars compared to overall roll rate and see what we do on aftermarket stuff. Once you do that its amazing how things start to make sense.

Factory Mitsu set the sway bar to be 50% of roll resistance front and rear. I now run (IIRC) 30% front and 33% rear.

 

All said and done, you have to get the balance right. A little more spring in the front, a slightly stiffer roll rate from the front bar helps keep the inside rear down. PU bushings help in reducing the delay in the bar being effective. Going too stiff front spring rate def will loose grip on uneven surfaces. Actually front or rear :-) . 4 wheels have more grip than 3.

If you then look at the rear, with all 4 on the ground, a more effective LSD will help under power and a tuned ACd ecu will make it even better. The stiffer front will help reduce the weight transfer or dive. At that point, as many find out, the stock bar is fine in the rear, although you might want to use PU bushings to ensure the bar quickly takes a set.

As others have noted having an adjustable mount for the front bar or adjustable front bar and rear bar further allows fine tuning.

 

It's not that simple, though. If you end up putting more load on the front outside tire in the process, you may end up worse off with four wheels on the ground. The front outside tire is the most heavily loaded tire on our nose-heavy cars in a turn. Anything that increases the load on that tire is going to have to come with enough of a performance improvement to offset any resulting reduction in grip from that tire.

 

Ooo! Ooo! Call on me! It's the square-root of gravity in radians. Now can I have a hall-pass? I gotta pee somethin' fierce.

 

 

I understand your thought process here however I'm not sure what the takeaway would be regarding the process of reducing load on the outside front tire?

Wouldn't adding more bar and/or spring in the front help to reduce the load on the outside tire since the car will not be able to squat as much? I understand there is a fine line where you can only go so stiff until it start impeding grip with that tire on an uneven surface for example but in an effort to reduce load on the outside tire how else would that be accomplished?

 

That's a pretty good summary of Evo handling issues in a nutshell. I know there are a lot of variables in play, but the front tires take a beating on our cars.

 

Even if I believe that it's only a start, I can walk you through the thinking behind posts like Construct's.

You are trying to turn. You have a front-heavy car, so you need maximum front-end grip (and the rear doesn't matter so much). You get maximum grip from a pair of tires when they have equal loads (due to the fact that the force-to-grip function bends down). If you don't do anything, the outside front will have a much higher load than the inside front due to lateral weight transfer as soon as you're turning. Therefore, you want to move some load from the outside front to the inside front. Luckily, we have a chassis that is strong enough to allow us to twist the front end of the car using the rear suspension. Ergo, you want add roll resistance to the rear, not the front, to get your car to turn.

 

Suspension compression (squat) is a function of load on that particular wheel and the effective net spring rate at that wheel. By increasing the spring or adding more bar you can reduce the suspension compression directly, but that doesn't necessarily help us out. What we really want to do is reduce the load on the most overworked tire.

When you enter a corner, you end up with weight transfer applying more load to the outside tires. Changing spring rates and bars doesn't alter the amount of weight transferred from side to side. Actually it might make relatively tiny change if the effective ride height changes, but let's assume equal ride heights across all of our spring and bar setups.

So we're stuck with a certain amount of weight transfer to the outside tires for a given cornering force, no matter how we change our springs or bars around. What we can change, though, is how that load transfer is distributed front to rear. The Evo is a front-heavy car, so the front tire is going to be loaded more than the rear tire. We have identical tires in the front and rear, so this uneven loading is not an ideal situation.

Now consider what happens when you increase the size of your front bar: A greater fraction of your roll stiffness will then come from the front of the car. Remember that our side-to-side weight transfer is still the same, so if we increase the roll stiffness up front then we're effectively asking the front outside tire to take more of the corner load. That's the exact opposite of what we want.

So instead, consider what happens when you increase the size of the rear bar: A greater fraction of your roll stiffness will come from the rear of the car. Again, weight transfer is the same, so what really changes here is how that weight transfer is distributed between the front and rear of the car. This setup now asks the rear outside tire to work harder, but that's okay because it's a lot less loaded to begin with. We end up with a more even (though still not equal) load distribution front-to-rear, which keeps the front tires happier.


In reality there are a lot of other factors that come in to play that must be considered as well, but this is the general idea. Body roll isn't inherently a bad thing, but on our Mac-strut cars we start losing camber once the front suspension compresses enough. As such, we want to keep that camber loss in check by keeping the roll within a reasonable range.

Increased rear roll stiffness can put the rear inside tire up in the air, especially when breaking (or deceleration) forces induce forward weight transfer as well. In my opinion, this isn't a bad thing as that tire wouldn't be doing much for you on the ground anyway. It can actually be detrimental if loading that tire indirectly increases loading on the front outside tire. However, there are (at least) two scenarios where having the rear inside tire in the air does become a problem:

1) If the rear inside tire remains unloaded as you apply throttle. The acceleration should induce a rearward weight transfer, which should be enough to plant the rear tires. If the rear tire remains unloaded, though, then you're limited in how much power you can put down on corner exit.

2) If you end up compressing the front outside shock so much that you're riding the bumpstop. This isn't so much a problem with the rear wheel lift as it is your overall suspension setup. The bumpstop is a very nonlinear high-rate spring, which means the suspension won't have enough compliance to work effectively in that corner. The end result will be a skittish, edgy ride if the surface is anything other than glass smooth. More spring up front will alleviate this, as will more bar. The downside to more bar is that the front has to shoulder a higher fraction of the weight transfer. The downside to higher spring rates is reduced suspension compliance.


Disclaimer: I'm not an expert on this, but this is my current understanding of Evo suspension setup.

 

I agree with the above with regard to the effects of load and weight-transfer on squat, but all of this is leaving out something rather important, but an order of magnitude more complicated: namely, the effects of suspension geometry on squat.

Start with just coasting through a corner. How much the car rolls (i.e., how much the outside squats and the inside rises) depends on both weight transfer and the heights of the roll (instant) centers, which is determined by the suspension geometry.

Similarly, how much the tail sinks and how much the nose rises when you accelerate depends both on weight transfer and the amount of anti-lift and anti-squat in the front of rear suspensions.

And then there's braking, where you also have the effect of braking torque on the hub. That can add (or subtract) some anti-lift and anti-dive.

(Note: some cars use pro-lift geometry in the front, instead of anti-lift, so all of these things have a sign which can be negative.)

Now, I'm not arguing that you have to start all over with geometry added in. What I'm really suggesting is that you focus on load and not squat. If you don't mind one of my stupid jokes: when it comes to assessing load on a car with a non-linear suspension, squat doesn't mean squat.

 

Fortunately for most of us, classing rules forbid us from making serious geometry changes so we don't have to worry about any of that.

But I agree that the initial focus should be on loading instead of suspension compression. I think too many people focus on suspension compression and rear wheel lift because they're highly visible, whereas you can't directly see the loading of your tires. Then people end up chasing solutions for rear-wheel lift at the expense of handling.

Excessive suspension compression is bad, though. If you're riding on the bumpstops, you're going to have a bad time. Likewise, if you constantly blow through your suspension travel and end up on the wrong side of our unfriendly camber curves you're going to be sacrificing some grip.

 

the "4 tires on ground must be better than 3 tires on the ground" thought process is very tough to get out of peoples heads even if it is completely wrong for evos.