i am very use to the idea of putting higher spring rates on cars that are front heavy, but it seems there are alot of track junkies that like the softer front, higher rear spring rate.
is there a simple explanation/ benefit to this?

 

decrease understeer

 

Well, that's part of it. The other part is the significantly lower motion ratio in the rear. The front is around .96 while the rear is down around .84. That means that springs are only around three-quarters as effective in the rear.

 

This is smart. I was just going to say, stiffer to help rotate the car better.

 

Thanks.

To complete the thought, please note that I got the "three-quarters as effective" part not from Dan Quayle math, but from the fact that you square the motion ratio to get the multiplier for converting spring-rate to wheel-rate. So it's .96 x .96 vs .84 x .84, which is about 4:3.

 

would that mean if i had higher spring rates in the rear (ex. 3k higher) , it would ride balanced as not being rock hard only in the back?

 

Nope, because you're not taking the fact that the car is front-heavy into account.

Let's do some rounding to make this easier. Let's assume the front corner weights are 1000 each and the rears are 750 each. So, ignoring the motion ratios, we're going to want more front spring than rear spring, simply because the front of the car weighs more.

The technical piece that you're missing is the natural frequency. This takes all of the relevant bits and combines them into one value. (If you want a quick definition, the natural frequency is the number of times per second that the car would bounce up and down if it didn't have shocks. Higher means stiffer and less body-roll, but it also means that more of the bangs and bumps will be transmitted from the wheels to the chassis and, therefore, less mechanical grip.) Wallowing Cadillacs from the 70s had natural frequencies down around 1.00. A dedicated road-racing car will be in the mid 2s. A daily-driven autocrosser will be right around or slightly above 2.00.

But, wait! There's more!

There are two reasons to run a natural frequency that is higher in the rear than front. On a performance car, you do it to make the car a bit loose. On a street car, you do it to prevent the car from pitching (rocking forward and back) when it runs over bumps. A standard rule of thumb (for pavement) to get both of these is to set the natural frequencies of the front and rear to about 2.0 and 2.2.

So, using our rounded off numbers, what spring-rates do you put on an Evo X to get these natural frequencies? Answer: right around 10/10. Sound familiar? Remind you of the default rates for lots of coilovers for an Evo X? Good. That means the people building them did the math and are offering a set-up that matches the standard advice.

Edit: people who run on dirt need to ignore all of this. You would never, ever, run a natural frequency anywhere near 2.0 on gravel.

 

In case you were wondering and don't want to do the math, the OE springs on an Evo X provide natural frequencies of about 1.35/1.45. The best, IMO, OTS replacement springs for our cars (i.e., GTWorx Tarmacs) raise these to about 1.65/1.75. But don't start ragging on GTWorx for stopping way below the 2.0/2.2 that might be suggested for serious autocrossing; the shocks would not be able to control them and the car would be very hard to drive.

Conversely, if you're wondering how to set up a car for gravel, the OE frequencies are actually pretty good, although the KYBs will not last very long; on really rough surfaces, just disconnect the front swaybar and go. If you rallycross on relatively smooth courses, the GTWorx springs are probably better in terms of frequency, but they won't make us a set that maintains the OE ride-height (I asked), so that's out. Thus, OE springs with the Bilsteins from an MR might be the best budget option. If you shell out for coilovers, the neat trick is to set the car up using only the springs, then reconnect the swaybars for the tarmac. We can't all be swapping the entire suspension, depending on surface, can we?

 

Iowa, most informative thread I've read in a long time!

I just had to say thanks for asking re: stock height GTWorx springs I have the GTWorx now and love them, but have no desire to be as low as I am honestly. Just didn't like the relatively soft stock ride at autocrosses.

 

You're welcome. And, if enough people ask, he'll make some Tarmac springs that are the same rate but only about half the drop (he said 10 mm), but you'd need to convince him that enough would sell and I have a funny feeling that not enough would. I started a thread about it a while ago and got pretty much no response. Then I blew (more than) this year's budget on the rally brakes, skidplates, and 15" Braids, and let it all go. I think I'll just end up getting Hotbits DT2s from Works.

 

Iowa999: informative read, thank you. Question: how does sway bar selection factor in all of this, & particularly rear diff options (rearranging stock diff plates particularly). Many owners that rearranged the stock rear diff plates implied that because of the 'tame' behaviour the stock rear diff has, it makes it necessary to use stiffer rear spring rates/thicker sway bar to counter the lack of oversteer that the stock rear diff should produce. What are your thoughts on that?

 

We can have a discussion of swaybars (but not right now, please, as it's almost dinner time), but I'd like to say (admit?) this about rear diffs. When I first got the Evo and started reading this forum, I was really surprised at all the threads about upping the rear swaybar and/or flipping the plates and/or putting a Cusco RS in the rear. All the threads were about "getting rid of the understeer" and I just couldn't sympathize. Yes, I know that I'm a huge (excessive) fan of the math, but this is one place where my experience over-rules all that. My past is in a 2G DSM and various Scoobies. When I hear people with Evo Xs complaining about understeer I come close to saying "what you talkin' 'bout, Willis?" because this car - bone-stock - is one of the best handling cars I've ever driven. If you use both feet, you can make it do anything you want. Most of all, when the AYC kicks in and the helical is clawing, the car seems to be pulled to the apex, as if by magnetics or magic. In the DSM, in contrast, I used to call apex cones "fang cones." A 2Gb Talon has these silly fang-like bumps on the lower front fascia, almost two feet in from the side. I'd point a fang at the cone at the apex and feed in power and the car would push on out to the right line. In the Evo X, if you do that, you'll flatten the cone or be on the inside grass. Anyone complaining about understeer in an X needs to spend a few years in a 2G DSM or an old WRX. Those things are pigs. This car is merely heavy.

 

Its a given that the Evo X is a better handling chassis out of the box. However, based on my personal experience auto-xing the Evo 8/9 chassis does experience understeer in certain situations. I never thought it was a weakness or made it a worse handling platform. You merely have to develop your style of driving to accommodate the "push". There is less margin for error with understeer because understeer is safer.

I have upgraded my diff with the Weir plate mod and it does make the Evo perform differently.
Running the higher spring rates in the rear is an added benefit and makes it so the chassis gains more traction in the rear. I know some Auto-Xers that run high spring rates in the rear and softer spring rates in the front with the Weir 12 plate mod. What they have found through trial and error is that the OEM rear swaybar is stiff enough. There is no need to upgrade the rear swaybar because the upgraded diff adds more traction to the rear wheels and the addition of the higher rear spring rates negates the body roll. There is now though more margin for error. The rear will be prone to oversteer in high speed transitions and high speed corners.

 

I love you for that statement

it amazes me how bad some drivers are out there that think this car understeers heavily

seriously you have to be doing something really wrong to say understeer and/or body roll is holding you back. GTWorx got their springs right and the whole system with the suspension works 47823987429 better in tandem than any cheap-level coilover

it is amazing once you get comfortable with a setup that you can just use throttle to bring the nose into the apex when you think you'll plow past it with understeer

 

I only autocrossed an 8 or 9 a few times, but the X seems very different to me. If you drive an X like other AWDs, it doesn't seem to work well. It took me three events to really get used to the AYC, although it probably would have taken less if I hadn't (a) blown one event to the pure joy of drifting and (b) played around so much with ACD settings.

The two things that seem different are how the X doesn't seem to need anything near to the same amount of trail-braking to have good turn-in and you don't have to lift when the car starts to push in a sweeper. The first difference was easy to adjust to, since it's like a WRX with camber plates. (In case you don't know, Scoobies have the steering rack in front, so if you arrive at events and slap the camber plates over to full negative, you also get a bunch of front toe-out. Rough on tires, but great turn-in.) The second difference I'm still not used to. I still get at least the urge to lift when I feel the car drifting wide. But on the occasions when I trust the AYC and feed in more power, instead, with maybe a light tap on the brakes, the car pulls inward. In some ways, I'd say that seven years in a 2G DSM is causing some pro-active interference. If I hadn't driven a pig for so long, I'd be better at driving this car right now.

As to stiffening the rear (with either more bar or more spring), it kind of depends on why it works. If it's helping rotation by cooking the outside rear, then I'm not a fan of this. I'm from the school that says that you should never give up grip; maximize grip and drive the car as it is. So, if the higher rear roll stiffness is holding the inside front down, I'm all for it, but not if you're cooking the outside rear. And I especially say this when you have an AYC that uses the outside rear to rotate the car. In other words, I'm worried that too much pre-AYC thinking is going into setting up Xs. Yes, the chassis is incredibly strong (especially compared to the overcooked pasta that was used to make a 2G DSM with a sunroof), so the outside rear can be used to hold the inside front down, but is that what people are really doing with their upgraded rear bars and 10/12 springs?

[apologies for the babbling post; not my best prose]