Its actually pretty similar to a couple spots at 0:15-0:22. Where there's the hopping around. Now that I'm thinking about it, I think I may only be happening on heavy throttle while turning.

 

no ****? it was a gross over-simplification because you still didn't get it until I had to explain it to you.

Again, no ****; I wasn't talking about that at that time, was I? No.

Exactly, now think about it a little more, I think you almost got it....
If you're worried about spring weight and size, why did you even make this thread, mr. "i wanna use an extra spring which will require dual spring spacers and thusly add more weight to my car"?

News flash: you're not that good of a driver that weight from extra spring length will affect your times.

 

Wow man, You are so off base I dont even know how to respond.

It sounds like you think I have an issue with how much compression travel I have? My tire would kill my fenders before I run out of travel, coil bind is not an issue so going to a longer spring would only serve to move my preload perch down yet I would still be set right just over zero preload. I have DROOP issues...You know, where the spring is being un-loaded...

About the spring weight, duh... Not saying I am trying to save weight, no where did I say I was. Just mentioned the compromises between going longer, especially if you don't need that much longer.

 

You don't say.

 

Whoa didn't know 10yr olds were allowed to comment on here.

Small as in how? Insignificant? Perhaps. Physically? Not a chance. But this isn't a fitness forum, so I'll avoid details.
I'm not all worked up so if you're the one crying to a mod, you don't know me; there's no reason to get all worked up on the interwebs and your comprehension of my e-tone is misconstrued.

Either way, I'll leave since miss nancy and aunt fancy don't want me here.

 

Alright everyone chill the **** out and relax.

A longer spring will not gain you any droop travel if you're sticking with the same spring rate, ride height, and preload. For a single spring with 0 preload your droop travel is the same as the compression of the spring due to the corner weight.

A helper or low rate tender is the tool for this job. I am a proponent of having usable droop travel and attempting to keep all 4 tires on the ground. It's not the end of world if you lift a wheel, but keeping them planted is a good thing IMO. That said it's certainly up to some discussion as to exactly how important it is.

Now everyone be cool.

- Andrew

 

I agree with Andrew. For a car to most effectively do its job; all four wheels planted will help it get there. The Evolution (and other AWD) responds a lot better when all four are in contact. I prefer that on FWD/RWD cars as well.

Exactly my point. You're coming off as a 'roided out ***.

Did you just "you don't know me"? You also need to get your **** straight. I was a SuperMod for a long time here.

What people perceive becomes their reality. You want to write as if you have a massive stick wedged in your ***. Then you are perceived as a miserable human being.

Just mind how you respond to others. People are here to ask and learn. There is a right way to post and there is a "I'm a complete ****tard" way to post. Which are a you?

 

Thanks for chiming in here Andrew

I guess since I have an idea I might as well test it. If it doesn't help much then easy enough to go back. Gonna take measurements tonight to see how much travel I have and order up some 60mm Swift or Eibach tender's.

 

No problem. Definitely figure out how much bump and droop travel you have now, and see if you have any total stroke to spare. Remember you'll be trading some bump for droop travel.

The Swift tenders are pretty nice. I don't think I've used the Eibach ones.

- Andrew

 

Great topic and I have been considering the exact same change. I'm currently on 560#/8" 780#/6" springs and have been considering a rate change along with the tenders in the rear.


I think 0:15 and 0:22 is exactly what the OP is describing? I also get the same type of behavior under certain circimstances.

Griceiv, what spring rates are your running in that video? Have you changed since?


Going back a bit though, hopping can often be due to overly stiff rear spring rates.

 

My theory on the hopping is once a wheel completely leaves the ground the wheels start gaining inertia. When it comes down any slop in the system (rubber bushings, tires, flex) gets taken up then releases causing the wheel hopping. I'm thinking the motion is separate from the shock travel which is why when it starts it keeps going. An un/under damped system.

Thats why my solution is to use a heavy enough tender to push through the sway bar and keep the wheel on the ground while soft enough to be completely compressed at rest.

 

Again, great topic, so I started to run some basic numbers. I haven't measured everything so I'm making some guesses here based on where my car was last time out. I might try to pull off some suspension pieces and weigh them as my car is all tore up anyway and it would be pretty easy. Calculating out natural frequencies, the front sits around 2.45Hz and the rear at 2.8Hz. Again, the motion ratio and unsprung weight was approximate, but I bet I'm pretty close and the frequencies are fairly accurate. The natural frequencies are pretty reasonable with the higher rear frequency. The separation is even reasonably aggressive at 0.35Hz which suggests it would probably be a little tail happy at higher speeds, which matches how the car feels from the driver's seat.

My car is sitting at ~15% higher natural rear frequency on a car that is ~60/40 weight balance. Even with that spring balance and the Shep rear diff, I would still like a little more rear bias on handling then what I had. Going stiffer in the rear would likely only cause more hopping issues though.

Low speed is where the car has troubles and it's likely just the fact that 60% of the weight is up front and inertia is the dominant factor at low speeds? Maybe it's just not realistic to think I'll ever get this car to handle like a 50/50 car does?

But we can always try to improve things.
My current thought is a slightly stiffer rear sway bar as I'm not THAT far off on balance. When the tires are on the ground, it could improve things, but the stiffer rear bar is going to want to lift the inside tire more now. This method would be in line with the idea that if all 4 tires are on the ground then it’s going to have better grip. This would go along with the idea of having to overpower the swaybar with the tenders.


If you are right on the oscillations being due to flex in the suspension mounting points, then I believe I will have that covered as I'm going to spherical bearings front and rear, along with a solid rear diff that I already have in there.

 

Dallas,

my 2c - coming from a background with FSAE and then BMW autocross suspension knowledge.

Your description of the problem: i have an issue with the inside rear running out of droop travel and getting picked up, then the outside loosing grip and dropping the inside. This happens in pretty quick succession causing either a hopping or oscillation depending on other corner conditions. Increasing rear rebound helps slow the oscillations in the rear but its also making the car a little unstable in slaloms. With any roughness on course I'm getting a lot of instability which obviously is hurting overall times.

Eric (R/TErnie) and I saw the exact same issue at PGP with his car last year. It was worst when he was still on stock diff and high spring rates with Skunk2 dampers, I forget the exact rates but they were in the 600-900 range.


There were two issues contributing to this:

• once the unloaded rear tire is picked up, with a rear diff without much preload, the tire spins up since it has much less resistance than the outside driving tire, and the RPM increase at the inside tire goes back through the transmission to the engine, so it sees less load and drops some boost (probably the suckiest part)
• once the unloaded rear is picked up, the only damper which offers roll damping is the loaded front - the car is basically rolling across an axis from the unloaded front tire to the loaded rear tire

Stepping back to lower spring rates on the Skunk2 and making no other changes did reduce the problem noticeably. The issue therefore is inadequate front damping for high spring rates. Remember that ONE FRONT DAMPER ALONE has to have enough low speed rebound and ESPECIALLY compression to control 4x coil springs and one fairly large front anti-roll bar in roll. He went to Ohlins dampers and a 12 plate diff and it made large improvements, including elimination of the hopping.

Another thing - simple rule of thumb for autox and road race suspension. Spring selection dictates how far the suspension moves. Dampers dictate everything else. Dampers especially dictate any behavior which is time- or frequency-related.

You can make those rear springs any length you want, but they're going to behave exactly the same. Unless you can reduce load transfer across your Evo by lowering the CG, weight reduction, or driving slower (heh) then you will always pick up rear wheels. At least for autox with an EVO on high spring rates, you must devise a car setup which handles well with only three wheels down at corner apex.

One other thing, about springs and tenders. You intuitively know how a spring works and what it does. It stores energy. When that spring has a load of 0 lbs, it is uncompressed, and the car's suspension is at full droop. If that happens, which it does, what does it mean? It means you have no vertical load on that tire. Adding a very low spring rate helper will not add vertical load to the tire. Your car has ~1050lb on each front tire and ~650lb on each rear tire. Your rear unsprung weight is ~110lb, so you really only have 540lb of the EVO supported by each rear spring. It is not hard to calculate and realize that 1.2G lateral acceleration will easily move 540lb laterally across an axle of a car.

Sorry for it ending up so long. I did notice that you have a real rear diff so everything I've mentioned doesn't apply to your situation.

 

I'd have to check at home what my N.F.'s are, but at 700/900 the car is just a touch too tail happy in places and just right in others. Im pretty sure Im trying to run too much rear rebound to control the oscillation giving me a bit too much movement in transitions.

What is your front camber and toe set at?

One other thing to mention cause I see it locally a bit, if you are already understeering then you cant use the throttle to oversteer. The front is slipping and has lost grip so the throttle will overwhelm that end first. Im a big proponent of "almost" never trail braking. The car is to front heavy to brake and turn at the same time so I always brake - throttle - turn. So Im nearly always turning ON throttle.

I dont initiate rotation when I need it, by then its too late. I set it up before and only do it to get the car pointed about where I want it and shoot through it. Point - shoot - point - shoot.

 

Nucci, do you guys have any events coming up soon where you and Ernie will be at?

I didnt think much about the front controlling the oscillation but I am running the fronts at 6/8 set by feel on turn-in. If you have an event up there I'd gladly drive up and try less rear rebound and more front plus get your guys' opinion.