03whitegsr

iTrader: (8)

I haven't thought about it from that angle really, so this is just "back of the napkin? thinking right here. I see two possible situations here:

1. With rear anti-roll bar
In the case of no helper with "0" preload on the spring, after your ~1" of droop travel is used you have exactly 0 pounds of weight on the actual tire. I'll call it the "instant lift point" although there is probably a common name for this? At this point, your roll-stiffness only relies on the outside spring and the roll bar, correct? The roll bar is trying to compress the spring to balance the torque on the bar from the other side.

Compare that to the case of with the helper spring at the same "instant lift point." Nothing really changes as the rollbar is still trying to compress the spring and it's going to be trying to compress the main spring, as there will be enough load to completely compress the helper provided it is a low rate helper.

Net effect, nothing really as the roll bar will keep the car from using the extra droop travel. I could see this being a waste of money...
However, this is simply considering steady state cornering on a smooth track. What happens when you toss a mid-corner bump into the mix?

2. Without a rear anti-roll bar
In this case, I can potentially see what you are talking about since the roll bar is not there to produce a reaction load to compress the spring. However, without the rear bar, once you reach that "instant lift point" doesn't your roll stiffness no longer depend on the wheel that is in the air anyway?

Adding the helper spring means you actually get a little more roll stiffness after the drop travel point where without the helper the tire would otherwise be in the air. It would be very minimal though as the rate is going to provide very little reaction load.

Net effect, the tire stays in contact with the ground for an extra 1.25" of travel. While it's going to have minimal load on it, it is still on the ground adding at least A LITTLE extra grip.



One thing I have noticed is that it seems like the rear end hops when it lifts and then I try to apply throttle. I have the Shep rear diff upgrade so i do have additional lock-up in the rear. My theory, when you loose all grip on one tire, even with the higher lock up, you might still be overpowering the clutches and it starts doing funny things, causing the hop. This is the main reason I'm trying to keep the tire on the ground, I am hoping it helps with this issue...along with the additional lock-up of the TRE upgrade (yes, I sent my Shep diff to TRE...).

I am very interested if your experience contradicts this thought pattern though or you have a different thought process. Experience >> Theory

griceiv

iTrader: (2)

the point of interest is the load that the helper spring unbinds. Once you go below that load your combined spring rate for that corner drops dramatically. If you assume that the new 2-spring rate << main spring rate your roll stiffness on that axle decreases to just over half of it's previous stiffness.

If you have a big sway bar installed and it's enough to keep the tender blocked, then what's the point of the tender other than just adding unsprung weight? You won't get any more droop travel in roll and you'd have to have both rear tires nearly unloaded before the tenders became active again in heave. Not to mention they eat up lots of shock travel and collect rocks.

If your spring unseats when you jack the car up off the ground, and that bothers you, have the shock shafts shortened and/or add droop limiting spacers.

03whitegsr

iTrader: (8)

Agreed, the rate greatly softens when the helper unbinds. But the tire would otherwise be in the air anyway so wouldn’t the rate without the helper be 0 otherwise since it has nothing to react against?

Also agreed, if the rear bar is stiff enough to overcome the helper, I see no point in them either for normal conditions. This is why I’m also considering something more like a 5kg/mm assist spring. It would still be fully compressed at normal ride height, but the knee would only be about 0.25” into the droop travel when preloaded a reasonable amount. This would actually be stiff enough to overcome a roll bar to some degree and allow extra droop.

I have about 4" of shock travel in the rear. With roughly 550 pounds on each rear tire, I use up a whole 0.70" of that shock travel from the weight of the car, leaving 3.3" of bump travel and a maximum load of 3200 pounds per tire before I reach full compression. I could stand the car on one wheel and not reach that. Now, I realize that's not how it works, but I'm simply pointing out I have about 1" more compression travel then I would ever need with my current rates. I'd be running the tire into fender long before I used it up, so why not take advantage of it by making it droop travel?

I fully agree, it won't add much since the rate would be low. But even with a low rate, wouldn't it be better to have the tire on the ground with 50 pounds on it getting traction instead of having it dangling in the air and putting that 50 pounds on an otherwise already overworked tire?

I am using a body adjustable setup and can reach my current ride height and future ride height while maintaining some spring preload when freely extended. It’s not an issue of the spring rattling around.

griceiv

iTrader: (2)

you get more displacement for a given amount of load transfer with the helper (if it goes active), therefore your roll stiffness is lower. I think you're considering a load case well beyond when the helper unbinds. Compare the difference in roll angle with 100lbs of load on the corner with a knee at 150lbs. Yes the helper setup will have a comparatively more extended shock position, but that doesn't mean the no helper setup is off the ground at 100lbs of load.
The roll angle is a product of the load transfer, not the other way around.

50lbs left sitting on the inside rear tire is 50lbs that could have been used to reduce the load transfer on the front axle. The most overworked, and performance limiting tire on the whole car is the front outside tire, anything you can do to help it out is better. The rear tires are basically just along for the ride. In fact the unsprung weight of the inside rear tire/suspension is best used as a sprung mass counter balance to reduce load transfer on the front axle (aka hanging in the air).

*if you haven't noticed, I am in the pro-tripoding camp.

Dallas J

iTrader: (1)

This!

Ive been trying a bunch of different things lately that in a way counter what has been the more recent generally accepted evo setup and I'm definitely finding more rear bar, and get the inside rear unloaded to get the front inside more load.

The name of the game in getting front grip is reduced front weight transfer, everything else is secondary for me at this point (well, for now...). Evos are miraculous in transition and being able to accelerate through everything, but seriously lacks in front grip at steady state. I was in shock when riding in a STR Miata on 255 RS-3 at how much speed he had through a sweeper. No wonder they're so damn quick with such little power.

03whitegsr

iTrader: (8)

You definitely bring up an interesting thought process. I'm pretty sure you've built some SCCA Nationals winning EVOs, if I'm not mistaken which makes your thoughts a lot more valuable then mine.


Would you say a visual representation of what you are saying then would be a tetter-toter, with the pivot being a line between the front inside tire contact patch center and the rear outside tire contact patch center? Any weight hanging in the air on the rear inside tire then tries to lift weight off the front outside tire. To balance the weight, the pivot would then have to pick up that extra 50 pounds. Do I have that thought process about right?

I'm not one to be "pro" anything but I can't deny I've seen fast BMWs picking up the inside front tire, fast FWDs pulling the rear inside, and fast EVOs lifting the rear inside. just like a FWD.

My only concern is that it seems to be messing with the rear diff. Or maybe it's the combination of the rear diff and the VC center diff? Either way, I get some hop when trying to get on the gas with what feels like the wheel in the air.

griceiv

iTrader: (2)

yep, pretty much. except the unsprung weight of the rear is more like 90lbs. you gotta transfer all of the sprung corner weight before you can pick up the unsprung.

yeah, best to solve drivetrain problems with drivetrain solutions. You can get in to some weird places trying to fix the diffs with the suspension.

Dallas J

iTrader: (1)

It really does help to see "The Berry Mobile" in action. When you see evo's with a tire a foot in the air, it's too soft up front. Watch Berry/Grice drive through a sweeper and you may only see the inside just slightly lift at peak corner but as soon as they're pulling out of a corner lateral grip is reduced and its back to accelerating out of the corner. Quite possibly a testament to how stiff the car is, but listing to the man himself talk and going home, noodling, and applying it myself... well, its all making sense.

I really feel like I learned more in the last 2 months on Evo setup than I did in the last two years.

griceiv

iTrader: (2)

you mean like this?

03whitegsr

iTrader: (8)

Thanks for the info, definitely gives me something to think about. As soon as my wheels show up, I can start figuring out how much compression travel I can even use before running the tire into thing to help select a spring length and rate for the rear.

Not sure what the answer would be on the center diff as I have an 8. Retro fit ACD for the most tuning options?

griceiv

iTrader: (2)

The tire probably won't hit anything you can't fix. the limiting factor seems to be the upper ball joint maxing out it's angularity.

Dallas J

iTrader: (1)

Haha, yes! The difference between lifting and hanging.

sparky

iTrader: (1)

OP, did you upgrade to the Whiteline front endlinks as well? Or just the bar?

alleggerita

I don't think we disagree that much. My comment was focussed on a car that is street driven not a track focused car. The article refers to skidpad/smooth track performance - the closer you get to a go-cart the better. Not so great for road use but definitely works for track use.

BTW, the picture you posted: your car does not look super low, nor does it look like it tripods a lot. Like that both front and rear wheel are starting to come of the ground - no wonder you are looking for lower CG

03whitegsr

iTrader: (8)

As far as ride height, it is an action shot and not the static ride height. Even at that, it looks like it's probably a good 1.5" lower then where I'm at in the rear. If that's full droop, figure 3/4" or so more compression at ride height, that puts the fender lip probably about 3/4" from the lip of the rim. That's pretty low.

It's not "stance" but it actually goes fast instead of parking hard.

Not that it matters here but I measured all the front suspension mount points today and created a solid model. Still got a couple pieces left to model and then on to the rear.