griceiv

iTrader: (2)

hahah Sorry, i'll tone it down a bit.

ignore roll centers, lower the car as far as you can.

alleggerita

not good advice, especially for anything driven on the street

Dallas J

iTrader: (1)

Its a balancing act. Is the car stiff enough to control roll with a given ride height/roll center height? Making tire happy, first priority. Roll Center/ Roll moment are secondary.

Another thing to ponder, does a car roll around its roll center? Short answer, no. Roll center calculations assume each tire is doing the same amount of work.

alleggerita

Sure suspension is always a balancing act. The issue is that you have a McPherson strut suspension up front and a multi-link system out back.

So when you lower the car your front roll centre will disproportionately drop relatively to the rear roll center resulting in a steeper roll axis front to back - not good for understeer among other things, nor good for front roll resistance ... you just set up your car for more tripoding with all other things being equal.

Other issue with McPherson is the camber curve. With increasing drop, you will loose negative camber. Your front camber curve will go south on you much quicker than on the rear. Unless you adjust the geometry, this will really kill your tires and make it not drive so nice ...

Yes, you can control a car with too low a front roll center with sway bars and heavy springs. This will do OK on a smooth track but will drive horrible on anything but smooth roads.

Way better to address geometry to begin with, e.g. with RCK and adding caster to assist the limited camber gain of the McPherson strut .... lower to the point of not screwing up your geometry after that ... and then go with the spring rate you need ... and assist with bars as necessary.

Geometry does keep tires happy

griceiv

iTrader: (2)

well to be fair, I said 'as far as you can', which in a softly sprung street car might not be all that much lower.

I wasn't trying to say a RCK is worthless, just saying that the optimum ride height is as low as you can go. I would not trade increased ride height for better roll center geometry ever. Certainly getting better geometry without having to raise the car is a good thing.

griceiv

iTrader: (2)

I disagree. technically speaking, lowering the front roll center in relation to the rear roll center will make the car more oversteer. Also lowering the CG will reduce the total load transfer, and thus, reduce the amount of tripoding...all other things being equal.

Dallas J

iTrader: (1)

Marshall, I was thinking about something we were talking about earlier about natural frequencies either being constant or variable. Well I realized an assumption I made was wrong, you're right they are effectively constant though may have a slight shift with body roll.

alleggerita

Not if your roll center goes subterranean and your camber curve goes to pots with it. A low roll center to a point will give more grip but too low a roll center will kill your front geometry, as well as reduce roll resistance in the front which will ultimately result in understeer. All these variables interconnect and depending how far you move it is not a linear effect.

The main point remains: Unless you are totally changing your geometry with different suspension arms or moving pick-up points, lowering a McPherson front suspension too much - on an Evo or any car for that matter - will have pretty serious negative effects on how your car handles. And jacking it up will not improve it either. But it cannot go too far from its designed-in ride height without creating serious geometry issues - never mind spring travel.

chrisw

iTrader: (2)

bump up your front spring rates to reduce the body roll, lower the rear spring rates and put the rear bar at full soft or I have even debated removing it altogether.

Another trick to help to keep the inside rear from lifting would be to rebuild your rear diff to the correct factory spec. I did this and now I get a little oversteer before the inside rear lifts. I have been able to tune that oversteer with nothing more than tire pressure adjustments.

I browsed through your other responses, and if you are running a 700lb spring in front your close. IMO, you need to pay more attention to what the rear of the car is doing. You are lifting the inside rear because you don't have enough suspension travel to keep the wheel on the ground. So how to get more travel out of the rear suspension?

soften the rear sway bar, or eliminate it as I mentioned earlier.
soften the rear springs
lower the rear of the car

all of the above.

griceiv

iTrader: (2)

and yes, the slight changes in motion ratio due to suspension position will change the nat freq but it's pretty a small change in the evo.

Dallas J

iTrader: (1)

Ignore camber gain/loss. On a mac-strut car you don't start loosing camber till your control arm angle is equal to the kingpin inclination which on an evo is around 15 degrees, far beyond what we'll ever actually see.

I agree with some, disagree with others in this statement. Yes, increase front spring. The car is rolling over on itself and also diving during turning (im guessing you're not braking there right?). Its the diving part that causes the rear inside to need more travel to prevent lifting.

The part I disagree on is rear bar. I use to push for soft rear bar, big front. All that's doing is trying to get the inside rear down to do work. On the evo, rear grip is plentiful, we need front grip. So reduce weight transfer up front, and increase it in the rear (All in an effort to get the inside front to do more work). To hell with what the the inside rear is doing, we don't need it . Though, what you'll find when you get near the right rates and bar balance is the car will be flat enough to not really worry about lifting that inside rear.

The car that sparked my ideas for changes all came from watching Tom Berry's car (Setup by Grice now?). Comparing how his car reacts to mine is when the real a-ha moments came on for changes to my car. The setup isn't the traditional 700/900, Ill say that much

griceiv

iTrader: (2)

Then we'll agree to disagree. Some nice reading info.
http://www.rhoadescamaro.com/build/?page_id=764

My favorite quote; " If my competitor has better a-arm angles, better corner weights, better working geometry, but a higher CG height, he's going to be slower than me around the corner because he's going to have more weight on his outside tires."

03whitegsr

iTrader: (8)

Great thread

I have to agree, the RCK doesn't do a whole hell of a lot. It's only about 5/16" taller then stock. That means you can lower the car about 3/8" while maintaining the stock geometry. We need an option that moves the lower control arm down about 1.5" for it to have a real impact.

My car is doing the same on only moderately sticky 275s. Currently, the car sits at like 7.25" at the pinch seam to the ground in the rear and 6" in the front. Picture for reference The front control arms without the RCK sit about 1/4" higher on the ouside then the inside at this height (already passed level). It's maybe 1.5" lower then stock in the front, probably 1" in the rear?

Here is the direction I'm heading:
Lower the rear about 2.25" more and the front about 1" more, eliminating the rake and reducing CG
Increase front spring rate from 10kg/mm to 12 kg/mm, leave the rear at 14kg/mm
Add helper springs in the rear to increase drop travel by about 1.25"
Adjustable rear sway bar (110%-147% of stock adjustment range)
Hollow front bar that is about the same stiffness as stock

I'm not interested in doing this to try and build something to sell, but I have the stock front hubs modeled up and was working on a billet aluminum hub carrier/upright to drasitically change the front geometry. If there was serious interest from a couple people looking to get a limited production run together, I would get back on the design work. I'd actually like to consider a tubular, adjustable lower control arm too that would allow track width and caster adjustment on the arm.

griceiv

iTrader: (2)

skip the helper springs. The last thing you need is your rear roll stiffness to drop right when the car nears max lateral grip. Terminal Understeer guaranteed.

I'd also be hesitant to run aluminum front knuckles due to the heat from the brakes reducing the strength/fatigue life of the aluminum. I would do a fabricated sheet metal upright in a second though.

Dallas J

iTrader: (1)

Considering heavy springs I see 3 possibilities here.

1) Main spring only - With the spring rates we run I only see 1.2" of travel from full droop to static ride height. That doesnt give much room for a tire to deal with surface irregularities. What does this instant stop in tire travel do for grip on that end? Im guessing only bad things.

2) Main and Helper - Give more travel, but beyond that 1.2" of wheel travel the tire can only use its momentum to extend the shock. It will at least get rid of the sudden change in wheel travel speed but the rate is null at that point. So you have an large instant rate change from X to ~0.

3) Main and Tender/assist - Ok, here are a lot of option with the different rates of tenders but my thoughts were to run a low rate high travel for increased preload but limited fully collapsed load. Since if the car is rolled enough to take up that 1.2" (It'll actually be less due to main spring also compressing some amount before tender is fully compressed) of travel then that wheel is probably pretty well unloaded and not contributing nearly as much to the overall grip at that point. So thinking in that terms, the rate change will have less effect on roll stiffness and overall grip. Where I noticed a huge improvement was in bump recovery. If you have a course like we do (Both Packwood and PIR) then you need to be able to deal with damn near getting airborne.

Number 1 assumes you have adjustable bodies and can set the preload (Grice has this in his shocks). In my case, after switching from my AMRs to AST5100s I lost this. So I need to take up the space with something (Using Case 2 or 3) to get the car low enough without worrying about the spring unseating. I need something in there, and with a back to back test of no tenders vs tender in the rear made a difference in bump recovery and I didn't notice any issues with increased understeer (The car already turned like ****, didn't get worse though )