Dallas J

iTrader: (1)

So Ive stepped up my rates getting the car balanced and roll under control. But with that has come a reduced droop limit. To keep the spring seated without a helper spring I have the spring perch at about 1/4 turn of preload, basically as little as I can. The ride height is set by the lower adjustment.

Measuring the static distance from the wheel to the fender, then jacking the car up to measure max droop I only have 1 3/16 inch droop. Its really hard to explain in text but 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.

Basically I need to get the maximum droop on the rear. Ive talked with several nationally competitive guys in my club and we all agree its the key issue.

Thinking about it over the last week, with 3 events of driving it in its current state I have a plan of attack. Here's the process Ive come up to figure things out.

1) Jack up the car and completely loosen the spring perch.
2) Raise the wheel to its maximum height (basically right before it crashes into the fender)
3) Adjust the lower mount so the shock bottoms out right before the wheel can
4) Lower the car down and adjust the lower perch to get to the desired ride height
5) Raise the car and measure both the droop and the space between the perch and spring. This is the space I need to take up at full droop.

So then Id know how much space I have to take up (tender spring free length will need to be longer than this).

The question is, what spring rate do I choose? My gut is telling me to choose a rate soft enough that it fully compresses at static ride height but stiff enough overcome the sway bar and push the tire into the ground keeping all 4 down.

Am I on the right track here, over thinking it, only one having this issue?

My setup, AMR coilovers, 700ft/900rr spring, Whiteline FSB, Stock RSB, TRE Rear diff, 245 V710s. Reason behind kumho's are two-fold. One, the local venue is really abrasive and tight. The kumho's survive there very well and at slow speed it doesn't feel under tired. Two, I believe in developing a car on smaller less grippy tires before stepping up. Next year Ill be on 285 or 295 hoosier or goodyears.

griceiv

iTrader: (2)

I'm curious what you think you will gain by having more droop travel?

BigT

iTrader: (8)

Why not go with a taller spring?

Dallas J

iTrader: (1)

Im hitting the droop limit a lot on course which I think I causing a sharp ramp in load change causing the issues I mentioned above. Its pretty obvious when its happening as the car gets into a harmonic inside lift, rear slip, inside down, car grip oscillation. I can get rid of the oscillation running a bit more rear Rebound but then the car is unstable through slaloms.

Also on the street 1.187" of droop is very noticeable basically everywhere.

I'm not saying for sure this is the problem, its just my best guess right now after talking with others locally. I guess Ill give it a shot, its not like it costs much to try.

Taller springs wont help, a 8" 900lb/in spring will compress exactly the same amount as a 50" 900lb/in spring with a given load.

Stormin Norman

iTrader: (2)

OP, a longer spring can be the answer; it takes 900lbs to compress the spring one inch, so by having a taller spring, your overall height under load will be taller, thus giving more available droop.... unless I misunderstood your response to BigT.
Addressing the other aspect, I spent a long time thinking about whether to run tender springs or helpers with my setup since I have a tall gap and only 6" springs... I chose helpers as I wanted to sit lower, and yes, I don't need that much travel and my springs aren't that stiff. What's your spring length now? I'm assuming it's 7". Anyway, I've seen "light" linear tender springs, progressive tender springs, and 1300lb tender springs; obviously your tender springs should be lighter than the main spring. But, you could always try a 250-550 progressive spring, or just a 500 or 600lb tender spring, if that's your issue. Just keep in mind that tender springs are about the same price as main springs, so your cost is going way up vs. just buying longer and/or heavier springs.

griceiv

iTrader: (2)

I would recommend against helper springs. Think about what happens to your rear roll stiffness when the helper spring becomes active (unbinds) in roll and its effect on handling balance. If you truly feel you are affected by your limited droop travel the only way to address it is to soften the spring rates front and rear if you want to preserve your balance.

Stormin Norman

iTrader: (2)

Helper springs collapse at like 20lbs, they should be seated fully and the main spring under load when the car is on the ground. Should never become uncompressed and if it does, it will help keep the main spring seated.

Dallas J

iTrader: (1)

The idea behind using the Tender spring is it only becomes active in a range where the droop would have previously been bottomed out. Ok, actually their would be a small amount of overlap depending on the tender vs main spring split but its pretty small.

Using the tender would give extra travel and help push the wheel back to the ground through the stiffness of the swaybar and keep the inside rear planted and at least in the game. Right now the spring rate is effectively going from 900lb/in (times MR^2 of course) to 0. With the tenders there will be a decreased spring rate but its not on a main loaded spring and only in the situation where the wheel would previously have lifted. At least thats the idea.

Stormin, Youve got it a little wrong. With the two height adjustments and no helper/tender you have to have some amount of preload to keep the spring seated. Height is then adjusted at the bottom. So as soon as you set the car down regardless of how long the spring is, youre moving the spring. Their is no travel before the main moves. The point of me running the tender is to get a bit of travel without going to a Zero rate spring, but still have it fully compressed at static ride height.

griceiv

iTrader: (2)

As soon as the tender becomes active your roll stiffness decreases dramatically and suddenly resulting in monster understeer but only when the car is under heavy lateral loading. The result is a car that will always understeer at the limit and likely be very loose in transitions after you try to apply all the fixes to get rid of the terminal understeer.

Attempting to solve inside rear wheel lift on a car with 60+% front weight bias anyway other than moving the CG rearward is not going to do you any favors. The only argument for doing so is to help out an under-performing rear diff, not a problem I see you having according to your mod list.

Also, why don't you try increasing front shock stiffness. It's the outside front wheel that is letting the inside rear wheel lift and oscillate. you wouldn't end up with a transitionally loose car.

Stormin Norman

iTrader: (2)

I'm fully aware, but you will compress the spring less if you have a longer spring. Then you can drop your perch and still allow for a longer droop without the spring unseating.
And unless your spring is coming unseated with that hop, you really don't need them, and you will certainly know when your spring is coming unseated.

Dallas J

iTrader: (1)

I can try going stiffer on the front, but I think you're missing the fact that these should at most barely come in to play on a normal corner. Its the offset bumps and jumps we have littered through Packwood Im trying to control. This last weekend I had the entire rear end leave the ground over a bump on power. I feared for my axles life!

So you asked what am I expecting more droop to help, its to keep the wheels on ground due to bumps no body roll. Regular sweepers I only barely have the inside wheel bobbing up unless its off camber or bumpy.

Dallas J

iTrader: (1)

How will using the same rate with the same load make a spring compress less regardless of its length? 900lb/in is 900 no matter the length. If you have exactly 0 pre-load but no unseating of the spring the load will be applied as soon as the wheel moves.

Mine moves ~1.1875in. The fact that spring length doesn't matter is why I can say I have about 570lbs load (assuming also a 0.73 MR).

griceiv

iTrader: (2)

Perhaps packwood is getting worse, but I've driven the berrymobile there and I don't recall having any issues with the bumps. We have less droop travel than you do.


Are you talking about stuff like what happens at 0:15 in this video?
http://www.youtube.com/watch?v=y_9cCVgBWFs

Stormin Norman

iTrader: (2)

Jesus christ, how are you missing this?

Alright, lets make this simple for you.

Say you have two 100lb/in spring, one is 1" long, one is 2" long. You put 100lbs on each. The 2" spring will be collapsed 1", making it 1" tall under load. The 1" spring will also be collapsed 1" under load, but since it's only 1" long, it's fully collapsed. Get it now?

Dallas J

iTrader: (1)

First of all, a 1" spring cannot collapse 1".

Second, by your own hypothetical example both collapse 1". If the strut is fully extended at Zero preload, then they are compressed 1" by the load. That means they can also only un-compress (droop) back to that starting point before you're back to fully extended.

The only thing a longer spring gives you is more range of compression before coil bind (a greater load range), and in a race car the compromised is size and weight.