spring rate changes

[coolbeans]

I've finally got a couple seasons of autocrossing under my belt and am looking for suggestions. The car has been one progression after another toward racing. The car started as a street driver, as a result I've done everything twice. It's an ls1 car with 6 speed and a quafie r200 with wolf racing 1/2 shafts, adjustable a a-arms all the way around and adjustable tension rods. I've sectioned the struts and am running coilovers with 225F/250R springs. Also wilwoods all the way around, weld in camber plates in the front and none in the rear but run about 2.2* camber in the rear with a arm adjustment. I'm looking to increase the spring rates. I've noticed that some are running more spring in the front and some are running more spring in the rear. What makes the determination? I haven't stiffened the frame yet and was wondering how stiff you might recommend? Also am I doing the right thing by running poly bushings? The car weighs right at 2850 lbs with driver and is 1475 in the rear with me in it and 1/2 tank gas. Any input would be appreciated. This photo shows some of the issues I'd like to address. The picture was taken on a 50 mph transition. The car has way to much lean. I also experienced the rear of the car breaking loose at higher speed corners. I'm running Tokico adjustables all the way around. 1" sway in the front and 5/8" in the rear lossened up a bit.

Edited November 27, 2011 by coolbeans

[TimZ]

First of all, that's a really good looking Z - nice job!

 

I'll leave the spring rate/sway bar recommendations to the guys who are racing regularly, but I do have a couple of other pieces of advice...

Most importantly, before you go all out making this into a racer, you should decide where you really want to end up with this car:

A: still enjoyable on the street, but respectable at an autocross

B: Competitive in EM, all else be damned (last I checked it was kind of hard for a streetable car with, say, an interior, to be competetive in EM)

 

You didn't really say in your post which direction you were leaning towards, but just looking at the car in the pic and reading between the lines, it sounds like maybe you would still like to drive the car on the street. If that's the case, then I'd caution against going too extreme with spring/bar rates - I started down that path some time ago, and found that I was really not enjoying driving the car around any more. I've since backed off and find the car much more enjoyable overall (I guess maybe I'm just getting old).

 

Also, what's the condition of your local roads? If they are pretty smooth, the you can most likely live with some higher rates. If not, then the lower rates will make the car more controllable in the real world.

 

I'm sure others will mention this, but if you do plan on going a lot stiffer then you will want to stiffen the unibody accordingly. Of course you'll also want to pay attention to what's legal in your class, etc.

 

Finally, when you mentioned that you've noticed the rear end breaking loose at higher speeds, was this on trailing/constant throttle, or was it power-induced? These aren't necessarily caused by the same things.

[h4nsm0l3m4n]

Based on what I've read on this board the illumina setup you currently run is about the limit for spring stiffness without beginning to strengthen the chassis. Also 250 lb is about the limit for how stiff a spring you can run with the illuminas (although some have run stiffer without issues). I run 275F, 330 R with Megan shocks looks like I might have just a little less roll.

The rates are probably too stiff for the chassis, but oh well I guess.

 

As far as front/rear spring rates, I'm not an expert, but from what I understand, what youre ultimately doing when you "tune" the spring rate is changing the natural frequency of the suspension. Unsprung weight, corner weights, motion ratio, etc all influence this same natural frequency, but are typically not design parameters - spring rate is. It is a general rule of thumb to have the rear natural frequency be a little higher than the front. This is because the front hits bumps first and the rear needs to react faster. Once you choose a natural frequency you can live with there are equations you can use to back-calculate the spring rate necessary to achieve it. They are available in various books or online resources. My calculations have always yielded that the Z requires a stiffer rear spring than the front so I cant explain why some people run stiffer springs in front, but I'm sure they are legitimate. I'll let them speak for themselves.

 

Finally, as mentioned before, I think youre going to run into trouble competing with the crazy stuff they have going on in Modified class. Especially if you want to keep some resemblance of "streetability" for your car. Unless youre set on running in EM, or have a previously unspecified mod on your car that forces you into that class, XP would fit your car perfectly. Also, unless I missed a recent update, I hear next year SM class is going to be allowing "any engine" to be used in cars that otherwise fit into that class. Either of these classes would fit your car well and give you plenty of competition to run against.

Edited November 27, 2011 by h4nsm0l3m4n

[tube80z]

I've finally got a couple seasons of autocrossing under my belt and am looking for suggestions. The car has been one progression after another toward racing. The car started as a street driver, as a result I've done everything twice. It's an ls1 car with 6 speed and a quafie r200 with wolf racing 1/2 shafts, adjustable a a-arms all the way around and adjustable tension rods. I've sectioned the struts and am running coilovers with 225F/250R springs.

 

You have a really nice car. It would be a shame to try and hack it up to run in EM. As mentioned below you have two options for classes, XP and SM. SM doesn't allow for any chassis mods outside of strut bars and a basic cage as I understand it. XP allows more freedom on the chassis but you can't mess with the firewall. Your car could fit in either class and probably be fun to drive and locally competitive. To be nationally competitive is a whole another story and would require a completely different direction I'm afraid.

 

If you like to drive your car on the street and do track days then I'd look at SM. If you have to put it on a trailer and run on slicks and do autox/track days then XP is probably a better match. EM will require that fabrication become your hobby and you'll most likely end up with a silhouette of a car. If you really want to run in EM I'd either fund a rusted out junker or just build from scratch and put a fiberglass funny car style body on.

 

As mentioned below the best way to pick springs is by frequency. For a more streetable car I'd look at springs in the 2.5 to 2.75 Hz range (most likely 300 to 400 lbs/in). You'll also need inserts that can deal with this and ideally you'd want something custom valved. It's not as big of an expense as you might think and money spent up front is going to be better than buying three of the same part. I'm sure you already know that pain.

 

For an XP car on slicks it really depends a little on the tires. I'd probably start at 3 Hz for a sedan/GT based tire and look at going stiffer. If you run the FA tires then I'd look at springs in the 3.3 to 4.5 Hz range (that's very stiff and requires very good shocks). And to run rates that high you really need to have a cage that ties the car together.

 

So the easy route would be to start in SM and then progress up. But truth be told you won't be much faster in XP than in SM unless you do some serious work to make all the new bits work together. There are a lot of really smart people on this board that can help point you in the right direction to go faster. But the ROI quickly goes south.

 

For bushings you really want to look for other options than poly. The stiction is way to high and that hurts grip. Where ever you can it would be good to change over to rod ends/monoballs. Again this all depends on class.

 

We're all happy to share what we know around here. Hopefully this helps.

 

Cary

[tube80z]

It is a general rule of thumb to have the rear natural frequency be a little higher than the front. This is because the front hits bumps first and the rear needs to react faster. Once you choose a natural frequency you can live with there are equations you can use to back-calculate the spring rate necessary to achieve it.

 

That rule of thumb doesn't apply to race/high performance cars. It's really intended for ride (flat ride) for not causing discomfort. For racing we are more concerned with balance in the corners and control. As long as the car doesn't hop off the ground in a straight line it's okay. It's also perfectly acceptable to have the same frequency front and rear or the front stiffer. In a high HP car that often is quicker.

 

Cary

[coolbeans]

That rule of thumb doesn't apply to race/high performance cars. It's really intended for ride (flat ride) for not causing discomfort. For racing we are more concerned with balance in the corners and control. As long as the car doesn't hop off the ground in a straight line it's okay. It's also perfectly acceptable to have the same frequency front and rear or the front stiffer. In a high HP car that often is quicker.

 

Cary

Thanks for all the input. The car was initially built for street fun and a friend of mine turned me on to autox, damn him. I've got about 700 miles on the car since finishing it and 650 of those have to do with racing, either driving to or racing. I'm confused about the XP class. Me and a buddy built z cars, his with a traditional SBC and mine with ls1. After our 1st season we were told we HAD to go to EM class by a veteran member because we were not running corperate motors? I have adjustable A-arms so I believe that may keep me from SM class? Being new we did not question the switch but I would certainly prefer XP. I'll have to do some research to prove this guy wrong I guess. As far as the car goes I don't mind it being for race mostly, I've got other projects for street duty. The picture there is while slowing down and that is when I experience the loss of control in the rear. I did have about an 1.25" rake in the car front to back measured at the seams. I've adjusted that to 1/2 ", lowering the rear. The car is pushing a little over 500HP One of the surfaces we run on the most is concrete like shown in the pic, this is where I notice the problem but it is also alot faster track. The poly bushings are only on the inboard side because I'm running TTT control arms and tension rods. Thanks everyone

[coolbeans]

Thanks for all the input. The car was initially built for street fun and a friend of mine turned me on to autox, damn him. I've got about 700 miles on the car since finishing it and 650 of those have to do with racing, either driving to or racing. I'm confused about the XP class. Me and a buddy built z cars, his with a traditional SBC and mine with ls1. After our 1st season we were told we HAD to go to EM class by a veteran member because we were not running corperate motors? I have adjustable A-arms so I believe that may keep me from SM class? Being new we did not question the switch but I would certainly prefer XP. I'll have to do some research to prove this guy wrong I guess. As far as the car goes I don't mind it being for race mostly, I've got other projects for street duty. The picture there is while slowing down and that is when I experience the loss of control in the rear. I did have about an 1.25" rake in the car front to back measured at the seams. I've adjusted that to 1/2 ", lowering the rear. The car is pushing a little over 500HP One of the surfaces we run on the most is concrete like shown in the pic, this is where I notice the problem but it is also alot faster track. The poly bushings are only on the inboard side because I'm running TTT control arms and tension rods. Thanks everyone

After a little research I've discovered we could have been racing XP all along. Back to the spring issue. Do springs advertise the Hz range on the springs? I did not notice that on the eibachs. A couple of threads I read where they were running stiffer springs in the front had something to do with the quaife they were running and pushing issues. They also brought into the conversation the whole sway bar size in the front and no bar in the rear topic. I had loosened my rear swaybar the last couple races and think that was a mistake? Where exactly do the tokico illuminas lose their ability to perform? 250# 275# 300# 325# 350# ? If I run 300# 325# would I then need frame stiffening and new struts? and are the bulldog frame rails enough or would you just build your own?

 

Thanks

[JMortensen]

Nice looking car. Agree though, way too much body roll.

 

Frequency of the springs relates to the spring vs the sprung weight and motion ratio, so a 200 in/lb spring might be a 5Hz spring on a formula car or a 1Hz spring on a heavy car. Read the FAQ on brakes/wheels/suspension/chassis. Particularly Dan McGrath's post here: http://forums.hybridz.org/index.php/topic/63492-suspension-tech-motion-ratio-unsprung-weight/

 

There is a lot of information to be had here. The one thing I would suggest is that if you go really stiff on the springs you'll probably need to do some pretty significant chassis reinforcement, otherwise the chassis will just flex, rather than the spring working. That's a whole other rabbit hole, but search "chassis reinforcement" and you'll find some good info on that as well.

 

Good luck and keep us posted.

[johnc]

First off, get the rule books. XP is probably you best place to run and you can be competitive locally. You have no chance nationally against cars like Fred Zusts's Monster Elise but that's a whole different world.

 

As mentioned above, upgrade the shocks and reinforce the chassis before bumping up the spring rate.

[heavy85]

Like has been said first decide how far you want to take it. Making it not very comfy on the street vs cutting it up into a racecar is two totally different things.

 

You say the back end has issues under braking? If so toe in is your friend. Helps drastically settle the rear under braking IME. 225/250 or vice versa can be very fast locally with a good driver. If you are still early in the learning how to drive curve < say 3 years of consistent racing then just tweak what you have and learn to drive it before going way stiff.

 

Cameron

[RebekahsZ]

My z-car is set up pretty much the same as yours and i love it. It drives and autocrosses "great." I don't have any pictures of it on a course, so it may look all screwed up. What I see in your picture is that you are probably hard on the brakes and turning at the same time, which will upset any car. I bet your car looks much more composed in other pictures where you have already slowed the car before entering a turn and then are driving the car thru the turn. Check it out, I bet things are not as bad as they look in this picture. Now on the class thing: screw 'em. You kind of turned your back on classes the day you put a v-8 in your car, along with composite body panels. Classes were meant to slow you down! Be free and run in an exibition class (if your club will allow it) and just enjoy the satisfaction of beating everybody and walking away without a trophy. Spend more time at the drag strip and make new friends, we rednecks hate classes-it is hard to hear someone complaining about what class you are in when they are in your rear-view. Strangely enough, the best drivers in my club beat us all in stock cars, sometimes on street tires.

[cnunes]

weld in camber plates in the front and none in the rear but run about 2.2* camber in the rear with a arm adjustment.

 

I also experienced the rear of the car breaking loose at higher speed corners.

 

Just wanted to throw out there that I've always heard the track width is usually greater in the front than the rear to reduce oversteer (rear breaking loose) or promote understeer by placing more load on the outside front tire. Therefore, adjusting rear track width just to affect camber may not be advisable? A wider rear track width could cause oversteer, right?

 

However, if the loose rear problem is indeed related only to trailing throttle or braking, I second the rear toe-in comment...but as RebekahsZ said, I try to get most of my braking done in a straight line, because my car used to oversteer (and spin out) in sharp autocross turns unless I was on the gas as I turned the wheel to shift weight over the rear tires. That was greatly reduced by the 1" sway bar you have, but I'm still running the stock rear bar so my car presently has a very different balance than yours. As a free test, you could try removing the rear bar altogether and see if that helps predictability? The car will feel slower (more understeer) but it may be easier to drive which sometimes results in faster times. And if disconnecting/removing the rear bar helps, you'd know that in order to use the rear bar you have, you may need to step up to a stiffer front bar like the 1 1/8" deal that Stepan has. Yes, increasing spring rates will reduce body roll but tuning the (anti)sway bars can have a substantial effect as well without dramatically affecting the ride quality.

 

Just my two cents...

 

P.S. That is a indeed a beautiful Z you have there!

[johnc]

Just wanted to throw out there that I've always heard the track width is usually greater in the front than the rear to reduce oversteer (rear breaking loose) or promote understeer by placing more load on the outside front tire. Therefore, adjusting rear track width just to affect camber may not be advisable? A wider rear track width could cause oversteer, right?

 

 

Actually, increasing track width reduces the load on the outside tire. The further the tires are from the CG the less affect the CG moment arm has on them. A simplified version of the weight transfer calculation is: Weight transfer = (Lateral acceleration x Weight x Height of CG ) / Track Width. The wider the track width, the lower the weight transfer.

[cnunes]

Sorry to hijack, but John, can you please explain your point a little more? I understand that increasing track width will reduce weight transfer across that axle, but I was referring to the effect of changing rear track width in relation to the front track width. Front to rear track width differences should have an effect on oversteer/understeer. The way I picture it in my head is a three-wheeler with the single wheel in the rear (narrowest rear track width possible). In this extreme example, during a left-hand turn, the front right tire will be taking on more of the lateral load than if the car was square. If the same car had an equal width rear track, some of that cornering load would be distributed to the right rear tire, allowing for more overall traction available thus faster cornering. However, I think I had the effect on balance reversed: wider rear track width compared to front promotes understeer and wider front track width yields oversteer. (The opposite example would be a tricycle which will understeer.) So since the OP has oversteer, you DO want to increase rear rear track width by adjusting the arms out to induce some understeer, correct? John please chime in again if I am incorrect!

[johnc]

I don't look at it exactly that way. Increasing track width at one axle doesn't cause something to happen at the other axle.

 

Again, in a simplistic way of looking at it, increasing track width increases overall grip by reducing weight transfer. It has the same effect as lowering the CG. Increasing front track increases grip at the front. Increasing rear track increases grip at the rear. If your car has a tendency to understeer, increasing front track will reduce that tendency. If your car has a tendency to oversteer, increasing rear track will reduce that tendency.

 

I don't think about reducing track width unless I'm at some kind of limit (rules, mechanical) and its the only way to restore some balance to the chassis. For the S30 we tend to run more front track width as an easy way to reduce some on the built-in understeer in the chassis and help turn-in. We also run into mechanical limits (fenders) in the back regarding rear track width.

[coolbeans]

I'll go ahead and do the frame stifffening and contemplate what stiffness of spring to use. I currently run 225 tires in the front and 235 in the rear on 9.5 and 10" rims. considering new rims for the front and using the same size tire. Is there any more opinion on stiffer front springs versus stiffer rear springs? Thanks for all the great advise and things to think about.

[ZT-R]

I am running 250 rear rates with tokico and with the car jacked up it still rides like complete crap, unless im on a newly paved road. Maybe i need to go to 150 or something.

[RebekahsZ]

Hmmm...wheels and tires? What brand and aspect ratio of tires you running? I'm running 225/50/15 BFG R1 all the way around (they were on sale). I think for autocross go with either BFG R1 or Hoosier A6. I've never understood the concept of "staggered" wheels and tires on a sports car that you plan to drive at the lateral traction limit. They do look cool, that's for sure! But, I wonder if they are really necessary for a car w/ approx 50/50 weight distribution. I wonder if your car is rough just due to your tires? Those are awefully wide rims for such small tires-perhaps your car drives rough because of really short, stretched sidewalls? My car feels great on any road with the same spring rates-I'd rather drive it than any other car I own, including my wife's minivan. The ride is firm, but nice, neither rough nor rigid. The only reason I could see for staggering wheels is if there is a big weight distribution problem to be masked. If it was due to high power (which I think is what we all subconsciously assume), then you would only want the stagger when exiting a turn, but not all the way through the turn. So, the stagger could have you upset going into the turn, understeering through the turn, then perhaps finally balanced flogging it out of the turn, but only if you are like a totally amazing driver. Perhaps JohnC could comment on the "staggered" concept and when we should and shouldn't stagger our wheels and tires? I recall that his alignment settings don't suggest a staggered setup.

Edited December 4, 2011 by RebekahsZ

[heavy85]

Search here for staggered. Yes it makes a difference and can be needed if you have enough power. Tuning is all about balance and theres a thousand ways to get there. If you have the wheels and super wide fenders big rears may be fastest but you have to setup the car o take advantage of it. I run softer rears but thats road racing not autox and i dont run a rear bar. No magic answer. If you have different offset wheels front to back even with the same tires (square tires but not wheel) simply switch them front to back and you can see the difference.

 

PS - speaking of track does anyone know where one can get proper wheel spacers? Everything I can find are generic swiss cheese or super thick with the dual sets of studs. I just need quality basic 10ish mm spacers.

 

Cameron

[Leon]

Bigger tires in the back will increase understeer.