Chassis Setup (corner weights)

[260DET]

It's informative to look at a ten year old thread, particularly the spring rates talked about and more particularly using heavier springs at the back. Now in 2014 I know of no S30 in my part of the world that has heavier springs at the back, all are front biased to various degrees.

[clarkspeed]

I think John Thomas runs 100lb ballast in passenger floorboard in FP. He's a small guy, but that car is light! On the other hand, he could probably win a Nat championship in an El Camino.

 

Dan if you ever need help with set up you can swing over to Orlando.

[74_5.0L_Z]

I am planning to go run the Regional SCCA event at South Georgia Motorsports Park on March 15th and 16th.  Unfortunately at regional events, the SCCA does not allow passengers.  So I will be adding ballast to the passenger side floor.   I ran this past weekend at Sebring, running the car both with and without riders.  The car is so much easier to drive with a passenger.

 

And yes, I think John Thomas could win a national championship in any given class even if he was blindfolded and driving a lawnmower.

 

Clarkspeed, I may just have to take you up on that offer just so that I can socialize with a fellow HybridZ lunatic.  Besides, I want to check out your car.

[jt1]

Which way gave better times, passenger or no passenger?

[74_5.0L_Z]

^{With Passenger by a long shot.  This car has way more than enough power and braking to accelerate the extra mass.  My next mod is going to be the addition of a tall rear spoiler to help plant the rear of the car at speed.}

Edited February 13, 2014 by 74_5.0L_Z

[clarkspeed]

Dan, Tom and I will be at Sebring in 2 weeks with SVRA. You should come down. I can put you on crew list.

[gira]

Wow, there can be a million opinions here. Making a Z car handle well is not that hard. Making a Z car handle great is hard. Generally speaking, the compression and rebound adjustment is a great tuning tool, but only applies to corner entry and exit. It's job is to control the speed at which the weight transfers. AND, the driver has to be experienced enough to say, "the car understeers too much at turn in", or "I get on the gas and the front end doesn't work." Many people don't constantly run the car on the edge for long enough to accurately feel these things. I prefer the Koni race shocks. They have been reliable and we put them on the shock dyno and they're surprisingly consistent. When you lower a Z, other things happen. I can't get my race car to push. Our cars exploit the fact that they really dig out of the corners, torque and rear grip. I'd ditch the rear bar and switch the spring rates to have a higher spring rate in the front. UNLESS your car pushes real bad already. Most Z cars inherently oversteer, but it really depends on how much you have changed the ride height and other things. Front grip has a lot to do with how bad the roll center had gotten when you lower the car. Our EP car has a roll center spacer that's about 5 inches tall. Swaybar adjustments are always the easiest way to make quick adjustments.

 

Hope this helps,

Greg

[74_5.0L_Z]

Hey Greg.  It's good to hear from you, and I will gladly accept any input that you have on setting up a Z car.

 

Here is where I am at:

 

Total Weight:  2632  (Front Weight: 1278, Rear weight 1354), (Left Weight: 1359, Right Weight 1273)

 

Roll Centers:  Front 1.9 inch,   Rear 4 inch

 

Spring Rates: Front 500 lb/in, Rear 425 lb/in  (I am changing the rear springs to 400 lb/in before the next event).

 

Sway Bars: Front 20mm with shortened arms, Rear none

Struts:  Front, Koni 8610-RACE   Rear, KONI 8610-1149

 

Alignment:

CAMBER:  FRONT  -2.2, REAR -1.9

TOE:          FRONT 1/8" toe out,  REAR 1/8" TOE IN

CASTER:   FRONT 6 Degrees

 

TIRES:  275/35/15 Hoosier A6 mounted on 15 x 10 wheels at all four corners

 

Power:  396 rwtq at 4000 rpm, 386 hp at 5950 rpm (greater than 350 ft-lbs of torque from 3000 rpms to 6000 rpm all measured in 4th gear.

 

Gear:  3.36 CLSD,  1.94 2nd gear in tranny (7000 rpms is 73 mph)

 

My current situation is that I cannot put the power down off the corners unless I have a passenger.  The addition of a 120 lb passenger(my wife) makes the car SO much easier to drive.  By that I mean that I can squeeze the throttle without the rear end wanting to take the lead.  I am not under the impression that I should be able to hit the gas indiscriminately, but I would like to apply full throttle more quickly than I currently do.  Toward that end, I am softening the rear springs to try and mimic some of the effects that I get from adding a passenger.

 

Adding the passenger does many things: 

 

First, the added weight in the passenger seat balances the car left to right.

Second, the added weight adds rear percentage.  Without the passenger the car has 51.4 percent rear weight, and with the passenger, the car has 52.6 percent rear weight.

Third, the added weight lower my rear natural frequency and changes the "magic number."  By that I mean the percent of front roll resistance minus the percent of front weight percentage increases (5.4 to 7).

 

At my next event I cannot take passengers, so I plan to play around with the lower rear spring rate which will lower the rear natural frequency and the rear roll resistance and hopefully improve off corner traction. 

 

I have been lowering the rear spring rates incrementally.  I went from 450 lb/in to 425 lb/in, and saw some improvement.  Next, I will try the 400 lb/in and see how it goes.  As it is, I am nowhere near a push condition so I think I am going in the right direction.

 

One place that I have not done any tuning is the shocks.  Currently, I run box-stock Koni 8610s.  I run them all on their softest rebound setting.  They seem to have too much rebound and not enough bump.  I am planning to get them revalved soon, but first I want to know where to start on the valving.

 

Thanks for listening to my rant.  Please respond with any useful input.

 

Greg, someday I would like to check out your EP car.

Edited February 21, 2014 by 74_5.0L_Z

[gira]

Dan,

 

Hi, sounds like you've been working on this awhile. A few quick observations. The left and right weight should be much lower on the priority list. Any competition production car will have rules, so it is what it is. I would wager John Thomas added ballast to meet a minimum weight requirement.  Some things in race cars can't exactly be explained on a piece of paper, believe me. So, make sure your corner weights are 50/50. Next, assuming two cars  on a road course are identical, a car of your weight with an additional 120lbs will never win. So, the moral is you need to tune that out if the passenger makes you feel more comfortable. You have a pile of torque, and probably too much to stab the throttle all the time, but that's half the battle. From what I have heard the Koni race shocks start to get a bit tired up around 500 lbs, and especially if you have the rebound set at the softest. If you want more bite out of the corners, you have to get the weight back there, less rebound front, less compression rear. No rear bar? Let me know. What size front bar? You will want to keep a split in the spring rates, generally speaking. What kind of roll cage? Try the double adjustable race shock. That's what I use. What is the car used for?

 

I'll be at Sebring next weekend hoping to help Clark and Tom out with their Z's.

 

Talk with you soon,

 

Greg

[tube80z]

I think more than anything it's the rear percentage.  On my FP autox car it was faster with ballast than without.  I tried changing springs, played with shocks, etc. to compensate and it didn't seem to change much on putting power down.  When I took Rouelle's seminar I asked this question.  The short version he gave was that you need rear percentage to help put power down.  He also reiterated that the tire has a traction allowance.  If you use all of it for lateral traction you have none for longitudinal.  So setting a car up if you can get the front to take more roll resistance you can free up some for the rear.  You can do this a number of ways but for me I found lowering the end of the car not working was better than changing springs or bars.  The downside is if you go too far the you lose turn in.

 

Taking this to the extreme of the EM monsters.  A friends LS2 powered locost (used to be a 260Z) is always faster running rear ballast (0.8 seconds on a 44 second course).  Lower CG the more weight percentage you're going to need on the rears.  While it's true that a lighter car will always be faster for courses where acceleration and braking are important you may find that the added weight will improve the times.  The new car will be built to take this into account.  

 

For the shocks you'd be better off dropping some compression rather than adding rebound.  That may sound backwards but my experience has been adding rebound often reduces traction but often feels better.  My advice would be to try dropping the rear ride height 2 to 4 turns on the rear perches and see if that helps.  Or if you are too low in the rear possibly raising the nose.  

 

Hope this helps,

Cary

[74_5.0L_Z]

I really only use the car for autocross though I hope to start doing track events soon. 

 

I have the rear about as low as I can get it without rubbing the tire on the inside of my SubtleZ rear quarters.  I am doing my best to tune out the oversteer without adding ballast,  but I will if I have to.  I plan to install the new rear springs before the next event, and I will also have some ballast that I can add if required. 

 

 

I am trying to decide where to add the weight.  The easiest place will be on the floor behind the passenger seat.  Or if I am a little creative, I could add it between the differential and fuel cell.

[heavy85]

How significant is playing with rear track width in putting power down? Dan, I, and others with rod-end rear suspension can adjust this fairly easily but I have only run one event with them so havent yet. Have you ever tried a gear type LSD? I switched several years ago from a Nissan clutch diff to a cheap OBX gear type and thought it was a good improvement. Last thing is rear camber as I was getting inside rear wear last year at only around 1 deg of camber - you ever try less to maybe help with putting power down?

 

Its really quite interesting that after many years of tweaking my car its currently very close you yours (2675 lb, 365 whp, 275 tires, just switched but yet to run from 425/375 up to 500/425 springs, Koni RACE single adj, stock front bar, no rear bar, same toe, etc). Im running this set-up in hillclimbs then change camber and run a street tire class on track. I've found too that aero is huge on a road course.

 

Cameron

Edited February 21, 2014 by heavy85

[RebekahsZ]

I'm watching this thread to learn. Is a bigger rear tire or a lower tire pressure of benefit? I think many of us with big torque zs have the same problems with corner exit oversteer

[gira]

Autocross, sorry I hadn't picked that up. I though we were talking road racing. Autocross is an entire different animal as tire life has no play, regarding weight versus a road race. The ballast idea is interesting. The less negative camber idea is good, you will have a better contact patch (while going mostly straight) with less negative camber. This may affect your steady state cornering. Most single adjustable shocks are rebound adjustments only, as this is what controls the spring. It is true that the tire can only take so much. With this in mind you might alter your line so the turning and accelerating is minimized. I have autocrossed a  bit, but not in a car with high HP and not at a national level. Bigger tire yes, lower tire pressure may be a good thing to try. I did drive Bill Coffey's Z car recently. I guess he has over 300 HP and it did not have this problem of putting the power down, just sayin.

 

Neat information and lots of cool ideas.

 

Greg

[JMortensen]

How significant is playing with rear track width in putting power down? Dan, I, and others with rod-end rear suspension can adjust this fairly easily but I have only run one event with them so havent yet. Have you ever tried a gear type LSD? I switched several years ago from a Nissan clutch diff to a cheap OBX gear type and thought it was a good improvement. Last thing is rear camber as I was getting inside rear wear last year at only around 1 deg of camber - you ever try less to maybe help with putting power down?

Just a word against adjusting track width with rod ends. There is a rule of thumb that says 1.5x the thread diameter in the tube, so that leaves you with something like 5/16" adjustment IIRC. You can push it farther than that, but we have had threads in the past where control arms failed in this area (AZC chromoly arms). Also, droop limiters in the back should help that inside tire wear.

[clarkspeed]

Hey this forum is getting interesting again!

 

Dan I think you are on the right track.  My setup is not much different than yours.  I run a little higher front RC (I hope you measured to the ball joint center!), lighter springs, smaller front sway bar, and custom valved Bilsteins.  I haven't autocrossed in a few years but I would think you would want a little softer setup than a road race one.  My rear frequency is 125cps and that seems to be the upper limit for me and it looks like you are about the same.  I wouldn't want to autox my setup.

 

My car didn't start rotating and biting out of the corner until I got rid of the rear sway bar and softened my shocks.  I was running 100 compresion and that was too much.  Looking at the internet, those Koni's look like about 160 in compression. So they are more or less acting like additional spring, which would be ok if you were driving a stock based car. The 8611's I think will adjust compression to something more reasonable.

[clarkspeed]

I also run bias ply tires, so almost no rear camber and that helps a lot.  Back when I ran radials I tried to run as little rear camber as possible and still keep the temps under control.

[tube80z]

Not trying to derail this thread but a number of us in the NW were running around 3 hz with the front 5 to 10% higher.  What was odd to me was that as we started to develop setups that were mostly spring and little to no bar we didn't need to change much with grip changes.  When we ran more bar and a lot softer in the springs we'd have to chase the track.  And when it was cold or wet the bars had to come off and often the car wasn't very much fun to drive.  

 

I got this chart from Eric Purcell a number of years back.  When I first heard about it I thought people were smoking something.  But having tried this it's worked very well for me.

 

Spring rate based on corner weight.

Sedans and radial DOT tires  -- .5 to .7 

Production and light GT cars -- .8 to 1

Medium weight, 200 to 300 HP cars GT cars -- 1 to 1.3  (FP)

Heavy high HP GT cars -- 1.3 to 2 (XP/EM)

 

Hope this helps,

Cary

[JMortensen]

Christ that would be like a 1000 lb spring on my car. 

[tube80z]

Christ that would be like a 1000 lb spring on my car. 

 

I've never ran higher than 1.3 myself (725 lbs/in).   But that was on a really bumpy hill and I was amazed at how stable the car felt.  But you can see the trend as you start to look at adding underbody downforce.  It does require a really good shock to pull it off.  The bilsteins you have didn't work for us once past 400 lbs/in springs.