May have figured out corner exit wheel spin issue ... now how to fix?

[heavy85]

http://mcscc.org/Gallery/album41/DSC0085?full=1

 

Not enough rear droop travel it appears. Going to test bigger front swaybar and moar rear downforce (front biased now). These are the Koni RACE struts with no rear swaybar, 425/375 springs, and on RS3's. Im getting around 1.4 lateral g's which explains the lean. Maybe need to test the clutch LSD again instead of the gear type. Tried 500/425 springs earlier this year but didnt like the feel. Other options for longer struts? Something else Im missing?

[G-E]

Have you got rollcenter adjusters up front?

[RebekahsZ]

The tokicos droop farther, but I like my konis better.

[clarkspeed]

If you are double adjustable try softer compression. Then try softer rear springs. 325 or even 300. Very small front ARB. Thats what it took to get mine to hook up. I run a Quaife and when you lift a tire you instantly loose 1 - 2 seconds. I still have to be careful over curbing.

 

Nice Aero.

[heavy85]

If you are double adjustable try softer compression. Then try softer rear springs. 325 or even 300. Very small front ARB. Thats what it took to get mine to hook up. I run a Quaife and when you lift a tire you instantly loose 1 - 2 seconds. I still have to be careful over curbing.

 

Nice Aero.

Single adj set full soft in the rear. I too went from 'big' 1" front bar to modified (for more stiffness) stock bar but just finished reinstalling the bigger front bar to hopefully help keep the rear flat. Will test that next race. Why smaller front bar? I originally went there to attempt to help reduce inside front wheel lock-up but seems counter intuitive to help the rear hookup?

[heavy85]

Have you got rollcenter adjusters up front?

Dont know what adjuster you are referring to but have some adjustability.

[clarkspeed]

When you are running relatively stiff springs, controlling body roll with ARB's is not so important.  You really only need the bars to balance Front and Rear moments (understeer/oversteer).   So if you removed the rear bar, it shouldn't take to much stiffness in the front bar to balance out.  I reduce front bar stiffness until I get some good oversteer and then put one adjustment back in. 

 

In Mark Donahue's Unfair Advantage he talks about setting up his cars by changing spring rates with no ARB.  He only adds the ARB so he can fine adjust at the track.  And if you notice most pro level cars run pretty light bars.

 

On track it is very important to rotate the car and get on full power as soon as possible. If you are having problems like you describe, reducing rear spring rate should help plant the rear.  But there are other factors involved like tire size and stiffness so it's hard to say what spring rate will work.  But I know plenty of other fast Z's that run a large delta between front and rear rate.  You just have to try and see what works for you.

[G-E]

These things:

 

[heavy85]

Clarkspeed - understand but that photo shows Im unloading the inside rear tire. Softer rear is only going to hurt that so think the main issue is I need to figure out how to keep the inside rear on the ground.

 

G-E yes I have those

[RebekahsZ]

I'm not a suspension guru, but our cars are very similar, so i'd like to learn along with you. With the V8, I have to be very careful with throttle application even in a straight line or I lose traction in back. All the more exiting turns. Mine was very touchy toward oversteer even before the konis when i had lots of droop with Illuminas. Do you currently have problems with oversteer even if the throttle is held constant thru the turn, or just when you start to add throttle as you exit (like the title says)? I so want a couple of dedicated hours on a skid pad , but I've never gotten to play on one. It doesn't address our limited droop, (and I'm sure you've done this), but I did notice improvement by dropping the tokicos to full soft and dropping rear tire pressure significantly. A big inprovement in controllability was made by reprograming my DBW such that more pedal input is required for each degree of throttle body opening. This year (if I can ever get to the track) I have obtained some wider rear wheels and tires to run a little stagger. I also may drop the tail some, which builds in more droop. Hey, that's an idea: drop the rear static ride height a couple inches and see if it helps. 1.4g is awesome-I doubt seriously if I am anywhere near that.

[johnc]

The Quaife diff requires suspension compromises to work properly. When ran the diff on my ROD I had 375 front and 325 rear springs with a 1" front bar and a 5/8" rear bar. The front bar was full stuff and the rear bar was full soft. My big advantage was triple Penskes with a low speed compression adjustment. That let me stand up the outside rear to keep the insider rest loaded.

 

Switching to an OSG Supetlock let me focus on suspension tuning for faster corner entry and mid corner while keeping a good corner exit.

[tube80z]

I may not be following all this but if you don't have enough rear droop travel you don't want to soften the rear.  You need to stiffen it up.  If you lower the spring rate in the rear you'll get more suspension travel used up from weight transfer.  A lot of the hillclimbers in the NW used to soften the rear of the car so they would put down the power better.  I have convinced a number of them to use much stiffer rates and it works.  When you start using aero you also need to run stiffer and change your driving style to accomodate.

 

To keep the splitter down and working harder to match the rear wing you can trail brake to the middle of the corner.  Then onto the gas but you need a stiff rear end to keep the car from squatting too much and upsetting the extra grip you are getting from aero.  If you watch classes with wings and splitters you'll see often see this.  I think you'll find that the diff will be kept loaded this way too.

 

Cary

[74_5.0L_Z]

I have been battling a similar lack of rear traction on corner exit.  This year in response to that problem I have done several things. 

 

First, I raised the front by 0.75" to raise my front roll center. 

 

That seemed to help, So I then began putting successively lighter springs in the rear.  I began the year with 500 lb/in front and 450 lb/in rear and a 20mm front sway bar. I then switched the 450 lb/in rear for 425 lb/in.  This change decreased my rear frequency, decreased the rear contribution to lateral load distribution, and also slight lowered my rear roll center.  It was a noticeable improvement.

 

Next, I replaced the 425 lb/in springs with 400 lb/in springs in the rear.  Again, this made a noticeable improvement.  However, again I changed several things by putting the lighter springs in the rear.  Lower frequency, lower contribution to the lateral load distribution, and lower roll center. 

 

With the changes that I have made, I have made an extremely noticeable improvement to my corner exit traction.  I have also not yet reached a push condition that can't be driven around.  At the last autocross event, a friend captured video showing me lifting the inside front tire and carrying it for about ten feet.  The car also now squats much more noticeably in the rear.

Edited August 19, 2014 by 74_5.0L_Z

[1vicissitude]

Sorry if this comes off a simple dumb solution, but if droop is your problem it sure looks like you could lower the car quite a bit. Ofcourse the maximum droop limitation will still be there, but if the car was lower there would be more travel before full extension.

 

Curious, are the Konis you are running the longer or shorter ones? If I am not mistaken isn't there a longer version beside what people commonly section struts for? At your ride height, maybe that would be more ideal.

Edited August 17, 2014 by 1vicissitude

[G-E]

I'm still convinced this a roll-center issue, either needs a taller rca up front to raise rc, or a lower rc in back...  ride height can accomplish this to some degree, so you could try lowering the car in the rear and raising it in the front as suggested

[JMortensen]

Good input here. I continue to believe that HLSDs are weak in terms of lockup. There is a tradeoff to a tighter diff, but that's the direction I'd go. Looking at Dan's pics, it really does look like the splitter's effectiveness will be compromised by running much softer rear springs.

[74_5.0L_Z]

Cameron,

 

What are your other suspension settings?

 

How big is your front sway bar? 

How much caster are you running? 

How high are your front and rear roll centers?

[heavy85]

There are short and long Konis? These are 1437-RACE

 

This is what Im going to test. From 18 mm (link mount modified to be ~1" shorter than stock) to 1" bar at the firmest setting (I think Susp Tech bar?). Lowering rear roll center via eccentric pivot bushings from highest to lowest setting. Adding a bunch of wing. Forgot but I also noticed a particular corner exist in a ~80% throttle max lateral accel kink that you could notice wheel spin that suddenly caught up quickly at corner exit. Kink of like slipping clutch feeling that all of a sudden grabs but I know its not clutch so assume its the diff unloading.

 

Caster is about 6 deg, tried 7 but that was way too loose on slow corners. 25" tires and front control arms point down from center 2 deg and rear arms point down from center 9 deg whatever that ends up in roll center height. Havent got it back on the ground yet to recheck rear control arm angles with the new setting. Front toe 1/8 out rear toe 1/8 in. 51% front weight depending on fuel. 50% cross. Rear tire temps are 10-15 deg higher than front.

 

Ride height is a little tall for a particular hill climb that is rough on the exhaust.

Edited August 19, 2014 by heavy85

[tube80z]

Lower the rear.  Your RC is too high if I understand the control arm angle.  You want to lower the end that needs traction unless you're hitting the ground and upsetting the cornering balance.  For the really bad bumps add packers and tune the bumpstops.  If you have a lot of lock on try and winding the wheel sooner or less caster to reduce cross weight change.  For a fun afternoon put the car on scales, if you have them, and raise and lower each end and turn the wheels and watch the weights change.  Change caster and see the difference.  On our cars we often use a lot of caster to help low speed turn in.  This can cause catch and release on corner exit as you unwind the wheel.  Spacers to change the track that add more scrub will make this worse.  

 

Maybe a cool winter project would be a simple subframe in the rear and new arms that allow you to raise and lower the pickup points.  I have some pics if you're interested.

 

Cary

[clarkspeed]

Some interesting comments here.  Dan I'm probably not too far off your set up, and I keep my RC's in the 2-3" range, higher in the rear.  I'm not lifting a front wheel but very close to doing so.  I know that 4 wheels planted on the ground is desired for faster cornering speeds, even when dirt track racing, but trading some of that front grip for rear traction really helps a Z car if you have some HP to put down and are pushing a Quaife.  Call it an old school set up and I know a few other fast guys that run this way.

 

As always, Cary has some good insight and probably has the correct recipe for getting traction AND maximizing cornering grip.  I can only say what worked for me and I don't run any real aero. I may be at a bigger disadvantage, I just discovered my rear strut housings that came with the car were cut 1" too short. I don't have much travel at all to max droop.

 

I think we are all in agreement that you shouldn't tune this with a sway bar.  A 1" solid bar on the front with no rear bar should be a recipe for serious understeer. When removing the rear bar, I caclulated the reduction in rear stiffness and then sized the front bar for the same reduction. Came out almost perfect balance.

 

Are you running a Quaife?  Yes when it drops it feels just like a clutch slip and takes a second to catch up, which seems like minutes when someone is on your bumper.  Very uncomfortable.  I'm sure you want it fixed right now.

 

I agree with Cary, you may be a little off on RC's but I can't say that would cure your problem.  Just guessing and assuming you have stock based components, I think your rear RC is probably 3-4"+ and front is JUST about ground level.  I don't really like to go that low on a strut suspension. I put together a chart of control arm angle vs. RC height for my car.  I set my front RC's (ride height) with a digital angle gauge on the CA.