Rear wheels jumping under heavy braking

Optimiser

Hi from downunder.

During very heavy braking (with cold brakes), eg: avoiding an accident, the rear wheels and therefore rear of the car bounces and jumps around from left to right and back again like a short oscillation. It's quite unerving.
I have a Cusco 1.5 LSD, Whiteline anti sway bar (on softest adjustment), Whiteline adjustable sway bar links, Whiteline diff bushes, Hardrace camber arms, traction rods, tow arms and MCA Red true type colivers (On soft setting for DD). This has only seemed happened since having the "true type" coilvers, but doesn't seem to occur on the track? I'm also using DBA 4000 series rear rotors with Endless pads.

Does anyone have any ideas why this might be happening?

Regards.

jchammond

Do you have your alignment specs? As I know if your rear toe is by some chance negative, this will cause the rear to dance around...especially when slowing down from trip digit speeds.
Otherwise I’d be checking for a loose suspension component.

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Optimiser

Thanks jchammond

Please see attachment for my last wheel alignment specs...

Attached Images

370Z_Wheel alignment specs_Nov 2017.JPG (208.4 KB, 28 views)

jchammond

Looking like plenty of toe on rear, so rule that out.
You listed rotors & brakes on rear; I know a badly warped rotor could cause this...but probably unlikely on your Z.
Unsure about your Diff & the 1.5way, as some lock under load & unlocks on decelerations?
Does this happen only while brakes are cold & then normal after warming up?
As you stated on track, all is good...but I’m sure that is consistent hard throttle & braking.
You may have developed some hot spots on rotors.
I’m sure others will chime in & help

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Rusty

I would say it's your settings for DD on your coil-overs. They're too soft. Add a click or 2 to the rebound.

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fryzia23

agree with Rusty, soft suspension with aggressive pads cause too much nose dive/weight transfer and rear end gets very loose. Adding more rear downforce would help. I had the same problem with swift springs matched with carbotech pads. Very scary on the street and basically undriveable on the track. Stock nismo wing. If the stiffer settings won't do it then you will need to upgrade to stiffer spring setup.

Rusty

Also add a click or 2 to the compression in the front.

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BGTV8

I reckon Rusty has this right - needs more rebound at the rear as the shocks are allowing too much pitch towards the front which unloads the rear

Optimiser

Thank you all very much!

I've currently got the coilover on about 2 clicks from the softest out of a possible 12 clicks.
I will increase the stiffness (I'll try the middle setting) on all 4 corners as suggested and see how it is and report back.
It would be great if that's all it is.

Thanks again everyone!

BGTV8

You could also give Josh at MCA a call and see what he thinks. (07) 3865 5091

Optimiser

Thanks for that BGTV8!

madwi

Off topic, do those endless pads play nice with the DBA rotors? I had heard somewhere that they dont really like each other.

OptionZero

A couple risers here found that the compound in the endless does not like the dba rotors

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BGTV8

Depends on the compound. I had a lot of success with Endless MX72 compound which was a great compromise "fast road" as well as "track day" pad but Endless ME20 was simply too aggressive for the DBA rotors I use and caused a lot of shuddering.

Optimiser

I have the Endless MX72 pads as BTGV8 mentioned and I can't believe how long they last. Even after some track days and some years of use they still have 80% pad left on the front and interestingly 50% pad left on the rears. Not sure why the rears are wearing faster than the fronts.

I'm using the DBA T3 5000 (2) piece slotted rotors on the front and the finish on the rotor surface looks perfect although the rears (DBA 4000 T2 1 piece slotted rotors) which have worn faster do show a little "grooving" and not as smooth as the fronts, but nothing bad at all.

I am on my second pair of DBA front rotors because the first ones warped slighty after first track day, (not due to the pads) but I think that has more to do with how the DBA rotors handle the weight of the 370Z which I've also heard isn't the best.
Next time I will go for Z1's (2) piece Akebono rotors which I've been advised will deal better with the 370z's weight and are less likely to warp.