Mid-corner oversteer advice

[dre99gsx]

Looking for some feedback here. I'm noticing many of my corner speeds are slower than some similar E46/E92 M3s on typical 100-200TW tires. So, this last event, I worked my way up to increasing my corner entry speeds to see what happens.

I tried a few techniques on some corners, letting off brakes a bit earlier, a little less trail braking, etc. What I noticed is the faster I come into the turn, the sooner I need to add steering input to help point the car to the apex.

What this does in effect is cause the rear end to want to suddenly kick out around mid corner where I'm holding maintenance throttle. It seems I have more than ample front end grip...

Here is a step by step process of what I am working with:
To catch ya'll up, I'm on Toyo RR 255fr/295rear. My alignement is -3.6/-2.2 camber fr/rear, 0toe/-1/8" toe fr/rear. I'm on HR 25mm front sway at medium stiffness, stock 3.0si rear sway sports (19mm). KW Clubsports on 10k(560lb) fronts/ 700lb rears. I have about 2mm rake.


- I corded my Toyo RR last event, so I have new RRs up front, but 24HC old RRs in the rear. Yes, this is probably the issue, but I read everywhere that RRs can handle over 30HCs with not-to-much degradation. Perhaps everyone is wrong..
- Ignoring the heat cycle difference, I found that I'm wearing a bit of the sidewalls on these tires. I originally was running 26psi Cold ~ 34psi hot. I have since started 30 cold, reaching 40psi hot. I was wiping out sometimes on lap 1-2 on warm up because the too low starting pressures. I learned my lesson.
- At NJMP Thunderbolt Carousel, I can start at 55mph. As I give it slightly more gas (just a hair) to induce understeer, the rear slowly kicks out. I then went to full stiff on the the front ARB, added 2 more psi to front tires, and added 1.5deg more to rear wing. This helped reduce that oversteer to where I can feel it going out and correct it with much more confidence. So, step in the right direction.
- I'm planning to experiment further with an 18mm rear sway from the 3.0i which should help induce more front end push. I'm also considering going to -2.8deg camber in the rear.

So, am I reaching some inherit limit of the Z4 platform vs the E46 with regards to suspension geometry/loading/dynamics? Meaning, no matter what I do, given the weight/balance of this platform, it will be very difficult to extract faster consistent laptimes because it's just the mechanical limitation?

My confidence is just not there knowing that the rear end is unpredictable on mid corner. The strange thing is she grips real well at corner exit with power.

[maupineda]

Try removing the rear sway. You have already very stiff springs. That will help with traction. Next, you may want to back off some front camber to level things out front and rear.

[wdb]

It's funny you bring this up. Very recently I was talking to a BMW geek and racer, who tracks an E46 and autocrosses his Z4. I mentioned thoughts of tracking mine and he said it wasn't the best platform for it and that he preferred the E46 for track use. One big reason was the wheelbase difference. So when you posted this I looked them up.

E46 M3: 2725mm
E86 3.0si: 2495mm

[dre99gsx]

Originally Posted by maupineda

Try removing the rear sway. You have already very stiff springs. That will help with traction. Next, you may want to back off some front camber to level things out front and rear.

I'll add this to my list, thanks.

It's funny you bring this up. Very recently I was talking to a BMW geek and racer, who tracks an E46 and autocrosses his Z4. I mentioned thoughts of tracking mine and he said it wasn't the best platform for it and that he preferred the E46 for track use. One big reason was the wheelbase difference. So when you posted this I looked them up.

E46 M3: 2725mm
E86 3.0si: 2495mm

Ok, so explain Cayman GT4: 2482mm ... Is the short wheelbase that big of a factor when one of them is a killer track weapon? I do agree and have read the E46 is a better track car because of the wheelbase. Interesting to see you brought that up.

[F360C]

Try removing the rear bar... i havent run one since i went past 650lbs rear springs

[dre99gsx]

I was planning to go from 19mm to 18mm bar, but I could remove it completely. What are the downsides on the Z4 with respect to handling elsewhere? I'm planning to up my springs evenly a bit more as well.

[Michael9218]

Did you have this issue before you mix matched the front and rear tires? I would never put fresh fronts with old rear tires. That just seems like a recipe for disaster. Another reason for running square set ups. I run 275's.

My car is very balanced. No understeer, no oversteer. No rear bar, large GC front bar. I run higher spring rates, but the spread front to rear is about the same (700/850).

The wheelbase may be an issue given the front engine placement, which is why I moved mine back 3.25". Seemed to help. But, as pointed out above, some very capable cars have shorter wheelbases.

[dre99gsx]

I did not, but then again, I wasn't trying to increase my corner speeds explicitly either. 24HCs on the rears could be considered 'old', they are only 1 year old tires. But, I wasn't expecting this drastic of traction loss with just 24 on these RRs.

You may be right. But, I want to be prepared with some options for next season and am assuming new tires will fix it. If not, then I can play with the rear bar, removing it, and see where that goes. One thing at a time as they say.

Good to hear you are balanced with no rear bar, that gives me confidence to give it a shot as well. I will be at 672/784 rates next season (I do have aero), not as high as you but at least it is good to see I can potentially go higher.

[Vanne]

I wouldent loose the rear sway, just means your rear isn't set up properly if your feeling more confident with no rear sway. Have you corner balanced your car?

[Michael9218]

Personally, I think you introduced the issue when you put fresh front rubber on the car and created an imbalance of grip.

You also didn’t mention damper settings. Turn in and mid corner rear grip can be improved with adjustments to your dampers. For example, take out a click of rear rebound to let the rears grip under weight transfer entering a corner.

[dre99gsx]

Originally Posted by Vanne

I wouldent loose the rear sway, just means your rear isn't set up properly if your feeling more confident with no rear sway. Have you corner balanced your car?

I corner balance during every string alignment. I'm leaving the sway in but will drop it as a last-ditch experiment next year to see how it behaves.

Micheal, I'll add the rebound adjustment to my list. I agree, this is probably tires. I'm on KW Clubsport.

Settings are (clicks open):
rebound 5/16 front, 6/16 rear
bump 3/12 front, 3/12 rear

Not full stiff, but close to it. I kept rear rebound less than front for precisely what you mentioned, but I have more adjustment is needed.

Just imagine going 55mph on a round-a-bout / skidpad. You slowly add 1.. 2... 3 more mph just adding 10/15% gas. Eventually, you would expect the front to push. Instead, the rear of my car just starts to hang out. Not abruptly, but swiftly. That's an example of what occurs.

[Michael9218]

Match up the tires and if you still have a problem start turning knobs. Experiment. Too many people get adjustable dampers but never adjust them.

FWIW, That rear bump looks pretty stiff if it’s 3 from full stiff…but YMMV.

[F360C]

Originally Posted by Michael9218

Match up the tires and if you still have a problem start turning knobs. Experiment. Too many people get adjustable dampers but never adjust them.

FWIW, That rear bump looks pretty stiff if it’s 3 from full stiff…but YMMV.

Really agree with this... trying diagnosis handling on worn tires will make you chase your tail.

What diff are you running?

Also assuming your rear subframe is solidly mounted and rear trailing are bushing are new? Both solidly mounting the rear subframe and spherical rear trailing arm bushings made a noticeable difference in rear grip for me. I also went for the rear subframe bushings that pull the subframe up in the car a couple mil to try and correct suspension geometry for lowered cars. Not sure if the geometry correction makes much difference but the Z4 GT3 did move the subframe up a bunch so i guess it cant hurt.

[dre99gsx]

Originally Posted by F360C

Really agree with this... trying diagnosis handling on worn tires will make you chase your tail.

What diff are you running?

Also assuming your rear subframe is solidly mounted and rear trailing are bushing are new? Both solidly mounting the rear subframe and spherical rear trailing arm bushings made a noticeable difference in rear grip for me. I also went for the rear subframe bushings that pull the subframe up in the car a couple mil to try and correct suspension geometry for lowered cars. Not sure if the geometry correction makes much difference but the Z4 GT3 did move the subframe up a bunch so i guess it cant hurt.

This issue did become more apparent after going for new RRs in the front, so I don't disagree with the statement. It will be the first thing I do next season.

Rear subframe is mounted with OEM bushings to chassis. I am on all sphericals for trailing arm lower and upper arm, including RCAB, all new since 5 years ago (no more than 2000 miles since?...). Interesting on moving the subframe up, any more detail on those bushings? I would suspect a few mil (must not be more than 5mm ?) isn't going to do much.

Also, I'm on the 3.0si diff 3.64 Auto (188k ring) with a Quaife in it.

[F360C]

Originally Posted by dre99gsx

This issue did become more apparent after going for new RRs in the front, so I don't disagree with the statement. It will be the first thing I do next season.

Rear subframe is mounted with OEM bushings to chassis. I am on all sphericals for trailing arm lower and upper arm, including RCAB, all new since 5 years ago (no more than 2000 miles since?...). Interesting on moving the subframe up, any more detail on those bushings? I would suspect a few mil (must not be more than 5mm ?) isn't going to do much.

Also, I'm on the 3.0si diff 3.64 Auto (188k ring) with a Quaife in it.

Here is a link to the bushings...

https://cmpautoengineering.com/colle...32378928365703

I would highly encourage doing some sort of solid subframe bushing in the rear.

[dre99gsx]

Originally Posted by F360C

Here is a link to the bushings...

https://cmpautoengineering.com/colle...32378928365703

I would highly encourage doing some sort of solid subframe bushing in the rear.

Site says sold out. Any other recommendations? I do like the fact that it raises the subframe slightly. And remember, my rear subframe is non-M.

https://www.ecstuning.com/b-turner-m...dr4680014~tms/ these are more expensive, but don't mention anything about height change.

[CTMP4me]

Originally Posted by dre99gsx

letting off brakes a bit earlier, a little less trail braking, etc. What I noticed is the faster I come into the turn, the sooner I need to add steering input to help point the car to the apex.

What this does in effect is cause the rear end to want to suddenly kick out around mid corner where I'm holding maintenance throttle. It seems I have more than ample front end grip...

Toyo RR 255fr/295rear. My alignement is -3.6/-2.2 camber fr/rear, 0toe/-1/8" toe fr/rear.

My confidence is just not there knowing that the rear end is unpredictable on mid corner. The strange thing is she grips real well at corner exit with power.

You're rolling a lot more speed into the corner now, off throttle (presumably with a 1-way LSD = open), and it's snapping mid corner / at apex - tbh this seems to be more an issue of technique / driving style and not your setup. IMO your alignment is on point, you're running huge 40mm tire width stagger, and flat rake - contact patch and weight balance should favor rear traction.

On the car side of things I would try (in order):
- softer front rebound, softer rear compression (encourage weight transfer towards rear on accel)
- if this is a higher speed corner, add wing angle
- check that your LSD is working and has fresh fluid
- add 0.2-0.3 degrees more rear camber
- think about what "maintenance throttle" means for torque in your car. you could be having the same issue as early S55 cars that made huge torque at 2-3K rpm = hard to manage and be smooth at "maintenance throttle"

On driving style side:

You're doing the classic "turn in early with speed", you're getting exactly what that approach should give. Without seeing a video / throttle and brake traces, this isn't some performance coaching advice, but I would try to enter corner a bit slower, gentle brake release, roll it in off-throttle, use throttle to catch rear end if it steps out (off-throttle oversteer advice only lol). Don't use maintenance throttle please
Add trail braking into the mix as you get more comfortable with weight transfer and depending on corner speed (yes to low speed, generally no to >100 mph corners)

[lelandr]

Originally Posted by dre99gsx

Just imagine going 55mph on a round-a-bout / skidpad. You slowly add 1.. 2... 3 more mph just adding 10/15% gas. Eventually, you would expect the front to push. Instead, the rear of my car just starts to hang out. Not abruptly, but swiftly. That's an example of what occurs.

Quite frankly, this seems like ideal behavior. As long as the rear end comes around progressively and controllably on power, this is a good thing. My M Coupe does this, and my previous E30 M3 track rat did it as well.

Do you get rotation under trail-braking at turn-in as well?

[Westersund]

Originally Posted by dre99gsx

Ok, so explain Cayman GT4: 2482mm ... Is the short wheelbase that big of a factor when one of them is a killer track weapon? I do agree and have read the E46 is a better track car because of the wheelbase. Interesting to see you brought that up.

The Cayman GT4 have the engine in the middle, more weight over the rear wheels and it probably have a better total aero package than your car.

As many have already said, remove the rear sway bar. Going faster into the corner and trailbraking won't help you because you will transfer more weight to the front wheels. Also I would recommend a ramp/clutch style diff instead of your Quaife.

[Westersund]

Originally Posted by Vanne

I wouldent loose the rear sway, just means your rear isn't set up properly if your feeling more confident with no rear sway. Have you corner balanced your car?

That's not really true in all cases. If your rear springs are stiff enough you won't need the rear sway bar, especially on tracks with uneven surface. Got that advice for my M3 by Wiborg Engineering (Look them up they know their stuff ) who calculated the spring rates for my car and re-designed the front geometry.