- Mar 10, 2018
Hey first of all, thank you so much for this detailed input!You seem to be reading all sorts of stuff and imagining up problems that don't exist lol... Also keep in mind that a lot of the above recommendations should only be taken as starting points.
The temperature differential you'll see across the tire is going to vary by the track you are running on; therefore, changes to get the ideal alignment, tire pressure, and suspension stiffness are all going to be track specific as well. There are too many variables for anyone to give you a meaningful adjustment. You're temperatures aren't off so drastically that there is some glaring suspension issue. The temps are pretty low though, but, I guess low is good for the street tires you are running. You can try dropping tire pressure a bit. 38/36 is a bit high for me usually. I like to see 34-36psi hot. I usually go on at like 26-28psi.
With that being said, @ -2.44* I really don't think you should need any more front camber on 240tw street tires. Forget tire temps and look first at how your tires are wearing. All tires have wear bars that show you the ideal usage per the manufacturer. Tweak things like tire pressure to make full use of your tires first.
@-2.2* I think your rear camber might be a bit high for 240TW tires. More rear camber can help keep the rear stable in corners (keep the car more oriented toward understeer) but driving on the inside edge of the tire decreases your ability to put down power. I'd drop more toward -1.75* to -2.0*.
For reference, I had perfect tire wear F/R with 200tw tires and -2.4*/-1.75*. When I moved to stickier 80tw r-comps I needed to dial in -2.6*/-2.0* to achieve the same wear bar target. Tire sidewall stiffness also plays into this... A soft sidewall tire is going to deflect more and the scenario might call for more PRESSURE rather than static camber to get ideal wear.
Caster changes aren't really going to make you any faster. Speaking conceptually, caster can help offset camber loss during cornering and peopel generally bump the caster up a bit. But, caster changes also affect steering feel/feedback. You need to understand the complex relationship between scrub radius and king pin inclination to understand how caster changes might affect the car. Suspension and wheel setups/offsets can drastically change the affects of caster changes from car to car. Stock is around 6.x*? With my setup, my caster floor is 7.6*. I could probably get it close to 10* but aggressive caster settings don't generally agree with daily driving. Further, I've read aggressive caster is generally not a good thing on a BMW. I'd keep it under 8*, personally, but experiment with it yourself.
2) A softer sway bar does not help dynamic camber gain. While you do get better camber gain due to more STATIC suspension compression (braking), you also experience more camber LOSS in dynamic situations (cornering)... You need to consider the NET change, and, in general, a softer suspension is going to net you more dynamic camber loss. That doesn't necessarily mean that a soft suspension setup is"slower" or "worse," but, you need to recognize this affect and recognize the trade-offs. If you want less dynamic camber loss then you would've wanted to move to a STIFFER sway, but, I would NOT be looking at sway bars as a primary method of changing the dynamic camber curve. The affect of sway bar changes on camber is completely overplayed. The car still rolls and you still lose camber in roll. Suspension travel is pretty much always dictated by bump-stop engagement unless your driving a purpose built track car with teeth chattering spring rates. Hence, why we all run fairly high static camber for track duty regardless of sway bar stiffness.
Also consider driving style! Are you trail-braking into the corner, or, are you heavily braking and loading up the front tires? Driving style might need to change drastically depending on how you set the cars balance up. You generally tweak sways to balance the car. As you noted, less front bar produced more front grip for turn-in, but, the balance changed produce too much forward bite and the rear got loose. You might want to try the M3 front sway as a compromise. Keep in mind that the rear sway is basically STRAIGHT. It is very effective. The front bar has many curves. Therefore, you can't just compare the diameters of the front/rear bar to determine "balance."
1) Same as above... lowering the car generally produces a net camber loss in cornering. There are other reasons why we lower cars, but, reducing camber loss in dynamic roll is generally not one of them.
3) This is actually a fairly common/reasonable option... Or, you can just get better camber plates if you really think you need more camber.
4) I'd also agree that making the camber links longer is not advisable without actually understanding the dynamic suspension changes that result. More static camber might be great, but, messing up the cars suspension geometry does't sound all that great to me...
Have you considered moving to a better tire lol? Put a street-able tire like an NT-01 on the car and you'll pick up several seconds alone... Maybe then you'll be able to pinpoint any other tweaks you might want to make. You aren't going to be setting lap records on street tires. That's just facts.
I should have been more clear, that I wanted to improve my laptimes specifically on the Ningbo circuit shown above. I do understand that each track need different setup... same for rain vs sunny track days. I am just trying to get the basics right, and optimize for one track before I move on to the next. I am obviously very new in this regard, and want to learn.
As for the tires, I did run 240tw up front, but then had 180tw on the rear (same brand and type though). This was bugging me, so yesterday I swapped the tires to 200tw both front and rear (an old set of AD08R I had lying around), to take that difference out of the equation. Now, you are right that my temps were low, but as stated, I did not push the car on the straights or in braking. Instead I went through all the turns at 90-100%, and just cruised on the straights. My approach here may have been off.
I also agree that tires matter the most. The people I compete against typically use Trofeo R or Cup 2. I am probably going for Trofeo R next as I have heard the NTs are very noisy. But I am very much undecided on that still. I however didn't realize tires with more grip would require me to start over on the camber adjustments. Great input!
As for front caster I believe I am at 7.5 deg right now (EDIT no 7.2). Also I use ET35 wheels up front to reduce kingpin offset a bit over stock Z4.
So what I conclude from your post:
1) I should consider custom work on the strut tower (better camber plates exist for the stock E89 only if running coilovers)
2) I must select the right tires first, and only then start collecting data (both temp and tire wear) on the exact track that I want to improve lap times on.
3) I probably need less camber rear, and should forget about swapping swaybars to adjust camber.