high speed rear stability

[M3 Number 86]

Originally Posted by chillindrdude

off topic, but do you think the differential runs cooler at the track all nannies off versus MDM or DSC modes activated?

Originally Posted by nicknaz

My GUESS is that it doesn't matter because the brakes are the ones getting worked by the DSC/MDM, but drifting/not drifting would matter more for wear / heat on the diff

I'm exaggerating, but if you aren't full DSC off, you're not even using the diff at all.

[Boss330]

Originally Posted by M3 Number 86

I'm exaggerating, but if you aren't full DSC off, you're not even using the diff at all.

Doesn't the diffs torque vectoring come into play here? Transferring power to the wheel with most traction (like the outside wheel in a corner). Or does this only happen when you have tire slippage (wheelspin)?

[M3 Number 86]

Originally Posted by Boss330

Doesn't the diffs torque vectoring come into play here? Transferring power to the wheel with most traction (like the outside wheel in a corner). Or does this only happen when you have tire slippage (wheelspin)?

I don't think it's like a torque vectoring system unless that's what the ediff does now. I always thought it will lock when you need the traction aka wheel spinning at the limit.

[CanAutM3]

Originally Posted by Boss330

Doesn't the diffs torque vectoring come into play here? Transferring power to the wheel with most traction (like the outside wheel in a corner). Or does this only happen when you have tire slippage (wheelspin)?

I am not sure it does torque vectoring either. I believe it is only a locking differential with electronic control. The ECU can anticipate when locking or unlocking is needed therefore improves the reaction time over the previous mechanical unit.

[Boss330]

Originally Posted by M3 Number 86

I don't think it's like a torque vectoring system unless that's what the ediff does now. I always thought it will lock when you need the traction aka wheel spinning at the limit.

Originally Posted by CanAutM3

I am not sure it does torque vectoring either. I believe it is only a locking differential with electronic control. The ECU can anticipate when locking or unlocking is needed therefore improves the reaction time over the previous mechanical unit.

Yeah, seems you both are right. At least no mention of torque vectoring in the tech article about the "Active M differential"

http://f80.bimmerpost.com/forums/sho....php?t=1018024

But from the way I understand it the active diff is active in any mode (or DSC on/off). The diff of course does work allways, that's the whole point of having a diff. And also the active part should be active during any spirited driving, regardless of DSC on or off. And the active diff is more proactive than just locking up after detecting wheelspin. According to BMW it reacts to a lot of inputs and locks up before the event takes place (based on steering input, yaw angle etc).

[CanAutM3]

Originally Posted by Boss330

Yeah, seems you both are right. At least no mention of torque vectoring in the tech article about the "Active M differential"

http://f80.bimmerpost.com/forums/sho....php?t=1018024

But from the way I understand it the active diff is active in any mode (or DSC on/off). The diff of course does work allways, that's the whole point of having a diff. And also the active part should be active during any spirited driving, regardless of DSC on or off. And the active diff is more proactive than just locking up after detecting wheelspin. According to BMW it reacts to a lot of inputs and locks up before the event takes place (based on steering input, yaw angle etc).

I agree

I think what M3 Number 86 implied is that when using DSC, it kicks in so early, you cannot fully appreciate what the e-diff can do. He might have "exaggerated" a little in his post .

[M3 Number 86]

Originally Posted by CanAutM3

I agree

I think what M3 Number 86 implied is that when using DSC, it kicks in so early, you cannot fully appreciate what the e-diff can do. He might have exaggerated a little in his post .

Correct - I think I even said "exaggerating but..."

Still aren't really using the diff unless right up next to traction limit which is for sure not being reached if nannies are on. Especially in this car with the torque it has and reading how many people think that it is way too intrusive at the track.

[dclowd9901]

Are any of you running the stock adjustable suspension in sport + mode? I hammered the shit out of it at Nürburgring and the only floating I got was on the occasional bumps, but never on hard braking. In fact, the stability under braking has saved my ass on more than one occasion when I misjudged speed into a turn.

[Appeltje]

Seems I'm one of the few who didn't notice this. I've done Texas World Speedway, on the straight my speed was +145mph and after first breaking point before going into the first corner I haven't notice any instability in the rear. Maybe because the bushing were still 'fresh' as the car had driven less the 2000 miles

[TRZ06]

Originally Posted by Appeltje

Seems I'm one of the few who didn't notice this. I've done Texas World Speedway, on the straight my speed was +145mph and after first breaking point before going into the first corner I haven't notice any instability in the rear. Maybe because the bushing were still 'fresh' as the car had driven less the 2000 miles

The rear sub-frame doesn't have any bushings. It's directly bolted on

[Racer20]

I didn't have this problem at all yesterday braking from 140 down to 50mph. My car is bone stock except for the brake fluid.

Adaptive suspension in sport+, 19" PSS, and iron brakes.

The most likely culprits here are:

Alignment
Braking technique
Front/rear brake force distribution (from pad selection)
Tires

I would be very surprised if the stock set up has this behavior inherently . . . from the couple replies above me, it sounds like completely stock cars don't have the issue.

[Racer20]

Originally Posted by CanAutM3

I don't think it is only the tires (if at all). I am running NT01s that have quite stiff sidewalls and still observed the instability.

Agreed. Getting the lateral compliance of each part of the rear suspension to work together as a system is a fundamental part of OEM chassis development. BMW wouldn't have overlooked this.

In the specific case that TRZ06 is talking about, it's more likely that the XP12's are just too much pad for the PSS.

[CanAutM3]

One more data point.

I was at the track yesterday with brand new NT01 at all four corners and new RS29 pads in the rear.

During my first session out, the car would get terribly unstable braking into turn-10 to the point where I struggled to keep it in a straight line. I had to position the car almost mid-track out of fear of putting a wheel on the grass. This is the worst I have ever experienced it.

However, as of my second session, everything became perfectly stable. The car remained arrow straight braking into 10 and I could hold the proper line again.

To me, this is a good indication that the instability is related to the interaction between tires and brakes and is not suspension related. Once the tires were broken-in and brakes bedded, everything became perfectly stable.

[nicknaz]

Originally Posted by CanAutM3

. Once the tires were broken-in and brakes bedded, everything became perfectly stable.

I guess that is why there is the "drive nicely for 500 miles until the mold release compound wears away"break in warning for tires?

I experienced something similar with my RE11 tires, where the car would slide/grip/slide/grip a bit in mid corner until after some more driving and heat cycling.

by the way, what were the ambient temps? Could you have had cold/damp track + tires in the AM that warmed up ?

[CanAutM3]

Originally Posted by nicknaz

I guess that is why there is the "drive nicely for 500 miles until the mold release compound wears away"break in warning for tires?

I experienced something similar with my RE11 tires, where the car would slide/grip/slide/grip a bit in mid corner until after some more driving and heat cycling.

by the way, what were the ambient temps? Could you have had cold/damp track + tires in the AM that warmed up ?

500 miles ... I certainly cannot pamper them that long, that's about half the life I get out of them .

But you are right, you need to scrub off that first layer for the tires to generate their maximum grip, which usually hapens within the first 10 track miles.

The temperature was around 75degF and the track was dry. But we did have significant fog during our first session that was at 9am. So it is possible there was a humidity film on the tarmac. The track did not feel slippery at all though. I did my best lap of the season (1:52.20) on that session despite the wobbly rear end in turn-10 .

[TRZ06]

Also keep in mind that in addition to the release mold that brand new tires have, they also have a lot of tread on them and that causes tread squirm, especially noticeable in transitions or when threshold braking.

Thats why racers have their tires shaved down. Tires in general feel more stable as they wear down. You may lose a little grip, but the stability is better with tires that are near the wear bars.

In my experience with tires like the RE-11 , it takes about 1,000 to 1,300 miles before they really settle in. Tires that have a higher treadwear rating may take longer.

[nicknaz]

Originally Posted by TRZ06

In my experience with tires like the RE-11 , it takes about 1,000 to 1,300 miles before they really settle in.

thanks for the info!

[MovingChicane]

Older thread I realize, but since I just created a new thread about the same (or similar) topic, I thought I'd add my experience here too.

I'm getting, and really don't like, the rear-end float this 2016 M4 (non-Comp. Pack) is providing. Where I'm seeing a difference between what you all have been discussing and what I'm experiencing, is that this is occurring NOT under braking but on higher-speed (~60mph) elevation w/direction transitions - larger "mounds" and not "bumps." So I don't have front-end weight transition or dive as part of the formula. I think it's caused by soft springs, poor shock calibration (instant compression response and too much rebound), and alignment.

[TRZ06]

Originally Posted by MovingChicane

Older thread I realize, but since I just created a new thread about the same (or similar) topic, I thought I'd add my experience here too.

I'm getting, and really don't like, the rear-end float this 2016 M4 (non-Comp. Pack) is providing. Where I'm seeing a difference between what you all have been discussing and what I'm experiencing, is that this is occurring NOT under braking but on higher-speed (~60mph) elevation w/direction transitions - larger "mounds" and not "bumps." So I don't have front-end weight transition or dive as part of the formula. I think it's caused by soft springs, poor shock calibration (instant compression response and too much rebound), and alignment.

Close, not enough rebound and then the shocks don't do a good job of stopping the rebound.

It needs both quicker rebound and a shock that controls that rebound better.

I'm really pissed at BMW for letting an M car leave the factory with the sub-par stability in the rear.

There is just no excuse for an M car to not be fully stable, especially with the shock technology out there today in much lesser cars.

[BMW M4 PWR]

It's definitely the alignment. Make sure you've got slight *rear* toe-in (-.02) at each corner and you're good to go

[FLOSS M]

I find the car to be pretty stable in breaking down the straight.

Stock suspension
Dinan monoball and rear toe links
M perf aero
R888
RS 29
Euro mdm - sport plus