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      07-27-2020, 02:01 PM   #1
Soul Coughing
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Steering Angle with Different Degrees of Throttle

Hey Everyone,

Hope i can explain myself correctly with what I am feeling with the car. I have a 2017 BMW M3 Competition.

I come from a supercharged Honda S2000 (have owned it for 14 years) and have taken the car to the track quite a bit. I have a decent understanding of how it feels nearer to its limits and how throttle application can influence the balance and feeling of the car, but driving the M3 nearer its limits has me a little perplexed.

When I am on a constant radius turn at 7/10- 8/10s, with the same angle set on my steering wheel, and I start to add more throttle to the car, it makes the car want to turn into the turn - when i let off the throttle, it wants to go wide of the turn. It feels like the weight is moving backwards under load, and then under deceleration the back is lightening up and its toeing out. This happens taking left and right turns on roads with no bumps and no weird camber changes.

In the past when that's happened to me, I've checked the tire pressure and that's usually been the fix - on the S2k under compression, the early cars would toe out on the rear, which would induce oversteer if you weren't super smooth. On the M3, I've checked the tire pressure - which is fine - and there's no apparant damage to the suspension. I haven't done an alignment since buying the car though.

My question is: Is this normal? Is this part of the ZCP suspension? Part of the torque vectoring LSD? Does this happen to other people? Any help would be appreciated. Thanks!
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      07-27-2020, 06:58 PM   #2
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That turn in feeling is the dif.

I've only ever felt that on cars with e diffs (f80, evoX), never on a mechanical diff car (s2k, evo8/9, sti)
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      07-28-2020, 05:49 AM   #3
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I struggle to folllow you 100% because you seem to be contradicting yourself.

It is vehicle dynamics 101: under deceleration the weight moves forward and the car has more oversteer/less understeer; under acceleration the weight moves rearward and the car has more understeer. When pressing harder on the throttle, the understeer can shift to oversteer as the rear tires have more slip.

Is this with DSC fully off? If not, it could be nannies playings tricks on you.

BTW, the F8X does not have a torque vectoring diff. It is just an electronically controlled limited slip.
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      07-28-2020, 07:23 AM   #4
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Quote:
Originally Posted by MrSmartyPants View Post
That turn in feeling is the dif.

I've only ever felt that on cars with e diffs (f80, evoX), never on a mechanical diff car (s2k, evo8/9, sti)
This makes sense, thanks!

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Originally Posted by CanAutM3 View Post
I struggle to folllow you 100% because you seem to be contradicting yourself.

It is vehicle dynamics 101: under deceleration the weight moves forward and the car has more oversteer/less understeer; under acceleration the weight moves rearward and the car has more understeer. When pressing harder on the throttle, the understeer can shift to oversteer as the rear tires have more slip.

Is this with DSC fully off? If not, it could be nannies playings tricks on you.

BTW, the F8X does not have a torque vectoring diff. It is just an electronically controlled limited slip.
My apologies if I didn't explain myself correctly. I am not at the limits of adhesion. DSC is on, but I am not adding enough throttle to induce wheel spin; I am just progressively adding throttle and then slowly letting it off, and while in a turn where I am not changing the angle of the steering wheel, the car will act like I've added more steering angle (if i am adding throttle) - and less steering angle when I am letting off the throttle. I just don't know if this is a 'feature' of the car, or if there's something wrong with it.

I guess I will go back and see if anything changes if I change the modes and turn off the DSC.

And thanks on explaining about the e-diff!
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      07-28-2020, 07:52 AM   #5
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Yup DSC on is your problem, it definitely does some funky things and is hyper intrusive even when off throttle.
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      07-28-2020, 09:47 AM   #6
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Quote:
Originally Posted by Soul Coughing View Post
This makes sense, thanks!



My apologies if I didn't explain myself correctly. I am not at the limits of adhesion. DSC is on, but I am not adding enough throttle to induce wheel spin; I am just progressively adding throttle and then slowly letting it off, and while in a turn where I am not changing the angle of the steering wheel, the car will act like I've added more steering angle (if i am adding throttle) - and less steering angle when I am letting off the throttle. I just don't know if this is a 'feature' of the car, or if there's something wrong with it.

I guess I will go back and see if anything changes if I change the modes and turn off the DSC.

And thanks on explaining about the e-diff!
Thanks for clarifying what you are experiencing, now I better understand your confusion since the car behaves opposite to what is normally expected.

The way the e-diff is programmed, it will only accentuate the normal on-off throttle behavior of the chassis. I doubt it is the e-diff that causes what you experience. My guess is that it is the DSC that is creating this behavior, as it applies the brakes to counter the natural over- and under-steer. The F8X is a superbly well balanced chassis that responds precisely to throttle steering inputs, but it needs DSC fully off to be truly enjoyed and appreciated.
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      07-28-2020, 01:21 PM   #7
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The DSC in our cars is real nasty. You may not be at the limit of grip but minor variation of angular velocity in your inside and outside wheels will have the DSC frothing at the mouth.
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      07-28-2020, 10:14 PM   #8
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DSC/MDM are a nice safety net, but I firmly believe they should be disabled ASAP. MDM is incredibly aggressive on the F82, it will reinforce some very bad habits and remove a lot of learning opportunities(spins) when your speeds are very slow due to skill. Spins only get more dangerous as you get faster.
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      07-29-2020, 01:34 PM   #9
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+1000 on turning off dsc

Maybe dsc interpreted your lift throttle as “please save me” and dialed in some understeer
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      08-21-2020, 08:24 PM   #10
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Interesting to have come across this post.

A few months back at an HPDE at Roebling Road, my instructor had me hold the steering angle mid turn on the exit to Turn 1 (he actually placed a couple of fingers on the steering wheel to make the point) and he asked me to accelerate smoothly out of the turn. The car actually turned in (tightened the turning angle) just as you describe. There was no tire slip (no trailing throttle oversteer) nor DSC engagement. I felt the rears (RE71R) grip and the car rotated into the turn. It was pretty amazing.

I have been looking for more info on this technique but no luck so far. It seems perfect for long turns with a closing radius on exit. On that day, it gave me a couple more MPH on exit.

I have been meaning to reach out to my instructor and this was a good reminder. Will post again if I can find a tech article reference.
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      08-22-2020, 08:03 AM   #11
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Steering Angle with Different Degrees of Throttle

Quote:
Originally Posted by Daytona_550 View Post
Interesting to have come across this post.

A few months back at an HPDE at Roebling Road, my instructor had me hold the steering angle mid turn on the exit to Turn 1 (he actually placed a couple of fingers on the steering wheel to make the point) and he asked me to accelerate smoothly out of the turn. The car actually turned in (tightened the turning angle) just as you describe. There was no tire slip (no trailing throttle oversteer) nor DSC engagement. I felt the rears (RE71R) grip and the car rotated into the turn. It was pretty amazing.

I have been looking for more info on this technique but no luck so far. It seems perfect for long turns with a closing radius on exit. On that day, it gave me a couple more MPH on exit.

I have been meaning to reach out to my instructor and this was a good reminder. Will post again if I can find a tech article reference.
I have literally noticed the exact same thing.

On 360 degree merge on ramps if i keep a neutral throttle or let off the car dials some understeer and hates to turn in.

The moment i mash the throttle a bit the front end bites hard and the car turns on a swivel at a much higher speed.

It seems that adding throttle helps the car turn in quite easily.

My personal opinion is with DSC or MDM on what we are experiencing is the car nannies are dialing in understeer as the aggressive steering angle + letting off throttle might be signalling an oh fuck situation.

The moment the driver adds throttle the car goes back to being completely neutral and turns in as expected.

I have tested this theory out somewhat going 210 kph down a bend with dsc fully on vs all nannies off.

In the dsc on situation if i let off as i am coming into the turn my speed drops a bit and the car understeers ever so slightly so i add more angle to keep it straight or stomp the gas to help it become neutral.

Under WOT stepping into the turn with DSC fully off the car just bites and takes off even when i let off it will steer right in line and stay neutral.

Long story short if you are confident as a driver and stay on throttle in corner the car leaves you alone, if you pussy out it tries to save you when you dont need saving in the case when you have dsc on. Now if you do go too aggressive in corner the nannies will kick in and slow your roll down so i would keep them on.

I honestly like the neat party trick they do though, having my passengers close their eyes as i am in a corner with my foot off pedal and seemingly struggling to turn her in, then by magic i accelerate even harder and to their surprise the car bites down instead of sliding off the road and combusting into a
Million pieces
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      08-22-2020, 01:28 PM   #12
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Got a hold of my instructor and he described it as the same thing that is covered in this article as "slipping rear tires".

http://racetrackdriving.com/driving-...ttle-steering/
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      08-22-2020, 01:35 PM   #13
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Yeah, you can use the throttle to rotate the car. In a similar manner to using the brakes to rotate the car, it's something best reserved with a bit of experience. I used to stab the throttle on my Miata to rotate it, we had it setup to be very neutral to minor understeer, as it was an endurance car with a rotating roster, but stabbing the throttle caused the rear to slip a little more and rotate the car.

Been struggling with rear grip on the M4 until I swapped to the Toyo RRs. With the RRs I was able to rotate the car through a heavily cambered corner at my track that I'm very comfortable with with holding partial throttle and jabbing it lightly to slip the rear slightly. Really makes you feel badass manipulating the car with the throttle. Tried it with my cooked NT01s, and drifted up the hill instead.
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