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      04-05-2012, 05:09 PM   #1
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car control question

not sure how to ask this, but here goes.
if i take a corner and push a little throttle to throw the tail out a little, what can i do to soften the moment the rear tires gain traction?
basically right now, when the rear tires gain traction, the rear violently buck left and right a couple time.
making it hard to launch out of corners.
is it because my driving technique (likely), my lack of LSD, my soft suspension, my lack of m3 bits on the rear? or all of the above?

any answer from race pros (not the straight line race) is appreciated.
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      04-06-2012, 12:36 PM   #2
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The answer is FAR more complicated that one can answer on a forum. You're much better off taking a few car control clinics that most BMW CCA chapters offer throughout the year to learn this stuff.

But I'll take a stab at it.

Imagine your car as a 3 axis pendulum, with 2 of the axis attached by springs. The car is capable of rotating or swinging in these three axis: It'll rock left and right, it'll dive front and rear, and when it spins, it'll pivot on the axle that has grip. The two axis that are controlled by springs are the rocking left and right, and the dive and squat front and rear.

If none of this stuff makes any sense, pick up a toy model car and imagine it moving in space. If you're familiar with flying, these three axis are pitch, yaw, and rotate.

In a car, since two of the axis is limited and controlled by springs mounted on dampers, and the tire's contact patch acting as control for the rotation of the car while the tires act as the spring, the end result is when the parameters of the car's operation is pushed beyond the limit of a single component to control, any component, the end result is OSCILLATION, which is the violent rocking back and forth you describe.

If you're still reading at this point, the simple answer to your question is yes, all of the above, some of the above, and none of the above. If this doesn't make any sense, I'll try to confuse you more...

The three axis rotation on a car works as a system. Every single movement is determined by movement of the other axis. How the car yaws depends on both the pitch and rotation. So yes an LSD will likely help since it controls the distribution of power to both rear wheels (controls pitch, the squatting and diving of the car, as well as the rotation, the rocking left and right of the car to a lessor extent), therefore it will ultimately control the yaw of the car. M3 bits in the rear limit the movement of the suspension and increases camber, thus altering the way the car's yaw rate in the middle of the turn, hence altering the pitch and rotation...etc. I hope you get the idea.

Ultimately though, NONE of the parts mentioned above controls the 3 axis like how YOU should be able to control the three axis. If your input overwhelms any of the linked parameters, it will start a chain reaction that ultimately results in oscillation. In other words, take a car like a Formula 1 race car. It's got, by comparison to normal street cars, almost unlimited grip and power. Yet put it in the hands of an untrained driver, it'll probably spin out before it leaves the pits, despite all the parameters of the equipment being magnitudes higher than what a typical street car is equipped with. At that point, ALL the equipment in the world is already installed on the race car to prevent the parameters being overwhelmed, yet those parameters will still be overwhelmed by any novice driver that do not know what they're doing.

IF you're still reading. If none of what I said above made any sense, stop now, go attend a few Car Control Clinics. None of what I say now will make much sense nor help you control your car better. But if you're interested, I can sort of explain to you what is happening.

As you exit out of a turn, weight of the vehicle is transferred to the outside wheels and forcing the outside springs to compress. And as you accelerate, weight of the vehicle is further transferred to the back of the car, making the outside, rear wheel bearing the brunt of the weight of the vehicle. So the rear, outside springs are likely fully compressed, or compressed more than the other corners, and the dampers doing its best to prevent it from REBOUNDING as you exit the turn. Now, as you apply MOAR throttle the rear wheels will eventually break loose and start spinning. At this point, since the rear tires are spinning freely you are no longer accelerating as quickly as you were prior to the tires breaking loose. The end result is, some of that weight comes off of the rear axle, and all that energy that was stored up in the compressed spring is allowed to rebound and send that force back the other way as the rear continues to lose traction from being overpowered by the engine.

This part is the slide.

What comes next, as you catch the slide by counter steering, weight of the vehicle starts to transfer to the INSIDE tire, and now the inside tire all of a sudden regains grip and catches, sending your car's polar momentum going the other way. Combined with the forces being unloaded by the rear outside springs plus the inside tire regaining traction, the two forces add and multiply and again overwhelms the rear tires, now going in the OPPOSITE direction starting an oscillation of the yaw of the car.

This part is the "tank slapper."

Now, in a car that is equipped without an LSD, relying on rear brakes to simulate the effects of an LSD, and DTC acting on its own to prevent the owner of the car from killing him/herself, in the above situation DTC and the e-Diff will act by providing forces moving in the OPPOSITE direction of the forces being applied in a spin. DTC will cut power to the rear to attempt to minimize the wheel spin to the OUTSIDE of the turn, while e-Diff will attempt to brake the inside wheel to prevent power from being diverted from the INSIDE of the turn, and this all act in conjunction to create that bucking sensation instead of a car spinning freely out of control one way or another on a sprung car.

This stuff is hard to visualize. It is best practiced on a big empty parking lot, ideally wetted down with soapy water to decrease grip to allow for prolonged spins. You know, like a Car Control Clinic from a BMW CCA chapter.
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      04-06-2012, 12:46 PM   #3
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Thanks Hack, Never thought of the relation of the balancing side to side while from to back is being effected.

I learned by experimentation and feel but understanding the physics of it is always a benefit to understanding why something is happening and, hopefully, how to control and, even more hopefully, to create more speed with it.
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      04-06-2012, 02:09 PM   #4
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wow, thanks for the write up. understanding the physics of it is very helpful.
i understood about 50% of the physics before reading your write up.
i've been to driving ed both held by the cca and pca (the porsche club) and continuing to learn.

so am i just taking the car too far? the bmw cca instructor usually suggest that i exit a little slower (apply throttle a little later), the pca instructor seems to think i need to be on the ragged edge of the limit. i guess there are a lot of driving technique.
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      04-08-2012, 11:28 AM   #5
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Neither are correct, yet both are right (BMW CCA and PCA instructors).

What you lack is SMOOTHNESS. Think again of the physical model. Then think of exiting the corner at the highest speed possible as a BALANCING ACT of all the forces involved. Since more weight on a rubber surface increases its grip against the road, you want as much weight as possible on the outside drive wheels. But in doing so, you also load up the springs, which has a natural tendency to want to rebound. In order to do so, you must be able to keep applying force to the rear outside wheel without going over the tire's static friction limit and force it to spin. One way to do so, is to apply throttle later (CCA). However, that means you're not putting as much weight on the rear outside wheel as possible and therefore not using the maximum available acceleration (PCA).

The key here, is to understand that the available grip, or the actual condition in which the tires stay in static frictional forces rather than kinetic frictional force, varies dramatically depending on a few parameters. First, if you move the weight of the vehicle onto the outside rear tires too quickly, it's going to dramatically lower the difference between static and kinetic friction. Second, if you apply too much throttle too quickly, it's going to again lower the static friction to kinetic friction. In order to maximize grip, you have to transition the weight as smoothly as possible but as quickly as possible, likewise with the throttle, you need to apply as much throttle as possible, yet as smoothly as possible but as quickly as possible.

It's all a balance.
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      04-09-2012, 03:40 PM   #6
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Hack...you are the man. Great write up! I know who to come to with questions from now on. Thanks for that!
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      04-09-2012, 04:21 PM   #7
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Quote:
Originally Posted by The HACK View Post
The answer is FAR more complicated that one can answer on a forum. You're much better off taking a few car control clinics that most BMW CCA chapters offer throughout the year to learn this stuff.

But I'll take a stab at it.

Imagine your car as a 3 axis pendulum, with 2 of the axis attached by springs. The car is capable of rotating or swinging in these three axis: It'll rock left and right, it'll dive front and rear, and when it spins, it'll pivot on the axle that has grip. The two axis that are controlled by springs are the rocking left and right, and the dive and squat front and rear.

If none of this stuff makes any sense, pick up a toy model car and imagine it moving in space. If you're familiar with flying, these three axis are pitch, yaw, and rotate.

In a car, since two of the axis is limited and controlled by springs mounted on dampers, and the tire's contact patch acting as control for the rotation of the car while the tires act as the spring, the end result is when the parameters of the car's operation is pushed beyond the limit of a single component to control, any component, the end result is OSCILLATION, which is the violent rocking back and forth you describe.

If you're still reading at this point, the simple answer to your question is yes, all of the above, some of the above, and none of the above. If this doesn't make any sense, I'll try to confuse you more...

The three axis rotation on a car works as a system. Every single movement is determined by movement of the other axis. How the car yaws depends on both the pitch and rotation. So yes an LSD will likely help since it controls the distribution of power to both rear wheels (controls pitch, the squatting and diving of the car, as well as the rotation, the rocking left and right of the car to a lessor extent), therefore it will ultimately control the yaw of the car. M3 bits in the rear limit the movement of the suspension and increases camber, thus altering the way the car's yaw rate in the middle of the turn, hence altering the pitch and rotation...etc. I hope you get the idea.

Ultimately though, NONE of the parts mentioned above controls the 3 axis like how YOU should be able to control the three axis. If your input overwhelms any of the linked parameters, it will start a chain reaction that ultimately results in oscillation. In other words, take a car like a Formula 1 race car. It's got, by comparison to normal street cars, almost unlimited grip and power. Yet put it in the hands of an untrained driver, it'll probably spin out before it leaves the pits, despite all the parameters of the equipment being magnitudes higher than what a typical street car is equipped with. At that point, ALL the equipment in the world is already installed on the race car to prevent the parameters being overwhelmed, yet those parameters will still be overwhelmed by any novice driver that do not know what they're doing.

IF you're still reading. If none of what I said above made any sense, stop now, go attend a few Car Control Clinics. None of what I say now will make much sense nor help you control your car better. But if you're interested, I can sort of explain to you what is happening.

As you exit out of a turn, weight of the vehicle is transferred to the outside wheels and forcing the outside springs to compress. And as you accelerate, weight of the vehicle is further transferred to the back of the car, making the outside, rear wheel bearing the brunt of the weight of the vehicle. So the rear, outside springs are likely fully compressed, or compressed more than the other corners, and the dampers doing its best to prevent it from REBOUNDING as you exit the turn. Now, as you apply MOAR throttle the rear wheels will eventually break loose and start spinning. At this point, since the rear tires are spinning freely you are no longer accelerating as quickly as you were prior to the tires breaking loose. The end result is, some of that weight comes off of the rear axle, and all that energy that was stored up in the compressed spring is allowed to rebound and send that force back the other way as the rear continues to lose traction from being overpowered by the engine.

This part is the slide.

What comes next, as you catch the slide by counter steering, weight of the vehicle starts to transfer to the INSIDE tire, and now the inside tire all of a sudden regains grip and catches, sending your car's polar momentum going the other way. Combined with the forces being unloaded by the rear outside springs plus the inside tire regaining traction, the two forces add and multiply and again overwhelms the rear tires, now going in the OPPOSITE direction starting an oscillation of the yaw of the car.

This part is the "tank slapper."

Now, in a car that is equipped without an LSD, relying on rear brakes to simulate the effects of an LSD, and DTC acting on its own to prevent the owner of the car from killing him/herself, in the above situation DTC and the e-Diff will act by providing forces moving in the OPPOSITE direction of the forces being applied in a spin. DTC will cut power to the rear to attempt to minimize the wheel spin to the OUTSIDE of the turn, while e-Diff will attempt to brake the inside wheel to prevent power from being diverted from the INSIDE of the turn, and this all act in conjunction to create that bucking sensation instead of a car spinning freely out of control one way or another on a sprung car.

This stuff is hard to visualize. It is best practiced on a big empty parking lot, ideally wetted down with soapy water to decrease grip to allow for prolonged spins. You know, like a Car Control Clinic from a BMW CCA chapter.
very cool write up; its always an interesting read especially when physics are applied. From your writeup I understand more how KW coilovers allow drivers to really customize the drivability of their car to their personal liking for optimal performance. I see people on these forums with KWs and what not but I never understood why people got them (especially people that don't track their car).

My question is, I have 15mm spacers in the front and 20mm spacers in the rear of my car (stock sports suspension on 161 style wheels). In the midst of cornering (in the midst of a drift or just grip driving), am I applying excessive stress on my suspension? I've read that spacers equate to having a more aggressive wheel stance (except i'm missing the extra rubber from larger tires). But I'm worried that if I track my car with these spacers on, I will be applying excess stress on my suspension during hard driving.
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      04-09-2012, 08:05 PM   #8
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thanks for the full fledge explanation. i do have problem with my throttle application. especially with the tune on. i guess i should've stick with stock map for races.
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      04-09-2012, 11:03 PM   #9
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Quote:
Originally Posted by spacemonkey1112 View Post
very cool write up; its always an interesting read especially when physics are applied. From your writeup I understand more how KW coilovers allow drivers to really customize the drivability of their car to their personal liking for optimal performance. I see people on these forums with KWs and what not but I never understood why people got them (especially people that don't track their car).

My question is, I have 15mm spacers in the front and 20mm spacers in the rear of my car (stock sports suspension on 161 style wheels). In the midst of cornering (in the midst of a drift or just grip driving), am I applying excessive stress on my suspension? I've read that spacers equate to having a more aggressive wheel stance (except i'm missing the extra rubber from larger tires). But I'm worried that if I track my car with these spacers on, I will be applying excess stress on my suspension during hard driving.
I would never track a car with spacers on unless it's 15mm or larger and it's bolted directly to the hub, and the wheels bolted to the spacers. That's just me though. And even then I'd prefer bolting on wheels with the proper offset rather than spacers since all you're doing is adding 2 more shear surfaces. Outside of that, 15-20mm spacers are adding what, less than 1" to an 18-20" arm? The additional forces, while not ideal, is not THAT significant. Not as much as say, putting on a set of R-Comps can potentially do to stress your suspension.

As for how spacers and different offset affects how the car handles, I've written extensively on this topic too. Feel free to read what my opinions is on the topic of using offset to widen the track/stance of the car.

Keep in mind what I wrote applies mostly to cars driven on the track. Most cars driven on the street comes but to about 2/10th of what typical stock suspension can do. When most people on these forums post about how their suspension transformed their cars, they have little to no idea that at the LIMIT of what these cars can do, most of the time the junk they're putting on their car is actually DECREASING the amount of grip because they don't know how to properly set up the car to optimize grip for their equipment...Since most of that needs to be done through testing at track.

So most of this stuff I wrote, at best, are theoretical. It doesn't apply to 99.95% of the drivers here except for the few that actually takes their BMWs the the track to be driven as how Car-Gods intended.
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      04-10-2012, 03:27 AM   #10
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Quote:
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I would never track a car with spacers on unless it's 15mm or larger and it's bolted directly to the hub, and the wheels bolted to the spacers. That's just me though. And even then I'd prefer bolting on wheels with the proper offset rather than spacers since all you're doing is adding 2 more shear surfaces. Outside of that, 15-20mm spacers are adding what, less than 1" to an 18-20" arm? The additional forces, while not ideal, is not THAT significant. Not as much as say, putting on a set of R-Comps can potentially do to stress your suspension.

As for how spacers and different offset affects how the car handles, I've written extensively on this topic too. Feel free to read what my opinions is on the topic of using offset to widen the track/stance of the car.

Keep in mind what I wrote applies mostly to cars driven on the track. Most cars driven on the street comes but to about 2/10th of what typical stock suspension can do. When most people on these forums post about how their suspension transformed their cars, they have little to no idea that at the LIMIT of what these cars can do, most of the time the junk they're putting on their car is actually DECREASING the amount of grip because they don't know how to properly set up the car to optimize grip for their equipment...Since most of that needs to be done through testing at track.

So most of this stuff I wrote, at best, are theoretical. It doesn't apply to 99.95% of the drivers here except for the few that actually takes their BMWs the the track to be driven as how Car-Gods intended.
HAHA my friend, you've become one of my favorite persons on this forum. You're explanations are the best (+ you have howland and wooden as your avatar so of course you know whats up!!! hahaha)

And I agree, I don't think I'll every know what is optimal for my car unless I test it on a track. Thanks for all the insight, I bookmarked a few pages now cause of you haha Someday when I have the funding, I'll be able to do a bit of trial and error to get the most out of my car. For now, I plan to make a trip to buttonwillow to see what my stock sports suspension is capable of and make notes as to how the car behaves and proceed from there. Do you have a recommendation as to tires to use other than the RFT that come with e90s hahaha My car is nearly bonestock and have been thinking about taking off the spacers just to have peace of mind that I didn't screw things up with the geometry of my suspension (at least when I'm ready to see a track).
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