What causes weight from front to back on acceleration?

[avosough]

I've been getting increasingly annoyed with the amount of body roll--not from side to side in cornering, but from front to back as the weight redistributes during gear changes (6MT) and when braking to a stop. It is extreme enough that I feel like I have to slow down my gear changes even under regular acceleration from a red light in order to stop from getting tossed into the seat belt.

My suspension is stock except for:
Michelin PSS tires on the rear
Continental DWS tires on the front (not ideal, I know...)
M3/1M front control arms
Whiteline rear subframe inserts (I think I need to make sure these are still torqued properly)

I've also got E46 M3 transmission mounts, dual cone intakes, and JB+. I'm not putting down any extreme amount of power.

Is there a certain bushing or shocks or springs that would be the most likely culprit for front to back roll? It seems like the more I do to this car the more unsettled it gets.

[Pissoff]

Bad dampers!

[MightyMouseTech]

The only thing that will change roll to the rear (the proper term is "squat") is increasing rear spring stiffness.

Although, lowering the vehicle will reduce the amount of weight transferred to the rear as you are lowering the roll center.

It is basically the same as roll side to side, just front to back instead.

[Pissoff]

He is having extreme weight transfer under both braking and acceleration. Change springs all you want problem not solved until good dampers are put on. From his question his biggest issue is nose dive.

[135TX]

Originally Posted by Coleroad

Bad dampers!

Originally Posted by MightyMouseTech

The only thing that will change roll to the rear (the proper term is "squat") is increasing rear spring stiffness.

It could be your dampers I'd venture to say that may be most of what you are feeling. Ultimately though the springs are pretty soft on the car and Mighty is right you would probably need to get higher spring rates ( both front and rear though) which would help keep the chassis stable on both accel and decel..

[MightyMouseTech]

Originally Posted by Coleroad

He is having extreme weight transfer under both braking and acceleration. Change springs all you want problem not solved until good dampers are put on. From his question his biggest issue is nose dive.

A damper does not affect the AMOUNT of dive, and/or squat, at all.

You could completely remove the rear shocks and it would squat the same amount.

[Pissoff]

Originally Posted by MightyMouseTech

A damper does not affect the AMOUNT of dive, and/or squat, at all.

Take your rear dampers off see how bad the weight transfer gets when the spring are uncontrolled.

[MightyMouseTech]

Originally Posted by Coleroad

Take your rear dampers off see how bad the weight transfer gets when the spring are uncontrolled.

You are talking about two very different things.

[135TX]

Originally Posted by MightyMouseTech

A damper does not affect the AMOUNT of dive, and/or squat, at all.

You could completely remove the rear shocks and it would squat the same amount.

correct me if im wrong, it wouldnt affect how much you squat/dive but rather how fast or controlled that motion would occur.

A higher level of dampning would therefore slow the weight transfer especially during the shifting, which was a concern of the OP. So although under hard braking or extreme accel you may reach the same height level, the transfer would be slower, thus less likely to jostle you about.

However I agree you need better springs and dampers, but if his dampers were stock and worn out this could effectively contribute to the OPs undesireable behavior.

[Pissoff]

No, no I'm not. Learn before speaking.


Shock and Strut Warning Signs:
Excessive Bouncing

Coil springs are part of your vehicle’s suspension system. If your vehicle is designed with shocks, the coil springs are mounted between the wheels and the frame of the vehicle. Struts, on the other hand, are essentially a coil spring and shock absorber all in one piece. When you drive over a bump, your car bounces on the coil springs, regardless of whether you have shocks or struts. The shock absorbers in both cases keep your car from bouncing nonstop. So if you drive over a bump or a patch of rough road and your vehicle continues to bounce, it is an indication that you require a shock or strut replacement.

Nose Dives/Rear End Squats Dipping

Your car should be steady and stable at all times – whether you are braking, accelerating, or turning – so that you can maintain control as you drive. Shock and struts help keep the car steady.

When braking, if the front end of your vehicle, otherwise known as the nose, of your car dives, it is an indication that your shocks and struts need to be replaced, or at least evaluated. Another sign that your shocks or struts require service is if the rear end of your vehicle “squats” when you accelerate. Additionally, if you make a turn and the vehicle dips drastically to one side, your shocks or struts may need to be replaced.

Tire "Cupping"

If the struts or shocks on your vehicle are bad, the tires will literally bounce up and down as you drive. Each time the tires hit the ground, bits of rubber can get scraped off. This is commonly referred to as “cupping” or “scalloping.” So instead of having a consistent treadwear pattern across the tire, you might have some tread, a smooth patch, some tread, a smooth patch, and so on. This is because the holes left by the missing rubber tend to get smoothed over as you drive.

Leaking Fluid

Inside every shock and strut is a piston and hydraulic fluid. When you drive over a bump or a rough stretch of road, the shocks or struts compress, causing the piston to push against the hydraulic fluid, which absorbs the force of the piston. Sometimes that fluid that can leak out of the shock or strut. Believe it or not, a little leakage is perfectly normal and will look like a small wet spot on an otherwise dry shock or strut. Leakage that you need to worry about will make most of the shock or strut look wet and oily.

[Pissoff]

And I'm not arguing that stiffer springs would be an upgrade. I am saying neck tossing just from shifting is caused by the dampers being bad.

[MightyMouseTech]

Originally Posted by Coleroad

No, no I'm not. Learn before speaking.

You are right, I have only been a licenced tech for 15 years and before that a mech engineer. I have not read anything on the topic.

[Pissoff]

OP, just google signs of worn shocks and struts.

[Kgolf31]

Dampers with compression adjustments will help control the amount of weight transfer. The shock resists compression and makes the wheel "force" up back into the car body

[MightyMouseTech]

Originally Posted by Kgolf31

Dampers with compression adjustments will help control the amount of weight transfer. The shock resists compression and makes the wheel "force" up back into the car body

I would say it more slows the body motion in response to weight transfer. It is very easy to calculate the weight transfer. The only thing involved is the height of the center of gravity (the center point of the weight of the vehicle), the weight, the roll center height (suspension design) and the acceleration.

The springs affect the bodies reaction to the weight transfer.

The dampers affect the rate at which the rear springs react to the transfer.

[Kgolf31]

Originally Posted by MightyMouseTech

I would say it more slows the body motion in response to weight transfer. It is very easy to calculate the weight transfer. The only thing involved is the height of the center of gravity (the center point of the weight of the vehicle), the weight, the roll center height (suspension design) and the acceleration.

The springs affect the bodies reaction to the weight transfer.

The dampers affect the rate at which the rear springs react to the transfer.

No argument there. Think we're talking about the same thing.

Dampers are all about the rate at which things change. Like Rebound and Compression.

Compression + Springs + a ton of other variables plays into squat. There is a noticeable difference on squat from a shock that's full soft versus stiff (you can notice it in a turn when you try to put power down especially).

You can see the minor squat here - #700 Springs and 6/12 Clicks of Compression

[MightyMouseTech]

Originally Posted by Kgolf31

No argument there. Think we're talking about the same thing.

Dampers are all about the rate at which things change. Like Rebound and Compression.

Compression + Springs + a ton of other variables plays into squat. There is a noticeable difference on squat from a shock that's full soft versus stiff (you can notice it in a turn when you try to put power down especially).

You can see the minor squat here - #700 Springs and 6/12 Clicks of Compression

I believe we are.

To the OP, if you have the stock springs still, they are variable rate in the rear on the 08's. So they are very soft for the first bit of travel. The later cars they went with a stiffer linear rate spring.

You want to get a stiffer linear spring and then match it to an appropriate damper.

[Ginger_Extract]

Springs control pitch/roll, shocks control rate of up and down movement. For a street car, if you're feeling excessive body motions, more likely than not, it's the shocks. If you're really pushing the car hard, i.e. autocross/track, then you're more than likely noticing the soft spring rate.

[John_01]

To control the dive and squat, the dampers, springs and bump stops ALL play a part.

With your tuned 135i, I think you probably over powering the stock suspension. The stock dampers are tuned for comfort on RFT tyres. They are too soft from a performance point of view.

Springs control the amount of dive and squat. For BMW suspension the polyurethane bump stops are also involved. Since the springs are quite soft, the car will be ride on the bump stops when driven moderately. Especially the front bump stops will have a major workout if you drive your car on the track.

Dampers control the rate of the body movement. If your dampers are ineffective you will notice the movement a lot more. The car will be "under-damped" which means it will rock on the suspension like a ship in a rough sea.

[Dackelone]

Perhaps also you need to do a "CDV delete". I suspect its your worn suspension(springs and shocks).

The rear subframe bushing bolts are a one time use bolt. Meaning they strech once torqued down(that first time). IF you reuse them, they can break off and let your suspension move around.

[ShocknAwe]

Correct me if I'm wrong (been a while since college physics and I'm pretty far from an engineer), but in the most basic sense possible:

If the damper slows the time for the same measured displacement to be traveled, then they directly affect the acceleration of the components involved, and as a result the force, torque, and rotational quantities as well.

People tend to feel that. So the rest of the details would be technical semantics to the layperson.

[chris82]

Originally Posted by MightyMouseTech

A damper does not affect the AMOUNT of dive, and/or squat, at all.

You could completely remove the rear shocks and it would squat the same amount.

Are you sure about that? When I adjust rebound/compression in my car, even one click can make a difference in how much squat or dive I get.

*Edit: it's the speed at which the weight transfers, so after reading your other comments I think we are in agreement.