CMP Solid Subframe Raising Bushes. The Pro's & Con's

Having received questions in past through forums, Facebook and emails asking for further clarification on the function of our unique solid subframe bushes bushes, we thought we'd create a tech article specifically for them to break down the function into simple pro’s and con's. 

Jumping right into it, we'll start with the simple pro’s and cons of solid bushes in comparison to the factory rubber or poly bushes then delv into what make ours bushings unique as opposed to others.

Solid bushes:

PRO’s:

-The subframe is no longer an isolated rigid body beneath the chassis but rather acts as a singular combined structure. This one change has several benefits for grip, wheel hop, chassis rigidity & body roll.

- Wheel hop: Undesirable roll moments about the prop shaft and rear axles is eliminated meaning as the driveline builds in torque bushing deflection no longer occurs in the subframe (just diff) creating better response to throttle inputs and greater feedback to the driver.

- Grip & stability: The benefit here is that the suspension pickups remain constant relative to the chassis while before, they would shift as the subframe does due to bushing deformation under torsional and lateral forces (driveline and cornering G’s). By achieving more constant geometry the vehicle is more consistent and predictable providing greater stability and predictable control of the back end as geometry remains more constant.

- Chassis rigidity: Now that the subframe and chassis are no longer two isolated rigid bodies but rather a single combine structure, the subframe contributes its rigidity to the chassis creating greater overall chassis rigidity between the 4 subframe mounts. Much like a strut brace between shock towers. In essence, for the RACP between the 4 rear subframe to flex, so must the subframe as the induced moments are experienced by both bodies simultaneously.

- Body Roll: Anybody roll acting on the chassis due to cornering G’s has to transfer through the subframe bushes before it can be resisted by the rear wheels. Due to the elasticity and elastic modulus of the rubber/poly bushes, deformation occurs when transferring this force much like those mentioned above. As a result, the chassis theoretically has a greater roll angle than the subframe beneath it due to this creating more roll in the chassis. The bushings themselves are like a spring so deformation is relative to the roll moment thus reducing the chassis response to roll till sufficient deformation occurs.

- Lastly: Given the springs are mounted on the upper control arms the control arm itself can be analysed as a simply supported beam between two hinge connections. Therefore, as the suspension is loaded up there is a reaction at either end (subframe & RTA). In addition to the driveline and lateral forces, the sprung weight of the car also acts to cause bushing deformation at the subframe bushes and between resulting in further geometry changes, body roll, etc etc.

In short, stepping up to solid subframe bushes has many positives for handling and from first hand experience running these in our daily driven M3 for the last few years, does not impact much at all on Noise Vibration or Harshness (NVH) as everything mounted to the subframe is still isolated by it’s own respective bushings eliminating the transfer of diff whine or just harshness from the wheels.

You get massive improvements in handling and feel for the back end without any compromise in drive-ability. 

For those who may be thinking, if it’s so great, why didn’t BMW do it that way originally? Surely BMW knows best!!! The answer is they did, just not for the E46 chassis.
If memory serves correct solid subframe bushes from factory first occurred in the 3-series line up on the E92 M3 GTS. We suspect that from the first hand experience testing that car, the design was then further developed to a hard mounted subframe with no bushings at all in the F8X/F2X 2 & 3 series.

Now the con’s of the solid bushes:

The con of the solid subframe bushes varies depending on the car however, we consider it fairly substantial given the issues the E46 chassis has with fatigue related cracking at the subframe mounts and the greater Rear Axle Carrier Panel (RACP).

Essentially, the forces acting on the subframe and thus subframe mounts & RACP are impulsive forces which is best described as a change in momentum. Imagine a collision (impact between two objects).

As a result, the force acting on a body is dependent on the rate of acceleration (F = M * a). For the acceleration to be reduced, the rate of change has to occur over a longer period of time. This can be done by creating deformation (imagine a spring, compressing, building up in force then expanding, creating a varying force over a prolonged period of time). This rate of change is known as dampening.

This is exactly how a bushing works to eliminate the harshness of the road from the chassis.
By swapping a rubber bushing for something solid or bearing, you eliminate much of that deformation and thus dampening create much shorter period of time for the rate of change to occur. As a result, the acceleration is much quicker thus creating a higher peak force.

Force on area makes stress. I higher peak force results in a higher peak stress acting on the chassis. 

The cracking and failure that occurs in the RACP is a result of fatigue. A structure is generally design to a ‘fatigue life’ which means the predicted period of time till fatigue will occur. Some things can be designed to an infinite fatigue life by keeping designed stresses below the fatigue limit however, often fail due to other variables such as material imperfection etc. This fatigue life is dependent on the severity of abuse (stress) and the amount of cycles it occurs.

By reducing the dampening of impulsive forces you increase the peak stress acting on the chassis thus reducing the fatigue life. Meaning, failure is both more likely to occur and occur sooner.

This is essentially the only negative to solid mounting the subframe and as mentioned above, only bears any significant on chassis that suffer existing fatigue issues such as the E46. Most cars do not suffer from fatigue even with ridiculous increases in power and thus opting for solid subframe bushes has no negative. 

To combat this negative, the structural rigidity must be increased to withstand the increased peak stresses to the point that such fatigue related failure is eliminated. This is why we encourage these bushes be paired with E46's fitted with additional topside chassis reinforcement.

Without going too off topic, plates provide localised rigidity at the subframe mounts however, not throughout the structure in other areas also within the load path distributing the same stresses. So failure is inevitable between the plates and wheel arch as well as beneath the rear bench seat along the chassis rail, and around the various anchor points present.

This is the reason the CMP RACP reinforcement kits are on the more substantial side as they intend to not only correct the design flaws to prevent failure occurring but continue to do so even after such modifications have been made or substantial increases in power.

That in essence covers all the need to know pro's and con's of solid subframe bushes than can in theory be applied to any vehicle. 
Moving onto where our solid subframe bushes differ specifically from other solid subframe bushes is the raising characteristic.

We'll start with the negative on this as it’s short and simple,

Con’s:

- Much like when lowering a car with double wishbone suspension, the camber geometry changes to be more negative due to the 'camber curve'. This is completely normal and occurs constantly as the wheel rises and falls in the guard over bumps and under body roll. 
By raising the subframe upward you create the effect of raising the car without the ride height actually changing. The new angle of inclination of the rear arms moves upward at the inbound end. 
This produces an opposite effect to the above of creating more positive camber.


If the car these bushings are fitted to has been lowered ~13mm, (1/2”) or more, you’ll find that the change in rear camber from both is equally opposite and you'll remain roughly in the same range of adjustment as before. 

If the car is still at factory ride height, the resulting change in camber geometry limits negative camber adjustment to less than ideal for sporty driving. We'd like to imagine that if you've purchased these subframe bushes this likely isn't your first suspension modification however, if your car is still are factory ride height, adjustable camber arms are required to dial in more negative camber.

To be clear, the need for adjustable camber arms when fitting these bushes only exists when the car is still at stock ride height.

Now onto the PRO’s of these bushings over other solid bushes.

Pro’s

- reduced body roll, increased rear wheel grip.

-Reduced Body roll: The purpose of the bushings is to change the angle of inclination of the rear control arms thus changing the roll centre (an imaginary axis in which the body rotates about under cornering forces). The bushings do not intend to alter the wheel position relative to the chassis and thus the centre of gravity remains essentially the same. the result of bringing the roll centre close (higher) to the centre of gravity is the reduced roll moment.

The roll moment like any other moment is the force multiplied by the perpendicular distance (T = F * d). Body roll can be reduced by either reducing the weight (F = M * a (in this case a = G force)) or by reducing the distance perpendicular to the G force by either lowering the centre of gravity or raising the roll axis.

This is where these bushings offer a performance advantage against other bushes.

Increased rear wheel grip: By reducing body roll you reduce the weight transfer between the two rear wheels making them more equally loaded. The principle here is simple in that two tyres of equal loading will provide more grip than two identical tyres with unequal loading.

So in short, in addition to all the positives (and negative) solid subframe bushes offer, the CMP bushes provide even more positives with the only negative (requiring adjustable camber arms) existing for cars at stock ride height.

Moving onto some other points of interest. 

Fitting underside reinforcement plates to the subframe mounts increases the distance between the centre of gravity from the roll centre creating additional body roll and less rear wheel grip based on the theory above.
Our solid subframe bushings aren’t intended to counter just the change due to fitting reinforcement plates but further improve geometry for better stability and reduced body roll.

We hope this deeper explanation of the functionality of our solid subframe raising bushes helps to answer any questions you may have. 

If you still feel uncertain about any aspect of the product feel free to reach out to us! Our contact info is below. 

Email: info@cmpautoengineering.com

Facebook: https://www.facebook.com/cmpauto