When you find these kinds of Mushrooms, it’s bad news. No, we’re not talking psychedelic fungus, but rather a common problem affecting the BMW E46 3-Series (as well as the BMW E36 3-Series). Unfortunately, the fact is, the OEM strut towers of your BMW E46 3-Series are a weak point of the car’s suspension and chassis, and so the forces acting on your shock absorbers or coilovers are also acting on the strut towers, flexing them slightly and slowly but surely stretching them out. Ironic that despite its great handling, the wear and tear on the BMW E46 Suspension itself is the problem.

The end result of which is more stretch than the elastic of Peter Griffin’s Tighty-Whities. Take a look at this example Mushrooming on an E46 with just 36k miles on it (Image via Rogue Engineering):

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So what’s happening here?

BMW Engineers have long been in a struggle to shave weight wherever possible as they’ve added more and more features to the 3-Series – starting back with the E36, with the advent of CAD design and accurate fatigue modelling, BMW created a lightweight chassis that could withstand the load of the STOCK suspension without deflection or deforming, allowing BMW to make the materials of the shock tower area itself as thin as is possible while still retaining the strength to withstand the load of the car’s projected lifespan.

What this means is that because of the thin materials, if you increase the loads on the towers (ie by installing stiffer, lower springs or coilovers that transmit more load than the OEM shocks), the shock towers can’t stand up to the forces it is being subjected to. With the car sitting lower, the potential for bottoming out the suspension is much greater as well – so when the car bottoms out on something like a pot-hole, the force thrusting upwards causes the strut tower to “mushroom” open, bending the strut mount studs outwards.

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Fortunately, there’s a simple solution to the problem. Simply fitting a set of these OEM BMW E46 shock tower reinforcement plates corrects this issue and gives the strut towers the strength they need to withstand the stresses sent through the chassis and prevents the issue. Of course, if your shock towers are already mushroomed, unfortunately it’s too late for reinforcement plates and more aggressive means will be necessary to correct the mushrooming.

At just $18.99+shipping for a pair, these are so cheap, as a piece of OEM BMW PREVENTIVE MAINTENANCE, there’s really no excuse not to have them – and infinitely cheaper than fixing your strut towers once they’ve already mushroomed. Sold in pairs of two – it’s important to remember ALL FOUR of your strut towers suffer from this same structural flaw – these are something you should STRONGLY consider before converting to coilovers.

What’s that squeaking noise in the trunk?

As we’ve mentioned, the rear towers as just as problematic – flaws in the original BMW design leads to the failure of the bushing as the inner metal sleeve and rubber bushing separate, causing speaks, rattles, clunks and deflection of the rear shock mount. Rogue Engineering offers a fantastic set of beefed up rear shock mounts that can actually cope with the stresses placed on your suspension by the less-than-perfect roads in the United States.

Rogue Engineering’s Rear Shock Mounts fit all E30 through E46 models, so these are a great upgrade if you have either of the two previous generations of 3-Series – and this includes M3 models.

To correct the weakness of the rear towers, Rogue Engineering developed their own Rear Shock Mounts that mount to the top of the rear shocks with 30%-stiffer-than-oem bushings instead of race-only spherical bushings sometimes used. At $98.95, these mounts are also a relative bargain, compared to the cost of repairing the rear tower once damage has already occurred.

Interested in ordering a set of OEM BMW Front Strut Tower Reinforcement Plates for 1999-2006 BMW E46? Click here to get yours..
Interested in ordering a set of ROGUE ENGINEERING Rear Shock Mounts? Click here to get yours.

Story by Nicholas Gregson

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