Strut Tower Suspension Trickery - 2014 BMW 328i xDrive Gran Turismo Long-Term Road Test
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2014 BMW 328i xDrive Gran Turismo Long-Term Road Test

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2014 BMW 328i xDrive Gran Turismo: Strut Tower Suspension Trickery

April 24, 2014

2014 BMW 328i xDrive Gran Turismo

I noticed something interesting when I was poking around under the hood of our 2014 BMW 328i xDrive Gran Turismo the other day. It wasn't the engine or any system associated with it, but it is something that's shared with another newly redesigned BMW product, the 2014 Mini Cooper.

What am I going on about? It has to do with the washers under the heads of the bolts that hold the front suspension struts to the body. They're super thick.

There's nothing new about a washer under the head of a bolt. They help spread the load out and they prevent galling when the bolt is wrenched tight.

But the extra thickness we see here has another purpose that isn't obvious to the untrained eye.

The sheet metal of the typical strut tower is pretty thin. This bolt doesn't need to be very long to punch through the other side and engage the threads.

But that's not ideal because of the way bolted joints work. For a given tightening torque, the clamping force of a bolted joint has to do with how much the bolt stretches (and they do stretch) as it's tightened down. But bolted joints also tend to relax slightly as the various layers in the metal sandwich seat and settle on a near-microscopic level. Some of this happens right away (which is why you should go through the star pattern of wheel nut torqueing two times) and some happens over the course of use, especially if there's vibration involved.

Here's the kicker: For a given amount of relaxation, shorter bolts lose a greater percentage of their overall clamping force than longer ones. Longer fasteners produce bolted joints that are more stable and stay tighter over time.

This thick washer and the longer bolt that goes with it increases the distance between the head of the bolt and the threads below by what appears to be a factor of two or perhaps three.

Why not just tighten them to a higher torque value to start with? You'd over-stress the bolt and possibly even snap the head off, that's why. And you'd still have the relaxation susceptibility of a short bolt.

Cool as this is, we're not looking at some new BMW innovation. I first saw something like them on Toyota products when I worked at their Arizona Proving Ground in the 1990s, and the idea is much older than that.

2004 Toyota Prius

Here's a picture of what Toyota calls "performance nuts" on our departed 2004 Prius long-term test car. These special nuts are as broad as a washer across the bottom and they lack internal threads until you get halfway up. This initial lack of threads is crucial because that's where the extra bolt-stretch length is gained. That's what allows this nut to do the same job as BMW's thick washer but without an extra part to keep track of in the assembly plant.

Here's a picture of what Toyota calls "performance nuts" on our departed 2004 Prius long-term test car. These special nuts are as broad as a washer across the bottom and they lack internal threads until you get halfway up. This initial lack of threads is crucial because that's where the extra bolt-stretch length is gained. That's what allows this nut to do the same job as BMW's thick washer but without an extra part to keep track of in the assembly plant.

2004 Toyota Prius

And these are the lower strut mounting nuts and bolts. It's hard to see the benefit here, though, because the bolts are quite long to begin with. I suppose it's because lower strut clamps of this sort are apt to scoot around in the course of use, which can lead to changes in wheel alignment. I guess they're looking for any improvement they can get.

So now you know what you might be dealing with if you're doing a DIY maintenance project and run across a thick washer or a weird-looking nut of this sort. And you'll know you should make sure to replace them as you found them because they have a specific reason for being.

Dan Edmunds, Director of Vehicle Testing @ 5,565 miles

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