In my youth, I thought – what could be easier than the repair of the undercarriage repair. Electronics – that’s something else! Unfortunately, the repair of the undercarriage is too complicated for many mechanics. Or – their qualification is not appropriate. 

This time – an entry regarding the large bearing of the front axle (big bearing or something like that). 

Here is the image of the front axle of E60:

And here – the image of the F32 LCI front axle:

In both cases, this spare part is called hydrobearing. 

What’s so special about this spare part?

As it is already clear from its name, there is some kind of liquid in this spare part. But I remember my discussion with one of the most experienced mechanics in our region, who strictly asserted: “It’s the urban legend. There is no oil in it”. Accordingly, there are no problems with using cheaper analogs and non-BMW products. I did not manage to convince him of the opposite. How is it, actually? And for what is this “hydro” necessary?

When replacing the tires of my F32 in the authorized service, the service specialist informed me that there were signs of liquid leakage from these hydro bearings. No, no cracks are (yet) visible. Obviously, some microcracks have appeared. Without thinking long, I purchased new BMW spare parts and made a replacement. Even more – I had a reason to perform this repair immediately.

A little while ago, I noticed that when braking slightly at a speed of around 80 .. 100 km/h, the steering wheel vibrates slightly. At the same time – the problem disappears when pressing the brake pedal stronger. Or braking from a higher or lower speed. The car recently had a BMW M4 series brake system installed, and all undercarriage parts – without any inadmissible movements; the wheel bearings without inadmissible movements (above the acceptable) too. The tires are new and balanced. What happened?

Anyway, there is a problem, and it is quite serious. We have to understand that the power steering works in the “opposite direction,” too. If the steering wheel is vibrating only slightly, then “before” the power steering, this vibration is much more powerful! Such powerful vibration will unavoidably and quickly destroy all wheeling parts!

This time, to explain the problem, I will mention two examples – I believe everyone has seen them. And first of them – even “enjoyed”. 

A first example:

I believe everyone has used a shopping cart with a damaged wheel. The wheel is braking – it creates increased resistance, but at the same time – it vibrates!

A second example:

Why is there a motorbike in the image? I assume that everyone has seen the video on the internet when the motorbike driver is braking slightly, and at the same time, the steering wheel starts strongly shivering from one side to another. In the best case – either the speed of the motorbike is reduced, and the vibration disappears by itself, or – the driver knows what to do (release the brake and add the acceleration). If not – such resonance ends with the fall. To solve this resonance, modern motorbikes need electronic auxiliary systems and specific steering shock absorbers (don’t confuse them with the shock absorbers of the front axle). 

Regardless it’s a shopping cart, motorbike or car, when braking, there appear forces which try to oscillate this hub “wheel/undercarriage”. It is physics. You can ask the physics teacher (or someone else who has learned in depth this science) to draw the force vectors. I will not burden you with the force vectors (but a long time ago, I learned physics in depth, and after slightly refreshing my knowledge – yes, I could draw it).

But this time, how is the hydro bearing solving this problem? 

Our first thought could be – if there is no unacceptable free movement in the undercarriage, such resonance can not appear. True. BUT. There is one “but” – to make driving maximally comfortable, the manufacturers (and BMW is no exception) introduced rubber bearings in the suspension. The task of these products is to reduce the impact of uneven road coverage. So – for us to not feel the potholes and pebbles on the road. At the same time – these bearings create the option for the resonance to appear. Yes, we can try to “optimize” these bearings – from one side, they are made soft enough to absorb the impact of the pebbles; from another – to not allow this resonance. Soft bearings absorb the pebbles but allow for the resonance to appear. Hard bearings don’t allow the resonance to appear but – don’t absorb the hits of the pebbles. 

What is the solution? Hydro bearing!

Hydro bearing combines a perfectly soft bearing if it (the bearing) is deformed relatively slowly (the car drives on the pebble or hits the pothole), but at the same time – it is hard if the forces applied are swiftly changing (the resonance is created). How do we implement this? It is enough to create two separated cameras, between which the liquid, which is filled in the hydro bearing, should move. Or the liquid of the appropriate viscosity should be chosen, which will absorb the energy of a swift vibration and will turn it into heat. Exactly so simple!

The result of the repair.

For the experiment to be correct, I did not perform any other repairs (actually – there was nothing else to be repaired), only replaced the hydro bearing. The result? The resolution has disappeared radically! Not the slightest sign that such would tend to appear. 

My conclusions?

This is another time when I can say – BMW spare parts are more expensive (sometimes – only slightly), not only because “BMW wants to earn a lot of money”. No, these spare parts are different from the analogs. They are different even from OEM products. Is it worth “saving” some EUR 15.. 25 on one hydro bearing to install some bearing of the “known” manufacturer without “hydro”? No, in my opinion, such an option is not even worth considering.