Why do so many cars have uneven front tyre wear?

I have been to look at a rather nice Mercedes Benz which was offered for sale today and I subsequently pronounced it to be in very nice condition too. One of the comments I had made about the vehicle was that the front tyres are wearing unevenly – something I see on the vast majority of vehicles which I inspect for purchase.  The key to looking at front tyre wear in these circumstances is deciding whether the cause is maladjustment (which is of course easily corrected) or distortion in the body shell / chassis (which nearly always means that you should run away. Quickly.)  There is rather more to this judgement than can easily be explained in a blog article but here are the rough and ready guidelines:

If a tyre is worn more on one edge and that wear is completely smooth to the look and to the touch, then there is probably a fault with the camber angle.
If a tyre is worn more on one edge and that wear is rough to the look and to the touch then the there is probably a fault with the wheel alignment (and possibly with the camber angle as well)
The critical thing about wear which indicates an alignment problem is in deciding whether it has been caused by maladjustment (ie: the “tracking is out”) or distortion (ie: the body shell or suspension components have been distorted in an accident) Generally speaking, if only one tyre is showing evidence of misalignment, or if both tyres are showing different evidence of misalignment then it’s more than likely a distortion related problem and will often be impractical to repair economically.  If both tyres are showing evidence of misalignment and look mostly like mirror images of each other then it is most likely an adjustment problem and is easily sorted by the right person…. if you can find that person…

Please remember: This is a generalisation and there is a bit more than this in practise when it comes to making the correct judgement!

But why do the vast majority of used cars seem to have so many tyre wear problems? Well a lot of them have been involved in accidents, and that is why they are now for sale.  As to the remainder, let’s take a look at how the front wheels have to align:

Wheel Alignment

The picture on the left shows how the front wheels can align with the steering in the straight ahead position.  Nearly all modern cars are adjusted to have the front wheels with “toe in” so that they are ever so slightly steering towards each other.  It is also usual for modern cars to have “negative camber” where the wheels are inclined inwards at the top. Cars built in the 1960s and earlier often had “positive camber”.  All these angles are so slight in practise that it is hard to tell them with the naked eye of course.

Ackermann Geometry

 

When the vehicle starts to turn a corner however, different things need to happen:  The wheel on the inside of the turn has to describe a tighter arc than the wheel on the outside of the turn as can be seen in the image to right.  This was first described by Rudolph Ackermann in 1818 and the theory bears his name to this day.

All cars for road use have their steering linkages arranged in order to provide for this geometry.

Steering in Action

 

This animation of a vehicle steering system probably shows how it works infinitely better than I can describe things! (With the sole exception that the wheels are turning backwards!!)

 

 

Tie Rods

 

Here lies the key to where it all goes wrong: The point at which a garage or tyre specialist adjusts the front wheel alignment or in popular terminology, “does the tracking”.  The adjustment occurs at the tie rods which are coloured red on the right hand picture.  The crucial thing is that the tie rods are exactly the same length as each other or the geometry shown in the animation above won’t work.  (The “Center Link” labelled on this picture directly corresponds to the red bit of the animated picture.)

The trouble with vehicle alignment gauges is that nearly all of them measure with the steering in the straight ahead position. This means that if a technician is lazy or lacks knowledge he can adjust either one of the steering tie rods in order to obtain a correct reading on the alignment gauge.  However, if as a result the tie rods are now not exactly the same length then the Ackermann geometry will have been destroyed and the vehicle will wear tyres unevenly.  Not only this but the steering wheel will no longer be in the straight ahead position on a straight road – as was the case with today’s Mercedes.  Some modern equipment holds the steering wheel in the centre position in order to ensure that the technician adjusts things correctly, which all works well unless someone has previously re-fitted the steering wheel to a maladjusted system……

The solution when this happens is to remove the steering rack bellows and to set the distance between the inner and outer ball joints so that they are exactly the same for each side of the car.  And I do mean exactly. I use a large vernier caliper in preference to a ruler for this.  Having equalised the length of the tie rods the wheel alignment is then set (often in the case of Mercedes Benz a spreader bar needs to be used first, which pre loads the steering) and any adjustments are made identically on each tie rod. Once the Ackermann geometry has been restored and the wheel alignment set correctly the steering wheel can be re-fitted in the correct position if it has been previously disturbed.

Lastly – a quick footnote:  I am used to working with steering and suspension but I am not at all used to producing nice drawings and animations, so I have taken the pragmatic solution and um… “borrowed them” to illustrate my point.  If I have used one which is copyrighted then please feel free to contact me via the blog!

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