I have worked with tyres (Firestone) in my early years and I have always been interested in them and how they interact with the vehicle. You may have read an earlier post I did where I described the tracking gauge I made myself.
You may know (so apologies if I'm telling you how to suck eggs) the appearance of the tread itself can tell you quite a lot about what's making a tyre wear - as long as that tyre has not been moved around on the vehicle of course. So lets consider we are sitting on the ground, in front of the OSF (driver's side front) wheel looking back towards the car. The first thing to consider is the overall wear pattern. Is it even across the width of the tyre or more worn in some areas than others?
Again apologies if I'm being too simple here but heavy wear in the centre but not on the shoulders is usually down to over inflation and the reverse, heavy wear on both shoulders but not the centre will be under inflation. This of course is easily rectified and eliminated in future by a weekly tyre pressure check.
Now, again sitting in front of the wheel and looking at the tread, perhaps left (outer) shoulder or right (inner) shoulder of the tyre is worn. This might be due to the suspension and wheel hub (perhaps because of wear or damage to suspension) failing to present the face of the tread squarely to the road surface. It might also be due to the tracking being incorrectly set or wear in the steering mechanism.
You tell us that it's the inner edges of the tread on both front tyres on your car which are wearing prematurely. So now have a very careful look at the tread pattern on these tyres. Lets consider the Off Side (drivers side) first. Ok, we can see wear of the inside shoulder but now look carefully at the treads over the width of the tyre. There will be, on most designs, bands of tread separated by water dispersing grooves which run right round the tyre. Look carefully at each of these bands, especially the edges of each band (some call them tread blocks). The inner and outer edges of each of these bands of tread should look the same (exhibit the same wear pattern) if your tracking is set such that it is allowing the two wheels/tyres to roll down the road parallel to one another. If you have this situation but with excessive wear on the shoulder then it's because the tyre is not being presented to the road surface squarely and it's likely the Camber angle on that wheel is incorrect. (top of tyre/wheel leaning in too much) If both front tyres are showing the same wear pattern (you seem to be saying both of your fronts are worn on the inside) then this is unlikely to be the reason as it's unlikely you will have an identical camber problem on both sides.
Given your wear pattern, it's more likely that your examination of the tread bands will show the inner edge of each tread block to be rounded slightly and the outer edge to be lipped. This wear pattern will form when the tyre is spending some, or all, of its time NOT rolling parallel to it's pal on the other side. Bear with me on this please. I know you believe your tracking to be correct. The wear pattern described will come about if both front wheels are toed out - ie splayed out (by even a small amount) The most likely reason for this is that the adjustment, made by adjusting the track rods where they screw into the track rod ends, is incorrect. But you know yours is right don't you, because it's been checked. By the way the opposite will be seen if there's too much toe in - ie the front wheels are "snow plowing" (wear on outside shoulders and outsides of each tread block rounded with inside edges lipped). It's not unusual to find when there's too much toe out that the inner shoulder of the O/S wheel is more worn than the inner shoulder of the N/S and where too much toe in is the problem the outside of the N/S tyre will be more worn than the outer shoulder of the O/S. This is simply due to road camber. - Think about it for a minute but just ask if you can't figure it out.
So now, perhaps, you're sitting thinking well, I do see some signs of this "lipping" effect but I know my tracking is correct, so what's going on? We have to face up to the fact that if the treads are lipping then, at least for some of the time, the two wheels/tyres are not rolling parallel! Having done more wheel alignments than I've had hot dinners my first port of call would be "show me the read out" Modern aligning rigs can produce a before and after read out which you should always request. There are many things which can interfere with the accuracy of a wheel alignment but the machine (if in good order and calibrated) can't lie. Also the technician doing the job MUST first do a thorough and rigorous check of all suspension ball joints, rubber bushings, steering swivels (macpherson strut top mounts being one such) and look for accident damage. If we assume the machine thinks it's correct then it's likely that the wheels are conforming when the vehicle is standing still but, and I've found this to be not uncommon, The suspension arms on most cars today use rubber bushings where they pivot on the chassis/subframe. These can look to be in good condition visually but in use, because of age and use, they can become "squishy" and overly flexible - There is a thriving business in "performance" poly bushes which are generally stiffer for the performance minded. If this is the problem then when just rolling along the track will be more or less OK but under braking, especially anything more than gentle braking, The drag of the retarding wheel will drag the front of the arm outwards, the bushes compress excessively and allow the wheels to toe out so giving the effect you are seeing. It's quite difficult to diagnose this one with certainty but it can be remarkable how different an otherwise OK handling vehicle can feel after renewing its suspension bushes!
A chap I used to know had a Gunson Trakrite. - I've just googled it and its still being made, now identified as a Gunson G4008 Trakrite - It consists basically of two plates with ball bearings between them (think they're ball bearings?) which you drive one wheel of the car over. The other wheel is still rolling along the ground so if both wheels aren't "tracking" parallel then the top plate slides sideways and registers it on a pointer type gauge. I've used it a number of times and found it very good because, regardless of what the vehicle manufacturer quotes as a toe in figure, the gauge will pick up if the wheels aren't rolling parallel! (I want one, It's on the wish list) I know some performance extremists intentionally introduce toe to alter handling but we're talking standard vehicles here so all i'm wanting is for the tyres to wear as slowly as possible!
I'm beginning to see spots in front of my eyes now! so maybe it's time to stop. We haven't even thought about Caster, King Pin Inclination, Toe Out on Turns and a whole lot more but this has been a start.
