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Hi, I have a BMW M3 and previously never really noticed much (if any) oversteer under braking.

The car went for 4 new tyres (upgrade to Pilot Super Sports) and they also did a full 4 wheel alignment. A proper computer setup, not old fashioned lasers etc.

They made some minor adjustments front and rear.

Now from nowhere, when I go into a corner under braking I get oversteer. Its very progressive, but I never noticed this before!

The back end mid corner also FEELS different, like its loading up more, but it does not oversteer any more, just feels different.

Do you think they have messed up my alignment some how? Or could change in tyres (to ones with more grip) have done this?

Thanks

Kind of sounds to me like brake balance, obviously the master cylinder setup is all hard-set. Are you rear brakes all working nicely? no sticking pistons etc?

What tyres did you have on before? Is the measure identical between the old and new?

Did they give you a printout of the alignment? Normally they should give you the initial alignment and the final alignment.

Sounds like a rear toe issue to me.

Well if you've changed the tires you have retuned the balance of the vehicle. Since it is no longer on the tires BMW developed it for, it won't work the way they expected. You could try mucking about with tire pressures.

It looks like they have reduced the rear toe to its bottom setting. Presumably in the interest of saving tyre wear.

So a reduction in rear toe would result in a more sensitive back end?

Do you have toe-in or toe-out in the rear? I'm guessing you should probably take it back to the alignment shop and have them check the rear. It might have slipped a little once you drove it around.

Stupid question but, did it have four equal wheels before?

It wouldn´t be first car to use different wheels on each axle

sure a reduction of toe in at the rear will reduce rear end stability .Dynamic toe Change under load (!)is very often substantial so if your new tyres have more grip you will inevitably create more force in the Suspension and this leads to more deflection -alas the car may with increased tyre grip develop a new characteristic in Terms of understeer or oversteer.

One little hint here :are you really talking about oversteer ? or is it the tyre building up force as it is going into slipangle? The felt going away or give you feel may be a tyre characteristic ...and if the grip /slipangle relation is flattening out very smoothly it may be difficult to read the tyres ideal slipangle which you may misinterpret as "oversteer" To really go quick the car HAS to slide ,ideally it will do so according to corner Speed (more oversteer in slowcorners and a move towards understeer in the quick stuff).
From the facts you gave it is not clear if you like or not like the new characteristic of your machine .The idea of saving tyres with rear toe adjustments seems a bit far fetched to me -did you wear the outer edges of your rears before ? Very unusual for a Bimmer .And there are an awful lot of M3s out there -my personal experience ends with E46 .. and i have to admit I have a story up my sleeve with that one: I was immersed into a Amateur Team competing at the Nordschleife a decade ago ,developing ohlins dampers for the green hell .One Weekend the car was prepared by some unknown guys and i simply did not have the time and order to recheck all nuts and bolts after a rear finaldrive housing Change.
The race started and the car was unexplicably going quicker and quicker over the first stint even though our Driver was complaining about a strange feeling coming from the rear.I have to add the car had rose Joints fitted to the complete Suspension over winter and a Motorsport Body with weld in Cage.From then on it was a rather hard Task to get the tyres working .The guys stayed in their comfort Zone and did not push bejond .
So with the Information of something mushy in the rear plus good laptimes I went out watch the car on track before calling him in ...What i saw was most alarming :On turn in it was possible to see the rear tyre going from toe in to toe out and back -that Suspension was about to fall off ...I called him in instantly we fixed the bolts and the mysteriously found Speed was also gone ..( we sorted this only later with damper Pistons and valving adjustments).
As your midcorner is not affected (that should be steady state ) I´m sure it has to do with increase loads through higher grip Levels and a bit of wipereffect coming from the tyre : If you are not preloading the tyre and your sidewalls are soft (I don´t know These tyres you have,sorry) it is well possible you experience something like "camber" thrust but of course coming from the Change of toe - as you apply the Brakes the question is do the tyres pull in or out ? Remember you had the rears under engine load the Moment before you applied the Brakes -I ´d think that is a toe out Situation ..

Hard to start without knowing the series...E30, E36, E46, E90????
One general thing on all 3series is, that you will mess up the toe if you do not load the car for the alignment. The values BMW gives are all with 75kg on the backseat and this changes a lot. If you align the empty car you will automatically end up at the toe limit.

as always everyone tries to sell his own experience knowledge and ideas and the poor devil hs to collect and filter the milk and get rid of the nonsense.

Let´s sum it up :

BMW M3 - depending on Model this might be semitrailing rearswingarms or multilink(?) These daysoper rear wheel drive

You changed tyres to a new brand and presumably a new grip Level this is not a economy choice.
You had the car aligned by someone who had Access to a Commercial alignment rig -A String Setup still to this day is not worse than anything you see out there in the field -it may be a bit quicker in handling but more precise ? If you don´t know the repeatability of the numbers i have my doubts .
The guys changed your rear Suspension Settings to reduce tyre wear -(we do not know if you had tyre wear Problems indicating wrong toe Settings ) We have no idea if the expert changing the toe Settings adhered to BMWs proceedures to condition the car for alignment .
You Claim the Problem (?) is just under braking ,staedy state (midcorner)Balance feels unaffected.
Your description of turn in oversteer is too vague to me .you give the Impression of having fear the car is on the verge of snapping out the rear end but it does not do this ...instead it does stick .


From there one can work through the possible reasons and can quickly erase the ones which cannot account for the new behaviour...it´s work .

Everything considered it may be a good idea to revert to the old Settings to get that uncertainty out of the equation before asking yourself 3 years what else could be the reason.