are new tires causing my rear to act squirrely at speed?
 

a few weeks ago i got rid of my rear pilot super sports (225/40/18) and replaced with falken azenis rt615k+. I do plan to use a set of 4 falkens all around but i am considering going with staggered tires and wanted to be sure of fitment (tire guy said 235/40/18 would not fit but i am a little skeptical of that and may end up with spacers if needed to make it work). car is on stock dampers, but eibach pro springs lowered about an inch. camber is roughly neutral in the front and there is some slight camber from the drop in the back (my guess is about ~1) - i have not touched the camber.

my issue is that at speed (50-60mph+) on the interstate the back end has a feeling of being pretty "floaty" like the rear tires are worn or losing traction and lift-off oversteer is pretty noticeable going into higher speed banks - the kind of banks where the pilot super sports felt very planted. the newer tire setup is engaging traction control on the rear wheels pretty quickly (i can't tell for sure if i am losing setup or traction control is acting over aggressively, but the speeds are too high for me to test it safely with traction off). this effect is somewhat lessened when the tires are warm, but it does not go away completely. lower speed cornering and off-the-line traction seem to be as good or better than ever.

my guess is that this probably has something to do with mixing tire compounds (mpss are max perf summer and falkens are extreme perf summer tires) and perhaps the falkens have stiffer sidewalls than the michelins, leading to more nose dive and weight transfer leading to liftoff oversteer? just an uneducated guess. i'm thinking it was a mistake to mix compounds and may just sell off the super sports (they are still at ~50% tread) and go with falkens all around sooner than i anticipated. i just want to see if there is maybe something else I may be overlooking in the suspension that is causing this effect.

can someone more knowledgeable than me chime in?

Mismatched tires front and rear have caused similar issues for me.


Side wall stiffness
Michelin > Falken


Just because the UTQG number is the same or close to the same does not mean tires are compatible. I have tried combos that have worked and some that do not. It is best to have the same tire and size all around.

sure. the other issue i'm wondering is if the falkens are undersized. they are both 225/40/18 but if the falkens run smaller then I can see why perhaps the f/r balance of the car feels off. 235/40/18 might have the reverse effect with understeer, but i figure that would be a less pronounced problem on a rwd car.

It is ALL about the tire compound, not the width.


I have 215 front and 245 rear tires. But they are the same tire, and have no issues.

i'm not disagreeing with you, but why i've got the stickier, lower treadwear tires on the back. shouldn't there be MORE grip than the outgoing MPSS?

Now you're saying on the interstate and "banks" so it's a little confusing if you're actually pushing the tires or if this is leisurely interstate speed (or if there are two scenarios). I can say that in my experience, the Azenis have a lot of grip, but when they let go, it is not in any way gradual, it is a greasy feeling. The sidewalls of those tires are also insane, which may be contributing to the imbalance you're feeling; the fronts will be squishier, and the rears being stiffer is forcing more body roll since the tire carcass won't budge.

I've run the Azenis a lot in my life, and can say they're really a purpose built tire that are not good all around like the MPSS or other tires ( StarSpec Z1 back in the day, RE-71, etc) but their grip when heated is incredible. I would bet that your traction control is coming on because either the tires aren't heated, the tires haven't been through heat cycles yet, and/or they're engaging more suspension roll which is triggering the loss of traction on the road surface.

Well, my money is on an off alignment and uneven tire pressure.

I just skimmed read this, but try rotating rears to front and front to rears and see what that does.

it's alignment, you might think -.5 degree is nothing. but it changes how the car feels dramatically when the left and right aren't sitting the same.

First question, what are the tire pressures?
Second question, what are the tire and wheel sizes?
Third, has an alignment been performed since it was lowered?

1. to be honest i've been stupid and not checked the tire pressure on the new tires yet. the fronts are almost always sitting at recommended tire pressure. the falkens were recently installed by discount tire but who knows if they did a good job with inflating
2. both sets are 225/40/18. wheels are 18x8, +48 offset enkei raijins
3. rims and springs were done by the previous owner. i believe he did get the car aligned (like i said, the car behaved well before the new tires went on. the previous rears did not have in my opinion overly uneven treadwear)

I will get this checked, though i am leaning towards getting new tires first. doesn't make sense to me to get it aligned if i am planning on swapping in new tires in the near future and will need another alignment. better to just bit the bullet on new tires and alignment all at once. would this also be a good time to get bolts on the fronts and dial in some camber?

yeah, freeway interchanges and onramps. I have noticed very good grip at lower speeds on hot days/and after the tires have gotten warm from spirited driving. the car can take a lot more mid-apex throttle before the tires break traction - completely the opposite at higher speeds. i chose the azenis to be a dual duty DD/autox and track tire. the autox here in san diego is on really crappy pavement and i was worried about chunking with a more street oriented tire.

I'm going to be "that guy" and ask why you are considering staggering your wheels? Do you make significantly more power than stock? If not, I would advise against a staggered setup, and just stick with a square setup.

Staggered will make the car more prone to understeer, and shorten tire life due to the inability to rotate front-to-rear. If you are doing it because you want an aggressive stance, then just put the same size up front for an even MORE aggressive stance!

my plan was to upsize to 235/40/18 all around but i got trolled into thinking it would not fit. and so i got 225/40/18 to match the current fronts. My thinking at the time was to run this michelin/falken setup for a while and when it was ready for rotation move the 225 falkens to the front and do a slight stagger with 235/40 or 245/35 in the back. Then for the next rotation, add new rears to make it square again at 235 or 245. Within this time i will also probably be upgrading from springs to mid tier coils like the CSG tein flex A/Z. i know its not ideal but live and learn.

Yeah, first thing is check the pressures. I recommend getting a cheap mechanical pressure gauge and keeping it in the glove box to everyone, particularly if they are sensitive to handling properties of the car. My guess is the pressure is too high. Measure and set when cold, then drive around a bit, and measure when hot. The different tires will likely heat differently and you may have to adjust to balance the pressures. I personally like ~38-37psi F-R when the tires are warm on both my setups (17x7, square: 215 'winter' and 225 summer).