Vitamin-SC

Car: 1999 Sc400
Let me start by saying before I installed the wheels I was on stock 16 "rims and tires or all oem stock for 99 sc400 .Ok got a set of stock IS350
Staggered wheels 18 x 8 & 8.5 since i'm not lowering the car.I purchased 2 new 225 45 18 for the front ,the rear came with a set ,255 40 18 used but had great tread so I went with it .
My old setup "16" the car ran true up to about at most speeds 70 and above+
Since installing the wheels the Rear of the car is Very unstable in the rear at speed 50+, higher the speed the more unpredictable it becomes ,to a point Its unsafe .I can only imagine how it would be on a Rainy Day on the Interstate highway
best way to describe it would be ,under inflated tires ,blown shock /strut . Its drifting side to side like the rear has steering
so far I've only done visual inspection at bushings and bounce test shocks ,looked for anything broke,or missing bushings "nothing obvious"
I dont really want to just throw random parts at it
The car has been well maintained Has 240k miles on it I've only had it my possession 9 months

logix7

did you have an alignment done since changing the rims? it couldnt hurt. and how much tread is actually left on the rears?

scsexy

iTrader: (1)

put your stock rims and tires back on and see if it goes away would be the most obvious thing to do in my eyes. It would eliminate the wheels as the concern. If it does go away you know the new rims are bent or tires are bad.

Vitamin-SC

No alignment yet but rears have about
6-7/32
​​

Vitamin-SC

Just check wheels of or play in bearing passenger fine but drivers side has just a smidgen of play gonna address that then get alignment after that check tire I notice a slight dip in the center of both tires

PsykotiK

if you are running two different tires front and rear that are different speed ratings then its gonna feel loose

Zerodrag

iTrader: (5)

Semi-easy way to troubleshoot would be to just swap the rear wheels back to stock and see if the issue goes away. If it does, then it would do with the new set of rear wheels/tires.

From what I recall, an alignment normally isn't necessary when only swapping the wheels/tires.

mrmj2u

iTrader: (2)

Vitamin-SC

had rear balanced before putting them on
and as far a tires the front are all season and the rear are summer tires that are directional
Ohtsu fp7000 front new rear ohtsu fp8000 rear {used }were on rear rims when purchased

shajbot

iTrader: (3)

If it's determined that the new rims are causing the instability, you should get them some new(er) tires and better brand. Get them balanced again and get an alignment afterward.

SpectraBlu

iTrader: (1)

I don't know how cold it is where you're at but, summer tires don't do so well under 45 degrees. I have summer tires on my GS and it feels like a completely different car when the temperature drops.

Duck05

Check the hub size, too......

RudysSC

Same thing happened on my V8 car back in the day when it got sticky tires thrown on. I highly recommend a good 4-wheel alignment. Also, be sure to check your rear wheel bearings and all bushings in the rear before having the alignment completed. Tires are simply exacerbating a more predominant issue from the sounds of things.

Vitamin-SC

is 350 wheels there hub centric with lexus lugs a well

Brucelee28

Doesn't hurt to get the wheels rebalanced sometimes. I had a wheel balance once then had to get it rebalanced again. Sometimes the first guy doesn't do it right or missed a spot.