I just put a new set of 255/40-17 Toyo R888R on my SVO, replacing a worn out set of the same tire in 245/40-17. Same wheels, 17x9's. The 255 is the widest tire I have ever put on the car. The problem I am having is that it is crazy twitchy when going straight, and seems to have quite a bit of bumpsteer which it did not exhibit with the older tires. The alignment is for racing, and it has about 3.5 degrees of negative camber. I do not know what the caster and toe settings are, but none of that has changed with the tire change. I know that the caster was "as much as you can get" and the C/C plates are as far rearward as they can go. The bumpsteer was dialed in a while back when I last changed the ride height.
I'm at a loss to understand why the car has developed this twitchy manner with just the tire change. It is almost scary driving on the street, you really have to pay attention to how it tracks on the road. I am going to a track day tomorrow at Barber and I'd like to see if the hive has any suggestions of what to look for. I have two other sets of tires that I will take with me, both of which have a few heat cycles and have known handling characteristics. Both are 225/50-16's, one set of Hoosier R1's and a set of Toyo R1R that I use in wet weather. I can put one of those sets on if needed, but I would really like to figure out the issue with the R888Rs.
On an old FOX body or SN95, bad inner tie rods can make the car feel like the steering wheel is on the head of a pin.
This is usually a side effect of wide, sticky rubber. The road is not flat and often has troughs were everyone drives. When the inner or outer edge of the tire gets more pressure than the other, the additional leverage that the wider tire provides can make it feel twitchy.
You may also need a touch more toe. When you toe-in you're basically compensating for the fact that the tires toe-out a bit from the drag on the surface deforming the tire. More tire sometimes needs more toe.
Or wasted tie rod ends.
On miatas the alignment alone will cause wandering and twitching. Just a side effect of doing performance stuffs.
Tom1200
SuperDork
9/11/21 3:47 p.m.
Sounds like 255 is a bridge to far. It could be a combination of the 10" tire on a 9" rim and the extra 10mm of tire is enough to exacerbate an existing condition that is not noticeable with the 245s.
The G35 is like that on new sticky rubber. It is also on an autocross alignment. Drive it a couple of 100 miles and it will probably go away as the tires wear in.
The others have pretty much covered it but you might try going up on the front tire pressure a bit.
Thanks for the recommendations. I happened to read the GRM article on suspension changes and the binding sway bar caught my attention. I pulled the links to see if my bar (which is a huge Steeda bar) was moving freely, and it was pretty much bound up. I reworked it and lubed it, but, alas, no improvement in my immediate problem.
The way the front end is aligned I can actually see that when the car is static in my shop that with all that camber there is only about half of the tire (the inner half) that is contacting the ground. So the car is tracking on the inner half of the tread unless it gets loaded by the road. I'm thinking that maybe LESS pressure in the front might help. It is 32 PSI now, which is my usual starting point for the R888Rs. I also think that some toe-in would help. I wish I knew what it was set to.....
I am hoping that after the first session tomorrow it might improve as the tires take a set and wear a bit like Toyman01 said, we shall see. Barber is a lot of turns, so, except for the front straight I probably won't notice it too much. From what I can tell from the street, turn-in seems sharp, not sure what will happen after that point. Easy to go to my backup tires if it is too hairy.
Your tires are not too big. That amount of camber is going to make for some interesting events with imperfect pavement, however, especially on a street. Try 26 lbs cold . . .
So at the track I lowered the front pressure to 28 to start and gave it a try. Pretty twitchy at first but as I built heat and speed it came around pretty nicely and the tires performed well. Still more touchy on the straights than the smaller tires but manageable. Makes me slow down my inputs which is probably a good thing anyway. Measured temps in the 170's to 180's across the left front with a probe after one session, which is pretty hot for me, so I was working them pretty good. All in all an OK day, until my turbo decided to take a dump which caused me to devolve into an old Fox body Mustang with a Pinto engine, but with less compression... i.e. pretty slow.
