Need help with handling issue
 

I recently changed out my rear tires/wheels to get a better aspect ratio look. After the swap I noticed the car drove a bit differently but expected that as the new tires are 1" taller than the old ones. All was good until I met up with a blacked out Z car on the highway yesterday and did an off ramp run with him. When exiting the off ramp I quickly got well into the 3 digit mph, then felt the handling get pretty unsettled. Once out of the curve and on a staight line all was good. I cannot describe well the feeling other than the car did not feel tight and planted like it used to, but rather squirmy or soft.
What can cause this?
The tires are now 1" taller which throws off the front/rear bias.
The tires have a shorter sidewall which should tighten it up.
The tires are Nitto drag radials on rear, Fusion performance street on front. Chose the Nitto because the tread pattern looks very similar to the Fusion. The 15's were Pirelli P-Zeros.

Here is before and after:



http://forums.pelicanparts.com/uploa...1450033730.jpg

http://forums.pelicanparts.com/uploa...1450033795.jpg

same compound tires?

I would at the very least check the rear camber & toe. I wouldn't be surprised if the front changed also. Maybe it's just the angle you did the photo, but the back appears to have positive camber.

No suspension changes were made so camber didn't change (probably the picture).
Yes the compound on the Nitto rears are very soft as they are drag radials, could this be the problem?

Lower the rear and corner balance to see if that will fix it.

Drag radials are made for straight line performance. What you were probably feeling was the relatively soft side wall allowing the tread to shift side to side.

Sorry for the off topic but those wheels and tires look so mean! Are you running 15x9-11's?

I may do that. Really didn't want to have to lower the rear as it looks good as-is.

If that is true these tires must go! I hit the corners hard when running the mountains as I know my car well, I need good feedback from the tires.

First set of wheels/tires was 15x8 and 15x12 running 225/50 and 345/35 Pirellis.
Swapped out the 15x8/225s for 16x9 245/45-16. That combo worked very well but looked odd (to me).
Second set of wheels/tires are 17x12.5 running 315/35-17

Anyone else running Nitto's having the same issue?
Anyone else running 16's up front and 17's out back having an issue?

Mine did the same. All is good now that they are worn down a little. Back to go kart like.

Lowering it won't reduce the drag radial side wall flex and it's a lot of work if you have to re index the spring plates and then do a 4 wheel alignment.
Those tires aren't made for cornering.

Call the tire maker or place you bought them and see if they have any suggestions like maybe running higher pressure in them.
I think you need to get new summer tires made for handling and high speed cornering,

Brian, see if the car will wiggle while pushing side to side in the back. If NO stick with suspension changes. Up a little in the front and down and in the back.

T-bar suspension can be finicky if not right.

Holy crap man, your tires could not be more different front to rear! Definitely the cause

Are they brand new?
Mold release agent is slippery unit it wears off.

Was it could out?
Tires may heat up at different temperatures.

This will change the contact patch too, it's best to do some tire temps to see what the pest PSI to run is. Pyrometers are cheap these days and after a short spirited run, you can get some baseline temps.

If you need some input on what to do once you get baseline temps, let me know.

Doing over 100 with so so street tires and a soft sidewall drag tire ... case closed, right?

The diameter difference was a way that the teams in the old days got more tire on the ground, skirting the rules that limited width, instead going with larger diameter.

Drag tires are not for exit ramps. The side walls are to soft.
I never mix match tires.

Yes good idea to play with the pressures. When I first started running huge rear tires 20+ years ago I noticed that I needed to run similar pressures front and rear rather than the factory bias.

The tire diameter front/rear bias is the same now as it was when I ran the 15" wheels, I don't think that is the problem. I ran a 1" bias on the Red Rocket as well.

I went out this afternoon (70°F) and hit the mountains hard to really try to better feel what is going on. What I am seeing/feeling is the tires stick - they do not slide like the 345's did, and they just don't feel the same. After rounding a few of my favorite corners I started cranking up the throttle and the tires simply do not slip. I think this may be it, by not slipping and giving me that feedback they are working the suspension harder and I'm not used to that. The unsettled feeling I get could be because I don't know what is going to happen when it does slip. Also tires are brand new which may be at least part of the problem. I'll wait a bit and see if they get better. Got to say I don't like it that these tires don't slip, and I don't know what it would take to get them to do so. There is an abandoned air strip close by where we used to do auto-x, I think that is where I need to go next.

Rods(RSR) - I'm pretty sure I saw a picture of your gray beast with these Nitto tires out back. If so how are they doing for you?

What are we missing here? The two extremely different kinds of tires are definitely the cause!!

Drag radials are drag slicks with grooves to pass DOT. Even at 100PSI, the sidewalls will be squirmy.

Lucky you didn't lose control complately! They are squirmy with front engine cars!

Possibly, but not yet proven. The fronts stick, there is no understeer. I used to have oversteer and now that is gone. Others on this site run drag radials and I am hoping they chime in. I know tire types can be vastly different in characteristics which seems logical, however I have not had this problem in the past and have run mismatched tire types front/rear many times as the sizes I need are not available in all makes.

I don't think these drag radials are slicks with grooves, they are sticky street tires with a street tread pattern which can be seen in the picture. The pattern is nearly identical to the front tires which is why I chose them. I would think cheater slicks would have a much more simple tread design.

I have no issue getting rid of these tires if they are the problem but I sure don't want to buy something else and still have the same problem. I don't deem them unsafe, they just feel funny.

I have run drag radials, they are made to have soft sidewalls to absorb torque and flatten the contact patch. Hence, "wrinkle wall".

They were never intended for a corner, I assure
you.

Your car will never handle well with those NT555R's. A'int gonna happen :)

When I posted about using different front and rear tires - even though from the same manufacturer/brand/ rating - people posted that it was likely to cause a wipeout at anything over 5mph ;).

They are not wrinkle wall, that would be ridiculous on a road handling car. The wall height is 4.3" and the tire is stretched on a 12.5" rim. The 345/35-15's I had on previous to these had 4.75" wall height, more wall flex, and puckered on the 12" rim.

I'm going to guess from all the info given that the squirmy feel is due to the tires not letting go but having a thick soft tread that is moving side to side as I traverse the corners. The sidewall is not flexing, but rather the tread itself.

.

Does sound like tread block squirm. Shaving the tire wound reduce tread depth and decrease the tires slip angle. Or just put more miles on them until settled in.

With a very soft compound on tread blocks the car can feel like it is driving on blocks of butter ie squirmy. I have not had this experience in a car, but driving karts on wet tyres (very soft compound) on dry track has the feeling you describe. They don't let go - the compound has plenty of bite - but the chassis rolls around a lot. A less rigid feel than on a harder compound. Less feedback in terms of connection with the asphalt. Not my cup of tea. If you had same up front I think you would find it quite disconcerting.
Regards
Alan
Alan

That is EXACTLY what I am trying to describe!
It looks like everyone is correct in one way or another. These tires have upset the handling, how much of it is the compound and how much is the front/rear ride height bias is not determined.

I guess I need to look for a different type of tire. When I bought these I did not know they were drag radials - a friend of mine told me that when he saw them on the car (he uses the exact same tires on his 1000hp Corvette). His car is never driven hard in a corner so he had no info for me on the problem I was having, all he said was "you ain't gonna break THOSE loose".
SO, what tire do you guys recommend?

I think the Toyo R888 tires will work well for you. Make sure to mount them on the front AND rear!

If you want something less sticky, more affordable, more comfortable and still an all around high performance tire I also suggest Bridgestone Potenza RE760 Sport.

I have enjoyed;
Nitto NT-01's - on the 930
Toyo R1R's - on the 86, 3.2 .... I don't they offer sizes for your set up
Potenza RE760's - on the 89 Euro M5
I didn't much like the R888's which I tried on an 88 M6 then took off and gave to a buddy.
If it was me Brian, I would put on a set of NT-01's.
Good grip and predictable at the limit,
Dave

I have enjoyed;
Nitto NT-01's - on the 930
Toyo R1R's - on the 86, 3.2 .... I don't they offer sizes for your set up
Potenza RE760's - on the 89 Euro M5
I didn't much like the R888's which I tried on an 88 M6 then took off and gave to a buddy.
Other guys like the R888's I just couldn't get them to work for me.
If it was me Brian, I would put on a set of NT-01's.
Good grip and predictable at the limit,
Dave

1) Mismatched tire baloney?
Some tire combinations work well together others not so much. It certainly is one variable, but not the definitively cause without further investigation. I hate when people preach that tires of different makes or models will send you off into the weeds as if turing the key becomes an ejection button. A matched set of known quantity will eliminate this variable, but will it resolve the handling concerns or has it more to do with the new tire size?

There are certainly some combinations that are less than ideal, and especially at the limit, but others that work well. You have to become your own tire engineer and analyze it. Playing with tire pressures or having a second set to try and experiment are a cople of ways of diagnosing.

Having said that, while waiting for my new front tires to arrive, I drove 300 miles on street rears and R compound fronts. For liability reasons I am certianly not recommending it, but after many twisty roads, when I got to my destination I did a few vigorous parade laps (certainly nowhere near the limit but spirited none the less). That has to be the absolute worst combination on a 911, but I was fine at reasonable speeds.

2) Tire squirm
I'd run them for a while just to put some miles an heat into them.
The 4 yokahama S drives that I put on my wife's miata were really weird handling with not much traction until we had about 200 miles on them. Why?
-Heat cycles to finish curing the rubber?
-Mold releas compound to wear off?
-too tall tread blocks?
Who knows but they sorted themselves out after a few miles and became excellent tires.

3) Suspension?
If this is your first set of super wide rear tires on a 911 they will feel much different due to the offset and affect on geometry. They will also put more force on your suspension. On the race track with 4 matched tires, my car fells like on turn in there won't be much traction, but then they take a set and rail through the corner. There is always a lot more than I expect once the car sets on its suspension. I have 30 rear torsion bars, and my adjustable rear bar set to mid setting. My racing buddy says my car is not evil but feels like it could use a bit more rear. It really seems to want to squat on the outside rear corner with these big boots.

4) Tire diameter
Another guarantee to send you into the weeds (or not)?
Back in the day some of the race teams ran bigger rears to get more tread on the ground, skirting the tire rules that limited width. (Contact patch can be increased by diameter or width). It does introduce another variable of weight bias though.

These variables all likely have an effect but not one of them is definitely the cause. What else was changed at the same time? the tire brand/model/traction/ width/diameter/other?

https://stacks.stanford.edu/image/ii.../0/default.jpg

I can tell that from a lot of responses, folks have not run drag radials.

I don't care what pressure you run nor how long you break them in, they will not handle as well as a dedicated handling tire. The sidewalls and tread is designed to flex to absorb torque and widen the footprint under load.

Crikes.... :)

^^^ THIS ^^^

I've learned this lesson, as well. I'll never mix and match tires again.

Yes, but is that the only change?
I'd hate to see him buy more tires and still not be happy with the result.

P.S. Kenik
NEVER mix and match? Always a disaster?
...or just recognize that it is an untested combination and some tires may not work together as well as others?

http://forums.pelicanparts.com/uploa...1450108423.jpg

There's a 3rd element, the tire compound. Drag radials only need a light hazing to get sticky. They're not like road racing tires that take awhile to heat up. You spin them, they're sticky. You roast them, they turn into gum and become greasy.

Brian,
I've got 'mismatched' tyres on my track car. After lots of experimentation I have ended up with Nitto 01s (255) on the front and Kumho V700 (275) on the rear. The Kumho is a soft tyre with high tread wear factor. This is the best balance and handling the car has had. It has swapped the oversteer for a slight under steer at the limit (which is my preference). I don't know how the Nittos will go in the wet (They say for dry only).
Regards
Alan

Guys, the thing here is the F&R tires of the OP'er are RADICALLY different...

Not all that dissimilar to a snow tire on front and a Z rated street tire on the back. Just a bad combination, bad idea, etc ... sorry...

Agree and disagree. You can throw the complimentary front tires that Nitto recommends for the rears, and it will still be crappy in handling.

It's a friggin' drag radial! :)

Yeah I wouldn't run a Nitto streeter on a HP 930 either. And totally different F&R tire construction is just insane in corners.

But then again I'm a dip**** who runs Kumho V710 track tires on the street because they feel sooo good. NO RAIN, lol.