New rear tires installed - now have control problem

[I Grok NSX]

I have just replaced the two rear (worn out) Toyo T1-S 255-40/17's tires with Toyo T1-S 245-40/17's (stock size). I am now experiencing control and handling problems. The back end is just plain "Loose" is the best way that I can describe it. The back end seems to have a mind of it's own.

I had the tires checked and there seems to be nothing wrong with them. They are balanced (no problems there - very smooth), the lugs are torqued to 80 ft/lbs, the air pressure is 40 lbs (I have tried playing with different pressures but to no avail), the tires are mounted in the correct direction (tripled checked!). Can't think of any thing else to check.

Also, the front tires are a bit larger than stock - 225-45/16. I've searched and found that a ratio of more than 5% will affect handling....could this be the problem? From my calculations, the difference of the tires that were removed and the new tires, size wise, is not more than 5%.

Help!

 

[sjs]

New tires do take a little while to break in so that may be part of it.
Assuming that the fronts are also T1-S then I would expect some difference due simply to width (as opposed to the +- 5% diameter issue). You now have the fronts significantly closer in width to the rear than intended, which will make a significant change in balance towards oversteer. Without driving the car it is difficult to know what it's really doing, but together those factors could be what you feel.

 

[I Grok NSX]

Hi SJS -

Yes, the front tires are T1-S's. The rear tires now have about 1000 miles on them so they are well broken-in. (I drove to Las Vegas )

I'm going to replace the fronts with stock size Toyo's and see if that will do the trick. The car is so "Unstable" now, it is not fun to drive at all - really kind of scary. If the new tires don't help, I'm going to set the car up as it was (I still have the old tires - going to use them for Auto-X) to eliminate all other possibilities of mechanical failure/computer failure. And speaking of computers, I did reset the computer by pulling the fuse for a minute. Did not help - and running with the TCS off doesn't do a thing. Sigh....

 

[sjs]

Well it certainly sounds too extreme for the things I mentioned, but you seem to have taken the rights steps so far. Alignment issues could certainly cause the same symptoms but just changing tires wouldn't alter the alignment. May be a combination of f/r tire balance with prior alignment problems. For example, if you are running zero camber on the back to improve tire life the back grip will be reduced. Likewise zero rear toe or worse yet slight toe out in back which can be very spooky.

 

[I Grok NSX]

Alignment is a possibility....but.... the car was fine before the new tires. No other changes were made except the installation of the new stock size tires on the OEM wheels. I pulled the wheels off the car my self and installed them on my own, (the folks at Costco did the tire mounting and balancing) so, unless it's necessary to re-align the rear after a different size tire is installed, no one messed with the alignment.

But that is a good issue....and, if all my attempts to fix this problem are exhausted, I'll have an alignment done. I'll keep this thread updated after the new OEM size tires are installed.

 

[NSXDreamer2]

is that just me? but I don't think the 225/45/16 is stock at all, it's bigger and also wider, so you have gained more traction.. thus the rear is more twichier...

 

[I Grok NSX]

is that just me? but I don't think the 225/45/16 is stock at all

NSXDreamer2 -

You are correct - stock is 215/45/16, but what I was trying to convey is that the size I have on the car now is 225/45/16.

And I hope you are correct about because of this difference, the control problems exist. Calling a Toyo tire place tomorrow and hopefully they have the OEM sizes in stock.

And if the OEM size tires correct the problem, all the better because I do need a set of front tires for Auto-X .

 

[NSX-Racer]

I'm pretty sure the size is not the problem. I used the same size combo (with other tire brands) for a long time on street and track - no problem. I wanted to reduce understeer a bit with this combo and had the impression that it helped.

New tires should (at normal driving) never react in that evil way you described it only because they are new. I would look at another issue: Some tires (same size, same type) come in different versions for different cars - e. g. there where different Michelins MXX 3 for Porsches. Can't remember exactly but there is an additional sign like "N 0" or "N 1" behind the type name (you may find it in Luds FAQ in the tire section).

I once had "Porsche" tires at the rear (emergency replacement after one of the originals had gone flat) and experienced about the same as you - the NSX was nearly not driveable. Sidewall stiffness, carcasse construction, compound - it was a completely different tire - just designed for the needs of a Porsche 911.

 

[I Grok NSX]

Just found out that Toyo does not make the stock size fronts for the NSX .... so, that leaves me with either buying the same size Toyo's that I took off (255/40/17) and try to sell the Toyo's that I have on the NSX now at Ebay (or someone here on the forum), OR.....switch to Michelins Sport Pilots which are available in the sizes I'm looking for.

 

[sjs]

Although I agree that from the description the change was too sever to be just the width and/or newness of the tires, those things do make a noticeable difference and I was suggesting that a combination of them can be quite significant. Combine that with what may already have been marginal alignment settings and things can get pretty scary. In fact just the 225 front 245 rear makes quite a difference, improving balance IMO but not what most people are comfortable with.

It is certainly true that Porsche specific tires are bad news on most other cars, but I don't believe that's the case with these.

If the problem isn't related to the last thing you changed (it usually is) then you need to check the alignment and suspension in general.

 

[JAYS NSX]

Well, I guess that this clarifies what women have been claiming and men have been denying for ages; size DOES matter!!!

 

[NSX-Racer]

jmjrdh1: At least two people in the forum (one of them is me) could tell you that smaller sizes sometimes work better - e. g. on the track. Maybe you should make a poll (just for yourself with all girls and women in the neighbourhood). You would be surprised. Oh well: Alignment counts too...

 

[NSXDreamer2]

I just think of one thing, could it be some suspension parts got lossen?? ie.. the toe link, sway bar linkage...etc??

I'm just brain storming of what's happening with your car.
oh, and if the tires are as new as less than couple hundred miles, the handling could be a little nervous... but as soon as the "mold coating" or whatever you called it wears down, it will be ok.

 

[BB]

I would suspect your problem is simply due to the new tires and that overtime things will get better.

I had the exact same experience when I replaced only the rears on my car a few years back. When new, the Dunlop SP9000's I was using have quite deep tread for a performance tire. When I first drove my car with the new tires thru some twisties I was shocked at how awful the handling felt-- vauge and loose like the rear end would break free at any moment, though it never did. I was very unhappy. After several thousand miles things got much better and they began to feel more like the old tires.

I don't have any experience with the Toyo's you mention, but I would suspect that any tire that gets good milage on the NSX (such as the Dunlops) will tend to have deep tread that feels squishy when new-- not the best for giving a good solid "feel".

 

[NSX-Racer]

Sorry, but I wouldn't use a tire on the NSX that feels good "after several thousand miles". Way too much downside even for a long lasting rubber.

 

[JAYS NSX]

Way too much downside even for a long lasting rubber.

Hey NSX Racer!

First you want me to poll the women in my neighborhood, and now a comment on your DISLIKE of long lasting rubber?!? Are we talking about cars here or what??

 

[I Grok NSX]

Problem solved. Ended up putting stock size (Michelin Sport Pilots) on the front and now ....no more problem. Amazing! I measured the stock and the old tires that were removed and the size difference is not much, but apparently enough to make a difference to a sensitive suspension.

I do still have TOYO's on the back but the combination seems to work. I don't like to run different brands of tires on any of my cars and will change the TOYO's out when they are worn and since I don't really push the X to it's limits, I'm hoping it won't be a problem.

Thanks to all your responses and suggestions.