I changed the rear tires on my Z51 with non-runflats. I know I am going to get super flamed for this but I changed the tires with the Toyo proxies TR-1. The problem is now my car is TOTALLY unstable on the highway, the rear end decides what it wants to do at random intervals. In one instance while accelerating onto an onramp I nearly lost control when the tires hit a little variation in road surface. My question is... is this caused by the runflats up front and non-EMT in the rear (different sidewall flex up front and back)? Could this be caused by suspension setup that was intended for a stiffer sidewall? Or Faulty tire? Or Plan crappy tire overall? I have to do something because the car is really not safe to drive.

 

Are the new tires the exact correct size for the car? Was any alignment work done at all at the time of tire swap?

 

I don't have any direct experience with this but my guess would be that it's because of the mismatch. Kind of like running radials and bias-ply together. This is a big no-no. Just an educated guess at this point. Try the fronts and see what happens.

 

You need to have identical tires, brand and AGE in all four corners.

Shouldn't replace only two tires, need to do all four at a time.

EMTs are not neccessary nor are the OEM sizes.

and alignment is highly recomended, especially if the ware patter looks funny.

 

No flames from me. But please let us know the outcome. I would not expect it to be that squirly except in spirited driving (meaning cornering) from mis-matched front/rears. The problem is that you can't really know unless you actually change the fronts to matching tires.

Good luck.

 

Sounds like the runcraps in the front are your probleem!!


I am running all michilen ps2s with no problems !

 

I don't disagree with you in an ideal sense, but in real life I've had different tires, but still in matched pairs, front and rear, and never had any sort of problem.
Now, they were always all radials, or all runflats. Just different brands and/or ages of tires front and rear.
I'm thinking that he's either experiencing bump steering now, or it's the mismatch of sidewall stiffness that's throwing him off.

 

A stiffer sidewall would equal a higher spring rate in front = more understeer. Softer side wall in back would equal a lower spring rate = more oversteer.

So less air pressure in front and more in the back may balance things out. Sounds like you could have some alignment issues though.

http://www.rogerkrausracing.com/overundr.html

 

When we did the same type thing with the 1998 Indy Pace Car it was almost impossible to drive. Had to go to all four tires being the same type and style tire.

Change the front tires before you lose the car and cannot control it.

 

How about an alignment. If rear toe is off, even a little, the car will be all over the place.

 

I would start simple and go from there.

First thing to check is the tire pressures. I just changed to the BS 050a tires on my car and when I drove the car after the install it was all over the place with no traction front or rear. I checked the pressures and fronts were 32 and the rears were 30. I dropped the rears to 27 and the fronts to 28. As the tires heat they gain about 2 lbs of pressure. So instead of running around on tires at 34 /32 I am running aorund with 30 / 29 heated. This made all the difference in the world in the way the tires performed.

The one thing I had to adjust to was the spring effect I gained from non-runflats. With a softer sidewall the car gets more suspension from the tires. Not bad just different.

I have never mixed non-run-flats and run flats so I can not oomment on the effecct that may have had.

 

This problem is caused by the mismatch of the tires. They silmply have different handling results. See the post by robvuk, he is right.

 

Could very well be. EMT have very stiff sidewalls, vs normal tires have forgiving sidwalls.

 

Let me amplify this:: On a high performance sports car, you have exactly 2 choices when it comes to needing new rubber.

1) you can put on pairs of the exact same tire you are replacing on this axle
2) you can replace all 4 tires

You must never replace a single tire (with more than a thousand miles (or so) on it).

AU N EGL will argue that 1) is unsafe, and to a certain degree, he is correct--the only sure fire safe thing to do is to replaces the worn axle with tires made in the same week as the worn out tires being replaced. However, if you are simply a street cruiser who does not get close to the limits of your car, tires of the exact same brand, type, size are essentially interchangeble over time--as long as you stay away from the limits--which I suspece few of us actually do...

If, on the other hand, you enjoy leaning on the suspension, feeling the sideways forces press you into the bolster, then option 2) is the only safe thing to even consider.

The right time to align a car is just after a new set of tires has been installed. Have the air pressures set at operating pressures (35-36 PSI street; 40 PSI road race track) before the alignment and back to 30-32 PSI as you drive out onto the street.
Cars SHOULD be aligned with the drivers' weight in the drivers seat and any constant passengers' weight in the passenger's seat. If you ever want you car corner weighted, just after installing new tires it the time to do it.

 

I don't think it is tire pressure related, it could be alignment related, perhaps the newer tire has more rubber and might be affecting he camber. With that said, I am going to get them replaced with four ps2. I cannot really justify my purchase, however, I am too scared to put the Toyo on the the front and find out that the issue is still the same and then I will have invested in 4 crappy tires. Even though they are cheap ($300*2 rear) $600 is still expensive to me. The one thing that I am pissed off about is the EMT's last for about 10,000 mi (MAX for me) so it going to cost be $1600.00 in tires every year. This may not seem like a lot to some people, but it is to me.

 

It's the EMT - no EMT mix more than the different brand. The handing differences between the two are enough to seriously effect the balance of the car. The sidewall flex - no sidewall flex combination on the same car is not a good thing. I noticed the same thing on my C5 when I put track tires on the back at the strip. It was BAD.

 

on my new zo6 i also have noticed the car tracking with differant road surface but its not the fault of the kind of tires.
for all of u old hotrod guys you sould remember when you have a stiff suspention and wide tires there was always some tracking or pulling issues ( remember air shocks and 50s jacked up)
with 35s on the front and 30s on the rear and a performace suspention(stiff) you will always have some tracking this is amplifide with a short stiff sidewall

 

I have the Toyo TR-1 all the way around with no problems. My guess, it's the miss match.

 

I have the same problem. With 15,000 on the Supercar tires (Z51) the rears were down to the cord. Even with worn out tires the car was rock solid at speed. I can't deal with purchasing new tires every year so I bought the EMT's that come on the non Z51 cars. Since the fronts still had some life left I only bought the rears. With only the rears (remember these are G/Y EMT's) the car darts all over the road. At 100 mph it takes both hands to keep the car on the road. This is mostly when I hit bumps and grooves in the road. I hope installing the fronts to match the rears will fix the problem. Discount called this afternoon and they just came in. I'll report back tomorrow. I hope to get at least 20,000 miles out of these stock non Z51 C6 tires. I do lots of track events so I get all the fast cornering I want at the track (I run Hoosiers at the track). I don't have to have the stickest tires for the street but in fyi, the new rears hold as well as the Supercar tires did except when they were brand new!

Jerry

 

You all have to understand, the OE tires were designed specifically for the car, and was the result of years of fine tunning to get the balance right, so if you intermix different tires, even if they are both EMT tires (Supercar EMTs front and base Eagle F1s rear) you will have problems. The tires have different constructions and different compounds. Days/weeks were spent at the Nurburghring to get the balance right, even at extreme handling limits, let alone low speed or highway speeds. I dont think is forum has an appreciation for the engineering, technology, time, etc. that goes into developing a good all-round chasis/tire combination........which all goes into the OE components (tires included). You assume the risks involved or trade-offs associated with changing tires and types.
And please keep the runcrap comments to yourself Burtonbl103, I can assure you its not the RUNFLATS problems, actually they are Extended Mobility Tires, get it right!