[gsxrgessler]

Well...the title pretty much sums it up. I picked up a set of used wheels/tires on the forum here and they have 245/40/18 on the rear, and 235/40/18 on the front.

My 4C is really mad, I presume that this tire size mis-match is the culprit. It just says something like "help me, help me please, I'm injured and I won't let you change any of my modes". Well, not really, but there is some sort of message that says that the suspension needs calibrating. Can't change from comfort to sport or "super-race-ready" mode.

Actually the code says: "chassis settings service required", sorry that I got carried away up there!

Anyone know what I should do?

Oh yeah, when I was starting out from a stop, the front tires spin for a second, then AWD kicks in. Very odd. At the moment, the AWD seems flawless though.

PS...buying new tires isn't a good option for me unless someone wants to pick up a pair of these from me for a reasonable price... Wife has put the ka-bosh on car upgrades for a while...

Cheers.

Modified by gsxrgessler at 10:08 PM 3-9-2007

 

[gsxrgessler]

Re: (AHI)

Any suggestions AHI on how to resolve it?

I was thinking...there is about 0.3" difference from the center of the wheel to the ground. So if I bump up the front tire pressure to about 38 and run the rear at like 32....I wonder if that will alter this height enough so that the 4-C thinks that the tires are the same radius.

As for the AWD...I presume that this doesn't engage unless slippage is detected...which of course with the different tire sizes, perhaps it is trying to engage all the time....????

Help.

 

[gsxrgessler]

Re: (RainyS60R)

I just did my best to play with pressure, looks like a 1/8" difference in radius is the best I can get to. That's 42 up front and 28 in the rear... WOW, that's a lot of PSI difference. Yeah, the staggered size isn't what I would want in an ideal world, but they're what I've got. The fronts are literally nearly brand new, the rears (245's) are about 90%.

Maybe I can have the rears shaved? That may be the best short-term option.

BTW...1/8" is around a 1% difference in diameter on a 26" tire OC (approx).

If I'm buying tires, I'm going to a different tire...i.e. summer only tire or R compounds.

Yes, wider rears will probably make it understeer a bit more, not my desire by any means, but as I said, the deal was too good to pass up, and so this is what I have. M&S rated tires weren't on my list either...

Thanks for all your help, keep it coming.

According to the dealer, 3/32 is the max front & rear difference. They didn't say radius or diameter...but I'm guessing it's diameter (doh!)

Modified by gsxrgessler at 9:04 PM 3-10-2007

 

[gsxrgessler]

Re: (SilverS60RNYC)

UPDATE:

the issue was a wiring harness that had worn through on the front axle...the tires were within the Haldex spec range according to the guys I talked to. So, I fixed the wires, the fault immediately went away...and I mounted up the new tires I had ordered in my haste. Anyhow, the new sneakers are Falken RT615's, which are awesome IMO.

Cheers. http://********************/smile/emcocktl.gif

 

[gsxrgessler]

Re: (ljjones)

Actually, it is dependent on radius, not perimeter. Imagine army tank tracks with a 17" wheel running inside of it. How many revs does the wheel turn over a given linear distance? Then imagine the 18". It's radius dependent.

If this doesn't make sense, sorry, I'm not a great teacher, I'm a much better engineer...

The tire pressure did influence the radius significantly, (i.e. 1/8" or so) but as I mentioned, the issue wasn't the tire size, it was the broken accelerometer wiring harness.

You can run 235/40's and 245/40's on the R without issue.

 

[gsxrgessler]

Re: (Needsdecaf)

Yes, c=pi*d. or 2Pi*r=c

But I'm sticking to my guns, radius is boss here (within some small amount of measurement error of course).

According to this: http://www.1010tires.com/TireS...ubmit

I prepared a little comparison for everyone's reference.

Here we go.... 235/40/18 vs 245/40/18

The radius is 12.70 vs. 12.85"

So the change is 12.85-12.70 = .15
The percentage is .15/12.85 = .012 OR 1.2%

The revs per mile is 818 vs. 809 (according to 1010 tires)

So the change is 818-809 = 9 revolutions per mile.
The percentage is 9/818 = .011 OR 1.1%

In either event, 1.1% or 1.2% difference is within range for the Haldex.

The key is revolutions per mile, as that is the Direct measurement that we need to be cognisant of.

That was fun.

 

[gsxrgessler]

Re: (ljjones)

Hey, wait a second whose side are you on? ha!

and don't forget, pi=3.14159265358979323

That's all I can remember off the top of my head, too many cocktails since college...I knew 120 places back then.

So, best measure from my perspective is to go by revolutions per mile, which is readily available for most tires. That's what your "car" is going to "see". In any event, three weeks to my first track day, where I intend to increase the # of revs per mile, and also add a second vector to the revs per mile measurement...my intention is for that vector to be perpendicular to the first measurement, though I'm sure someone can work up something in the imaginary realm that will confuse me even more.

Now where did I leave my pocket protector....??? http://********************/smile/emthup.gif

 

[gsxrgessler]

Re: (ljjones)

Interesting. So, the basis of that would be that the tread of the tire, or rolling surface has to cover a given amount of ground in each revolution? Which you are presuming would equate to one revolution, or 360 degrees of rotation of the axle?

This would be true if the tire was a perfect disc. But irregularly shaped objects (tires) don't follow rules like 2*pi*r = circumference. That only applies to perfect circles.

So even though the tread may travel it's full length on the ground, the axle might be 1% shy of 360 degrees of rotation, or in the case of running low pressure, but axle may have rotated further than 360 degrees (versus was a PERFECTLY round disc...i.e. a steel disc...would have traveled).

The rolling circumference you mention would be equal to the tread length, circumferentially around the tire I presume?

Hang on, I need a few more cocktails...
http://********************/smile/emcocktl.gif http://********************/smile/emcocktl.gif http://********************/smile/emcocktl.gif http://********************/smile/emcocktl.gif

 

[gsxrgessler]

Re: (ljjones)

Ow. My head. Okay, so let me get this straight. I'm back to the tank tread.

No no, I've got a new one. a hula-hoop, with a 5 gallon buck for a "rim". The 5-gallon bucket turns 360 degrees, the hula-hoop advances by the diameter of the bucket (plus the thickness of the hula-hoop) x pi.?

This is a fun topic. Because I could still be wrong (of course in your eyes I am wrong...but in my eyes, you're wrong). The reason the sidewalls don't wrap around the wheel is because the difference...i.e. that last 0.25% or so is eaten up by tread squirm and sidwall flex...and the real radius of the tire.

Okay, I've thought on this one enough, and I have to go to work tomorrow, so no more cocktails. Time for ta-dum....GOOGLE!

from tire-rack " If one tire is low on pressure, its circumference changes enough to roll at a slightly different number of revolutions per mile than the other three tires. "

http://www.tirerack.com/tires-techpage-1/44.shtml

Tire Tech - Tire Pressure Monitoring Systems at The Tire Rack.If one tire is low on pressure, it will roll at a different number of revolutions per mile than the other three tires, and alert the vehicle's onboard ...
http://www.tirerack.com/tires-techpage-1/44.shtml - 64k - Cached - Similar pages

Okay, maybe one more... http://********************/smile/emcocktl.gif

http://********************/smile/emcocktl.gif and that's it.

 

[gsxrgessler]

Re: (ljjones)

I agree.

So then, now that I already sold the 235's...anyone interested in a set of 245's? Buyer beware...you have approximately a 1% difference in rolling diameter, though that is within spec for the Haldex...

Happy motoring, and watch those tire pressures!!!!!
http://********************/smile/emcocktl.gif

 

[gsxrgessler]

Re: (parkstephen)

To answer the question on 245's all around...no, it's not bad as far as the car computer is concerned. There could possibly be some tuning parameters that affect the system response time that are affected...but this is only in the hypothetical sense. I would say that there is ZERO affect by upsizing like this.

I also believe that running the 235's up front and 245's in the rear should not pose any issues either, considering the 1.1% difference in circumference...or distance travelled per mile.

And yes, Minnesota rules! Mash, we should do some ice-race events together...!

Back to drinking... http://********************/smile/emcocktl.gif