HRDROKN

I had just put on some new Hankook V12's... 275/37-19 rears and 245/40-19 fronts and the fronts look like the diameter is too big! The specs indicated the fronts to have a 26.7" diameter and the rears are spec’ed at 26.6" diameter, but with my eye I'm getting 26.75" front (close enough) and 26.4" for the rears... a little off and you can see it!

On top of that my slip light came on? Again indicating that the rears are smaller than the fronts by a bit! Had to turn the VDC off.
Anyone have a similar discrepancy in tire diameters?

tejasg37x

What are the widths of the wheels? That can make a difference...

HRDROKN

19x8.5 and 19x10....

tejasg37x

The little bit of extra width on the rear could cause it to be a little smaller in diameter...we're only talking about .25" (6.35mm) difference from the tire spec.
Hankook's measurement of 26.6 diameter is on a 9.5" rim...that's .5" (12.7mm) narrower than your tire on 10" wheel that stretches an extra .5" (12.7mm) more. I know it's not much, but maybe enough to cause .25" (6.35mm) difference in diameter. That's only .125" (3.175mm) in radius. If the tire sidewall changes in direction in or out, it has to change the diameter too...even if it is very small.

Plus you already started off with the rear tire being smaller in diameter than the front.


I've always found that tire specs are not always 100% accurate either.

HRDROKN

So if OEM diameter is slightly larger in front (27.0") than the rear (26.7"), then the ECU obviously knows this as the rears are turning more revs per mile, but how does the ECU know... is it learned or is it set and that’s it?

Reason that I ponder this.... I bought the car with non standard tires, so what I had was a smaller front (25.8") vs a 27.0" rear and had no VDC slip indicated. So was this just a product of the size (smaller front vs larger rear diameter) or did the ECU learn the change in dia?

Still trying to figure the problem....

tejasg37x

I believe it's set up in the ECU from the factory dependent of the original tires.
I can tell you this from my experience with my 350Z. It came factory with 225/45/18 front and 245/45/18 rear. The ECU knows what the difference should be between front to rear tire diameters; if you deviate too much from that ratio, it will causes VDC problems.
I want to say it's around 3% of what was originally on the car.
Someone correct me if I'm wrong.
If you were to put tires on of the same size diameter all the way around on the Z, you would get VDC issues. You can go with taller or smaller diameter tires, but the difference between front and rear diameters has to still be within the same %. Does that make sense?

What size tires was your car supposed to come with from the factory? That's what matters the most...then there is a % of the original difference between front to rear tire diameters you can be off from that sizing without getting VDC problems. Anything over that % and you will have problems.

HRDROKN

Yes... 3% sounds like the number that gets thrown around.

The factory calls for a

225/45-19 @ a diameter of 27.0" on front and
245/40-19 @ a diameter of 26.7" on the rear...

My car (used 2008) came with...

245/35-19 @ a diameter of 25.8" on front and
255/40-19 @ a diameter of 27.0 on the rear... and no VDC issues.

So I changed to...

245/40-19 @ a diameter of 26.7" on the front and
275/35-19 @ a diameter of 26.6" (although the actual is closer to 26.4")?

And now have the slip light coming on.

HRDROKN

I have an appointment with Infinity to take a scan to see if any issues are present... in the mean time I've been doing quite a bit of searching and I did find this from the G37-Tech site...

Interesting as this is the way the tires were when there wasn't any VDC issue!

So I'm thinking that a switch back to a 245/35-19 (25.8" diameter) could be the resolution. Funny that the factory Bridgestone's are slightly larger in front... seems that the VDC would always be thinking the smaller rears were slipping (more revs/mile). I'm going to have to see if the service guy has any real thoughts on this... or the standard "we can't find anything that could be causing this".

tejasg37x

yeah, I've always thought the rear tires have to be larger in diameter...except in cases where they are the same...my X has all four tires the same size.
But I see the coupes with sizes like yours where the front is actually slightly taller and think WTH?

HRDROKN

OK.... It looks to me (as tejas indicated) that the reason for the smaller (26.4") measured diameter in the rear is that I'm running them on a 10" wide wheel, compared to the spec of 26.6" on 9.5" wheel. Still less than the mystical 3% difference, but appears to be enough to upset the VDC.

So my options at this point are switching to a 245/35 front or a 285/35 rear tire... obviously the rear is the more expensive option.

Opinions?

HRDROKN

As suspected.... the dealer was of no help!

Doggy

iTrader: (12)

Coupe or sedan?

HRDROKN

Coupe...

I was just looking at my original tire size (non-stock) of 245/35 and 255/40... and that is nearly 5% difference in revs/mile with no VDC issues! If you look at the diameters it is the rear that was larger at 27" vs the front at 25.8"... so it would appear that this combo (larger rear) has less effect on the VDC, which seems to make sense to me as the slower rotating rear is further from the speed of the front.

Now I'm thinking that the factory uses the smaller rear as to keep the VDC (spin control) closer to the point of activation as a safety factor... much like the understeer by design.

Coprolite

Did you check your fluids? There is the infamous VDC brake fluid symptom that pops up every 5 minutes on this forum...

HRDROKN

Yes... fluid, pads, etc. all good.