Is VDC designed around 4 wheels of equal diameter, or to work with the actual front/rear diameter difference that exists on our stock equipped cars? Speculation could abound, but I'd really like to hear from someone who knows for sure. Why's it so important to me? The difference between 225/40/19 front and 245/40/19 rear (stock on many VDC equipped cars) is substantial, and using it as a starting point (instead of zero) radically changes the landscape of alternate size combos, makes some commonly prescribed combos seem less than ideal.

 

(F)225/45R18 and (R)245/45R18 is actually the stock tire/wheel size.

 

 

We're truly in the realm of speculation here. Only the Nissan engineers who programmed the parameters of VDC know the answer for sure (and they can't answer anyway due to confidentiality agreements). But VDC does use the wheel speed sensors to measure rotational speed and the 3% rule was discovered by trial-and-error (mostly by members on this board). The big picture is whether it's 17', 18" or 19" wheel size, examine the overall rolling diameter of the wheel/tire combo you are considering and make sure the rears are either equal to, or slightly larger (3% rule) than the fronts to allow VDC to function.

 

>> (F)225/45R18 and (R)245/45R18 is actually the stock tire/wheel size.

The size I quoted came stock on many G35s. And regardless, the height difference between the sizes you quote is even greater than the size I quoted, so VDC equipped Z owners should be even more concerned about VDC's frame of reference.

 

Not sure what you are talking about. You seem to be preoccupied with height differences, which VDC doesn't factor in. The two tire sizes have only a 0.7" difference in rolling diameter (F- 25.9" overall rolling diameter, R- 26.6" ORD) which is about a 1% difference. So whether you have a G or a Z with an FM chassis, there is some leeway in tire sizes before VDC causes any problems.

 

w 19"s most use 35 profile all around or 35 fr 30 r. I used 35 and loved it

 

>> examine the overall rolling diameter of the wheel/tire combo you are considering

Looking simply at overall diameter gives you the exact same answer, ie a 3% increase in diameter = 3% increase in circumference (aka rolling diameter).

>> make sure the rears are either equal to, or slightly larger (3% rule) than the fronts to allow VDC to function.

If VDC were designed to function with all wheels being equal, why would it function well if rear wheels were slightly larger but do poorly when fronts were slightly larger? It just so happens that on VDC equipped Gs and Zs the rears are around 2.5% taller. Given the actual stock sizings and the empirical experience (you mention) that taller on back works better than the opposite, woudn't it be reasonable to assume (our) VDC was actually designed to work with taller rear wheel, not equals? Perhaps even designed to work with a size difference pretty close to what our cars were sent from the factory with?

Once again, whether we use equal or 2.5% higher in rear as a starting point for reference has huge implications for tire sizing options.

I'm heavily leaning towards believing it's designed around taller rears, especially after reading that taller in rear is part of the "3% rule". This would all make perfect sense. Tires of equal diam would work -they'd be only around 2.5% off from design spec, whereas tires that were only marginally taller in front would be brushing up against or exceeding 3% difference from spec.

And... I Just read several accounts of people running 245/35/19 and 285/35/19 with no problems. That's a 4.3% taller rear tire! Well outside 3% if we were to use equal as a starting point.

 

>> You seem to be preoccupied with height differences, which VDC doesn't factor in.

It's my understanding that VDC uses wheel speed sensors to help determine if slip is happening. Differences in wheel diameter will affect relative speed of wheel sensor rotation, so will also affect VDC operation.

 

I give up..

 

op... make trip to japan and ask the engineers.... you seem to know it all so why are you asking on a forum?

 

also been a member for almost year and half and only 10 posts, 4 of them in here........ hell of a contributing member

 

Let me try, wheel speed sensors will read a normal condition when stock tires are mounted. Stock tires consist of a few combinations and the G35 combo you mention follows the same rules as the Z. So yes changing the OD ratio between front and rear will effect VDC. Forget the percentage crap, most people screw that up, for VDC to fully work you want a rear tire no more than 1 inch taller than the rear but not equal.

 

I have no empirical data to support the "3% rule" one way or the other.

However, therein lay the reason I use a quick Excel spreadsheet to do all my tire height calculations. The baseline data I use is the STOCK 225/245-45-18 set-up and every tire I have plugged in from 17" to 19" shows the overall diameter AND difference in actual height deviation in inches and mm's from the baseline dimensions.

Of course, since it's in Excel, I can always change the base tire dimensions and then I can sort the tires by size or height or price or...??? if I wanted to add those factors in.

The point: Never considered factoring in a percentage differential between F&R because I just go with the tires CLOSEST to stock heights for proper front and rear stagger. I could add that in in about a minute.... but, I don't have VDC so who cares? Hahahahahaha....

Why a spreadsheet? Well, because as often as I shop for tires for one car or another, I get tired of using a calculator every other day.

You don't believe me? I bought three sets just this week...







Mic

PS Sorry to derail the thread, I was just amazed at my lack of self control. Carry on now.

 

but you have ABS , it looks for a difference from normal condition in the wheel speed sensors.

 

Ahhhhh, too true.... either way, I always stay with stock height tires to avoid avoidable issues.

I'm averse to chasing dumb problems. In cars and in life. Heh heh.

 

The answer is the bold one. The sensor assumes a 2.7% diameter difference between the front and rear (3% gets used to make things simpler). At this 2.7% number, the computer is happiest. After some trial and error, it's been theorized that you can take that 2.7% and increase or decrease it by 2.7% giving you a range of 0% - 5.4% where the computer will still be ok.

I personally haven't discovered the limits of the VDC behavior as I've always purchased tires with setups as close to that 2.7 number as possible.

 

This has been an odd thread with the OP insisting on parameters that make no sense.

I’m in agreement with Mic. You avoid problems with VDC, TCS, and ABS by keeping close to the OEM tire diameters (front and rear).

I have violated this axiom by running all-square with snow tires, but finding the right size required some experimentation.