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 and 245/40/19 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.

 

It's as simple as your car will fight you in every turn thinking the tires have lost traction because of how fast/slow they spin. Ask those that spent a thousand on new tires what it was like to have to buy new tires again... Lol.

 

I fully understand the importance of VDC, what a pain in the *** it can be, and the importance of tire size for it to not over react. Perhaps if you re-read my post you'll see that I was simply asking how one very specific aspect of VDC works.

 

Or perhaps you were unaware 225/40/19 and 245/40/19, tires that vary in height significantly, are what come stock on many VDC equipped Gs? Does one use the difference between these 2 sizes as the reference starting point? I would certainly think so. They differ by 2.4%. If we assume that VDC is OK within 3% of it's designed center point, why would infiniti take a system designed around all wheels having equal circumfrence and install it in their flagship sport model knowing that it was already 2.4% off-target? That's nearly 3% where it definitely becomes wonky. At 2.4% away from design spec I bet VDC kicks in much more quickly than it does at spec- hard to imagine something so fundamental eluding all the minds at nissan

 

Calm down kiddo. You are greatly exaggerating the height differences. 3.54" sidewall vs 3.85" sidewall is hardly anything to get your panties in a bunch over. Plus that is unloaded. It is never the same when mounted and installed on a car. Two exact same tire sizes can easily have different heights when installed on a car depending on sidewall construction. Runflat tires have significantly more steel weave in the sidewall than a summer tire. Even if you had exact same size, the runflat would be taller than the summer tire when installed.

Plus why in the world are you assuming 3%? What made you think diameter even has anything at all to do with VDC? You are completely off base. VDC is a stability system that uses many sensors throughout the vehicle such as the ABS sensor, steering wheel angle sensor, and yaw sensor to measure what you are telling the vehicle to do versus what it is actually doing. When those two things do not match, the VDC kicks in. 0.3" tire height difference doesn't matter in the least bit. Finally, VDC doesn't kick in at all if you just disable the yaw sensor.

You sound like a first year engineering student.

 

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Overall diameter of the tires has a lot to do with it.

OP, I don't think anyone knows how how the system was programmed, nor do I think its an issue. I purchase tires based off "equal" specs and it hasn't failed me yet.

 

The tire OD has no direct input into VDC system. Tire OD can attribute to slip and overstear, but that is a function of friction more than OD. The only way OD would impact the VDC system is if it was greatly able to throw off the wheel speed sensors (same way tire size can impact speedometer). As long as you keep the same size wheel on left and right of the car, the yaw sensor will not be impacted by tire OD.

Plus the car has been out for 12 years now and no one has ever had VDC issues because they put new wheels on. OP is worrying about a non-existent issue.

 

WhosUrBuddiee wrote:

>> Plus why in the world are you assuming 3%

Google is your friend. Many people cite it as a truism. I don't, in fact I'm questioning an aspect of it, but I'm at least aware of it.

>> no one has ever had VDC issues because they put new wheels on

Really. Try googling something akin to "vdc problems tire size" and you'll find many reporting VDC surfacing when they've changed wheels (ie overall rolling diamters).

>> The only way OD would impact the VDC system is if it was greatly able to throw off the wheel speed sensors

No kidding? So forget about all the other issues you mention (regardless of whether they were addressing anything I was really saying/asking or not), and just remember these words you wrote. Perhaps if you just remember this you can understand why I'm concerned about tire diameter differences and what diameter difference (if any) VDC was designed to function with.

>> You sound like a first year engineering student

At least I was being polite. You spout off info unrelated to my question attempting to make my question seem foolish, make a derogatory comment towards me, then say something that validates my original concern, and I sound like a first year engineering student? That all makes you appear mighty sophomoric.

 

I was afraid that was your train of thought. The 3% guide you are thinking of is related to changing stock tire sizes, it has absolutely nothing at all to do with difference between front/back on staggered setups. Also it is based on brake performance and speedometer accuracy. It isnt based off VDC. My last vehicle I changed it +21% without issue.


Ive seen acouple threads in past with people guessing, but again no real correlation to changing tire size and causing VDC problems. I have seen FAR more people on here with 20-22" wheels with 6-7% difference, again no issues. Plus that Nissan has never published anything saying to limit to 3%. People just guessing.



Slip is based on traction, not OD. When I said greatly change OD to throw off VDC, I truly meant greatly, like people with donks. If you install 32" donks, you will probably run into some VDC issues. If you go from a 245/40 to a 275/40, it is inconsequential.

You applied made-up guidelines you found on Google, incorrectly and questioned the engineering of the manufacture. Yea, I'd say that constitutes foolish.

 

>> Slip is based on traction, not OD.

And how is traction gauged? Is it not gauged, at least in part, by the wheel speed sensors? If the control unit senses the rear wheels are turning at a (sufficiently) higher rate than the front, wouldn't that be seen as a loss of traction? The wheel sensors can communicate this information, and wheel diameter has a large impact on wheel sensor speed of rotation. So if front/rear diameters were different than VDC programming were expecting, why wouldn't it affect VDC operation?

 

You are grossly overestimating the impact of tire diameter on speed. Plus you are assuming slip is base on set values. Since the G comes in multiple wheel and tire sizes from factory, it would be logical that slip calculation is based on relative/learned values.

Basically to question their engineering basis, you would first have to know their engineering basis. Which you don't, so this entire thread is useless.

 

So you're saying I could throw my 285/75/16's on the back of my car and I won't have any issues with the VDC??? I call BS. I would like to see these links of people running 8% difference between the front and rear
without issue.

 

The dudes a troll. Just loves to argue and act superior, forget about getting any useful info from him.

 

I'd be impressed if you could even fit a jeep tire on there. If you jacked the back up 6 inches, your car wouldnt even be driveable. VDC would be the least of your worries.

I do not know of everyones tire combinations on here. Personally I ran 245/30/20 and 285/30/20 for years without a single issue (tried a 305 but couldnt fit it on the car). A buddy of mine ran 275/25/19 and 325/25/20 on his Z06. Nearly 7% difference and not a single issue. At the meets, I have seen several C6 guys running 345/25/20 rears.

Problem is, you are starting with an arbitrary value (3%) that you have no idea if is even valid. Then you are trying to apply it to a G35, which you have no idea how their VDC is programmed. Guess however you want, in the end you still will never know.

 

The 3% tolerance is the generally accepted rule for front/rear tire diameter/circumference differences. Not sure where that specific number came from at this point, but it had a lot to do with trial and error over the past 12 years or so. 3% isn't a set parameter, it's a tolerance range.

Looking at stock tire sizes, the Sedans had same size tires front/rear, or 0% stagger, and the coupes ranged between 1.5-2.7% front/rear differences. (17" = 1.5% 18" = 2.7% and 19" = 2.4%)

It's the Traction Control System that gets out of whack when there's a huge difference in tire diameter. Per the FSM: "TCS Function - The wheel spin of the drive wheels is detected by the VDC/TCS/ABS control unit from the wheel speed signals from the four wheels, so if wheel spin occurs, the rear wheel right and left brake fluid pressure control and engine fuel cut are conducted while the throttle value is restricted to reduce the engine torque and decrease the amount of wheel spin."

In other words, slip is detected by comparing the front and rear wheel speeds. A larger rear tire outside of the tolerance is falsely flagged as wheel spin by the VDC/TCS/ABS unit since it is spinning faster than the front.

One problem with the FSM is that, when referencing tires, it only states: "Are air pressure, wear, and size within standards?" without defining what the standards are.

Looking at the tires sizes posted by WhosUrBuddiee, he ran with a 3.7% difference on his G. His friend with the Z06 only had a 3.9% circumference difference (not 7%), but it's irrelevance because it's a Z06, apples to oranges comparison.

If you're curious about ratios, I use http://www.miata.net/garage/tirecalc.html for tire calculations. I personally run 245/35/20 & 285/30/20 on my G35 which works out 0% circumference difference.