Okay, I've read from numerous folks on these forums that the rear tires must be between .5" and .75" larger than the front tires in order for ABS to work correctly. From some of the tire sizes I've been seeing lately, it seems like people aren't too worries about keeping within those tolerances.

For example:

factory Track rims and tires:
front = 25.97" diameter
rear= 26.68" diameter
That's a .71" difference. Okay, that works.


Nismo rims come with 245/40/18 in the front and 275/40/18 in the rear, correct? That gives us:
front = 25.71" diameter
rear = 26.66" diameter
That's a .95" difference.

What's up with that? Aren't the Nismo sizes listed above what they recommend with their rims? If so, is the .5" to .75" difference requirement actually necessary? If you do a 245/265 combo, you have a .64" diff, which is within range. A 255/275 combo gives you a .63" diff, which is also within range. But a 245/275 combo doesn't cut it.

(Although I guess a 245/265 or 255/275 combo might not be bad, since from what I've been reading it would help reduce understeer better than the 245/275 combo.)

So what's the deal, folks?

 

I run 265/35-18 F and 265/35-18 R for my race tires, and I drove about 100 miles on them yesterday to and from the AutoX and didn't have any problems with ABS, VDC, or TCS. Of course, I never had a chance to find out if VDC or TCS still worked (since I race with them off and drove sedately to and from the event) but ABS definitely did.

I'm sure the other features would be affected somewhat, but I have no idea how much.

-D'oh!

 

I say it...because before the car was released nissan usa said that is how it should be. If you think about it...the computer knows that the factory wheels are:
factory Track rims and tires:
front = 25.97" diameter
rear= 26.68" diameter
revs per mile = 800.92 front 779.61 rear
The ABS uses that data to accurately hit & release the brakes at a given speed based on that. Now if you do a 245-40-18 & 275-40-18 your revs per mile = 808.71 front 780.20 rear. Sure..this isnt quite within spec...but also not enough to completely set off the system. Now if you go crazy and do 255-35-20s all around...I think you'll have issues. slam on the brakes in wet and it will try to hit the fronts more than the rears...although it should not since they are the same height. I think Nissan is most worried about keeping a stagger. Race only applications where vdc and abs are disabled...obviously not an issue. In the end , it is your car, and your decision to make. I only wish to make sure you are informed.

 

I think you (or others) are mistakenly interpreting what Nissan says. The front to rear stagger is needed for VDC to work correctly, but has no (or little effect) on ABS operation. You quoted a Nismo wheel/tire difference out side the limits - wouldn't it make sense then that that information corresponds with the Nissan warning about use on "vehicles without VDC" installed? I think your statement above affects VDC operation and not necessarily ABS operation - which is dependent on wheel rotational sensor readings among all four wheels. When one wheel locks up, (rotation speed goes down), ABS releases fluid pressure to that corner to get rotation speed back up to match the other wheels.

 

Thanks for the responses. Good clarifications on ABS vs VDC uses of the diameter differences.

So it would seem that it's a safe bet to try and get the revs per mile as close as possible to that of the stock rims. Seems reasonable. Anyone find any fault with that logic?

 

Looking at the service manual, BOTH VDC and ABS use data gathered from the wheel sensors on each hub.

That being said, I still dont have a problem running the same size tires to/from the track or on the track, either with VDC on or off, but wouldn't recommend it for everyday use.

 

For those of you running the same tire size all around, do you do that to reduce (or eliminate) understeer? Do you find that the car is neutral in that situation?

From what I've read, it seems that proper suspension and swaybars will go a long way towards reducing understeer. (Per some of Raceboy's posts from his track time.) I'd almost be concerned that having the same front and rear tire width might actually give you some oversteer!

 

I have all the mods below, including 275/40/17 all alround. I have had no problems. I always turn off TCS since it is very dangerous (does anyone know how to disable it permenantly?). I had no problems with ABS on track - lap after lap.

 

The problem with people saying there are no problems with their ABS is that they are making subjective observatons. As far as they can tell, the ABS still works, and they slow down quickly, as they did before. However, I wouldn't be surprised if their actual stopping distances are negatively affected due to the improper stagger. They won't be able to tell in the driver's seat if their car now takes an extra 8 feet to stop from 60 mph. So then they come on the board here and say "There's nothing wrong with my ABS, works fine!". The reality is, they don't really know that. Only proper before and after controlled testing would tell us what staggers, if any, affect ABS activation. I am applying my argument to ABS only, not VDC or TC, since these last two are of no concern to me. My enthusiast only has TC, which gets turned off every time I get into the car, but I definietly don't want my ABS to be negatively affected by an improper stagger, whether I'm on the street or the track. So in reality, the real answer is 'nobody knows' (well, Maybe Nissan does, but we'll never get that info). We need a test car, something like the Stoptech testing done by that Nick fellow a little while back. That would yield some definitive answers, not just subjective opinion of "my ABS works fine with 255 all around" or whatever the case may be. I'm not trying to sound like a jerk, just trying to show that as of now, it's all opinion, not fact.

 

What, nodoby has anything else to say about this? C'mon guys, this is important stuff here!

 

I'd love to provide some input, but I'm afraid I have no idea as to what the facts are. Wait... that's why I started this thread!

I have been doing some thinking about what others have posted here and about this topic in general. I think ChinaClipper and DiscountTireDirect posted some good thoughts up above. It seems reasonable to assume that VCD would be adversely affected, but I'm not so sure on the ABS. Here's my reasoning:

On the models that have the 225/50/17 front and 235/50/17 rear tires, the difference between the two is only .39". On the other hand, the 225/45/18 front and 245/45/18 rear combo has a difference of .71". You can see that the 225/235 combo does *not* fall within our requirement of .5" - .75" difference, while the 225/245 combo does.

for reference:
225/50/17 = 25.86"
235/50/17 = 26.25"

225/45/18 = 25.97"
245/45/18 = 26.68"

On all the 225/235 models, there is no VDC. On all the 225/245 models, VDC is present. So that leads me to believe that VDC depends on the 225/245 difference. However, ABS is present on all models, regardless of wheel size.

This certainly isn't definitive proof or anything. Just some food for thought. But heck, if Nissan itself said that the stagger difference matters, than I'm inclined to believe it.

 

more food for thought, on the G35c auto, there is VDC, and it comes with the 225/235 17" combo above, AND the 225/245 18" is optional.

 

I like this line of thought. But my interpretation on Nissan's .5-.75" recommendation is that is holds true for each wheel size, be it 17 or 18. In other words, the acceptable range for the 17 inch wheel equipped vehicles will be on the lower end, compared to that for the 18 inch wheel equipped vehicles. So the .5-.75" range is arbitrary in the sense that it depends on the stock diameters. I think that what Nissan is saying is that if you have a base model and change wheels/tires, make sure the new combo is within .5-.75 inches of the stock diameter difference of .39 inches. And this range will be slightly lower than the acceptable range for the track model, for example, which will be .5-.75 inches from .71". Gosh I sure hope what I just said made sense.

But getting back to what I quoted above, from your last post, that sounds very reasonable. It also begs the question of whether or not ABS is truly affected. I really need to know the truth about this stuff.