A wider front tire

[1HOT NSX]

My car is ready for new tires and I'm switching from OEM Yokos to Bridgestone SO3s. I want to keep the original wheels so I must change the size of the front tire from the OEM 215-45-16 to 225-45-16 - the closest available size. This tire is just a bit taller at 23.9 in diameter as opposed to 23.6 for the original - 872 rpm (revolutions per mile) as opposed to 879 rpm for the original. These changes are too small to affect the TCS and rear tires are available in OEM size so they won't change.

Here is my question: What do you think this change in front tire size will do to the handling, if any?

 

[nsxtasy]

I don't think the change in size will change the handling at all.

However, I would expect that the change in tire model from the OEM tires to the S03 will affect the handling, reducing that "feel" of precision cornering that the OEM tires excel at.

Let us know how you like it.

 

[sjs]

I like the tires, but you should take a few things into consideration. For starters, I think the size change may affect overall balance because the fronts will be wider and probably grip a bit better. If so then of course it will shift towards less under-steer (more over-steer.) To me that’s a bonus on an otherwise stock suspension but it’s not for everyone.

It’s also important to note that the RPM spec is affected by rim width. The value of 872 is based on a 7.5 inch rim. On a 7 inch rim (the recommended minimum) it will pucker up slightly and lower that number, but still not enough to be a problem with TCS. However, that probably puts you close to 24 inches which means you raise the front by nearly 0.2 inches relative to the rear. That doesn’t sound like much but one of my long-standing complaints about the stock NSX suspension is that they are dead level on new tires. As the rears typically wear twice as fast as the fronts you end up with the rear of the car lower than the front. Not much, but enough for me to see it, which looks dumb, and more importantly is aerodynamically bad generating lift at high speeds causing the front to be light. The stock setup is marginal IMO, but starting 0.2 inch high in front would make it worse.

So, that’s the long-winded explanation of why I would go to the 255/40/17 rears instead. Like the fronts they will be on the minimum recommended rim which isn’t ideal, but you will retain more of the familiar balance and keep the car closer to level. The down-side is that they give you 1.2% taller effective gearing and weigh slightly more, but life is full of tradeoffs.

 

[nsxtasy]

Originally posted by sjs:
one of my long-standing complaints about the stock NSX suspension is that they are dead level on new tires. As the rears typically wear twice as fast as the fronts you end up with the rear of the car lower than the front. Not much, but enough for me to see it, which looks dumb, and more importantly is aerodynamically bad generating lift at high speeds causing the front to be light.

Over the life of the tires, on average, the difference in wear rate does not change the relative height (front vs rear) or the aerodynamics of the car unevenly.

I'll start with a quick note, that the radius of the tire, and thus the height of the car, changes by 1/4 inch from the tread depth of a new street tire (10/32") to the tread depth of a street tire that's at the treadwear indicator bars and ready for replacement (2/32").

You are correct that the rear tires wear faster, typically twice the wear of the front tires. Thus, if you start out with new tires front and rear, the rear wears twice as fast, and the rear of the car gets lower relative to the front. By the time the rear tires are ready for replacement, the rear has lost 1/4 inch of height, and the front has lost 1/8 inch of height, so at that point, the rear has gotten 1/8 inch lower than the front. However, when you then replace the rear tires with new ones, at that point the rear gains 1/4 inch of height, and the rear is then 1/8 inch higher than the front. The rear continues to wear faster than the front, so that that difference diminishes (but the rear continues to be higher than the front), until both sets of tires are ready for replacement, at which point they are then even.

Thus, on average, the rear is higher than the front the same amount of time that the rear is lower than the front, and the difference is never more than 1/8 inch.

If you don't start out with new tires at both ends as in this example, you could have a difference in height front to rear due to treadwear that, at some point, is greater than the 1/8 inch here. However, in any case, the fact that the rears wear faster than the fronts is still going to make the rear higher than the front, on average, as much of the time as the rear will be lower than the front.

In the example above, the front to rear difference in height due to treadwear varies between 1/8 inch and zero and averages 1/16 inch. In any case, the difference will never be more than 1/4 inch, when you have new tires at one end and worn tires ready for replacement at the other. (Although that particular scenario is unlikely to occur; you would never replace one pair of tires without replacing the other pair if they are also ready for replacement at that time.) I'm not sure exactly how noticeable even a difference of 1/4 inch is. Perhaps your eye is more able to notice such differences than mine.

[This message has been edited by nsxtasy (edited 15 December 2002).]

 

[1HOT NSX]

Originally posted by sjs:
So, that’s the long-winded explanation of why I would go to the 255/40/17 rears instead. Like the fronts they will be on the minimum recommended rim which isn’t ideal, but you will retain more of the familiar balance and keep the car closer to level. The down-side is that they give you 1.2% taller effective gearing and weigh slightly more, but life is full of tradeoffs.

Thanks sjs, this is very helpful. I wonder what would unbalance the car more, a touch taller up front leaving the rear unchanged or taller all around changing the gearing slightly. Also, what do you think would look closer to stock.

I look forward to other members opinions and especially from someone who has tried this before. Thanks.

 

[1HOT NSX]

Thanks nsxtasy, I hadn't read your post when I posted the above. What you say makes a lot of sense and perhaps the difference in height between the front and rear is not worth worrying about. I'm of the school of thought that stock is usually better because a lot of thought and engineering went into every decision made for production. In this case, I'm just curious about the SO3s feel and performance on the NSX so I'm willing to risk it. I recently drove an S2000 with SO3s and it just felt wonderful.

 

[sjs]

Hey Ken, what’s that old bit about sticking the bottom half of your body in the fridge and the top half in the oven so that on average you’re at a very comfortable temp?

I’m not sure the “average” is what counts, but you are correct about the probable worse case being less than my description suggests, good point. Perhaps a bit more than you suggest though because the tires seem to wear at a faster rate as they get thinner so my first rears are toast when the fronts are down no more than 30%, and the second set gone before the fronts are really finished. Yes a small difference but I can see it, and more importantly I can feel it at high speeds. Another consideration may be that the rear of the car is much heavier than the front. Might this lead to the rear “sagging” slightly more with age? Plus, most of our efforts to lighten the car is up front, removing spare tire, smaller battery, etc. At the rear we bolt on more weight with SCs and turbos (but probably save on exhaust). These factors can also contribute to a nose-up attitude. So as I said, starting with fronts that are already taller by any amount just seems like a bad idea.

 

[nsxtasy]

Originally posted by sjs:
Hey Ken, what’s that old bit about sticking the bottom half of your body in the fridge and the top half in the oven so that on average you’re at a very comfortable temp?

Hey, you said that the faster rear wear makes the rear lower, when the fact remains that it can make the rear higher as much of the time as it makes the rear lower. So some of the time your point is true, but some of the time your point is exactly the opposite.

Originally posted by sjs:
Perhaps a bit more than you suggest though because the tires seem to wear at a faster rate as they get thinner so my first rears are toast when the fronts are down no more than 30%, and the second set gone before the fronts are really finished.

Then maybe you get three sets of rears to a set of fronts. In which case the point remains the same.

Factoid: In my experience, I do get roughly three sets of rears to a set of fronts. I was partway through my ninth set of rear tires when I replaced my front tires for the third time. Stock Yokohama 15"/16" tires, with a fair amount of track usage.

Originally posted by sjs:
Another consideration may be that the rear of the car is much heavier than the front. Might this lead to the rear “sagging” slightly more with age?

If the car is level when the tires are new, I don't see why the rear should be any lower as time goes by.

Factoid: My car is roughly level, with the height of the front jacking tabs above the level garage floor approximately the same, front and rear.

Originally posted by sjs:
Plus, most of our efforts to lighten the car is up front, removing spare tire, smaller battery, etc. At the rear we bolt on more weight with SCs and turbos (but probably save on exhaust). These factors can also contribute to a nose-up attitude.

Any modifications to a car from stock can have effects in many areas aside from the ones targeted by the modifications.

Originally posted by sjs:
starting with fronts that are already taller by any amount just seems like a bad idea.

I don't dispute this... but a lot of people are doing it. When people use aftermarket wheels and/or aftermarket tire sizes, they are often changing the height (outer diameter) of the tires unequally. And I bet a lot of folks are increasing the front tires more than the rear tires, rather than the opposite. However, the change that 1HOT NSX is suggesting - increasing the outer diameter of the front tire by 1.5 percent - is relatively minor. I don't think it's a bad idea to increase the rear to 255/40-17 as well; in fact, I think it's a good idea, since that increases the rear outer diameter by 1.3 percent, close to the increase in the front.

Here's another perspective on the magnitude of the change and whether or not you (1HOT NSX) ought to worry about it. You're considering increasing the treadwidth in front by 10 mm and the outer diameter by 1.5 percent, and you may or may not be doing the same in the rear. There are a lot of people (myself included) who have taken a '91-93 NSX, and put '94-01 OEM NSX wheels and tires onto it, and thought nothing of it. Anyone who has done so has increased the treadwidth in front by 10 mm and the outer diameter by 2.4 percent, and increased the treadwidth in the rear by 20 mm and decreased the rear outer diameter in the rear by 0.6 percent. So the changes you're considering, 1HOT NSX, are so minor that they are much less significant than the changes that the NSX designers themselves did to the wheel/tire sizes in 1994.

 

[1HOT NSX]

Thank you sjs and nsxtasy. I think I'm going to do both, 225 for the front and 255 for the rear, unless someone else comes up with a compelling reason to persuade me not to. I'll let you know how I like it .

 

[docjohn]

I had been using the sizes you are entertaining but at that time I had HR springs and the taller fronts rubbed the inner fender on full lock and did give the car a little nose up look due to increased diameter.I'm sure as the others have said that at stock ride height you'll be fine.