tire load index - sidewall stiffness?

Hey guys,

Picking a 'max performance' summer tire for my 05 civic si...

The size I'm looking to buy, has a 45 series sidewall with a load index of "87" (1201lbs), the size I currently have on the car has 50 series sidewall with a load index of "94" (1411lbs), and I understand what the #'s signify.

A few of the honduh forms (with their questionable knowledge) recommend getting the 50 series size 'because the increased load index will translate to a stiffer sidewall and much less sidewall rollover at the same psi'.

My hunch is the 45 series will have plenty of sidewall stiffness for my 2,800lb car, and the tire's load index doesn't translate as directly to the sidewall stiffness/less rollover as they believe, if anything I'll get greater performance from the 45 series's shorter height & less rotating mass...

?

I don't believe your buds from the Honduh forum are right. As I understand it the increased air capacity of the 50 series is what gives it a higher load capacity and has nothing to do with the sidewall stiffness.

Some tires are built with a stiffer sidewall regardless of size.

This is a commonly misconceived thing.

Sidewall stiffness is relative. If you're comparing the same exact tire - one with a higher load rating than the other - the sidewall will most likely be stiffer on the higher load rated tire... but not as a function of supporting weight. Loads on tires are fully suspended by air (ok, maybe 10% of it is supported on the sidewall) but the heavier-rated tire is stiffer because of the extra material needed to contain the higher pressures that are associated with the higher load rating.

Load rating is more a factor of the max PSI the tire can hold, not the actual rigidity of the tire. Its just that tires which can hold more PSI tend to be stiffer. But... it doesn't matter which one you use, chances are the proper inflation required will be within a few PSI regardless of whether or not you're using 315/25/20 or 215/65/16.

A good example - take for instance my truck. It had Toyo A/T tires with an E load range. (80 psi max) I replaced them with the same tire in D range. (65 psi max) Both tires require about 42 psi to suspend my truck properly, but the E range tires had more ability to carry more weight because they could hold more air.

So, in a nutshell, as long as the tires you choose can support the weight of your vehicle without exceeding the max PSI, you're good to go.

As far as handling vs sidewall stiffness/load rating... its usually assumed that the effects are similar to aspect ratio changes... a stiffer sidewall from a higher load index will somewhat act like a shorter sidewall - less deflection in cornering at the risk of a less progressive slip ratio. But, in the case of stiffer sidewalls, the trade off isn't as favorable. The handling gets skittish faster than it reduces cornering deflection. Not really a good trade off.

Compare the tire's weights. Many times shorter aspect ratios are heavier due to the carcass stiffening required to get it to hold together at its rated speed.

In general, don't use load rating to choose the tire's aspect ratio. It has minimal affect on the actual handling. The real answer is to pick the "right" aspect ratio and I promise there will be enough weight capacity for a Honda

Thank for confirming my hunch guys, some good info! btw - in this instance, 45 series sidewall is more than 1lb less than the 50...

carguy123 wrote: I don't believe your buds from the Honduh forum are right. As I understand it the increased air capacity of the 50 series is what gives it a higher load capacity and has nothing to do with the sidewall stiffness. Some tires are built with a stiffer sidewall regardless of size.

You're right, carguy. One of the playing factors in a tire's weight capacity is the volume of air in the tire.

For the same tire size, load rating is directly proportional to max PSI. When comparing different tire inflated volumes, its not uncommon to see higher load ratings in larger tires even with the same max PSI.

paul wrote: Thank for confirming my hunch guys, some good info! btw - in this instance, 45 series sidewall is more than 1lb less than the 50...

Good to know. One thing they don't publish is moment of inertia. I'm sure its not a big deal in the two tire sizes you're considering, but it has to do with where the weight is in the wheel/tire.

Many times when you plus-size wheels, you add a lot of weight. Careful wheel and tire choices can eliminate that extra weight, but it still comes at a cost... the weight is the same, but you have shifted the bulk of the weight further away from the hub. Regardless of keeping the same rotational weight, the net result is a negative for rolling inertia. It will require more torque to overcome the same weight.