What to expect with a 235/40/18 upgrade?

[PtonDad09]

Planning to keep my current ride height on my Sportwagon so how much ride deterioration can be expected.
Also concerned with increased torque steer and wandering in truck ruts.
Currently running stock 215/50/17 Michelins.

[posttosh]

QUOTE (DaveR @ Jul 5 2009, 07:20 PM)

Planning to keep my current ride height on my Sportwagon so how much ride deterioration can be expected.
Also concerned with increased torque steer and wandering in truck ruts.
Currently running stock 215/50/17 Michelins.[/b]

Turning to your specific question, the main concern would be the clearance of a 235mm wide tire from the upper control arm (at the front pair) when the wheel is turned. Even when the OEM 215/50R17 tires are mounted, there is scant clearance; with a 235/40R18 tire, you should be concerned about rubbing. A lower offset wheel would increase clearance, but would alter the steering geometry (the steering axis would no longer meet the road at the midpoint of the tire width). As you note, you would also have an inch less of sidewall to flex at each wheel, so there is less spring action in the tire, giving a harsher ride and increasing the likelihood of edge lift under lateral stress. Also, the wider and shorter contact patch would be more prone than the OEM tire to being steered by irregularities in the road surface -- the obverse side of quicker steering response from inputs at the other end of the system.

Out of curiosity, why do you (your topic line) consider a 235/40R18 an "upgrade" from the 215/50R18 size?

[PtonDad09]

QUOTE (posttosh @ Jul 6 2009, 08:42 AM)

Turning to your specific question, the main concern would be the clearance of a 235mm wide tire from the upper control arm (at the front pair) when the wheel is turned. Even when the OEM 215/50R17 tires are mounted, there is scant clearance; with a 235/40R18 tire, you should be concerned about rubbing. A lower offset wheel would increase clearance, but would alter the steering geometry (the steering axis would no longer meet the road at the midpoint of the tire width). As you note, you would also have an inch less of sidewall to flex at each wheel, so there is less spring action in the tire, giving a harsher ride and increasing the likelihood of edge lift under lateral stress. Also, the wider and shorter contact patch would be more prone than the OEM tire to being steered by irregularities in the road surface -- the obverse side of quicker steering response from inputs at the other end of the system.

Out of curiosity, why do you (your topic line) consider a 235/40R18 an "upgrade" from the 215/50R18 size?[/b]

Thanks for the response. As far as clearance it seems from searching these forums that there are many who have this tire without any rubbing problems.
It seems to depend on the particular brand.
I have a track on a set or RX8 18x8 wheels which I am considering. My main goal is to improve handling and steering response with the side benefit of improved ascetics from the 18 inch wheels. The car has only 20k so I will wait awhile on a shock upgrade until needed.


Wouldn't the shorter sidewall lead to better overall handling?

[posttosh]

QUOTE (DaveR @ Jul 6 2009, 10:12 AM)

Wouldn't the shorter sidewall lead to better overall handling?[/b]

As with most things in life, there are trade-offs. The shorter, stiffer sidewall will give a more direct, "quick," feel to the steering. Too direct, too quick, and you edge into twitchy.

The wider, shorter front-to-back, contact patch on the pavement makes the tire less self-steering in the direction pointed and more responsive to turning input, whether that turning input comes from the steering wheel or from irregularities in the pavement.

The wider footprint gives a longer path for water trapped under the tread to go to be fully evacuated, so a wider tire is more prone to hydroplaning than a narrower one at any given speed.

A longer, more flexible sidewall provides more springiness and a softer ride, and less susceptibility to damage from road hazards, than a shorter, stiffer one does. In terms of cornering, a tire with a supple sidewall can be designed to "extend" the contact patch up the lower portion of the sidewall when the tire shape distorts under lateral load, while short, stiff sidewalls tend to provide more sudden transitions at the borderline where traction is lost, as the entire tread tends to stay flatter as it lifts off as a unit.

It is all a question of which side of the compromises you prefer.

[Romaninus]

I have 04 wagon- just got new wheels 17x8 48 mm offset (17x7-OEM) wraped with 225/50 R17 General Exclaim UHP-maximum wheel with for this tire is 8" so with tall side wall it's spread and hight still the same as a OEM 215/50 R17 but tire has wide contact patch. Ride softenes almost same but cornering and respond alot better then OEM combo. As clearance go-front almost stay the same-on the back I roll fenders just to be safe. May try next time 235/40 R17 will see how long this tires will last me. Load raiting affect how soft side wall going to be higher load most likely will give you stiffer fill.

[PtonDad09]

QUOTE (posttosh @ Jul 6 2009, 06:54 PM)

As with most things in life, there are trade-offs. The shorter, stiffer sidewall will give a more direct, "quick," feel to the steering. Too direct, too quick, and you edge into twitchy.

The wider, shorter front-to-back, contact patch on the pavement makes the tire less self-steering in the direction pointed and more responsive to turning input, whether that turning input comes from the steering wheel or from irregularities in the pavement.

The wider footprint gives a longer path for water trapped under the tread to go to be fully evacuated, so a wider tire is more prone to hydroplaning than a narrower one at any given speed.

A longer, more flexible sidewall provides more springiness and a softer ride, and less susceptibility to damage from road hazards, than a shorter, stiffer one does. In terms of cornering, a tire with a supple sidewall can be designed to "extend" the contact patch up the lower portion of the sidewall when the tire shape distorts under lateral load, while short, stiff sidewalls tend to provide more sudden transitions at the borderline where traction is lost, as the entire tread tends to stay flatter as it lifts off as a unit.

It is all a question of which side of the compromises you prefer.[/b]

Would you say I'd be better off with slightly stiffer springs and keep the stock wheel tire combination?
The Michelins have plenty of life left in them so there is no rush to replace them.

[Romaninus]

RB springs for wagon will not slam it down, but give you nice ride ..Invest in sway bars!!! This is hard to beat mode. First I installed rear Progress sway bar -lil difference but better then stock,then I got front RB bar- that changed everything !!! I think for wagon (havy back) Progress bar on the rear better then RB bar ( OEM wagon thiner then sedan bar -I guess wight difference ) Front swap is hard but I did it in 3 hours alone-second set ofhands will be a big plus in this install.

[dmlsupra]

I just install 235/40/18 with RX-8 wheels, and honestly, it's a great upgrade. I have yoko es100, I don't have any rubbing issues.

I feel a huge difference in handling. I used to run stock 18'' with pirelli

[posttosh]

QUOTE (DaveR @ Jul 6 2009, 10:38 PM)

Would you say I'd be better off with slightly stiffer springs and keep the stock wheel tire combination?
The Michelins have plenty of life left in them so there is no rush to replace them.[/b]

It all depends on what your priorities are in the changes.

If the rear is riding low, then XAM's advice on springing and sway bars would be the place to start; worry about the wheels and tires later.

If you want to improve road handling, so that you can leave the Infiniti G37 coupes in the dust when they have to slow down for the curves, then a simple tire change, using your current OEM wheels, to Hankook Ventus V12 evo ($85/each in the 215/50R17 size at tires-easy dot com) or Vredestein Ultrac Sessanta ($140/each in the 225/45R17 size at performanceplustires dot com -- West Coast -- or slightly more at Euro-Tire -- East Coast, depending on shipping costs to your location) would give you far more bang for the buck than any other cash expenditure of the same order of magnitude.

[smokeU6s]

posttosh,

CND just placed the Hankook Ventus in 2nd place in a tire test, and the price can't be beat.

Whats the estimated tread life?