cooney

I am planning to replace the original Dunlop Sport Maxx tires on my IS350, 225/40-18 front and 255/40-18 rear, with Michelin Pilot Sport PS2 tires, and wonder about going to the same size tire on each axle (stock wheels), either 245/40-18 or 235/40-18.

I had an early E36 BMW M3 which ran the first Michelin Pilot Sport tires, 235/40-17 on all four corners (BMW later went to wider tires at the rear), and was very happy with that setup. In addition to going to Michelin, I want to increase the contact patch on the front axle, to see if I can get more steering feel -- I do miss the M3 in that department.

Increasing front tire width and decreasing back should reduce understeer, somewhat offset by the fact that the diameter of the tires will increase in front and decrease in back, albeit not much (I've read that taller wheels in front and shorter in back tends to increase understeer).

Is 245/40-18 too wide for the front wheel wells of the IS350? If so, will 235 work? If I do this, will subsequently installing after-market springs & shocks, or coilovers, create problems?

Would you think decreasing the rear contact patch (going from 255/40-18 to 245, or perhaps to 235) will affect handling much?

Am I crazy to not just replicate the OE sizes?

Thanks for your help.

cooney

tqlla3k

iTrader: (1)

I think you will rub your front tires with 245/40/18s

I would go with 235/40/18

ES3

You might wanna check with your dealership before you do anything that doesn't have a staggered (wider) tire in there rear, the ABS and VSC are all calculated based on a staggered setup. Sure you can minimize the stagger, but there needs to be something there I believe. Tire Rack is a good source for the info too!

Zanorath

You'll be fine as long as you don't change the rolling diameter on the wheels. Keep in mine the stock setup has a slightly larger rolling diameter since the 255/40 sidewall is based on 40% of the 255mm rear vs the 225mm front. So if you go to a non-staggered width setup you may want to run a 35 profile in front and a 40 profile in back. However doing this will still make it so you can't rotate your tires without dismounting and just doing a side to side rotation. Also, typical stock runs on the 18's which have an 8 inch width front and an 8.5 inch rear wheel so the actual metal will still be staggered. I don't think its a great idea, but in terms of possible I don't think there will be any real problems, your traction control would be more sensitive since it wouldn't be set to a stock ratio. I'm not sure what the tolerance is though, I doubt its less than 1% though which is what your difference would be by matching diameters.

If you *did* go for non-staggered going with a 245 width would probably be best to retain as much of your rear traction as possible, though you'd probably need to drop to a smaller profile and/or do the heat gun to the front fender mod.

Zanorath

P.S. In terms of handling I recently went with a 235 front on a new rim and notice quite a bit more feedback. Its most likely because the wheel is heavier though so it takes more force to move it around, not exactly a performance orientated 'upgrade'.

ES3

So essentially would you say it's not possible to come up with a tire combo that would allow you to rotate tires from front to back, without messing up traction etc? That's how I understood it.

Joe Z

iTrader: (10)

I'm basically in the same boat..

I will be using my car on the freeway a lot and I wanted to put the same size wheel & tire combo on all four corners.

My first thought was to go with OEM wheels 18x8 with 225/40/18 on all four

M second thought was the OEM 18x8.5 with 235/40/18 on all four..

Either way I will have to buy 2 new 2008 wheels and 4 tires so that I can easily rotate front to back with no issues...

I'm really not looking forward to buying tires once a year if you know what I mean..

Any Lexus Tech / Mods have any useful input on this??

What are the technically disadvantages or long term issues that may arise to the VDIM or other systems... (if any) ???
&
Have ANY IS350 owners done this yet??


Thank you in advance!

Joe Z

Zanorath

I think using the words 'messing up' is going too far. It will just make things more sensitive. If in stock form its expecting the back tires to be traveling 1% faster, then the safety features will kick in at different rates than stock.

Theory time:
Lets say traction control kicks in when there in a 5% difference in speed between the wheels. The ECU reads this as the wheels slipping. In this case since you're matching the tires the ECU will read that your fronts are slipping by 1%, that would mean traction control would kick in at 6% now instead of 5% on the rears, and 4% instead of 5 on the fronts. I guess in my other posts I was making it sound like the TRC would kick in sooner, but really its making it later unless its your fronts that are slipping, which is not a typical situation on a rwd. That being said, at most its making your ecu 20% more sensitive, probably not the biggest of deals. Most tire shops will recommend anything within a 3% range, our cars don't really come with a huge stagger.


But why ; ;
Staggered setups come hand in hand with a sport sedan, and the 350 has the power to utilize the extra rubber on the ground, plus it makes the car look meaner. Seems like a downgrade to me, I suppose it'll help gas mileage be decreasing the amount of tire touching the ground, and save a little money by rotating.

gshb

actually, a guy like me could use a non-staggered setup for track days. often times a track configuration puts alot of strain on either the front left or front right tire. being able to rotate these around means longer life. i do expect some tire chirping in full auto mode due to less grip, but once moving i dont expect thinner tracks to be an issue.

cooney

Regarding tire size:
and
Is the front wheel well that tight? A 245/40 tire is barely three quarters of an inch wider than a 225/40 tire (20 mm = 0.79"), i.e., less than 0.40" closer to the fender. I appreciate that the IS350's wonderfully small turning radius must mean that there is a lot of steering travel, but I guess I'm surprised that the tolerances are that tight.

Regarding staggering,
For me it is not about being able to rotate front and back, or to ever need maximum rear traction (except in an emergency stop), but to try (a) to increase steering feel (bigger front contact patch), and (b) to modestly reduce understeer (reducing the back contact patch contributes to that).

Zanorath's suggestion about going to a 35 aspect ratio in front makes some sense, as the overall front diameter would decrease a bit, which is also better to reduce understeer, but I worry about ride harshness and tire vulnerability with a lower aspect ratio tire.

I confess I had not thought about the effects of all this on the electronic nanny. I gather that can't be adjusted.

Perhaps I should just stick with what I have, size-wise.

Thanks for all the input. I appreciate it.

cooney

Zanorath

I feel that if you go to a better tire than the OE you'll get more comfort even with a smaller profile. I recently 'upgraded' to heavier rims on a smaller profile and feel the comfort is better than stock. That could be because I'm comparing old vs. new tires though.

Understeer is mainly controlled by the VDIM system, I believe its set to understeer on purpose. Making the contact patch smaller in the rear and larger in the front would make the car physically prone to more oversteer, but that just means that the nanny is going to kick in more and correct it. I'm currently on 235 front and 275 rear and the nanny still kicks in when I'm trying to test its limits. The second there is any 2 wheel slip it kicks in right away, I can seem to get away with a tiny bit of 4 wheel slip before it stops me, I assume thats because it doesn't know whats happening if all 4 wheels are slipping evenly. The only real way to get it to understeer less would be to turn off the nanny (foot dance with my car, I believe its easier on 07's). Though, I don't recommend this unless you're on a controlled course, for the most part the nanny pisses me off, but there have been rare times where some weird road condition that I was unaware of will cause the ecu to take control, those rare times wouldn't feel so rare if each was an accident instead of the nannys protection =P

As for how tight it is, I'm not sure typically people that run a 245 are also running on 19's, that being said, most 245 owners still have problems on a 35 profile with a 19" wheel, which is very close to the rolling diameter of a 18" on a 40 profile. It also has to to with the offset of the wheel, a wheel thats tucked in more in the wheel well, like the stock wheels do, will me more prone to rub against the inside of the well. An aftermarket wheel thats flush with the fender will be more likely to clip the plastic guard just inside the bumper. A perfect offset fixes most of the problems, but thats something thats built to the wheel, so you either have to know enough about measuring you wheels insides to request a certain exact offset (if available) or trust that the wheel manufacturer has enough ability to make it fit just right for your car based on schematics at best, guesswork at worst.


Personally, I think the only real benefit to matching would be rotating, and you'd still need to buy 2 matching wheels to match whichever of the other two you keep otherwise you'll still have slightly uneven wear even with the exact same tire.

carchitect

iTrader: (1)

The GS 18" wheel is the 18 X 8 that is used on the front of our cars. I think the 245 should work. I did have a client with an IS250 with the rears replaced with front wheel/tires. The vehicle didn't seem all that different with 225s all the way around. I'm sure the 350 would feel much more different. If you're planning on doing this I'd say go for it. I'd get a set of front wheels, 2 tires and tire pressure sensors. Have the dealership program the 2nd set as the two front ones on the car and the two new ones you've purchased. You're TPWS will work fine and if you wish to use the wider rears you can then go back to the main tire set on your TPWS. Note that the TPWS can only detect/register 4 tire sensors at one time so if you're using the front set you already have you may want to install some different valve caps (like chrome) on your 2 new wheels/tire pressure sensors so you can tell at a glance the ones that are registered to both main and 2nd set.

I think that running a IS350 with smaller rear tires will be OK once the car is in motion. The torque reduction at higher gears won't cause wheel spin. The 1st and 2nd gear might be different and your VDIM might be a little sensitive to it since the 225/40R18 and 255/40R18 is diameter staggered and while the system won't have issues with ABS/TCS w/o wheel spin, the factory set thershold maybe about 15% tire slip before engagement but with the non-staggered set the system might engage as low as 5% slip.

EyeKutr

iTrader: (5)

Can the 18x8.5 be installed on the front without issues ??? This ebay link has non-staggered sets

sailing217

245's on the front will rub when turning hard. Dealership rotated my tires at an oil change is how I found out

I would stick with OE widths, and get a second set of wheels/tires for auto-x. You could find some lighter wheels too especially if you move to 17"

cooney

Thanks for all the information.

I've about decided to keep the staggered setup and only upsize the front tires a bit, to 235/40-18. .But one last question -- the guy at the tire store said that going from the stock 225/40 to 235/40 will cause the ABS warning light to stay on. .Has anyone heard of that? .Makes no sense to me.

Thanks again,

cooney