: Michelin PS2 275/40ZR18 on the rear



CrashCadillac
06-08-07, 09:25 AM
Does anyone know what actually happens when you stuff a wider tire in a rim than it was designed for?
Michelin gave this response: “We do not recommend putting the 275/40ZR18 Michelin Pilot Sport PS2 in a wheel with a rim width less than 9 inches wide. A smaller rim size can cause irregular wear, poor handling, cracking, and other issues. We do offer the Michelin Pilot Sport PS2 in a 245/45ZR18.”

Others have already done this – so it can be done. But what actually happens? Can anyone offer a more technical answer?

thebigjimsho
06-08-07, 09:42 AM
Nope, that's about it. Physically, you can mount that tire on a stock rim. But it's not worth it. There is no benefit except for what some people call a cool factor. They think the tire bulging over the rim is cool. I think it looks liks crap. And your handling is not going to be any better. Extra rubber does not help if the wheel width has not changed.

Plenty have done 245/45 fronts and 275/40 rears on stock wheels. Just don't waste your money on PS2s to do it.

rand49er
06-08-07, 10:08 AM
Okay, going from 245 to 275 is of no benefit.

But, what about going to 255 or 265? (Not sure if PS2s come in these sizes, but guess I'm just wondering about this in general. :confused: )

The Tony Show
06-08-07, 10:25 AM
You'll actually have more sidewall flex by stuffing a wider tire on a narrow rim, which will result in a poor contact patch on your outside rear tire under hard cornering. The possibility also exists for the rim to stress the sidewall and cause a belt seperation inside the tire.

Most guys who have gone to staggered setups are the drag racing sort -If you widen the rear wheels and put a staggered setup on it can be good for that. As far as road racing though, I think staggered is a poor choice as it promotes understeer (greater contact patch = higher lateral grip), and we don't need any more understeer than we already have.

GMBOUND
06-08-07, 11:12 AM
Tony how bout you do a Forza simulation for him?

Luna.
06-08-07, 11:37 AM
Agree with BigJim and Tony's comments.

While you will get more rubber on the road with a 275 vs. a 245 (on an 8.5" rim), there are other considerations that, in my mind, make it not worth it. The tire really has to fight to not rollover the rim, resulting in extra sidewall stress, you aren't getting the full benefit (rubber on the road) of the 275 tire, as it's squeezed on a smaller rim, etc.

I applaud Tony for mentioning understeer with wider tires in the rear. I believe this to be true. It's probably best to run the same width tire all the way around for roadracing; I think Mallet runs 275s all the way around for that matter. For a car as heavy as the V, which already understeers more than it oversteers, that isn't necessarily a good thing.

thebigjimsho
06-08-07, 11:45 AM
Agree with BigJim and Tony's comments.

While you will get more rubber on the road with a 275 vs. a 245 (on an 8.5" rim), there are other considerations that, in my mind, make it not worth it. The tire really has to fight to not rollover the rim, resulting in extra sidewall stress, you aren't getting the full benefit (rubber on the road) of the 275 tire, as it's squeezed on a smaller rim, etc.

I applaud Tony for mentioning understeer with wider tires in the rear. I believe this to be true. It's probably best to run the same width tire all the way around for roadracing; I think Mallet runs 275s all the way around for that matter. For a car as heavy as the V, which already understeers more than it oversteers, that isn't necessarily a good thing.Which is why while I'm running 275/40 18 PS2s on widened stock rims, I'm still tracking the V on 245/40 Hoosiers all the way around...

V-Love
06-08-07, 11:46 AM
As far as road racing though, I think staggered is a poor choice as it promotes understeer (greater contact patch = higher lateral grip), and we don't need any more understeer than we already have.

Man, I'm going to be so much faster now that I know that. Plus, I'm ordering tires today. Thanks.

thebigjimsho
06-08-07, 11:46 AM
Although, I do see you can get Hoosier R6 rubber in a 255/40 18 size. You don't see that size too often on a race tire...

LV_V
06-08-07, 11:55 AM
In addition to all of the above, throwing a 275 on the rear stock rim will also add unnecessary rotating mass to your rear wheels (weight of 275 > 245). And its on the outside of the wheel, even worse.

lawfive
06-08-07, 02:54 PM
i run staggered but got my rear rims widened 3/4" for 275's more expensive more unsprung weight more understeer looks cool though so i get laid a lot full stop

LV_V
06-08-07, 03:31 PM
Well as long as it gets you laid more then I think everyone should do it! If I didn't care about getting laid I would be driving a Prius...

thebigjimsho
06-08-07, 04:02 PM
i run staggered but got my rear rims widened 3/4" for 275's more expensive more unsprung weight more understeer looks cool though so i get laid a lot full stopI'm guessing you're posting remotely...:alchi:

Dreamin
06-08-07, 05:51 PM
...interesting how the people who haven't tried 275's on stock rims are the most opinionated about them...

CTSV_Rob
06-08-07, 05:56 PM
...interesting how the people who haven't tried 275's on stock rims are the most opinionated about them...
They are just trying to justify why they didn't do it. :stirpot:

thebigjimsho
06-08-07, 07:52 PM
They are just trying to justify why they didn't do it. :stirpot:Because I'm not stupid.

Look, I used to own a '92 SHO which had 215/60 16 tires on 6" wide wheels...stock! I've driven SHOs which had 235 rubber on 6" wide wheels. And I know the difference from 1 to the other. If you want to go tooling around on stock rims with 275 rubber, that's cool. But I'll be faster...

CTSV_Rob
06-08-07, 08:02 PM
Because I'm not stupid.

Look, I used to own a '92 SHO which had 215/60 16 tires on 6" wide wheels...stock! I've driven SHOs which had 235 rubber on 6" wide wheels. And I know the difference from 1 to the other. If you want to go tooling around on stock rims with 275 rubber, that's cool. But I'll be faster...
I'm 245's still, just wanted to see if I could get a rise out of someone.

BTW, I don't think running 275's on a 8.5" rim will determine if your stupid. There are many things that could like driving a Ford possibly... :bouncy:

thebigjimsho
06-08-07, 08:24 PM
I'm 245's still, just wanted to see if I could get a rise out of someone.

BTW, I don't think running 275's on a 8.5" rim will determine if your stupid. There are many things that could like driving a Ford possibly... :bouncy:Easy to say now. But Ford had a Yamaha engine that could not be touched by another V6 until about 7 years after SHO production started...

CTSV_Rob
06-08-07, 08:41 PM
Easy to say now. But Ford had a Yamaha engine that could not be touched by another V6 until about 7 years after SHO production started...

I was just yanking your chain, awesome car in it's day.


...But I'll be faster...

I'd still race you even though your V may be quicker!

thebigjimsho
06-09-07, 01:23 AM
I was just yanking your chain, awesome car in it's day.



I'd still race you even though your V may be quicker!HAHAHA Bring it!

CTSV_Rob
06-09-07, 01:48 AM
:burn:

psycho43142
06-10-07, 05:43 PM
I believe if you check the CTSvr's run 275/35's up front and 335/30's in the rear if wider tires caused understeer wouldn't they heve went away from this. There are alot of cars that come this way from the factory, albeit with different width rims, so inherently it must not be a bad idea. I'm also of the belief these cars handling is very nuetral to loose and only push if going into a corner to fast.

Chris

thebigjimsho
06-10-07, 05:55 PM
I believe if you check the CTSvr's run 275/35's up front and 335/30's in the rear if wider tires caused understeer wouldn't they heve went away from this. There are alot of cars that come this way from the factory, albeit with different width rims, so inherently it must not be a bad idea. I'm also of the belief these cars handling is very nuetral to loose and only push if going into a corner to fast.

ChrisEvery car is different. Of course you can have a car that has a staggered setup and that will oversteer. But ours came pretty neutral from the factory. Adding rear rubber is going to add understeer. So, playing with air pressure, ride height, spring rates, alignments, sway bars can all get back that neutral feel.

But, apples to apples, an identical V going from 245 F+R to 245 F, 275 R will gain understeer.

mbiker97_old
06-10-07, 07:54 PM
I believe if you check the CTSvr's run 275/35's up front and 335/30's in the rear if wider tires caused understeer wouldn't they heve went away from this. There are alot of cars that come this way from the factory, albeit with different width rims, so inherently it must not be a bad idea. I'm also of the belief these cars handling is very nuetral to loose and only push if going into a corner to fast.

Chris

How about the extra 100hp that they mention on the same website? Or suspension setup? You can not compare the 2 vehicles in any way.

CTSV05
06-11-07, 11:57 AM
its time for Us to replace tires, I'm going to a 255/45 18, same KDW's. The tire is just a touch taller, .39" overall, and 4 tenths wider, nothing dramatic but should change the look a bit.

This will be all four tires, I like rotating them.

The Tony Show
06-11-07, 12:36 PM
The CTS-VRs have a much lower center of gravity and full race suspension (in addition to the 100 extra hp mentioned above). Kind of tough to compare them to a street CTS-V.

CrashCadillac
06-11-07, 01:23 PM
Thanks for all the sage advice on tires and rims!

I will stay with 245’s all the way around. I am tempted to widen the rims but I’m not crazy about using spacers. I read in previous posts that if you only go with rims a half an inch or so wider with 1/8” or less spacers - there is no problem with stud length, wheel balance, that sort of thing. I plan to keep the car forever so if/when I go to a 427, I will probably need to purchase 10” rims with the right offset for 295’s in the rear. V wheels can be widened by an inch and an a half but then that gets into some significant spacers. I have the Cadillac split spoke accessory wheels and I love the way they look, so I will stay put for now.

Going to PS2’s should be a huge improvement over the Dunlop Direzza DZ101 tires I now have.

psycho43142
06-11-07, 05:17 PM
Maybe you need to look at the rules for world challenge, lowered yes but only bolt on suspension mods and bushings not as drastic as you might think. I also believe that there are a few members here who have as much if not more power than the r's 500 horse at the crank. I'm probably going to order a set of the optional wheels in the next week or so to put a set of 275/35 Hoosier A6's all the way around to use at AX events, yes on stock 8.5 inch wheels. People are running 285/30 A6's on 8 inch wheels for the Pontiac Solstice. I don't think I would try a tire that wide for the street but for the 6 min of runs at an AX I think would be fine.

Chris

The Tony Show
06-11-07, 05:33 PM
Maybe you need to look at the rules for world challenge, lowered yes but only bolt on suspension mods and bushings not as drastic as you might think.
Chris

With all due respect, I've photographed every inch of the World Challenge car up close and personal. You'll be hard pressed to find much on that car that's stock past the V badge. Pay attention to the upper control arms and toe control rods in the front suspension pics -Those are some high dollar, pieces.

http://www.cadillacforums.com/forums/attachments/cadillac-cts-v-series-forum/28795d1174003402-cts-vrs-sebring-today-diff-pics-dscf0145.jpg

(Thanks MarkH for the rear diff pic- I didn't have any of those on my laptop)

mbiker97_old
06-11-07, 08:27 PM
but only bolt on suspension mods and bushings not as drastic as you might think

Bolt on mean nothing as a racing company can easily make anything bolt on.

psycho43142
06-11-07, 09:42 PM
Yes, espicially Pratt and Miller with GM footing the bill. I never said the bolt ons were cheap but that it was all basedand 0n the production car not a one off custom race car chassis from the ground up like say a Trans Am/GT1 car. A few members have the Ground Control coil over kit and that alone would allow you to mix and match spring rates to get the car balanced again with a wider rear tire which would also allow more traction off the corner to get up the straight and that's what driving these cars on a road coarse is all about, brake early, turn in late, apex late and make the straights drag strips.

Chris

thebigjimsho
06-11-07, 11:08 PM
http://www.cadillacforums.com/forums/attachments/cadillac-cts-v-series-forum/28795d1174003402-cts-vrs-sebring-today-diff-pics-dscf0145.jpg

So who took this picture while you stood at the front of the car?