Drawbacks to 245's on 7.5" rim

[jayro]

Hi all,
I have a set of 16x7.5 rims that I am thinking of using for some r-comp tires. I was looking on tire racks and they were saying that I could fit 245/45's on the rims, but that it would be the limit. What would be the drawbacks on running this wide of a tire on the 7.5" rims? They do have them in 225's also, but if I'm gonna spend the $ I would like to get the most I can out of them. Thanks.

 

[Toolman]

Which brand? Some companies 225's are as wide as others 245's.

 

[jayro]

Which brand? Some companies 225's are as wide as others 245's.

I was looking at the Hankook's. Car is mostly autox'd, but I would like to get to a track once or twice also.

 

[RonPorter]

Look at these pics:

These are 215/50/17 Hankook V12 tires on 17x7.5" rims.

They are PERFECT. Note that the rim width equals the tread width (look carefully to see the dirt patch on the tread). And the sidewalls are perfectly vertical.

You cannot get a better combo than this. For a 225 tire, an 8" rim does the same thing.....BTDT.

Get the 225. The 245 is WAY too wide for that rim. You gain nothing by doing that, except sloppy handling, and you won't get any benefit from the 245 width.

The 225 on that rim will handle better, and have better steering response.

 

[jayro]

You cannot get a better combo than this. For a 225 tire, an 8" rim does the same thing.....BTDT.

Get the 225. The 245 is WAY too wide for that rim. You gain nothing by doing that, except sloppy handling, and you won't get any benefit from the 245 width.

The 225 on that rim will handle better, and have better steering response.

Thanks for the info. I am currently running 225's on them and they do fit well. So basically you are saying that due to the ballooning effect my contact patch would not be any bigger with 245's? I'm looking for maximum lateral grip.

 

[RonPorter]

Thanks for the info. I am currently running 225's on them and they do fit well. So basically you are saying that due to the ballooning effect my contact patch would not be any bigger with 245's? I'm looking for maximum lateral grip.

For maximum grip, you want the situation in my pics.

When you bend in the sidewall. it loses rigidity and you lose contact patch.

Plus, the (too narrow) rim pulls the tire tread in, so you also lose tread contact patch.

Narrower tires on a PROPER rim will outperform a too-wide tire on a narrow rim.

Probably not the best example, but, on sticky 215/45/17 tires on 17x7.5" rims on my LGT, the only guys that usually outran me at SHO Convention track events were guys on track rubber.

 

[jayro]

For maximum grip, you want the situation in my pics.

When you bend in the sidewall. it loses rigidity and you lose contact patch.

Plus, the (too narrow) rim pulls the tire tread in, so you also lose tread contact patch.

Narrower tires on a PROPER rim will outperform a too-wide tire on a narrow rim.

Probably not the best example, but, on sticky 215/45/17 tires on 17x7.5" rims on my LGT, the only guys that usually outran me at SHO Convention track events were guys on track rubber.

I can see what you are saying. If the contact patch isnt any bigger due to the rims pulling in the tread then the wider tires do no good. R-comps run about $500 more than a set of star specs in the same size. Are they THAT much stickier?

 

[RonPorter]

I can see what you are saying. If the contact patch isnt any bigger due to the rims pulling in the tread then the wider tires do no good. R-comps run about $500 more than a set of star specs in the same size. Are they THAT much stickier?

Oh boy, this opens up a whole new discussion!!!

In any case, you ought to be running -1.5 - -2.0 camber on your car if you want decent cornering. I have done this on a few SHOs and the LGT, and with regular tire rotations, doesn't **** the tires.

Now, to really get the most out of track rubber, you need all the negative cambe rthat you can get. like -3.0 or more, if you can.

When I put the coilovers on the LGT, I had them set to -2.0 with RS2 & then Azenis 615 rubber. I had considered getting track rubber, and use the adjustable camber plates to dial it up to maybe -3.0. The folks informed me that, if you just change the camber, you have to readjust the toe, because it changes with camber adjustments. This was more than I wanted to mess with.

If you just want decent handling for occasional track forays, and not a serious track car with track rubber, get the camber to -1.8 - -2.0 or so, and get some Dunlop Star Specs, Hankook RS3s, or Azenis 615Ks, and you are good to go.

 

[SHOspazz92]

I disagree 100% with Ron.

Run a 40 series tire, you will be fine, especially for Auto-X.

Run -2.5 Front, -1.75 rear.

Don't waste your money on Hoosiers, your not fast enough to Max them out anyways (They are maybe 5 tenths faster in the hands of a very good driver then the other R-Comps). Try V710's, or better yet, the Hankook Z214's in the C71 Auto-X compound, (That's what I run).

-Sam

 

[RonPorter]

A 100%??? Your post doesn't sound like it!!

 

[jayro]

I disagree 100% with Ron.

Run a 40 series tire, you will be fine, especially for Auto-X.

Run -2.5 Front, -1.75 rear.

-Sam

I can get them in a 45 series. It says the 40 series won't fit a 7.5 inch rim. I am running -2.25 up front and -1.25 out back.

 

[Shoaz]

I ran 245 and effectively 245 tires on 7.5" rims competitively for a long time. I had a set of 8" and 7.5" rims, ran the same tires on both (i.e., treated both wheel sets the same) and could never tell the difference between the wheels.

IMHO, 245s are fine on 7.5" rims unless the manufacturer spec says it's not.

And, yes, good, R-comps are better than Star Specs, but won't last as long. For bang-for-the-buck if you're not competing the sticky street tires are often a better choice than R-comps. YMMV, one size does not fit all.

 

[jayro]

Oh boy, this opens up a whole new discussion!!!


If you just want decent handling for occasional track forays, and not a serious track car with track rubber, get the camber to -1.8 - -2.0 or so, and get some Dunlop Star Specs, Hankook RS3s, or Azenis 615Ks, and you are good to go.

This is not my dd. It is my toy that sees mostly autox.

 

[SHOspazz92]

A 100%??? Your post doesn't sound like it!!

Let me Clarify, I disagree 100% on a 245 tire on a 7.5'' or 8'' wheel not fitting, and making the car handle "sloppy".

Here is a picture, comparing your tire fitment, to mine. 245/40/17.

Looks about the same.

-Sam

 

[Toolman]

While we all know Ron is a big proponent of proper sized tires, let us not forget that every manufacturer is different, for wheels and for tires. Some wheels listed as 7.5'' wide are wider than wheels listed as 8'' wide, and some 245's are not as wide as some 225's, and different diameters and aspect ratios will have different actual tread widths for a listed size. There really is no magic number that can be applied to all combinations. So you really have to find out what particular tire will work with your particular wheel.

To illustrate:
A 225/50-13 Hoosier R6 has an 8.5'' tread width, versus a 225/50-15 which has an 8.8'' tread width, versus a 225/50-16 which has a 9'' tread width. Even within the same diameter there can be small differences. Same R6, a 225/40-17 versus a 225/45-17, but with a 0.1'' different tread width.

A 245/45-17 Kumho V700 is 9.1'' wide. while a 245/45-17 R6 is 9.4'' wide. So the 225 R6 is actually closer in size to a 245 Kumho.

 

[thebigjimsho]

In Indy back in '05, I was running aftermarket Mille Miglia wheels on 245/40 18 Hoosier GAC slicks. I kept the tires and ditched the wheels to get another set of CTS-V stockers. It was all to get an extra .5" of wheel width, from 8" wide to 8.5" wide.

Did the 8" wide wheel **** my handling or the benefits of those Hoosiers? No. Those 245 Hankooks will fit and be fine on your rims. Will it be worse than 225s? Probably not noticeably. But wider rims AND wider tires would be better...

 

[RonPorter]

Let me Clarify, I disagree 100% on a 245 tire on a 7.5'' or 8'' wheel not fitting, and making the car handle "sloppy".

Here is a picture, comparing your tire fitment, to mine. 245/40/17.

Looks about the same

-Sam

Ah....no.

You have a good 0.5 - 1.0" of tread hangin' out beyond the rim width.

I'm sure they work fine for you. What I'm sayin' is that if you had the same tire with a treadwidth that matched the rim, the performance would be the same.

That extra treadwidth is unsupported and won't perform as well as it could.

 

[RonPorter]

While we all know Ron is a big proponent of proper sized tires, let us not forget that every manufacturer is different, for wheels and for tires. Some wheels listed as 7.5'' wide are wider than wheels listed as 8'' wide, and some 245's are not as wide as some 225's, and different diameters and aspect ratios will have different actual tread widths for a listed size. There really is no magic number that can be applied to all combinations. So you really have to find out what particular tire will work with your particular wheel.

To illustrate:
A 225/50-13 Hoosier R6 has an 8.5'' tread width, versus a 225/50-15 which has an 8.8'' tread width, versus a 225/50-16 which has a 9'' tread width. Even within the same diameter there can be small differences. Same R6, a 225/40-17 versus a 225/45-17, but with a 0.1'' different tread width.

A 245/45-17 Kumho V700 is 9.1'' wide. while a 245/45-17 R6 is 9.4'' wide. So the 225 R6 is actually closer in size to a 245 Kumho.

So, doing what I say, is to keep the rim width within an inch of the treadwidth whatever each may be.

 

[SHOspazz92]

Ah....no.

You have a good 0.5 - 1.0" of tread hangin' out beyond the rim width.


I'm sure they work fine for you. What I'm sayin' is that if you had the same tire with a treadwidth that matched the rim, the performance would be the same.

That extra treadwidth is unsupported and won't perform as well as it could.

Are you kidding me? It's hardly any more then what you have posted, and no where close to .5'', none the less.

Now, my 275 R-Compounds on the other hand, those baloon out on 17x9's.

But as you can see, I'm using EVERY inch of that tire. So I'll keep with them.

-Sam

 

[RonPorter]

Are you kidding me? It's hardly any more then what you have posted, and no where close to .5'', none the less.

-Sam

Armed Forces have bad optometrists??

Look again. 1/2" MINIMUM.

Just sayin' that, if you had the same tire in a width that matched the rim width, you would perform as well, if not better.

Plus have sharper steering response.

FWIW, I went through four sets of rims on the LGT. Stock 17x7", Rota 17x7.5", DT-brand-I-forget 17x7", Drag 17x7.5". All with 245/45/17 tires.

Without a doubt, steering response was noticeably better with the 7.5" rims. That was my test, and it confirmed what I learned with tire & rim widths on other cars.

After my track use and other driving, the RS2 and 615s on the 7.5" rims were trashed, BUT wear was very even, all the way across the tread width.