What is the widest 18" tire you can fit on the stock front fenders?
What is the widest 18" tire you can fit on the burnout front fenders?

I will fit the rims to the tires. I do NOT want a stretched look tire. Pics would be nice if you have them.

What is the largest back spacing for the rear to get it all the way inside without rubbing on an 18" rim with a NON STRETCHED TIRE?

 

Dependent somewhat on suspension setup, camber, spring/shock setup, etc.. You can run a 295 up front with a 10-10.5" when with a +48-52 offset. For the rear a 305-315 is achievable with a 18x10.5-11" wheel with a +44-
- +50 offset. Rolled fenders of course, and aftermarket rear trailing arms offer more inboard clearance for higher offsets.

Check this thread out:

https://www.rx7club.com/showthread.php?t=892901

 

To provide answers to the tire fitment questions we would need to know what negative camber you will be running and do mean completely stock fenders or would you consider a fender roll on the stock fenders. Stock rear shock style or coilover?

Personally, I run 18x10.5 +38 with 265/35-18 all around, completely stock rear fenders and the lip rolled under on the front fenders, -2.4 deg front and -2 deg rear camber.

That fitment may look "stretched" to you though it is recommended by the tire manufacturer. You could use the same offset and a narrower wheel if you are going for a certain look over maximum grip.

With autocross and spirited driving on 11k springs/tripoint front bar I wear the inside and outside of the tires evenly, but do leave a bit more rubber in the middle.

I have on order 18x11 +45 and 295/30-18 all around.

This fitment places the outer edge of the wheel/tire in the same spot relative to fenders as what I have now and comes inboard as far as you can without having to use an aftermarket rear trailing arm, offset rear coilover bushing and bash the body of the car in inside the rear wheel arch.

45 offset is the magic number for maximum tire fitment, but it is hard to find wide wheels with that deep of an offset.

 

My car

 

I have aftermarket trailing arm and I have Tein HA Coil Overs.

I want to fit:
295/35/18 on 18" x 10" offset?
335/30/18 on 18" x 12" Offset?

I know it wont fit in the rear outer side but I have someone to flare it.
I am more concerned with the front.

 

What are your goals? You do realize that larger diameter and wider tires hurt straight line grip?

I see a lot of people going wider and wider which is great for turning, but at some point it might start to effect straight line grip.

 

My goal is to have a street car that doesnt look odd because it have 245's and 335's. want something a little more even.

 

Ummm...what? Every drag racer, pretty much ever, seems to disagree with that. More contact patch = more traction.

 

take the entire sentence into context.

larger diameter and wider tires

Mehcanisms of Tire grip.

Friction
Mechanical Keying
Adhesion

As they are madr from deformable rubber, tires dont exactly follow basic friction theory. So to just say that F=mu*r is strictly wrong. Although it basically does floow this during the elastic range.

Tires only give grip when they are at the correct temperature. this is the most important thing you can remember about tires .


Now on to business.

Tires are the most important part of the suspension, either they are designed to fit it, or the suspension is designed around the tire.

People who say wider tires make more grip because ;''there is more rubber on the road' are wrong. They are both wrong that it makes mroe grip and that there is more rubber on the road.

When you make a tire wider, you alter the contact patch to be wider , but it reduces in length. So depending on sidewall stiffness, a wider tire can actually give less rubber on the road.

The main reason for tires being the size they are is actually heat management. Wider, low sidewall tires will cool better than narrow tall tires. If you can;'t get a tire up to temperatie it will give no grip, if you get it too high you will cook the rubber and ruin the set of tires.

Back to contact patch, you can safely assume that contact patch stays roughly the same area with wide or narrow tires (as long as the load stsys the same). Narroe will have longer contact patches and wide tires will have shorter.



The reason why F1 tires are wide is primarily so that they dont cook (remember they arent just wide, they are fairly high sidewalled), but they ten dto have wider contact patches because they will give better grip going round corners.

A wide tire will generate more lateral force per slip angle making cornering better. F1 cars DO NOT have wide tires for linear acceleration.


Conversely (Mike im goin to have to disagree with you here) drag racers acutally use the tires not becuase of the width, but the tallness. As we know a wider contact patch gives better cornering performance, a narrow but long contact patch is what you want for linear acceleration.

so strangely, drag racers will actually be better with narrow tires. So why do they use wide tires? (remember the most important gip aspect of tires) Temperature! They want as longer contact patch as they can get, but need the width for cooling. (eith 4000+ horsepower you do kind of build temp rather well)

But if you look at the contact patch shape between say, an F1 car or drag racer. (both are considered to use wide tires). The F1 patch will be wider and shorter for good cornering, the drag patch will be longer and narrower (relatively) for good linear acceleration.


So to sum up: Wider tires are not always better. They dont always give better traction. It depends on the car, the situation, the conditions.

Eg. Rally cars use wider tires when on tarmac rallys, and use (surprisingly) very thin tires on ice rallys.
F1 cars used to use narrow tires until aero began to be used in the 60's.
Drag racers acutally want tall tires, width is there to stop the tire being destroyed.

TEMPERATURE!!!!!! rawr.

As example I can think of is formula student cars, they used to use 8inch tires but couldnt get them up to temperature. They switched to 6.5inch and got more grip because they can get them up to temp.

 

I have aftermarket trailing arm and I have Tein HA Coil Overs.

I want to fit:
295/35/18 on 18" x 10" offset?
335/30/18 on 18" x 12" Offset?

I know it wont fit in the rear outer side but I have someone to flare it.
I am more concerned with the front.
__________________

Issue with front tire height-

On my car shown above I would not be able to run the taller 295/35-18 up front as it would rub the rear of the front fender arch (where the mud flap is) and the front of the wheel arch at the fender liner.
My front tires were .3" taller than stock 25" and it clearanced the mudflaps turning. I have to periodically take the mudflaps off and remove any debris between the body and mudflaps so they don't rub- its close.
I also have an issue with the top of the front tire rubbing the fender liner on bumps when I am driving hard with my 11k springs at the height shown. Taller heavier wheel/tire will exacerbate this problem.

I believe 45 offset is correct up front with a 295 width tire (obviously, as thats what I ordered). But I think you will find a 285/30-18 or 295/30-18 is required for the shorter height up front.

Issue with rear tire height and width-

The rear is more forgiving of a taller than stock tire. One issue to watch though is that when you try to put a wide tire in there the taller tire will rub the unibody more in the spot you have to bash in and rub more on the trailing arm as it is angled out toward the chassis side.
With a 305, 315 or your desired 335 you will have to move outward from the ideal 45 offset that would center the tire between the suspension and the wheel arch.

I plotted long and hard on how to fit a 18x12 with 295/30-18 all around and it wasn't going to happen without a rear fender pull or aftermarket trailing arm and offset shock bushing. I didn't want to pull and the trailing arm wasn't allowed in my class so I went with 18x11.
Blah blah, but what this means for you is I took lots of measurements and lets see if I can get them to benefit you as well.

My 18x10.5 +38 at -2 deg camber was 18mm from my shock lower mount (the body of the coilover below the threaded part) at droop and 21mm from it at ride height. This tells me it will be closer on full compression, so we have to leave a little room.
I decided 5mm was enough relative to the droop travel versus compression travel. In my case that meant I would use a 18x11 +45.

In your case it would mean you would use a 18x12 +30, but since you want to run a tire wider than the rim you have an additional clearance issue. 18x12 +30 with 335 looks about right.
The wheel will stick out 38mm or 1.5" from what you see in the rear of my car in the pictures and the tire a couple more mm. Plus since you seem to be focused on straight line traction you will probably want -1deg camber instead of -2 so that is another 12mm out at the top of the tire.

You will need an add on rear flare.

You could gain a little room inboard by using an offset shock busing or inboard shocks in the hatch with a rod where the stock shock mounts, but you are going to be right at the edge of what you can fit with your aftermarket trailing arms anyways, so its not worth the couple mms.

Here is a good site to play with different fitments-
http://www.willtheyfit.com/index.php...&Submit=Submit

 

In your case it would mean you would use a 18x12 +30, but since you want to run a tire wider than the rim you have an additional clearance issue. 18x12 +30 with 335 looks about right.

Sorry, should read "18x12 +20 with 335 looks about right." as shown in the wheel fitment site link.

 

SO I spoke to HRE and they say:
front needs to be measured since they dont have anything solid for this application
rear is 18" x 11" +51 with 315/30/18

recommend front to be 18" x 9" with 265/35/18

What do you guys think?

 

A common fronts is 18x10 +50 with a 285/30. You have a little bit of wiggle room on offsets and widths , as Blue TII described above, but that will work.

 

this also doesn't effect the scrub radius.

 

So would the rear fit 18" x 11" +51 with 315/30/18 Michelin PS2

Tein HA Coil overs and aftermarket trailing arms.

Front will be 18" x 10" 285/30/18 +50

Sound about right for a stock body?

 

Front will be 18" x 10" 285/30/18 +50

Front will fit with rolling the front fender lip under and at least neg 2 deg camber.

So would the rear fit 18" x 11" +51 with 315/30/18 Michelin PS2

The rear wheel may need a 5mm spacer for the rim not to hit the shock if you want to run some negative camber in the rear.

The 315 rear tire will probably need another 5mm (so 10mm spacer total) not to rub the unibody on the inside front with zero toe-in. If you want toe-in you will probably need a thicker spacer.
On the outside edge the rear tire will require a fender roll with a pull or cut and add a flare.

 

My $0.02 on the backspacing - 7.25 inches.

The ideal is often stated to be 7.5, but that depends a bit on the tire.

My FD has Forgestar 18x10+50 all the way around, with Hoosier 285/30/18.

To prevent rubbing on both the inner fender the trailing arms (Improved Racing) I had to install 5mm spacers on the rear, effectively reducing the backspacing by about .25 inch.

Fenders are stock, but the lips are rolled.

Something to keep in mind.