[JustinHEMI]

When I'm done with these Pzeros, I'm probably going to want to go to 295/30/20 in the rear while staying OEM size up front.

Any concerns with that? I think the 285-295 increase should be negligible in terms of changing dynamics, but wanted to see if I should be thinking about bumping the front from 275 to 285 as well.

Thanks!

(Not interested in 305s)

[M2siast]

Why do you want to do that specifically? We can discuss it based on your goals.

One thing to note is that the rear will be raised a bit, increasing the rake, since the tires will be taller in the rear. But not by much.

[Tag]

Originally Posted by JustinHEMI

When I'm done with these Pzeros, I'm probably going to want to go to 295/30/20 in the rear while staying OEM size up front.

Any concerns with that? I think the 285-295 increase should be negligible in terms of changing dynamics, but wanted to see if I should be thinking about bumping the front from 275 to 285 as well.

Thanks!

(Not interested in 305s)

You will induce more understeer with that setup. You want to match the stock setup if you go wider. So, if you want 295 in the rear you should put 285 in the front. You can also go with a square setup which will make the car more playful but also might be a little too twitchy. Regardless of what sizes you go with your biggest hurdle will be the overall diameter of the tire, you want it to remain as close to stock as possible.

[babym_gen]

I wouldn't worry about understeer that much, Porsche runs some crazy staggered setups on the 911 GT3, like 315R/265F.

Stay within the traction limits of the front tires and you will be fine. Add some negative camber to improve surface patch contact.

[JustinHEMI]

Originally Posted by M2siast

Why do you want to do that specifically? We can discuss it based on your goals.

One thing to note is that the rear will be raised a bit, increasing the rake, since the tires will be taller in the rear. But not by much.

Because 285s are stretched a bit on these wheels.

Yes there is a little difference, but not enough to matter, IMO.

Originally Posted by Tag

You will induce more understeer with that setup. You want to match the stock setup if you go wider. So, if you want 295 in the rear you should put 285 in the front. You can also go with a square setup which will make the car more playful but also might be a little too twitchy. Regardless of what sizes you go with your biggest hurdle will be the overall diameter of the tire, you want it to remain as close to stock as possible.

That's my concern, how much more understeer? Negligible? Enough to consider upsizing the front too?

Originally Posted by babym_gen

I wouldn't worry about understeer that much, Porsche runs some crazy staggered setups on the 911 GT3, like 315R/265F.

Stay within the traction limits of the front tires and you will be fine. Add some negative camber to improve surface patch contact.

Thank you!

[JustinHEMI]

Here's upsizing the front. Seems like bumping both may be the best option.

[Tag]

Originally Posted by JustinHEMI

That's my concern, how much more understeer? Negligible? Enough to consider upsizing the front too?

Impossible to give you an exact figure but there will absolutely be more understeer. All I can tell you is I would not go that route.

Originally Posted by JustinHEMI

Here's upsizing the front. Seems like bumping both may be the best option.

I like to look at official BMW specs to figure out what difference in the overall tire diameter is acceptable. In this case we can look at the OEM 1000M wheel package. It comes with Front 285/30R20 / Rear 295/25R21 tires. The stock setup is Front 275/35 ZR19 and Rear 285/30 ZR20.

Let's do some number crunching.

275/35/19 = 675 mm diameter
285/30/20 = 679 mm diameter
Difference = 4 mm

285/30/20 = 679 mm diameter
295/25/21 = 681 mm diameter
Difference = 2 mm

There you have it. I would not deviate from these figures. You don't want the tire diameter to be more than 4 mm larger than stock when you upsize your tires. Of course there might be some more wiggle room but you may then start rubbing, etc. Notice, you need a taller wheel in the rear to get close in diameter. If you try to put a 295 on 20" rear wheel the overall diameter will be 656 mm which is far too small. This is also why BMW had to put a larger wheel in the rear to accommodate a wider tire.

[JustinHEMI]

Originally Posted by Tag

Impossible to give you an exact figure but there will absolutely be more understeer. All I can tell you is I would not go that route.



I like to look at official BMW specs to figure out what difference in the overall tire diameter is acceptable. In this case we can look at the OEM 1000M wheel package. It comes with Front 285/30R20 / Rear 295/25R21 tires. The stock setup is Front 275/35 ZR19 and Rear 285/30 ZR20.

Let's do some number crunching.

275/35/19 = 675 mm diameter
285/30/20 = 679 mm diameter
Difference = 4 mm

285/30/20 = 679 mm diameter
295/25/21 = 681 mm diameter
Difference = 2 mm

There you have it. I would not deviate from these figures. You don't want the tire diameter to be more than 4 mm larger than stock when you upsize your tires. Of course there might be some more wiggle room but you may then start rubbing, etc. Notice, you need a taller wheel in the rear to get close in diameter. If you try to put a 295 on 20" rear wheel the overall diameter will be 656 mm which is far too small. This is also why BMW had to put a larger wheel in the rear to accommodate a wider tire.

The bolded will depend on wheels and offset, no?

My new wheels are 19x10 ET14 front, 20x11 ET11 rear. OEM are 19x9.5 ET20 front 20x10.5 ET20 rear.

I'm missing something, for 295/30/20, I'm coming up with 686mm, not 656mm?

[babym_gen]

I have a crazy idea I may try when I get the car:
Keep the same front (275/35r19)
Go bonzai in the back with 325/30r19
It works because the total diameter on 325/30r19 is the same as stock.
Once you lower the rear tire pressure to adjust for the bigger width, you will get crazy acceleration from 0-60 ans on track you won't get as much power-on oversteer when getting out of slow corners.
But you need 12x19 wheels for this.

This setup will also look better because the sidewalls will have the same thickness between front and rear. And the ride should get comfier.

I don't know if the 325 will fit but if you have 1" between the tires and suspension, I think it will. You just need to find a 12" wheel with a reasonable offset (less than ET44).

Downsides are increased *static* understeer, tramlining and reduced brand choice when it comes to tire (the big ones all make 325 tires both for street and track).

[Charley135]

Originally Posted by babym_gen

I wouldn't worry about understeer that much, Porsche runs some crazy staggered setups on the 911 GT3, like 315R/265F.

Stay within the traction limits of the front tires and you will be fine. Add some negative camber to improve surface patch contact.

that is a bad example, there is not weight on the front axle of a gt3 vs an m2

[Tag]

Originally Posted by JustinHEMI

The bolded will depend on wheels and offset, no?

Offset doesn't matter when it comes to the overall diameter of the tire. Wheel size does matter because it dictates what tire size you'll run. It all has to do with size of wheel you are putting the tire on and the tires aspect ratio.

Originally Posted by JustinHEMI

My new wheels are 19x10 ET14 front, 20x11 ET11 rear. OEM are 19x9.5 ET20 front 20x10.5 ET20 rear.

I'm missing something, for 295/30/20, I'm coming up with 686mm, not 656mm?

In my example it was a 295/25/21 not 295/35/20.

I used this tire size calculator: https://www.wheel-size.com/calc/

[JustinHEMI]

Originally Posted by Tag

Offset doesn't matter when it comes to the overall diameter of the tire. Wheel size does matter because it dictates what tire size you'll run. It all has to do with size of wheel you are putting the tire on and the tires aspect ratio.



In my example it was a 295/25/21 not 295/35/20.

I used this tire size calculator: https://www.wheel-size.com/calc/

OK I guess I'm not sure what you're saying then. I'm not going to 20/21, or 20/20, I'm staying 19/20 with the wheel specs noted.

I also used that tool for the scenario I'm considering and it seems like it's good to go;

Front: OEM to 285/35/19.
Rear: OEM to 295/30/20.


OR

I might just live with the "stretch."

[Tag]

Originally Posted by JustinHEMI

OK I guess I'm not sure what you're saying then. I'm not going to 20/21, or 20/20, I'm staying 19/20 with the wheel specs noted.

I also used that tool for the scenario I'm considering and it seems like it's good to go;

Front: OEM to 285/35/19.
Rear: OEM to 295/30/20.


OR

I might just live with the "stretch."

I don't blame you for being confused. Figuring out wheel/tire fitment and offsets is confusing AF. I do however think if you reread this again as if you were reading it for the very first time it might click.

[JustinHEMI]

Originally Posted by Tag

I don't blame you for being confused. Figuring out wheel/tire fitment and offsets is confusing AF. I do however think if you reread this again as if you were reading it for the very first time it might click.

Will do thanks!

[JustinHEMI]

Originally Posted by Tag

Impossible to give you an exact figure but there will absolutely be more understeer. All I can tell you is I would not go that route.



I like to look at official BMW specs to figure out what difference in the overall tire diameter is acceptable. In this case we can look at the OEM 1000M wheel package. It comes with Front 285/30R20 / Rear 295/25R21 tires. The stock setup is Front 275/35 ZR19 and Rear 285/30 ZR20.

Let's do some number crunching.

275/35/19 = 675 mm diameter
285/30/20 = 679 mm diameter
Difference = 4 mm

285/30/20 = 679 mm diameter
295/25/21 = 681 mm diameter
Difference = 2 mm

There you have it. I would not deviate from these figures. You don't want the tire diameter to be more than 4 mm larger than stock when you upsize your tires. Of course there might be some more wiggle room but you may then start rubbing, etc. Notice, you need a taller wheel in the rear to get close in diameter. If you try to put a 295 on 20" rear wheel the overall diameter will be 656 mm which is far too small. This is also why BMW had to put a larger wheel in the rear to accommodate a wider tire.

Originally Posted by Tag

In my example it was a 295/25/21 not 295/35/20.

I used this tire size calculator: https://www.wheel-size.com/calc/

Here's part of what's confusing me with some of your input (which I appreciate by the way, it's the point of the thread, to help me ensure I think through all the possibilities!)


You said putting a 295 on a 20 would be far too small. That's a big part of my confusion and when re-reading, I'm still confused on that.


Also, out of curiosity, why would you use that package to compare to? Is it specified for G87?

[JustinHEMI]

By that calculator for rears;

285/30/20 to 295/30/20 = +6mm

For front;

275/35/19 to 285/35/19 = +7mm



In both cases the calculator yields positive results.

[Tag]

Originally Posted by JustinHEMI

Here's part of what's confusing me with some of your input (which I appreciate by the way, it's the point of the thread, to help me ensure I think through all the possibilities!)


You said putting a 295 on a 20 would be far too small. That's a big part of my confusion and when re-reading, I'm still confused on that.

It has to do with the combination of wheel size and aspect ratio of the tire. Let's look at the rears for example. If you use the tire calc and select 295/20 on a 20" wheel you get 626 mm which is too small compared to stock which is 679 mm. If you go up a couple of sizes in aspect ratio of 295/25 20" you get 656 mm which is still too small. Go up another size to 295/35 20" and you get 715 mm which is too big. So for a 295 wide tire no aspect ratio will come close to stock on a 20" wheel.

Now do the same thing for a 21" wheel. 295/20 21" will be 651 mm, still too small. But, if we go up to the next aspect ratio size we're golden. 295/25 21" is 681 mm, only 2 mm more than stock.

[JustinHEMI]

So 295/30/20 isn't too big.

[babym_gen]

Originally Posted by Charley135

that is a bad example, there is not weight on the front axle of a gt3 vs an m2

That's a plus for the M2!
Bigger rear tires = more understeer
More weight on the front = more oversteer

So all in all, less static understeer than the 911 with the bigger rear tires.

[Tag]

Originally Posted by JustinHEMI

So 295/30/20 isn't too big.

It's too small.

Your wheel wells will have a bigger gap and your spedo will be off. If you want to run that setup by all means, go right ahead.

[JustinHEMI]

Originally Posted by Tag

It's too small.

Your wheel wells will have a bigger gap and your spedo will be off. If you want to run that setup by all means, go right ahead.

Not by much according to the calculator.

285/30/20 to 295/30/20 = +6mm

[Squidget]

I was taught that about 4% diameter difference is the limit. Don't know if that's still the rule of thumb these days.