Wheel size question

[ahtim]

Hi guys, since the stock size on 1m is 245/35/19 and 265/35/19, which means the rear is slightly bigger, if I am going to change Tyre size to 18 , do I have to make sure the rear is slight bigger or can it be the same or can the front be a tiny bit bigger?

I saw that the safety car with 255/35/19 and 285/30/19, which means the front is tiny bit bigger,

I am thinking to get 255/35/18 with 295/30/18, or 265/35/18 with 295/30/18
Or 245/35/18 with 275/35/18

Thanks in advance

[PRTSB1M]

Originally Posted by ahtim

Hi guys, since the stock size on 1m is 245/35/19 and 265/35/19, which means the rear is slightly bigger, if I am going to change Tyre size to 18 , do I have to make sure the rear is slight bigger or can it be the same or can the front be a tiny bit bigger?

I saw that the safety car with 255/35/19 and 285/30/19, which means the front is tiny bit bigger,

I am thinking to get 255/35/18 with 295/30/18, or 265/35/18 with 295/30/18
Or 245/35/18 with 275/35/18

Thanks in advance

If you look at weight distribution of the 1M, I think it is something like 51/49 or 52/48; meaning a tiny bit of more weight towards the front. A shorter tire in the rear will drop the rear axle and likely balance this out to perfect 50/50, while original size allows rear axle to break loose at lower speeds to "excite" most buyers. I have the 255/35 and 285/30 "1M safety car/M3 GTS" setup on my stock wheels now and it definitely feels glued to the road where original tires would slide around on command. I am waiting for 265/35 and 295/30 tires to arrive for my 18s.

[blue135]

Comment seems backward to me - bigger tire will be harder to break loose than smaller tire. What am I not seeing?

[PRTSB1M]

Originally Posted by blue135

Comment seems backward to me - bigger tire will be harder to break loose than smaller tire. What am I not seeing?

I'm not sure what you mean by "bigger". The contact patch (how much rubber touches the road) of any tire will not be bigger simply by putting wider tires on the car, only the shape of the contact patch will change.

To make more rubber touch the road, you must put more pressure on the tire; either through applying more weight to the axle or changing the mechanics/geometry of the suspension. If you don't understand, maybe you should ask BMW why they used this setup on their track focused version of the M3?

[CarJunkie]

Originally Posted by blue135

Comment seems backward to me - bigger tire will be harder to break loose than smaller tire. What am I not seeing?

I agree with you. A shorter tire is the same as putting a shorter gear set in the rear differential. You will travel a lower speed at the same engine RPM and have more mechanical advantage from drivetrain to road. My square set up has a rolling diameter similar to stock on the front and much smaller than stock on the rear 285/35/18 all around. Despite having treadwear 100 tires on them (Toyo RA1) I break traction more easily than I do with my stock sized PS4S in 1st and 2nd gear.

[ahtim]

Originally Posted by PRTSB1M

If you look at weight distribution of the 1M, I think it is something like 51/49 or 52/48; meaning a tiny bit of more weight towards the front. A shorter tire in the rear will drop the rear axle and likely balance this out to perfect 50/50, while original size allows rear axle to break loose at lower speeds to "excite" most buyers. I have the 255/35 and 285/30 "1M safety car/M3 GTS" setup on my stock wheels now and it definitely feels glued to the road where original tires would slide around on command. I am waiting for 265/35 and 295/30 tires to arrive for my 18s.

Thanks bro, I will get 265/35/18 and 295/30/18,

[ahtim]

Originally Posted by CarJunkie

I agree with you. A shorter tire is the same as putting a shorter gear set in the rear differential. You will travel a lower speed at the same engine RPM and have more mechanical advantage from drivetrain to road. My square set up has a rolling diameter similar to stock on the front and much smaller than stock on the rear 285/35/18 all around. Despite having treadwear 100 tires on them (Toyo RA1) I break traction more easily than I do with my stock sized PS4S in 1st and 2nd gear.

What size front are you using?

[CarJunkie]

Originally Posted by ahtim

What size front are you using?

Same as rear (square setup)

[PRTSB1M]

I think this better explains how the rear axle sitting lower and the front axle sitting higher affects roll and overall grip.

https://suspensionsecrets.co.uk/roll...d-roll-moment/

[ahtim]

Originally Posted by PRTSB1M

I think this better explains how the rear axle sitting lower and the front axle sitting higher affects roll and overall grip.

https://suspensionsecrets.co.uk/roll...d-roll-moment/

This is complicated science, in terms of physics, I have another FR car with 285/30/20 and 315/35/20, and I cant use any Tyre other than pss or ps4s, I guess changing the rear to 325/30/20 should make it more grip too , and I can then use cup2

[PRTSB1M]

Haha, for sure, it starts and stays heavy for the most part. If you scroll to almost the end there is a more simple explanation of how pushing to the limit results in wheel lift at either the front (RWD) or the rear (FWD) inside wheel during high-G turns.

When the two roll centres are joined together by a line from the side view (the neutral roll axis) it will either incline towards the rear or decline towards the rear. A car that has an inclining NRA towards the rear, like most front wheel drive cars, will tend to lift a rear wheel during cornering if the angle is steep enough. If the NRA is declining towards the rear of the car, like in the application of a rear wheel drive car, then the car will tend to lift the inside front wheel during cornering if the angle is steep enough once again. Lifting wheels does also depend on the roll stiffness of the roll bars and coils spring rates as well but as most race cars are stiff enough to lift wheels it proves to be a good visual representation to remember which way the roll centres should be set up on your car.

If I understand it correctly, the more a car's NRA is biased towards the front, the more likely it will lift rear wheel. For more rear traction, put more pressure on the rear tires, make the NRA slope more towards rear axle. The front tire being taller and the rear tire being shorter will do this.

BMW mastered and designed M cars with controlled loss of traction at rear wheels, and provides MDM to not lose control. E9x was first M car to come with taller rear tire. The E9x M3 rear differential slips a wheel first before locking and was inherited from E46 M3, so you can see the iteration of engineering towards this goal of controlled traction loss.