[lixiang7349]

Guys,
I need to pick on your brains.
I just did some calculation and turns out my front and rear wheels want to move at different speed.
I have 2019 M240i xDrive with 405M performance wheels out of the factory and these are the specs:
7.5 x 19 225/35R19Y front. 8 x 19 245/30R19Y rear
When I put this in the online calculation (https://tiresize.com/speedometer-calibration/)
when front wheels want to travel at 65 MPH, the rear wants to travel at 63.93. I mean won't that hurt the performance of the car given it's AWD?
Would having aftermarket square setup give better performance?
Am I missing something here? I mean why would BMW engineers do this?

[dradernh]

Originally Posted by lixiang7349

I mean won't that hurt the performance of the car given it's AWD?

First, I am no expert; so, I recommend speaking with experts, like those at the Tire Rack. No affiliation, mind you.

My understanding is the principal AWD tire consideration is that of having (nearly) equal tire diameters. In the Michelin PS4S, a front 225/35ZR19 has a diameter of 25.2" and a 245/30ZR19 has a diameter of 24.8". Is 0.016% too much difference? I don't know; again, I recommend consulting professionals – they know this stuff cold.

Regarding "hurt[ing] performance", it might help if you define what you mean by 'performance'.

[lixiang7349]

Originally Posted by dradernh

First, I am no expert; so, I recommend speaking with experts, like those at the Tire Rack. No affiliation, mind you.

My understanding is the principal AWD tire consideration is that of having (nearly) equal tire diameters. In the Michelin PS4S, a front 225/35ZR19 has a diameter of 25.2" and a 245/30ZR19 has a diameter of 24.8". Is 0.016% too much difference? I don't know; again, I recommend consulting professionals – they know this stuff cold.

Regarding "hurt[ing] performance", it might help if you define what you mean by 'performance'.

I think you meant 1.6%.

What I mean by hurting performance is that I believe the performance should be maxed when the diameter is perfectly matched with 0% difference instead of a small difference, at least by design. In practice things vary a little due to manufacturing.

Think about how bad the performance would be if you have a 5% mismatch.

I was thinking maybe it's because there's 60% torque rear and 40% front so that's why they make them different? But I still believe the diameter should be matched by design to max performance.

[cooolone2]

Rule of thumb is no more than 2% difference in diameter and you're good to go!

[marcthedark]

They don't do it for technical reasons (I'm sure) but for optics/customer demand. A 255 in the back would probably get close to equal (at least for 18 inch) but 245 is the largest tire they would put on the rim.

[Mason Hatcher]

I am just a simpleton, so explain to me how for any given MPH the front and rear tires can possibly be traveling at different mph. Different rpm I can see.


What am I missing?

[dradernh]

Originally Posted by lixiang7349

I think you meant 1.6%.

I did mean 1.6% - thanks.

[MassNerd]

Originally Posted by Mason Hatcher

I am just a simpleton, so explain to me how for any given MPH the front and rear tires can possibly be traveling at different mph. Different rpm I can see.


What am I missing?

"Traveling at a different mph" is not the best way to describe it. What the OP is talking about is concern that the wheels are spinning at a different RPM. Car moving at speed, but wheels of different diameter means wheels spinning at a different RPM which can be tough on the differentials.

[cooolone2]

Originally Posted by Mason Hatcher

I am just a simpleton, so explain to me how for any given MPH the front and rear tires can possibly be traveling at different mph. Different rpm I can see.


What am I missing?

Your missing the part that you're looking too deeply into this. It's real simple as the diameter between the front and rear being 2% or less traveled over a distance eqates to less of a difference than matters.

Have you considered the gears and ratios between front and rear? That the gear boxes themselves have deviation tolerance? Is why 2% is a defined spec. If you wanted to get them close as possible, you'd want Front 225/35/19 - Rear 255/30/19 = -0.8% or 6 revolutions per mile... Lol. Just not sure if the 255's will rub in the rear

[lixiang7349]

Originally Posted by cooolone2

Your missing the part that you're looking too deeply into this. It's real simple as the diameter between the front and rear being 2% or less traveled over a distance eqates to less of a difference than matters.

Have you considered the gears and ratios between front and rear? That the gear boxes themselves have deviation tolerance? Is why 2% is a defined spec. If you wanted to get them close as possible, you'd want Front 225/35/19 - Rear 255/30/19 = -0.8% or 6 revolutions per mile... Lol. Just not sure if the 255's will rub in the rear

Originally Posted by cooolone2

Your missing the part that you're looking too deeply into this. It's real simple as the diameter between the front and rear being 2% or less traveled over a distance eqates to less of a difference than matters.

Have you considered the gears and ratios between front and rear? That the gear boxes themselves have deviation tolerance? Is why 2% is a defined spec. If you wanted to get them close as possible, you'd want Front 225/35/19 - Rear 255/30/19 = -0.8% or 6 revolutions per mile... Lol. Just not sure if the 255's will rub in the rear

Well I can just do 225/35r19 for all 4. Problem solved.

[lixiang7349]

Originally Posted by MassNerd

"Traveling at a different mph" is not the best way to describe it. What the OP is talking about is concern that the wheels are spinning at a different RPM. Car moving at speed, but wheels of different diameter means wheels spinning at a different RPM which can be tough on the differentials.

Originally Posted by cooolone2

Your missing the part that you're looking too deeply into this. It's real simple as the diameter between the front and rear being 2% or less traveled over a distance eqates to less of a difference than matters.

Have you considered the gears and ratios between front and rear? That the gear boxes themselves have deviation tolerance? Is why 2% is a defined spec. If you wanted to get them close as possible, you'd want Front 225/35/19 - Rear 255/30/19 = -0.8% or 6 revolutions per mile... Lol. Just not sure if the 255's will rub in the rear

Originally Posted by MassNerd

"Traveling at a different mph" is not the best way to describe it. What the OP is talking about is concern that the wheels are spinning at a different RPM. Car moving at speed, but wheels of different diameter means wheels spinning at a different RPM which can be tough on the differentials.

No the angular frequency of the front and back are force to be the same but different diameter will result in different speed traveled on the ground which is a problem.

[Mason Hatcher]

Originally Posted by cooolone2

Your missing the part that you're looking too deeply into this. It's real simple as the diameter between the front and rear being 2% or less traveled over a distance eqates to less of a difference than matters.

Have you considered the gears and ratios between front and rear? That the gear boxes themselves have deviation tolerance? Is why 2% is a defined spec. If you wanted to get them close as possible, you'd want Front 225/35/19 - Rear 255/30/19 = -0.8% or 6 revolutions per mile... Lol. Just not sure if the 255's will rub in the rear

I don't think you meant to quote me.

[Mason Hatcher]

Originally Posted by MassNerd

"Traveling at a different mph" is not the best way to describe it. What the OP is talking about is concern that the wheels are spinning at a different RPM. Car moving at speed, but wheels of different diameter means wheels spinning at a different RPM which can be tough on the differentials.

Thanks for confirming. Like I wrote, I get that different size tires travel at different RPMs.

[liebetrau88]

So you're concerned with what BMW engineers had decided as being ok? On an AWD system, the only thing that you'll notice is that one set of tires are going to wear quicker until it reaches the same diameter as the other tire, and then from there, it will remain pretty damn close to the same.

[lixiang7349]

Originally Posted by liebetrau88

So you're concerned with what BMW engineers had decided as being ok? On an AWD system, the only thing that you'll notice is that one set of tires are going to wear quicker until it reaches the same diameter as the other tire, and then from there, it will remain pretty damn close to the same.

Right if only there were a way to get rid of that wearing before it performs well!

[dradernh]

Originally Posted by lixiang7349

Right if only there were a way to get rid of that wearing before it performs well!

Having the tires shaved will do that for you.

What do you mean by "performs well"?

[lixiang7349]

Originally Posted by dradernh

Having the tires shaved will do that for you.

What do you mean by "performs well"?

Originally Posted by dradernh

Having the tires shaved will do that for you.

What do you mean by "performs well"?

Would two voltage sources with different voltages do well if they are put in parallel? It's the same principle.

[NISFAN]

You are right to be concerned, in a geared drivetrain there is no 'tolerance' regarding rotational speeds. In the case of xDrive vehicles, the wet plate transfer case clutch becomes the 'adjuster' by slipping. This of course wears it out over time. The more differential in axle pair wheel speeds, the more slippage per mile.

How does BMW arrive at 'the tolerance'? Easy, number of wear rotations stated by manufacturer divided number of rotations capable in life span of car. Life span for BMW being 100,000 miles.

Performance wise, a slipping clutch adds more drivetrain losses, but as this is road speed dependant, it only takes effect at high speeds. Not particularly noticeable unless the salt flats is your usual playground.

Coming back to the OP's question, yes same size tyres all round is the best engineering solution.

[lixiang7349]

Originally Posted by NISFAN

You are right to be concerned, in a geared drivetrain there is no 'tolerance' regarding rotational speeds. In the case of xDrive vehicles, the wet plate transfer case clutch becomes the 'adjuster' by slipping. This of course wears it out over time. The more differential in axle pair wheel speeds, the more slippage per mile.

How does BMW arrive at 'the tolerance'? Easy, number of wear rotations stated by manufacturer divided number of rotations capable in life span of car. Life span for BMW being 100,000 miles.

Performance wise, a slipping clutch adds more drivetrain losses, but as this is road speed dependant, it only takes effect at high speeds. Not particularly noticeable unless the salt flats is your usual playground.

Coming back to the OP's question, yes same size tyres all round is the best engineering solution.

Originally Posted by NISFAN

You are right to be concerned, in a geared drivetrain there is no 'tolerance' regarding rotational speeds. In the case of xDrive vehicles, the wet plate transfer case clutch becomes the 'adjuster' by slipping. This of course wears it out over time. The more differential in axle pair wheel speeds, the more slippage per mile.

How does BMW arrive at 'the tolerance'? Easy, number of wear rotations stated by manufacturer divided number of rotations capable in life span of car. Life span for BMW being 100,000 miles.

Performance wise, a slipping clutch adds more drivetrain losses, but as this is road speed dependant, it only takes effect at high speeds. Not particularly noticeable unless the salt flats is your usual playground.

Coming back to the OP's question, yes same size tyres all round is the best engineering solution.

Exactly. That's why I'm getting a set of 4 forged wheels 18x8.5 235/40R18 in about a week. 🥳

[ImStrokerAce]

Originally Posted by lixiang7349

Exactly. That's why I'm getting a set of 4 forged wheels 18x8.5 235/40R18 in about a week. 🥳

As someone who has considered this as well; how did you find this potential solution?

[nioh_lbbm2]

Just a bystander here but I think op is overthinking this too much. The difference is tiny and you also have to keep in mind the AWD system is rear biased on these cars right? So the rears wear differently regardless

[ImStrokerAce]

I am of the mind that any reduced mechanical stressors are a benefit. One of course has to find a balance between costs in time and money and said stressors.