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      03-31-2021, 10:04 AM   #1
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Rear Axle Transmission. M3 and M4 - explained, Episode 16

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      03-31-2021, 10:15 AM   #2
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I followed this until about 5:00 when he talked about his favorite detail - the left output shaft being slightly larger. I thought nothing that followed even began to address what?, why? or how?
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      03-31-2021, 10:40 AM   #3
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Quote:
Originally Posted by rak299 View Post
I followed this until about 5:00 when he talked about his favorite detail - the left output shaft being slightly larger. I thought nothing that followed even began to address what?, why? or how?
If the driveshaft rotates clockwise, then the left wheel should have more torque than the right wheel. By using a thicker diameter output shaft on the left, the right can catch up to the left. This means there should be better traction.

I believe it's angular momentum in physics
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      03-31-2021, 11:09 AM   #4
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Quote:
Originally Posted by ashtaron14 View Post
If the driveshaft rotates clockwise, then the left wheel should have more torque than the right wheel. By using a thicker diameter output shaft on the left, the right can catch up to the left. This means there should be better traction.

I believe it's angular momentum in physics
appreciate the breakdown and the drawing, help me understand because i was confused too
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      03-31-2021, 11:11 AM   #5
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Quote:
Originally Posted by ashtaron14 View Post
Quote:
Originally Posted by rak299 View Post
I followed this until about 5:00 when he talked about his favorite detail - the left output shaft being slightly larger. I thought nothing that followed even began to address what?, why? or how?
If the driveshaft rotates clockwise, then the left wheel should have more torque than the right wheel. By using a thicker diameter output shaft on the left, the right can catch up to the left. This means there should be better traction.

Should be called angular momentum in physics
I thought the left axle took more toque because the ring gear is on that side?
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      03-31-2021, 01:00 PM   #6
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Quote:
Originally Posted by rak299 View Post
I followed this until about 5:00 when he talked about his favorite detail - the left output shaft being slightly larger. I thought nothing that followed even began to address what?, why? or how?
Actually he does explain it fairly well, albeit in very technical terms. What I understand is that the two output shafts are designed with a different stiffness to minimize harmonic behaviour in lower traction conditions; read reduce axle hop.
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Last edited by CanAutM3; 04-01-2021 at 02:10 PM..
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      03-31-2021, 03:38 PM   #7
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Quote:
Originally Posted by CanAutM3 View Post
Actually he does explain it fairly well, albeit in very technical terms. What I understand is that the two output shafts are designed with a different stiffness to minimize harmonic behaviour in low traction conditions; read reduce axle hop.
Yes, should be spot on. The torsional rigidity is higher for the left axle, leading to different harmonic behaviour both in amplitude and frequency, avoiding under equal load (diff lock) both axles amplify each others distortive dynamics resulting in hop. Is indeed a clever trick!
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      04-01-2021, 02:41 AM   #8
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This is one very VERY advanced differential.
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      04-01-2021, 04:56 AM   #9
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This is one very VERY advanced differential.
Yea, it's a pretty good electronic LSD unit but don't let this video make you believe that it's something new; it's literally the same identical technology used in the F8Xs M cars, of the last generation.

F80 M3 / F82 M4 Active M Differential: Deep Dive and Interview https://f80.bimmerpost.com/forums/sh....php?t=1018024

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      04-01-2021, 05:22 AM   #10
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From some of the comments made by journalists in that the G80 seems to be able to accelerate even with high yaw angles (better than F8x), I was expecting to see some torque vectoring technology. Twin clutch packs. But nope, just the same carry over diff from F80.

The programming that controls diff function is extremely important, perhaps they have taken this to another level? Or maybe it is a just the wide 4S that helps.
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      04-01-2021, 06:07 AM   #11
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Originally Posted by Poochie View Post
Yea, it's a pretty good electronic LSD unit but don't let this video make you believe that it's something new; it's literally the same identical technology used in the F8Xs M cars, of the last generation.

F80 M3 / F82 M4 Active M Differential: Deep Dive and Interview https://f80.bimmerpost.com/forums/sh....php?t=1018024

.
And first introduced in the F1X M5/6
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      04-03-2021, 08:25 PM   #12
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      04-27-2021, 06:15 AM   #13
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Quote:
Originally Posted by CanAutM3 View Post
Actually he does explain it fairly well, albeit in very technical terms. What I understand is that the two output shafts are designed with a different stiffness to minimize harmonic behaviour in lower traction conditions; read reduce axle hop.
This was first implemented over 10 years ago by Cadillac in the CTS-V, so it's nice to see BMW finally getting around to it. Cadillac were the first to finally put a damper (pun, lol) on the IRS "axle-hop" dynamic problem. It's a ridiculously complicated non-linear dynamics problem with many different degrees of freedom. We've all felt it when accelerating hard in an IRS car and one gets the stuttering wheel hop shaking. There is a "wind up" of energy when the tire has grip that eventually (talking in very short timeframes here) is unleashed causing the tire to break traction only to then recycle with another wind up cycle, etc, etc.

Cadillac discovered that if they employ the proper ratio of torsional stiffness between the two IRS half-shafts (based on that huge number of variables overall dynamics problem), they can allow for absorption/release (so to speak) of said energy and greatly reduce the cyclic build-up/release cycle. Said design was first on the gen2 CTS-V but then also shared with other GM cars with an IRS.
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      04-27-2021, 06:18 AM   #14
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Quote:
Originally Posted by CSBM5 View Post
This was first implemented over 10 years ago by Cadillac in the CTS-V, so it's nice to see BMW finally getting around to it. Cadillac were the first to finally put a damper (pun, lol) on the IRS "axle-hop" dynamic problem. It's a ridiculously complicated non-linear dynamics problem with many different degrees of freedom. We've all felt it when accelerating hard in an IRS car and one gets the stuttering wheel hop shaking. There is a "wind up" of energy when the tire has grip that eventually (talking in very short timeframes here) is unleashed causing the tire to break traction only to then recycle with another wind up cycle, etc, etc.

Cadillac discovered that if they employ the proper ratio of torsional stiffness between the two IRS half-shafts (based on that huge number of variables overall dynamics problem), they can allow for absorption/release (so to speak) of said energy and greatly reduce the cyclic build-up/release cycle. Said design was first on the gen2 CTS-V but then also shared with other GM cars with an IRS.
Agreed, this isn’t new breakthrough tech.

Maybe axle hop was more of an issue on the G8X so they had to remedy it.
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