Why is rear end so squirrelly in straight line acceleration?

[Poochie]

Originally Posted by CanAutM3

Say what ??

I comfortably fit 305 rear tires on my M4 with room to spare, as do many other owners. Some managed to fit even wider tires.

I know some folks squeeze in larger tires but in reference to the person I was replying to, typical stock BMW M4s come with 275s in comparison to the 305s in a stock Mustang.

[hellokitty]

Originally Posted by CanAutM3

Say what ??

I comfortably fit 305 rear tires on my M4 with room to spare, as do many other owners. Some managed to fit even wider tires.

What wheels?

[CanAutM3]

Originally Posted by hellokitty

What wheels?

11" wide ET44.

[CanAutM3]

Originally Posted by Poochie

I know some folks squeeze in larger tires but in reference to the person I was replying to, typical stock BMW M4s come with 275s in comparison to the 305s in a stock Mustang.

Stock, M4 comes with either 275 or 285. Further, tread width varies greatly depending on the tire of a given nominal section width, so section width is pretty meaningless.

[TideMD]

Originally Posted by Poochie

I know some folks squeeze in larger tires but in reference to the person I was replying to, typical stock BMW M4s come with 275s in comparison to the 305s in a stock Mustang.

Wait... but that's not what you said. You specifically replied the M4 "only can accommodate 275 max," which is patently false.

[Poochie]

Originally Posted by TideMD

Wait... but that's not what you said. You specifically replied the M4 "only can accommodate 275 max," which is patently false.

Jezz, you guys are touchy..

Ok, fine, I stand corrected.. The point was the Mustang can accommodate larger tires stock than the M4, hence the better traction. I honestly wasn't trying to take anything away from your car.

My apologies; I will bow down tonight, say three Hail Marys and ask for forgiveness..

[Beemerm3]

Sometimes I don't notice my tires slipping when it's finished slipping. But yeh most of the time redlining from 2nd to 3rd. And high rpms in 3rd. I gotta be careful more.

[Faceplant]

If you have stock power and the grippy tires this shouldn’t be an issue. My comp spins first fairly straight, second only if it’s cold. Both both gears I can stay full in and just use steering to keep it in line. These cars have a hard torque hit. My previous m3 came from snow country and on all seasons the car was hard to handle. Iv driven lots of high hp rwd cars. Proper factory tires they are not hard to handle at all imho.

[jmg]

Originally Posted by dezzracer

2018 M3 CS. Seems like with all the hi tech performance based electronics loaded into this car (especially the M e-diff) that one would be able to launch this car without the rear moving around side to side so much. With TC turned fully on it's ok because the tires are barely spinning but in MDM mode the rear ends gets so squirrelly side to side. Geez, I can launch my 500 hp 04 Mustang Cobra better and straighter and it has ZERO electronic aids. I don't get it :

My M3 CS launches in both M Dynamic and DSC OFF just fine in a straight line with some driver intervention. Keep in mind the CS has more torque down low than your mustang and I bet has a little more factory toe front and rear. Factory suspension is certainly tuned for different tasks than a 2004 Mustang.

[Fast4d]

I was pushing 700hp on my fox body (turbo) on 275 drag radials. I had a torque arm and panhard rod.

the F80 needs tires. wider and stickier.

you should also limit boost in lower gears. the rear end seems to react very quick. maybe it's due to solid mounting? I don't really have a problem keeping the back straight launching.

[dezzracer]

Well in the end the fix for me (the OP) is leaving all traction control nannies on now. The car will definitely launch straight then but of course the program does intervene if wheels spin too much which is Ok . I think because the car was new to me I just liked running around with MDM mode engaged and having some fun with it. There is really no need for MDM mode on the street.

So yeah...after thinking about everything that was discussed in this thread I've just kind of changed my thought process on how this car works as well as my driving style on the street. Changed my M button settings too. Now I have to engage MDM mode separately (if I want it) which is no biggie because if you have MDM programmed to your M buttons it asks you to confirm it anyway which requires two pushes.

Thanks for all the replies. It provoked a lot of thought on my part and changed the way I drive the car. Good stuff!

[Mendoteach]

Mine is also much squirrelly-er in MDM mode than than traction control to off. Maybe the diff. can cut power to each side and is too invasive?

[CanAutM3]

Originally Posted by Mendoteach

Mine is also much squirrelly-er in MDM mode than than traction control to off. Maybe the diff. can cut power to each side and is too invasive?

The only thing the diff can do is lock-up the axle, it cannot "cut power" from side-to-side.

[Mendoteach]

Thanks for the clarification. I would like to know more about how the diff. works. Looks like from pictures I have seen it has a couple "clutches" that look like a motorcycle's clutch. So, when not locked up, in a straight line, can one side slip more than another (passively) and when TC is off those "clutches tighten things up?

If anybody can point me to something on the tech the diff. uses I would appreciate it.

[CanAutM3]

Originally Posted by Mendoteach

Thanks for the clarification. I would like to know more about how the diff. works. Looks like from pictures I have seen it has a couple "clutches" that look like a motorcycle's clutch. So, when not locked up, in a straight line, can one side slip more than another (passively) and when TC is off those "clutches tighten things up?

If anybody can point me to something on the tech the diff. uses I would appreciate it.

The F8X e-diff has one multiplate cutch. How much that clutch pack locks-up determines how much the left and right rear axles are locked together. This is all controlled by an electric motor managed by GHAS control unit.

Yes, it could be theoretically possible for the e-diff to be un-locked and have the DSC system individually apply the left and right rear brakes. I however doubt that is the case, since this is how a primitive/basic "active differential" works. The F8X has a diff that can actually lock-up and it is much more efficient from a dynamics stand-point to have the diff locked-up when pulling away in a straight line.

IMO, why folks feel the rear to be "squirrelly" with MDM is mainly because of how the DSC system operates. When DSC detects oversteer, it will apply the front brake on the same side as the slide. Since MDM allows for some wheel spin, when the rear moves to one side, the DSC applies the front brake on that side sending the rear the other way. It applies the left and right front brakes in succession while attempting to keep the car in a straight line. With DSC fully off, the is much less side to side movement. When using full DSC ON, the DSC cuts back on the power more aggressively stopping the rear wheel spin altogether and hence the sliding around.

Screenshots from the F8X technical docs:

[spuntyb]

This is what I *like* about my F80 to be honest. That said, it just takes some practice with modulating the throttle, and I would throw on some MPS4S rubber. Helped my tuned car some.

[Mendoteach]

CanAutM3 thank you very much for the info. on the diff. Your explanation of the "squirelly-ness" in MDM vs. TC off is very logical. Never thought about the front brakes helping out in a straight line. You would think they would only be active if steering there was a certain minimum of steering angle input.

[Spun Crankhub]

1st gear is super short and should not be hard for you to keep in a straight line
2nd gear is the easiest to get the wheels spinning and rear end getting loose, so its up to the driver to modulate pressure on the accelerator to allow for a shift at 7k+ rpm
3rd gear you can start to apply more pressure but still run the risk for some wheel spin and side to side action if you press your foot down too hard
4th and up and now I am assuming you are not having any problems with controlling the car

[Beemerm3]

I was going really fast on a curve road and there's a slight bump on the road and my car just spin along with the curve road. Dsc off too. Felt kind of nervous at times. But still got grip on the road at least.

[hellokitty]

Just wanted to revisit this topic.

Anyone replace their toe arms to aftermarket and diff bushings (solid or poly + turner bracket) to address these rear end issues?

The above mods worked wonders for my 335i on keeping the rear end straight, but wondering if anyone who has done these mods confirm the same impact for the f8x.

[maddmatth]

The E90 335i has soft subframe and diff bushings though. This car a solid mounted subframe, and diff bushing should be pretty stiff, there should be minimal flex in the rear.

However, the diff bushing on the early models is fluid-filled and prone to cracking. The later revision is poly. People who find the back-end slipping around might have a worn diff bushing?

I drive my car hard and have taken it to the track, I can still keep it straight even when lighting up the rear wheels - although this wasn't the case when I first got the car on the 2015 software, it was much better when updated at the dealer to the 2020 revision. So it may be worth a SW update - or just get a coder to flash the CS/GTS software which is even better.

[hellokitty]

Originally Posted by maddmatth

The E90 335i has soft subframe and diff bushings though. This car a solid mounted subframe, and diff bushing should be pretty stiff, there should be minimal flex in the rear.

However, the diff bushing on the early models is fluid-filled and prone to cracking. The later revision is poly. People who find the back-end slipping around might have a worn diff bushing?

I drive my car hard and have taken it to the track, I can still keep it straight even when lighting up the rear wheels - although this wasn't the case when I first got the car on the 2015 software, it was much better when updated at the dealer to the 2020 revision. So it may be worth a SW update - or just get a coder to flash the CS/GTS software which is even better.

I have a 2017 so my diff bushing is the old rubber one. It's not cracked, but I was going to go with the turner bracket and get two of the revised poly ones to lock it down.

The only other thing I can think of is toe change on hard acceleration which is why I am going with SPL toe arms.