[RS61]

Hey guys. I've fiddled quite a lot with my 2021 Comp. M Performance suspension with an 8mm drop. Front upper camber arms (from BMW direct). Dinan ball joints. JB4 so about 700 or so bhp.

The suspension work was done by a very reputable shop that is basically the go-to place for alignment, and they spent several days aligning it. The car tracks totally straight, all the way to the speed limited, and braking is totally straight too, even on track where I'm maxing the brakes. On track where I feed in the acceleration carefully out of each corner, the car is fine.

However if I'm cruising on a fast road or motorway, and then suddenly accelerate hard, the whole car intensely rotates anticlockwise, I have to correct by turning the steering wheel to the right to stay in lane, but that doesn't help because the car keeps rotating and ends up losing traction. If DSC is on it'll sense the rotation and cut the power. If I'm in 2wd then the entire car will end up in a high-speed skid, even at very high speeds.

A bit of an obvious question, but I'm guessing everyone else's M5s can just be floored at any point, and they'll boost and go in a straight line, right?

I've tried two sets of tyres, so I don't think it's that, and the alignment is rock solid in all other situations, and has just been done by the best shop in the UK, so I don't think it's that. Any ideas for what it might be?

[micvite]

Originally Posted by RS61

Hey guys. I've fiddled quite a lot with my 2021 Comp. M Performance suspension with an 8mm drop. Front upper camber arms (from BMW direct). Dinan ball joints. JB4 so about 700 or so bhp.

The suspension work was done by a very reputable shop that is basically the go-to place for alignment, and they spent several days aligning it. The car tracks totally straight, all the way to the speed limited, and braking is totally straight too, even on track where I'm maxing the brakes. On track where I feed in the acceleration carefully out of each corner, the car is fine.

However if I'm cruising on a fast road or motorway, and then suddenly accelerate hard, the whole car intensely rotates anticlockwise, I have to correct by turning the steering wheel to the right to stay in lane, but that doesn't help because the car keeps rotating and ends up losing traction. If DSC is on it'll sense the rotation and cut the power. If I'm in 2wd then the entire car will end up in a high-speed skid, even at very high speeds.

A bit of an obvious question, but I'm guessing everyone else's M5s can just be floored at any point, and they'll boost and go in a straight line, right?

I've tried two sets of tyres, so I don't think it's that, and the alignment is rock solid in all other situations, and has just been done by the best shop in the UK, so I don't think it's that. Any ideas for what it might be?

Depending on how hard the car is "squatting" you're changing the geometry quite a bit. Also are you sure they aligned it right with using weights and all that? Could also be your engine mounts are caput and the torque of the engine is physically pulling the car one way or the other.

[SenorFunkyPants]

Sounds like something amiss with the rear suspension.
Simplest would be that the rear wheels are not toeing in an exactly equal amount side to side.

[pbonsalb]

The OP says the alignment is good because a reputable shop did it. It is possible something is broken that does not show up during an alignment—it takes serious load like the windup from full acceleration to expose the problem. I’d put the car in the air and look and do some testing with a prybar.

If the front to rear tire size difference is not the same ad stock, the transfer case could be a factor. But then I am not sure why there is a problem in 2WD with TC fully off.

[RS61]

Yeah the alignment shop is the best of the best, recommended by literally everyone over here. Spent 20 mins on the phone with them today talking it through.

Definitely possible that something has knocked the alignment out. But we talked about how if one of the rear toes was out, it wouldn't be just during the acceleration that it would show up, you'd have a sense of it in general while driving. They said that in 95% of cases it ends up being the tyres, but there's 5% chance that something at the back is amiss, maybe a damper that behaves differently during the squat.

One thing they suggested trying is to swap the rear wheels around to see if it changes the dynamic. Would definitely confirm that it's not the tyres, and that there's nothing wrong with the actual wheels. Seems an easy test so going to give that a go.

[SenorFunkyPants]

Originally Posted by RS61

Yeah the alignment shop is the best of the best, recommended by literally everyone over here. Spent 20 mins on the phone with them today talking it through.

Definitely possible that something has knocked the alignment out. But we talked about how if one of the rear toes was out, it wouldn't be just during the acceleration that it would show up, you'd have a sense of it in general while driving. They said that in 95% of cases it ends up being the tyres, but there's 5% chance that something at the back is amiss, maybe a damper that behaves differently during the squat.

One thing they suggested trying is to swap the rear wheels around to see if it changes the dynamic. Would definitely confirm that it's not the tyres, and that there's nothing wrong with the actual wheels. Seems an easy test so going to give that a go.

I had an E92 M3 that had a slight mismatch of toe in at the rear and it did drive quite oddly....especially on acceleration where the rear would tend to steer the car. It was a while ago but IIRC going from accelerating to sudden closed throttle also felt weird.

The rear suspension typically has some static toe-in if OEM.
Under acceleration the rear wheels tend to toe-out, reducing the amount of static toe-in...but maintaining enough that the car tracks in a straight line.
If the rear wheels have a different static toe-in side to side you can get rear wheel steer...especially under acceleration.
If the rear static toe-in is correct but the rear wheels toe-out a different amount under acceleration then that can also cause rear wheel steer.
If that makes sense.
Wheel swap seems a good idea to start with.

[NoMods]

Several days to do an alignment? Lol.

[RS61]

Originally Posted by SenorFunkyPants

I had an E92 M3 that had a slight mismatch of toe in at the rear and it did drive quite oddly....especially on acceleration where the rear would tend to steer the car. It was a while ago but IIRC going from accelerating to sudden closed throttle also felt weird.

The rear suspension typically has some static toe-in if OEM.
Under acceleration the rear wheels tend to toe-out, reducing the amount of static toe-in...but maintaining enough that the car tracks in a straight line.
If the rear wheels have a different static toe-in side to side you can get rear wheel steer...especially under acceleration.
If the rear static toe-in is correct but the rear wheels toe-out a different amount under acceleration then that can also cause rear wheel steer.
If that makes sense.
Wheel swap seems a good idea to start with.

This sounds very very relevant, thank you! Mine definitely feels weird going from full to closed throttle too - dramatic changes both ways. Floor it and it rotates anticlockwise, suddenly take the foot off and it rotates back clockwise. Is there any way of measuring how much the wheels toe out under acceleration? It definitely feels like the geometry is changing when accelerating hard, and that the rear toe changes pushing the rear out to the right each time.

[M5 London]

Originally Posted by RS61

Hey guys. I've fiddled quite a lot with my 2021 Comp. M Performance suspension with an 8mm drop. Front upper camber arms (from BMW direct). Dinan ball joints. JB4 so about 700 or so bhp.

The suspension work was done by a very reputable shop that is basically the go-to place for alignment, and they spent several days aligning it. The car tracks totally straight, all the way to the speed limited, and braking is totally straight too, even on track where I'm maxing the brakes. On track where I feed in the acceleration carefully out of each corner, the car is fine.

However if I'm cruising on a fast road or motorway, and then suddenly accelerate hard, the whole car intensely rotates anticlockwise, I have to correct by turning the steering wheel to the right to stay in lane, but that doesn't help because the car keeps rotating and ends up losing traction. If DSC is on it'll sense the rotation and cut the power. If I'm in 2wd then the entire car will end up in a high-speed skid, even at very high speeds.

A bit of an obvious question, but I'm guessing everyone else's M5s can just be floored at any point, and they'll boost and go in a straight line, right?

I've tried two sets of tyres, so I don't think it's that, and the alignment is rock solid in all other situations, and has just been done by the best shop in the UK, so I don't think it's that. Any ideas for what it might be?

That sounds crazy and dodgy.

Sounds almost like the rear subframe is loose and when large torque is applied rapidly or removed, the whole rear axle setup is doing a twist. Carefully have all the rear subframe mounting points checked and also the rear diff mount points, while you're there.

PS: I presume you're referring to Centre of Gravity? They are on another level. I hear good things about Suspension Secrets too.

[M5 London]

Originally Posted by armoredsaint

Several days to do an alignment? Lol.

Yes because top establishments don't mess around. If they deem you require new suspension arms or bushes, for example, they'd have to order them in.

[RS61]

Yep exactly, Centre Gravity. Suspension Secrets are great too, but newer. They did some stuff for me at a track day once and were very good too. But Centre Gravity are so good they have a 3 month waitlist to book in. And yeah, that's exactly how it feels, something at the rear subframe moves under intense torque, but is totally fine otherwise.

[M5 London]

Originally Posted by RS61

Yep exactly, Centre Gravity. Suspension Secrets are great too, but newer. They did some stuff for me at a track day once and were very good too. But Centre Gravity are so good they have a 3 month waitlist to book in. And yeah, that's exactly how it feels, something at the rear subframe moves under intense torque, but is totally fine otherwise.

Was this issue ever resolved at all?