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M40i brakes - design flaw uncovered
- Jim S
- Member
- Posts: 395
- Joined: Mon Dec 26, 2016 9:59 pm
- Location: Ayrshire, Scotland
M40i brakes - design flaw uncovered
I think I may have uncovered a design flaw in the M Sport brakes fitted to the new Z4 M40i and other Z4 G29 models fitted with the same brakes. Stones are far too easily getting trapped between the brake disc rotor and the metal heat/protector shield behind the rotor.
In the space of only a few weeks and 800 miles of taking delivery of my brand new Z4 M40i this problem has occurred four times. When it happens it sounds absolutely horrendous. The noise is a very loud high pitched squeal and it sounds like the wheel is just about to fall off. I have also noticed that it usually only happens when the car is slowing down close to stopping. I have a theory for this, which I will explain later.
There is no jack in the car to lift up the wheel to see where the stone is trapped and you either need to try to free it by driving, sometimes several miles and causing damage by scoring the disc rotor, or call out the BMW breakdown service.
In many years of motoring in BMW's and several other brands of cars this is the first time I have ever experienced such a problem. I believe that there is a fundamental design flaw in the structure of the M40i brakes as I will explain below.
Most modern cars have disc brakes and they can develop a lot of heat so the manufacturers incorporate a metal shield right behind the brake rotor to prevent unwanted heat transfer to brake pipes, ball joints and other heat sensitive parts of the car. The shield is also there to prevent stones, debris, water and dirt getting into the brake assembly. Normally, this shield is bent over the top of the rotor to close off the gap where debris and stones can fall into the brake assembly. On the M40i both the front and rear metal shields do not bent over the rotors. In fact the front ones actually bend away from the rotor creating an even bigger gap for stones and debris to fall into the brake assembly. The gap is large enough for small stones and debris to pass through but anything larger than about 1cm (1/2”) in size is likely to get trapped, causing the problem that I am describing in this review.
In the UK, many of the roads are covered in stones washed up out of pot holes and decaying road surfaces so it is very difficult to avoid getting stones thrown up into the wheel arches of our cars. I have also noticed that the Z4 M40i is very prone to throwing up stones and this might have something to do with the 19" Michelin Pilot Super Sport Performance tyres. These two things are exacerbating the problem.
On examining the location of the brake rotors on the M40i it is difficult to understand how the stones are getting into the rotors as the wheel rims are extremely wide and the rotor assembly is near the centre of the rim. Also, the rotors are quite large in diameter so the space between the top of the rotor and wheel rim is quite small, making it very difficult for stones or debris to get thrown directly into the brake assembly.
My theory is that some stones are being thrown onto the wheel rims and the centrifugal force of the wheels rotating causes the stones to stay on the wheel rim until the car slows down at which point they fall down into the gap between the brake rotor and metal shield. Any stone or debris larger than about 1cm is likely to get trapped, particularly since the design of the metal shield does not prevent the stones and debris getting into the brake assembly as it should.
I believe that the problem outlined above is a design flaw in the Z4 M40i brakes and BMW should stand up and take responsibility and offer an immediate remedy for the affected cars. I have already requested my Dealer to take this issue up with BMW.
I would like to hear if any other Z4 G29 owners have had similar problems with their car.
In the space of only a few weeks and 800 miles of taking delivery of my brand new Z4 M40i this problem has occurred four times. When it happens it sounds absolutely horrendous. The noise is a very loud high pitched squeal and it sounds like the wheel is just about to fall off. I have also noticed that it usually only happens when the car is slowing down close to stopping. I have a theory for this, which I will explain later.
There is no jack in the car to lift up the wheel to see where the stone is trapped and you either need to try to free it by driving, sometimes several miles and causing damage by scoring the disc rotor, or call out the BMW breakdown service.
In many years of motoring in BMW's and several other brands of cars this is the first time I have ever experienced such a problem. I believe that there is a fundamental design flaw in the structure of the M40i brakes as I will explain below.
Most modern cars have disc brakes and they can develop a lot of heat so the manufacturers incorporate a metal shield right behind the brake rotor to prevent unwanted heat transfer to brake pipes, ball joints and other heat sensitive parts of the car. The shield is also there to prevent stones, debris, water and dirt getting into the brake assembly. Normally, this shield is bent over the top of the rotor to close off the gap where debris and stones can fall into the brake assembly. On the M40i both the front and rear metal shields do not bent over the rotors. In fact the front ones actually bend away from the rotor creating an even bigger gap for stones and debris to fall into the brake assembly. The gap is large enough for small stones and debris to pass through but anything larger than about 1cm (1/2”) in size is likely to get trapped, causing the problem that I am describing in this review.
In the UK, many of the roads are covered in stones washed up out of pot holes and decaying road surfaces so it is very difficult to avoid getting stones thrown up into the wheel arches of our cars. I have also noticed that the Z4 M40i is very prone to throwing up stones and this might have something to do with the 19" Michelin Pilot Super Sport Performance tyres. These two things are exacerbating the problem.
On examining the location of the brake rotors on the M40i it is difficult to understand how the stones are getting into the rotors as the wheel rims are extremely wide and the rotor assembly is near the centre of the rim. Also, the rotors are quite large in diameter so the space between the top of the rotor and wheel rim is quite small, making it very difficult for stones or debris to get thrown directly into the brake assembly.
My theory is that some stones are being thrown onto the wheel rims and the centrifugal force of the wheels rotating causes the stones to stay on the wheel rim until the car slows down at which point they fall down into the gap between the brake rotor and metal shield. Any stone or debris larger than about 1cm is likely to get trapped, particularly since the design of the metal shield does not prevent the stones and debris getting into the brake assembly as it should.
I believe that the problem outlined above is a design flaw in the Z4 M40i brakes and BMW should stand up and take responsibility and offer an immediate remedy for the affected cars. I have already requested my Dealer to take this issue up with BMW.
I would like to hear if any other Z4 G29 owners have had similar problems with their car.
Last edited by Jim S on Mon Jun 03, 2019 8:33 am, edited 1 time in total.
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- Senior Member
- Posts: 2671
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- Location: Lincoln
M40i brakes - design flaw uncovered
Wow! How annoying. That would be a nightmare in Lincolnshire where they are now into ‘the ‘laying loose chippings’ season. Hope BMW responds positively and that you get something sorted out quickly.
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- Lifer
- Posts: 11121
- Joined: Tue Feb 10, 2015 10:27 pm
M40i brakes - design flaw uncovered
Uh oh....E89 wheel cracks.....G29 stone traps....and so it goes.
The very best of British OP.....
The very best of British OP.....
M roady...OEM CSL’s, strut brace, Remus back boxes, ZHP
MR2 MK 2
E89 35i project car...mapped 365bhp, M4 stoppers & wheels, KWV3’s, H&R front ARB, M3 front arms, strut brace Eisenmann cat back race exhaust, VRSF downpipes inbound
E89 35is
G29
MR2 MK 2
E89 35i project car...mapped 365bhp, M4 stoppers & wheels, KWV3’s, H&R front ARB, M3 front arms, strut brace Eisenmann cat back race exhaust, VRSF downpipes inbound
E89 35is
G29
- Jim S
- Member
- Posts: 395
- Joined: Mon Dec 26, 2016 9:59 pm
- Location: Ayrshire, Scotland
M40i brakes - design flaw uncovered
Thanks IRD. I have passed a copy of my review to my dealer so that it can be sent directly to BMW Technical for action. I will post their comments when I receive them.
- buzyg
- Legend
- Posts: 26605
- Joined: Fri May 20, 2011 6:11 pm
- Location: Cornwall
M40i brakes - design flaw uncovered
That must grate on the nerves. My ///M brakes occasionally do that, but they have always cleared them selves within a few miles. Hope it gets sorted quickly.
- Jim S
- Member
- Posts: 395
- Joined: Mon Dec 26, 2016 9:59 pm
- Location: Ayrshire, Scotland
M40i brakes - design flaw uncovered
Thanks buzyg, will let everyone know how I get on
Current: 2019 Z4 M40i (G29)
Previous BMW's (all new):
2017 520d MSport xDrive (G30)
2016 Z4 20i sDrive SE (E89)
2014 428i MSport Gran Coupe (F36)
2009 530d SE Gran Turismo (F07)
2007 525d SE (E60 LCI)
2005 523i SE (E60)
2002 320d SE (E46)
1982 520i (E28)
Previous BMW's (all new):
2017 520d MSport xDrive (G30)
2016 Z4 20i sDrive SE (E89)
2014 428i MSport Gran Coupe (F36)
2009 530d SE Gran Turismo (F07)
2007 525d SE (E60 LCI)
2005 523i SE (E60)
2002 320d SE (E46)
1982 520i (E28)
- Jembo
- Lifer
- Posts: 6034
- Joined: Sat Jun 25, 2011 11:59 pm
- Location: 1066 Country
M40i brakes - design flaw uncovered
Fingers crossed this post will be sorted long before it reaches the lengths of the E85/6 broken rear spring thread
Joined the dark side with a ///M Hell Bronze Sepang Coupe
Ex Aug 2005 Maldives Blue 3 litre cruizer, lots of toyz,
Lifer 54
Ex Aug 2005 Maldives Blue 3 litre cruizer, lots of toyz,
Lifer 54
- Jim S
- Member
- Posts: 395
- Joined: Mon Dec 26, 2016 9:59 pm
- Location: Ayrshire, Scotland
M40i brakes - design flaw uncovered
Let’s hope so Jembo
Current: 2019 Z4 M40i (G29)
Previous BMW's (all new):
2017 520d MSport xDrive (G30)
2016 Z4 20i sDrive SE (E89)
2014 428i MSport Gran Coupe (F36)
2009 530d SE Gran Turismo (F07)
2007 525d SE (E60 LCI)
2005 523i SE (E60)
2002 320d SE (E46)
1982 520i (E28)
Previous BMW's (all new):
2017 520d MSport xDrive (G30)
2016 Z4 20i sDrive SE (E89)
2014 428i MSport Gran Coupe (F36)
2009 530d SE Gran Turismo (F07)
2007 525d SE (E60 LCI)
2005 523i SE (E60)
2002 320d SE (E46)
1982 520i (E28)
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- Lifer
- Posts: 4069
- Joined: Sat May 25, 2013 8:27 pm
- Location: Stourbridge
M40i brakes - design flaw uncovered
Is it possible to swap the left right shields, so lip is over the rotor.
- Jim S
- Member
- Posts: 395
- Joined: Mon Dec 26, 2016 9:59 pm
- Location: Ayrshire, Scotland
M40i brakes - design flaw uncovered
That might be possible. I am starting to wonder if my shields have actually been fitted the wrong way round. Worth asking that question to the dealer. Thanks for pointing that out flybobbie
- Jembo
- Lifer
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- Joined: Sat Jun 25, 2011 11:59 pm
- Location: 1066 Country
M40i brakes - design flaw uncovered
Great suggestion Fred - suggest best done by the main dealer so as not to void any warranty
Be a bit of a laff if turned out was how were supposed to be fitted & the whole line is wrong way round
Joined the dark side with a ///M Hell Bronze Sepang Coupe
Ex Aug 2005 Maldives Blue 3 litre cruizer, lots of toyz,
Lifer 54
Ex Aug 2005 Maldives Blue 3 litre cruizer, lots of toyz,
Lifer 54
- Mr Tidy
- Legend
- Posts: 24060
- Joined: Wed Jul 02, 2014 12:18 pm
- Location: North West Surrey
M40i brakes - design flaw uncovered
That's a shame OP, as otherwise you seemed very pleased with your G29.
I hope you can get the issue sorted.
I hope you can get the issue sorted.
Coupes because stunning!
Current - Silver Grey MC, Imola Red heated Nappa & carbon trim. Aeros, H & R Coil-overs, 224s, OE Strut brace, Nav, cup-holders, DSP Hi-Fi, pdc, cruise, MFSW, no CDV! E90 330i daily
Gone - Montego Blue
Gone - Ruby Black
Current - Silver Grey MC, Imola Red heated Nappa & carbon trim. Aeros, H & R Coil-overs, 224s, OE Strut brace, Nav, cup-holders, DSP Hi-Fi, pdc, cruise, MFSW, no CDV! E90 330i daily
Gone - Montego Blue
Gone - Ruby Black
- Jim S
- Member
- Posts: 395
- Joined: Mon Dec 26, 2016 9:59 pm
- Location: Ayrshire, Scotland
M40i brakes - design flaw uncovered
Otherwise, the car has been great. Just a pity about the problem with the brakes. I know of two other UK M40i owners who are having exactly the same problem. Let’s hope BMW comes up with the solution ASAP.
- mr wilks
- Legend
- Posts: 21898
- Joined: Wed Jul 04, 2012 10:02 pm
- Location: Lancashire
M40i brakes - design flaw uncovered
Id be happy if that was the one & only flaw you've uncovered
3 ZMRs
3 E89s
5 Si coupes
5 Si roadsters
997 C4
TTRS
F82 M4
MK7 Golf Gti
current Bmw 6 Gran Turismo + Golf R
3 E89s
5 Si coupes
5 Si roadsters
997 C4
TTRS
F82 M4
MK7 Golf Gti
current Bmw 6 Gran Turismo + Golf R
- ben g
- Lifer
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M40i brakes - design flaw uncovered
I work on an engine production line and that happens more often than you'd think many a time we have had to reject half a lines worth of engines (approx 150) because of silly errors like this
Mind you, the guys who get the overtime to fix the problem love it
Red Soft-top Sterling grey 3.0i - Z4M Front, Eibachs, Polybushed, Clear headlights, Triple clear spot rear lights, Shadow Chrome 107's, Sport MFSW, ZHP.