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      06-01-2019, 11:24 AM   #1
Jim S
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Z4 M40i 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 (Rocks) 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.


Last edited by Jim S; 06-03-2019 at 03:48 AM..
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      06-01-2019, 11:42 AM   #2
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People have been complaining about this on some of the M cars for years. Doesn't appear that they have any interest in addressing it.
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      06-01-2019, 11:45 AM   #3
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Now, the general temperatures of the trunk interior on the other hand... damn near cooked my lunch on the way to the office
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      06-01-2019, 11:46 AM   #4
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Quote:
Originally Posted by SgtGigglebox View Post
People have been complaining about this on some of the M cars for years. Doesn't appear that they have any interest in addressing it.
Shame on BMW. Let's see what they do about my complaint.
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      06-01-2019, 11:50 AM   #5
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Originally Posted by SgtGigglebox View Post
Now, the general temperatures of the trunk interior on the other hand... damn near cooked my lunch on the way to the office
Maybe fry your eggs then
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      06-01-2019, 12:00 PM   #6
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Quote:
Originally Posted by SgtGigglebox View Post
People have been complaining about this on some of the M cars for years. Doesn't appear that they have any interest in addressing it.
The brake shields on the early production M3/M4 were revised by BMW due to this problem. I had this happen on my 2015 M4, and after the brake shields were replaced with the new version it never happened again. There have been enough complaints on the new Z4 about this that I am sure BMW will do something.

And Jim, thanks for being our beta tester on the new Z4!
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      06-01-2019, 12:15 PM   #7
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Originally Posted by unit View Post
The brake shields on the early production M3/M4 were revised by BMW due to this problem. I had this happen on my 2015 M4, and after the brake shields were replaced with the new version it never happened again. There have been enough complaints on the new Z4 about this that I am sure BMW will do something.

And Jim, thanks for being our beta tester on the new Z4!
Thanks for the info "unit". Hoping to post a 1000 mile review soon.
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      06-01-2019, 04:00 PM   #8
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M brake shields are shared between all Z4 and G20 3 series models equipped with the optional M brakes. If there is a design flow, I am sure it will be identified and addressed very soon. I checked BMW's latest part catalogue and the part numbers haven't changed since October 2017.

Which corner seems to be the most affected?
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      06-01-2019, 04:07 PM   #9
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Two definitely at the front, one each nearside and offside. Hard to tell for the other two as I was in the car on my own and the stones came out after about 7 miles driving. Could have been front or rear, again one on each side.
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      06-01-2019, 04:22 PM   #10
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Quote:
Originally Posted by Jim S View Post
Two definitely at the front, one each nearside and offside. Hard to tell for the other two as I was in the car on my own and the stones came out after about 7 miles driving. Could have been front or rear, again one on each side.
You've had this 4 times already! Sounds serious. Is there any possibility it's the same stone and sometimes gets stuck somewhere then becomes loose or not a stone at all?

I've had an incredibly loud intermittent noise in my early F30 335i and after hours of diagnosis by BMW's support engineer, it turned out to be a plastic grease-stop cap that became loose and ended up inside the driveshaft! Driveshaft had to be replaced!
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      06-01-2019, 04:31 PM   #11
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Quote:
Originally Posted by tmehanna View Post
You've had this 4 times already! Sounds serious. Is there any possibility it's the same stone and sometimes gets stuck somewhere then becomes loose or not a stone at all?

I've had an incredibly loud intermittent noise in my early F30 335i and after hours of diagnosis by BMW's support engineer, it turned out to be a plastic grease-stop cap that became loose and ended up inside the driveshaft! Driveshaft had to be replaced!
No, Im afraid it has been separate stones. The dealer took out the second one from the front offside and showed it to me. It was a round rock about 1/2 in diameter.. Thanks for your input anyway.
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      06-01-2019, 07:10 PM   #12
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Not hopeful this will be addressed soon. The issues has been around for several years on the M3/4 as noted above. Nothing would have stopped BMW to revise the design for the new G platform cars.
My M40i sits at the dealer for me to pick up once I return from vacation. I hope i don't share the same experiences as JimS when I pick her up.
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      06-01-2019, 09:29 PM   #13
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Quote:
Originally Posted by German1967 View Post
Not hopeful this will be addressed soon. The issues has been around for several years on the M3/4 as noted above. Nothing would have stopped BMW to revise the design for the new G platform cars.
My M40i sits at the dealer for me to pick up once I return from vacation. I hope i don't share the same experiences as JimS when I pick her up.
While M3/M4s were niche cars, the 3 series is BMW's bread and butter and the M-brakes are optional across the board on all 3 series models. I doubt BMW can afford not to listen.
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      06-02-2019, 08:07 AM   #14
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Quote:
Originally Posted by tmehanna View Post
While M3/M4s were niche cars, the 3 series is BMW's bread and butter and the M-brakes are optional across the board on all 3 series models. I doubt BMW can afford not to listen.
I'm trying my best to get BMW to listen. Need a few others to do the same if they experience the same problem, which is extremely likely from my findings.
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      06-02-2019, 04:49 PM   #15
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I had the same issue driving the car home from the dealer. I was lucky, only had to drive a couple of feet, very slowly, breaking often, to get rid of the stone.
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      06-02-2019, 05:02 PM   #16
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Originally Posted by CeEl View Post
I had the same issue driving the car home from the dealer. I was lucky, only had to drive a couple of feet, very slowly, breaking often, to get rid of the stone.
Sorry to hear that, but I am not at all surprised. Most likely it will happen again. You need to report it back to your dealer with instructions for them to record it in BMW's system if we are to force BMW to come up with a solution to the problem.
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      06-03-2019, 08:46 AM   #17
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Post by Jim S Mon Jun 03, 2019 1:22 pm

Here is the response from my Dealer regarding the Technical Issue that I raised regarding the M Sport Brakes problem. Please note that we need other owners to report this problem to their Dealer as well in order for this issue to get passed onto BMW Germany for action.

BMW response to M40i stones in brakes problem
Monday 03 June 2019

Good morning Mr Stewart,

Many thanks for getting in touch.

Following your most recent visit to the workshop, we raised a technical case with BMW directly.

This was their reply:

Thank you for this case, we have not had this brought to our attention on the G29 before. We need cases like this reporting to be able to ask AG to view this complaint. I know you say that this vehicle is not on site, But with the next instance of this nature can we have pictures of the rear disc and the clearance distance and any damage this has caused. Evidence of what has caused this complaint. I take it the other vehicle that the customer is referring too has 2NH M sport brakes like this vehicle?

Following your email(s) from the weekend, we have added your comments verbatim onto the case for them to view.

Once BMW technical have had an opportunity to digest your comments, they may reply, however, it would appear they require more instances to occur from multiple vehicles in order to present a case to AG (Germany) for any potential action.

I will be in touch if they respond regarding your case or in general relating to the complaint.

Kind regards

BMW Dealer Service Manager
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      06-04-2019, 02:33 PM   #18
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Hi all,

Delivery of my Z4 will be in a week or two. I also have the M-brakes so I hope I wont have the same problem...What I would like to know:if there are rocks sitting on the inside of the rim while driving there should have to be scratches noticeable on the paint of the inside of the rim....
Did anyone who has this problem check the paint of the inside of the rim?
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      06-04-2019, 03:07 PM   #19
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Quote:
Originally Posted by kv1980 View Post
Hi all,

Delivery of my Z4 will be in a week or two. I also have the M-brakes so I hope I wont have the same problem...What I would like to know:if there are rocks sitting on the inside of the rim while driving there should have to be scratches noticeable on the paint of the inside of the rim....
Did anyone who has this problem check the paint of the inside of the rim?
I doubt very much if the rocks would scratch the rim paint. This is not something that is likely to happen very often. However, it would only take a single rock getting into the brakes every couple of months or so to cause severe inconvenience to the owner.

The Z4 M40i is the only car that I have ever owned that has had this problem, and I have owned several BMWs and other brands in over 50 years of driving.
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      06-04-2019, 06:16 PM   #20
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I'll have to take a look when I get the car; I have a metal fab shop so I'm sure I could modify the shields if need be.......the hoist helps too. Or I could just take them off; I'm not convinced they do anything useful anyhow - never seen them on a track car or racecar or any of my 41 motorcycles.

Dave

Last edited by DPelletier; 06-04-2019 at 06:35 PM..
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      06-04-2019, 08:22 PM   #21
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This isn't a Z4 only problem... other cars with the M brakes have the same issue.

For what it's worth: only happened to me once so far.

I am disappointed in BMW saying they never heard about this... well, maybe not on the US M40i since it is so new... where is ABC News when you need them?
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      06-05-2019, 05:33 AM   #22
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My dealer sent me the latest response received from BMW UK regarding the problem with stones getting into the M Sport brakes on my M40i. I am taking the car in for the requested inspection tomorrow.

"BMW UK Technical have replied to the further information we sent them this morning. To paraphrase, as the vehicle is a new model, there will be some feedback generated to AG, however they do not expect a solution introduced to prevent stones from being picked up by the wheels. They would like us to confirm the brake components fitted to the car are correct and to check the brake shields, even though they believe they cannot be fitted incorrectly. Is there a possibility the car could be made available for us to check the parts fitted are correct and we can reply to the Technical case? This will give us an opportunity to take some images and measurements that they requested in their initial reply.

it is worth pointing out there have been no other issues of this nature reported to BMW UK, other than your own. We copied and pasted your entire email to the technical case, so they are aware of all your comments."
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