Bike rack attachments for E86

[exdos]

I'm wondering if anyone who has used a bike rack on the Z4 Coupe can help me?

I'm presently making an easily removable rear wing for track use for my Z4MC and I want to attach it to the boot lid using brackets which hook around the bottom of the door and around the glass at the top. I've made a simple template as the basis of the design as shown in the photo below and the arrows show the points of attachment. If possible, I'd like to use commercially available bracketry that is known to work with the Z4 Coupe boot lid. I can easily fabricate a bracket that fits around the metal lip at the bottom of the lid, but at the top of the lid the glass is completely unsupported at the edge and I'd like to see the different solutions that manufacturers of bike carriers, and the likes, have produced that deal with this issue.

Thanks to anyone who can help. Links to photos of appropriate hardware will be greatly appreciated.

 

[CornishRob]

Have you looked into the roof rack option?

I know that there isn't one for the Z4, but I have seen someone has used the system from a 7 series and it works perfectly, apparently!

 

[CornishRob]

Here is the Z post thread on a roof rack for Z4m.

http://www.zpost.com/forums/showthread.php?t=564114&page=2

Annoying as the guy who did it didn't leave many specific details about how he did it. Also seems two of the pictures have been removed, but think I might hav them saved on my PC somewhere if you're interested.

You might be just the chap to work out a roof rack for the E86!

 

[Taz]

I tried this option:

 

[exdos]

CornishRob Thanks for the input. I don't want anything attached to the roof because I'm trying to make the fitment as aerodynamically "clean" as possible. The fitting hardware must attach to the boot lid only.

Taz, Again, thanks for the input. That's the Saris Bones rack which suggests that the rear window can take a certain amount of loading and it has two straps which come over the exposed top edge of the glass and is the basis of a means of attachment. How did you get on with it? Was it a satisfactory solution and did you have any problems? I don't want to use long straps along the full length of the rear window, because, no matter how tight they might be, they will have a tendency to "flutter" at speed. Using very short straps at the top edge of the window, which would attach to the rigid part of my wing support that goes down the full length of the rear window, is definitely one of my options if I have to fabricate all the fittings myself. How does the strap of the Saris Bones attach over the glass at the top of the window? I'm thinking of looping a strap around the boot lid hinges, so that there would be no lateral movement.

I've just come across an OEM ski/bike rack here: http://bmwfans.info/parts-catalog/E...L-N/browse/universal_accessories/rear_carrier which fits as below:

 

[srhutch]

Won't you have issues with the forces bring generated against the glass edges?

 

[exdos]

Won't you have issues with the forces bring generated against the glass edges?

That's precisely the issue I'm wanting to avoid and that's why I want to see what the manufacturers of bike racks have already devised.

 

[srhutch]

Won't there be less stresses on the bik racks though as they won't be giving downforce?

 

[Taz]

i only tried it once. what i found was it was not balanced. too much weight on the rear and it tries to lift off the glass - if that makes sense

 

[Taz]

it wants to tip up

 

[exdos]

Won't there be less stresses on the bik racks though as they won't be giving downforce?

A bike rack inherently has the weight of the bikes it's carrying, whereas a rear wing will only develop any downforce with an increase in speed. How much downforce my wing might develop can only be guessed at this moment, but test-driving with different adjustments of the wing in different configurations will show what feels "balanced" for the car. I don't intend to go too aggressive. The finished wing may not be positioned quite so high as the top of the template in the photo.

The photo below shows the way that I'm looking at this. I intend to restrain the wing supports where the red arrows are pointing and the wing supports will be rigid. There will be feet (shown as pink) which will be rigidly attached to the wing supports that will spread the load of the wing laterally on to the horizontal and vertical surfaces of the boot lid. When the car is moving, there will be aerodynamic forces acting both downwards and backwards on the wing (shown as blue arrows), and provided that the front of the wing support is firmly anchored at the top of the boot lid, then this point should act as the axis of potential rotation of the wing support, where the downforce on the wing should press the wing support against the boot lid (shown as the green arrows). Provided that the wing supports can be firmly anchored to the boot lid, then all aerodynamic forces acting on the wing, should result in downforce rather than unwanted movement of the entire assembled wing unit. I also intend to fit a front splitter to maintain aerodynamic balance of the car at both ends. I've also got adjustable suspension which can also assist in balancing the car with increased aerodynamic forces.

As I see this project, the success will depend upon having rigid attachment points at the top and bottom of the boot lid. The finished wing unit will have two wing supports in the position of the template on both sides of the boot lid, and at the top there will be a wing bolted to both supports; the lateral feet on the supports will prevent lateral swaying of the units, therefore the assembly should be as rigid as a table.

 

[Paulwirral]

John , what about a roof spoiler - rear wing package like on the old csl batmobile , the roof spoiler would probably be permanent but it would give you plenty of options mounting wise for a detachable rear wing ? I can only see something attached to the window ending in tears at high speed .

 

[exdos]

HI Paul, for a functional spoiler to work properly on a car shaped like the Z4MC, it really needs to be placed at the trailing edge of the boot lid. I'm really trying to avoid, at all costs, making any permanent fitments to the exterior bodywork.

Provided the rear wing of a similar design to my template is firmly attached at the bottom of the boot lid, just above the bumper, the drag force acting against the wing in the horizontal plane is producing a leverage force at the bottom of the boot lid, and since the wing support is the length between the points of the two red arrows, the force acting on the restraint at the top of the window, should be similar to the force required to stop a loaded sack truck from wanting to stand upright, as in the diagram below. Experience of wheelbarrows and sack trucks suggests to me that the forces acting at the top of the rear window might not actually be that great and that the force will be pulling upwards off the window, rather than downwards on to it. Taz said that's precisely the situation with the Saris Bones bike rack he tried.

 

[tertius]

Is it really needed?

 

[exdos]

Is it really needed?

If you don't know already, I really enjoy experimenting and making stuff for cars and playing with set-ups etc. and discovering how all these things work (or don't!). I do it all for the intellectual challenge and I will not permanently modify any OEM part on the car unless I have a spare undamaged part set aside. For me, it all adds to the enjoyment of the car. Just owning and driving the car isn't enough for me.

I have a roof wing and front flippers on my Z3MC and it handles far better with that kit on the car than without, so I'm curious to see how aerodynamic parts affect the handling of my Z4MC.

You have an ACS rear wing: have you driven the car around The Ring without it? If so, any difference?

 

[Bing]

John, I have nothing to contribute to this, being completely non-techie and, to compound by ignorance, a roadster owner... I do love that the title of the thread is totally different to the topic though, requiring people to read it before commenting, by which time the haters have been scared off by the science :lol:

I can imagine that 'Proper Rear Wing for Coupe Possible ?' Would be full of :scratchhead: and :crazydude: by now

Topic diversion ends.

 

[exdos]

John, I have nothing to contribute to this, being completely non-techie and, to compound by ignorance, a roadster owner... I do love that the title of the thread is totally different to the topic though, requiring people to read it before commenting, by which time the haters have been scared off by the science :lol:

I can imagine that 'Proper Rear Wing for Coupe Possible ?' Would be full of :scratchhead: and :crazydude: by now

Topic diversion ends.

Bing, although I ultimately want to develop a removable rear wing for my Z4MC, I hoped that I could draw on the knowledge of those who've used bike racks, since that's the actual information I'm really looking for. How are bike racks for the Z4 Coupe attached because I want to copy the tried and tested methods? If I do progress with the project, then I'll do a separate write-up with "rear wing" in the title.

I've not come across any haters in this thread. Yet!

 

[tertius]

Is it really needed?

You have an ACS rear wing: have you driven the car around The Ring without it? If so, any difference?

No, I haven't. I didn't add the ACS wing and if it wasn't bonded on I'd remove it. I'm extremely dubious whether it has any real benefit at the sort of speeds I drive. I'd also be interested to know whether the increase in drag is in reality offset by the increase in downforce.

Since I've had the car I've spent as much time removing (what I consider) unnecessary modifications as I have adding new ones ...

 

[inkey$]

Interesting. Look forward to seeing this thread develop.

Used a Saris Bones on my old MC. Secure and no issues but you do need to ensure the weight is distributed to the feet as opposed to the straps.

 

[Paulwirral]

Good call about the point of downforce being on the rear of the tailgate , you know I'm familiar with wheel barrows there will be a stress on the glass which will increase as your speed increases , but like you say , it's actually going to be up over , I've never really looked at a coupe but I would be dubious about putting any reliance on glass , I've broken toughened glass shower screens just by lifting them up .
There's probably only you a good enough driver to appraise any difference it will make , my only experience with aero was a f2 stock car with a one sided wing , I built it , bolted it on and I reckon I was slower due to drag and weight , it did seem to work on a shale track though , through the corner acting as a wind break !
I'm sure you'll come up with something .