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VagBremTechnic Big Brake Kit for E85/E86 Z4 'M' models

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VagBremTechnic Big Brake Kit for E85/E86 Z4 'M' models

Post by BMWZ4MC » Wed Jul 18, 2018 9:41 pm

The spigot is to centre the wheel when fitting it. Compressive forces between wheel and hub are generated by the bolts and it’s friction at that interface that transmits drive to the road. The spigot doesn’t keep the wheel on the car.
As for 5mm spaces being poorly machined leading to imbalance, that risk is true of any spacer or wheel.
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VagBremTechnic Big Brake Kit for E85/E86 Z4 'M' models

Post by TomK » Wed Jul 18, 2018 10:51 pm

BMWZ4MC wrote: Wed Jul 18, 2018 9:41 pm The spigot is to centre the wheel when fitting it. Compressive forces between wheel and hub are generated by the bolts and it’s friction at that interface that transmits drive to the road. The spigot doesn’t keep the wheel on the car.
I might be wrong (actually probably am) but I thought the idea of hubcentric (which these cars are, like most) is that they are not lugcentric, i.e must be centrally supported by the spigot. The wheel bolts are not designed to take the shear loads unless centrally located?
Either way, with a 5mm you will still be located on the spigot, but if your bolts decide to back out a touch say (has happened to me once upon a time), there's not a lot of meat left on the bone.
BMWZ4MC wrote: Wed Jul 18, 2018 9:41 pm As for 5mm spaces being poorly machined leading to imbalance, that risk is true of any spacer or wheel.
true, but I'd say the risk is higher with one that is not hubcentric. Say for instance the 5mm spacer hasn't been machined perfectly to ensure it is centered with equal distance around the hub and then slides over when tightening the bolts.
RedUn wrote: Wed Jul 18, 2018 9:09 pm I used to think that but some OEMs (Porsche) run a 5mm spacer from the factory
Do you know the depth of those spigots? Perhaps they are larger, I see talk of 7mm (not hubcentric) which would be pretty on the limit with these hubs.
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VagBremTechnic Big Brake Kit for E85/E86 Z4 'M' models

Post by BMWZ4MC » Fri Jul 20, 2018 4:22 am

If you were to loosen all the wheel bolts on a wheel without a spacer, a stationary car might be able to stand on the hub spigot. However, as soon as you introduced torque from accelerating or braking, the wheel would rotate on the hub until the bolt holes met the bolts. The bolts would either then shear or transmit the torque between hub and wheel. I would expect any lateral forces from cornering with loose bolts to be carried through the bolts unless the wheel was a virtual press fit on the spigot.

I do take your point about the potential for a spigot not being long enough to centre a wheel if a 5mm spacer is used with a wheel with a chamfered centre bore. However, I’m not sure if I’ve encountered wheels with chamfered centres. Also, the chamfered heads on the bolts will tend to centre the wheel on the hub just so long as you progressively tighten each bolt sequentially, not one at a time.

Anecdotal evidence I know, but I’ve used soft compound track tyres with 5mm spacers over hundreds of miles high G corners as well as full throttle starts and heavy braking without incident.

Edit - Did you replace your wheel bolts after running with them not torqued up? The bolts are intended to see tension loads only and they might have looked ok macroscopically but still have been exposed to huge uncontrolled stresses.
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VagBremTechnic Big Brake Kit for E85/E86 Z4 'M' models

Post by RedUn » Sun Jul 22, 2018 12:50 pm

TomK wrote: Wed Jul 18, 2018 10:51 pm
BMWZ4MC wrote: Wed Jul 18, 2018 9:41 pm The spigot is to centre the wheel when fitting it. Compressive forces between wheel and hub are generated by the bolts and it’s friction at that interface that transmits drive to the road. The spigot doesn’t keep the wheel on the car.
I might be wrong (actually probably am) but I thought the idea of hubcentric (which these cars are, like most) is that they are not lugcentric, i.e must be centrally supported by the spigot. The wheel bolts are not designed to take the shear loads unless centrally located?
Either way, with a 5mm you will still be located on the spigot, but if your bolts decide to back out a touch say (has happened to me once upon a time), there's not a lot of meat left on the bone.
BMWZ4MC wrote: Wed Jul 18, 2018 9:41 pm As for 5mm spaces being poorly machined leading to imbalance, that risk is true of any spacer or wheel.
true, but I'd say the risk is higher with one that is not hubcentric. Say for instance the 5mm spacer hasn't been machined perfectly to ensure it is centered with equal distance around the hub and then slides over when tightening the bolts.
RedUn wrote: Wed Jul 18, 2018 9:09 pm I used to think that but some OEMs (Porsche) run a 5mm spacer from the factory
Do you know the depth of those spigots? Perhaps they are larger, I see talk of 7mm (not hubcentric) which would be pretty on the limit with these hubs.
I can't remember the depth (it looks about the same!?) but on the Porsche the spigot is not a full spigot, it's machined away to save on weight, it's basically like a castellated nut if that makes sense? I suppose there's even less surface area there though which makes me think they aren't that worried about it?
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VagBremTechnic Big Brake Kit for E85/E86 Z4 'M' models

Post by TomK » Mon Jul 23, 2018 2:43 pm

BMWZ4MC wrote: Fri Jul 20, 2018 4:22 am If you were to loosen all the wheel bolts on a wheel without a spacer, a stationary car might be able to stand on the hub spigot. However, as soon as you introduced torque from accelerating or braking, the wheel would rotate on the hub until the bolt holes met the bolts. The bolts would either then shear or transmit the torque between hub and wheel. I would expect any lateral forces from cornering with loose bolts to be carried through the bolts unless the wheel was a virtual press fit on the spigot.
Edit - Did you replace your wheel bolts after running with them not torqued up? The bolts are intended to see tension loads only and they might have looked ok macroscopically but still have been exposed to huge uncontrolled stresses.
Thanks, i'm beginning to understand the concept of a bolted joint, and yes therefore the spigot is really just for locating.
RedUn wrote: Sun Jul 22, 2018 12:50 pm I can't remember the depth (it looks about the same!?) but on the Porsche the spigot is not a full spigot, it's machined away to save on weight, it's basically like a castellated nut if that makes sense? I suppose there's even less surface area there though which makes me think they aren't that worried about it?

Seems BMW chose to run 5mm spacers on the front of the E92 GTS too so I'll get me coat :lol:
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VagBremTechnic Big Brake Kit for E85/E86 Z4 'M' models

Post by TomK » Mon Jul 23, 2018 5:39 pm

Interesting explanation of it all from Apex, who I guess know their stuff.
Eddy@ApexRaceParts wrote:Mon Jul 23, 2018 2:43 pm
5mm spacers are fine in most cases as they can keep the car 100% hub-centric. It seems there is a LOT of misinformation about what hub centric really means, and how spacers effect a hub centric fitment.

Just because 5mm spacers do not have an extended hub, does not make the wheel assembly non-hub centric. As long as hub material is making contact with the center bore, the assembly will remain hub centric, and in most cases a 5mm spacer works as there is enough of the factory hub sticking through, that everything remains centered.



Hub centric means the hub keeps the wheels centered. The lugs hold the wheels to the car. This is important to understand, because the hub itself holds very little load on it's own. It's simply a guide to keep the assembly centered as there is a lot of slop and play with BMW lugs. Many cars on the other hand are lug centric. Meaning the hub is meaningless and the lugs don't have the same slop. They can center the wheels on their own. Two different designs, both work. BMW chose one.

The important thing to remember about the above is that the hub on a BMW only has to keep things centered, which is not a complex task. To do that, it only needs a few millimeters of contact with the wheel. Most BMW hubs have about 10mm of hub extension. If that sticks into a wheel 10mm, then you can use a flat spacer so long as there is still a few millimeters of material making contact with the wheels center bore.

Some wheels have a chamfered edge on their center bore, while others don't, and that chamfer can very in depth as well. That has an impact on how deep the hub has to reach to make proper contact with the center bore. In most cases a 5mm spacer will still have a few millimeters of center bore contact even on a wheel with a chamfered center bore.

An 8mm spacer on the other hand would not leave enough hub to keep the wheel centered since most wheels have a chamfered edge that is a few millimeters deep, that would make it impossible for the remaining factory hub material to make contact with the wheel. Because an 8mm spacer is flat (no attached hub extention) people tend to pool them all together and make generalizations.

The spacers shown in that thread above are NOT a universal spacer. They are still a bmw specific spacer as they are 5x120. It doesn't need an extended hub, as the factory hub does that work, and it would be impossible to create 5mm spacer with an attached hub.

In Summary:

The chamfer of your wheels are the determining factor of whether or not a 5mm spacer will work.
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VagBremTechnic Big Brake Kit for E85/E86 Z4 'M' models

Post by MrPT » Mon Jul 23, 2018 6:24 pm

Cheers for posting that - really good info.

Really does emphasise how important the centre bore contact is, not just for spacers but for anybody looking at non-standard alloys. I wonder how many people are using 5 x 120 aftermarket wheels with no hub contact?! :?
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VagBremTechnic Big Brake Kit for E85/E86 Z4 'M' models

Post by Bodyboarder » Tue Jul 24, 2018 11:09 pm

..... so we are now saying a 5mm spacer should be good to go :wink:

Where are people buying said spacers and longer bolts ?

Thank you

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VagBremTechnic Big Brake Kit for E85/E86 Z4 'M' models

Post by Mister T » Tue Jul 24, 2018 11:44 pm

I like the H&R Trak+ ones

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VagBremTechnic Big Brake Kit for E85/E86 Z4 'M' models

Post by BMWZ4MC » Wed Jul 25, 2018 4:21 am

I make my own from old baked bean cans.... :D
Otherwise, H&R spacers from eBay are usually a reasonable price.
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VagBremTechnic Big Brake Kit for E85/E86 Z4 'M' models

Post by MrPT » Wed Jul 25, 2018 7:36 am

BMWZ4MC wrote: Wed Jul 25, 2018 4:21 am I make my own from old baked bean cans.... :D
Grub-centric?

:tumbleweed:
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VagBremTechnic Big Brake Kit for E85/E86 Z4 'M' models

Post by Bodyboarder » Thu Aug 02, 2018 1:34 pm

Calling MrPT!!

Right , got my 5mm spacer on , along with longer bolts . No issues whatsoever.

Please see below pics . It would be massively helpful to me if you could take a measurement from the same spot on your setup . I’m hoping there’s a 1mm difference. I’m measuring 56/57mm from disc to the inside of the spoke ( see pic with what spoke )

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Image

Image

Really appreciate if it’s at all possible :thumbsup:

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Post by Bodyboarder » Sat Aug 04, 2018 8:15 am

Bump for MrPT :oops:

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VagBremTechnic Big Brake Kit for E85/E86 Z4 'M' models

Post by MrPT » Sat Aug 04, 2018 7:12 pm

Bang on 55mm using the same method.
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VagBremTechnic Big Brake Kit for E85/E86 Z4 'M' models

Post by Bodyboarder » Sat Aug 04, 2018 7:14 pm

MrPT wrote: Sat Aug 04, 2018 7:12 pm Bang on 55mm using the same method.
Ah that’s great , really appreciate it :thumbsup:

Now know that the calipers will fit :D

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