Single Front Disc Brake

[Bob_Roller]

Got a question for owners of R65's with a single front disc .

I was checking RealOEM.com for brake rotor bolts, only 2 are listed, one a 70 mm and the other 100 mm in length .

There is a part in the picture, a washer that goes on each side, one on the disc and the other on the nut side .

I'm just wondering if these washers take up the thickness of a second rotor hub or carrier .

So that should make the washers about 4-5 mm thick .

Item number 16 in the illustration .


http://www.realoem.com/bmw/showparts.do?model=0364&mospid=51897&btnr=34_0595&hg=34&fg=11  

[tvrla]

Rob,

Those spacers are very thin stainless. I thought they were NLA, but according to that source it appears they're available.

The first came on the 74 models with the disc brakes, and ended sometime in the late 70s I believe. I heard they were discontinued due to corrosion behind them.

I noticed the same thing - only two lengths of rotor bolts are listed. Odd...

[Bob_Roller]

Well, so much for my theory !!!!!!

[Mike V]

Bob,

Are these the spacers you're referring to?

-Mike V.

[Bob_Roller]

Alright, I should have checked this first !!

The 70 mm length bolts are 70 mm long from the bottom of the bolt head, to the end of the bolt .

The threaded area of the bolt is 30 mm long .

The 100 mm length bolt is 100 mm long from the bottom of the bolt head to the end of the bolt .

The threaded length of the bolt is 20 mm long .

So, the same bolt is used for the single and twin disc on snowflake wheels, the extra 10 mm of threaded length makes then usable for both brake versions .

[Bob_Roller]

Yes Mike those are the ones .

[Bob_Roller]

I forgot, that I had a spare 70 mm bolt, mistakenly ordered it last year for my '82 LS, the threads were not in good shape .

I have the rotors off of the '84 LS, I just found the bolts from them  this morning , it was actually pleasant in the garage this morning, it was 85 F (29C.) at 0700 .

[Mike V]

Bob,

You probably already know about this link.  I wanted to share with those who aren't aware of it.  I refer to it a lot since all this detail stuff is necessary if you're ever doing a restoration or extensive repair that requires ordering parts, hardware, etc.  I find this link very helpful with almost everything you would want (or not want) to know about fasteners.  Seems like I forget things from one week to the next and if I can remember where I store and organize my reference information ... it turns out to be a good day.

Anyway ... for what its worth;

http://www.boltdepot.com/fastener-information/Measuring/Measure-Length.aspx

-Mike V.

[Bob_Roller]

The one thing I don't like about the extra 10 threaded length of the snowflake wheel with two rotors, is that now all of the shear forces from one rotor are now in the threaded area of a bolt .

Standard aviation practice, is that you don't subject the threaded area of a fastener to shearing forces, the threads concentrate forces into a small area, increasing the possibility of the fastener shearing in that area .

Good example of this on our bikes, is the large countersunk head bolts that hold the center stand on .

I've had 6 instances where one of the bolts has sheared, as I was putting the bike in the center stand .

But I've never heard of this being a concern due to broken bolts before on this particular area, so I guess the bolts are strong enough .

[Mike V]

Bob,

Can you reverse the bolts so the shear forces of the rotor are put upon the shoulder area of the bolts?

-Mike V.

[Bob_Roller]

No matter which way you have the bolts installed, one rotor will be in the threaded area .

The thickness of the rotor hub or carrier is .325 in. (8.3mm), the thickness of the hub on a snowflake R65 wheel is 1.65 in. (42mm) .

The unthreaded length of the 70 mm bolt, is 1.57 in. (40mm) .

So with one rotor and the thickness of the wheel hub, it's 1.90 in. (48 mm) .

So, even with one rotor , the threads are inside the wheel hub, by about .315 in. (8mm) .

So, it's quite obvious in this application, that my concern of this, is pretty much unfounded !!!!!!!!!