Replacing OEM screws on brakes ... any risk?

[bikermeow]

Hiya ...

My bike came with rusted bolts on the front brake calipers (Allen key type) and on the rotors (torx type). They appear to be of a dull galvanised silver finish. can I replace them with stainless steel bolts with the same thread size?

My mechanic said something about the rotors heating up, which makes stainless steel ones unsuitable.

Views?

Thanks,

Meow

[mdouglas]

I've replaced disk and caliper bolts on several of my boxers and they've all been absolutely fine. I ride all year round and cannot tolerate the bike looking like a corroding pile of crap after 1 winter season.

[baltm604]

I would have no concerns about the bolts holding the caliper.

Now the bolts holding the rotors to the wheels are another story.


I overheated the rear rotor so badly that the bolts came out with no effort after the heat had burn away the red loctite!!! A bolt that holds heat, like stainless, would be a bad thing I think.

Bill

[boxermania]

bikermeow.....

The bolts that afix the caliper and the rotor have a specific design strength. Both of the applications are predominantely in shear (the forces acting upon try to cut through the bolt). I agree with your mechanic that replacing the sandard bolting on your rotors with SS could lead to problems. Of course bear in mind that your mechanic has to contend with potential liability issues, so when you don't know play it safe.

Allow me to expand:

Most SS bolting is manufactured out of the 300 series (304, 309, 321) or what is refered as the Austenitic series in the lingo. The coefficient of thermal expansion is approximately 50% greaqter than that of the comparable steel components, 10.2 x 10-6/F degree vs 7.2 x 10-6/F degree for steel.

What this means is, as the rotor gets hot, specially after some heavy use the SS bolting is going to grow ~50% more than the steel counterpart. The end result being:

The taper at the head of the bolt will generate more force on the taper on the rotor. Problem: Potential for radial cracks on the rotor

The bolt will elongate more than the steel one. Loosenes of the rotor to the wheel mounting, likely

The bolt will expand more against the threaded bosses that hold the rotor assembly to the wheel. Cracking of the threaded bosses that attach the rotor to the wheel....OUCH, costly and dangerous

Of course the same applies to the caliper with the exception that the caliper does not get exposed to the temperatures of the rotor.

The bottom line is:

If your type of riding and the use of the brakes don't heat the rotors beyond 250 F you are probably safe with the use of same strenght SS bolting. I would not recomend them if temperatures are above 250F

Hope this helps..............

[bikermeow]

Wow ... that's really in-depth analysis.

Darn .... was really hoping that the stock bolts could go. The stainless steel ones I have in mind are really cheap where I come from, and somehow having bolts which rust easily on the BMW is not very nice, but I suppose that safety first.

Cheers

Meow

[mdouglas]

All of which is great. Except the rotor bolts on our bikes aren't even remotely tight to the rotor. So they can grow considerably with no impact. Unless the US bikes are made different, my rotors are located by some large stainless steel bushes (note SS - put there by BMW) and wavy washers. Its called a floating disk. I have had my bike on a track with stainless bolts all round and no issues.

[boxermania]

mdouglas.....you are absolutely correct regarding the "floating" nature of the BMW rotors. There is some clearance built in, where the bolt attaches to the rotor, that allows the rotor to align itself to the brake pads, reducing the potential of lateral missalignment between the rotor and the pads.

My post dealt with three potential problems: At the head of the bolt, elongation of the bolt and most importantly where the bolt threads onto the wheel, which is an aluminum alloy. The last one is the one that I would be concerned the most.

Am I correct in understanding from your post that you have replaced the factory bolting with SS and then race the bike, or just use it ocassionally on track days. If your answer to the above is yes, you have provided proof that the exchange can be made.....

[beemer-biker]

mdouglas.....you are absolutely correct regarding the "floating" nature of the BMW rotors.

My '02 R doesn't have floating rotors. I just checked. I had a '98 R1100RT which did. I thought they did away with them for, I believe, the '02 model year?

[collyers]

My helicopter has a floating rotor, but my '02 R1150R does not. If it bugs you muchly, paint them rustoleum black or silver & look the other way.

[mdouglas]

Occassional track days only.

I'm very surprised to hear people claiming they don't have floating disks. They may think they don't but I bet they do. Every exploded diagram I've seen in any Boxer Repair Manual, including 2003 onwards EVO Integral ABS use the same system - a top hat shaped bush and a wavy washer. They don't feel floating to the hand but they are.

[beemer-biker]

Occassional track days only.

I'm very surprised to hear people claiming they don't have floating disks. They may think they don't but I bet they do. Every exploded diagram I've seen in any Boxer Repair Manual, including 2003 onwards EVO Integral ABS use the same system - a top hat shaped bush and a wavy washer. They don't feel floating to the hand but they are.

Nope, just checked again. No top hat, no wavy washer, just tightly bolted directly to the rim. I had the tophat/wavy washer on my '98 RT, so I do know what to look for. Maybe the US spec bikes did away with them, but not the Euros? I am not going from a manual, just plain old eyeball check.

[mdouglas]

If your brake disks are secured like this:



then they are floating. The large washer looking thing under the bolt head is the "top hat" bush I referred to. It is deeper than the disk is thick. So when the bolt is done up tight, the disk is not being pinched against the wheel and in fact can move from side to side by a small amount. There is a thin, wavy washer (which you cannot see when everything is tightened up) that exerts a spring load against the disk to hold it still (so it doesn't chatter around).

[MikeCam]

Installing rotors on front wheel; have all parts, etc. Is there a tightening torque value for the rotor (with top hat and wave washer and ABS ring)?

Boxermania, mdouglas, dean, cyclerob, pat, dallara, darth, lance, bob, all???

[boxermania]

Sorry Mike, but I don't have the info with me, nor I remember. If you don'y get an answer soon call the dealer, they shouldn't have a problem telling you.

Since they are lfoating rotors, you don't have to worry about criss-crosing while torquing down, however I would torque down at least in two steps, to make sure the bolt relaxes.

I would also advise you to put a dab of Locktite Blue to insure that the bolts won't come loose.

Good luck............

[CycleRob]

MikeCam,

My CD manual says 21 Newton Meters for front and rear rotor bolts. Use Blue Loctite too.

Here's the page and it shows the "TopHat" bushing and wave washer too:

[MikeCam]

What a great resource! Thanks Rob. Helps tremendously.

[Photoguy]

Breathing new life into this old thread as I found my brake bolt heads had rusted badly over the winter. Got a small can of Rustoleum silver aluminum color and a small brush and painted them up after hitting them with an abrasive pad. While I was at it, I hit the bolt heads on the front of the engine cover as well as these seem to get a little corrosion especially the lower bolts. The paint has made them shinier than normal, but I suppose a ride or two will calm that down.