Potential catastrophic brake failure.

[riffer]

Isn't "replace the bolts every time the calipers are removed" the official factory stance ? I don't bother doing this when replacing pads but tend to do this when popping out pistons etc. I've never bothered with loctite either.

Agreed. Now that Supercheap stock flange head high tension bolts for $8 for six there's not much of an excuse really.

[Warr]

Agreed. Now that Supercheap stock flange head high tension bolts for $8 for six there's not much of an excuse really.

Spot on thinking there riffer.. I have to admit to personal experience of loosing caliper bolts and having trouble replacing them with high tensile ones..

[Flange]

Specified torque for that bolt is 19 ft/lb. Assuming that the bolts which Suzuki use are around 8 grade, this is a pretty reasonable torque setting for the size. I concede that it's possible that by using a whiff of low strength Loctite (rather than a clean dry thread) I might be overloading the bolt slightly, but I'm confident that this is not a factor in this case.

My practice is to tighten torque-specified bolts with a spanner or ratchet to the estimated torque, then finish them with one of two high quality torque wrenchs I own.

As for replacing the bolts upon every removal, I've seen no mention of this in either the owners' nor the workshop manual, and it's not something which I'm aware of being routinely done.

It's interesting that Tokico used to fit 10mm bolts to the predecessors of the SV's callipers (as fitted to ZX7s for instance) but saw fit to reduce this to 8mm on the SV's iteration. I would be quite confident that they've done their homework before moving to 8mm bolts, and would suspect that in my case the problem is a bad batch of bolts. The question is, how big was this batch ?

The breakage occurred on the trailing edge bolt and the waisting occurs between the end of the shoulder and the point at which the thread engages in the calliper.

Brian D'marge's observation of rust is very good. The rust occurs within the final area to let go as witnessed by the fresh, clean area vs the outer contaminated areas which I would suggest have been parted for some time. I'm not much on metallurgy but wonder if the rust indicates an improperly mixed batch of metal ? Note that the rust is only in a couple of small spots rather than spread across the clean area.

Neither the calliper nor the fork mounting lug holes are deformed.

I take great comfort in your advice AllanB - do ya reckon a piece of 4x2 through the rear wheel would be the appropriate safety measure here ?

Thanks for the interest and suggestions Team.

Cheers
Flange

[The Stranger:Me]

Spot on thinking there riffer.. I have to admit to personal experience of loosing caliper bolts and having trouble replacing them with high tensile ones..

Did you use a torque wrench and set them to the correct torque?
Just wondering, because I have never seen a bolt tightened with a torque wrench to the correct torque yield or fall out yet.

Just seen the post above. There's my first time.

[Warr]

Did you use a torque wrench and set them to the correct torque?
Just wondering, because I have never seen a bolt tightened with a torque wrench to the correct torque yield or fall out yet.

Just seen the post above. There's my first time.

Lets just say... to protect the innocent .... no a torque wrench wasnt used... nor any sort of spanner...
Finger tight wasnt the intention ... but these things happen

[Brian d marge]

Brian D'marge's observation of rust is very good. The rust occurs within the final area to let go as witnessed by the fresh, clean area vs the outer contaminated areas which I would suggest have been parted for some time. I'm not much on metallurgy but wonder if the rust indicates an improperly mixed batch of metal ? Note that the rust is only in a couple of small spots rather than spread across the clean area.

Cheers
Flange

Max preloads may have it right , from the pictures I cant tell , and I am unfamiliar with the type of bolt ,

A crack leading to brittle failure, Yes there is crystalline structure visible by the rusty area that "step" has suddenly failed ( I can make out if that grey -crystalline area ) is at 45 deg indicating shear stress failure

The outer edges I look like Fatigue failure bought on by a stress raiser such as thread root damageand a cyclic load ( brakes ) ... The pictures I am looking at seem to show the beach marks , and have been opening and closing for a while hence the rust ,, , The Question is though , those bolts should be infinite life., ( sorry brain is fried after trying to mesh a brake lever ,,cant remember stress/no of cycles graph name )

Now on to Torque wrenches, even the most expensive wrench is still a best guess , bolts work the same way a rubber band does, so measuring the stretch is a better way, Are BMW still using the turn of the wrench method ?

In a shear Joint ,,,( dont quote me Im tired ,,,I will check tomorrow if needed ) The higher the preload in the bolt the better the fatigue resistance of the nolt , What COULD have been happening is that either the bolt was elongated before , or not been tightened up properly lowerering in fatigue resistance

Me I am a locite fan ,,,

Stephen

0.02 cents and time for bed
nite

[imdying]

Isn't "replace the bolts every time the calipers are removed" the official factory stance ? I don't bother doing this when replacing pads but tend to do this when popping out pistons etc. I've never bothered with loctite either.

Naw, just the disc bolts.

Not much you can do, have the wrench checked, replace the bolt, cross all the appropriate fingers and toes.

[CookMySock]

The outer edges I look like Fatigue failure bought on by a stress raiser such as thread root damageand a cyclic load ( brakes ) ... The pictures I am looking at seem to show the beach marks , and have been opening and closing for a while hence the rust ,, , The Question is though , those bolts should be infinite life.

Assumption! There might be a vibration problem here. What else has been dissasembled or modified in this area?

It is possible the loctite went off before the bolt was sufficiently tight - the main reason I do not like loctite.

Now on to Torque wrenches, even the most expensive wrench is still a best guess , bolts work the same way a rubber band does, so measuring the stretch is a better way, Are BMW still using the turn of the wrench method ?

Agreed. Thats the only way to properly set any fastener, and the reason why using a higher grade bolt often doesn't work.

In a shear Joint ,,,( dont quote me Im tired ,,,I will check tomorrow if needed ) The higher the preload in the bolt the better the fatigue resistance of the nolt , What COULD have been happening is that either the bolt was elongated before , or not been tightened up properly lowerering in fatigue resistance

I'm with mister d'marge on this one. More likely the latter due to loctite giving a false reading to the torque wrench. Be careful with loctite.

Theres a lot more to nuts and bots than meets the eye.

Steve

[erik]

Note that the intact bolt is showing clear signs of stretching just out from the shoulder area - the waisted area is over 1mm thinner than the area which threads into the calliper. The other two mounting bolts are showing the same signs to a lesser extent.

The breakage occurred on the trailing edge bolt and the waisting occurs between the end of the shoulder and the point at which the thread engages in the calliper.

Are you sure that's waisting? Wouldn't it break at the thread rather than stretch at or near the shoulder where it's larger diameter? And unless the bolt was made out of mild steel, wouldn't it fracture before stretching that much?

[Flange]

Are you sure that's waisting? Wouldn't it break at the thread rather than stretch at or near the shoulder where it's larger diameter? And unless the bolt was made out of mild steel, wouldn't it fracture before stretching that much?

You are correct Erik, the breakage did occur at the thread.

The waisting I refer to is not the reduced diameter portion of the shoulder, rather in the threaded portion outwards from the shoulder. You will notice that the narrowest area of the threaded portion occurs at around the third and fourth thread out from the shoulder.

Cheers
Flange

[kiwi cowboy]

Max preloads may have it right , from the pictures I cant tell , and I am unfamiliar with the type of bolt ,

A crack leading to brittle failure, Yes there is crystalline structure visible by the rusty area that "step" has suddenly failed ( I can make out if that grey -crystalline area ) is at 45 deg indicating shear stress failure

The outer edges I look like Fatigue failure bought on by a stress raiser such as thread root damageand a cyclic load ( brakes ) ... The pictures I am looking at seem to show the beach marks , and have been opening and closing for a while hence the rust ,, , The Question is though , those bolts should be infinite life., ( sorry brain is fried after trying to mesh a brake lever ,,cant remember stress/no of cycles graph name )

Now on to Torque wrenches, even the most expensive wrench is still a best guess , bolts work the same way a rubber band does, so measuring the stretch is a better way, Are BMW still using the turn of the wrench method ?

In a shear Joint ,,,( dont quote me Im tired ,,,I will check tomorrow if needed ) The higher the preload in the bolt the better the fatigue resistance of the nolt , What COULD have been happening is that either the bolt was elongated before , or not been tightened up properly lowerering in fatigue resistance

Me I am a locite fan ,,,

Stephen

0.02 cents and time for bed
nite

Whot's this rubber band and turn of the wrench you speek of?

[Brian d marge]

A screw clamps in the same way an elastic band does ,,by stretching , and the turn of the nut is ,,snug fit plus half a turn,

Stephen

[Flange]

A screw clamps in the same way an elastic band does ,,by stretching , and the turn of the nut is ,,snug fit plus half a turn,

Stephen

Logic would tell me that this system would require absolute consistency of tensile strength of the bolt ?

[FROSTY:Me]

Flange I wonder -have you had tyres/wheels fitted at a tyre shop. --Thats just a classic symptom of rattle gun overtightening

[Flange]

Always done tyres myself Frosty. Likewise all mechanical work.

DangerousBastard vocalised an interesting question - what's been modified in the area? The original brake rotors were replaced very early in the bike's life when one of the factory rotors went sloppy on it's carrier. The current rotors are Moto Master Flame rotors. Likewise the OEM pads were replaced at the same time with SBS Dual Carbon items. Further, I've modified the fork valving, fitted linear springs and am using non-standard fork fluid. So, you could say that the whole area is pretty well "modified".

Does any of this contribute in any way to the failure of the calliper mounting bolts? One would have to have a pretty closed mind to say that it's impossible, however looking at it analytically I can't pinpoint any causitives.

The rotors are of a fairly agressive pattern which I could imagine producing some vibration, but the brakes feel smooth. That's not to say that there isn't a vibration at a frequency beyond my senses - possible, but not very likely I think.

The pads are less sensitive than the OEMS, less "grabby", more progressive, so I think that these pads would be easier on affected metals.

Does the current braking system produce more braking power than standard? Possibly but the standard SV system is powerful enough to either overcome tyre adhesion or to stand the bike on it's nose, so the modified system shouldn't be capable of generating greater braking stresses on the bike. In any case my riding style is pretty easy on brakes.

Have the spring/damping mods produced a harsher action which transfers higher "G" loads to the callipers? To the contrary, these mods have made the forks more compliant - the ride is much better than standard.

It still comes back to faulty bolts I feel...

Cheers
Flange