Swingarm failure - torque arm mounting point

[kebab19]

Had a bit of an incident this morning.
Normal road riding conditions probably doing 50 or so at the time.
Pulled front & rear brakes to slow down, heard a fairly loud crack then realised that the rear brake pedal was solid and wasn't working anymore.  I was about 2 miles from home so limped back and then had a look in the driveway.
I was disturbed to see both the rear caliper and the torque arm had rotated up round the top of the swingarm.  I thought the torque arm securing bolt had either come off or broken but then took a look under the swingarm where the torque arm mounts    Brake fluid all over the back wheel after the line got twisted round.
These swingarms are alloy so it can't be due to corrosion.  I have no idea how this could have happened, everything was secured safely.  Could it be down to the bluespot rear (or silverspot in my case)? Metal fatigue?
I realise some people will suspect I was acting an eejit, locking the rear at 80 or something, but this wasn't the case.  Sobering stuff....
Anyway, I now need a replacement swingarm, RHS chain adjuster and rear brake line now  and new pants

[fazersharp]

Glad that you are safe mate - but I don't know why people want to go changing the original back calliper and arm anyway and your experience is another reason not to do so.

[Fazerider]

 
Scary. Lucky it didn't lever the back wheel off the road as the arm pivoted round. Guess I'll take a look at mine tomorrow to check it's not threatening to do the same thing.
Good luck with the repairs.

[His Dudeness]

Lucky escape alright. Any chance the bolt was over tightened and that might have cracked the lug on the swingarm?

[Paulfzs]

Powdercoat could hide anything, as above maybe it was too tight, suppose you might get away with making a bracket and drilling the swingarm if youre feeling brave...


This is why i kept the original torque arm.

[His Dudeness]

You could get it ali welded if you wanted

[tweetytek]

If you were doing 80 when that happened your lucky the whole thing didn't go through the wheel and send you flying into the air in a different direction to the bike. Glad you are here to do the show n tell. I won't be fitting a fancy torque arm/blue dot now.
Cheers and stay safe

[unfazed]

Fitting a blue dot caliper would have nothing to do with it as it is fitted to the standard mounts.

Fitting a non standard torque would be a more likely culprit.

Another culprit would be, not loosening the caliper side and swinging arm side torque arm bolts when adjusting the chain.

[Skippernick]

Fitting a blue dot caliper would have nothing to do with it as it is fitted to the standard mounts.

Fitting a non standard torque would be a more likely culprit.

Another culprit would be, not loosening the caliper side torque arm bolt when adjusting the chain.

Sorry don't get that unfazed, why would you need to do that? If the torque arm is undone but the calliper bolts aren't and you realigned the rear wheel/chain tension the torque arm would still be in the same place. It's all a fixed point.

[His Dudeness]

The angle of the torque arm changes as you adjust the chain. You really notice it when you go from an old stretched chain to a new one.

[fazersharp]

Fitting a blue dot caliper would have nothing to do with it as it is fitted to the standard mounts.

Fitting a non standard torque would be a more likely culprit.

I always thought (from reading past posts )that if you were fitting a blue dot then the OEM arm was the wrong length and so you also need a different arm to match.

[tweetytek]

And thus if you don't loosen the calliper end torque bolt, the arm is stressed under tension, and this applies a shearing force to the other end of the torque arm mount... where yours has sheared off !!

[fazersharp]

If you look at this picture the area around the mount point on the face of the swing arm looks ground off a little, also the welding on the mount point dosent look like a factory neat job,has this already been re welded in the past I wonder, I also see a different shock and dog bones as well as the non oem caliper and arm .
I wonder if its a combo of all these things was just too much of a change in speck for the OEM mount point
 

[unfazed]

The angle of the torque arm changes as you adjust the chain. You really notice it when you go from an old stretched chain to a new one.

And thus if you don't loosen the calliper end torque bolt, the arm is stressed under tension, and this applies a shearing force to the other end of the torque arm mount... where yours has sheared off !!

Eureka 



Nothing to do with suspension

[fazersharp]

The angle of the torque arm changes as you adjust the chain. You really notice it when you go from an old stretched chain to a new one.

And thus if you don't loosen the calliper end torque bolt, the arm is stressed under tension, and this applies a shearing force to the other end of the torque arm mount... where yours has sheared off !!

Eureka 



Nothing to do with suspension

Hmmm -- I pulled my wheel back a few months ago by about 2mm (yes) do you think I should now go and loosen off the caliper-side arm bolt, and then re tighten to realease said stress

[tweetytek]

Mmmm

[fazersharp]

Sorry I dont get you tweety ?

[tweetytek]

LoL
Soz. I was pissin around a bit. 4give.
Loosen the bolt AND the calliper hanger bolt, THEN adjust chain, and then tighten up to the torque specs

[fazersharp]

Ok, I sort of knew I should losen the arm bolt but from my memory of the manual there is no need to lossen the caliper hanger bracket.
 
EDIT or by caliper hanger do you mean caliper hanger side of the arm.

[midden]

Should loosen both ends of the torsion bar for adjustment 

[unfazed]

Should loosen both ends of the torsion bar for adjustment 

You are correct Midden, it should have read "not loosening the caliper side and swinging arm side torque arm bolts when adjusting the chain"

Unfortunately 90% of people including mechanics do not 

I have changed the post to reflect your correct observation, it is obvious from the comment  that you too loosen the bolts as do I and a few others on the forum. 
What bother me most is the number of mechanics who do not loosen these bolts or understand why you should 

[unfazed]

I am glad kebab19 was ok with nothing more than shock and the lightening of his pocket to repair the damage.

Great advantage of this forum is that by him posting what happen and the comments of how and why will prevent it from happening to others 

Thankfully we learn a little everyday and some days what we learn may help us live longer. 


Kebab19, you are probably aware, but in the off chance you are not, the foxeye swinging arm is different to the boxeye ones.

I personally would also change the torque arm and caliper bracket just to be absolutely confident.

[fazersharp]

Im still scratching my chin on this whole tourque arm thing.
How come then if you should loosen it (both ends ) before you pull back the wheel-------- how come you CAN still pull back the wheel without loosening said arm.
And what exactly is the tourque arm providing tourque against or what tourque is it countering, I always thought it was to stop the caliper spinning around the disk when you brake --- but isnt it bolted in place?

[tweetytek]

Im still scratching my chin on this whole tourque arm thing.
How come then if you should loosen it (both ends ) before you pull back the wheel-------- how come you CAN still pull back the wheel without loosening said arm.
And what exactly is the tourque arm providing tourque against or what tourque is it countering, I always thought it was to stop the caliper spinning around the disk when you brake --- but isnt it bolted in place?

edited out my response: changed my mind !  kebab , you have answered your own question and also seen the results of not having a torque arm or having one that isnt to spec, non OEM, or has been stressed out of normal parameters

try to visualise in math terms...

imagine a triangle made of tough steel rods connected by bolts at each corner. One side is your vertical on the swingarm, one side is from the vertical to the caliper along the swingarm horizontal, and the final side is the torque arm. Now imagine pushing and pulling on the calliper , what do you think is going to happen to the shape of the triangle. Push hard, force, its going to "bend" the triangle and eventually one side is going to bust (or a corner).

Now, undo some of the bolts and the triangle now starts to change its shape, its still a triangle but the angles are changing  -and thus the shape - as you move the corners around pushing and pulling. It moves freely now but there is no rigidity - things can move around the pivots (corners) and so can the calliper !!!.

Tighten up the bolts again and you have a rigid triangle shape. Try to bend it now or apply forces too great, and it will break but under normal parameters the shape remains rigid and the pivots stay where they are (the calliper doesnt move!)

So if one of the sides is weak (a non OEM torque arm or an arm not to spec) then the forces applied when braking are going to try to move that triangle shape and the weak point (the arm) is going to give or a pivot (the mount) is going to shear

When you adjust your chain you are changing the wheelbase and the shape of the triangle; loosening the bolts allows the calliper to rotate slightly around the pivot and bring the triangle into a slightly different shape and moves the torque arm slightly (the swingarm "sides" of the triangle cannot move). Without allowing the calliper to rotate, you are trying to apply those forces to the triangle without allowing the shape to change in reciprocation and thus "blow" the triangle out of shape or make it come loose at the weakest point - a pivot point (the swingarm mount in your case).

Its all very minor adjustments but the sum of the parts is greater.
 
. Voila!

You have your answer

[fazersharp]

Errr forgive me but I will read that again another day --------- when Mr Tyskie is not with me