Rear brake torque arm, frame mount vs. swing arm mount.

[Arnie]

I'm not sure we can come to agreement on all part of this, but I'll give it one more shot.
And, I'm sure you'll poke holes in my explaination. :-/

We (mostly) seem to be in agreement that the brake caliper bracket can rotate on the axle if not constrained by the torque arm.  This is good, as it IS the case.

The rear wheel and the various components that rotate with it have a certain amount of inertia or momentum.  When the caliper squeezes the disk most of the momentum is translated to a linear force acting through the torque arm.  If the torque arm is connected to the swing arm it will  become a triangular structure and as long as all the pieces are strong enough to not bend then there will be no effect.  So my initial statement was wrong.

However, if the torque arm is connected to the frame and the caliper is underslung then the torque arm is in tension when braking and this will help limit the rear of the bike from squatting.

Arnie

[Pat Conlon]

Sorry Arnie, I did not mean to slam you. I wanted to show where I agreed and disagreed with your statements.

Remember the squeeze that Harvy was talking about? The squeeze between the swing arm and the caliper bracket when the axle nut is torqued. We all agree that this squeeze will slow down, or resist (but not stop) rotation of the unrestrained caliper bracket as it rotates around the axle.
Keep this thought for a minute.

Now if your brake torque arm (or stay arm) is connected to the frame here's a test you can do to see what I mean:
1) Loosen the back axle nut.
2) Put the bike on a center stand (or other method) to raise the rear tire off the ground. Do not use a swing arm stand as you will see why in a minute.
3) Support the rear wheel with a block and disconnect one side of the 2 dog bones, so now you can move the swing arm up and down without fighting the shock.

Now move the wheel/swingarm up and down looking at the position of the rear caliper. Notice as you move the swing arm up the rear caliper rotates (moves) forward towards the front of the bike, and as you move the wheel/swingarm down the caliper rotates (moves) to the rear of the bike.
This is the trapezium that Harvy was talking about.

**The fact that the torque arm is anchored to a spot on the frame that is different from the location of the swing arm pivot bolt, means that the caliper will move back and forth as the swing arm moves up and down**

Ok so you say, "What's the big deal if the caliper moves back and forth when the swing arm moves up and down?"

Here's the rub (get it?) All is fine when the axle nut is loose. The swing arm moves up and down, the caliper rotates forward and back, nice and easy, no problem.
Now with the dogbones still disconnected....Tighten up the back axle nut, remembering the squeeze that Harvy was talking about?  Now move the swing arm up and down. Feel the difference? The friction from the squeeze on the caliper arm is fighting the free range of motion of the swing arm.

If the torque arm is connected to the swing arm, the caliper bracket and caliper stays still (in one spot) as the swing arm moves up and down.

[carsick]

You guys use the rear brake?
I am very tempted to stick my hand in this blender of a conversation, but the weather's too nice outside to be sucked in. I do congratulate all parties involved for keeping a civil tone, and diplomatically trying to convince dissenters that they just don't understand. I was going to disagree with a few points, then realized I don't know what the hell I'm talking about and couldn't amass sufficient evidence to prove my position. I am enjoying the lively discussion, keep up the good work folks!

[Mark Olson]

thanks to pat for clearing that up.

my head was starting to hurt

[Arnie]

Pat,

I didn't take your response as a "slam", so no need to apologize.  I am OK with people disagreeing with me and supporting their position.  I might even learn something. :-)

Trapizius is a muscle.  Trapizoid is a quadralinear figure that if regular looks like a truncated triangle.  I don't know what a trapizium is.

Arnie

[Pat Conlon]

 I don't know what a trapizium is.

Touche mi amigo....LOL, neither do I... but it sounded bitchen, huh?

[Firehawk068]

I don't know what a trapizium is.

Arnie

A trapizium is a device used during various high-flying acts in the circus......but only if it is held in a gymnasium....

[simi_ed]

Trapazoid

[rlucas]

Pat,

I didn't take your response as a "slam", so no need to apologize.  I am OK with people disagreeing with me and supporting their position.  I might even learn something. :-)

Trapizius is a muscle.  Trapizoid is a quadralinear figure that if regular looks like a truncated triangle.  I don't know what a trapizium is.

Arnie

Trapezius/trapezoid. I don't know what a "trapizoid" is. 


Gaudere's Law: Any post containing a correction to an error in spelling or grammar will itself contain an error in spelling or grammar. ("Gaudere" was a moderator on the Straight Dope Message Board). Also known as "Muphry's Law", intentionally misspelled.

St. Rossi the Pedantic

[Pat Conlon]

I am not familiar with the GSXR wheel swap.......
I have a question for you.
Does the caliper mounting bracket pivot on a bearing of some sort?
By torquing the axle nut tight you are locking it in place otherwise, and having the brake torque arm mounted to anything other than the swingarm will want to push the caliper backwards, and pull it forwards when the swingarm moves up and down.
Just want to make sure you are not about to embark on some sort of unsafe riding experience.

There is a bush in the brake mounting bracket which allows movement.

Connecting the brake torque arm to a fixed part of the chassis allows the brake to not affect the suspension as the wheel moves up and down and for the braking force to be isolated from chassis movement.

Arnie

Hey Arnie, I'm sorry I missed this fact. 
If the GSXR caliper bracket does have a bushing, then my discussion (pontification) on the squeeze friction inhibiting the swing arm travel does not apply.
  The bushing will take the squeeze and not the caliper mounting arm. I was unfamiliar the the GSXR bracket.
The FJ bracket does not have this bushing feature nor does the YZF1000 Thunder Ace brackets which I am familiar with...
You will still get a little bit of drag from the GSXR bracket rotating over the bushing but with a bit of Moly Lube it should be no big deal.

Again, sorry about that.... 

[Travis398]

You guys use the rear brake?
I am very tempted to stick my hand in this blender of a conversation,

All right I'll stick my hand in, don't those GSXR's calipers mount to the frame.
seems like if it was a big "no no" they would have changed that back in the 80's

Not saying it is better or worse than mounting to the swing arm, but......come on...........

[the fan]

The 91-92 GSXR1100 caliper mount has a pivot built in to allow it to freely rotate around the axle as the wheel moves, provided it is properly working of course. It really does not matter where it is attached provided it is able to move completely through its range of motion without binding. if the bushing seizes you can run into serious handling issues if it is attached to the frame rather than the swing arm. It's fairly unlikely that this will happen if carefully installed and maintained but it can be a concern.

There is very little reason aside from cost to design a system like the suzuki but it does work well. The yamaha stock parts must be attached to the swingarm as they do not have this bushing and are not free to rotate. When modifying a bikes chassis all aspects of the mod need to be taken into account.

[Harvy]

I am not familiar with the GSXR wheel swap.......
I have a question for you.
Does the caliper mounting bracket pivot on a bearing of some sort?
By torquing the axle nut tight you are locking it in place otherwise, and having the brake torque arm mounted to anything other than the swingarm will want to push the caliper backwards, and pull it forwards when the swingarm moves up and down.
Just want to make sure you are not about to embark on some sort of unsafe riding experience.

There is a bush in the brake mounting bracket which allows movement.

Connecting the brake torque arm to a fixed part of the chassis allows the brake to not affect the suspension as the wheel moves up and down and for the braking force to be isolated from chassis movement.

Arnie

Hey Arnie, I'm sorry I missed this fact. 
If the GSXR caliper bracket does have a bushing, then my discussion (pontification) on the squeeze friction inhibiting the swing arm travel does not apply.
  The bushing will take the squeeze and not the caliper mounting arm. I was unfamiliar the the GSXR bracket.
The FJ bracket does not have this bushing feature nor does the YZF1000 Thunder Ace brackets which I am familiar with...
You will still get a little bit of drag from the GSXR bracket rotating over the bushing but with a bit of Moly Lube it should be no big deal.

Again, sorry about that.... 

Ahha........OK now that makes sense......... I was also unaware of this......so the hanger can rotate about the axle freely with the axle torqued, and the stay bar can pivot at the front the back mounting points.
That being the case.........I withdraw my concerns also.

Harvy

[WestOzFJ]

It really does not matter where it is attached provided it is able to move completely through its range of motion without binding.

It DOES matter where it is attached. It makes a HUGE difference.

If the brake reaction arm is mounted to the swing arm then the torque reaction is ultimately transferred through the suspension and results in rear wheel squat. Rear wheel "hop" under high braking load is caused by this reaction of compressing the rear suspension which then unloads, loses traction, suspension drops back down due to no braking effect/no torque reaction, tyres contacts the road again so braking force torque reaction back on and so the oscillating cycle starts again....

When the arm is mounted to the frame the 100% of the torque reaction is no longer transferred through the suspension although, some percentage or component of the total force may in effect transfer which will depend largely on the angle of the arm and where it mounts particularly in relation to the swing arm angle and pivot point.

To demonstrate this, there was a 1970's experiment on Barry Sheen's grand prix Suzuki whereby the FRONT calipers were mounted on floating radial mounts and the torque reaction arms went vertically to mount on/under the lower triple tree. Therefore upon suspension compression the calipers rotated freely and were in effect suspended unsprung weight - one of the reasons they tried it. When heavy front braking force was applied the front of the bike didn't dive - it actually ROSE UP because as the calipers tried to rotate, the torque reaction was transferred vertically and converted to suspension movement according to the forces now at play.

So lets transpose that experiment to the rear and to demonstrate the potential, imagine a hypothetical where a true floating rear caliper that (let's say) sat in the 9 o'clock position and the reaction arm were to mount vertically upwards to the rear subframe.... The caliper would become (to some degree) unsprung mass as it's now supported and the braking force would raise the back of the bike upwards actually creating artificial mass and therefore increasing traction.

It makes a HUGE difference where it's mounted - it's all about directing that force...

[ian.z9]

Interesting discussion.The rear brake has a nice progressive action and no real noticable affect on the rear suspension action.I hope I'll be as happy with the front end when I upgrade that.