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Adjustable brakes on new Commencal???

EVRAC

Monkey
Jun 21, 2004
756
9
Port Coquitlam, B.C., Canada
Something's not right with this:

It still features adjustable length chainstays, but now also has a three position rear caliper position. Rather than add a floating caliper, Commencal has given the rider three choices of rear caliper placement so they can tune how the bike sits up or squats under rear braking, depending on the course they're racing on.


How can different brake positions affect brake squat if the caliper is bolted to the swingarm. It's still the same rotor radius and angular deceleration. It will also cause the same torque on the swingarm. I think they're out to lunch on this one. Could be to do with the adjustable cs length, and the pr guys totally bungled it.
 

ZHendo

Turbo Monkey
Oct 29, 2006
1,575
44
PNW
the adjustable chainstay idea is pretty cool, but i too am skeptical of their claims regarding different levels of squat in the braking. i don't think it's possible to change the squat features unless it has some semi-floating design integrated into it that is somehow adjustable.
 

jonKranked

Press Button, Receive Stupid
Nov 10, 2005
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ok let me try to explain this as clearly as possible...

when a disc brake is activated, it creates a force tangent to the point the caliper is contacting the rotor, and the vector direction of the force will be relative (or is it opposite?) to the direction the wheel is spinning. By changing the point at which the caliper contacts the rotor, you change the angle of the tangent direction of the force. this will affect your suspension performance based on the angle between the swingarm motion and the tangent direction of the brake force.
 

davep

Turbo Monkey
Jan 7, 2005
3,279
0
seattle
Nope.

If that were the case, you could eleminate brake induced squat on a single pivot by simply 'pointing the vector' at the pivot.
 

jonKranked

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Nov 10, 2005
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Nope.

If that were the case, you could eleminate brake induced squat on a single pivot by simply 'pointing the vector' at the pivot.
that couldn't work because the angular motion of the swingarm and the rotor are counter to one another (because of the location of the 2 axes in relation to one another). 2 opposite circles with 2 opposite forces that would only be truly parallel (and opposite) one another for only an instant at a single point. also, the angle (to ground normal) of the braking force changes as the bike moves through its travel, so its dependent on that as well.
 

jonKranked

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Nov 10, 2005
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a side note on the above... with a fixed caliper the tangent angle of the braking force is dependent on the swingarm's position in its travel.

with a floating caliper brake mount, the point at which the caliper contacts the rotor stays in essentially the same position (and hence the angle of the braking force changes very little) regardless of where the bike is in its travel. this makes the braking forces fundamentally independent of the suspension. the downside? weight++
 

Fulton

Monkey
Nov 9, 2001
825
0
Something's not right with this:





How can different brake positions affect brake squat if the caliper is bolted to the swingarm. It's still the same rotor radius and angular deceleration. It will also cause the same torque on the swingarm. I think they're out to lunch on this one. Could be to do with the adjustable cs length, and the pr guys totally bungled it.
where did you quote this from? sounds like bs to me.
 

Steve M

Turbo Monkey
Mar 3, 2007
1,995
23
Whistler
that couldn't work because the angular motion of the swingarm and the rotor are counter to one another (because of the location of the 2 axes in relation to one another). 2 opposite circles with 2 opposite forces that would only be truly parallel (and opposite) one another for only an instant at a single point. also, the angle (to ground normal) of the braking force changes as the bike moves through its travel, so its dependent on that as well.
Davep is correct as usual. You're totally forgetting that whatever force is generated at the caliper has to be opposed by a force at the axle, otherwise the wheel would translate relative to the swingarm, which clearly it can't. This creates a couple moment that is totally independent of the caliper's position anywhere around the rotor. The other force at work is the horizontal force acting through the axle (equal to the tyre's tractive force) which is what provides your actual deceleration. The moment generated by this is a product of the IC/pivot height over axle (which might be zero) multiplied by the horizontal deceleration force. If this is zero, the only pro-squat effect will be from the couple moment generated by the caliper/axle reaction pair. Neither of these components are affected by the position/orientation of the caliper on the swingarm or relative to the axle. The Commencal claim is simply false.
 

kerbdrop

Chimp
Nov 24, 2007
3
0
ok let me try to explain this as clearly as possible...

when a disc brake is activated, it creates a force tangent to the point the caliper is contacting the rotor, and the vector direction of the force will be relative (or is it opposite?) to the direction the wheel is spinning. By changing the point at which the caliper contacts the rotor, you change the angle of the tangent direction of the force. this will affect your suspension performance based on the angle between the swingarm motion and the tangent direction of the brake force.
I hope you are joking? :twitch:
 

jonKranked

Press Button, Receive Stupid
Nov 10, 2005
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Davep is correct as usual. You're totally forgetting that whatever force is generated at the caliper has to be opposed by a force at the axle, otherwise the wheel would translate relative to the swingarm, which clearly it can't. This creates a couple moment that is totally independent of the caliper's position anywhere around the rotor. The other force at work is the horizontal force acting through the axle (equal to the tyre's tractive force) which is what provides your actual deceleration. The moment generated by this is a product of the IC/pivot height over axle (which might be zero) multiplied by the horizontal deceleration force. If this is zero, the only pro-squat effect will be from the couple moment generated by the caliper/axle reaction pair. Neither of these components are affected by the position/orientation of the caliper on the swingarm or relative to the axle. The Commencal claim is simply false.


thanks for the clarification :thumb:
 

ChrisKring

Turbo Monkey
Jan 30, 2002
2,395
4
Grand Haven, MI
Davep is correct as usual. You're totally forgetting that whatever force is generated at the caliper has to be opposed by a force at the axle, otherwise the wheel would translate relative to the swingarm, which clearly it can't. This creates a couple moment that is totally independent of the caliper's position anywhere around the rotor. The other force at work is the horizontal force acting through the axle (equal to the tyre's tractive force) which is what provides your actual deceleration. The moment generated by this is a product of the IC/pivot height over axle (which might be zero) multiplied by the horizontal deceleration force. If this is zero, the only pro-squat effect will be from the couple moment generated by the caliper/axle reaction pair. Neither of these components are affected by the position/orientation of the caliper on the swingarm or relative to the axle. The Commencal claim is simply false.

Good analysis. I was sketching out the forces and moments because I was sure that the axle was providing an opposing force. Thanks for saving me the time.
 

dhkid

Turbo Monkey
Mar 10, 2005
3,359
0
Malaysia
i am very surprised that commencal would say something like that, i really hope its not coming from them.:disgust1:
 

EVRAC

Monkey
Jun 21, 2004
756
9
Port Coquitlam, B.C., Canada

P.T.W

Monkey
May 6, 2007
600
0
christchurch nz
Does anybody remember the 1st protype Tomac 204 single pivot(the 1 after the Lawall frame)It was displayed with a so called floating brake,but the lever arm was conected back to the swing arm near the pivot.:banghead:
The article said somthing along the lines that when Doug Bradbury was questioned about this,he replied that seeing as the torque rod pointed at the main pivot it transferd braking forces back into it????WTF:crazy:
Right up to that point i used to admire an lust after anything that Doug designed.Funny that it disapeared on the production frames tho:clapping: