[thaflyinfatman]

Ok, could someone please explain to me exactly what causes brake jack/squat (in terms of vectors etc)? And how does a floating brake (optimised of course) solve this problem? I'd appreciate the use of a singlepivot as an example because I don't understand moving ICs etc and what effects that has (what defines the Instant Centre anyway?)

My theory at the moment is this:
When the rear brake is applied when the bike is moving, the contact patch at the bottom of the wheel tries to move backwards in relation to the bike (ie tries to maximise the horizontal distance between the contact patch and the pivot). Given that the contact patch shifts relative forwards on the wheel as the wheel moves upwards, if the pivot is fairly low, it won't make much difference. However if the pivot is high (BB7 etc), the distance can increase considerably more, making the wheel try to move upwards, which breaks traction, skids, and feels like it's locked out.

Is that right (or close to)? I'm at a loss as to understand how a floating brake changes things...

Thanks

 

[JohnMc]

It's how the rotational force (torque) of the wheel is fed into the suspension when the brake clamps down. Not anything to do with the rearwards force exterted at the contact patch, all though that is a separate force at work.

Imagine the suspension with no spring, hanging on a work stand. With the brake locked, turn the wheel. Will that motion do anything to the suspension? On a single pivot bike - yes - it will roatate the arm forward, compressing the suspension. On a good multilink suspension, no - the hub does not rotate (or very litttle) as it moves through the travel, so there is no way for brake forces to affect it.

A floating brake adapter is basically putting the brake mounting on a small multi-link suspension - so it no longer rotates (as much) when the suspension compresses.

 

[thaflyinfatman]

Ah, I think I get ya. So the placement of the caliper anywhere on a swingarm (other than on a floating brake), won't actually make any difference, correct? The swingarm as a whole will try to rotate in the same direction that the wheel is spinning?

I still don't really get how a floating brake works though... could you elaborate on that?

 

[ohio]

Originally posted by thaflyinfatman
Ah, I think I get ya. So the placement of the caliper anywhere on a swingarm (other than on a floating brake), won't actually make any difference, correct? The swingarm as a whole will try to rotate in the same direction that the wheel is spinning?

I still don't really get how a floating brake works though... could you elaborate on that?

Yes to the first paragraph. For the second question, I definitely recommend a search.

The short simplified version, and sort of repeating what John said, is that there are two types of forces of forces at work: rotational (moment) and linear (translational).

1. Rotational - braking forces try to turn the rear wheel and whatever it is attached to (caliper and whatever the caliper is mounted on). If the system rotates as it moves through it's travel, the forces will push the system in the direction of that rotation. A floating brake arm eliminates the rotation... picture a floating arm at 12 o-clock at the bottom of it's travel. At the top of it's travel, it's still pointed at 12 o-clock even though the axle may have traced an arc. Make sense?

2. Linear - the braking forces also push the wheel/caliper system backwards. If the axle path is not perpendicular to that force, the braking forces will push the system along that path, e.g. on a BB7 with a floating brake, the axle path is still rearward, so if you push backwards on the tire, the system will try to translate back AND up, causing some suspension compression. This is generally a more predictable and tractable reaction than (1).

 

[JohnMc]

Single pivot bike:

When the brake is applied the wheels' torque is transferred into the swingarm. There is nothing to directly counteract the swing arm rotation so it turns in the direction of the wheel's rotation - forward, and compresses the shock.

Multi link bike - like my Ellsworth Truth:

The brake caliper is mounted on the rear 'seat stay'. When the brakes are applied this recieves the wheel's torque just like the C'dale does, but the force is just fed into a force that attempts to compress the upper linkage. The torque has no way to feed into the shock to compress or extend the suspension.

Brake Adapter:

Here's a floating brake adapter on a single pivot (big tavel) bike. With the caliper mounted on the arm it would turn the arm and have the same problem as any other single pivot bike. But the caliper is mounted on a little upright that pivots on the swingarm, and as the suspension moves up and down it is held in a pretty straight up and down position by the locating rod that reaches all the way back to the frame. So the torque of the wheel is fed through the rid as a push, instead of through the swingarm as a twist. Its sort of like a little multi link setup, only just on the brake caliper, not on the entire rear wheel assembly.

 

[ohio]

Originally posted by JohnMc

Multi link bike - like my Ellsworth Truth:

The brake caliper is mounted on the rear 'seat stay'. When the brakes are applied this recieves the wheel's torque just like the C'dale does, but the force is just fed into a force that attempts to compress the upper linkage. The torque has no way to feed into the shock to compress or extend the suspension.

Well, this is not entirely accurate. Most 4-bar bikes have MUCH more neutral braking than single pivots, but they don't by definition. The Id, and Truth both exhibit some brake-induced compression, as do most FSR 4-bars. Some, like the GT LTS and Yeti/Schwinn Lawwill bikes exhibit brake extension. The closest I've seen to true neutral interactions are the Dare, and some iterations of the Lawwill design.

The same can be true with a floating caliper, and I HAVE seen some poorly designed ones, but in general it's much easier to achieve ideal geometry with a seperate flaoting brake mechanism than one integrated into suspension actuation.

 

[Steve from JH]

Originally posted by ohio



2. Linear - the braking forces also push the wheel/caliper system backwards. If the axle path is not perpendicular to that force, the braking forces will push the system along that path.

Not according to standard motorcycle theory. The path of the axle can be perpendicular to the force acting on the ground at the contact patch, in other words the pivot can be horizontally in front of the rear axle, and you can still have a lot of brake induced compression. What matters is the trajectory or path of the ground contact point. If you draw a line from that point to the pivot, the trajectory of that point is perpendicular to that line.

A bike with a low pivot near the BB, like a Kona for example, could very well have just as much compressive torque as a BB7 (without a floating link). That's because both pivots could lie on the same line from the contact point.

A floating link, it seems to me, is less analagous to a four-bar linkage and more analagous to chain torque, only without the kickback. A chain line that duplicates the line of the floating link will produce the same moment, but extending rather than compressing.

 

[ohio]

Originally posted by Steve from JH
Not according to standard motorcycle theory.

Steve, we've been through this exact same discussion before...

Your first statement is partially correct. The contact patch analysis only holds under the condition of a locked wheel. The axle path condition holds only under a completely freely rotating wheel. Like I said, I was giving him the SIMPLIFIED VERSION.

As for the chain/cog analogy, as I explained to you before, you are failing to analyze the system dynamically. While a chain ALWAYS pulls on a cog at a perpendicular to it's radius, one linkage does NOT always apply a force to a second linkage perpendicular to a line through its pivots. I don't know what else to say about this, except that your reasoning is simply wrong.

 

[Steve from JH]

Originally posted by ohio
I don't know what else to say about this, except that your reasoning is simply wrong.

I won't say anything more about this except that it's not my reasoning. It's the reasoning of those who design and tune motorcycles for formula racing.

 

[thaflyinfatman]

Originally posted by ohio
Yes to the first paragraph. For the second question, I definitely recommend a search.

The short simplified version, and sort of repeating what John said, is that there are two types of forces of forces at work: rotational (moment) and linear (translational).

1. Rotational - braking forces try to turn the rear wheel and whatever it is attached to (caliper and whatever the caliper is mounted on). If the system rotates as it moves through it's travel, the forces will push the system in the direction of that rotation. A floating brake arm eliminates the rotation... picture a floating arm at 12 o-clock at the bottom of it's travel. At the top of it's travel, it's still pointed at 12 o-clock even though the axle may have traced an arc. Make sense?

2. Linear - the braking forces also push the wheel/caliper system backwards. If the axle path is not perpendicular to that force, the braking forces will push the system along that path, e.g. on a BB7 with a floating brake, the axle path is still rearward, so if you push backwards on the tire, the system will try to translate back AND up, causing some suspension compression. This is generally a more predictable and tractable reaction than (1).

Ah, I get it now! Thanks! Very good explanation.

And for the record, I spent over two hours searching this board (yes using the search function ) and another one, before I posted this. I couldn't find an answer that I understood.

 

[ohio]

Originally posted by Steve from JH
I won't say anything more about this except that it's not my reasoning. It's the reasoning of those who design and tune motorcycles for formula racing.

There is not a credible motorcycle engineer on earth that analyzes a 4-bar linkage under braking forces as the same system as a chain and cog. You told me your reasoning the last time you posted and I explained to you why it was wrong.

 

[ohio]

Originally posted by thaflyinfatman
Ah, I get it now! Thanks! Very good explanation.

Glad I could help, Grav. I didn't exactly give you the textbook version... so if you start designing you should probably check out some texts, but otherwise it's useful info to know.

 

[Steve from JH]

Originally posted by ohio
There is not a credible motorcycle engineer on earth that analyzes a 4-bar linkage under braking forces as the same system as a chain and cog. You told me your reasoning the last time you posted and I explained to you why it was wrong.

The book I have in hand is MOTORCYCLE DESIGN AND TECHNOLOGY, a product of the engineering division of Aprilia Motorcycles. On facing pages, 78 and 79 there are two virtually identical diagrams showing the same angle generated by the line from the contact patch to a theoretical point. One of them is of the lower tension run of the chain and its relation to the swingarm and the other is a floating link mounted on the bottom of the rotor and its relation to the swingarm. They are comparing the chain braking effect with the floating brake link effect. The text for the floating link diagram says, "The case is very similar to chain/gear-drive and the compression effect on the suspension can be regulated on the basis of the position of the point of linkage to the frame."

That's all I was talking about. I tried to make a distinction between this and braking on a 4-bar linkage, where the axle is mounted on the floating link and not concentric with a pivot of the linkage. The distinction is basically the same point you are making.

As for the point about axle path I refer you to this article by Tony Foale:

http://www.tonyfoale.com/Articles/Dive/DIVE.htm

If you scroll down to the part about rear braking (fig. 8) you will see a horizontal swingarm of two different lengths and the observation that there is considerably more anti-rise (compression) with the shorter swingarm because of the difference in angle of the line from contact patch through pivot. That's the other point I was trying to make

 

[ohio]

Originally posted by Steve from JH
"The case is very similar to chain/gear-drive and the compression effect on the suspension can be regulated on the basis of the position of the point of linkage to the frame."

If you scroll down to the part about rear braking (fig. 8) you will see a horizontal swingarm of two different lengths and the observation that there is considerably more anti-rise (compression) with the shorter swingarm because of the difference in angle of the line from contact patch through pivot. That's the other point I was trying to make

Fair enough. The cases look very similar but cannot be analyzed the same way.

As for the anti-rise action resulting from swingarm length, this due to CoG height and weight transfer, not the isolatedd effect of the braking force on the wheel I was referring to, but yes it is entirely correct. It should be noted however, that the relative height of the center of gravity is MUCH higher on a bicycle than on a motorcycle.

 

[Steve from JH]

Originally posted by ohio


As for the anti-rise action resulting from swingarm length, this due to CoG height and weight transfer, not the isolatedd effect of the braking force on the wheel I was referring to, but yes it is entirely correct. It should be noted however, that the relative height of the center of gravity is MUCH higher on a bicycle than on a motorcycle.

The percentage of anti-rise obviously depends on the location of the center of mass. But the absolute torque on the rear suspension, including the reaction force at the ground, is the same regardless of where the COM is.

The method for calculating the percentage that Tony F. is using in the old article I referenced is outmoded and in fact wrong. Percentage anti-rise should be found by dividing the tangent of the angle of the line from contact patch to pivot by the ratio of height of COM to wheelbase.

For bicycling pedal efficiency 100% anti-squat (the reverse of anti-rise) is not at all desirable. It will make the COM rise on the windup to the acceleration and rise during the acceleration because of the rising front end. Maximum energy efficiency occurs when the COM does not move vertically at all.

 

[rbx]

Originally posted by ohio
Yes to the first paragraph. For the second question, I definitely recommend a search.

The short simplified version, and sort of repeating what John said, is that there are two types of forces of forces at work: rotational (moment) and linear (translational).

1. Rotational - braking forces try to turn the rear wheel and whatever it is attached to (caliper and whatever the caliper is mounted on). If the system rotates as it moves through it's travel, the forces will push the system in the direction of that rotation. A floating brake arm eliminates the rotation... picture a floating arm at 12 o-clock at the bottom of it's travel. At the top of it's travel, it's still pointed at 12 o-clock even though the axle may have traced an arc. Make sense?

so the floating caliper is not rotating because the upper arm(link/rod) is always blocking it at wathever point during the travel?

 

[Rustmouse]

I have to draw pictures (badly) to make this clear. I'm using a single pivot style, cause it's easiest to show (and within my art abilities)

The wheel rotating in the direction of the arrows gets stopped by the brake at point A. This transfers all the torque to the swingarm. The energy tries to continue rotating the entire assembly (by hitting the brakes, you in essence make the wheel a solid piece of the swingarm) This means the entire assembly is going to try to rotate (arrow B). The pivot (at the x) cant move, the swingarm can only move at point C, towards the shock. so it makes the swingarm go up and in (red arrows)

If you keep applying the brakes, the shock compresses and you get brake jack.

Some designs are better than others at preventing this.

A floating caliper mounts to an arm that comes from the axle and hooks to the frame somewhere other than on the swingarm. This transfers the energy to the frame (usually at the top tube) instead of into the swingarm. So the suspension is free to travel.

Hope this helps.

 

[Rustmouse]

The floating disc works by mounting the caliper on an arm that is free to pivot at the same point as the axle (keeping the same distance from the hub no matter where it pivots) the rod takes the force of the braking and transfers it to the rest of the frame (in a single pivot design, usually on the front shock bolt) instead of rotating the swing arm, like the previous drawing, it's pushing forward on the top tube. this pretty well stops brake jack.

The floater in this picture connects at the axle and actually hangs off the arm in blue. since it's mounted at the same center as the disc, no matter where it rotates, it just rotates around the disc. The green arm takes the pressure from braking and transfers it.

the super 8 is a good example of a bike that really needs a floating disc... with the single swing arm, all the braking energy turns into a huge lever pushing the arm up.... with the floater, it doesnt move the arm at all. (so you still have full suspension travel to deal with keeping the wheel in contact with the ground over the bumps)

multi link suspensions are designed to minimize this without the use of a floater. They do this by changing the direction that the torque can make the springs go... so instead of allowing the rotation to move it, rotation hits the top suspension pivot and is transferred to the frame of the bike.

 

[Repack]

With a single pivot bike in a stand, with no shock, spin the wheel and hit the brakes. The wheel/swingarm will swing up and slam in to the frame.
On a linkage bike, a rep dumbed it down for me and said that the forces have the same effect as pushing a door straight towards the hinges-it won't move because there is no sided to side force. Obviously you can push a door from the sides, but a suspension linkage makes it impossible to exert that kind of force. Same principles but don't ask me to explain the bike physics. FSR bikes also keep the distance between the axles about the same throughout the travel. This means that the suspension isn't having to work against tire traction, making things mo better.