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"Brake Jack" - Explanation??

28K views 52 replies 20 participants last post by  DannyHuynh 
#1 ·
Could please someone layout how exactly brake-jack occurs in a step-by-step fashion?
I have tried reading on the subject and I kinda think I know what's goin' on. I have a faux-bar link on my bike and I look and try to visualize the phenomenon, but i just can't quite get it.
 
#27 ·
RED5 said:
The issue with brake jack/squat has always seemed to be more of an issue with the inability of the suspension to react properly when the brakes are applied.
Absolutely agree there and I would include the front suspension in that too. In general the bike handles like crap on the brakes verses off. I would include motorcycles also and that is smooth pavement. In the moto world they do not call it the "thrill uphill" for nothing.
 
#28 ·
NormanPCN said:
Absolutely agree there and I would include the front suspension in that too. In general the bike handles like crap on the brakes verses off. I would include motorcycles also and that is smooth pavement. In the moto world they do not call it the "thrill uphill" for nothing.
True, however it is a much larger issue with the rear suspension, at least the front is still able to track the terrain when being used instead of skipping over the bumps. Unless you run some sort of floating brake mechanism, like the Kona DOPE system or the Bullits floater.
 
#29 ·
RED5 said:
I don't think any one has ever had issue with a bike squatting when applying the brakes. The issue with brake jack/squat has always seemed to be more of an issue with the inability of the suspension to react properly when the brakes are applied.
RED5 said:
When you grab the brakes on a suspension bike the suspension has a tendency to lock/stiffen. Watch the Kona video I posted above, it will make it much more clear.
No no, I got it, you're just saying "When you do X, X doesn't happen." You're saying two completely contradictory things in the same sentence. I think you just don't really understand what brake squat is. Our explanations left something to be desired obviously.

When a bike "squatts" it is exactly that "stiffening" feeling you're describing. The reason for this is because the brake causes the link, the arm, whatever its connected to to rotate (equal and opposite reaction). Essentially, the swing arm is taking the force of the wheel spinning onto itself. This causes it to rotate, which pushes into the suspension. All things equal, this would result in the back end of the bike sinking down.

The reason you're getting confused here, is because at that same time the riders instant center pitches forward and so weight is removed from the rear wheel at the same time as the brake attempts to compress the wheel. This is especially compounded by the fact that usually when you brake, you're using the front brake too, which causes you to pitch forward even more. And so the shock extends despite "squatting" from the brake.

The reason your fork or shock wont react as well under braking is because its deeper in the travel then it would otherwise be. This results in the feeling of the spring being stiffer then normal because you're deeper in the travel then you otherwise would be. The deeper you are in the travel, the less room there is for the shock to move and the stiffer it is, and so the result is your suspension doesn't track as well.

Again, like I and davep said, the vast majority of bikes squat under braking. The reason your suspension doesn't track as well is because of brake squat. There are other benefits that you may not notice, but I'm sure if you rode something with truly no squat you would learn to appreciate (such as not feeling like you're getting pitched forward PLUS the springrate being off because your instant center is so much further forward).
 
#33 ·
William42 said:
haha, well, I think this thread is pretty much over, theres not really too much to debate. I think red5 just didn't know what the terminology refers to, and now he does. Still good information for somebody who doesn't know what brake squat or brake jack is!
actually there are a few over simplifications and thus incorrect conclusions. (geo can have a significant effect...anti-squat as shown to pro-squat, depending)...

then there is the true net effect on the chassis when things like mass transfer, traction force, etc are taken into account....
 
#34 ·
William42 said:
No no, I got it, you're just saying "When you do X, X doesn't happen." You're saying two completely contradictory things in the same sentence. I think you just don't really understand what brake squat is. Our explanations left something to be desired obviously.

When a bike "squatts" it is exactly that "stiffening" feeling you're describing. The reason for this is because the brake causes the link, the arm, whatever its connected to to rotate (equal and opposite reaction). Essentially, the swing arm is taking the force of the wheel spinning onto itself. This causes it to rotate, which pushes into the suspension. All things equal, this would result in the back end of the bike sinking down.

The reason you're getting confused here, is because at that same time the riders instant center pitches forward and so weight is removed from the rear wheel at the same time as the brake attempts to compress the wheel. This is especially compounded by the fact that usually when you brake, you're using the front brake too, which causes you to pitch forward even more. And so the shock extends despite "squatting" from the brake.

The reason your fork or shock wont react as well under braking is because its deeper in the travel then it would otherwise be. This results in the feeling of the spring being stiffer then normal because you're deeper in the travel then you otherwise would be. The deeper you are in the travel, the less room there is for the shock to move and the stiffer it is, and so the result is your suspension doesn't track as well.

Again, like I and davep said, the vast majority of bikes squat under braking. The reason your suspension doesn't track as well is because of brake squat. There are other benefits that you may not notice, but I'm sure if you rode something with truly no squat you would learn to appreciate (such as not feeling like you're getting pitched forward PLUS the springrate being off because your instant center is so much further forward).
Trust me I'm not confused. My point was that the "squat" isn't the issue, it's the stiffening action of the suspension. Let me put this another way.Say your bike squats everytime you grab the rear brake and the suspension didn't stiffen, I doubt anyone would still have issues. My point was it's not the squatting thats the issue, but the suspension stiffening up. Believe me I have quite a bit of knowledge regarding brake jack/squat. Which is exactly why my 07 Coiler has a floater on it.
 
#35 ·
davep said:
actually there are a few over simplifications and thus incorrect conclusions. (geo can have a significant effect...anti-squat as shown to pro-squat, depending)...

then there is the true net effect on the chassis when things like mass transfer, traction force, etc are taken into account....
Sure, if I were to design a bike just based on the above, it'd ride like poop. But then again, as a simplification for getting a concept off its to far off the mark either. There are lots of variables, but all things equal, thats basically what brake squat is.

RED5 said:
Trust me I'm not confused. My point was that the "squat" isn't the issue, it's the stiffening action of the suspension. Let me put this another way.Say your bike squats everytime you grab the rear brake and the suspension didn't stiffen, I doubt anyone would still have issues. My point was it's not the squatting thats the issue, but the suspension stiffening up. Believe me I have quite a bit of knowledge regarding brake jack/squat. Which is exactly why my 07 Coiler has a floater on it.
You clearly do not know what squat is. I'm not trying to be mean, it took me awhile to figure it out, but the "stiffening" action that your describing IS squat. It is the function of the brake inducing force into the shock that is not transmitted from bumps in the ground causing the shock to stiffen up and lose its ability to react to and absorb bumps.

I think you may be thinking along the lines that when you grab the brake, it makes the swingarm more resistant to movement. That is not the case at all. Grabbing the brake does nothing to the swing arm other then try and push it up, which in turn compresses the shock. The stiffening feeling is brake squat. I don't know how else to say that. Keep thinking about it, it'll come.
 
#37 ·
so would I be correct in calling a bs on trek's ABP in reducing brake jack, as it should actually have less brake squat than their previos designs. Thats not to say it's braking performance isn't better than before, just that it is wrong to say it has no brake jack, instead it has more anti brake squat.
 
#39 ·
William42 said:
No no, I got it, you're just saying "When you do X, X doesn't happen." You're saying two completely contradictory things in the same sentence. I think you just don't really understand what brake squat is. Our explanations left something to be desired obviously.

When a bike "squatts" it is exactly that "stiffening" feeling you're describing. The reason for this is because the brake causes the link, the arm, whatever its connected to to rotate (equal and opposite reaction). Essentially, the swing arm is taking the force of the wheel spinning onto itself. This causes it to rotate, which pushes into the suspension. All things equal, this would result in the back end of the bike sinking down.

The reason you're getting confused here, is because at that same time the riders instant center pitches forward and so weight is removed from the rear wheel at the same time as the brake attempts to compress the wheel. This is especially compounded by the fact that usually when you brake, you're using the front brake too, which causes you to pitch forward even more. And so the shock extends despite "squatting" from the brake.

The reason your fork or shock wont react as well under braking is because its deeper in the travel then it would otherwise be. This results in the feeling of the spring being stiffer then normal because you're deeper in the travel then you otherwise would be. The deeper you are in the travel, the less room there is for the shock to move and the stiffer it is, and so the result is your suspension doesn't track as well.

Again, like I and davep said, the vast majority of bikes squat under braking. The reason your suspension doesn't track as well is because of brake squat. There are other benefits that you may not notice, but I'm sure if you rode something with truly no squat you would learn to appreciate (such as not feeling like you're getting pitched forward PLUS the springrate being off because your instant center is so much further forward).
You've got it wrong; Red5 has it right. And you shouldn't accuse someone of misunderstanding the terminology and concepts when you've just misused the term "instant center" twice. You meant center of gravity or mass.

You misunderstand what causes the brake squat because you're leaving out half of the action/reaction relationship. The friction at the brake pads creates a torque trying to rotate the wheel backwards. The reaction is that the forward angular momentum of the wheel tries to rotate the swingarm forwards. The two cancel each other out.

That means you can consider the swingarm and wheel (or floating link and wheel on a four bar) as though they were tied together into a single rigid unit. What's then left to consider is the backward acting force at the ground and its relationship to the pivot point for the wheel/swingarm unit. And you have to consider the relation between the rider/bike center of mass and the force at the ground. The first tries to compress the suspension; the second tries to extend it.

The first moment--the brake squat-- is greater the higher and more rearward the pivot point is. On single pivot bikes with a fairly high pivot there will be about enough squat to just cancel out the extension caused by the forward weight shift. You won't find many bikes that will actually squat from rear braking. A bike like the OP's will probably stay right at the sag point from rear only braking.

But that bike braking at the sag point will not handle bumps nearly as well as if it were coasting. And it's not any deeper in its travel than normal. The reason the suspension stiffens is explained on the Trek website referenced by gotboostedvr6 up above: http://trekmountain.typepad.com/king...explained.html The explanation is kind of buried in their promotion of their "active brake pivot" so I will summarize.

The farther back the pivot point, the more the brake/swingarm or brake/floating link unit rotates during compression. As that unit rotates, the contact patch has to move forward around the tire. The ground force, pushing back against the contact patch is trying to keep the patch from moving forward. The two tendencies are fighting each other. The result is suspension stiffening. The more brake squat, the more stiffening.

To conclude: a bike with a lot of brake squat will stay right about at the sag point from rear only braking (which is a good thing) but the suspension will stiffen and behave badly (obviously a bad thing). A bike with a far forward instant center, as on a nearly parallel four bar linkage with the caliper on the floating link, will extend a lot from rear only braking (a bad thing) but will not noticeably stiffen and will track the ground better (a good thing).
 
#41 ·
Flystagg said:
so would I be correct in calling a bs on trek's ABP in reducing brake jack, as it should actually have less brake squat than their previos designs. Thats not to say it's braking performance isn't better than before, just that it is wrong to say it has no brake jack, instead it has more anti brake squat.
As should be clear from reading through this thread, different people mean different things by "brake jack". I think the term should just be dropped.

The Trek bike with ABP moves the caliper from the chain stay to the seat stay. Now the braking pivot is the instant center of the linkage. That's way out in front.

The bike will have little brake squat. The rear will rise up from weight shift under rear only braking. The suspension will not stiffen and will track well.
 
#42 ·
Its taking me a bit to get my head around where we disagree Steve, so I'm going to start from the basics and you can tell me where I'm going wrong. I don't claim to be an expert, and if somebody can show me how I'm wrong then I'll believe them, but your explanation isn't really doing it for me. And yes, you're right, I should have said COG, lets skip IC for now though.

Brake squat is the tendency of the suspension to sink in under braking.

Brake squat can occur at the same time as the shock extends. Heres where my explanations get tricky and hard to understand I think: The bike will "squat down" even as the shaft extends, because the shaft is extending due to shift in riders weight (forward). There are two parts to the mix here (and I'm going to assume you're not pedaling, because I have no idea how to explain what happens at that point). There is the brake/wheel/contactpatch whatever pushing the swingarm to compress the shock, and there is the forward shift in riders weight as they slow down (which unweights the shock, causing the spring to want to decompress and extend).

I think we still agree at this point?

If the sag point compresses, the the riders shift in weight is less then the brake induced squat.

If the sag point decompresses, the riders shift in weight is more then the brake induced squat.

The reason the bikes rear wheel stops working so well is because its suddenly oversprung when you apply the brakes. Squat attempts to keep the bike steady in its travel and keep it balanced without excessive pitch forward, but supposing your weight is 70-30 or 60-40 rear wheel-front wheel, when you brake it suddenly changes to 50-50 or 40-60 R/F. You have less weight over the rear wheel, and so whatever spring (or air pressure, whatever) you were running that was the correct springrate for you at a regular sag point and regular balance is suddenly too stiff. The suspension cannot react to bumps as efficiently.

Hypothetically, your springrate should change throughout the normal movements of the bike. For regular riding, it should be (hypothetically) a 400 lb spring. Then, under braking, it should be a 350 or 375 lb spring. Then, as you stop braking and start pedaling, You should be back to a 400 or 425 pound spring.
 
#43 ·
William42 said:
Its taking me a bit to get my head around where we disagree Steve, so I'm going to start from the basics and you can tell me where I'm going wrong. I don't claim to be an expert, and if somebody can show me how I'm wrong then I'll believe them, but your explanation isn't really doing it for me. And yes, you're right, I should have said COG, lets skip IC for now though.

Brake squat is the tendency of the suspension to sink in under braking.

Brake squat can occur at the same time as the shock extends. Heres where my explanations get tricky and hard to understand I think: The bike will "squat down" even as the shaft extends, because the shaft is extending due to shift in riders weight (forward). There are two parts to the mix here (and I'm going to assume you're not pedaling, because I have no idea how to explain what happens at that point). There is the brake/wheel/contactpatch whatever pushing the swingarm to compress the shock, and there is the forward shift in riders weight as they slow down (which unweights the shock, causing the spring to want to decompress and extend).

I think we still agree at this point?

If the sag point compresses, the the riders shift in weight is less then the brake induced squat.

If the sag point decompresses, the riders shift in weight is more then the brake induced squat.

The reason the bikes rear wheel stops working so well is because its suddenly oversprung when you apply the brakes. Squat attempts to keep the bike steady in its travel and keep it balanced without excessive pitch forward, but supposing your weight is 70-30 or 60-40 rear wheel-front wheel, when you brake it suddenly changes to 50-50 or 40-60 R/F. You have less weight over the rear wheel, and so whatever spring (or air pressure, whatever) you were running that was the correct springrate for you at a regular sag point and regular balance is suddenly too stiff. The suspension cannot react to bumps as efficiently.

Hypothetically, your springrate should change throughout the normal movements of the bike. For regular riding, it should be (hypothetically) a 400 lb spring. Then, under braking, it should be a 350 or 375 lb spring. Then, as you stop braking and start pedaling, You should be back to a 400 or 425 pound spring.
I don't think I disagree with any of this. You're saying that when braking the load shifts to the front so that the rear is oversprung and the front is undersprung. That's always true, no matter what kind of rear suspension you have.

What I was talking about is something additional to this. The more the caliper carrying link rotates during a given amount of suspension travel, the more the wheel gets a forward rotational force. The braking force at the ground is in opposition to this. It's also true that the more the caliper carrying link rotates during a given amount of travel, the more brake squat you have and the less the rear will tend to rise up from the forward weight shift. So more brake squat means more interference with suspension performance.
 
#44 ·
Steve from JH said:
I don't think I disagree with any of this. You're saying that when braking the load shifts to the front so that the rear is oversprung and the front is undersprung. That's always true, no matter what kind of rear suspension you have.
Ok at some point I must have implied something I didn't mean to, that was all I'm trying to get at. Brake squat is the tendency of the shock to compress under braking (barring shifts in rider weight). Brack jack is the tendency of brakes to decompress under braking (barring shifts in rider weight)

Loss of traction is due to change in COG (shifts in rider weight) and the resulting change of correct spring weight. Brake squat is one tool used to keep the bike even and stable in its suspension despite the loss in traction because of the shift in COG.

I think that (in complication with everything else I've said) sums it up pretty well. While I see what your saying about the change in caliper position in relation to the rotor, the amount of leverage enacted on the shock because of speed, bumps, and shifts in COG under braking effect the suspension to a way greater degree.

Correct me if I'm wrong here though, you're one of the people I got this all from so I'll take your word for it if you show me how it works!
 
#45 ·
William42 said:
You clearly do not know what squat is. I'm not trying to be mean, it took me awhile to figure it out, but the "stiffening" action that your describing IS squat. It is the function of the brake inducing force into the shock that is not transmitted from bumps in the ground causing the shock to stiffen up and lose its ability to react to and absorb bumps.

I think you may be thinking along the lines that when you grab the brake, it makes the swingarm more resistant to movement. That is not the case at all. Grabbing the brake does nothing to the swing arm other then try and push it up, which in turn compresses the shock. The stiffening feeling is brake squat. I don't know how else to say that. Keep thinking about it, it'll come.
Actually, I have a very good understanding of brake squat/jack. But whatever I give up!! You do not seem to be grasping what I'm getting at and I haven't the patience to keep trying to explain myself, because frankly it's not really that important to me. :madman:
 
#46 ·
the simple explanation is..... Momentum wants to be conserved... Rear wheel is moving at a certain speed and when the brakes are applied, wheel slows down but momentum must be conserved. That motion will transfer into lifting the rear wheel up as the wheel cannot spin as fast. When rear is lifted up, suspension is compressed to do so and there, you have less travel and because of most suspension rates, they run stiffer in that range.
 
#47 ·
mrpercussive said:
the simple explanation is..... Momentum wants to be conserved... Rear wheel is moving at a certain speed and when the brakes are applied, wheel slows down but momentum must be conserved. That motion will transfer into lifting the rear wheel up as the wheel cannot spin as fast. When rear is lifted up, suspension is compressed to do so and there, you have less travel and because of most suspension rates, they run stiffer in that range.
kind of? whats moving wants to keep moving, so theres definitely a transfer of energy, but the reason it'll push into the suspension isn't because it can't spin any more and moves up, its because wheel is transmitting its spinning force into spinning the swingarm rather then itself. If thats what you're getting at the yes.
 
#48 ·
William42 said:
kind of? whats moving wants to keep moving, so theres definitely a transfer of energy, but the reason it'll push into the suspension isn't because it can't spin any more and moves up, its because wheel is transmitting its spinning force into spinning the swingarm rather then itself. If thats what you're getting at the yes.
yea William... you hit it right on the money... :thumbsup: :thumbsup: :thumbsup: :thumbsup: :thumbsup:
 
#49 ·
still kind of

mrpercussive said:
yea William... you hit it right on the money... :thumbsup: :thumbsup: :thumbsup: :thumbsup: :thumbsup:
this
"wheel is transmitting its spinning force into spinning the swingarm rather then itself"
should read
"wheel is transmitting its spinning force into spinning the axle carrier rather then itself"
the axle carrier maybe the swing arm or the seat stays
and it may not be obvious at first glance which it is.
 
#50 ·
mrpercussive said:
the simple explanation is..... Momentum wants to be conserved... Rear wheel is moving at a certain speed and when the brakes are applied, wheel slows down but momentum must be conserved. That motion will transfer into lifting the rear wheel up as the wheel cannot spin as fast. When rear is lifted up, suspension is compressed to do so and there, you have less travel and because of most suspension rates, they run stiffer in that range.
Virtually no bikes actually compress in the rear from rear braking. So that can't explain the stiffening.

The capture of angular momentum of the wheel, which is what you're describing, is a very minor effect because the wheel doesn't weigh much.

The compressive torque from rear braking is almost entirely caused by the resistance to slowing of the entire bike and rider, which weigh much more.

This resistance produces a backward acting force along the ground line against the contact patch. Since that force acts beneath the pivot point for the wheel and wheel carrying link, it causes a forward torque, clockwise viewed from the drive side, on that link.

The longer the distance from the axle to the pivot point (the IC in the case of a four bar linkage), the less the compressive force is.

The compressive force is at its most not much more than the extending force caused by the forward weight shift from deceleration.

If you add any significant degree of front braking, all of this discussion becomes unimportant and it doesn't matter much what kind of rear suspension you have.
 
#51 ·
I agree with those who critise the marketing men. Ride a mountain bike hard in singletrack especially downhill and on loose rocky surfaces and you have all sorts to deal with. So get the latest wizz bang suspension design with loads of gimmicks etc and this alone will NOT solve your bike handling issues, which are SKILL factor. Strange how some guys on hardtails with V brakes on the rear nail some of the hardest downhill tracks where other guys on VPP / maestro / ABP etc etc are left way behind! They have SKILL suspension and BIG GRINS!
 
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