Brake Jack, some one explain the right way
- Thread starter mack
- Start date
Brake jack is when the using the rear brake causes the rear of the bike to rise under braking and the suspension becomes less effective.
Brake squat is the when the rear of the bike squats under braking.
Brake squat is the when the rear of the bike squats under braking.
Brake jack (in general) is when using the rear brake causes the rear suspension to become less active (or sometimes inactive). Depending on pivot locations, the braking forces can cause the suspension to either compress or extend under hard braking, which will cause your rear wheel to start bouncing around and rendering the majority of your rear suspension useless.
Ill add to what Salami said.
Brake jack is never good. Brake squat can be good, but like many things, too much can be a bad thing.
Contrary to popular belief, single pivot bikes can not feature brake JACK. (unless you are riding backwards I guess!)
dw
Brake jack is never good. Brake squat can be good, but like many things, too much can be a bad thing.
Contrary to popular belief, single pivot bikes can not feature brake JACK. (unless you are riding backwards I guess!)
dw
You there, evil ninja guy, you explain now!
Am I correct when I say that these phenomena are due to the following:
if you had your bike on a bike stand and remove the shock, and you articulate the suspension up and down, the brake caliper would move relative to the brake rotor..... or the caliper would not follow the movement of the rotor.
So then while you are riding, you apply the brake, because the caliper is stopping the rotor, this would tend to almost lock out the suspension momentarily.
Whether this would cause squat or jack, I don't know.
Then if you introduce a floating caliper, it allows the caliper to follow the movement of the rotor through the travel, thus reducing the effect.
is this close to accurate?
if you had your bike on a bike stand and remove the shock, and you articulate the suspension up and down, the brake caliper would move relative to the brake rotor..... or the caliper would not follow the movement of the rotor.
So then while you are riding, you apply the brake, because the caliper is stopping the rotor, this would tend to almost lock out the suspension momentarily.
Whether this would cause squat or jack, I don't know.
Then if you introduce a floating caliper, it allows the caliper to follow the movement of the rotor through the travel, thus reducing the effect.
is this close to accurate?
My understanding of this goes like this:
Since the brake caliper is stopping the wheel it needs something to push against with whatever force is generated in the slowing down of the rear wheel. The thing it pushes against in this case happens to be the caliper mount of the rear swingarm of the bike. This force has to have something to push against it. It's that every action has a reaction thing you learn in physics. In the case of a single pivot such as a bullit that force is in the direction of the shock (more or less...) which in turn causes the shock to compress. Now when you attach a solid arm to the caliper and to a solid point on the mainframe of the bike the force is no longer directed at the shock but rather it byepasses it straight to the tubing on the mainframe thus eliminating any brake induced squat.
Since the brake caliper is stopping the wheel it needs something to push against with whatever force is generated in the slowing down of the rear wheel. The thing it pushes against in this case happens to be the caliper mount of the rear swingarm of the bike. This force has to have something to push against it. It's that every action has a reaction thing you learn in physics. In the case of a single pivot such as a bullit that force is in the direction of the shock (more or less...) which in turn causes the shock to compress. Now when you attach a solid arm to the caliper and to a solid point on the mainframe of the bike the force is no longer directed at the shock but rather it byepasses it straight to the tubing on the mainframe thus eliminating any brake induced squat.
if you have a bike on a bike stand.....then spin the cranks and then lock the rear brake, the bike thuds forward as all the energy is passed from the brake and literally makes the bike jump forward. I am guessing that this is what Zaskar riding is explaining. And when the bike jumps forward all the energy is passed through the swingarm first, thus compressing the shock
Brake jack is some stupid little thing that the reviewers at Mountain Bike Action complain about way to much, and make it look like it is so much worse then it is. I have always ridden single pivots/ no horst links and no floating brakes and they have all been fine, I am now on my first bike with a floating brake a Yeti DH9 and I really don't notice any huge difference. People who think there bike gets brake jack really bad get it, because they are always waiting for it to happen. People who don't think about it don't notice it, if you ask me just ride the bike and don't think about. Brake jack certainly isn't holding any of us back from going faster, we are. A pretty good way to look at is grab your rear brake and bounce on the bike, you may see the suspension is slightly stiffer, but I don't think I have ever seen a bike where it completely locks out the back end. Thems are my few words.
I'm serious, do this if you ever get a chance. Ride a lawill bike (schwinn straight 6/8 or a yeti) without the brake mount. i.e. one of the older frames that allowed you to mount the caliper to one of the swingarm components. Ride (not fast, you will die) down a sidewalk or road and grab just the rear brake. That is without a doubt the finest example of true brake jack mankind has ever created.Originally posted by mack
Ok, this has been explained before, but i still have no idea what it is???? or how the hell it works. so can some one tell me the right way how it works, if they dont i will deny its existence.
The way dw explained it is in line with how I have heard other engineers explain it- they all say that it is too complicated to explain fully, but gave some examples of it. A Balfa mechanic told me that single pivot bikes squat. To see it, take the spring off the shock, put the bike in a stand, spin the wheel up, and hit the rear brake, The force will drive the wheel up until it bottoms. On other bikes, (I have been told) that the changing wheel base has a tendency to fight the suspension. A geberal rule of judging this effect on your bike is to grab both brakes while sitting and not moving, and then bouncing up and down. The more travel you can get before the wheels try to brake loose, the less your suspension will be effected by braking.
It made sense to me, but I always take information like this with a good ole grain of salt. The FSR and floater equiped bikes I have tried that on had very little change in wheelbase, unlike the single pivots I have owned/ ridden.
It made sense to me, but I always take information like this with a good ole grain of salt. The FSR and floater equiped bikes I have tried that on had very little change in wheelbase, unlike the single pivots I have owned/ ridden.
what, no ones even mentioned the changes in relative wheel velocity from caliper rotation vs. wheel path/displacement?
yet another forshadowing of Balfas fate....
nor does anyone care
thats why i wonder why people keep bringing up this topic ....
I was trying to hint at that. I wasn't terribly clear in my little diatribe....
ahh, missed that post. You know as an engineer, i expect you to use more convoluted sentences to hint at technical superiority....
Like Toshi's "conservation of angular momentum"...or whatever it was...
how 'bout this?
"ENTROPY!!"
Fvcking engineers....
you need to use maxwell's equations and laplace transforms and jacobians and fourrier series and ohm's law and newton's gravitational constant.
This is dead wrong, brake jack will not occur with the bike sitting still. You can observe it a bit spinning the wheel in a stand and grabbing the brake(maybe), but really the only way to tell is to know what your talking about and ride the bike. It mostly becomes in issue in rack garden sections and such where both the brake and the rear shock have to do alot of work, of course one easy solution is use less brake:devil:
Brake jack/squat is definately a real thing - my 222 didn't have a perfect floating setup, but it was noticable better under braking then my DHi - on the DHi I do have to go slower into sections because I wont be able to scrub off as much speed when I need to
Same here. My frame locks up when braking and the back skips around, All I hear about these days is pedalling performance, when will they focus more on braking? I want to brake as late as possible but still have my rear on the ground to slow me.Originally posted by - seb
Brake jack/squat is definately a real thing - my 222 didn't have a perfect floating setup, but it was noticable better under braking then my DHi - on the DHi I do have to go slower into sections because I wont be able to scrub off as much speed when I need to
actually, lots of BS and omissions in that. Its actually a very poor analysis and thesis on braking. Like the rest of the paper, his 'proofs' and analysis are actually meaningless; it reads very much like an MBA article (err, thats the magazine i mean)
I didnt say it wasnt, I just said that guys suggestion of how to find it was completely wrong and made no sense at all. My only point was I dont think there is any real scientific way to measure brake jack except to know what you're looking for and ride the bike.Originally posted by - seb
Brake jack/squat is definately a real thing - my 222 didn't have a perfect floating setup, but it was noticable better under braking then my DHi - on the DHi I do have to go slower into sections because I wont be able to scrub off as much speed when I need to
sure there is, with enough sensors and data gathering with some analysis tools....