[9.8m/s/s]

I'm riding a 6.5 and 6.5 single pivot frame now and I happen to love the way it feels, but I continually hear people talking about brake jack as a negative. Let's assume, for the sake of arguement, brake jack does exist. The only place I'd be remotely worried about it is a downhill rock garden. (Climbing or flat- your not on your brakes anyway. On a smooth downhill I'd be mainly on my front brake, and a little squatting in the back wouldn't change much.)

But a downhill rockgarden is the exact place I'd want my bike to act like this. Squeezing the rear brake and inducing brake jack would seem to have two huge upsides. One, it brings the rear wheel up to the frame for me so I can concentrate on staying back after my front wheel has rolled over a large obstacle. Second, once my rear wheel clears that obstacle, if I'm still on the brakes, the angle of my fork will be more slack because I've bunched up my rear travel to make it easier to roll over the next obstacle.

Is there anything I'm missing? This is my first big heavy bike, so I'm not accustomed to all the other suspension designs on a big bike, but this thing seems handle anything I throw at it.

 

[Alandrex]

It locks up your suspension. Its not helpful in rock gardens. When the brake jack happens it uses your travel which leaves less travel for the tricky rocks. It just makes your bike handle harshly over the rough. Its more bad than any bit of good thats why bike companies try to reduce the jacking.

 

[Deweydude]

single pivot bikes should not have brakes, (or wheels for that matter)

 

[standard235]

You shouldn't have posting ability! HA!

 

[9.8m/s/s]

Alright, so I shouldn't have posting ability, sp bikes shouldn't have brake's, and brake jack bunches the suspension up and companies try to design away from it. Anyone have any real reasons to counter the points I originally posted?

 

[Deweydude]

I think he may have mean't me

And this is what I mean by sp bikes should not have brakes or wheels.


no worries of any kind of "jack" at all mate, when there ain't enough friction to the ground to create it.

take any single pivot bike off wheels and put it on ski's and it'll feel like twice the bike it ever was.:thumbsup:

sorry off topic I knowrft:

 

[Brown_Teeth]

Why waste that POS frame when a POS snowboard will do way better? Must be a mensa thing? LMFAO!

 

[wyrm]

BrkJ doesn't help the frame squat the way you are thinking it will. The natural tendency of the suspension would return/rebound back. To what ever point the frame rebounds with the brks still on will be the point where the next hit will be absorbed with brks still on.

BrkJ also forces the front to dive much harder while brking. Fork dive is not good.

 

[baconstrip]

Squat vs Jack...

As Go-Ride describes rear braking (Squat, & Jack).

There are three basic terms used to describe how rear suspension is affected when the brakes are applied. The terms are:

Brake jack - describes a suspension that rises up when the brakes are applied. As the rear brake is applied the rear suspension expands and pushes the riders weight forward as the suspension stiffens.
Brake Squat ö describes a suspension system that lowers when the brakes are applied. As the rear brake is applied the suspension is compressed moving the riders weight backwards.
Neutral ö describes a suspension system that is not affected when the brakes are applied. As the rear brake is applied the rear suspension does not rise or compress, but rather stays in the same attitude as if the brake was not being used.

You have probably already figured out that "brake jack" is a bad thing. As you apply the front brake the bike will have some fork dive, if you then have a rear suspension that jacks up all of the riders weight is going to the front of the bike and over the bars. This is not good in any situation.

XC bike (3" ö 4" of travel) manufactures believe that the optimal rear suspension will stay neutral under braking forces. This is probably correct in most XC applications. With less rear suspension a bike will stop better if the rear wheel can track smoothly over the ground. A neutral braking suspension will do this best.

In DH bike applications I believe that a rear suspension that has some "brake squat" is the superior design. We know that as the front brake is applied the fork is going to dive some. This moves the riders weight forward. But, if the rear suspension has some squat as the rear brake is applied this will help to offset the fork dive and allow the rider to stay balanced between the wheels. This is even more important as you are going down a steep hill that is already trying to move your weight forward.

As you can see some suspension designs that are optimal for XC bikes are not necessarily the best choice for DH bikes. I hope this article helps to clear up some of your suspension and braking questions.

GO-RIDE

So what type of suspension has what ????

Also, most all bikes posses the Squat/Jack to some degree under some conditions.. I have two single pivot bikes, and I know it exist, but really don't notice it that much. As I have said in the past, when you go over the bars, while going through the rock garden, was it your skills (or lack of) or was it the mechanics that created the issue. Most of the time it is the skills...

 

[Raptordude]

I have a 7" Single Pivot DH Bike with 8" Rotors...amd brake jack still remains a mystery.

I really can't exactly come to a conclusion as to how it affects my performance. However, I can't say that the claim of brake jack is completley BS, because it makes scientific sense, but as a rider...what the hell AM I supposed to feel braking hardcore under insane conditions that use most of my travel? At that rate, it's a little hard to judge what exactly your braking "charecteristics" of your rear wheel.

Here's a example: I am riding Stick n' Stones at Northstar, I am decending down a part containing large 3-4 ft. boulders, steep and techie. I am about to sh!t my pants in fear and adrenaline...yet in the back of my mind I am thinking "Dude! My brake is TOTALLY skipping because I get major brake jack".....(Notice the Sarcasm)

I don't really understand it, but I can't really make any claim as to weather or not it makes a difference or not, its like a 180 dollar gamble. Some say it works, but I have yet to feel when my brakes really skip over larger hits where I am in 100% control...usually I tend to learn how to use my brakes in varied conditions.

 

[9.8m/s/s]

As Go-Ride describes rear braking (Squat, & Jack).

There are three basic terms used to describe how rear suspension is affected when the brakes are applied. The terms are:

Brake jack - describes a suspension that rises up when the brakes are applied. As the rear brake is applied the rear suspension expands and pushes the riders weight forward as the suspension stiffens.
Brake Squat ö describes a suspension system that lowers when the brakes are applied. As the rear brake is applied the suspension is compressed moving the riders weight backwards.
Neutral ö describes a suspension system that is not affected when the brakes are applied. As the rear brake is applied the rear suspension does not rise or compress, but rather stays in the same attitude as if the brake was not being used.

You have probably already figured out that "brake jack" is a bad thing. As you apply the front brake the bike will have some fork dive, if you then have a rear suspension that jacks up all of the riders weight is going to the front of the bike and over the bars. This is not good in any situation.

XC bike (3" ö 4" of travel) manufactures believe that the optimal rear suspension will stay neutral under braking forces. This is probably correct in most XC applications. With less rear suspension a bike will stop better if the rear wheel can track smoothly over the ground. A neutral braking suspension will do this best.

In DH bike applications I believe that a rear suspension that has some "brake squat" is the superior design. We know that as the front brake is applied the fork is going to dive some. This moves the riders weight forward. But, if the rear suspension has some squat as the rear brake is applied this will help to offset the fork dive and allow the rider to stay balanced between the wheels. This is even more important as you are going down a steep hill that is already trying to move your weight forward.

As you can see some suspension designs that are optimal for XC bikes are not necessarily the best choice for DH bikes. I hope this article helps to clear up some of your suspension and braking questions.

GO-RIDE

So what type of suspension has what ????

Also, most all bikes posses the Squat/Jack to some degree under some conditions.. I have two single pivot bikes, and I know it exist, but really don't notice it that much. As I have said in the past, when you go over the bars, while going through the rock garden, was it your skills (or lack of) or was it the mechanics that created the issue. Most of the time it is the skills...

This post had what I was looking for. I mistermed the title of this post, I was thinking brake squat all along. I was only thinking of the rear of the bike, not the fork. I think everyone can agree that there is almost no situation where you want your fork to stiffen up on you. Thanks for the replies everyone.

 

[Dan Gerous]

And this is what I mean by sp bikes should not have brakes or wheels.

This is not a single pivot design, it's a URT (united rear triangle) and that should have never existed in the first place. A single pivot bike has the BB on the main frame, not on the swingarm... Just a precision.

 

[todd_freeride]

play around with rotor size, I got pissed at my zone a couple weeks back, because all the bike would do was the rear wheel would lock up. was tired of it, so for one I took the original brake off (HFX-9) and replaced it with a MX2 (much better) and the rotor size of 160mm it now stops better and doesnt lock up. I dont know why this works, but it does.

 

[XSL_WiLL]

Uhhh.... todd? That wasn't the question... Do you have any idea what you're talking about? You feelin' okay bud? You seem pretty off today.

BTW, the HFX-9 has very limited modulation. The MX-2's modulation can be adjusted by the position of the static pad, which in term affects how far the rotor has to be pushed.

 

[sriracha]

I really can't exactly come to a conclusion as to how it affects my performance..

i was in the same boat a few years back, when i had a bullit. i never thought about brake jack out on the trail, it never bothered me and i didn't feel that it slowed me down any. most brake jack can be compensated by the rider. that is until i upgraded to a bike that didn't have brake jack, then i noticed how much smoother the thing was braking down steeps into corner hot.
brake jack is one of those things you don't notice until you ride a bike that doesn't have brake jack.