wheel rotation invokes swingarm rotation when the brake is applied.
(the ground skidding past the tyre is an input force, which acts around the pivot, the swingarm rotates).
your seesaw model is not valid.
sorry.
let's say you're slowing at 0.1g, and weigh aroudn 100Kg, the force at the tyre/ground contact patch will be 100N.
100N acting at a distance equal to the height of the pivot above the ground (0.35m)= 35Nm.
and that's 35Nm of compressing torque - not insignificant (based on 0.1g)
the ground skidding past the tyre is an input force, which acts around the pivot, the swingarm rotates
well since that's inevitable if you want to slow down, it's hardly the fault of the suspension design.
Brake jack is the action of the actual brakes on the suspension, not the ground.
Ahh yeah silly me, been riding/cycle mechanic for 16 years and I don't know what brake jack is, sorry oh wise one 
I know there isn't much difference between the Trek and the Five when it comes to brake jack, I was there when I rode em.
I don't know why people see it as a bad thing anyway. You can use it to your advantage once you are used to it as it weights the front wheel giving it more traction.
flow - MemberI don't know why people see it as a bad thing anyway. You can use it to your advantage once you are used to it as it weights the front wheel giving it more traction.
It also steepens the head angle and reduces traction on the rear wheel
Who cares what the rear wheel does, it will (pretty much) always follow the front wheel. Nothing wrong with getting a bit loose!
"Though the statement about a flexing force between caliper and dropout seems unlikely. The swingarm will be designed to be strong enough to transfer this force away from the area
Not really - the swingarm will flex minutely because it's very stiff. But increased stiffness doesn't transfer the force away, it just resists it.
It'd be like standing half way along a seesaw (between the seat and the middle) and pulling up as hard as you can on the seat handle. No matter how hard you pull the seesaw is never going to tip!"
But you can make it tip by suddenly pulling up whilst standing on it, akin to the brake being suddenly applied. And the seesaw would mainly move as a single entity, as well as there being a tiny amount of flex around the point on which you're standing. But the seesaw would move, as would the swingarm in your example.
Sorry to go on, but it's a pretty poor example anyway, because the caliper on the swingarm provides nothing like the resistance to turning that the human does in the seesaw example. A seesaw is in fact a very bad analogy.
I just read your posts Molegrips, it seems your the one who doesn't have a clue what brake jack is.
Brake jack is when the rear suspension is compressed under braking, making the rear feel rigid and reducing suspension function. Its called brake jack because it gives the bike a jack hammering effect over bumps, its nothing to do with the suspension extending.
Single pivots are not the only suspension design that suffer from it, Four bar is to, hence why the EX I had felt just as bad as the Five.
Chain growth isn't just a problem with single pivots either, VPP and DW link also suffer.
If you use use selective braking (not braking over rough sections) its not a problem, like I said you can use it to your advantage too.
flow - MemberI just read your posts Molegrips, it seems your the one who doesn't have a clue what brake jack is.
Brake jack is when the rear suspension is compressed under braking, making the rear feel rigid and reducing suspension function. Its called brake jack because it gives the bike a jack hammering effect over bumps, its nothing to do with the suspension extending.
Single pivots are not the only suspension design that suffer from it, Four bar is to, hence why the EX I had felt just as bad as the Five.
Chain growth isn't just a problem with single pivots either, VPP and DW link also suffer.
If you use use selective braking (not braking over rough sections) its not a problem, like I said you can use it to your advantage too.
retro83 - Member
Not taking the piss, just thought it was funny appearing at this point in the thread.
What you're referring to is brake squat (compression), not brake jack (extension). Jack as in 'jacked up', not 'jack hammer'
The reason people get confused about it is because it is damned confusing!
http://www.rotorburn.com/forums/showthread.php?35572-quot-Brake-jack-quot-an-explanation.
I didn't read that I don't have time, but, brake jack is definitely compression not extension.
Brake jack and brake squat are the same thing.
I didn't read that I don't have time, but, brake jack is definitely compression not extension.
No, it isn't. Maybe this is why there is so much confusion around on the topic.
http://www.youtube.com/watch?v=3sDot8FHtpg&feature=related
http://trekmountain.typepad.com/king/2007/08/abp-explained.html
I was wrong, it is in fact just stiffening of the rear suspension under braking reducing suspension movement, nothing to do with compression or extension.
Either way ABP didn't noticeably work compared to the Five and that is what they are trying to address.
Its called brake jack because it gives the bike a jack hammering effect over bumps, its nothing to do with the suspension extending
Got a source for that? Cos everything I read says otherwise.
The fact your Trek felt the same as your Orange just goes to show suspension interacting with caliper braking forces is not important. The roughness you feel is caused by your wheel trying to slow down.
That even happens in a car. Brake hard over bumps and it makes them feel really rough. THAT is unavoidable.
Snippet from Titus website:-
4 BAR LINKAGE DESIGNS: AKA HORST LINK
There’s never been a more hotly contested suspension design on the market then the 4-Bar linkage design with the Horst Link. This design has been around only slightly longer then the original Outland VPP, but its instant success and many attributes have made it the most valued and fought over suspension design in the world. The original design was developed by Horst Leitner at AMP research. The driving goal behind the design was to isolate braking forces from affecting the suspension performance. The first generation Horst bikes did not stiffen up or lift (brake jack) under hard braking. They were and still are to this day “fully active”. The icing on the cake was that the original bikes pedaled better then anything else at the time.
Marketing bumph ^^^
I should use my rear brake more so I can join in these exciting debates
Well so far the replies have shown that:
1/it's caused by the suspension compressing during braking
2/""""""""""""""""""""""""""""" extending """"""""""""""
3/ it's got nothing to do with the above
glad that's all clarified then !
I'm right - ignore everyone else.
He doesn't have a clue, and I give up.
It's caused by bad braking and useless riders, I'm so good it never happens to me.. 
Doesn't it just encourage you to choose a better place to brake rather than slamming them on in the middle of a rock garden?
He doesn't have a clue
I definitely do 
If you don't brake on the rough bits how else can you make the braking bumps bigger.That is the idea is'nt it?? 
This topic has been closed to new replies.
Advertisement
Advertisement
Advertisement
Advertisement
Advertisement
