Skip to content

Ask an Expert Front end twitch under braking Technical

You are in... Forums > Ask an Expert > Technical > Front end twitch under braking

Got something to say?

Got something to say?

Go to most recent reply

Kabinlad

Joined:

May 13

Posts: 2

Kabinlad says:

Front end twitch under braking

Hi,
I have a problem with my Honda cbr600fy that i hope someone on here can solve.
 As soon as i touch the front brakes whilst riding i get a slight twich of the front end to the right, its not enough to completely unsettle the bike and the brakes pull the bike up fine.
There is no pulsing or spongy feeling at the brake lever either.
The brakes have had new pads put in and a complete brake fluid change, the front forks seem ok too.
Anyone got any ideas?

Reply to this Topic  
  • Posted 2 years ago (12 May 2013 14:35)

Post a message in Technical

Fields marked with an asterisk * are required

   

Please note. You cannot submit more than 4000 characters as a message.

Upload image(s) from your computer (up to 3 images)

  1.  
  2.  
  3.  

Terms of use

Use of our community areas and forums is subject to important terms of use. By joining our community and using the features you agree to be bound by these terms. See terms of use below. 

Cancel
kcmc

Joined:

Jan 08

Posts: 7533

kcmc says:

Honda CBR 600 FY

Hi Kabinlad and welcome to the site:smile

Could be head stock bearings,and or slight twisted forks,also check the front tyre for uneven wear and the tyre pressures are correct.Good Luck

KCMC:tongue:

Reply to this Topic
smidget

Joined:

Nov 07

Posts: 2424

smidget says:

As per KC

 plus have the calipers been cleaned and serviced and has the left caliper still got a little air in it. 

Reply to this Topic
eatcs01

Joined:

Aug 02

Posts: 3271

eatcs01 says:

cbr braking

If the CBR has floating discs, then maybe one of the discs has seized bobbins.

Reply to this Topic
Kabinlad

Joined:

May 13

Posts: 2

Kabinlad says:

Honda CBR 600 FY

Hi,
Thanks for the quick replies
The head race bearings feel fine and as i have no handling issues i think the forks are ok, tyre pressure is ok too.
The calipers were cleaned but not serviced ie seals etc but the pistons moved freely.
Its possible air got in to the system when i bled them but since then i have bled the brakes twice just to make sure all the air was out.
The twitch feels like one caliper is engaging before the other.
The calipers are connected individually with ss hoses to the master cylinder so if it is air whats the best way to get the sucker out?







Reply to this Topic
smidget

Joined:

Nov 07

Posts: 2424

smidget says:

Kabinlad

you could try tying the brake lever back (so the brakes are held on)and leave it like that overnight this sometimes allows any air in the system to find its way to the master cylinder.

If that does not work you could bleed the brakes backwards, from the caliper to the master cylinder, you need an old fashion pump oil can (Clean) and don't forget that you will need to remove excess fluid from the master cylinder.

Have on hand a large jug of water (which I do whenever I bleed brakes) to flush away any brake fluid that gets onto the paintwork should this occur but with care it won't.  

Reply to this Topic
tim8061

Joined:

Apr 10

Posts: 115

tim8061 says:

SIngle discs?

Loads of bikes have single front discs and they don't pull to one side so why should something with one caliper have that affect ?

Reply to this Topic
Arthurshandy

Joined:

Mar 13

Posts: 26

Arthurshandy says:

Biased bike

I'm struggling to see how air trapped in one banjo could allow one brake to operate first? The two lines are connected and so are subject to exactly the same pressure, at the same instant. Whether the same line pressure exerts the same pressure on each disc is another matter, but that wold be a problem at the caliper end of the system.


Air would be an issue, but not that issue surely?

 I wouldn't so easily dismiss a problem at the headstock. A bike with intrinsically good geometry might act generally ok with slight play. But that play (or tightness) might possibly show itself under strong braking. Although I have to admit, you would think it would not pick the same side to twitch each time. But then again the camber of the road is always the same side...

Maybe a front tyre with a scrubbed sidewall?

Badly misaligned wheels?

Are we talking the exact same corner each time? Or each and every time the brakes are applied?

Check all front end fasteners are torqued up ok too.

[This Reply has been modified by the Author]

Reply to this Topic
bbstrikesagain

Joined:

Nov 08

Posts: 879

I'm with Arthurshandy

Air would mean squidgy lever, it wouldn't deflect the bike's course.


If head stock and wheel bearings are smooth, with no play and no binding or notchiness, then alignment or even a loose, bent or broken part?

Trail self-steer is amplified when you touch the front brakes, which normally just straightens the bike out, but if it's out of alignment it will head off elsewhere.  Take the brakes off and the forces you're using to keep it straight may be so subtle you won't notice them...

Reply to this Topic
smidget

Joined:

Nov 07

Posts: 2424

smidget says:

Arthurshandy

Air in a hydraulic brake system means an imbalance in the efficiency in any line where the air is trapped, air is compressible where hydraulic fluid is less so.

As an example, on a car brake system where all components are good and all seals are also good, introduce air into one leg of that system, now place it on a brake tester and you will find that there will be less efficiency in that leg of the system. This translates to an on road result of the car pulling to one side.

One sided disc on a bike, there is only one caliper and disc therefore there is no other disc inputting into the retarding of the wheel. 

Now introduce a second disc which exerts a different force to the wheel and you then have two forces of unequal forces working on a wheel which is under gyroscopic forces this unsettles the wheel which in turn unsettles the bike until the gyroscopic forces are reduced.

Now this is only my own understanding.

Hope you solve your problem soon.  

Reply to this Topic
bbstrikesagain

Joined:

Nov 08

Posts: 879

dual circuit

diagonally split systems can be imbalanced by air trapped in one side, e.g. in CARS etc.. BUT bikes have only a single circuit feeding both front calipers, the pressure is common to both, regardless where the air is trapped both calipers experience the same pressure and apply the same force.

Reply to this Topic

Page

Compare Insurance

Save money by comparing quotes. It's quick and easy

Motorcycles for sale

 

It's only £13.99 to advertise your motorcycle on MCN

Sell your Motorcycle

Motorcycle pricing tool

New! Find used bike prices