knock off

[F|sh]

can some one explain this to me.

I have noticed that when i touch the pedal and then hit the brakes that the pedal feels better(re harder) could this be pad knock off?

Also how can i calculate the size of the master cylinder required for my brakes.

I have a 13/16 non boosted master cylinder and after last weekends racing the pedal is way to soft.

Does fluid choice also effect my pedal hardness? i can feel the fluid getting hot as the pedal travel was longer.

this is my front brakes so some one gets an idea 40mm 4 spots.

http://groups.msn.com/_Secure/0TQDjArAXfZ0U5vquE5k!2k09NCr*om16oIVj61wNJncozS9Bk XNRxjrbCng8mXh0OAWXg319isVldKaA1uvDdZRvrW7uh2SKUX! cis8wGfWaNWhzlFrzIQ/brakes%20002.jpg

my rears are 38mm single pots sorry for size. They do work as they were a bit blue.....

http://img124.imageshack.us/img124/1613/dsc004175wt.jpg

[Lonx]

I have noticed that when i touch the pedal and then hit the brakes that the pedal feels better(re harder) could this be pad knock off?
I think this occurs due to the stab of the pedal pressuring the system a little before you actually brake. In a perfect world I think it should make no different, but as far as I know its a pretty common thing to touch the brakes lightly before you really need to, if you haven't hit them shortly beforehand. Happens with most cars... but someone may say completely otherwise so I'm happy to get a better explanation.

Also how can i calculate the size of the master cylinder required for my brakes.I'm going to need to know the same :) Cheers for saving me a thread....

Does fluid choice also effect my pedal hardness? i can feel the fluid getting hot as the pedal travel was longer.As far as I know again, it'll affect the pedal hardness when heat increased as you're describing. What fluid are you currently using and how long ago was it bled? From the reviews & comparisons I've found in the past, Motul 5.1 is a pretty good option if you can get it, and its compatible with any Dot 3 or Dot 4 compatible brake system. Otherwise Castrol Response has done me good in the past... both are cheap & very affordable ($9 - $13/bottle).

[F|sh]

im using a PBR fluid but seriously looking at going back to my other brake fluid. Brakes done recently.

[260DET]

Your pedal softness appears to be fluid/heat related so I'd look at that first, before changing the MC. Do you have cool air ducted to the fronts? Sounds like you have brake knock off, never experienced it myself.

[wheelnut]

have you tried bleeding brakes after each run, i always end up with air at nipples after a run. ive found ferodo dot4 really good.

[F|sh]

260det the pedal is soft even beforewe had out. No ducting to the front rotors. I will investigate it

Wheelnut we have and it did make some difference.

[260DET]

Too small MC by itself won't give a soft pedal, just excessive pedal travel at all times.

[F|sh]

i was thinking of maybe a minute leak somewhere.

[oldcorollas]

to calculate new MC size is relatively easy.

if you want to double the force required to push the pedal (and halve the distance it travels), then you need to double the _area_ of the MC.

13/16 = 20.64mm
MC area = 3.334cm^2

halve pedal travel = 6.668cm^2
D = 29.18mm
= 1 5/32"?? something like that (stupid non-metric crap:P )

ie calculate MC area. divide by the % reduction in pedal travel (ie / 0.85 for 85% pedal travel), or multiply by increase in force required (115% force = x1.15) then work out diameter required from new MC area (A = PixR^2)

or somthing like that.... :huh:

[dattoman1000]

To increase the force at the calipers you actually reduce the size of the master which therefor increases pedal travel but decreased pedal effort

So to get a firmer pedal he needs to increase his master cylinder size but this will decrease the pressure at the wheels and increase pedal effort

As your running a stock Gem master of 13/16 with no booster you have 2 options

1. Go to the gym and work on those leg muscles and fit a larger master cylinder. ie Falcon 1"
2. Balance bar assembly with seperate masters tailored in size to suit your brakes. i.e 5/8" front 3/4" rear.

And if you have knockoff you need to find the cause and fix it or learn to drive around it

[F|sh]

cheers guys. Will let you guys kno what happens

[F|sh]

found a leak yesterday...

Also a pin is missing from the front rotors that holds the pads in place....

Maybe a air leak somewhere. Pedal gets hard when flushing but after a while its goes abck spongy?

[F|sh]

also found that he was play from the rod to the master cylinder adjusted it and seems better. hopefully. It feels better and i think the piazza calipers could be a problem

[edo]

I doubt you would get knock off from rotors that small.

[F|sh]

on 266mm front rotors?

[edo]

Yeah, any movement in the bearing/hub (& alignment of the rotor/hub) is amplified, as is run out as the rotors get bigger. You'd have to have something pretty far out of whack to be knocking pads back with rotors of that size.

[FatBoy]

I put a HQ master cylinder on mine and it works great Fishy. Dunno if that helps you though... :)

[Guido]

Yeah, any movement in the bearing/hub (& alignment of the rotor/hub) is amplified, as is run out as the rotors get bigger. You'd have to have something pretty far out of whack to be knocking pads back with rotors of that size.

I could imagine it... With my car there was very little movement noticeable with the stock Fiat brakes (being that they are ~45mm in diametre), but with the the 280mm rotors fark it wobbled a lot!! The standard Fiat hubs were so untrue it was a joke! Phil managed to machine them so they are far more acceptable but I have not taken out these pads to see of there is any knock off.

[rowdytoot]

F|sh- given that every gemini I and everyone else has driven seems to have this issue almost irrespective of what M/C, calipers, or discs they are running- perhaps it's a flex thing- firewall/hubs/ bearings ??

I know mine does the same, with new bearings, new discs, reco master cylinder.. I've adapted to the long initial travel now- unless you have just been on the brakes, and have learnt to just keep pushing- the firmness is down there somewhere! and it stops 'pretty well OK, kinda' for what it is..

:shrug:

Rowds

[F|sh]

yeah i have braced the firewall too.
I want to replace all flexible rubber lines with braide soon. I have noticed that the rear plate for the caliper could be flexing. Also noticed that the pedal box needs to be thrown or rebushed,

[rob323]

I have the same problem on my gemini series rally car, the only difference is that I have the standard TX gemini booster.

I finally got real sick of the "only brakes during the last inch of travel just before it hits the firewall" syndrome too. Yes, a light tap prior to braking helped a little but the pedal was still along way down before anything happened.

Since Xmas, I have tightened up the front wheel bearings ever so slightly (I can't believe how little preload they are supposed to have) which even though there is still a very small amount of play in the bearing, once it heats up, this dissapears. I have also adjusted the rod from the booster to the pedal and even preloaded it a little to remove the inch or so of slop in the pedal. This is probably detrimental to the master cylinder's health, time will tell. After doing about 180 km around an autocross track on Sunday, I finally have confidence in the brakes. Knock off is gone and the pedal only needs to be at half travel to lock the wheels up making heel & toeing much easier.

First rally isn't till after easter so we'll see how it goes then.

[rowdytoot]

thanks rob- that's kinda helpful, I might get the boys to try that for me if we do another rally in this car..

Rowds

[F|sh]

well fingers crossed that we have made some changes to it.

I just have to put a pin back in the caliper

[Raynman]

If this doesn't cure it, then look at brake springs (the ones that go inside the calipers). We fitted them to the legacy and it helped the knock off problem (rally car, with slide calipers)