Brake shudder under hard braking

Evening all,
So just done a brands TD and found the brake set up I have on this new to me bike is pants!!
I thought the forks were bottoming out first of all but after putting on a load of preload nothing changed
Pulled the calipers off cleaned and and replaced still the same
I think they are m4s with brembo pads and not sure what discs they are until I look properly
I'm now thinking the discs could be warped which I will check
Any thoughts ??

 

Yea discs could be warped bud, easy to tell if you just hold something against the disc like a DTI and keep it still then give here a spin.

Adding preload won’t stop your forks from bottoming out if they are however. Pre load is just the starting point or front ride height. The spring rate stays the same so if they are bottoming our they will continue to do it even with loads of pre load. It will handle horribly though.

If you can feel it through the lever then it is most likely the discs but you a more than capable and your gut feeling is usually right so don’t dismiss the forks as the issue. Have you not got a fork travel indicator

 

Does sound like the discs especially if you have checked / cleaned the calipers as a stuck piston and pad chatter creates a vibration too.
As @Bigmac says, get the front wheel off the ground and spin it, easier with 2 people too.

 

check the disk for a large mark where the pads were while it was stood? maybe looks like a surface rust spot or black/blue stain the shape of the pads.

i get this a lot on my Sprint commuter when it has sat, or put away wet or hot, and those areas can cause the pad fluctuation like a warped disk until they are worn down, and a bit of wire wool sorts it until next time.

 

Evening lads thanks for the advice as always
So had a quick look today and got s few pics unfortunately my bikes still in the Van's until the weekend when I can unload
But the discs do look a bit cruddy and I did on the track day use a bit of scotch brite on them but obviously not enough
When I can get it to the yard I will check to see if they are warped
Further to the bottom out scenario the indicator is where I finished my last flying lap

 

Looks like there's some high spots on the disc in that last picture (apart from the pad imprint) could be pad deposits possibly.
If so it'll probably need more than scotch-brite to get them flat again.
Is it fitted with suitable pads for track days?

 

I’d be looking at what pads are fitted and what type of material they are, those discs are braketech cast iron and should in theory be far from pants when used in conjunction with the right pad

 

Think I'd be changing those disks and throw a new set of pads in while I was at it. They don't look great IMO.
Fork marker looks just about right I would say although a pic from a better angle would be easier to tell.

 

i was trying to think of a better way to say it, and was having a brain fart as just could not put the words together.

nailed it, cheers

 

When I bought the bike the fella did say these discs were supposed to be good albeit now a bit cruddy
I have brembo pads in but not sure what spec but will pull them out quickly for s look
So.. can these discs be refurbished?
Secondly what pads would you guys recommend?
My last set up was nissin blacks with sbs duel carbon on Armstrong wavey discs which were excellent with the rcs19 master cylinder
Ie I still have the Armstrongs so that's my next port of call if I cannot solve the current issue
Cheers

 

Not entirely true about preload not preventing it bottoming out. Pre-load does not change the spring rate as @Bigmac said however if your starting point gives you more travel then for the same force the spring will move the same amount so if you had 100mm of travel before and you now have 110mm and the breaking force wanted to compress spring by 105mm, it would have bottomed out before you increased the pre-load but not now.

However there is no such thing as a free lunch as you now have less top out travel available to keep the front tyre on the ground when you drive out of the corner so it might run wide or wheelie more.

So a stiffer spring might be better as it would compress less under brakes and stay withing that notional 100mm and still have some travel so fork does no top out so badly on the power.

 

Worth noting on the Daytona forks the bump stop (according to JHS) is about 10mm up the chrome, so if your marker is there you don’t have much travel left

 

that sorta makes sense to me but i have 10.0 springs in the forks which were the same as my old bike and i had about 25mm of chrome showing so not sure if i was bottoming out?

 

does that still apply to forks that have the KTEC DDS cartridge kit in it?

 

so checked the discs today and i was getting a variant of about 40 thou on both discs with the calipers off but a little less +10 & -10 with the calipers on
I've also found a small weep on my brake lines as well which may have contributed to the shudder (not sure)

 

Is that 40 thou run out?
Not sure why having the calipers on or off would make a difference
If it is I think that's way too much, I think spec is 0.25mm, 40 thou is 1mm

As already suggested I think you'll be better off replacing them, even if they're not warped those faces will take a lot of cleaning up.
I don't do track days so other people will have better ideas on pads, aren't those the standard fitment brembo HH though?

 

Looking at the pics

I’d says your discs shot one more then the other just try and see if the calliper ain’t sticking

But like @wobblybiker99 said it’s gonna take a lot off cleaning up

I think your best option would be to replace the discs and pads for a new set but just make sure you buy something that’s made to be used on track as well as road

 

sounds like the best solution i think
cheers lads now to find some good pads !!!

 

Update:
So I have now changed my discs which I know are true but stillnit don't feel right even just pushing it
My other discs I had checked at Maidstone motoliner and they are true.
When I press the brake I can hear a little click so now thinking headstock bearing?
I've ordered new ones and just hoping it is just that as the last thing I can think of is a cracked headstock.
It has been crashed the frame as the lock stop has been broken off.
Is there anything the forks that could make a click when you press the brake lever?

 

Can you get a video?

If it's headstock you'll be able to feel it when you rock it back and forward with the front brake on.

If you have a headstock stand whip the top yoke off and check the bearing.

Only other think I could think of would be the discs or pads catch the pad retaining springs in the calipers?