Braking on a downhill - squeals and vibration

[GrumpySmurf]

Ultegra brakes (3000K old pads) on Ksyrium rims. Going down Welshpool road, I occasssionally feather alternatively the front and rear brakes. Not a lot, but just to slow it down from 70kph to 60kph. After a short while the brakes will squeal and vibrate quite badly - to a point where I wouldn't dare touch the brakes again.

I realise that things will get hot there, but is there anything else I should look into?

[rustychisel]

many possibilities, but vibration is a worry and suggests looking at the wheels themselves to see if

a) they're centred in the dropouts
b) the bearings aren't adjusted a little too loosely, which leads maybe to
c) QR levers done up sufficiently tight (do not need to be massively torqued)

Other than that either replace the pads or service the ones you have...

clean pads and examine for roughness in the braking surface. Make sure they're hitting the rim flush and are adjusted equal, many many bikes I see have the pads 3~4mm out of line with each other (in the vertical plane). Check toe in.
Clean your rims, warm sudsy water and an old dish scourer (nylon non-abrasive style is best) will work wonders in tandem with cleaned pads.

I presume the brakes aren't grabbing ... fouled or worn cables, or jammed calipers etc. Clean and lube all pivot points. Replace cables as required.

Or, a long shot, headset bearings loose or worn. Stand next to bike and hold front brake on whilst you 'rock' the bike gently back and forward. Any clicking or notching or movement between the forks and the frame suggests they're loose and servicing is needed.

[Aushiker]

Hi

Do you get the vibration issue when braking elsewhere?

Andrew

[GrumpySmurf]

No - none whatsoever. Only coming down those long big hills.

[Chaderotti]

Coming down that steep ass cul-de-sac near Kings Park I hold the brakes, towards the bottom when they start heating up the braking sound gets louder and louder... But I don't get anything which sounds like squeals nor do I get any bad vibes.

[giwi2]

i have ultegra brakes and they are 5000kms old. I have come done Welshpool Road numerous times at similar or greater speeds and have not had those problems - but then again i have not really used my brakes cos it is such a good hill to descend on.

That said I have used brakes on similar long hill descents due to other riders and cars etc but never had the vibrations you have mentioned. My immediate thought is loose calipers but if that was the case vibration/shuddering should occur at almost any speed depending on how loose.

[thomas_cho]

Chances are that the braking on the descent has overheated your pads, and formed a glaze.

That is likely the cause of the squeal. This glaze is slippery, and the pad is probably now slipping and grabbing on your rim, causing the shuddering.

I once descended and got the squealing. I went home and cleaned up the pads, roughed it up, and they were fine to use again.