Front disc - how hot is hot?

[SteveC#222]

How hot should a front brake disc be in normal use?

I've just MOT'd my XBR after 3 years of storage. Checked and serviced everything in the days prior to going, but on pulling up after it's first run in 3 years it was obvious the front brake was dragging a bit - rather hot. Fortuneately my mechanic is a sensible bloke who not only knew me well enough to know it would be sorted as soon as I got home but also realise that this was the shakedown ride.

So I've removed and cleaned the caliper ( new piston ordered as one is a little pitted), everything back together and tested on the drive and working fine, wheel spins freely.  Took it for a short 10 mile test ride and on getting back the front disc is still quite warm - not hot like before but quite warm - though the wheel still seems to spin pretty freely and it cools down within a couple of minutes.

So how hot should it get in normal use or do I still have a problem?

[Propellor]

Good question! But the answer would be dependent on lots of factors. With time to stabilise after heavy braking I wouldn't have thought much higher than around 30 C to 40 C. Pure guesswork though!

Just throwing ideas around, might pay to check there is sufficient space above the bellows in the reservoir, if you've put new pads and/or a disc in. If the xbr caliper is a sliding type I'd scrutinise the condition of the pins holding the pads. A warped disc might cause it, but you'd have spotted that spinning the wheel.

[Oldtimer]

I would say it would get f***ing hot, enough to burn your fingers if you touched the disc.

[chris22]

Blocked fluid return hole in the master cylinder? That is what caused red hot disc on my VF400.

[BrendanO]

Fwiw I think discs can run lots hotter than 30-40C. I have gone OW! when accidentally touching a CX front disc, but hat has a wee bit of drag.

[Propellor]

Blocked fluid return hole in the master cylinder? That is what caused red hot disc on my VF400.

That would certainly cause it to drag or even lock on. That must be pretty rare tho? You were unlucky!

Regarding temperatures, sure they get stinking hot. When the brakes are applied hard against a rapidly turning wheel! Steve was asking what is normal "stabilised" temperature, I assumed he meant "normal" road riding. There's no way they operate mega hot for any sustained length of time.... Unless there is a problem!

I ran old fiats back in the very early 80's. Very prone to caliper problems (I won't bore you with why, unless you want to know). I used to know which one was starting to seize by feeling the disc after driving "normally". The stinking hot one was the dodgy one. The other "normal" ones were about the temperature I suggested.

[SteveC#222]

Thanks for the input guys.

I should have a new ( stainless steel) piston and full seal & boot kit coming tomorrow, a new tube of 'copperslip' on the work bench and just been watching a couple of 'how to's'  on youtube so it's rebuild time!!

Hopefully that should sort it out.

[SteveC#222]

OK, caliper now fully refurbished and refitted.  I had thought it was the top, pitted piston that was probably sticking but on stripping the caliper the lower piston was well and truly stuck! It would move out under hydraulic pressure, but pulling it the last bit with pliers (wrapped of course!) was quite hard work.

Once inside, as suspected, I found the dust seal grooves were a bit of a mess - lots of crud, crap and salt corrosion. It took the best part of an hour to carefully clean them out. The sticking was almost certainly because the muck had distorted the seals out of the grooves. So everything out and cleaned, new seals and pistons back in ( suprisingly easy) and back on the bike and bled. It feels much better and the wheel spins freely enough.  I only need to road test it but as it's curently blowing at 40mph + and we've just had a thunderstorm I think I'll leave that for tomorrow.

[SteveC#222]

Took it for a quick run today and everything seems fine now. After a 10 mile run the disc was bareably warm to the touch and measured with SWMBO's meat thermometer it registered about 30 degrees.

[Moto63]

Hhmmmm that bloody propellor geezer got it about right then, always one smart aleccy kid in the class ain't,t there. Glad ya got it sorted tho, summer,s a coming

[Andy M]

Back when I was testing I've done hot performance work in the Alps where the disc was glowing cherry red. About 400C.

The temperature only shows how much work it is doing.

Glowing red after a few miles of flat roads where it should be doing no work simply shows it is dragging.

Andy

[Propellor]

Took it for a quick run today and everything seems fine now. After a 10 mile run the disc was bareably warm to the touch and measured with SWMBO's meat thermometer it registered about 30 degrees.

Good work Steve.

And a lucky guess for me   

Let the good times a roll ah!