Bouncing Brakes??

[brainbug007]

I'm hoping someone can shed some light on this one.. The haynes manual for my motorbike said I should replace most of the braking elements after 2 years which I've done. So I put new pads, discs, lines & fluid in. Now when I brake it's a bit weird and hard to describe. At speed it seems ok but as I slow down (like when approaching a red light) and my speed gets down below say 10/15 mph, the front brake doesn't seem to work properly and the front of the brake bounces up & down on the forks so to speak as it finishes slowing down and the rate of deceleration doesn't seem very good. I'm wondering is this just something like the disc & pads wearing in? Or perhaps is this what people refer to as spongy brakes and there's air in the brake line?

[flyerncle]

Check the disc for runout as this will cause problems and it sounds like the fault you have.

[brainbug007]

Can't say I know much about what that is and the description on wikipedia isn't very good... any points for what I'm looking for?

[ayjay]

I agree -- take the disc off again and make sure its seated properly at all points.

[Talonmotorsport]

In plain English and not in text book terms it means that the disk could be mounted on the p#ss or it is not completely flat causing the feed back that you are talking about.

[flyerncle]

Should have been more specific,40 years in the trade is begining to tell.

Basically if the the disc does not run true with the hub (wheel ) it is bolted to,when you apply the brakes the pads will bounce on and off and the fluid will move back and forth up the pipe to the master cylinder giving the on off motion you describe as the clamping force is not constant.

[brainbug007]

Thanks for all the replys guys. I've taken the disc off & put it back on carefully 4 times now and it still hasn't resolved it. I was looking at the disc closely and it does seem to be warped slightly as there's a ring worn into the disc from the pads and on one section the ring hasn't worn in :( Suppose that's what i get for buying a cheap replacement off the bay. Not only that when I went to put it back on the last time, one of the bolts has snapped off when I was torqueing it up :( Proper cheap bolts they use on these bikes. Even though it had an 8.8 stamp on the top of the bolt, seems unlikely it's actually a 8.8 bolt if it snapped before torqueing up to 10nm (which seemed awfuly low for m6 bolt?)

[flyerncle]

Torqueing a bolt will increase its length causing it to stretch and this is the root cause of failure of a reused bolt in some cases.

Jap bolts tend to fail if used more than once,how acurate is the torque wrench.

[brainbug007]

Good to know I guess that they won't hold more than once. I would hope my torque wrench is reasonably good as it's one of the professional ones from halfords. Is it right though that an m6 should only be tightened up to 10nm as that seemed awfully low for holding a brake disc on?

[flyerncle]

Quite possible and I would replace them with something of a known and better quality.

the screws that hold the injectors on some Audi diesel engines are unbelievably small for the job and so is the torque,about 4 nm or so but they work.

As for Halfords "Profesional " I would not get too excited,known makes are possibly better.

[alga]

Actually, 10 Nm (torqued dry) is ~75% of proof load for an M6 8.8 grade bolt.
If you lubed the thread and torqued to 10 Nm, you've reached proof load, give or take.

http://www.cncexpo.com/MetricBoltTorqueNm.aspx

Useful to keep in mind when torquing various bolts with unspecified torque: ~10Nm for M6, ~25Nm for M8, ~50Nm for M10, ~90Nm for M12.

[alga]

Is it right though that an m6 should only be tightened up to 10nm as that seemed awfully low for holding a brake disc on?

The brake disk is actually held down by the 4 or 5 wheel studs, the M6 bolts are there just to keep it in place when you take off the wheel. Sierra, for one, does not have these at all.

[ayjay]

[QUOTE=alga;77034]The brake disk is actually held down by the 4 or 5 wheel studs,

Remember BB is referring to a motorcycle disc

[brainbug007]

Well granted the halfords one isn't the best but it was the best I could afford at the time without spending silly money and was certainly better than the clarke ones from what people said.

Daft question probly but do bike brake discs have some sort of standarized numbering system similar to sprockets? I'm wondering basically if the same disc is used on other bikes in which case I could find the same disc in better quality for a different bike that's more well known for "performance" aftermarket parts?

[ozzy1]

Only the box it comes in usually has a part number on which you can cross reference to others.Manufacturers usually use the disks on a few of their models to save on costs.Link here for Demon tweeks maybe worth a look for a new disk.

http://www.demon-tweeks.co.uk/motorcycle/brake-discs

[flyerncle]

I should have mentioned I am a mechanic and have 40 odd years worth of tools so bought decent stuff and still have it and one wrench was silly money so its accurate.

There are different torque values for dry and lubed bolts and i have seen them come loose with disaterous results and Albert has pointed that out,Not the comming loose bit.

[brainbug007]

Good to know guys. This is probly a bad idea but.. while digging around to try and find a better quality version of my disc, i came across ecb's site and started digging around there. They had an article about fitting new discs and checking the runout. they basically said that if after trying the disc in all the various mounting bolt positions you still have runout to just adjust using washers as spacers? Do you think I could get away with this and not bother buying a new disc?

[robo]

The disc mounting face and register along with the wheel bearing holes are all machined at the same time so unless the hub is damaged or the disc mounting face has some crap on it it should run true. If the discs are new and a decent quality there should be no issues. It might be an idea to clock the hub for run out before you go much further, it might also be worth looking at the hub where the bolt holes are and see if they are pulled/raised through over tightening. That may be holding the disc off the flat register. If the hubs run true bolt the discs on and clock them that will eliminate all the guess work and save a shit load of time . All quick and easy stuff, worth checking.

bob

[flyerncle]

Nail on the head Bob,putting washer's or shims under the disc will make it worse, EBC are usually pretty good and have used them on MX and road bikes.

May solve your problem if you find complete wheel and replace it.

[robo]

Good to know guys. This is probly a bad idea but.. while digging around to try and find a better quality version of my disc, i came across ecb's site and started digging around there. They had an article about fitting new discs and checking the runout. they basically said that if after trying the disc in all the various mounting bolt positions you still have runout to just adjust using washers as spacers? Do you think I could get away with this and not bother buying a new disc?

I think you will find is they are refering to the caliper centralising, no way would they ask someone to put washers under a disc,

bob

[baz-r]

most bikes come with a thin fiber washer between the wheel and discs on pritty much all the bikes i have had. you havent lost or dobbled up on any of the holes by mistake? or fitted it with the wrong rotation (should have arrow if so) i always try to refit my disc in exactly the same alignment it was fitted to the wheel if im reuseing an old one
your problem does point to disc warping by the sound of it