Disc brake runout

[stuartb]

Anyone know the runout tolerances on front discs ?

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[Flyingscotsman]

Hi Stuart,
BMW manual says max 0.15mm at circumference for front disk on HP4.
BW Rob

 

[stuartb]

Thanks for that Rob, I was just being super lazy last night and couldn't be bothered going into my garage for my manuals..keep forgetting I can access the manual online [emoji17]...

I replaced both fronts a couple of years ago, they haven't done much mileage wise, tho they've been thrashed on track a fair bit, tried rotating both 180 degrees... still a very noticeable throb through the lever when I grab a handful and seems to be increasing, the rim is new as are the bearings, so process of elimination, pretty sure it's one of the discs.

My mates got a good engineering shop, I'll whip them off and get them onto one of his lathes and check with Dial Indicator.

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[stuartb]

I just bought these, the guy said I would never wear them out.. 34 mil thick V 5 mil, told me not to worry about the unsprung weight, I'd forget all about it after the first corner [emoji848]..

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[Wull]

Does the RS4 even go quick enough to warrant them lol

 

[stuartb]

Does the RS4 even go quick enough to warrant them lol

Cheeky bastard... [emoji1787] ..

So the backdrop is, ?1400 supplied and fitted from Audi, I Mic'd the discs when it was in the garage getting my rims done the other day (yup, already trashed one on a kerb last week).. random measurements in excess of 33 mil either disc, so won't need them till another 15 to 20k, but I will need them.... Geezer on Ebay was selling them plus pads which are ?400 alone [emoji17].. managed to snap up the whole megillah for ?700, he dropped them off at my house this morning, not bad from Worcester !! He's a service engineer for Aga's and the like and fortunately had a call in Ayrshire today, christ knows what the postage would have been, I reckon the discs must weigh 15kg each.

Pads are OE too Manufactured and stamped by Brembo, I've never felt a coarser material on a brake pad, I mean like 80 grit coarse..no wonder they chomp through discs..

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[stuartb]

So EVENTUALLY got the green light about my discs on Friday afternoon, which isn't what I wanted to hear, they are well within tolerance..was hoping it was a fooked disc !

Alex from Dumbarton Precision Engineering has a top quality shop full of high end machinery, so his lathes are all electronic, they measure Midgies Dicks down to Microns or have I got that the wrong way round, I want a job on the tools in there !!!!

Next head bearings, this needs to be a process of elimination now..

Hammer and screw driver are OUT this time around, I picked this up from Rab Carson, not sure if dogs will fit onto lock nut, ordered correct one from Ebay anyhow.. fingers Xd.. [emoji1696]

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[Flyingscotsman]

Thats a bummer that its not simple, but good that the discs are OK. Have you ruled out a sticking caliper piston?

 

[stuartb]

Yea Rob, overhauled the calipers last year, bought a set of Brembo seals, replacement OE pistons and retaining springs, they slide and retract from the discs perfectly well. Which reminds me how impressed I was with Brembos mounting gel for the seals.. amazing stuff !...

I'll pull the top bridge off and check the bearing out later tonight.

One thing I have thought of, I never settle the forks on the deck prior to tightening everything up, I do that with the bike at waist height on the ramp and the front on the paddock stand, maybe it just needs a good bounce.

Dave Moss has a good video on that, Shay sent me something similar in a link... my gut feeling it's either the bearing or a slight misalignment in the forks.

Rims only a couple of years old, you may recall.. (eyes roll in head ... ????) but I'll spin it up on my balancer and check anyhow..I've also previously whipped the disks off, swapped sides and rotated each 180 degrees, settled the calipers loosely and cable tied the lever then torqued the wheel up..all the old tricks..I'll post up what's the results.

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[Flyingscotsman]

Sounds like you're doing a thorough job as always Stuart. I'm intrigued to find out what's causing it.

 

[bagpuss]

Sorry if obvious but have you checked if all the disc bobbins are clean and have free play in them.

Mine used to clog horrendously when using GPFAX pads on track and cause horrendous drag with the wheels when I didn?t clean them often.

 

[stuartb]

Yea bobbins are tickety boo, checked them on and off the bike.. whipped off the bridge tonight to find the tool I got on loan of doesn't fit, was a Honda one so had suspected this, I had ordered one off ebay anyway so should get that tomorrow..

Alex had a dial guage on it, maximum run out was something like 0.08mm on one and a bit less on the other.. wanted it mounted on as true plane as possible to rule out any slop I got from the rim/bearings etc..

Took some pics when I was in.. those tool heads for the CNC machines are a fortune. I love places like this, took me back to an engineering course I done at Clydebank College, it's a gem of a place..

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[stuartb]

Sounds like you're doing a thorough job as always Stuart. I'm intrigued to find out what's causing it.

Muddled my way through it you mean haha.. and so am I..

I'll bet its either the head bearings or forks.. with the big money on forks, so I'm going to bounce the shit out of them.

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[Flyingscotsman]

Hope it's not the forks, could be expensive. Nice photos of the engineering shop.

 

[stuartb]

They just need a good bounce on the ground prior to nipping everything up, the forks are fine thankfully.

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[stuartb]

Tum te tum... paid an extra quid for faster postage, socket still isn't here [emoji58]

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[stuartb]

Woop woop.. socket arrived at long last !

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[PeterBrodbeck]

A runout of 0.08 mm is within acceptable limits. It's good to have a friend in such a useful shop with such useful machines).

 

[bananaman]

Used to work in a machine shop, interesting if you're that way inclined.
Bought myself a dial gauge to true up an abs ring, worked a treat.