Fitting new disks wiht dial indicator gauge?

MattSAU2XR8 · 20-02-2014, 03:10 PM

Hi Matt,

Hope you don't mind another technical question which may be of interest to other readers also. I've been reading around on the net and come across a few posts that describe checking the axial runout on both hubs and then discs with a dial indicator gauge when fitting new discs, and then trying the disc in different positions to get the least combined runout, if any is present. Theory seems to be that if the combined runout is reduced as close as possible to zero then there is less change of disc thickness variation developing as the rotor wears. Given I can buy a cheap dial indicator gauge for about $45 on ebay this seems like it might almost pay for itself on the first set of rotors fitted. Do you think this is worth considering or are new rotors and hubs so accurate now that there is unlikely to be any benefit.

Thanks for your thoughts,

Matt

jpblue1000 · 20-02-2014, 04:07 PM

Unless your prepared to have the new disks machined and replace the hubs as the indicator will show run out no matter which way you mount them as the bearings being older than say 1 month will have some minute amount of play I would not bother.
For a road car I wouldnt bother, neither would I bother for a track day car, a full on race car maybe especially if you compete at the front of the field but then your likely to replace the disks sooner than have to worry about disk shape changes.
I would suggest the work involved and additional cost for a non precision item such as brake disks is not worth it.

JP

**FLOORED** · 22-02-2014, 11:28 AM

Good questions and good answers there.
We did a tour of DBA's facilities at Silverwater some years ago and they would actually pull rotors off production and check runout in a temperature controlled room on a measuring tool worth thousands of dollars and get a printout. On the other hand there would be other manufacturers that wouldn't go to that extreme.
I have been in the industry quiet a few years and really the amount of new rotors i have seen that have any runout to be concerned with is very minimal but fitting them on used bearing hubs are a different story so if you have the tools and the time then why not do it.

gtfpv · 22-02-2014, 01:49 PM

fitter machinist here . forget it . if they arent true , they arent anygood . and any new brake discs with runout would be rare . just buy the new ones and clean your hub surface . if they are shaking then you have a very rare case of runout on a new disc , or another problem all together . if you bought a new window , would you check it with a light reflective gauge to measure equal lumanosity ?

MattSAU2XR8 · 24-02-2014, 02:44 AM

Probably not because I can see through it which tells me its luminosity. On the other hand I often check my 'pre gapped' spark plugs and all the gaps are always different :-)

TITO · 27-03-2014, 01:20 PM

Just bolt on new rotors and tyre rim etc with torque wrench and your done forget about the eBay gauge!

metookevin · 23-07-2014, 01:30 AM

Fitted new DBA rotors on AU XR6 & found left rear axle hub runout ( not rotor) to about 0.24mm as measured on perimeter of the rotor. Checked hub directly without rotor fitted and found corresponding runout. Drivers side rear hub is near perfect. All thoroughly cleaned on mating surfaces & conclusion is the hub has distortion that I now want to eliminate. I have fitted new goodyear eagle directionals tyres and want it fixed due to concern about tyre wear. Otherwise you would not know it exists.

The runout on the old rotor was not checked for comparison because my wheel nuts have a blind thread & stop short on locking rotor. I had not machined up spacers at that point. Wished I had made comparison because it was a bastard to get the rotor off & I wonder if I induced some distortion on my part.

To machine hub runout requires bearing removed, abs disc removed, studs removed in that order. Another day.....

Cheers

MattSAU2XR8 · 30-07-2014, 10:40 PM

I finally ended up getting myself one of these gauges and its been very informative. Executive summary is:
- New PBR slotted 322 fronts had 0.08 and 0.04 mm runout straight out of the box, on clean hubs, and not altered by fitting the disc in a different position.
- RDA 322 mm slotteds that have done 5000 km had 0.05 and 0.12 mm of runout.
- It seems possible to reduce a small amount of runout by sanding the back of the disc where it mates with the hub, i.e. from 0.08 to 0.04 mm, which may help reduce disc thickness variation down the track, and this would only require sanding the surface back by about 0.015 mm which is fairly easy given iron cuts easily.

Long version below.

I've been having some ongoing brake brake shudder after fitting RDA slotted front discs and Ferodo pads and bedding them in. I think now that the RDAs did have some runout when new (spinning the wheel after bolting it back up made a tch, tch, tch sound as though the pads were touching intermittently, although the old rears additionally warped during the bedding in process - The Ferodo front pads require higher pedal pressure which had been working the rears harder. So initially I planned to just replace the front again, but now it looks like I need rears as well.

To use the gauge for front discs you need to put some spacers under the wheel nuts to allow them to hold the disc against the hub, I used large sockets and pinched them up very gently with a socket bar. Ideally the sockets will be of larger diameter than the countersinks in the disc holes. Also a spare disc sitting on a towel or rag makes a good surface to mount the (magnetic) gauge on. For rears you'd additionally need some good wheel chocks since the handbrake would need to be off for the disc to rotate.

I'd initially figured it was the front brakes and tested the discs (5000 km old) and found that one had 0.04 mm of runout and the other 0.12 (which is still incidentally within RDA's fairly generous tolerance, DBA recommend no more than 0.04 mm). I took multiple readings at each point and repeatability was within 0.01 mm.

Next up I commenced fitting a new set of PBR slotted discs (made here in Aust) and found that one of them had 0.09 mm of runout which I could only reduce to 0.07 mm by refitting it in a different position. I'd already cleaned the hub (which has only done 11,000 km) and so next step was to gently sand back the inside of the disc where it fits against the hub, about 1/3 of the circumference related to the side of the disc that is 'sticking out' too far. Only had to do the outer part of the discs mounting surface since the hub has an inner and outer ridge that bear on the disc, not a flat plane all the way across. And because the iron has a rough surface its easy to cut it back a bit. I had been hoping for 0.02 mm runout but in the end I only managed to get both discs within 0.04 mm of true, at least as far as the outer face is concerned. Hopefully this will be good enough.

So I guess the moral of the story is that different companies specify different tolerances (eg. 0.04 for DBA and 0.15 for RDA) and not all new discs are true, i.e. 0.08 mm runout on brand new PBR discs, although whether this small amount would be noticeable when driving remains to be seen. I quite like the sanding idea because having bought new discs and fitted them up to the car, its too much of a headache to put the old ones back on in order to return the new ones and exchange them for more discs out of the same factory which may still have runout.

The other interesting thing I noticed is that I still have the vibration immediately after fitting the new front discs and some new rear Ferodo pads. In fact as soon as I got the brakes warmed up it was worse than before. So I think the shudder I was getting was in fact the rear brakes all along. So now I'm tossing up between DBA T3 rears or just upgrading to territory rears which are a heap thicker. Leaning towards the T3s since the car stops OK, shudder notwithstanding, and I'm trying to stop myself from doing excessive mods :-) I still have the RDA fronts I removed and may re-use them with a bit of sanding in the future, assuming changing out the rears solves my problem for now.

Life would be much simpler if Ford had fitted BA GT fronts and BF V8 rears in teh first place. At time of build this couldn't possibly cost more than an extra $200 and would have been a good selling point over Holdens :-( And then there wouldn't need to be brake threads on Ford Forums.

MattSAU2XR8 · 30-07-2014, 11:11 PM

On the topic of RDA, I was just fact checking for the above post and noted that:

1. RDA say that even 15 microns of disc thickness variation (0.0015 mm) can cause brake shudder,

http://rdabrakes.com.au/page/TB06.aspx (halfway down page...)

2. Maximum acceptable runout is 0.15 mm

http://rdabrakes.com.au/page/TB01.aspx (again halfway down)

Its not hard to imagine that if a disc has 0.15 mm of runout then the pads will have no trouble at all chopping back the high points to produce well in excess of 0.0015 mm of disc thickness variation......

I've emailed RDA to see what they think about all this

**FLOORED** · 31-07-2014, 08:29 AM

It also goes to show that cleaning the bearing hubs before rotor fitment is important as you could only imagine the runout then.

jpblue1000 · 31-07-2014, 01:51 PM

Did you measure hub runout due to casting/machining tolerances or or bearing movement? before fitting the new disks

JP

MattSAU2XR8 · 31-07-2014, 02:06 PM

No - didn't measure hub runout directly.

As I said above the readings were repeatable to within 0.01 mm, measured near the outside of the disc, or at least 5 mm in from teh edge, so 312 mm diameter.

Given that the disc runout was able to be reduced by up to 0.02 mm by reorienting the disc it seems fair to say that this much is due to hub runout.

Given that the hub diameter is about 150 mm and the disc diameter is 322 mm, and I was measuring just in from the edge of this disc, say at 312 mm diameter, the hub runout must have been about 0.01 mm.

Given that readings were repeatable only to within 0.01 mm it would not be possible to measure the hub runout with the gauge I have.

As for bearing movement, no, didn't measure, but as I said readings were repeatable to within 0.01 mm at each point, so possibly this 0.01 mm could represent bearing play. I was pretty happy that the bearings are not a major problem yet as the car has only done 11,000 kms and they still feel tight to rotate by hand. Also I think the hub is pretty much a sealed unit now, although not sure on this.

On the topic of DBA vs PBR vs RDA the RDAs are looking pretty good in hindsight - One was as good as the PBRs out of the box and the other could have been sanded back a tad to get close and they cost just over half the price.