JohnTB Posted May 25, 2017 Report Share Posted May 25, 2017 Anybody know what front disc run out should be ? I've been experiencing "judder" when hard braking and after getting a clock gauge on the front discs, I find a variance of 7 thou on near side and 9 thou on off side. Would this be down solely to distorted discs or is there something else that may affect run out ? Quote Link to post Share on other sites
Waldi Posted May 25, 2017 Report Share Posted May 25, 2017 John, Assuming you have minimal bearing clearance, 2 thou (0.05 mm) max. is what sounds about right to me. Dirt between disk and hub would be a possibility, but probably more likely when new disks are installed. Waldi Quote Link to post Share on other sites
john.r.davies Posted May 25, 2017 Report Share Posted May 25, 2017 John Worth reading this? http://stoptech.com/technical-support/technical-white-papers/-warped-brake-disc-and-other-myths It's by Carroll Smith, who has to be an authority. John Quote Link to post Share on other sites
RogerH Posted May 25, 2017 Report Share Posted May 25, 2017 (edited) Hi John, you could try a couple of things to see if the variation is on the disc or outside. Tighten the wheel bearing so that the very small free play is removed and measure again. The DTI MUST be lock solid with the disc - ie attached to the same structure with no movement. You could remove the disc and fit to a lathe. This will give a very accurate reading. How old are the discs. Discs can produce odd grain structure as they wear down. Swap the discs from side to side and see if the measurements follow them. Buy a cheap plain disc and measure that insitu. Roger Edited May 25, 2017 by RogerH Quote Link to post Share on other sites
Nigel Triumph Posted May 25, 2017 Report Share Posted May 25, 2017 According to the manual, maximum disc runout is a mere 2 thou for the TR6. Interestingly the Reliant Scimitar manual gives a maximum runout of 6 thou, and it uses TR6 discs, albeit with different calipers. My Scimitar GTE has 4 thou one side and 6 thou the other. Under heavy braking there's a slight judder... The runout measurements you have on your discs would be enough to explain the judder you've experienced. Nigel Quote Link to post Share on other sites
HSM Posted May 25, 2017 Report Share Posted May 25, 2017 Some points to bear in mind when fitting new drake discs, always clean the hub face before fitting new items, pay particular attention to the bedding in process , this involves a gradual build-up of heat in the disc & pad compound to a point where at a molecular level a thin layer of transfer film is laid down smoothly and evenly onto the disc surface. During this bedding in period , which can last for about 400 mls. it is important to avoid heavy braking ( unless in emergency ) . Rapid heat build up can create thermal shocks, which can cause surface cracking. Also when bedding in new pads & discs it is important to avoid sitting stationary with the brakes firmly applied as this can create hotspots, especially if the braking system hasn't had time to cool. The hot spots can create an uneven surface resulting in disc thickness variation ( D T V ). I always add an advisory note to my customers invoices regarding bedding in brakes after there replacement. Harvey S. Maitland Quote Link to post Share on other sites
JohnTB Posted May 25, 2017 Author Report Share Posted May 25, 2017 Thanks Gent's, some very good points, and thank you John for the link, which explained a lot. The bit about TR6 discs used on the Scimitar but with different calipers is pertinent in my case as I run with Toyota four pots (and have done for the last 15k miles). A scout around my garage revealed two new discs in boxes (oh the joys of old age and forgetfulness!) So, Roger, instead of swapping the discs around, I'll fit the new ones and see what happens. Watch this space. Quote Link to post Share on other sites
RogerH Posted May 25, 2017 Report Share Posted May 25, 2017 Hi John, have you got any of my bits in your garage as I'm also missing bits and pieces Roger Quote Link to post Share on other sites
JohnTB Posted May 27, 2017 Author Report Share Posted May 27, 2017 Well Roger, try as I may, I can't find your bits in my garage .... Fitted new discs after thorough clean up of hubs once the old discs had been removed. Torqued to 34ft/lb using method as close as I could to tightening head bolts. Prior to fitting, used fine file on the face where the discs fit (mainly around the edges of the bolt holes to ensure no edge) and also a thorough a emery clothing. Checked face with a known straight edge and all looked fine. Fitted hubs - no bearing play, but found run out of 6 thou on off side and 4 thou on near side. So, decided to press on in view of the Scimitar 6 thou comment above. Thoroughly cleaned the calipers, fitted new Mintex pads and went for a drive to bed in. Brakes work very well, with no judder at all, although no 'emergency stops' just yet. I think I've sorted it ! Many thanks for all responses. John B Quote Link to post Share on other sites
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