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MG MGB Technical - Brake disc runout
|Hello All, I am experiencing a problem with brake disc runout on the offside front hub on an 67 mgb roadster, causing lots of vibration under braking from high speed. The runout measured at the disc swept edge is 5.5 thou which is exactly the same when measured in the same place after turning the disc thru 180 deg on the hub. So this suggests to me that the disc is true and the hub is out (THE DISC IS NEW BY THE WAY) |
I have made sure the mating surfaces of both components are completely clean before assembly.
So my question is can uneven bearing wear cause the hub to run out and will new bearings cure the problem?
|Just getting my head round this--|
When you turned the disc 180 , did the high spot stay in the same position compared to hub position or move around 180 with the rotor
Has this been a long time issue or only since the rotor was replaced
|Bearing wear could well allow you to see runout, but I wouldn't expect it to be consistent, and I'd expect to see significantly more end-float than normal, and maybe at some positions of rotation and not others. While the hubs have been on and off I take it you got the oil seal collar on the spindle first, and checked the end-float?|
And as Willy why did you change the discs, and did the problem only start then?
on turning the disc thru 180 deg the high spot was in the same place, i.e. I marked the hub where the high spot was then turned the disc and bingo the high spot is still by the mark.
The end float is around 2 thou, which is where I normally try to set it. I changed the discs as the old ones were worn too much. The new discs are EBC with Greenstuff pads. I haven't had a vibration issue in the past in the 68000 miles the car has now done since a full rebuild. I can normally work things out logically, but I'm completely baffled with this one.
|Hello Graham, are you talking about judder, I guess you are. If with the previous discs you had no problems then the replacement disc seem to be the cause of your vibration. This is usually caused by disc thickness variation (DTV) it only needs 0.0005" to produce judder.|
I would refit your old discs and retest, if that cures the judder, then send the discs back to EBC.
|X2. Test for runout on the old disc as well as for vibration on the road. If no runout and no vibration then the new disc is not of merchantable quality.|
Incidentally, I know it's a lot of faffing around but you are likely to be arguing with the vendor. I'd also swap the new discs between sides and check for runout. If it stays with the disc then it's definitely faulty.
|Yeah, I'm leaning towards agreeing that the runout, although not ideal, may not be the cause of the brake shudder'|
As John explains, it is more likely to be a variation in rotor thickness to be causing your drama
If this rotor is running out as you have proven the hub has to be out of true a tiddle, but possibly always has been so not really a new problem
You could measure rotor thickness in several places around the rotor to try and find an error in machining
If you lock your micrometer at a nice slidey feel and run it round the rotor you might find a high or low spot
Second choice, which is the way I would probably go, is to have the rotors trued up with an on-car rotor machine
job which will make them 100% true
They would only take a few thou. off them, just enough to true them up
Are your caliper pistons nice and free moving
If you have a seized piston on one side of a caliper and not the other, this can cause shudder if the rotor is out a little like yours'
|There's a school of thought regarding the "running in" or bedding in of new pads and discs taking 100 miles or so, more with fewer braking episodes. Light braking only during this period. Heavy braking from new can cause patchy glazing and cause judder and squeal.|
|Hello Alan,when I worked for Mintex as a Road Test Technician, we used to bed in brakes for 800 miles mainly light braking and keeping the temperature below 180deg c.|
I am not sure that heavy braking from new causes "patchy glazing and judder". I'm keeping clear of brake noise as it is too complicated a subject for me.
|Read it somewhere John. The article said sometimes the glazing is a result of a hard pad not getting hot enough. I stick to standard, never had a good experience with Greenstuff.|
|Allan, surely that a pad problem not discs. Brake discs are generally made from cast iron and even when taken up to high temperatures thhe cast iron doesn't appear to suffer any changes in it's metallurgy. A very early Mintex racing pad had lots of copper in the mix, the copper was used as a lubricant at high temperatures. Once used in anger the copper came to the surface and it was difficult to get the pad working again. Thats what may be happening with "Greenstuff".|
Would not be entirely confident about the make up or consistency of the alloy of Cast Iron used to make after market discs. C I is basically Iron and 3.5% upwards of Carbon. Most of the carbon is in free form at the grain boundaries, wipe your finger across basic CI and it will be black. Carbon is a good lubricant but also to make tougher C I, often referred to as Malleable Cast Iron, Silicone, also a lubricant, is added which makes the free flakes of carbon at the grain boundaries "ball up", making the grain boundaries stronger, not sure if this Spheroidal C I is used for discs though . Also when C I is cast, the surface is rapidly chilled by the mould effectively hardening it. This surface layer is removed by machining, but it's depth can be somewhat problematic depending on the casting process and the original "over-size" of the casting. Finally the machining relieves internal stresses caused by the heating and cooling of the metal, resulting in warps and twists etc.., it should be done in at least two stages with a delay between them to allow the metal to "relax". Casting which have to stay really accurate, surface plates, machine beds etc. are given a rough machine then left to "weather", often for several months.
As you can see the metallurgy is complicated, the process wasteful of material and very time consumptive too. After market items, as we know, tend to be cheap...........mutually exclusive????
|That's very interesting Alan. Our car brakes|
department used to take samples of discs by drilling into the mounting fllange to test for the titainium content. The amount of titainium used to affect the friction level. That was for discs for prototype Mondeo's. It's all clever stuff, not that I understand any of it. LOL
|Thanks Guys, I will try a few suggestions, but unfortunately do not have the old discs to try, they went with the local scrap man. |
I can mic the disc all round to see if thickness is OK and I have aquired a spare hub to try. Will post any results asap, need to fix it before trip to LeMans in July.
|Not surprisingly others such as |
https://www.zeckhausen.com/catalog/index.php?cPath=6446_6443 and http://www.autoanything.com/brakes/how-to-bed-in-brake-pads
recommend something completely different, for initial bedding-in anyway, repeatedly braking to get almost to the point of fade.
|Paul, Have just been reading another "learned" dissertation recommending lots of heavy braking. It's a bit like Brexit. Everyone has an opinion............all different!!|
|Hi again, |
Have changed hub, but no difference outside edge of disc still runs out by 0.12 mm. When mic'd the disc is no more than 0.0005 out in thickness.
Now checking the calliper and pads.
|Graham,did you by any chance check the run out of the hub face where the disc sits. If you get a good reading then you will have enough data to convince the disc suppliers. Is the 0.005 inch or metric? as you have used metric measurement aswell.|
The 0.0005 is inches, as my micrometer is imperial, the dial gauge is metric.
I don't see how I can check the hub face as I can't get the dial gauge into a suitable position to read it on the spindle and I have nothing to mount the hub on off the car where I could check it.
I have, however, found something else that I'm unsure of:- I noticed that the calliper pistons have a slight cutout on the edge that mates with the pad. The orientation of this was facing towards the centre on one piston, but facing approx 90deg from centre on the other. Should these cutouts be lined up or doesn't it matter?
|The calliper pistons cut outs should be facing towards the centre of the hub as per the attached image. Not sure if this is the cause your problem, but it seems one of your pistons is not correctly positioned.|
|As long as you haven't wire wheels have the discs skimmed on the car, as already said this will true up everything, and if your problem is to do with run out on either discs or hub that will fix the problem, these bits of kit are very good, and most main dealers have them to cure brake judder on cars that are still under warranty, I have never seen an adapter for wire wheel hubs with any of these machines, one last point are you sure this brake judder is coming from the front brakes and not the rear ones as this can happen, a quick check is to put a brake pipe clamp on the rear flexi to cut out the rear brakes, then road test, A.T|
Have moved the calliper piston round so that both now face the centre, but why would it move round anyway, could it be moved by the vibration?
The vibration is definately coming from the front. I've looked at having the discs skimmed on the car and guess that could be a last resort, but thought I'd try all other avenues first.
Please explain the symptoms. you talk about vibration. Is that a squeal or a judder felt through the steering wheel. When I built my V8 roadster I had a real judder which was so pronounced steering control was an issue from high speed. I went through everything on the front end, bushes, discs, pads, rack, rack mounts the possibility of bump steer and changed the fluid to a higher race spec. All to no avail. In a moment of inspiration, more like desperation, I swapped the drums for the ones on my other B. Cured!!! Looking at the drums they both had "polished" darker patches which I assumed were causing snatching, but it was only felt through the steering. On examination these "polished" patches were corrosion, mirrored up by the friction material. The uneven braking at the back was steering the front, and probably putting slight pressure changes in the system, even though the rears only contribute 30% or so to retardation! The effect was even more pronounced when the brakes were hot.
Also where are you mounting your DTI to get your run-out reading?
It's judder through the steering wheel, which occurs mildly on normal braking but gets very violent when braking from high speed, as in leaving the motorway down the slip road.
I mount the DTI in a large heavy vice, which I can then position near to the disc, so not attached to the car at all.
Have to say I'd never thought of the drums as being a possible problem, but it's easy to check them, so thats one for the list. The reason I'm concentrating on the offside front is because of the 5 thou runout.
|Hello Graham, when you checked for disc thickness variations did you check both discs? BTW in an post very early today did you put tooo many "0", as 0.0005 would be very difficult to read on a standard micrometer.|
The thickness varied by no more than 1/2 of one gradation on the mic, so whilst it's not properly readable, you can make a judgement. I only checked the disc where I believe the problem is.
Have now turned the cutout on the pistons to the centre, given the pads a rub with emery paper and put a slight 45deg chamfer on the leading edge.
Cleaned the disc with fine emery paper and re-assembled.
First signs seem to suggest the vibration has gone, nothing showing on mild braking and then nothing on heavy braking from 60mph, apart from slight pulling to the left, which I believe will go once the pads bed in properly again. I also intend to remove the nearside pads and treat them the same to make sure all is balanced.
However, I'm not relaxing just yet as only time and a few more miles will tell, so thank you to all for you help and I'll keep you posted if anything changes.
|Hello Graham, I'm glad things are improving. I would deal with the other side so both side have had the same treatment ASAP. Get some miles in to bed in the pads and when it feels Stable, build up the speed and brake temperature, don't go mad doing this and it should condition the pads for some spirited driving.|
Just a late thought
There may be a chance that the new rotors had been treated with some type of rust preventative which had soaked into the rotor and although I am sure you cleaned them before fitting ,maybe there was a bit left at one spot on the rotor giving you the on-off braking (shudder) effect that you had------------
This thread was discussed between 26/05/2016 and 04/06/2016
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