Technical Brake Drum Ovality

[ianhunterv8]

Hi! I can feel the brake pedal pumping up and down as I stop and heavy braking comes with quite a vibration.

I have taking the drums off and checked them for ovality, albeit just with a set of calipers and the diameters are within 2 thou, taking two measurements at 90 degrees of each other. Only one drum was perfect!

Even so, I would have said that is too little to feel? Or is perfect the only correct answer? Or could it be anything else?

Ian

 

[jjacob]

Hi Ian,

You should check the condition of the brake hardware and springs. Also, it is very possible that one or more wheel cylinders have a sticking or frozen brake piston on one side of the wheel cylinder.

Make sure also that your wheel bearings are not out of adjustment (loose).

Replacement of all wheel cylinders and brake hardware is easy and cheap.
John

 

[ianhunterv8]

Wheel cylinders all look pretty new, linings and springs look fine as well - wheel bearings are a definite possiblity, will check those on reassembly. On the way back from Italy we had some mechancial problems and I saw the mechanic with a wrench on the hub nut - no idea why and Italian not good enough to ask him!

King pins had huge play and am in the middle of sorting that, which is why the drums are off and why I thiught I would look into the problem now.

I can't decide whether to reassemble everything and see how it drives or just buy four new drums?

Ian

 

[dragonwagon]

2 thou shouldn't be enough ovality to be an issue - it's more likely to be something else, possibly a loose or worn wheel bearing or more likely a damaged / failing tyre casing.
Tyre problems are quite common on rarely used cars in my experience

 

[ianhunterv8]

Lets go for the wheel bearing - the front tyres are new!

Ian

 

[Bambino]

What follows is just for fun, but there is also a serious side. Ian checked his drums for ovality by measuring two diameters and got a pair of results differing by only 0.002" - does this mean that the drums are in fact circular? Unfortunately not - he may have just got lucky as all he's achieved with his meticulous technique is fix 4 points on a circumference. There are many shapes of constant diameter that are not circles (eg: the 'piston' in a rotary engine) that when spun on a central point will give you a very bumpy ride but which when measured, give equal diameters. The only way he can be sure is to either use a set of known circular templates fitting them at many points on his circumference or, more commonly, spin the drum on a lathe or similar device and measure the eccentricity - tyre fitters do this as part of a wheel balance.

So where is the serious bit you ask. Twenty seven years ago today, the space shuttle Challenger lifted off for an orbital mission - 73 seconds later it exploded in a fireball killing all on board. The cause was found to be fragile, but also ill-fitting, o-rings between the fuel booster segments which allowed fuel to exit then ignite with tragic results. These fuel boosters were always intended to be reusable and when recovered, they were checked for damage prior to reinstallation and refilling. One of the checks was to determine their circularity and this was done by measuring 3 diameters, which, if they fell within a certain tolerance, were passed as OK. After the disaster all of the booster segments in use were measured using templates and many were found to be quite oval which meant that even if they were fitted with perfect o-rings, they would leak when they expanded after the heating that occurs on lift-off. Since that time, all recovered booster segments have been measured using calibrated circular templates.

Have a look here - http://www.qedcat.com/archive/53.html

Drum brakes are a bit different in that the rotary wear pattern will tend to reinforce the circularity (if the wheel bearings are OK). So in Ian's case it is most likely to be another cause for his brake judder, but it would be interesting to spin them on a lathe and see exactly how circular they are. If it was me and I was rebuilding the brakes, I'd get new ones.

Sorry to bore you with this at the end of your weekend - it's the Mathematician in me coming out

Regards,
Chris

 

[tjmra]

What Chris said.

No really, before my recent rebuild, my car used to dance around the road under brakes. Lot of shudder lot of rattling. Was exciting to say the least. I had the kingpins done, new tie rod ends, new suspension rubbers, new wheel cylinders and new shoes. Each one had a positive impact. I never replaced the drums and the dancing never really went away.

This time when I rebuilt the car, I replaced all the components again and fitted new drums. Car has never felt better at top speed or under braking. Not brilliant mind you but when you look at the set up it is only ever going to be OK.

Out of interest I measured my old drums and the new ones with a pair of calipers. The old ones seemed OK, but I knew from driving that they were not really round. I suspect you really need to get a specailist to measure to be sure.

Given that the crumple zone of these little cars finishes at the rear parcel shelf, I think a new set of drums would be a good investment.

Joe R

 

[ianhunterv8]

Thanks for the input guys - we did check the front drums on a lathe, but the central hole in the rears was too small, which is why we reverted to the caliper route for them.

Even with new kingpins, trackrod ends and correctly adjusted wheel bearings. the only thing that can make my brake pedal move up and down under braking is oval drums, so none of the above will make any damn difference I suspect. I will just have to dig deeper into my pockets and buy the drums.

I may just get it through it's MoT first just in case the problem has magically disappeared!

Regards

Ian

 

[Toshi 975]

The 500R and early 126 brake drums are more heavily ribbed on the outside diameter to make them more robust so I guess it must have been a known problem as the cars got faster and braking harder. I have got a spare pair of new old stock front drums tucked away somewhere complete with new bearings should anybody need them.

 

[ianhunterv8]

They could be just the job for me, do let me know how far away you are and how much you want for them?

Ian

 

[Toshi 975]

OK Ian , I should be able to look the bits out bye the weekend and will be in touch

 

[Toshi 975]

Hi Ian , I sent am e-mail via the forum at the weekend so not sure if you received it.

 

[Middlewich]

An old post I would like to re-open and get some thoughts.

My 500 failed its MOT due to the brakes pulling to one side (front) I stripped them down, cleaned and checked everything, even measured the springs to ensure they were the same size ( a free tip, these springs make a hell of a difference to the braking especially if they are new).

I tested it again but still pulled, so I swapped just the drums over, nothing else and the car now pulls to the other side when braking. I can only presume the two new drums are not identical?

Has anybody else experienced this?

I know the fix is find someone with a lathe, and a bit of skimming, but I am a bit amazed that new drums could cause such an issue.

 

[jackwhoo]

If the only parts you swapped side to side are the drums then it must be the drums. Unusual yes.
We're both drums from the same manufacturer?
Does the friction surface of each drum look identical?

 

[F123C]

When checking rear brake drums for ovality, it's often possible to mount the drum in reverse onto the hub, then use a pointer ( I've used a steel rule/socket extension clamped to an axle stand) to find the lowest spot and measure it with a feeler gauge. If trying this, make sure the drum sits flat against the hub, if not, insert washers between drum and hub to achieve this. Better still, is if you have a Dial Test Indicator and Stand, then you can monitor and measure the run-out as the drum is revolved.

Front brake drums can be checked if you have a spare stub axle. If not you can use a very large bolt or threaded-studding plus nuts and washers to hold the brake drum complete with hub and bearings in a bench vise, then check as above with a pointer and feeler gauges or a DTI and stand. (I know, not an ideal way to hold a front hub but better than nothing when working at home ).

AL.