brake drum (hub) truing

[thander]

Hi there GSers,

would like to ask if anyone has ever bothered truing a monolever rear brake drum.
My R80 G/S has developed the usual wobble on the foot pedal, so I took the wheel apart, chucked the drum on the lathe and had a go. Well, sadly didn't make it any better, so I would like to ask for some advice on setting up the lathe. The thing is that there is no obvious reference surface in order to have the thing centered...

Has anyone tried and pulled it though?

Cheers,
Thanos

 

[JayC]

You don't explain how you set it up or how much material you skimmed out. Do you know whether there is any allowance for anymore material to be removed after your first attempt? ? Do you know the tolerance limit before it becomes in need of replacement?

If there is still material to come out, and that would still render it serviceable, then you would probably need to pick up the PCD of the 3 mounting holes. Make a location fixture to hold in the chuck, use three dowels the diameter of the hub holes ( -.05mm on dia) and use this as your location. Ensure the dowels are inserted true , for squareness.

If it won't clock over the dowels accurately in a 3 jaw then grab the 4 jaw and clock it in to eliminate run out on both plains (vertical/horizontal) and machine within the serviceable limits. You MUST clock over the dowels to achieve concentricity.

The fixture would also require a through bolt for securing the hub to the fixture.

Alternatively, you may want to give it to your local machine shop, or replace the hub.

 

[Beemerboff]

On my G/S the maximum size is cast into the hub somewhere - mine is worn past that so I just save the rear brake for emergency use.

 

[England-Kev]

You never, never true a brake drum that is not built onto the rim!

And when it is, you then use a Brake Doctor...

 

[JayC]

Do you think brake drums are only manufactured and finish machined once a rim has been attached then? What you are saying is that if a wheel should require re-lacing then it must also have the drum re-machined to suit the rim.

 

[Beemerboff]

Probably, and then if you want them to work you machine the linings to suit the drum.

But there is more to it than that - the difference between a job from a mainly car Brake shop like my local Powerbrakes, and a motorcycle engineer like Martyn Adams, who trades as MBA is like night and day.

 

[JayC]

OP , I'm not sure if we are at cross purposes here, especially as you have added no further info to this thread, and the existing info seems a little lacking!

Is this solely a case of truing hub/rim runout (I suspect not) , or are you trying to skim the drum surface round again and then wanting to true the hub/rim? Did you measure the brake surface diameter for ovality before you started machining in the lathe?

"... chucked the drum on the lathe and had a go." Had 'a go' at what exactly? Please be specific what you have done and how you have gone about it?

 

[Myke Rocks]

The correct way to skim a brake drum

Also probably a waste of time in my experience, because the ovality seems to return after a heavy braking session:
Get a lathe big enough to spin the entire wheel.
Put a piece of say 40mm bar in the chuck.
Turn a centre on the bar.

Put a centre in the tailstock.

Mount the wheel on the wheelbearings between the centres now in/on the lathe.
Wheel has to be tied to the chuck. I cannot remember the name of the funny looking device with a hole in the centre, a bolt through the edge, and a hook on one end to do this with, I am sure others will tell you what it is called. However, it is easy enough to make something up yourself.

Go skimming.

But, as I said, my every attempt at this, whilst it worked, resulted in perfect drums at the time, which soon went oval again.
Do not forget to have over thickness brake shoes relined to compensate for the material you remove.
Myke

 

[JayC]

The Correct Way To Skim A Brake Drum - Take 2

Also probably a waste of time in my experience, because the ovality seems to return after a heavy braking session:
Get a lathe big enough to spin the entire wheel.
Put a piece of say 40mm bar in the chuck.
Turn a centre on the bar.

Put a centre in the tailstock.

Mount the wheel on the wheelbearings between the centres now in/on the lathe.
Wheel has to be tied to the chuck. I cannot remember the name of the funny looking device with a hole in the centre, a bolt through the edge, and a hook on one end to do this with, I am sure others will tell you what it is called. However, it is easy enough to make something up yourself.

Go skimming.

But, as I said, my every attempt at this, whilst it worked, resulted in perfect drums at the time, which soon went oval again.
Do not forget to have over thickness brake shoes relined to compensate for the material you remove.
Myke

'Carrier' , is the word that escapes you.

So, if I buy a new hub and want to lace an Akront rim to build a 'special' you are suggesting the 'new' drum needs re-skimming because it wasn't skimmed to the Akront originally? Hang on, I don't want Akront after all , I'll fit a wider rim , no, I don't like that either, I'll go back to the OE rim to suit the hub. How many skims is that now, I've lost count? Don't worry sir, your 'new' hub has now reached its limit and is scrap , so you can start again...now, which rim was it Sir required?

Hubs are manufactured without a rim attached, and on a fixture to maintain consistency in production. You then 'true' the rim to the hub when lacing the two items together.

 

[John Roberts]

'Carrier' , is the word that escapes you.

Others might be catch and plate.

 

[thander]

Hi there guys, thanks for all the replies. Sorry for not getting back earlier but I had been offline for some days.

My 'had a go' term stands for 'chucked the hub as true as I could and skimmed the drum until ovality disappeared."

Then, laced the wheel. I agree that the easiest way to go is skim a complete laced wheel, but of course the other way should work also. A properly laced rim on a true hub gets you a perfect wheel. If the hub is true to begin with and after lacing the brake is still oval, then repeat the lacing procedure since this is the cause of the problem.

Also, thanks for the advice on setting up the hub on the lathe, but they are not easily applicable on the specific case:
1. A device as the 'brake doctor' cannot be easily used on a R80 monolever hub, since there are no bearings and the respective true surfaces.
2. The 'dowels' option is also not optimum for the specific hub. The problem is that the only true surface that is used to center the hub on the bevel wheel shaft is on the inside of the drum. There is a little protrusion that fits inside the bevel wheel shaft. The hub mounting holes are not (i think) intended to center, and hence, are not accurately machined in order to be used as reference. Of course, there are many ways around this, such as actually finding out whether the mounting holes are actually true (e.g. with a turned off milling maching and a divider), or come up with a reference plate that will fit the hub from the inside and will be a press fit to a shaft passing though the hub center hole and chucked on the lathe). I must, however, notice that the mounting holes, even if not perfect, are, probably, true enough for the specific procedure.

The thing, dear ukgsers, however is that my problem seems to be something else. In fact, I chucked up the hub (on its own) on the lathe again, used the outer rim (the closest to the drum that seems like machined) or the tapered section of the centering protrusion and trued it as well as I could. Then, checked the drum and was also true. So, it must be something else, and here it is what I think it is:

If you take a look at what I have called 'centering protrusion' you'll see the marks from the female part of the bevel wheel axle. The wheel axle's female part is not a full circle, it is machined away except from the locations of the mounting holes. So, what I think happened is that, earlier in this bike's life it had been ridden with the rear wheel not properly tightened, ending up with these grooves on the hub that wont allow for proper centering. So, what I am facing is (most probably) not an oval drum but an offset wheel, depending of how lucky I am fitting the wheel!!! (that explains the effect appearing more or less severe at some cases, for no good reason)

The simple and obvious experiment, i.e. fitting the wheel loosely, tightening up the brake adjuster so that it'll 'center the hub' and then tightening the wheel mounting bolts really worked, no brake pedal waving (static experiment of course, since the wheel is not laced).

So, I am thinking of tig-filling of the marks on the hub and careful machining using as a reference the tapered end of the centering spigot (not very easy) or the true drum it self (somewhat risky...).

Will certainly let you know!

thanks,
Thanos