drilling holes in a read brake drum

I am ready to blast and paint my rear brake drum.

I was going to drill some holes in it for lightness and cooling and I am concerned about stress and balance. I was going to draw lines radiating out from center using the existing bolt holes as a guide and marking off centers with a compass using the existing bolt holes for the compass point, then stagger drill out to the edge, increasing hole size as I go.

Anybody have a good reason not to do this?

Anybody do it and can share insight?

...it's a sporty drum...not a stock K part.

...and if you must know, i was thinking about tapping some of the holes and using them to secure ballance weights...although I'm not sure if the size/effect will be ok for the placement of the weight closer to the center of rotation...if you know what I mean...

Hi Richie,

that's how many holes I managed to fit on a std Sporty/K drum: a lot!!!



Patrick

boy howdy! that's perfect. done and done.

thanks Patrick

depends where and how you ride. Drilling holes in the brake drum will admit road dirt and water. That probably isn't a great issue for most restorations, which aren't daily commuters, but it's there all the same.

same goes for the actual cooling effect, most restorations probably aren't ridden hard enough for that to be a problem.

so, it's nice if that's what you want to do, and for sprinters or other competition bikes it might be worthwhile, but I wouldn't do it otherwise

"Drilling holes in the brake drum will admit road dirt and water."
True - 1 puddle and you have no rear brake at all (none, zero) for a few miles. Dirt will embed in the lining and eventually mark the drum. If left for a while, retained water will rust the drum to the shoes.

"same goes for the actual cooling effect, most restorations probably aren't ridden hard enough for that to be a problem."
True - brakes have to be up to temp to work well, too cold doesn't help. Added cooling does almost nothing except for a very long hard stop (100-0) where it might fade near the end, or repeated hard stops (road racing), or dragging the brake down a long hill. For normal use it has no effect.
For cooling, air only goes from a high pressure area to lower pressure area. Simply making holes won't do anything - the air in the drum won't circulate. Normally, air enters through a scoop or slot in the backing plate (points forward, duh - hint: it doesn't have to be on the plate itself...), and exits through the drum holes, continually "flushing" the shoes with cooler air. The really trick designs have splitters and vanes inside the drum to make air pass both shoes before leaving.

More: http://victorylibrary.com/brit/2LS-2.htm#cooling

However, it does look the part, saves unsprung weight (if swing-arm) and gyroscopic weight.

well, exactly, you need a scoop to do any real use. But it looks nice, and for sprinters or other competition bikes used in the dry where lightness is everything it might be worthwhile

Hi all,

I would have thought anything going round with holes in it while moving forward will create air circulation within... Surely less than a scoop will allow, but some still...

Patrick

the small volume available withing the drum will prevent any air which DOES enter, from finding its way to where it is required - ie over the brake shoes. Panic is right.

I had a front brake drilled like that on a sprinter once, a BSA Bantam hub as it happens. The brake plate was cut away till there was nothing left but the centre boss and a cross-bar mounting the shoes, torque stay and actuating arm. It ran quite well, as well as a tiny thing like that can, anyway. But when we changed the brake plate for a stock one, due to distortion of the cut-away brake plate, it blistered the paint right off the hub in a days' racing.

I'm sure the air inside will move, but it won't go from cold to hot (which is what helps). I have a gut feeling that the cast-in ribs in some drums act as paddles to move the air but not sure how effective this is.
If there are holes on both sides (even flush with the vertical surface) there will be some pressure differential between left and right just because of how the bike is built, but that's the key: how to get as much pressure diff as possible without an obstruction.
There's an article somewhere about a guy who drills really small holes through the outer contact surface of the drum, which is touted as helping braking action (although I'm not sure this is valid). It would provide a great exit for air coming in through the center - as long it had nowhere else to go, and had to pass the hot lining on the way out.

drilling holes in discs isn't a new idea. But with a disc, you only have to cool the rotor which is moving in the open already

Interesting, Panic, Seems I read long ago somewhere that fading was due, in part, to vapor pressure buildup between the linings and friction surface. Small holes might alleviate that as well as aid cooling. Anyone else heard/remember this phenomenon, or is it just a senior moment?
DD

Hi all,

The cooling radial holes in a disc brake, in the webs, from center to periphery, that's what I was trying to say in an obtuse way in my earlier reply:

Anything turning and with holes in it, periphery or otherwise, will create centrifugal forces within, hence air circulation...

Panic, you could be well right with the vanes theory, the drum brake as a starting point for a centrifugal supercharger, see Miller/Offenhauser blower designs!!!

All the best from Patrick

"vapor pressure buildup between the linings and friction surface"

I think you're right - the theory is that some gas (bonding material vaporizing?) is released when the lining reaches temperature, and the shoe/pad coasts across this gas film, which is produced fast enough that cam/caliper pressure can't overcome it.
Venting the gas allows more contact.

The holes in a disk are to give the gas vaper a place to go. they are not for cooling.

These are the radial holes I was talking about, they are for cooling and use centrifugal force to circulate air...



Patrick

...and at the end of the day, I painted the drum without drilling. The drum break was just an interim solution to get a rolling chassis that I couldn't stop thinking about. I have a triple disk break setup that already has a bunch of holes drilled. If the drum works ok, I'll probably just leave it and use the disk wheels for something else. All my bikes end up as daily commuters in the warm months and none of them are show quality or even restoration quality, and none are raced. They are bits of bikes put together and put on the road. Most of the bits are the less desirable stuff left over from the first pickings. I think that's why I'm fussing so much about this one cause of the KR top and KHK bottom. I feel like I should be doing something more special with it.

The holes in a disk are to give the gas vaper a place to go. they are not for cooling.

That's funny, because just about every manufacturer who makes drilled brake rotors for either performance or racing state they drill the rotors first for cooling to provide multiple pathways for air........secondly for gas venting.....third to prevent pad glazing. See Brembo, Wilwood and Baer brakes just to name a few who state this about their product.

Drilling the brake drums has been around for decades......5+ that is.....I learned from a old hot rodder when I was having problems with my Mustang who was doing it on his old '40's cars. I've since drilled the drums on several of my cars and had no ill effects and I didn't notice any abnormal wear or failures. Shoe glazing and hot spots on the drums were reduced. There are a few sites that show the correct way to drill your drum.

Kurt

The picture got me thinking........

If you took the drum drilled like it is and put a short lip on the axle side of the hole (1/2 way around the hole) it would help Scoop the air when in front of the axle and help pull the air out when behind the axleas the air rushing over the lip would pull the air in the drum out.

Darn engineering school clogs the brain whith stuff.........

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