Want to vent my drum brake

thinking of drilling/fabricating a scoop on the faceplate side, drilling holes on the other side for cooling, don't care about weight so much: anybody do this/have problems/think it's a waste of time/have tips or warnings etc?

[CL450 drum on CB350]

 

Bump since this question was never answered. How important are cooling holes in the hub for ventilation? On one of my hubs I drilled 6 large (1.375" diameter) holes because monkey see, monkey do. But I'm wondering if I should give my other hub the same treatment or should I leave it as-is in case of rain? Will the 2520V compound cope fine with the heat of a non-ventilated hub? Will the large cooling holes create stress risers that can lead to cracks? (I did chamfer the edges).

 

i was thinking about this last night as well... although I don't think it's a legal modification for the class I'm considering, but I have to imagine any ventilation helps the drum stay cooler...

 

forced exhaustion

One of the keys to good brake ventilation is using any means to encourage evacuation of the hot air. Everyone uses an intake scoop to help the air in, but the challenge is to also find a way to encourage the hot air's exit. On double drum Fontanas, they exhaust the air thru holes in the perimeter of drum between the two sides, using centrifugal force to throw the air out...on a single sided drum, your options are fewer. I've machined breathing holes at about a 45 degree angle to the rotation so that the revolution of the drum will tend(emphasis on "tend") to draw air out of the drum. How effective(who knows)?

 

I cut large diameter holes in the front and rear conical BSA hubs. The rear looks like a pattern Manx, and I figure whatever was good enough for them should be good for the BSA. I was careful in sizing/locating the holes to stay awawy from the strength ribs and to leave what I thought was sufficient "meat" to allow the hub to perform its function. I've not been concerned about the possible decrease of braking in the rain. I mean, really? In the rain? What I am concerned about is the possible intrusion of gravel, sand and other debris that would adversely effect, i.e., prevent from working, the brake mechanism. I assume this is why my Norton front drum has a screen on the intake scoop. I'm sure whatever cooling effect resulted helps the brake, somewhat. How much? I don't know. The BSA conical hub brakes leave a lot to be desired for racing purposes. They are not much at the start of a race, and they are even less at the end. I've not attempted back-to-back vented vs unvented tests to try to gauge the difference. I think it is best to simply accept the fact that most period production (not Yamaha or Grimeca 4 leading shoe monsters) drum brakes basically suck for racing.

Besides, as Ted Hubbard was fond of saying, they only slow you down.

Cheers,

Dave

 

To clarify, I'm talking about a 2LS Honda CB77 200mm drum on a CB350, just like Charly.

I've seen screens made to fit holes drilled in the hub but it makes me think about cooling efficiency vs. keeping foreign debris out.

See http://cyclebutcher.blogspot.com/2011/09/vintage-race-brakes.html





^^^ not mine but something I'm considering. Also, at some point, simply going with a real 4LS race drum has got to be better than a worked over 2LS street drum. But it's hard pill to swallow $1500+ in one lump sum.

 

My Triumph uses a RamAir scoop casted into the backing plate(forward edge) to pressurize the drum assy. It vents the heat out at the back edge of the backing plate. This seems the best way to vent heat, with good air flow. I haven't tested it , i just think trying to flow air through spinning holes would be harder (drilling the hub assy.). Yes, any venting is going to help. I think drilling the holes in the hubs would cause more air turbulence. But it looks cool.

 

It's not easy to add a scoop to the 2LS Honda drum because of the linkage and brake stay arrangement. Doing so would require an entirely new backing plate. Supposedly, there is a guy in the UK making them, but they're reportedly of poor quality and he ignores overseas inquiries. I'm not going through the effort to design and fabricate my own backing plate so that leads me back to my original question.

I see some guys racing without vented drums and I'm just wondering how bad they are with modern friction compounds that are better capable of dealing with the heat. Clearly, they are less than ideal but are they terrible? Has anyone with large open vent holes ever had foreign debris foul their brake?

 

http://klemmvintage.com/bighorntech.htm

I found this article particularly interesting. The brake section is about 2/3 of the way down the page. It looks like they put some thought and experience into the venting.

The whole site is worth looking at.

Ken

 

Update;
My original CL450 drum on a V-1 CB362 was destroyed [probably at CMP] by overheating
which cracked the steel or iron liner after which it gobbled up a set of Vesrah shoes in one practice vibrating like crazy.
The replacement CL450 drum was drilled [before re-lacing or it won't fit in a small drill press] with 7/8" holes pretty much where indicated by cast-in circles.
The face plate was also drilled for a small alloy scoop which was riveted on.
No more overheating problem.

 

I chatted with Jim Wulzen and he recommended that I rotate the drum brake backing plate forward and weld in a new brake stay lug just aft of the short arm spline. This frees up enough room along the leading edge of the backing plate for a nice brake scoop.

Charly, can you share a picture of your front brake with scoop?

 

Max was here and did this with a smart phone...[Brenda won't let me have one]

 

Thanks Charly. That helps.

 

what forks are those?

 

CB450 with Racetech gold emulators and I think I ended up with old XL250 springs;
I tried a fork brace but [per Buff's advice] it didn't seem to make any difference with the short 35mm forks so I took it off.