Adding ventilation to drum brakes?

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Longarm9
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Adding ventilation to drum brakes?

Postby Longarm9 » Fri Feb 16, 2018 1:20 pm

Has anyone had any experience doing this?

I have a 1st gen Honda CR-V that I often take offroading. It's an awesome little vehicle that does amazingly well on the narrow, steep, rutted roads of BC's forests. The downside of the vehicle is that being what Honda calls "real-time 4wd," which is a fancy way of saying AWD, it doesn't have a true 4low gearbox, and thus although it has, I think, a lower gear ratio than most street cars, it doesn't have as much engine compression braking for descents as would be ideal.

In looking for methods to increase the braking efficiency and durability, the idea occurred to me of ventilating the rear drum brakes in order to maximize their efficiency and prevent brake fade. Apparently, I have discovered, this is an old hot rodder trick that's been in use for decades. I wonder if anyone has any experience with doing this, if they know of a good, affordable kit, do's and don't's etc.

Cheers!
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Dave Brown
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Re: Adding ventilation to drum brakes?

Postby Dave Brown » Fri Feb 16, 2018 4:08 pm

Old hotrodders trick that had no basis in scientific fact. We can add that to the old wives tales files, because if you do this, only an old (and slow) wife will be able to safely drive that vehicle again.

Ventilating drums (and rotors for that matter) LOWERS the brake efficiency. The whole idea of greater airflow is absolute bullshit, especially if anyone knows anything about fluid dynamics and boundary layer airflow patterns. (Basically, air cannot suddenly turn sideways and dart in through cooling holes at 90 degrees to the direction of travel. Plus, the airflow over a drum or rotor does not change if there are holes because the airflow right next to the moving part does not suddenly get reduced or break away. This can easily be seen by airliners that are going 600 miles per hour. If there is dust on the wing when they take off, there will be dust on the wing when they land. This is the boundary layer.)

The reason that it ends up actually lowering brake efficiency is that you lower the overall mass. Given that there is minimal to no greater airflow, brakes are cooled by mass. Lower the mass, the less the drum (or rotor) that can absorb heat.

That's why sniper rifles have heavy barrels. It has nothing to do with accuracy; it is to do with heat absorption and maintaining zero shot after shot. The heaviest duty brakes in the world are found on airliners and they NEVER ventilate their rotor/drum system. Race cars rarely have ventilated rotors. Ventilated rotors are STRICTLY for the looks.

Your only option is better drums and pads, or to look at how easy it would be to upgrade to discs. On some vehicles, it takes everything from hubs out; on others it is a bit simpler.

Good brake pads and shoes add more to the "feel" of the brakes than any other single factor.

As far as drums and pads, there are two (and sometimes three) quality levels available. I NEVER cheap out on brakes and I always get the best. If I were you, I would look at the top ultra premium line at NAPA.

All that being said, there will be little difference between the best rear brakes and the worst rear brakes in comparison to the fronts, because the rears contribute only a small portion of the total braking power. However, in your case, I would suggest you will find the highest end shoes and drums to be worth the extra money.

Ventilated? Not necessary, and probably detrimental to performance.

(Don't tell all the ricers with their fart can mufflers and anodized suspension parts that are several factors WORSE than factory.)

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Re: Adding ventilation to drum brakes?

Postby mack_silent » Sat Feb 17, 2018 11:00 am

Def stay away from the cheap China made brake components.
The rotors will warp prematurely and the pads/shoes are made from sawdust quality metals.
Even if you change out the parts at home (which I do), it's a hassle to replace brake parts so often.

I've had good luck with OEM parts, but for the price you can usually go up a level to performance level parts.
It's never worth saving $5 to $20, and going for the basement level stuff, as they will be worn out in months.
I learned this the hard way, when I ordered a set of OEM pads and got sent China knockoffs.

The rotors and pads lasted 3 months... I am light on my brakes, so that's a tiny fraction of what a quality set would've lasted.
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Longarm9
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Re: Adding ventilation to drum brakes?

Postby Longarm9 » Mon Feb 19, 2018 5:38 pm

CRV is built on the Civic platform, so the rear discs from a civic or I believe an acura integra are a drop in fit except for the lack of caliper brackets. Those would have to be aftermarket or possibly custom, I think. There are definitely guys who have retrofitted discs onto the rears of the CRV. Definitely a more involved process, though.
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Re: Adding ventilation to drum brakes?

Postby Longarm9 » Tue Feb 20, 2018 10:49 am

There's a factor that I neglected to specifically mention that I think is not being factored in here. The process also involves drilling ventilation holes around the outside edge of the backing plate for air to enter into.

Now, I was thinking about this, and I'm thinking that with such a setup, the heat generated by the brakes operating would increase the pressure inside the drum as the air expands, thus forcing the hot air out of the holes in the surface of the drum, aided by centrifugal force, which would in turn create suction that would draw the cool air in through the backing plate. It seems to me that the system would not rely on passive entry of air, which as Dave points out, would not work, but rather would create a kind of suction-vortex effect that would cause hot air (along with brake dust, water, etc) to be expelled from the drum while sucking cool air through the backing plate holes. It seems to me that especially at the slow speeds involved in off-roading, this effect should counter balance any loss of heat disspation caused by the very small loss of mass in the drum due to drilling the small holes for ventilation. The holes in the backing plate would not have any discernible effect on the mass/cooling equation, I would think, as the shoes don't act on the backing plate at all.

Given this extra factor, what are your thoughts Dave? I certainly don't claim to be an expert in fluid dynamics, but the notion seems sound given what I know about hot air expanding, air pressure effects, etc.

Edit: Another factor that occurred to me is that, unlike the surface of an airplane wing which is exposed to the full speed of the aircraft moving through the atmosphere, the air in the wheel well of a car is shielded by the front bumper, so I would think that the airspeed moving against the surface of the drum and backing plate would be significantly less than the airflow over the bodywork outside of the car. This would be even more significant at the speeds used in descending steep inclines while offroading, which is to say, as slow as humanly possible, which I think would all but totally negate the boundary layer effect Dave mentions, as the air in the wheelwells would barely be moving as the car crawls along.

Thoughts?
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Dave Brown
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Re: Adding ventilation to drum brakes?

Postby Dave Brown » Tue Feb 20, 2018 3:38 pm

For this to work, the brake drums would need to be a sealed system. And because they are not, there is no basis in scientific fact for hot air to be expelled outwards and cool air to be drawn in from the backing plate. Air is drawn is as required and all the openings in the average backing plate are a million times more sufficient to draw in what tiny little bit of air is expelled. Basically, this is what is labeled "junk science" and just has no basis in fact.

The same thing applies to cross-drilled rotors. They are for looks, period.

Backing plates are there for a reason. They help prevent water splashing on the pads, which is not that big a deal because it dries in seconds, but it can lead to an initial reduction in braking effect for that half second until they dry.

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Re: Adding ventilation to drum brakes?

Postby Punisher-One » Wed Jun 20, 2018 6:46 pm

The only thing drilling holes will do is allow mud water and other contaminants in and not only negatively impact your braking power but also prematurely rust and wear the components. I am an avid 4 wheeler and built my own 1-ton jeep fyi :)

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Longarm9
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Re: Adding ventilation to drum brakes?

Postby Longarm9 » Fri Jun 22, 2018 11:50 am

Punisher-One wrote:The only thing drilling holes will do is allow mud water and other contaminants in and not only negatively impact your braking power but also prematurely rust and wear the components. I am an avid 4 wheeler and built my own 1-ton jeep fyi :)


Oooh, awesome. I can perhaps pick your brain when I start doing all the mods I want to do.

I have big plans for this machine, but sadly I have a very tiny bank account, and those two don't mesh well. :((

I looked into it, and it looks like a disc conversion would be simpler and probably more useful anyway, so I may do that down the road sometime.

Another idea that has me intrigued is the notion of dropping the 148hp turbodiesel from the 2nd gen into the first gen engine bay...heh. That's just a thought, though. I think the diesels are rarer than hens teeth in Canada anyway.
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