Drum Brakes

[onegammyleg]

Originally Posted by jolt-tsp

so drums in the front will heat up very quickly and can degrade your braking performance in just one hard stop.

And just how many high speed emergency stops would one be anticipating in a high FE car ?
-- and that is why (if done properly) a drum setup could be viable on a FE car.


PS - I know a whole bunch of disc front ends that will fade to NOTHING in less that 1 hard stop.

[onegammyleg]

Originally Posted by jolt-tsp

The drum brake is going to be lighter and brake harder initially than a disc brake of equal size because their is a greater friction area. However, this causes heat build-up faster and degrades the performance very quickly.

I am confused with this logic.

If I turn this around you are saying that disc brakes are less prone to fade because they have smaller friction area.,, so less area is better.
If that true , I should cut half the surface off my brake pads , then ide have super brakes.

That just doesnt make sence.

[GasSavers_Ryland]

as I see it the area that rear disk brakes have an advantage is if you are doing alot of speeding up then slowing down (to go around corners?) or are takeing a fully loaded car down moutan roads, but a few days ago when I went over the rocky moutans in a civic with rear drums that was loaded slightly over the recomended gross vehicle weight, we had no problems at all slowing down or stoping at any point in time, as I see it they are strickly a stelling poing, a style point, like those little rear air fin tails, they are shiny, and look pretty on the rear, compared to my rusty drum brakes.

[DracoFelis]

Originally Posted by onegammyleg

If I turn this around you are saying that disc brakes are less prone to fade because they have smaller friction area.,, so less area is better.
If that true , I should cut half the surface off my brake pads , then ide have super brakes.

I thought that brake fade was largely caused by brake fluid boiling. If so, than one key factor in brake fade, would be how much heat is produced by brake friction (which isn't necessarily how much you slow down/stop with the brakes), and how much of that heat is removed by brake cooling (which is one reason some people think smooth hubcaps are a bad idea, because they trap extra heat in the brake system).

BUT, if you are worried about brake fade, it seems like you have one easy/cheap solution that many people totally forget about. If you have some reason why your system causes (or traps) extra heat in the brake line, just use a quality brake fluid and change it out every year or two. While doing so doesn't lower the heat in the system, it does an amazing job of raising the boiling point of your brake fluid. And remember, it's not heat (per se) that causes brake fade, so much as heat that exceeds the boiling point of the fluid in the lines! So by greatly improving the heat handling ability of the brake fluid, you increase the total capacity of the brake system.

NOTE: Changing the fluid out every year or two is even more important than using a quality fluid. While you can "get away with" running brake fluid for 10+ years, it's boiling point will be noticeably diminished with just a year or two (because the brake fluid attracts water, and water in the fluid greatly lowers the boiling point). So the "frequent" fluid changes are more important than the quality of the fluid used (although both help). Happily, even decent quality brake fluid is CHEAP (often under $10 for a quart, and many brake systems use less than a quart total), so there is no real excuse to forget to change the brake fluid!

NOTE: If it's a hassle (or a fair cost from your mechanic) to totally bleed the brake system, you can just take a $2 turkey baster (don't use it for food afterwards, or you will poison yourself) and suck out all the fluid you can from the brake fluid fill location (and then replace with fresh). This isn't nearly as good as a complete flush (in some cars you can only get around 1/3 of the fluid at a time this way), but it will still help a lot by (easily) replacing some of the brake fluid in the system with fresh. And mixing fresh brake fluid in with the old (i.e. diluting the old fluid with fresh) will still help to refresh the brake fluid (raising the overall boiling point, getting some dirt out of the system, and generally making the brakes work better). And using "the turkey baster method" to your brake system, is almost as easy as filling your washer fluid reservoir. Just be sure to have a real mechanic occasionally (every 5 years or so?) do a total bleed of the brake system (as there will be some fluid damage and dirt/junk that "the turkey baster method" doesn't properly address).

WARNING:
Do NOT use DOT 5 brake fluid in any system originally designed for any other brake fluid (i.e. most brake systems). It's OK to upgrade from DOT 3 to 4 (or even 5.1, which is not the same as 5), but don't use DOT 5 brake fluid as its makeup doesn't mix properly with the residue of other brake fluid types (and therefore improper use of DOT 5 fluid can cause your brakes to totally fail)!

Bringing this back to FE:

If the primary safety issue is "brake fade", than lowering brake fade should increase your safety. Conversely increasing brake fade may be a safety issue for people. However, the gains from improving your brake fluid can in many cases be larger than the losses by doing something that makes sense for FE. So if (and this may depend upon how well they are adjusted and therefore how much they drag) drum brakes "fade" quicker, you can still likely more than offset this by using fresh (higher quality) brake fluid!!!

Speaking of which, many people interested in FE use smooth hubcaps. Yet, it is well know than smooth hubcaps trap heat in the brake system, and therefore would cause the brakes to fade quicker. But smooth hubcaps also improve the aerodynamic air flow around the car, and therefore should help FE! So again, if you are at all concerned about increasing the brake fade by the hubcaps, changing your brake fluid is "cheap insurance" against brake fade.

BTW: If anyone is interested, http://www.valvoline.com/pages/produ...asp?product=51 is the brake fluid I currently have in my car. I picked it because its specs are much better than the cheap/average stuff, but still a lot cheaper (only about $8/quart at the local "AutoZone" store) than some of the super brake fluids marketed to the racing community.

However, if you really want to know about what brake fluids are "good", I suggest checking out this thread ( http://theoildrop.server101.com/foru...&Number=523531 )on the BITOG forums. When reading the specs in that thread, keep in mind that the "dry" specs are what you will get when you first put the fluid in the car, and the "wet" specs are what you have when the fluid is "old" (not changed for a while). As you see, even the cheapest fluid "dry" works better than all but the most expensive racing fluids "wet". Which is why its more important to change your brake fluid "frequently" (so you are running closer to the "dry" specs) than it is to use a high quality brake fluid (which has higher wet/dry specs overall). But doing both helps even more than doing one or the other.

[onegammyleg]

Originally Posted by DracoFelis

I thought that brake fade was largely caused by brake fluid boiling. .

Thats a good post about brake fluid , and even just on a maintainance point of view its good to replace your fluid every couple of years.
Old fluid absorbs water which leads to corrosion of brake calipers , cylinders , lines and of course the master cylinder.
Water in the brake fuild lowers its boiling point too.

Unfortuantely , faded brakes are not caused by boiling fluid , its just the gas from the overheated brake pads.

Brake fuild in a normal pasenger car doesnt get to the temps required to boil it.

Race cars where you can get the discs glowing RED ,, are a totally differnt story.

[DracoFelis]

Originally Posted by onegammyleg

Unfortuantely , faded brakes are not caused by boiling fluid , its just the gas from the overheated brake pads.

Now, I'm confused. Are you talking about gas (in the wheel region) between the pads and the drums/disks making the pads grip less well, or gas in the hydraulic line (meaning you have less pressure pushing on the brakes)? If you are talking about gas in the brake lines, how would the pads have anything to do with this, as the brake lines are sealed (with the pads on the outside of that seal)?

However, if you are talking about the pads overheating and gripping less well, I can see how that could happen. And you are correct, that better brake fluid won't do anything for overall pad grip (other than let you press the brakes in harder), as the brake fluid only protects the hydraulic brake lines (not the pads). So if your pads tend to not grip well (due to heat), your best bet would be to either lower the temp (better brake cooling, or adjust for less brake drag causing heat in the first place) or replace the pads with (higher quality) better gripping (when "hot") pads.

And if you are talking about the pads giving off some type of "slippery gas" when heated/used, I can also see how that could happen (although some pad materials would naturally be more susceptible to this than others, so higher quality pads could help in this area). However, in that case, the problem isn't the brake style per se, it's failure to vent the gas outside the wheel region, so it is able to stick around to cause problems. And while I admit that many (most?) disk brake systems are naturally better "vented" (then drum brakes), I don't see this as a brake style problem per se (but rather a problem with sealing the brake system up too much from the outside world).

But, I thought a serious problem with a lot of "brake fade", was loosing hydraulic pressure due to gas in the brake lines (and thereby having trouble putting enough pressure on the pads to get the desired braking). And that problem is generally a combo of the amount of heat in the brake lines, and the quality of your brake fluid. So that portion of the "brake fade" problem really can be helped (a lot) by just using fresh (decent quality) brake fluid...

[DracoFelis]

Originally Posted by theclencher

I suppose I ought to flush the brake systems in my stuff... some of that fluid is over 20 years old but it still works...

I used to be like you, because I didn't know any better. And I bet most of the population is the same as well (not changing their brake fluid, as that issue isn't even on their radar screen). And the reason why we can often get away with this, is that brake fluid will often last YEARS after it would be a good idea to change it.

But, it will work BETTER if changed (i.e. out with the old, in with the fresh). And the point is, that brake fluid (even fairly high quality brake fluid) is so CHEAP, it is silly to save a few pennies by trying to drive the fluid longer than you really should. Once I learned that fact, I went out and changed the brake fluid in all our vehicles. And even though the brakes worked before that, I was amazed how much smoother and easier the brakes worked from just changing the brake fluid all by itself (without any other brake "repairs"). Yes, it didn't solve all brake problems (I did have the mechanic lube up the springs/sliders in my brakes after that, to help even more), but just changing the brake fluid made a quite noticeable difference all by itself!

[Silveredwings]

I was just dropping in to see who was still beating this drum and I see some very good posts.

I too have become a fan of replacing all the fluid every coupla years. I generally run a good DOT4 (like ATE), but I use DOT5 (silicone) for a car that doesn't see too much action and I don't want it to rot away all on its own.

[onegammyleg]

Originally Posted by DracoFelis

Are you talking about gas between the pads and the drums/disks making the pads grip less well, or gas in the hydraulic line.
SNIP
But, I thought a serious problem with a lot of "brake fade", was loosing hydraulic pressure due to gas in the brake lines .
SNIP

Yup .thats it.

The pad material chosen for road cars is a compromise.
Pads are made of metals , fillers and resins.
The type and quantity of these parts determine the working heat range of the pad and consequently the braking system.
Choose a compound that is too cold (like suits a racing car) and it wont heat up to its rite range and stopping power will be very poor.

On the other extreme are normal road car pads.
They provide good stopping from cold (almost cold) but they have a lower top temperature limit.

When this limit is exceeded the pads start to expell gas.
This occurs of course at the frisction surface where it presses against the disc surface.
This gas (thankfully inert) has a lubricating ¨effect¨ between the pad and disc.
Discs that have holes or grooves machined into the surface allow a passage for the gas to escape - extending the effective brake system heat range.

In extreme duty applications a combination of vented discs and harder compound pads will be require to provide suitable stopping power under those expected conditions.

The signs that the pads have overheated is that you still have quite a firm pedal but it requires even harder pressure to pull the car up to a stop - and even then its stopping power wasnt impressive.

In rare cases that the fluid boils it is usually in systems with old fluid.
Brake fluid doesnt get the the same temps as the friction material or the discs but it can get hot.
Normal DOT 3 brake fluid out of the bottle has a boiling point of just over 200C , but in use this drops to about 180C.
Brake fluid is hygroscopic.
Over time brake fluid absorbs water from the atmosphere, which reduces its effectiveness by lowering the boiling point.

This can lower the boiling point to 100C , which is the same as water.
Under hard braking the water in the brake system can boil turning into steam.
Steam , unlike water or the brake fluid is compressable.

This will give an instantanious spongy pedal.

It could be so extreme that the pedal may drop to the floor and no braking effect at all.

This is of course an extreme example but possible.
The brake fluid companies would like people to belive it can happen to you at any time , but in reality , its quite rare as its from heat soak - long term high temperatures.
To get this heat-soak you would need many repeated hard braking applications.

But because the pads (in a normal raod car) will go off in 1 or 2 hard stops there is generally not enough time and heat to heat-soak the fluid to get it to boil.

[DracoFelis]

Originally Posted by onegammyleg

When this limit is exceeded the pads start to expell gas.
This occurs of course at the frisction surface where it presses against the disc surface.
This gas (thankfully inert) has a lubricating ?effect? between the pad and disc.
Discs that have holes or grooves machined into the surface allow a passage for the gas to escape - extending the effective brake system heat range.

In extreme duty applications a combination of vented discs and harder compound pads will be require to provide suitable stopping power under those expected conditions.

The signs that the pads have overheated is that you still have quite a firm pedal but it requires even harder pressure to pull the car up to a stop - and even then its stopping power wasnt impressive.

Thanks for the heads up. So it seems that there are two major factors in brake fade:

1) How the pads handle heat (as you described above).

and 2) How the brake lines handle heat (which is the factor that the newer brake fluid can help with, and which I described in a previous note).

And like many things in life, it seems that the one that will "bite you" is whichever one happens to be "the weakest link" (be it the pads, or the brake lines).