Brakes failing?

Dampsquid

I was out driving there yesterday, was driving over the slieve bloom mountains.

The brake on the car seemed to be working fine. Then when we go to the top of the mountain and started to come down the other side, I would be using the brakes alot more on the steep declines. After a while, maybe after braking 3 or 4 times in a row, the brakes became very light, so as when i pushed fully on them, they would't even lock up, just slowly slow the car down. And i do mean slowly.

Back on to level ground they seemed to be working ok again.

I was thinking that they were over heating (there was a burning smell for a while) when used alot on the way down the mountain, and this caused them to not work as good.

Has anyone got suggestions on what it might be? should i have them replaced asap. But they do work fine normally...

ColinM

Yeah, I've experienced that coming down some steep twisty mountain roads in the Alps. It is quite terrifying alright. The brakes overheat because they are being used constantly, but you are not going fast enough to allow sufficient airflow around them to cool them down. Get them checked out at your next service.

BrianD3

How old is the car? If it's old and and/or has done a lot of miles the brake fluid probably need to be changed. When you use the brakes a lot, your brake fluid gets very hot and may even boil. If it boils, braking efficiency will be seriously reduced. Over time, brake fluid absorbs water which reduces its boiling point so this is more likely to happen. So I would recommend that you get your fluid changed asap.

<edit> I just did a search on usenet for symptoms of boiling brake fluid and it would appear that if the brake fluid boils, the brake pedal goes to the floor and you get no braking at all which is different to the symptoms you described. Apparently, the brake fade you described may be due to the friction surfaces (pads, discs, shoes etc.) getting extremely hot and losing their efficiency. The quality of these components may have something to do with how much brake fade you experience eg cheapo pads may not be as good as pads recommended by the car manufacturer. Anyhow, I think the best think to do is take the car to a garage and let them check out the braking system including the fluid etc.

BrianD3

Seth Brundle

the best thing to do whilst coming down a steep hill is
a) don't keep your foot on the brake - encourages brake fade (gasses building up between pads and discs) and fluid temp increasing & even boiling.
and
b) brake in short heavy bursts, ie stand on the brake briefly giving maximum braking over the shortest period and therefore allowing the longest time between brakings for the contact surfaces to cool down
or
c) just go hell for leather down the hill, using as much of the road as possible and don't use brakes that much at all

Dampsquid

Thanks guys.

The car is a '98. It was service in march i think. Not sure if the brake pads were replaced then. must check my log book.

Its just that i never noticed it before, so i was a little worried.

RicardoSmith

You should use engine braking on hills and then you wouldn't need to use your brakes at all. It works less well in car with a smaller engine but it works. When you need to brake simply drop to a lower gear and you'll feel the engine slowing you down. You can also do this on the flat, it doesn't have to be on hills. You would use it a bit racing to save your brakes too. If you are finding you are having to change your brakes a lot then you should look at your braking/driving technique. Its likely very poor. Its also useful to give a tailgater the fright of his/her life since your braking lights don't come on and yet you slow down pretty quickly. A good driver drives smoothly and shouldn't need to accelerate or decelerate a lot. Their speed is relatively constant. You can tell a poor driver is they are forever on the brakes and constantly having to accelerate. In Racing drivers who are quickest use their brakes sparingly are the quickest and the best. That doesn't mean they don't slow down it just means they don't use the brakes to do it. They generally are reading the road ahead much better and are at the correct speed before they need to hit the brakes. In Karting you would flick the Kart sideways to slow down. Letting the drag of the tires slow you done. Hence "scrubbing" the speed. You'd never hit the brakes. In F1 is known for doing it too. In many cases you will notice other drivers brake light coming on and Shumaker's not. It because hes using Karting technigues even at 150mph!!!

BrianD3

In Racing drivers who are quickest use their brakes sparingly are the quickest and the best. That doesn't mean they don't slow down it just means they don't use the brakes to do it. They generally are reading the road ahead much better and are at the correct speed before they need to hit the brakes.

Sorry, but that's wrong. Yes, racing drivers do downshift when slowing for corners - but the reason is not for engine braking but so that they're in the right gear for optimal acceleration out of the corner. Take modern F1 for example - hit the brakes and it's like driving the car into a wall such is the stopping power. Why would anyone want to use engine braking in this instance? It would just mean that you'd spend more time slowing down and would have to slow down much sooner for corners. The result is that you'd be on full throttle for less of the lap => slower lap. F1 engines probably produce quite significant engine braking (I believe these engines have no flywheel) but it's nowhere near the retardation that they get from the brakes themselves. The whole idea of racing is to go as quickly as possible, not to save your brakes! Take someone like Alain Prost (4 times world champion) who had a rep for being an extremely smooth driver and used to think his way through races rather than going for outright speed all the time. I can guarantee you that he was still making serious use of his brakes!

However, on the public road it's completely different and I pretty much agree with your points about using engine braking, being as smooth as possible, and trying to anticipate and read the road. For the purposes of both safety and mechanical sympathy. Eg when approaching a corner, ease off the throttle well before the corner rather than barrelling up to the corner and slamming on the anchors. This is even more important when driving an oversized heavy vehicle such as a HGV or when towing a heavy trailer. Indeed I believe some HGV's have an "exhaust brake" which the driver can operate to accentuate the effect of engine braking.

BrianD3

RicardoSmith

Sorry I didn't mean that professional drivers don't use their brakes, obviously they hammer them till they glow red. But they use them sparingly. If in a long race where endurance is a factor and you have to save your brakes, in that situation you could/would use engine braking where possible, say into a slow series of bends where you knew you have a few seconds where you don't need max acceleration. F1 wasn't a great example especially as they now use those ceramic brakes that don't fade as much as the old ones did. But drivers still use alternative braking means. You might put the car sideways to slow down, but it also allow you to keep the foot planted keeping the revs up for the exit of the corner. You do it rallying, particularly in 4wd drive cars which don't change direction as quickly as 2wd cars. Theres a lot of variables involved all acting on the car. I was over simplfying the physics of it all to make my point. That "exhaust brake" sounds interesting though. Thats a new one on me.

Another point is the car acts differently under braking, as the suspension is under compression and thus the car handles differently. You might not want to do this either, which is another reason not to use the brakes or a good reason to use the brakes. To ride over a bumb or bumbs you might avoid braking just before them, so the suspension has maximum travel and thus rides the bumps better. For speed bumbps its faster to let the car go over a series of them at a constant speed without braking, instead of braking, accelerating, braking etc. I guess some cars have a slow suspension response so adding braking to the mix doesn't help.

BrianD3

OK then I think I see what your saying. Modern F1 was probably a bad example given the complexity of the cars, the electronics, the carbon brakes etc.

As for the exhaust brake, I don't know how it works maybe some of the HGV people (Silvera?) here can enlighten us

BrianD3

John R

Dampsquid, unless the brakes fade very rapidly I wouldn't be too concerned, if you say they work normally except on steep descents then there is no need to worry. Brake fade after steep descents is perfectly normal, all components have their limits and brake pads and disks are no exception, they just overheat. It is not necessarily a sign of faulty or sub-standard parts, after they cool down they will operate normally again. Higher performance brakes would fade less of course but probably not worth the extra cash unless you drive down mountains regularly.

All that stuff about racing drivers sliding cars to help braking is complete crap. Firstly the fastest way of driving on sealed surfaces is to keep traction between tyres and road at all times, note the huge amount spent on traction control in f1 for this purpose. Secondly by sliding you create a big increase in tyre wear which is a very critical factor in both f1 and rallying, far more important in gaining or losing time than the potential for brake fade.
In rallying sliding through corners is used on loose surfaces simply because better traction can be created by keeping the wheels rotating rapidly against the rough surface, nothing to do with brakes and brake fade

It has been a while since I paid close attention to f1 but I don't remember brake lights being used, they have had marker lights for a few years but brake lights?

Intentionally skidding is a very dangerous thing to do both in racing and on the public roads, it can be very difficult to accurately control the direction of your car even if you are expecting the skid.


Exhaust brakes (also called retarders) basically work by restricting the exhaust output and creating pressure in the engine making it work harder to rotate creating an extra engine braking effort. They are only used with diesel engines, from what I remember this is because petrol engines have a throttle valve that restricts air intake when the accelerator is off creating pressure in the engine whereas diesel engines do not, so require exhaust brakes to assist engine braking.

RicardoSmith

If you've ever been Karting or racing you'd know what I'm on about. I'm not taking about skidding. I'm talking about unsettling the car before a corner so you lose speed so that you can take the corner but not need to reduce the engine revs or use the brakes. Basically adjusting the atitude of the car is the most energy efficent way of doing it. every see a F1 team change the brakes during a pitstop no, but they change the tires a lot yeah? It makes sense to save the brakes as if you are having problems with them your in trouble. The tyres you can change. As has been pointed out its not really and issue anymore with carbon brakes but the energy management as applied to a racing car is still valid. Braking hard is still hard on the tyres so theres not a lot of saving there. Its not a big attitute change like you see in all the "mags" with cars at 45 deg to the direction of the corner, thats jsut stupid. Its more of a couple of degrees off line coming into a corner. I'll try and find an article as it relates to Schumacher . Theres one I remember about him in practice for the English GP that demonstrates the techniques but I can't find the link at the moment.

But I did find this article which it explains it better than I can. In this example their using brakes to change the attitude if the car but in a fast corner you can give the car a jiggle to offset it and it does the job.

No Diff' Handling

The more you can understand about why a kart handles as it does, the better equipped you are to get the most out of it. Take, for example, the fact that a kart has no differential. There are two major consequences of the fact that the two driven wheels always spin at the same speed:

1. The kart has a tendency to point in a straight line, making turning into corners difficult as the front wheels struggle to grip - "understeer" (or, if you're American, 'pushing'). Note that from a standing start a kart will accelerate far better with both front wheels pointing straight.

2. When cornering you either have the outside rear wheel spinning too slowly or the inside rear spinning too fast.

It is point (2) that forces the best kart drivers to adopt a distinctive driving style that can be seen among top-level F1 and CART drivers. On entering a corner a good kart driver will break hard in a straight line, then use the accelerator to spin the rear wheels slightly on entry, breaking their grip with the surface to allow the front wheels to steer the kart in better. Once the kart has rotated sufficiently the front wheels are straightened, pointing to the exit of the corner. This, when balanced correctly, allows the rears to grip and accelerate hard, and early, from the corner.

Suffice to say, getting this technique to work properly requires a huge amount of practice. It is worth bearing in mind for certain race situations however, particularly wet conditions where the grip levels are lower and understeer on entry is worsened.

I'd use it a lot in karting. Never had the chance of trying it something bigger that I wasn't scared of bending. But it works in the better sims like GPL GT2 etc.

kayos

Well if you do want to do something to reduce your brake fade you should look at getting braded stainless steal brake lines and a better brake flui (something like ATE Super blue). Its a relatiely cheap way to upgrade your brakes to help prevent brake fade....