|   how often do you change the brake fluid??nm. | 13
Jan 11, 2002 6:38 PM | | . |
| Basic advice | firecrotch
Jan 11, 2002 11:09 PM | | Although I do not own hydro discs, I have worked on them. Most manufacturers recommend a fluid change every two years at least. And even then, its really just to get fresh fluid in the system. Generally speaking, change or add fluid when it feels necessary. If your brakes feel mushy, that's a good time to check the fluid and bleed them (mushy feel indicates air bubbles in the system, which unlike fluid, is compressable). Brakes like Hayes and Hope are super reliable in my experience with customers who own them and go to the shop I work at, so they may never need bleeding for well over two years. And remember, always, always, always (nuff emphasis?) use the correct replacement fluid for your system. That means the right number DOT oil (if you use this) or the mineral oil FOR (different formulas of this) that specific brake from the manufacturer. DOT oil will absolutely ruin a mineral oil system, and mineral oil can do equal damage. Just don't do it. I've worked with customers who have screwed this up, and boy is it expensive! Check with your manufacturer for the scoop on fluid. |
| DOTs and minerals | bladteth
Jan 12, 2002 2:48 AM | | There are more than one DOT fluid. AFAIK we can have DOT 3, DOT 4,
DOT 5 and DOT 5.1. The first two are based on glycol and can be
mixed without problems. DOT 5 is silicone-based and therefore can't
be added to lower numbered DOTs. The higher number, the higher
boiling temperature. DOT 5.1 is very rare and not used in bicycle
brakes. Mineral oils are not higroscopic as the DOTs are and (if
properly bled) they don't have to be changed at all.
bladteth |
| DOTs and minerals | rjw
Jan 12, 2002 6:12 AM | | DOT 5.1 is very rare and not used in bicycle
brakes.
Incorrect. All Hope hydraulic brakes use DOT 5.1, although this can be changed to DOT 4 if you completely flush the system through. |
| DOTs and minerals | bladteth
Jan 12, 2002 6:43 AM | | I wasn't aware that Hope uses DOT 5.1. This brake fluid, like 3 or 4,
is based on glycol. The difference is higher boiling temperature
(about 70F over DOT 4) and even higher price and DOT 5.1 is rather
difficult to find in shops.
bladteth |
| DOT 5.1 | rjw
Jan 12, 2002 10:17 AM | | I had no problem finding DOT5.1 fluid here (UK), it's more commonly used on motorbikes rather than cars. |
| Agreed | HooKoo
Jan 12, 2002 11:02 AM | | I'm also in the UK and have bled my friend's Hope Mini with 5.1 which I happened to have lying around from the days when I prefered MX to MTB.
Alan |
| DOTs and minerals | Dougal
Jan 13, 2002 1:04 AM | | DOT 5.1 is the same chemistry as DOT 4, some are sold as DOT 5.1, some are sold as Super Disc Brake Fluid.
Check motorbike shops, car shops. I run Castrol Super Disc in my Hayes, it's DOT 5.1, cost me a dollar more than DOT 4 for a 500ml bottle.
Dougal.8m.com |
| NEVER!!! | Stan
Jan 12, 2002 5:24 AM | | If you have a properly designed brake system you should never need to replace the brake fluid. How often do you replace the fluid in your car? Never even if you have a problem with you car all you do is bleed the line you are having problems with.
If you start changing your fluid you will in most cases create more problems.
Stan |
| My answer is: | bladteth
Jan 12, 2002 6:52 AM | | Why don't people change fluid in their cars? Simple, because they're lazy and the drawbacks of water absorbed in the fluid are hard to experience in normal, everyday use. The problem is that moisture will decrease boiling temperature of the fluid and cause corrosion in brake lines and cylinders. That is why you have to change glycol based (DOT 3, 4 or 5.1) fluids in your brake system.
bladteth |
| And steam bubbles | Stefan
Jan 12, 2002 8:30 AM | | If there is water mixed in brake fluid you will get steam bubbles in the fluid on long descents because of the heat emission from the hot discs. These bubbles are compressible and the lever travel will increase - worst case = failure.
That is the case for cars and bikes. Change your fluid after 2 years! |
| Never??? | cbranney
Jan 12, 2002 10:04 AM | | Ok, so maybe Im weird but we change brake fluid in our cars a fair bit.... just to keep it fresh! But isnt DOT 5 or 5.1 total overkill for a pedal bike? I mean, we rally cars and we go to DOT % to stop the fluid boiling under extreme braking conditions ie: with competiton brake pads and pretty high speeds. On a very low weight bike I cant see that there would be anywhere near the same heat energy released even under the harshest conditions.
Assuming that all the kinetic energy will be turned into heat energy in the brakes (which I know is not the case but will do for comparissons), we can calculate the total heat energy generated under certain conditions, using Ke=1/2 mv^2
A 12kg bike travelling at 50mph will generate approx 2962 Joules of energy in coming to a halt
a 850kg car travelling at 70mph will generate approx 411358 Joules of energy in coming to a halt.
So Why would a bike need anything approaching DOT 5 brake fluid??? |
| Never??? | rjw
Jan 12, 2002 10:27 AM | | Don't forget that a bike and rider weigh somewhat more than 12kg.
There is also a lot less material in the rotor/caliper with which to dissapate heat.
More importantly the brakes aren't just used to stop from x mph once, most of the heat is generated when dragging or repeatedly applying the brakes on long descents. |
| i agree with what he said, and... | Nicko
Jan 12, 2002 12:02 PM | | in a bike system there is alot less brake fluid in use. in a car there is a lot more. so cars may generate more heat, but bikes will still generate a lot, but will have less fluid to heat up so a smaller amount of fluid could heat up and cause problems.however, i've personally never even heard of DOT5.1 being used in bikes, the highest i've heard of is i think DOT 4. good dirt.
~Nicko |
| air cooled | cbranney
Jan 12, 2002 5:16 PM | | However, the disc on a bikes brake is exposed to airflow, cooling not only the rotor but the caliper and the hydraulic lines as well. In a car, not much air flow passes through the wheels (and the stuff that does is very turbulent and traveling pretty slowly). Of course, If you have an F1 car with brake cooling ducts and scoops, and open bodywork things are different. But a bike will recieve much better cooling from the airflow round it.
I do accept the point regarding less amterial to heat up. However, you must remember that the converse is usually true, if it heats up more quickly it will also cool down more quickly.
However, the overlooking of the rider does change the figures somewhat. Taking a 100kg bike and reider combination the total energy is just less than ten times what I originally quoted.
Dont get me wrong, Im not saying that I know for certain that bikes dont need DOT 5 fluid. Ive never ridden a Bike with Disc brakes let alone tried to stop a full on DH rig from 50mph. But Im a mechanical engineering student (trying to find out more about bikes etc.), and to me it still seems like overkill. Im home for the holidays at the minute so I dont have any of my textbooks to get any info on specific heat capacities or anything like that, but Im just trying to figure out why its necessary to run DOT 5 fluid in a bike when the same fluid is overkill for some motorsport aplications.
So has anyone had problems with DOT 3 or 4 fluid boiling? If so, then I would agree that DOT 5 was justified. But if not, then to me its still unneccessary expense. |
| lol, its not about "stopping a bike from 50mph" | Jm
Jan 12, 2002 10:13 PM | | Its about extended braking on a run, where you are using lots of brake and never letting up. The run, course, or trail dictates how much braking you need, and there are ones where you must be braking the entire time to control your speed. Cars do not do anything like this, and there is no good comparision I know of. Being on the brakes on a DH run at northstar the entire time over 15 minutes will generate a LOT of heat, and i for one am glad I have DOT 5.1 in my brakes to keep the fluid from boiling. |
| just a few statements | Brakemeister
Jan 12, 2002 10:20 PM | | The air cooling effect on a bike is pretty non existing. If you rallye you have usually internal vented rotors... correct ?
The airflow of those babies is pretty impressive, even without ducts ( which you will loose on a rallye car anyhow )
I also cahnged brakefluid in the rallye car every 5 events or so ( 3 times a year ) but I also had always problems with the brakes. ( heavy SAAB 99 turbo, with a LOT of leftfoot braking )
But on my regular car I havent changed fluid in the last 5 years ( it doesnt weigh that much and it has BIG brakes ...lol )
Anyhow. Bicycle Brakes are much more delicate than the brembos on my car. Weight of the rotor for example !!! Just compare that a 100 gram rotor has to slow down a Bike rider going 50 mph downhill ..
ok a complete brake system on a bike is 350 grams ( well not everybody's but mine is ) On my car ????
Careful with DOT 5 .... silicone based fluid. Very Spongy.. I tried it once in my racecar.... I changed that stuff in a hurry back to DOT 4
DOT 5 and DOT 5.1 are totally different !!!
Thorsten |
| air cooled | Dougal
Jan 13, 2002 1:12 AM | | You'll learn more as your degree progresses, including more turbulent air actually cools better.
Cars also have ventilated disc brakes, where air passes between the braking surfaces to cool the brake. Bike rotors are also poor heatsinks, they just don't have the mass of material. Combine that with high friction pads because we don't have any power assist and you get some pretty hot brakes.
Yes my bike has boiled DOT 4.
Dougal.8m.com |
| Hope uses DOT 5.1 in their brakes (nm) | SS_MB-7
Jan 14, 2002 4:49 AM | | |
| You've obviously never riden down any hills | Jm
Jan 12, 2002 10:10 PM | | big several thousand foot decents, where you are on the brakes all the time. Oh sure there are hills where you don't have to be on the brakes all the time and only short quick braking is necessary, but then there are also ultra steep decents with turns and rocks and roots where you have to be on that front brake the whole time to control your speed and make the turns. I seriously doubt this is anything like racing a car where you are braking before turns. In dh mountain biking the rotors can actually get red hot with enough time on the brakes and right sunlight conditions(setting sun or night), it has happened.
Discs allow you to take biking into a new dimention and tackle these super steep hills because you can be in control all the time. This energy though is converted to heat and over a 10 minute DH run I am sure it is a staggering amount of heat. |
| Never??? | Dougal
Jan 13, 2002 1:08 AM | | A mate borrowed my bike and boiled DOT 4 out of my rear Hayes brake. Sure he was dragging the brakes but it still does happen.
Tandems are another story, these guys don't just boil fluid, they melt hoses.
Dougal.8m.com |
| Fair enough! | cbranney
Jan 13, 2002 10:23 AM | | Ok, thanks everyone, that what I was looking for. My bike currently only has V-brakes and I havent ridden too many serious DHs yet. I was on the brakes most of the time myself, but I didnt appreciate that people on more serious machinery would do the same thing. And the fact that anyone has boiled the DOT 4 fluid proves that DOT 5 is necessary.
As for red hot rotors - wow! Im sure that must be quite an impressive sight! |
| Seriously Dougal, give it a rest. | wc
Jan 14, 2002 5:56 AM | | You keep spreading this crap about bicycle disc brakes reaching rediculous temperatures and boiling your brake fluid. Frankly, I respect your opinion on other matters such as suspension but you are losing some credibility here. Please see my post a while back along the same lines here.
For those that don't want to read the old thread, here is the Cliff notes version.
From the Hayes FAQ:
"8. Is it normal for the disc to be discolored at the end of a downhill run? Yes, your brakes can generate up heat up to 300 degrees at the disc. This will discolor the disc, but will not harm the disc or change the stopping power."
Now don't you think that Hayes tested this time and time again and have included a bit of a safety factor to cover themselves? If your friend did indeed boil the fluid out of your brakes it was because there was an fair amount of water in there.
Some data you may find interesting: (fluid, dry boiling point, wet boiling point)
DOT 3, 401º F, 284º F
DOT 4, 446º F, 311º F
DOT 5, 500º F, 356º F
DOT 5.1, 518º F, 375º F
The dry boiling point is determined when the fluid is new and the wet bp is determined when the system contains 3% water by volume.
So if Hayes says their brakes will reach 300º F, then ALL of that heat would need to be transferred to the fluid with none of it going in to the air, hub, or other caliper parts and your system would have to have more than 3% water in it. Sounds pretty damn unlikely to me.
I won't even address your insanely high temperature claim at this time. |
| Good info | cbranney
Jan 14, 2002 3:43 PM | | Well, this adds an interesting new spin on things. I appreciate your info, as this quantitatively addresses the problem in a way I can relate to!
It also supports my original idea, so once again I appreciate your info! |
| And steam bubbles | Dougal
Jan 13, 2002 1:07 AM | | You don't actually get water in the brakes, the fluid absorbs the water which lowers it's boiling point. But the boiling point is still well above that of pure water/steam.
DOT 5.1 fluids start with a higher boiling point and take more water to lower the boiling point. All round a better fluid so why not use it??
Dougal.8m.com |
| re: how often do you change the brake fluid??nm. | Atomic
Jan 19, 2002 4:06 PM | | I'm using DOT 3, ride very hard, never got a problem... I think that's all! 5.1 isnt NECESSARY, if you can get it ok, but it wont change a thing... See ya! |
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