discs siezing when hot

[davehirst]

I have a set of shimano 3 series hyd discs on a gt karakoram, this week we took it to the blue mountains, where we did a lot of hills, when the brakes got hot the front would come on partially, when the bike was home and cool the brakes are fine.
Is this possibly to much fluid.
The brakes are new, replacing avids where we were having a similar problem.
It is the wifes bike and she is heavy on the brakes.
thanks
dave

[10speedsemiracer]

There may be a more appropriate pad compound for your particular usage. Does she experience fade and have the discs glazed or warped?
Generally a slightly harder pad compound is recommended for sustained heavy usage however the trade-off is less initial bite when cold.

[davehirst]

no pads and discs are ok, when every thing cools down all is ok

[Duck!]

Probably not excess fluid, because the Shimano oil is pretty stable thermally. Could be that as the calipers are getting hot the material is expanding & binding the pistons in the calipers.

The lower-spec Shimano brakes don't have a metal pad option, so really you should be looking at altering technique and letting the brakes breathe between corners.

[Defy The Odds]

I race my car a few times a year, and brake fluid usually does this in a car when it gets to it's boiling point but I doubt your fluid is reaching those temps.

The only other thing I could say is firstly try changing the pads to something more high performance and suitable. However as mentioned earlier the trade off is cold weather braking.

If this doesn't work bleed the brakes with Dot 5 or Dot 4 fluid of a reputable brand

[Trevtassie]

If this doesn't work bleed the brakes with Dot 5 or Dot 4 fluid of a reputable brand

Don't using DOT 4 or 5 fluid on Shimano brakes, they take mineral oil as Duck mentioned, different stuff.... you need the genuine oil.

[Defy The Odds]

Not sure of Shimano but I have seen recommendations for dot 4 or 5 on SRAM mountain bikes.

These fluids have a higher boiling point which can create brake fade

[MichaelB]

Not sure of Shimano but I have seen recommendations for dot 4 or 5 on SRAM mountain bikes.

These fluids have a higher boiling point which can create brake fade

Oils CANNOT be mixed and/or used in different systems - the seals will fail.

Even at 95kg + bike, I've never even gotten close to boiling my fluid even down European passes with steep descents. IMHO, the boiling point of the fluid is a not issue.

Pad fade due to high temps/poor braking is a more common occurrence

[Calvin27]

Not sure of Shimano but I have seen recommendations for dot 4 or 5 on SRAM mountain bikes.

These fluids have a higher boiling point which can create brake fade

Please don't tell the dude to put DOT in a shimano brake system if you are unsure!

You can buy finned pads for a little bit more cooling - they come in organic and metal.

[eeksll]

no one else has said it, so I am probably wrong here, wouldn't this symptom be caused by having a bit of air in the caliper?

[MichaelB]

no one else has said it, so I am probably wrong here, wouldn't this symptom be caused by having a bit of air in the caliper?

If anything, that will lead to a spongy lever and poor braking, rather than seizing on.

[Duck!]

If this doesn't work bleed the brakes with Dot 5 or Dot 4 fluid of a reputable brand

Don't using DOT 4 or 5 fluid on Shimano brakes, they take mineral oil as Duck mentioned, different stuff.... you need the genuine oil.

DO NOT EVER UNDER ANY CIRCUMSTANCES USE DOT FLUID IN A MINERAL OIL BRAKE SYSTEM!!!!! Sure-fire way to destroy the seals, leaving you with no brakes at all.

Not sure of Shimano but I have seen recommendations for dot 4 or 5 on SRAM mountain bikes.

Absolutely not DOT 5. SRAM specify DOT 5.1. DOT 3, 4 & 5.1 are all the same chemical base, so are interchangeable. DOT 5 is quite different, and must only be used in brake systems designed for it. As far as I'm aware, no bike brakes use DOT 5.

These fluids have a higher boiling point which can create brake fade

Actually not necessarily the case. Not all mineral oils are formulated equally. Shimano's oil has a higher boiling point than DOT5.1, and because it does not absorb moisture, maintains consistent performance. DOT fluids absorb water, which lowers the boiling point and degrades performance. Additionally, brake fade is not caused by boiling fluid; that's vapour lock and pressure loss. Brake fade is the loss of friction resulting from overheated and glazed pads.

[Duck!]

You can buy finned pads for a little bit more cooling - they come in organic and metal.

Not for those brakes. Metal compound & cooling fins are only options on higher-spec models.

[Duck!]

no one else has said it, so I am probably wrong here, wouldn't this symptom be caused by having a bit of air in the caliper?

If anything, that will lead to a spongy lever and poor braking, rather than seizing on.

Air is compressible, so would indeed cause a spongy feel and poor brake performance. If there was sufficient volume of air in the system to expand that much, the brakes would be in all probability not even be functional. Additionally, if any air was inside the system and trying to expand, it would tend to push the fluid up the line & back into the reservoir, where there is only atmospheric pressure acting against the expansion bladder. Path of least resistance & all that.

[eeksll]

no one else has said it, so I am probably wrong here, wouldn't this symptom be caused by having a bit of air in the caliper?

If anything, that will lead to a spongy lever and poor braking, rather than seizing on.

Air is compressible, so would indeed cause a spongy feel and poor brake performance. If there was sufficient volume of air in the system to expand that much, the brakes would be in all probability not even be functional. Additionally, if any air was inside the system and trying to expand, it would tend to push the fluid up the line & back into the reservoir, where there is only atmospheric pressure acting against the expansion bladder. Path of least resistance & all that.

thanks for the clarification.

[Trevtassie]

Finally, a little more on brake fluids,to complete the picture. DOT brake fluids, except for DOT 5 are hygroscopic, they absorb moisture from the atmosphere, it's actually almost a feature of the fluid. Water is bad because it boils at 100C, which can easily be reached in automotive brakes, boiling = steam, steam is a gas and is compressible = squidgy pedal . When a small proportion is mixed with brake fluid however the boiling point is lifted to close to the boiling point of the fluid, around 200C+.
Water used to get into brake systems on cars through condensation because they weren't sealed at the master cylinder , if it wasn't absorbed into the fluid it would travel down the brake lines to the lowest point, the brakes, where it could boil or corrode brake mechanisms. So the idea was brake fluid, although prone to absorb moisture, if changed regularly (which you should do every few years, or more often in the tropics) was a better bet than non hygroscopic fluids. It's not such a big issue nowdays because systems are much better sealed. Shimano use non hygroscopic mineral oil because like modern cars, their brake systems are sealed, water can't get in, and because there are advantages to mineral oil, like a long shelf life after being opened and the fact it doesn't eat paint like most brake fluids. There is some interesting reading here about the various brake fluids: http://www.stoptech.com/technical-suppo ... rake-fluid
In terms of your modern car, it's still worthwhile changing your brake fluid every 5 years or so at a minimum. Your average mechanic will look at you like you are a weirdo if you insist, but basically your brake internals will outlast the rest of your car if you do this.