You don't need to be an 'investor' to invest in Singletrack: 6 days left: 95% of target - Find out more
Morning,
Random question but I was riding (not very well but hey it was fun for me) my DH bike down a particular steep and long track and when I got to the bottom my rear brake pulled straight to bar and basically wouldn't do anything. I left it for a while and then it started working again. I am not a great rider and working on not brake dragging but what is the diagnosis? I assuming it's a temp issue somewhere. Thanks
Brakes are SRAM Guide RE with sintered pads.
Most likely you have air bubbles in your brake system and they need bleeding.
Dragging the back brake probably heated your fluid and boiled it. It cooled and returned to normal again. If you are using Dot4 swap to a Dot 5.1 which has a higher boiling point.
Or, less back brake dragging or use the brake on / off a little more...
Dragging the back brake probably heated your fluid and boiled it. It cooled and returned to normal again. If you are using Dot4 swap to a Dot 5.1 which has a higher boiling point.
Or, less back brake dragging or use the brake on / off a little more…
This. Also, fit a larger rotor if you can.
Out of curiosity, but not enough to Google it, what about the liquid being hot makes it all if a sudden compressible?
Brake fluid can absorb water. This lowers the boiling point. What will happen is that when you release the lever, the pressure in the system drops and the fluid will boil, then you will get a spongy lever. When it cools down, the fluid will cool and the brakes will work like normal.
If you're doing big descents, it's worth flushing the system and refilling it with Dot 5.1 fluid from a sealed container every six months or so. Also, giving the brakes a brief rest between corners instead of just constantly dragging them should help keep the pads cooler. Rather than just dragging them steadily, brake hard for a second or two, briefly release the lever, then brake hard for a second or two, and so on. Obviously, if you really need to brake hard, you should, but if you're just gently dragging the brake to control speed, giving the brakes some brief rests to cool the pads should help.
All very interesting thanks for the responses. Based on that school experiment where you fill a container with steam then close the lid and as it cools it crushes the container. My simple brain is now thinking but if it actually boils surely the expansion into gas, all be it compressible, would push the pads out and lock the brakes.
Sorry massive derail for the OP. I'll go Google later.
Thanks everyone. @thols2 I was just thinking the same thing. Time for an oil change with some fresh oil and try to improve my riding.
but if it actually boils surely the expansion into gas, all be it compressible, would push the pads out and lock the brakes.
The boiling point is higher when the fluid is under pressure. When you're applying the brakes, the fluid is pressurized and doesn't boil. When you release the lever, the pressure is released and the fluid boils. It may cause the brakes to drag, but it won't be enough to lock the brakes. There will be gas bubbles in the caliper so, when you squeeze the lever again, you just compress the gas without getting much braking force.
My simple brain is now thinking but if it actually boils surely the expansion into gas, all be it compressible, would push the pads out and lock the brakes
Brake fluid only boils when you are OFF-BRAKE (brake pressure will raise the boiling point so it only boils when at lowest pressure). If you're OFF-BRAKE there is a fluid pathway or vent, via the mastercylinder piston, into the fluid reservoir and that allows the fluid volume to increase, because of the vapour, and expand into the reservoir without the brakes locking.