Technical FAQ: Even more on disc brakes in extreme cold

Lennard Zinn gleans even more information from disc brake companies about how performance is affected by very cold temperatures

When I posted these answers about riding disc brakes in extreme cold, I figured that would be enough to say about it. But even after posting a lot of opinions by brake manufacturers last week, I continued to get some responses from other brake companies that either covered new ground or had a different take on the subject. Here are some of them.

First, I want to explain the mechanism for pad retraction in a hydraulic disc brake to ensure that some of the comments below are more widely understood.

Pistons in most hydraulic disc brakes are pulled back into the cylinder upon release of the brake lever by the untwisting of an O-ring seal that is square in cross section. This “square seal” or “quad ring” surrounds the waist of the piston and sits in a groove running around the bore of the piston cylinder. [You can see the square seal in cross-section as well as in the exploded view of a caliper in the drawings from “Zinn and the Art of Mountain Bike Maintenance” shown above.] The rear wall of this groove is generally perpendicular to the cylinder wall, while the leading wall of the groove often is at a lower angle to allow the square seal to flex outward against it.

When fluid is forced in behind the piston by squeezing the lever, the piston moves outward, and the internal diameter of the square seal will move with it, causing the seal to twist. When the rider releases the lever and hence releases the hydraulic pressure, the square seal will untwist back to its original configuration, bringing the piston back with it — as long as the seal holds, that is.

The flexibility of the square seal’s rubbery material is quantified by its “durometer,” a measure of how forcefully it pushes back against pressure applied to it. And durometer of an elastic material is affected by temperature.

Extreme cold can inhibit piston retraction in two different ways. First, it can make the square seal stiffer (i.e., increase its durometer) so it is less able to twist. Second, low temperature can shrink the materials in the caliper — the metal of the caliper body, the metal, ceramic, or composite material of the pistons, and the rubber (or rubbery material) of the square seals. This shrinkage will happen at different rates in the different materials and will thus affect their fit against each other.

Dirt around the pistons has a similar effect in reducing pad retraction by causing leaks around the seal and by increasing friction, since the piston is being asked to slide on grit. If you complicate the job of a dirty piston seal by asking it to perform in extreme cold, the combination can result in abysmal pad retraction. And seals that have become loose due to low temperatures are more susceptible to contamination from dirt.
― Lennard

From FSA:
Thank you for tackling this topic. As fat bikes become more and more popular, it’ll be necessary to explain to folks what is happening with their brakes in the cold. I’ve been testing disc brakes in the Wisconsin winters since 1998 and still can’t believe the difference in feel as the temps drop.

Your piece contains many valid points made by very experienced disc-brake veterans and riders. I completely agreed with most points and just wanted to pass on my two cents for what it’s worth.

1. The DOT-based systems will be less likely to get “sluggish” lever feel in the extreme cold, but system design will have more to do with the feel than anything.

2. The seal durometer part is kinda crap in that all the brakes I’ve ridden in the cold not only have slower retraction but also lose retraction as the pistons start slipping (lever stroke gets shorter) when the square seals get harder from the cold. Saying one is better than the other is a stretch.

3. Leaking can happen in either system, as well. Depends on the tolerances set up in the design. Oddly, cheaper brakes often are designed to have more “squeeze” with their rubber parts to compensate for looser tolerances and in turn aren’t as likely to leak as a high-end brake that is designed to have very little squeeze in order to feel light at the lever.

4. Metallic pads are for sure gonna work best, but will be noisy as hell in the cold and wet. They also need heat to melt the ice and snow, so high speeds are needed, which doesn’t happen often on fat bikes.

5. The carbon lever thing is no joke, as it won’t transfer the cold to the rider’s hands. The joke is that Tim [Abhold] at Hayes [Richardson used to be a product manager at Hayes Disc Brake -Ed.] used to give me a ton of crap when I would report that test riders wanted carbon levers for winter riding!

[It] was interesting coming from 13 years of preaching DOT fluid to folks [at Hayes] and then switching over to mineral oil with FSA’s system. [I] learned a lot from our Japanese fluid vendor, in regard to the hydraulic fluids and all the various additives that can be formulated for different applications.

We discussed cold weather specifically and played with different levels of, what I referred to as, “antifreeze” in the oils to help stabilize the fluid in cold temps. We talked about mineral oils used in airplane and snowplow hydraulics that need to be fluid down to -50F degrees. So, it is possible to make mineral oils that are amazing in the cold; the problem is keeping their boiling or flash point high enough to meet our needs and the various standards. We ended up with a very balanced fluid that keeps our brake functioning in -20F degree life-cycle testing, yet still doesn’t boil on the dyno and more importantly on the hill with big guys that go fast.
— Joel Richardson
Disc Brake Product Manager
Brand Manager for the Gravity MTB brand
Full Speed Ahead (FSA)

(Another response) from TRP:
The only thing I’d add [to Lance Larrabee’s comments] is that wintertime can make rotor and pad selection very tricky. Often we get “dry” conditions in the morning that are below freezing, followed with above-freezing wet temps later in the day; I prefer a two-piece rotor with metallic or semi-metallic pads [for this]. The metallic pads provide longer pad life, and the two-piece rotor is stiffer, mitigating resonance that can occur with wet conditions (it’s like the wet finger and wine glass trick — doesn’t work so well on your everyday drinking glass — the glass is thicker and does not vibrate/resonate/sing).
— Nick Riddle
TRP Product Designer

From Magura:
I’d recommend fully hydraulic disc brakes, no mechanical or semi-hydraulic brakes, as cables can freeze with water getting into the outer housing or the mechanical part of the caliper.

Viscosity of brake fluid or mineral oil will increase with lower temperatures, making the brake feel slow. Additionally the quad-ring in the caliper will lose elasticity with piston retraction getting smaller with lower temps up to the point, where no retraction is available anymore.

This is common on all hydraulic brakes, with some being more affected by lower temps than others.

There is a nice info on Wikipedia on brake fluid; see especially dry and wet boiling points.

Mineral oil has a boiling point of 180 to 190°C (356-374°F) and will maintain it over time, as it doesn’t attract water.

I was riding at around 0F, and my Maguras still worked. A bit slow, but still decent.
—Stefan Pahl
Magura Product Manager for Bicycles