Technical FAQ: Spreading disc pads and tubeless road failure

Happy Thanksgiving week, readers! This week we’ll address a quick question on disc brake pad installation and then look at a reader’s letter about his untimely tubeless tire blowout.

How do I expand my brake pad spacing?

Dear Lennard,
When I replace old disc brake pads on mountain bikes, I always find that the new pads never leave enough room for the caliper? I try and push the exposed pistons inwards but never with much success? Any tips to make this common procedure a little easier?
—Brian

Dear Brian,
Hydraulic disc brakes are self-centering. The pistons do not return to a fixed place; rather, they return the same distance each time. The pistons have an O-ring around their waist that is square in cross-section and sits in a square groove in the wall of the caliper cylinder. The O-ring twists when the piston moves out, and it untwists and returns the piston when the brake lever is released. When the pads wear, the gap between the pad face and rotor remains constant. So as your pads wore down, the pistons gradually moved further out of the cylinders to keep the same distance from the thinner pads to the rotor. This left insufficient room for new, thicker pads to fit in the space between the pistons.

Therefore, yes, you do need to push the pistons back. After you’ve removed the pads, use plastic tire levers or the box end of a combination wrench (10mm usually works) to push the pistons back into their bores. Park actually makes a special tool to push the pistons back while the pads are in (http://www.parktool.com/product/hydraulic-piston-press-pp-1-2).
―Lennard

A long tale of tubeless trouble

Dear Lennard,
Thanks for continuing to put out great in-depth material on bike tech; I have read everything you’ve written for years, and continue to learn each time. Your latest piece on Zencranks was particularly thorough, which I really appreciate. I’d worked on the Dynadrive cranks on the odd customer bike years ago, but never knew what the design was about.

Since you’ve been riding and talking about tubeless tires for road for years, I wanted to relay an experience that I’ve never heard anyone talk about. I’ve been on tubeless road for three years, after being comfortable with MTB tubeless for years. I’ve used Hutchinson’s offerings in 23mm and 25mm, along with Stan’s Ravens and Clement LAS and PDX tires, for everything from road racing to adventure riding. I use Stan’s rims, Alpha 340s and 400s.

Here’s what happened.

While riding my fifth Deerfield Dirt Road Randonnee, I punctured my front tire (Clement LAS, tubeless) on the fastest of the rough descent, traveling at about 60 kph. The road is covered in loose rocks (the segment is called “This is Awesome” on Strava), so it was important for me to slow down quickly as my tire deflated to avoid smashing my front rim and denting it. I was weighting my back wheel to ease impacts on the front, and braking consistently hard, but not skidding. I use long reach caliper brakes with Kool Stop Salmon (Scott Mathauser compound) pads on this bike. As I came to a stop and pulled to the shoulder, my rear tire hissed, and quickly lost all its air at the bead, right at my valve. This was a Stan’s Raven, tubeless, which I’d ridden thousands of kilometers on, trouble free.

I knew this was bad. I attended to the rear tire first, and was not surprised to see the bead destroyed. An inch long section, right at the valve, was shredded. The rubber was gone, exposing yellow bead chords. Its integrity was gone. I installed a boot and tube in a vain attempt to correct the issue, knowing it was unlikely to work. (Another rider informed one of my teammates later that he’d dealt with a similar issue by duct taping the inner tube to limit its expansion at the damaged bead location. Brilliant!) As expected, the bead was bulging out, trying to escape at maybe 15 psi. After repairing the front flat, I gingerly rode out the descent, then ended up riding another 10 kilometers on it, standing, weight on the front wheel, until a generous rider, Jamie, lent me a spare tire he was carrying. I then regained contact with my teammates, who were ahead trying to source a tire, and rode out the day, happy to be riding at all.

This was the first time I’ve ever experienced or heard of an actual bead blowout on a tubeless tire/rim combo. Clearly, the heavy braking overheated the rim, and an abundance of heat accumulated at the valve, overwhelming the tire. Normally, I’d not be braking at all on a descent like that, or I’d be alternating brakes to dissipate heat. However, this experience indicates that some situations could lead to blowouts.

My experience only solidifies two conclusions from the 2012 edition of D2R2: high volume tires are better, as are disc brakes. The bike I’d set up to ride, a 29er with drop bars, broke two days before the event, leading me to use my all-road bike instead (thanks to a teammate bringing it from Ottawa). This meant only 35mm tires, and rim brakes. Had I punctured the front the same way, and been on discs, I’d not have had any issue with my rear wheel. Taking this into account, I firmly believe that tubeless tires have a firm place on the road, but not on carbon rims if using rim brakes, and for steep descending with aluminum rims, preferably with disc brakes. I’d be curious to know how my Hutchinson tires would have held up to that heat, given their more robust bead construction.

Anyhow, I thought you might like to relay part of this episode to help riders understand the limitations of the tubeless design when using rim brakes.
—Matt

Dear Matt,
Thanks for your letter. I got these responses from Stan’s and Hutchinson regarding the rear flat you had that you attribute to heating.

From Pete Kastner at Stan’s:

We actually have had one of our employees and a few of our local riders race that same race.

It’s hard to tell without seeing the tire and rim.

I do not think it has anything to do with heat, but a more likely case is that the brake pads started to rub on the tire possibly only in one area due to a small amount of vertical hop in the wheel.

Once the bead of the tire is that badly damaged, it’s not possible to repair it.
Peter Kastner
System Manager, Stan’s NoTubes

From Stan Koziatek himself:

You would not generate that kind of heat on a descent on a gravel road.

My guess is he cut the sidewall just above the bead. If the Kevlar snapped, the tire would have gone flat instantly. He did say he had thousands of miles on the tire.
Stan Koziatek
Founder, Stan’s NoTubes

From a chemist at Hutchinson’s factory in Montargis who debunks tire failure due to heat and points to increased pressure as the sole culprit:

There is a well known law called Mariotte’s law (it was French) which says: P1V1/T1 = P2V2/T2 where P is the pressure in Pa (pascals), T is the temperature in K (Kelvin), and V is the volume in m3 (cubic meters).

As we can consider that the volume is the same, it gives: P1/T1 = P2/T2.

So an increase in temperature of 100 degrees Celsius will give an increase in pressure of 36 percent.

The maximum pressure for a tubeless (tire) is 5 bars [Perhaps this applies to cyclocross tubeless tires; maximum pressure for a road tubeless tire is 8.7 bars; recommended is 7.0 —Ed.]. If the initial pressure is 2 bars, you will reach the maximum pressure at 410 degrees Celsius (770 Fahrenheit). At this temperature the tire would have been burned since a moment (instantaneously).

So the pressure increase isn’t the most important problem. The first would be the fusion of the plastic tape put in the bottom of the rim so as to avoid the puncture of the tube by the spoke, which will come out near 100 degrees Celsius.
S. Labard
Hutchinson Pneumatiques

Americans generally call the law relating pressure and volume Boyle’s Law.

If there is a question of maximum tire pressure, Mr. Labard’s argument still holds even with different numbers. For instance, if Matt had been riding at 60 degrees Fahrenheit (288 Kelvin) temperature with 3 bars (43.5 psi) of pressure (pretty high with a 35mm tire), to exceed the road tubeless max of 8.7 bars would require a temperature of: (8.7 X 288)/3 = 835 Kelvin = 1,035 Fahrenheit. And if he had started with the same 2 bar Mr. Labard assumed, that would be 1,253 Kelvin, or a temperature of 1,796 Fahrenheight to get to 8.7 bars!

I think we can agree that none of these temperatures would have been reached; any of them would burn a pizza (or a tire) to a crisp. However, if it really only takes 5 bars to destroy a tubeless cyclocross tire, and he started with 3 bars inside, that would require a temperature of 404 Fahrenheit; I don’t know if that’s conceivable or not.

From Hutchinson PR:

There’s a lot going on in this email. The first thing I’d say is that using conversion kits to create “tubeless” systems using non-tubeless tires and/or rims has never been recommended by Hutchinson.

He refers to a Clement LAS tire, which isn’t tubeless, although the rest of his kit seems to be of the tubeless variety.

Hutchinson spent the better part of six years developing Road Tubeless and from day one has always recommended only using Road Tubeless tires paired with Road Tubeless compatible wheels. There’s far more risk of reliability trying to convert non-tubeless tires and wheels into tubeless wheels.

Not knowing much about the cyclist (weight, tire pressures, riding style, road surface seems extreme) also creates many unknown variables. We may be looking at no more than a situation of chance where the rear tire was cut at the bead by a rock or other sharp object and it was simply that which caused the tire failure. He’s using alloy rims so there’s not the consideration of carbon rims overheating.

Would Hutchinson tires have faired better in this scenario? Considering how different a Road Tubeless bead is in terms of materials, construction, and design than a normal bead on a tube type tire, I would venture to say yes. But cyclists are always pushing tires to their limits and with that comes the risk of material failure or puncture. Certainly the tire model would matter and the Sector 28 would be perfect (gratuitous product plug). Additionally, Atom, Fusion3, Intensive, and Sector 28 have a bead-to-bead puncture breaker to help support the casing and every Hutchinson Road Tubeless tire is production-line inspected by mounting, inflation, water submersion (checking for leaks) and then hanging mounted on a rack for 24 hours to test for slow leaks.

I think that from time to time cyclists believe tubeless configurations are puncture proof and all tires are indestructible until the tread wears through. Although highly puncture resistant considering their weight and structure, Tubeless tires still perform the same tasks as a tubed tire. Containing over a hundred psi with constant loading and unloading and torsional stresses using only millimeters or less of materials.

The tire is really the most amazing component on a bicycle.

Richard Goodwin
PR and Marketing Liaison, Hutchinson Tire North America

—Lennard