Why SRAM believe hydraulic brakes are the future

SRAM confirmed a few months ago that they were working on new hydraulic brakes for the road and they’ve now gone public with their new designs, one operating on the wheel rim and one disc version.

Hydraulic brakes have featured in mountain biking for some time and they’re now making headway in the road market too. A year ago, Cervélo specced Magura’s new RT8 TT hydraulic rim brakes on their P5 time trial bike and Team Garmin-Sharp riders have used the same brakes on their road bikes on many occasions. Plus, leaked photos have shown that Shimano are working on hydraulic discs too. 

We sat SRAM’s Product Manager Paul Kantor down and asked him about the development of their new designs…

So SRAM are finally launching the long-awaited hydraulic brakes. What’s the thought behind the new designs?

We liked the idea conceptually of putting disc brakes on road bikes but we didn’t know if they’d ride well and be any improvement on what we already had, so we made a hydraulic coupler [built into a stem] to standard mechanical levers, put it on a steel cyclocross frame and used it for four or five months.

I used to commute to and from work everyday with it and took it on my weekend rides and established that disc brakes lived up to the advantages that we had in mind. But the coupler was ugly and couldn’t be made shorter than 110mm so we shelved that idea and said that the system had to be fully integrated.

We spent a lot of time working out whether the smaller contact patch between a road bike tyre and the road [compared to a mountain bike tyre and the trail] makes a prohibitive difference. We quickly decided that we liked the idea and got to work on the engineering principles and on additional testing in the real world.

People often say that, when it comes to road bikes, it’s not stopping the wheel that’s the problem but preventing the tyre from sliding on the road when you brake hard. What would you say to that?

That’s not incorrect. There’s definitely some truth in that. I can certainly lock up my wheel with a carbon rim and a mechanical rim brake, but if I do that I’m still just sliding down the road and that sucks. Locking up the wheel isn’t hard but if that were our goal we’d just sell you a stick that you could ram in there and you’d be done!

Everything we’ve focused on is prior to locking up the wheel. We’d argue that with both of these hydraulic systems you can take off more speed with greater control prior to locking up the wheel. Some people refer to that as ‘modulation’. That’s a term we don’t really care for, but it’s fine. We’re really talking about decelerating substantially without locking up the wheel. Beyond that you get into tyre/Tarmac territory and that’s not where we at SRAM operate.

With the hydraulic rim brakes feeling so powerful for such little effort at the lever, why would people want to go for disc brakes on the road?

Our hydraulic disc brake has a higher braking force at every lever force than a mechanical brake on an aluminium or a carbon rim, and more than our hydraulic rim brake. You can provide quite a bit more force for less hand effort and that’s really what we like most about hydraulics.

We think that Red mechanical and Shimano Dura-Ace mechanical brakes are pretty comparable, but with a hydraulic rim brake you are able to exceed that braking performance. On a disc brake we can create even more force for the same lever effort. It’s much more consistent wet and dry too because we are braking on a steel rotor that’s consistent time and time again. That’s really where discs come into their own.

CEN [European Commettee for Standardization] requires that there’s not more than a 20% drop off between wet and dry on a rim brake and we improve that substantially on a disc brake. It’s more like a drop off of 5-8% in bad conditions. Plus, it’s a sealed system that’s consistent over time.

You can run a rim brake engaged at about 550W for 6mins and you’ll burst the tyre. Admittedly, you don’t see that a lot out in the field, but you do see it. You can run a disc brake at 900W for 11mins and the brake doesn’t boil and the tyre doesn’t burst.

Once you start adding up all these testing elements you start to see more and more opportunity for a disc brake to exceed what’s already out there.

So why not just go for discs, then?

There are millions of bikes in the market that have single-bolt caliper brakes. The rim brake allows someone to take their two year old Colnago and bolt on some new technology and move things forward, so that serves the existing customer.

We really like the way the rim brakes ride and we’re all about choice too. We like to put a couple of good options out there to allow the customer to choose what they want.

We may find eventually that hydraulic rim brakes do win out over hydraulic disc brakes, although I don’t think that’s going to be the case. I don’t think that discs will kill the rim brake, I think we’ll see the two options, but I do think that mechanical cable-operated brakes will go away.

People are sometimes worried that disc brakes will overheat and fade. Did you have to do a lot of work to make sure that doesn’t happen?

Where you might have trouble is with some big guy riding down the Stelvio for 45mins dragging the brake, and we were worried that we’d have boiling issues there. But what happens is that the brake reaches a steady state where the heat isn’t increasing. It really wasn’t an issue.

To prove that, we put a pressure transducer in the line and took brake temperatures – so we were measuring the brake temperature relative to the pressure – and hooked it all up to a computer. We never boiled the system.

What we did see is that when we decelerated from 50-60kph down to 15kph in 1-2sec increments on a switchback descent there was friction fade where you’d lose some of your coefficient of friction in the pad. That is much better than having the system boil.

We could induce this on a 140mm rotor so we went away from an aluminium backing surface on the pad and moved to stainless steel which dissipates that heat way better. So now we have a very small window where you could induce some friction fade if you were really trying to do it but we have had next to no issues with boiling the system. We were really surprised.

So, how do we decide between 140mm and 160mm rotors?

We recommend 160mm rotors front and rear for road use and 140mm is fine for cyclocross. We have tested 140s extensively but we like the margin of safety that 160s offer for the road.

What brake fluid do you use?

We use DOT 5.1 fluid, which has the highest boiling point of any DOT fluid. Making it is highly regulated so it’s consistent around the world, which we like. It means that if you bleed your brake in Thailand the fluid is going to be the same as if you did it in New Zealand.

Do you see any benefit in further developing mechanical disc brakes for the road?

We made the mechanical road brake [at Avid] 10 years ago and it was so far ahead at the time that there were no bikes for it. I think that a good mid-price mechanical has a home. Some people just like the cables so I think there’s a place for that.

Hydraulic disc brakes will come down in price over the next 3-4 years to a 105/Rival level. Then we’ll have to decide whether to make a fancy mechanical disc brake or whether we can push the hydraulic technology down further. That’s undecided right now. We have to see how the market reacts.

For more info on SRAM's new hydraulic brakes go to our story on the complete new groupsets.