Disc Brakes Are Coming to Road Bikes, But What About Wheels?
In part one of our series about disc brakes coming to road bikes we explored the challenges and changes of making brakes work for the different conditions road bikes will put them through. Now it’s time to talk wheels.
To get the skinny, we spoke with the following folks:
- Bill Shook, founder of American Classic
- Adam Marriott, category manager for Easton
- David Ripley, technical PR manager for Zipp
- Paul Lew, director of technology and innovation for Reynolds
- Paul Guebarra and others from DT Swiss’ U.S. offices
- Brian Roddy, president of Rolf Prima
UPDATED: Added comments from Rolf Prima.
While there is definitely some consensus on a few topics, there are also some differing opinions as to what the exact benefits will be and how wheels can best provide them. Fair warning, there aren’t many photos to gussy up this post, but the info is compelling if you’re anxiously looking forward to the day when you won’t need rim brakes on the pavement. Jump on in for the answers, listed in alphabetical order by brand…
BIKERUMOR: What changes to the rim will disc brakes allow for on road bike wheels?
AMERICAN CLASSIC: Rims don’t need brake tracks with disc brakes. This solves the heat build up problem with carbon and aluminum rims (more important for carbon). Heat build-up in carbon has been a big dilemma and most people know about possible carbon delimitation because the resin can soften and the rim can deform. This is a particular problem with carbon clinchers where the tire is pushing against the sidewall. If it happens unevenly, the bulge will drag the brake pad more, heating that area more and it becomes a self energizing problem. Even with tubulars there’s radial pressure and the rim can still deform from the heat build up in the resin. That’s why we don’t do a full carbon clincher. The people that most experience heat problems with alloy rims are tandems. Heat build up on the rim can cause the tire to expand and they can blow off the tire. People have developed drag brakes (drums, bands, etc.) for tandems to solve this at some level.
DT SWISS: The situation is similar to what has happened with MTB rims, they got lighter as the need for a braking surface was eliminated. There is hopefully more freedom of design such as the push for even more aerodynamic rims and more potential use of carbon rims without any drawbacks.
EASTON: Rims should get lighter and more aerodynamic by moving the brake from the rim to the hub. This will allow for rim shapes that are designed without the constraints of the brake forces or limitations of brake widths.
REYNOLDS: Disc brakes will allow the rim manufacturer to use lower Tg (Transition Temperature) resin systems. Lower Tg resin systems have the theoretical potential to lower the cost of the rim by 10%. However, manufacturing “special” disc brake rims (at a presumably lower volume than rim-brake models) may not produce the resin volume pricing necessary to benefit from lower pricing, and the effect may be an undesirable cost increase due to the purchase of a lower volume of resin for “special” disc brake rims. In the end, the net result is probably no change.
ROLF PRIMA: I think adoption to disc on road will be slower than MTB. With MTB, the benefits were clear and it still took a while. On the road, some applications will benefit and others not so much. As it happens, we will be able to shift to lighter brake walls and smoother shapes. This will trim rotating weight and allow more freedom in rim shapes. From a design standpoint, that opens a lot of doors for us on the rim, but produces some other challenges at the hub.
ZIPP: For now, we have not changed the layup from current design. We will test the design, once built on the disc platform, to determine what changes can be made. Initially maintaining the impact protection and overall durability proven by this design is paramount.
BIKERUMOR: How will disc brakes affect hub design? Do you see thru-axles and 135mm spacing becoming part of the design?
AMERICAN CLASSIC: American Classic has had disc hubs from the beginning. We have a proven system for disc hubs that is easily transferrable to the road. Fortunately the road bike makers seem to be going to 135mm spacing for disc which means that the hub technology for mtb will cross over to road. American Classic has thru axle technology already so we are ready to go when the fork makers decide what they are offering. It will depend on the speed of wheel changes. QR is probably going to be adopted faster than T/A. With road bikes, there’s an opportunity to reposition the disc caliper to a more appropriate place. With it mounted on the back of the leg right near the dropout, it tends to want to spit the wheel out of the drop outs. With road bikes, if you put it in front of the fork or changed the angle of the dropout so the forces that are put onto the axle will push the axle back into the dropout.
DT SWISS: The easy part of this line of questioning is the 135mm rear spacing. It doesn’t make sense to stay with 130mm designs. We all agree here at DT Swiss that using mtb length 135mm wide hubs are the easy button here and the natural progression. But, we could see something else become a standard much like 142mm has been adopted the past couple of years on the MTB side. For now, the easy part is using 135mm.
Thru axle designs? Well… we already have a 9mm thru bolt that customers can use to convert their 240s front hubs for standard open dropouts. On the rear wheels, why not? We push people all the time to convert out of standard QRs and instead use our RWS thru bolt system on 135mm rear wheels. It is the same weight unless someone uses some trick Ti axled QR or RWS. I think you want riders to “feel more stiffness.” The truth is that most of the arguments for increased “stiffness” in bottom brackets and frames and such are a manufactured feeling for most riders driven by marketing companies and media. Most riders don’t ride as hard as pro guys do to need or even know the difference in using a BB30 vs external BB cups. Having someone use a thru axle design is something that CAN be felt more so than the slight incremental “increases” in stiffness or bottom bracket changes. It just translates into faster response to the pedals.
How does it affect hub design? I think it puts even more importance in the need for a great hub. Our whole attention at DT Swiss in our wheels starts with our hub design. Anyone can throw a rim on their hub but the heart of wheel is the hub, not the rim. You will need to make stronger hubs. You won’t see disc hubs in the 150-200g range for a set. You will need to make the disc brake interface stout. The importance of a great hub that is going to last with easy maintenance is key. Durability- much like what people want to see on the MTB side.
EASTON: I don’t think thru-axles will be necessary on road bikes; thru axles solve a couple of problems which are specific to bikes with suspension; rear triangle stiffness and front wheel ejection under braking forces. Rear triangle stiffness is a much smaller problem with seatstays and without suspension linkages. I think we will see road forks with forward facing dropouts to help counter braking forces.135mm spacing does make the wheel stronger, not so much because of the brakes but because drivetrain manufactures keep adding more cogs which push the drive-side spoke flange further toward the center of the hub, this make the wheel less stiff and weaker, changing to 135 from 130 helps get back some of that stiffness/strength.
REYNOLDS: The increased load on the hub and axle may be better accommodated with a thru-axle design. 135mm spacing is a benefit to all rear wheel design, disc or non-disc. Generally, a disc brake system will require an increase in hub strength and mass.
ROLF PRIMA: For road, thru axles are overkill. Look at today’s front hubs. They are very light – only the essential material. To make a place to bolt a rotor on, hubs will get heavier. I think we can minimize it, but not eliminate it. This will eat up some of the weight savings from the rim. Our goal will be to make the hub as svelte as possible.
135mm is not necessary for disc although we like the idea of 135mm for other reasons. It would give us more space to add bracing angle on the drive side as Shimano joins Campy in going to 11 spd and moves the freehub body further inboard. We already have the best bracing angle in the business due to our patents, but we are greedy when it comes to stiffness and we always want more bracing angle and ever more stiffness, 135mm would give room for that. At 130mm with all the space the 11spd cassette takes up, the drive side spoke bracing angle is very steep. That all said, I think any transition to 135mm will be slow and a little bumpy. We have proto’d it already and we’re at the ready. Bring it.
ZIPP: Yes, 135 spacing will be the platform of choice. This will allow us to maintain optimal lateral stiffness, which is a consistent “ask” from our consumers and professional athletes alike. Thru-axles will certainly be something that should be explored over time. But, for now this is a road platform which dictates weight and overall size constraints will need to be kept minimal. Overall hub design changes will probably be dictated by the marketplace over time. With our experience with mtn bike hub systems we have a ton of data to work with in regards to heat build up/dissipation as well as energy force displacement. Time and market demand will probably dictate whether these hubs are more “road hubs converted to disc” or Mtn. bike hubs converted to road”.
BIKERUMOR: What about the wheel as a whole? What changes are we likely to see?
AMERICAN CLASSIC: Radial lacing does not work with a disc hub because the hub has to transfer torque to the rim. So the wheels will need higher spoke count and cross spoke lacing, otherwise the spokes will get pulled out when braking.
DT SWISS: Lighter wheels overall. The wheel will be heavier at the hub than current road wheels, but the advantage is better potential braking performance with less of a chance of warping carbon rims or blowing tires due to heat issues.
EASTON: Again rim shape can see major changes with the switch to disc brakes. I think there is a need for manufactures to rethink how we mount rotors to the hub as there is room for improvement in this area.
REYNOLDS: The spoke pattern will change. Crossing spoke will be required on both front and rear wheels. The aerodynamic impact will be undesirable with the loss of radial spokes on the front wheel.
ZIPP: Truthfully probably not a lot. Weights will climb slightly, but this will also allow us to work with different seal systems and different geometries to maximize the performance for Cyclocross and road. The different hub geometries will likely dictate different spoking patterns resulting in slightly different ride qualities or overall stiffness properties. But, none of which should be toward the lesser.
BIKERUMOR: Do you expect the wheels to be lighter? Stiffer? More Aero? Or do you see things going the other way?
AMERICAN CLASSIC: Disc hubs are heavier, there are more spokes. They will be stiffer but heavier. Removing the brake track from a rim is not a great weight saving. Disc rotors are not aerodynamic.
DT SWISS: All of the above. There can be more design than before. We potentially see more rim choices based on riding style much like MTB rims now, multiple widths and depths, but lighter all the way around.
EASTON: In a word, Yes. Moving brake forces to the hub opens up possibilities for gains in performance in every category. I’m not sure the wheels as a whole will get lighter but I see the rim getting lighter which will improve the ride quality and acceleration by moving some of the weight from the rim to the hub.
REYNOLDS: Generally, I think the wheel will be stiffer (due to the structural requirement to accommodate the disc brake system), heavier, poorer aerodynamics.
ROLF PRIMA: We are going to keep innovating and making our wheels more aero like we have over the past several years where we really revamped. Disc brakes will give new options, but it will take good engineering to make disc wheels more aero and as light. We’ll be there.
ZIPP: Lighter, we don’t know yet specifically as we will be adding some weight to the hub system, but may be able to offset with rim layup. Too soon to tell. Aero, about the only way that this would affect aero properties would be the ability to make rims wider by delimiting brake caliper limitations. But, that will take quite a bit of development and “tunnel/CFD time” to alter rim profiles accordingly to prove out the advantages. Could be fun! Certainly we can expect some stiffness gains on Cross type builds with 2x spoking patterns. As for road wheels with an aero focus maybe we would expect about the same as current.
BIKERUMOR: What do you see as the primary challenges in designing road bike wheels around disc brakes?
AMERICAN CLASSIC: It’s easy for us! It’s pretty much mixing and matching the right parts. You do need a rim strong enough to keep the spokes from ripping out now that all of the braking forces being transferred through them. We have prototypes working their way through our testing system and will show them at Eurobike and Interbike in Sept. 2012.
DT SWISS: Making the wheels will be the easy part, agreeing on some standards such as hub spacing and rotor diameter will need to be sorted out. If the frame does not increase to 135mm, your rotor clearance with the seat stay/chain stay will be an issue. Carbon frames can mold around the rotor/frame interference, but all others will be limited to 140mm rear rotors and some 160mm rotors. Also, if road cranks remain with the same spacing as current designs but the rear moves to 135mm spacing, you will have issues with small ring/smaller cogs usage. The chain will rub the big ring and in some cases hit the pick-ups on the chain ring causing the chain to possibly “jump” and be dangerous. Also the need to control the heat. How do you combat the heat from long descents?
EASTON: I think it removes several challenges by removing the constraints from the shape of the rim.
REYNOLDS: I think the challenge will be convincing the competitive cyclist that a disc brake system is worth the trade-off for increased weight, and poor aerodynamics.
ROLF PRIMA: Compatibility. When MTB’s transitioned, there was a period of time when frame attachment was not set, when lateral spacing was not set etc. In the late ’90’s Rockshox (remember those?), Formula Italy, Hayes, etc., all had different disc spacing. Hopefully we, as an industry, have learned from that and apply that lesson here. Also retro-compatibility. There are a lot of 130mm, non-disc bikes out there and we’ll need to support them with wheels but then they’ll be new disc brake bikes – 130mm or 135mm and we’ll need to support them. The fewer iterations the better for everyone – manufacturers, bike shops and riders.
ZIPP: Frame manufacturers! Just look at the current “standard” for BB systems. Nuff said.
BIKERUMOR: Are there different challenges for creating aero wheels versus standard or ultralight wheels with disc brakes?
AMERICAN CLASSIC: Disc technology is not aerodynamic. There is too much stuff sticking out of the side of the wheel (rotor, rotor mount, screws, caliper etc). Disc wheels are not ultralight either. They have to be strong (heavier) to withstand the torque of braking so that the spokes don’t pull out of the rim.
DT SWISS: Controlling the airflow with keeping the weight low. Generally the more aero the heavier the rotating mass, keeping the weight low and still achieving aerodynamic properties is the balancing act. Not all road markets will need disc brakes. There may be no benefit to TT and tri bikes with disc.
EASTON: Ultralight wheels will be more difficult as you have to add more material to the hub than you can take away from the rim. However you won’t notice the addition to the hub in weight like you would at the rim. Aero wheels will have to deal will tangential spokes vs. radial spokes which have a little more drag, however improved rim shape should overcome this and more.
REYNOLDS: In general, disc brake system wheels are inherently heavier and less aerodynamic. A disc brake system wheel will not compete with a rim-brake wheel for low mass or aerodynamic performance.
ROLF PRIMA: Sure, but we’re going to make superlight and aero wheels either way. That’s why we have great engineers, pros who test ride prototypes and a big testing budget. Having a rotor at the hub is not very aero, but at least diameters are likely to be small (in the front at least). We’ve spent the last few years really dialing in the rim sidewall to brake transition to smooth it out for aerodynamics so I don’t think we’ll see a quantum improvement in rim aerodynamics because of the switch to disc brakes. The challenge will be to not go backwards aerodynamically. As for weight, a rotor, bolts and mounting location are not going to come for free – weight-wise. Remember, we’re still talking clincher rims and high pressure tires so it’s not like we can really hog out what used to be the brake walls. That material was always doing dual duty – rim brake wear and retaining a tire. So we’ll save weight there but nice rims aren’t exactly fat in that area already. We’ll save weight in the rim and I think it will be reasonably close to offsetting with discs being a little heavier. It’ll come down over time.
ZIPP: As with any product that has multiple usage opportunities, there is no “one size fits all” design to optimize everything. As with our design focus now, we have three criteria that we focus on: We Optimize, Minimize or Maximize, but all within the optimal operating balance of the wheel. One trade off for minimizing weight, will take away our ability to maximize impact performance, and so on.
BIKERUMOR: When it comes to braking performance, what parts or design characteristics are most important?
AMERICAN CLASSIC: Handling the heat build up in the rotor is the most important part of the design. Overheating the rotor will cause the brake to fail. Road bikes go faster than MTB’s on a descent. If small, skimpy lightweight rotors are put on road bikes, then there is nowhere for the heat to go. On a DH bike, they have huge, heavy rotors to get more leverage and to dissipate the heat. Effort is made to keep heat from traveling down into the hub, so there’s really not much the hubs can do to mitigate the effects of heat build up. Heat at the hub would cause the bearing bores to grow, melt the grease and do other bad things. It’s much better to catch the heat at the rotor and spread it into the air. That’s why rotors are designed the way they are with spindly little arms.
DT SWISS: Strong spoke and hub interface to transmit the torque forces are important. Modulation performance is quite important when compared to power. Maybe ultra small rotors are not the way.
EASTON: Torsional stiffness is extremely important to braking feel, you can no longer use radial spokes as the brake is moved from the rim to the hub.
REYNOLDS: For a disc brake system, the hub and spokes are most critical. For a rim-brake system, the performance is all about the rim and the pads.
ZIPP: We have data-shared with our mountain bike counterparts within the SRAM team to “learn” which parameters need to be monitored most closely as we move into this. Disc tabs are certainly one area of intent focus, as they are the main stress carriers in the system. We cannot overlook function in our pursuit of form. We want to make a sleek and lean road racing hub, but it still needs to be as strong and durable as a mountain bike hub. Either way, this is a fun time to be in the bike industry as we get to play with new technologies and maybe even learn some new tricks.
BIKERUMOR: Anything else you’d like to add?
AMERICAN CLASSIC: Disc brakes make sense for mtb and cyclocross bikes because of the muddy, dirty environment. Rim brakes get clogged with mud and dirt, and then the rim gets destroyed from the dirty brake pads scratching off the rim surface (like high grit sandpaper). Road bikes don’t usually face mud and dirt build up on the braking interface. Disc brakes may work well on road bikes in very wet conditions however you may not want powerful braking in the wet because the reduced tire/road adhesion (sliding!). Wheels with rim brakes will always be lighter than disc brakes so I don’t think disc brakes will be put on all road bikes. We will see how this all shakes out.
DT SWISS: An added benefit is to be able to switch between wheels on a bike without changing pads (like going from carbon to aluminum rims) or adjusting brakes. And, I think that you have to change the mindset at least early with riders until the technology catches up. We have to have the message that lighter is not always safer. Get the right equipment for the right application. We constantly have that discussion with customers looking for the lightest combination of product. DT Swiss can make anything and shave tons of weight off but we choose not to so we can enhance the customer experience. We are in a discussion about the majority of riders not pro level riders. Pro level riders have the ability to use just about anything but the majority of people just want to go out and mirror what these guys are doing without the ability.
EASTON: We are really excited for more disc brakes on road and ‘cross bikes because better brakes give you the confidence to go faster.
REYNOLDS: I am a big fan of disc brake systems on all bicycles other than competition Road, TT, and Tri bicycles. Road, TT, and Tri bicycles will be resistant to embrace disc brake systems due to the increase in mass and poor aerodynamic performance.
ROLF PRIMA: Bring it on!
ZIPP: Just to recap, with the UCI broadening their horizons and allowing some new technologies in the sport, we are getting the chance to think outside our boxes and maybe learn some new tricks. Whenever you have the chance to redesign a product you get to start with a clean piece of paper (or at least you should, right?), allowing the opportunity to maybe do some things differently than current SOP. We may just find a way to change the game again… Time will tell, right?