Tech Breakdown: How 135mm Rear Hub Spacing Affects Road Bike Chainline & Shifting

There’s no doubt disc brakes are well on their way to the road bike market. A few major brands like Colnago and Specialized recently introduced production models, Volagi built their brand with them from the outset, and disc brake cyclocross bikes are pouring in.

But what does all this mean for the other side of the wheel? We’ve covered (here and here) some of the technical challenges and concerns of using disc brakes on road bikes and the changes required for wheels. But the conversation’s been missing one critical element of the shift. Virtually everyone that’s making or planning a disc brake road or cyclocross bike has 135mm rear hub spacing on the drawing board.

That’s 5mm wider than what road bike frames have been designed for. 5mm wider than what decades of drivetrains have been designed around. Sure, that’s only 2.5mm per side, which doesn’t sound like much. Until you consider that Shimano only needed 1.8mm to add an 11th cog.

We spoke with the engineers and product managers at FSA, SRAM, Shimano, Specialized, Parlee and Volagi to see how this will affect chain line, shifting performance and heel clearance on what could very well be your next road bike…

CHAIN LINE & SHIFTING

One of the biggest concerns we heard initially was how the chain line would be impacted at the extremes, particularly when cross chaining from the little ring to the smallest cog. Granted, most of us know better than to ride like this for very long, but it happens. And when it does, the chain could snag on the big ring, causing a mis-shift, jammed chain or worse if you’re really cranking.

Above is a standard 53/39 Ultegra crankset and cassette on my Moots cyclocross bike. It has 135mm rear spacing and disc brakes. I put the camera’s focus at the chainrings (left) and cassette (right), click to enlarge.

What I found in this gear combo was that my chain was dangerously close to the big ring’s teeth (highlighted in yellow). Click to enlarge and you’ll see it better. You can also see how close the chain is to the front derailleur’s cage. It’s not rubbing, thankfully, but -and this is a big but- my ‘cross bike has relatively long 423mm chainstays.

Where this becomes a real issue is with race-oriented road bikes with very short chainstays. The Specialized Tarmac has 405mm stays for most frame sizes, for example. It’s such a potential issue that FSA has created and issued a technical advisory bulletin with the following guidelines to help ensure proper shifting and clearance:

At top, the chart shows OK’d chainring and chainstay combinations for standard 130mm hubs. Below it, the same thing for 135mm hubs. The increased limitations become pretty obvious with all that red.

And it’s not just the big ring’s teeth that could catch the chain. Shift pins are generally used to help guide the chain up during an intentional shift. But, get the chain too close and it could help out on its own.

While Shimano wouldn’t get into much detail with regards to product designs and research regarding this matter, they did send over their standard statement with regards to compatibility. Dave Lawrence, Shimano road product manager, had this to say:

“Shimano has always been a leader in bicycling component engineering. Part of that success is due to Shimano’s ongoing communication with frame manufacturers. With Shimano’s CX75 and R515 mechanical disc brakes for cyclocross and road bikes, Shimano is well aware of the industry-wide move to wider rear spacing for road and cyclocross bikes. While this does affect chainline, currently Shimano is not changing its 430mm chainstay length guideline for ideal Shimano component performance. Rest assured though that Shimano will continue to speak with frame manufacturers.”

So, what does everyone else have to say? I asked a few standard questions, answers from each brand follow. What’s interesting is that some of them don’t see this as much of a challenge, and others mentioned things we hadn’t even thought of.

For example, most of us assume road and ‘cross bikes are only switching to 135mm hub spacing to make room for the rotors. Maybe. Maybe not. As with many things in life, it depends on who you ask. Thankfully, we ask a lot of people a lot of questions. And equally thankfully, they don’t get tired of it! Here we go…

THE QUESTIONS & ANSWERS

We spoke with Omar Sisson (head mechanic) and Robert Choi (Co-owner) of Volagi and Tom Rodi (Product Manager) at Parlee before putting together our formal question list. Our conversations with them are transcribed further down. For these questions, we reached out to the companies mentioned and Colnago, but didn’t hear back from them. For the others, the replies are from Jason Miles (FSA, Engineer), Scott McLaughlin (SRAM, lead drivetrain engineer), Charles Becker (SRAM, road product manager) and Ty Buckenberger (Specialized, carbon road bike project manager).

BIKERUMOR: How was chain line and drivetrain function (ie. shifting, cross chaining, etc) affected by increasing the rear hub width to 135mm? What measures did you take to ensure proper performance?

FSA: One problem we’ve seen is when you have really short chainstays and a large difference in chainrings, like, say, a 53-39. If you hit a bump, the large ring can grab the chain and cause a bit of ghost shifting if the derailleur is trimmed in such a way that its cage isn’t rubbing the chain when you’re on the smaller cogs, too. It may also cause the chain to drop to a smaller gear a bit slower than you’d expect.

We’re working on solutions, and our chainrings will have to change. The chain rub you may experience right now won’t be there forever. In the meantime, we make the RD460 wheelset that’s a 130mm road disc brake wheelset. MSRP is $499.99 and comes in at 1830g. It’s been spec’d on one of the Redline Conquest bikes in the past, we’ve been making it since 2006.

SRAM: First, it’s worth mentioning that Road/CX bike rear hub spacing did not change from 130 to 135 to accommodate disc brakes. Bike frame and wheel manufacturers decided that they could make stiffer frames and stiffer wheels with 135 rear spacing. And since they needed to design new road/CX wheels and road/CX frames to accommodate disc brakes they decided to use the occasion to also change to 135OLD. That said, front and rear shifting are effected because the drivetrain is optimized for 130OLD. We have proposed a solution that we hope to implement soon.

SPECIALIZED: We’ve done considerable testing to insure that the shifting wasn’t compromised on our current disc bikes (Roubaix and CruX). The chainstay length on those two models helps. The small/small gear combination on the Roubaix with 135mm dropout spacing isn’t any worse than a shorter chainstay bike with 130 spacing.

BIKERUMOR: Are there certain gear combos that don’t work as well or don’t work at all in this set up?

FSA: See charts above.

SRAM: It depends on the bike frame dimensions but cross chaining while in the small chainring is affected most.

SPECIALIZED: All the gear combinations are available, although not necessarily recommended. Like most bikes the small/small gear isn’t ideal. It functions, you may get some chain/front derailleur contact in that combination. The same contact occurs on most performance road bikes with 130 spacing and short chainstays.

BIKERUMOR: How did that affect heel clearance, particularly for people with bigger feet? What measures did you take to ensure proper clearance?

FSA: It’s not really our department, but if the frame manufacturers don’t account for the narrower Q factor of road cranks, then it could be an issue. In talking with frame manufacturers, we’ve actually been warning them about this for years. If you look at DH bikes that have really wide bottom brackets, it can feel like riding a horse. That’s a no-go on road bikes where you need to maximize power output, ergonomics and aerodynamics.

From an engineering standpoint, we understand why some people want to add the extra 5mm to make up for the non-driveside space taken up by the brake rotor. But, when you ‘re talking about 700c (also 29er) wheels, that extra little bit of bracing angle given to the spokes is so minimal that you’re not seeing a huge gain. At least not enough to offset the potential problems with moving to a 135mm rear spacing on road bikes. You can’t have wildly different spoke angles from one side to the other because the tensions would be too different, so the closer you can get to equal angles and tension, the better wheel you’re going to have. With a 130mm hub, you’ll start out with enough space to add a disc and still have a plenty strong, stiff wheel for road and cyclocross use. I’m not saying a 135mm hub can’t be stronger, but I don’t think it’s necessary for these disciplines.

SRAM: (Scott) Heel clearance isn’t effected with respect to drivetrain. This is definitely a question for frame makers.

SRAM: (Charles) I think heel clearance is not really the question, as nothing changed. Heels usually touch the chain stays. The real question should be about the cranks (near the pedals) running into the chain because the 11t is further outboard. And yes this is a concern on frames with short chain stays and 135 OLD.

SPECIALIZED: Again, the longer chainstay lengths on these models really helps with problems like this.

BIKERUMOR: What other technical, performance or engineering challenges had to be overcome or considered due to the extra 5mm hub width?

FSA: There’s always the chance your derailleur won’t adjust outboard enough to accommodate the extra 2.5mm on the drivetrain side, which could lead to chain rubbing even on the big chainring/small cog combo if your front derailleur won’t move out far enough.

We could very easily add 5mm to our crank spindles and supply them with spacers, but that’s not what people want. They don’t want wider Q factors. We can’t release any info on what we’re working on now. Our new D10 chainrings have offset chainrings for mountain bikes that let you use a double front derailleur on triple cranksets (Editor’s note: you might do this when you replace the big ring with a bash ring or something, effectively making your bike a 2×10). This may not be the best direction for road, but we’re looking into it.

SPECIALIZED: The main challenges are shifting performance and heel clearance along with everything that goes into getting a new model into production.

A CONVERSATION WITH VOLAGI

There were road and ‘cross bikes with disc brakes before Volagi broke onto the scene, but they did quite a bit to cement the concept into mainstream riders heads. Their original Liscio started out with 130mm hubs, but the second generation and the new Viaje steel road bike both have 135mm hubs. They also provide their own wheels, and shown above are their new 11-speed freehub bodies (right). Their splines are exactly 2mm longer than the 10-speed version, thanks to a slightly longer body and deeper machining at the back plate.

In ensure the cassette cleared the spokes, a 1mm taller spacer was required (right) to push the freehub body out from the hub shell slightly. It might seem like we’re getting off track here, but this illustrates that the new crop of 11-speed drivetrains are going to push the smallest cog even slightly further out on some bikes and wheels. So, in some cases, the change might be equivalent to another .25 millimeter or more on the drive side. Here’s what Omar and Robert had to say on the whole thing:

The chain line hasn’t really been an issue. It may have more to do with idiosyncrasies on the frame. In a perfect world, you’d make the adjustments to the design so that you’re only making 2.5mm movements on either side of the hub. So you’re only moving the chain line 2.5mm out, but the frame center line has to be perfect and that’s not always the case.

We’ve looked at a broad representation of derailleur and chainring combinations. That includes the FSA BB386 cranksets with compact and cyclocross chainrings, and the usual range of cassettes from SRAM and Shimano, particularly the ones we build bikes with.

We started to talk with FSA about it, but as we started building bikes and testing things, we didn’t have any issues. It hasn’t been a problem for us. We use KMC chains, if that matters, and so far we’ve only built up bikes with 10 speed drivetrains.

Sometimes it’s more of an issue just switching bottom brackets, but we haven’t noticed any issue even at the extremes of cross chaining. Or at least any more than you’d expect. (Editor’s note: Volagi’s bikes tend to have longer chainstays since they’re aimed at the endurance crowd).

We haven’t tested it with Shimano’s 11 speed Dura-Ace yet, but with our own hubs, we’re redesigning our freehub bodies and taking up that space.

We did have to make some adjustments to the brake mounts to improve heel strike clearance, mainly by moving the hose to the inside of the chainstay. And, for the hydraulic brakes, TRP got rid of the banjo and ran the hose directly into the caliper. For mechanical disc brakes, folks with larger feet (size 11 or 12 and up) could rub the dial on the Avid BB7 mechanical brakes a bit, but models from Hayes and others seem to alleviate this, even the BB5 Road calipers have better clearance because they don’t have that outside cap.

OTHER THINGS TO NOODLE ON

WHY NOT CHANGE THE BOTTOM BRACKET WIDTH?

Road bikes have used 68mm bottom bracket shells for ages, and even the newer BB30 standard uses that shell width. Modern crankset/BB standards like PF86 and BB386 have a wider shell, but the bearings are all internal, allowing for a generally similar Q-factor as bikes from thirty years ago.

When mountain bikes gained steam, many manufacturers settled on a 73mm bottom bracket shell while still using the outboard bottom brackets. Conveniently, this was 5mm wider, matching up nicely with the 5mm wider hubs, which worked out really peachy when disc brakes came on the scene.

So, why not just stretch the BB a bit on road bikes? Well, as Jason at FSA mentioned, roadies don’t want to feel like they’re straddling a horse. A wider stance just wouldn’t feel right on a road bike, physical performance could likely suffer and aerodynamics would definitely suffer. Maybe only a little, but heck, if we’re spending thousands on wheels and frames that promise a few watts or seconds over 40km, keeping things narrow is like free speed.

HOW MUCH DOES 2.5mm REALLY AFFECT HEEL CLEARANCE AT THE CHAINSTAYS?

As someone who’s personally worn through too much of his carbon fiber chainstays to feel comfortable, any amount is too much. But, if you want to put it into perspective and see if you’d be affected, tape two dimes to your chainstay where your heel passes it and see if they’re still there after a long ride. Or just look at your chainstay. If you see wear marks and scrapes that align with your heels, you’ll want to really pay attention to how this shakes out.

COULDN’T I JUST COMMISSION A CUSTOM BIKE AND HAVE THE CHAINSTAYS TUCKED IN?

Maybe, but let’s see what Parlee has to say. After all, they just introduced their new top-of-the-line Z-Zero with a disc brake option that’ll have (yep, you guessed it!) 135mm rear spacing. Here’s our conversation with Tom Rodi from the Massachusetts based custom builder:

Our feeling is that there’s nothing that’s really going to be a nightmare. It’s only 2.5mm per side, it doesn’t seem like it’s going to affect chain line too much. But, we do think with new 11 speed drivetrains, people are going to have to pay more attention to minimum chainstay lengths…but that’s not going to affect us too much since we don’t believe in super short chainstays.

There’s a little more of a kink in the chainstays on the new Z-Zero chainstays to accommodate heel clearance. And, on the Zero, we’re also trying to accommodate for wider rims.

We’re fortunate in that many of our customers will send their bikes back in after five to eight years to be repainted, so we can see wear patterns.

If you came straight back with the chainstays then made a severe bend out toward the dropout, it wouldn’t really solve the heel strike issue. Most of the wear we see is at the back 3/4 of the stay. One thing we did have to tweak over the years was moving the cable stop for mechanical groups closer to 6 o’clock.

In testing, it hasn’t been an issue, but like everything else, once we get more real world data, we’ll adjust as necessary.

WHAT DO YOU THINK?

Do you have heel rub issues on your current bike? Do you have a disc brake road or cyclocross bike? If so, how’s it shifting?