For the latest on SCS, skip ahead to this article right here.
Disc brakes for drop-bar bikes isn’t a new thing, custom builders have been making them for ages. However, the introduction of hydraulic disc brakes for drop bars the last few years has finally lead to industry wide adoption – every manufacturer now offers them on most road/CX/’gravel’ bikes, and some manufacturers have dropped rim brakes entirely from any drop bar bike other than pure road-race oriented bikes.
Discs on drop bar bikes has created some issues, especially at the rear end of the bike. Road bikes have traditionally used 130mm spaced rear hubs, whereas MTBs used 135mm or 142mm rear hubs. When the industry as a whole jumped into discs for ‘road’ use, they adopted the 135/142mm MTB standard. The wider hubs create packaging problems – the cassette is further outboard, meaning the chainline is no longer the same as with a 130mm rear hub. Specialized claims that Shimano would not sell road groupsets for bikes with road-race-short chainstays unless traditional road chainline was maintained, so they introduced a new hub standard called SCS (Short Chainstay System). Interestingly, no other manufacturer is doing this, even though they’re maintaining the same chainstay lengths, and Shimano is selling them groupsets – for example, Giant’s TCRs all have chainstays as short as 405mm, disc or rim brake, just like Specialized’s Tarmacs. I guess Shimano changed their policy.
So what is SCS? An SCS hub uses a 135mm width but places the cassette inboard to maintain the chainline of a 130mm hub. Why not just use a 130mm disc hub? Well, there’s fewer options, and it makes changing wheels more difficult…in theory. But, the SCS standard really screwed the pooch in that regard as well. Here’s the various issues:
QR bikes – Specialized makes a non-SCS hanger that allows the use of any 135mm 11spd road disc hub. This is pretty easy.
Thru-Axle bikes – use a 135×12 axle standard which is a bit rare. And, Specialized doesn’t seem to offer a non-SCS thru-axle hanger.
As soon as I publish this article I’m sure someone will say I’m wrong and show me pics of a non-SCS thru-axle hanger. I’ve also heard talk of a few aftermarket non-SCS hangers, but apparently some of those didn’t work, the cassette would hit the frame.
UPDATE Feb 2017 – I knew it! There is a thru-axle non SCS hanger, it DOES exist, and I have a handful in stock. These are OEM Specialized hangers, and will open up options to more hubs. Contact me for details.
The whole SCS system has caused a ton of confusion. For wheel builders, dealers, Specialized employees, and consumers. Supposedly SCS is going away. Hopefully so.
But, what do you do if you have an SCS bike and want a better set of wheels? If you have a QR rear end, get the non-SCS hanger and then any 11spd road disc hub you want. Easy. If you have a thru-axle, there’s two options really.
You can use a DT Swiss rear hub and a kit made by NEXT, which uses one 142×12 end cap and one custom end cap. If you use a DT hub with DT’s own 135×12 end caps the cassette will hit the frame, so the NEXT kit actually spaces the hub out to 137.5mm to avoid that cassette/frame interference so it spreads the frame a bit. If you go this route, if you eventually move the wheels to a non-SCS bike and change out the axle to a more standard QR or 142×12 you’ll have to re-dish the wheel. UPDATE :: Nevermind, looks like NEXT has quit making this kit.
If you want a true 135×12 SCS hub, you’ve only got one option – Hope’s SP24 SCS hub. As much as I love Hope hubs, there are some downsides to this particular hub. First, it’s a straight pull hub which I’m not a big fan of. It’s also a 6-bolt hub, so you can’t use Shimano’s very best rotors, and if you’ve got centerlock rotors you’ll need new rotors. And if you want to use Shimano IceTech rotors you can’t go smaller than 160mm, since Shimano doesn’t offer a 140mm 6-bolt rotor. But the biggest issue with this hub is that it’s a 24 spoke hub. I don’t often build 24H rear wheels, it’s just not enough spokes for most riders in most applications. 24H alloy rear rims tend to eventually crack under all but the lightest riders unless you use a really beefy rim. And 24 spokes doesn’t offer enough stiffness for heavier riders.
If you do want to use this hub, I’d highly recommend having it laced up to a Nox Composites rim. The reasons are twofold – stiffness, and spoke tension. The Nox rims are carbon fiber and much stiffer and stronger than an alloy rim, offering better durability and ride feel. And since they have an offset spoke bed, that raises the spoke tension on the low-tension side of the wheel, which improves durability and reduces the chances of the wheel going out of true.
Here’s an example. This is Andy’s Diverge, and I built him a set of wheels using the straight pull Hope hubs and the Nox Citico rims. We also added new 6-bolt IceTech rotors, and a set of Panaracer Gravel King SK 35mm tires. I set the tires up tubeless of course, and they measure 38mm on the wide Nox rims, pretty well maxing out the clearance in the Diverge frame. The wheels weighed in at only 1425g, and the wheel/tire package dropped 3/4lbs off Andy’s Diverge. Andy is a light weight guy, so the 24 spoke count combined with the stiff carbon rims will work out great for him. But if you’re a bigger rider, or don’t have the budget for carbon wheels, I would recommend buying a bike that does not use the SCS system.