The latest generation of Specialized bikes features two mounts with ISCG 05 spacing for chain retention devices. But with only 2 of the three mounting points as detailed in the standard, running a frame-mounted bash guard is not recommended.
The Stumpjumper EVO includes a chain guide to assist with chain retention out of the box, but if you’re concerned about chainring protection, this presents a serious challenge to mounting a bash guard or chain retention device with a frame-mounted bash guard.
As the Stumperjumper EVO features a low bottom bracket height — even in the higher of the two possible settings — after riding the bike on a range of trails, I’ve found myself smashing the chain and ring into a few obstacles.
I’m not clear on why the designers of these bikes made the decision to limit rider choices for chainring protection, it feels like a step backward in regards to frame design and usability, and the only real weak point in what otherwise is an excellent chassis. Bash guards are cheap insurance, and without the lower mounting point, none of the frame-mounted options are recommended.
While there are BB to ISCG adaptors, that wouldn’t make sense with two of the three mounts already on the bike, though if you were set on running a frame-mounted guard (like the models from MRP and E.13 pictured above) you could make it work. Your chainline would go all sorts of wack though, so I focused on researching and implementing a solution for our test bike.
Option 1: Raise the bottom bracket
The Stumperjumper EVO features a flip-chip that offers the choice of two bottom bracket heights. Setting it to the high position helped, but it’s still pretty low, and it would be nice if it were just a tad bit higher.
A few years ago I added offset reducer bushings to lower the bottom bracket of a bike that didn’t corner well. The same solution could be implemented and reversed to raise the bottom bracket.
Another option could be to install 165mm cranks (I already run 170mm) for additional ground clearance. This would help with pedal strikes, but the bottom bracket height wouldn’t change, and the chainring would remain vulnerable to impacts.
Option 2: A crank arm mounted bash guard
After a review of all the chain guides I’ve amassed over the years, it was obvious the three-hole models were a no-go. With only two of the three mounts, the upper two mounts would be receiving a significantly bigger brunt of any impacts of the bash guards and could result in damage to the frame that would not be covered under a manufacturer’s warranty.
Ironically, the only chain retention device I had that would work is a vintage MRP model, that features a throwback bottom bracket mounted backplate. However, it weighed much more than I was willing to add to the bike.
I am fond of how the bike currently rides, which makes these more of a nuclear option, so instead, I’ve shifted to evaluating crank mounted bash ring options.
Digging around the garage, I discovered I had a few laying about; unfortunately, none of them are compatible with modern 11 and 12-speed cranks. In the ever-evolving effort to reduce the weight of components, the 12-speed Shimano XT FC-M8100 crankset uses a direct mounted chainring, fastened by a lock ring to the crank arm. With no chainring bolts, there’s nothing to mount a bash guard to. Which isn’t a problem for most frames… provided they have proper ISCG 05 tabs.
Fortunately, I had recently acquired a Shimano SM-CD50 bash ring and a matching SM-CD800-I chain device to pair with it. Unfortunately, none of my go-to Shimano XT cranks would work, as the last three generations of the XT crank — 10, 11, and 12 — weren’t compatible.
In fact, after a review of the 2020 lineup, it appears the gravity-specific ZEE crankset (it appears the current Saint crankset also works) is the only option that still utilizes the 104 bolt pattern and can accept a bash guard in the location once utilized for the big ring of a triple. Fortunately, the ZEE FC-M640 crank exists, (they’ve available at Jenson USA for $100) though, at 906 grams, they do weigh a bit more than the components with the “trail” designation. (in comparison, the XT M81000 weighs 622 grams)
Fortunately for me, I’ve been holding on to a “vintage” SLX crankset with similar specs. I love this crank as it features many of the specs as the ZEE M640, but with less heft. What I love best about these cranks though, is the silver surface; this will continue to look good, no matter how much my shoes rub against it. (Shimano should bring this back!)
The SMCD bash ring only weight 32 grams, and it easily bolted to a Narrow Wide Race Face 104 BCD ring I had. The chainring I have is threaded and it made mounting everything up a cinch.
On the trail
As we’ve come to expect with Shimano engineering, the chain guide and bash ring combo works flawlessly on the trail when set properly. It was also quick and easy to set up.
The SMCD-800-I chain guide is especially impressive in terms of adjustability. Most chain guides include several sets of spacers that require trial and error installs to dial in the alignment and spacing to center the guide on the ring. The innovative Shimano solution features an integrated dial that allows precise adjustment and pairing with the SM-CD50 bash ring.
Honestly, it’s almost impossible to find documentation on this iteration of the Shimano Chain Devices, but I’m super stoked that the team at Shimano offers this solution. That said, if the Stumpjumper EVO just had a 3rd ISCG mount, none of this would have been necessary, so here’s to hoping it’s added to the new iteration of the Stumpjumper when it drops.
A brief history on ISCG
Before clutch derailleurs and narrow-wide chainring profiles were developed, most mountain bikes came equipped with multiple chainrings. It was a good day if you reached the bottom of a rocky descent and your chain was still on the rings, so many of us ran chain retention devices. At the time, they were still a gravity racing specific product, but we didn’t care about the weight, preferring performance. Sure we had to grunt up the hill on our 6″ travel bikes (at the time this was considered massive overkill, nowadays it’s just enduro) and our “heavy” tires with robust casings, but it was worth it for the descent.
Mounting a bash guard or chain retention device on mountain bikes used to be a major challenge. Spacing issues were the norm, and to installing a chain guide for the first time required stacks of spacers and a ton of patience.
The first reliable chain guide I recall ever owning was attached to the bike and held on using the bottom bracket. Installation always involved guess work, and if one only had to remove the bottom bracket up 2-3 times that was a quick install.
To simplify the issues with chain retention devices, the two main players at the time (MRP and Mr. Dirt) combined their efforts and created the ISCG (International Standard Chain Guide Mount) standard.
Of course, it wouldn’t be the bike industry if another standard didn’t come along a few years later to promptly replace the previously established standard, renaming the initial standard ISCG-03, or to keep things easy to remember, ISCG-OLD.
Our current standard, ISCG-05 was stronger and better defined to eliminate mounting issues with ISCG-OLD. Let’s hope it sticks around, because if it ain’t broke…