If you'd like to know more about the Behind the Numbers
series, aren't familiar with the terms being used or want to know why we're doing it then check out our Introduction article
for all the information.
First up in our Behind the Numbers series is the Specialized Stumpjumper EVO 29.
The Stumpjumper EVO uses a familiar suspension layout from Specialized, but with quite an unfamiliar geometry. The bike is a big departure from their usual conservative angles and lengths.
Stumpjumper EVO Analysis Details
Travel Rear: 138mm
Travel Front: 150mm
Wheel Size: 29
Frame Size: S3
COM Height: 1150mm
Chainring Size: 30T
Cassette Cog Sizes: 50T, 24T and 10T
Despite its short travel, when compared to the other bikes in the enduro category, it should gain buckets of stability and inspire confidence from its truly low, long and slack geometry.
The Stumpjumper bikes use shock extenders, which are a way of actuating the shock from a linkage system further away. They also enabled Specialized to maintain an uninterrupted seat tube, as the shock extender straddles the tube. However, these things seem to be growing in size across lots of brands, and while Specialized doesn’t win the award for longest extender, it’s still 69% the length of the shock.
While they have ditched their proprietary mount between the shock and extender, the shock extender is still going to increase the leverage on the shock shaft and internals. A bigger shafted air shock is going to help durability issues here, while adding vital adjustment options to the bottom out resistance. Sadly, shock manufacturers are now having to design for these increased side loads of MTBs, and hardly anyone seems keen to use spherical bearings.
Does the departure from the norm continue in the suspension? Have they added features to match the upped aggression of the geometry? Let's start to look closer at the characteristics of the suspension, beginning with the leverage ratio.
The Stumpjumper EVO has 9.5% progression in the high position and 9.7% in the low position, with an average ratio of 2.76 in both settings. It has a linear to progressive to linear curve.
Having the progression percentage under 10% is going to push the rider into the predicament of which to prioritize – small bump sensitivity or bottom out resistance.
Setting the bike up with a normal amount of sag will result in good traction, as the suspension is not compressing a massive spring. But the linearity of the leverage ratio means that the bike will use excessive amounts of travel on features of the trail that don’t require that much travel, including inputs from the rider. Pumping the bike will result in more energy going into compressing the shock rather than maintaining forwards momentum.
Getting to the end of travel with this shock setup is going to be possible without too much hassle, especially with the final portion of travel becoming very linear indeed. And given that the carbon models come equipped with a coil shock, this also won’t help out here. The coil shock leaves no adjustment options for the end of travel like on an air unit.
This low progression coupled with an already low BB will probably bring some pedal clipping problems in chunky terrain and miss-timed pedalling scenarios.
Alternatively, the rider could prioritize not bottoming out all day long and over-spring the bike to save their ankles. This will, however, result in less small bump sensitivity and will make the bike ride dynamically higher. Given the really low BB, this might not be too bad. There’s definitely room to move in this direction without adversely affecting the geometry. How much over springing is required will depend on the aggressiveness of the rider.
Despite the long links of the Specialized layout, which give smooth curves for the acceleration responses, the shock is driven off quite a small link, and so this creates some subtle bumps in leverage ratio curve. Smoother curves generally translate to more predictable bikes, so there could be some room for improvement here. The overall size of the bike and the smaller amount of travel could go some way to masking these shapes in the leverage ratio curve.
Moving the chip to the high setting gets rid of a touch of that end stroke linearity, but now introduces a bit of regression at the beginning of travel. While it’s not at all a big amount of regression, 0.7%, it is taking a step down the road of potentially creating a problem for the damping continuously going back and forth over this hump and the changes in shaft speed that it brings.
All gears in both chip settings are under 100% anti-squat, and around sag it’s hovering around 80% and then drops off quickly.
Coming back to over springing, if this is the case, the bike would be running less sag and so have a touch more anti-squat as you'd sit higher in the curve.
The linearity of the leverage rate, the high overall ratios, and now the always below 100% anti-squat are going to result in a very active package.
Pedal kickback should never be a complaint on this bike, but it's a belief that good anti-squat numbers are more important. This is a common trend among many of the longer established companies. Pedal kickback probably got prioritized at some point in the past, and the anti-squat suffered as a result. It’s a shame that this got carried through for so many generations of Specialized bikes, but perhaps things will change for future generations of Specialized bikes after seeing the layout of the prototype Demo that's being raced at the DH World Cups.
In any case, it's likely that the Stumpjumper's climb switch will come in handy, as the weight transfer from accelerating and the cyclical mass of your legs spinning are going to use some of the suspension travel while climbing. Further into the cassette, and more in situations that might entail sprinting, the anti-squat really drops off. When that number drops under 0% the bike will further help compress the suspension, rather than counteract the weight transfer. But it’s good that most people won’t be sprinting at almost full travel, and the mass transfer effect will be less at these higher bike speeds.
The curves are nice and smooth however, unlike, for example, the short link bikes in this category. This is due to the long distances between the pivots, meaning that each frame member that defines the instant centre is rotating through less of an angle, and doing the rotation less violently. The high chip position gives a small boost of anti-squat in all gears, and this, along with the steepened seat tube angle, should help a tiny bit with the pedalling performance.
The Stumpjumper EVO maintains low levels of anti-rise throughout its travel, which might go towards creating some vagueness at the rear contact patch when on the brakes. The mass transfer forwards under braking will shift the riders weight forwards and lessen the force at the rear contact patch, reducing the ability to feel what's going on down there and allowing the wheel to break traction more easily.
The increasing anti-rise throughout the travel, however, is good for giving more support to the rider in high energy braking situations that will push the bike to the end of its travel.
As is the case with the anti-squat numbers, combining the anti-rise and leverage rate is going to result in a very active overall package. I’m sure lots of riders would appreciate this active, soft, and travel-using ride. But, given the bike's intent, which is clear from its geometry, I’m not sure the bikes kinematic is as up to the job of charging as the geometry is.
Given the low and far forward instant centre that is generating the low levels of acceleration response, the axle path is almost entirely a forward one.
For any bikes without a fixed main pivot, the instant centre will be a point in space that can move around as the bike compresses through its suspension. You can find the instant centre by seeing where the lines of the chain stay and link cross. In the case of the Stumpjumper EVO at zero travel, it's a point out in front of the front wheel, just slightly higher than the line of the axles. As the bike compresses, the instant centre quickly dips below the line of the rear axle and so the rear wheel follows a forward's trajectory.
But in the grand scheme of things this shouldn’t provide too much of a perceivable disadvantage to the rider as, depending on their riding aggression level, they would encounter more perceivable factors to deal with elsewhere with the design.
Want to read about how the Stumpjumper EVO works out in the real world? Check out the review here
.Previous Behind the Numbers Articles:Introducing Behind the Numbers - A New Suspension Analysis Series