Behind the Numbers: Specialized Stumpjumper EVO 29 Suspension Analysis
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
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.
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.
Anti-Squat
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.
Anti-Rise
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.
Axle Path
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.
Final Thoughts
 | Overall, the Stumpjumper EVO looks mean, and the geometry is an exciting jump forwards for Specialized. But after delving into the suspension it seems that there's still some of that conservatism there. The gross result of the suspension characteristics will be an generally active ride that will need some tuning to handle the aggressive riding that the geometry will enable. In my opinion the bite of this EVO bike doesn’t quite appear match its bark—on paper anyway. |
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
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How rad would it be to set up a database with all current bikes..? Or at least an analysis of the Orbea Rallon.
I'm still not a fan of coil shocks on linear suspension designs, and I don't understand how people get to like the feeling, but heck, if you like the bike how it is, that's what counts the most!
linkagedesign.blogspot.com/2017/11/orbea-rallon-29-2018.html
The only issue I have is it's probably overdamped. It was definitely the case with the DPX2 (even with the lightedt of tuning), and much better with the X2 (medium tuning for compression, I may go for a light as it's fully open).
www.bikechecker.com
That said, they certainly help!
Cheers!
I ask because I have a brand new Orbea and have been debating putting a Hazard on it with a progressive spring...
I wonder why I never got to junior year???
The main issue with the Rallon and so many "superbike" like pivots, SC, Ibis etc... is their high AS/PK values.
Most of them try to balance high PK values with linear ratio and coil in order to get less forces counteracting suspension compression in the travel, but they mostly results in what you described : a harsh ride. Nothing to do with the shock. Only high PK consequence.
The only solution is to use a bigger front ring to decrease AS/PK.
That's why a lot of companies mostly communicate on their AS value, never on PK, cause they are strongly positively correlated.
Some companies still thinks that people wants bike that climb like a hardtail, even for enduro bikes so they boast the AS in detriment of suspension action.
Lotsa riders long shocking these so progressive coil could be a legit option.
Also maybe long shock with a high end air shock. I’ve found without a fairly adjustable shock that Spesh bikes can be pogo sticks. My Stumpy does well with a Topaz. Sag in upper 20s plus lotsa spacers for bottom out plus air spring curve chamber maxed at 200psi. Still kinda mushy on the pedals tho.
Good point Waki. Even on my Ripmo demo I toggled between med & firm LSC on the climbs. Loved how throwing in pedal strokes while descending equaled acceleration though! (Open LSC). Something I’m not used to on the Stumpy.
There was just no way of consistently making them to spec. At least at the price point where they are an option for mtb.
It would be great if MRP have somehow found a solution, but I wouldn't bet on it.
They go as short as 51mm stroke, and at 51mm you get about 18% progression. Obviously the longer strokes get more.
Anyone know how that compares to various air cans? Also, is that pricing and weight good for a coil? Its been so long since I sold the DH bike that I don't know anymore.
I own the alloy version of this bike and have sunk a lot of time and effort into turning it into my ultimate do it all machine.
Some of these stats make sense, some I don’t relate to at all.
From my experience, the stock dpx2 is no good for your heavier rider. Not enough rebound. Small bump sensitivity very poor.
I then moved to a cane creek IL coil at 57mm stroke. This gave the bike 163mm of travel with nil clearance issues. The bike was all the sudden the magic carpet ride I was after. However as the charts suggest, the progression at the end of stroke meant some bottom out moments and using a bit too much travel on square edge hits. Winding up the hsc helped however.
I went back to air, this time a Cane creek DB air with 55mm stroke for 154mm travel. Small bump sensitivity was nearly as good as the coil, mid stroke was fine and with just a single big band inside the air can enough bottom out control to be what I was looking for.
A fox x2 apparently fits by a bees dong. I think this is the ultimate solution for this bike.
TLDR: sick bike, insane geo, shitloads of fun. Put a big can air shock on and enjoy!
Coming from a downhill background I was comfortable with the “glued to the ground” feel which I couldn’t replicate until I changed shocks.
It’s more a preference thing than anything but on mtbr forums were there is a big community of us on these bikes, the bigger lads all had the same complaint. A lot of us went Coil and have been very happy.
Now they are ripping the big S with detailed analysis of why they are good for entry level riders and people who think they ride hard but really don’t. (Or maybe this is a way to set up why the new Enduro really is a big change...)
Pros of the design: Linear leverage ratio so the suspension is very active, great climbing sensitivity because the low levels of anti-squat, great braking sensitivity because the low levels of anti-rise.
Cons: Linear leverage ratio so you have to run higher spring rates or a more progressive shock, not a good pedaling platform because the low levels of anti-squat, poor braking traction because the low levels of anti-rise.
Notice that there were pros and cons drawn out of each characteristic.
In terms of this story, please keep in mind that it's NOT a traditional review. Kinematics are important, but not the be-all end-all of how a bike rides in the real world. Lots of other factors in what makes a bike good or bad.
I wouldn't say 50% anti-rise is bad, it just means that the suspension will push a bit to the ground under braking, and that you have to acount for it and adjust your body position.
Important bit: if you own an enduro and have shock problems, Specialized will send you an ohlins STX after the 2nd or 3rd time, if you call up rider care directly. These seem to hold up much better. Love them or hate them Specialized are pretty great with warranties.
Instead of having 160mm of travel, and having used up 65mm when at 30% sag, you have 138mm, and use 35mm at sag, and still achieve the same BB heigth and amount of usable travel as the bigger travel bike.
I guess the purpose of all this is to make the bike feel just as comfortable as a big travel bike, but also less planted, aka a nimble bike, that can stilll plow.
Compared to the Megatower for example (just taking it because it's another 160mm travel 29er that was reviewed yesterday), the BB height (in high) is 343mm and at 30% sage would sit around 295mm. The Stumpy's BB height is 328mm and at 30% sag would sit around 287mm. Again, only a difference of 8mm, though the Stumpy is lower, which confirms the low feeling of the bike.
That would only happen if the bike was completely linear, making 30% of shock stroke equate to 30% travel.
Since most bikes nowadays are quite progressive, 30% of shock stroke equates to more than 30% travel at the rear wheel.
Maybe tomorrow i'll try and work out how much travel 30% of shock stroke is on a bike with 160mm travel, that starts of with a 3:1 ratio and ends on 2:1. (Too busy rn studying eletricity and magnetism for tomorrow's exam)
I'd ride the bike to find out if there is something in the feel of the suspension that stands out. Only then check the numbers and see if there's an explanation for it.
The worst thing is being stuck between bottoming out all the time and having to run less sag which makes the bike tall and shit in the corners.
So in a way you can judge the ride by the numbers (they are what is defining the ride in the end), but of course you shouldn't get too hung up on them.
The bike is great, bit short for me but otherwise crazily capable and fun. Everything from backyard trail, long xc rides or DH lines in park. But putting yet more money towards trying to fix it or just buying new bike .... But I think you just broke me xD
Thanks for the tips!
Antisquat occur without a chain because the acceleration force the ground have on your tire will exerce a torque on the swingarm. This torque will oppose squat. The higher the instant center of your suspension, the more antisquat it will have.
For bicycles (and motorcycles) the chain forces are relatively large and create a counter moment (torque) about the rear axle that causes the shock to either sag (think High Single Pivot without an idler) or extend (think about having a giant cassette cog and how the chain would pull the wheel down). This adds or subtracts from the moment created by the tire force and the total result gives you the anti-squat number.
On a bicycle its pretty damn hard to accelerate the rear wheel (without using your hands) with no chain so there is effectively no anti-squat effect without it. Note that acceleration due to gravity does not have the same impact because there is no force on pushing on the tire (which is what is creating the anti/pro-squat moment).
Raw carbon and raw aluminium for the win!
Revalving your suspensions is always beneficial.
which rear shock?
This doesn't make sense. This regression you're talking about is tiny and only in the first 7-8mm of the stroke, so at sag it's not affecting anything, and I doubt it will ever be noticeable. Damping is not going to go continuously back and forth over it.
Thanks for doing this series though, it's really valuable to have linkage analysis. After this you guys can try nerding out over tires.
I'm a SJ evo 29 rider with a coil @52.5mm stroke, flat pedals, aggressive ex bmxer with a strong preference for pumping whenever I can.
I was hesitant to put a coil on, it's least least progressive leverage rate bike I've ridden in a while, asked a sponsored rider with the same style who was running coil on theirs, they said go for it, so I did. Feels great to me. Do I bottom out, yes. Do I care, no.
Numbers say neutral bike in terms of pedalling and kickback, agree with that, been put off masses of anti squat on previous bikes.
As in the other intro post, there's mention of feel versus numbers and importance but not seeing it being implemented here.
The bike review does not seem to echo what the analysis states.
And if Kazimer has to ride 600lb spring, gl to anyone else with bodyweight 20-30lb heavier, that shock is going to work overtime all the time.
He would have been riding at like 15% sag with his light bones.
650lb was too harsh for me.
I really enjoyed reading that, and there are a bunch of really interesting comments on this article. Would you consider adding some additional factors into the conclusion along the lines of some of the comments?
XYZ type of shock would be likely to work well with this linkage arrangement. Coil shock, air shock, one with high and low speed compression/ rebound adjustments etc etc etc
ABC type of tune is likely to be required to get the most out of this bike. Maybe link this with the below comment if applicable.
Whilst it is not explicitly mentioned in the assumptions section of the behind the numbers article I am assuming that the number used for centre of mass is an average number of Joe MTB at 75-80Kg. My limited knowledge on this has me thinking that if you are a very light rider or a very heavy rider the impact of the various curves is going to affect you and the setup of your shock in very different ways.
If this is correct (I appreciate I may be way off the mark here) could you comment if the type of set up being analysed is likely to suit and or hinder a particularly lightweight rider or a heavy one. And maybe if a lighter than average or heavier than average rider would need to consider more than simply air pressure/ spring weight changes to get the most out of the bike.
All you Specialized haters need to a find a new hobby.
And after trying the mattoc of a friend, I need that HBO on every fork now. Mattoc is at the top of my wish list for my next fork.
The harsh AF ride that I had with the DPX2 has given way to a supportive yet smooth ride with insane amounts of traction. I also tried a Float X2 which was a bit better than the DPX2 but felt kind of dead no matter how it was tuned. Progressive springs absolutely work.
was thinking of getting the Marz as well
but i'm only 70 kg - i think heavier riders might struggle with the dorado
It's not that it was a bad fork. It's just it was really supple at the top and I like forks with much more support. And I didn't spend enough time to know what to tune to obtain a better feeling. Pumping up air to max and closing every knobs didn't help much.
Mattoc on the other hand, which is claimed to have "dorado internals" is quite different and suits me better.
did you need any adapter to fit the MRP spring on the original collar ?
the evil is progressive up to around 40% travel , then quiet linear with a small ramp up @ end of stroke
i feel a progressive spring will be needed
I agree that the stock DPX2 was pretty awful on this bike.
I like the critical review, only problem you guys have is (if we are being scientific) we need similar analysis done on most other big hitters in the trail bike field! Or at least 3 others.....
It’s the nicolai for poor people
I own and love my Alloy EVO (650b version) but have noticed that it gets pretty mixed reviews. Try comparing the Vital and Pinkbike reviews, they are very different!
I think that it’s a progressive trail bike as opposed to a light Enduro bike. Reviewers seem to love it or hate it depending on what they are looking for.
It will lift the bb a little less then 20mm but you could run it in the low setting and fit some offset bushes to try and get it back down a little more as the low bb is part of its charm.
So given that I don't get on with Spec's Horst-link and given the "numbers" on this Specialized, what should I be looking for in a bike??? Higher leverage? Over 100% anti-squat?
I've really liked Yeti's SI and Pivot's DW...should I just stick with those b/c numbers are meaningless????
One thing's for sure though - Specialized's FSR suspension has endured as one of the top suspension formats for over two decades for good reason - because it's awesome. It's noticeably plusher than dual-link while descending and what it lacks on climbing can be fixed by a good shock. You can't say that about suspensions with built-in anti-squat...that anti-squat has a detrimental effect on active-ness that cannot be turned off, whereas you can always turn off a shock's compression damping.
VPP works well for me because climbing is where I need the most help. I can handle the harshness while descending at mach chicken because I need all the help I can get when the next climb comes along. Also, dual-link has this "popiness" about it when it comes to jumping...it's a suspension that loves to party. That "fun" is missing from four-bar suspensions.
My recommendation is always this - if you want your suspension to be descending focused, you want four-bar. If you want your suspension to be climbing focused, you want dual-link. If you love to jump off things, you want dual-link (especially DW-link). There are billions of other factors but in my experience, those are the most noticeable ones.
For the record I’m 148lbs and run the stock 550lb spring; so it’s way over sprung. I thought the air shock (on the alloy) suited the bike more and will probably swap to one.
Great series though, looking forward to the rest...............
I have ridden one of the new regular 29er Stumpys down a dh track though and found it had a major tendancy to rip my feet off the pedals over square edge bumps, lost my feet a few times which I've never had happen before on another bike!? Why would that be?
Super articles for the first two ... keep 'em comin' please.
F = m x a. E = m x c squared. W = F x D. Torque = F x radius x sin theta. Etc.
Well written and very insightful, really goes over this bikes suspension with a fine tooth comb. I've been looking into getting a Stumpy EVO Pro for a little bit now and while I love the geometry of the bike (mirrors my Transition Patrol), I could never get around the issue with having such a high leverage ratio. As a bigger rider who rides a bit aggressively, I know I'd have to be running a #650-#700 spring and that is just tough for any damper to deal with let alone the sacrifice of small bump sensitivity. After reading through this article and re-reading some sections twice, I'm starting to think the EVO isn't going to be the bike for me unfortunately. But I guess you could always just say f*ck it and just ride the damn bike and be stoked on it, short comings and all.... hmmmm... puzzling.
Keep this stuff coming -- it's fun to geek out and see the numbers behind things. I keep going back to an ABP bike because it feels more balanced for the way I ride. I would love to see an analysis on that and whether the numbers confirm what I feel. I loved my old DW Link 7point7, but there are so many variations on that design now (VPP, Maestro), it would be great to see a comparison of 2 bikes that take radically different approaches to that type of design (link positions and angles). I keep considering going back to a DW Link-type bike, but they are all so slightly different I don't know which one I would like!
Trying to figure out where I need to go damping wise.
There is no difference between Fox's 210x50 and 210x55 mm stroke shocks beyond the travel limiting spacers.
When you understand the curve, it help understanding the whole bike.
There are spherical bearings of many varieties, roller and plain. There are many types of self aligning bearings too, which don't necessarily have to be spherical bearings. If the way to reduce side loads on the shock is to provide a larger degree of angular freedom, then self aligning bearings are the solution. The same problem could also be solved by making the rear linkage stiffer. I am not a frame designer and do not know which would be better, but there are multiple ways to solve the problem of side loading shocks, and I suspect that spherical roller bearings are not the right option.
This isn't to say that using self aligning bearings in the appropriate places isn't a good thing, or using ball or roller bearings instead of plain bearings won't result in lower friction, because it will. However, I think it is important to be clear about which components we are talking about, what they do, and how changing them will improve the bike and the reason for and against the changes.
140 travel, low bb... what's the point? Then you wonder why racers have to run 180 forks on this thing to raise the bb and make it work on rough terrain. But I wonder why they always have mechanical.
Thanks