First Ride: 2022 Polygon Mt Bromo - A Relatively Affordable eMTB With Six-Bar Suspension
Polygon's first full-suspension eMTB could hardly be accused of following the crowd. Polygon is no stranger to unusual suspension layouts, having used the equally radical R3ACT suspension system in the past, but Polygon told me they have no new models in the pipeline with this design. Instead, the Mt Bromo uses a six-bar suspension design they call Independent Floating Suspension (IFS). The basic idea, as its name implies, is to control the wheel path and anti-squat independently of the leverage curve.
There are two models: the N7 ($4,399) and N8 ($5,999) - these prices have gone up since we recorded the above video. That still seems quite good value to me though, especially when compared to Specialized's new Levo.
There are two models: the N7 ($4,399) and N8 ($5,999) - these prices have gone up since we recorded the above video. That still seems quite good value to me though, especially when compared to Specialized's new Levo.
Polygon Mt Bromo Details
• Wheelsize: 29" only
• Travel: 160mm front and rear
• Motor: Shimano EP8, 504Wh battery
• Alloy frame
• 64.5° head angle
• 77-degree effective seat angle
• Chainstay length: 435mm
• Reach: 510mm (XL)
• Weight: 25kg / 55.1lbs (XL)
• Sizes: S - XL
• Price: $5,999 (revised up since recording the video)
• .polygonbikes.com
• Wheelsize: 29" only
• Travel: 160mm front and rear
• Motor: Shimano EP8, 504Wh battery
• Alloy frame
• 64.5° head angle
• 77-degree effective seat angle
• Chainstay length: 435mm
• Reach: 510mm (XL)
• Weight: 25kg / 55.1lbs (XL)
• Sizes: S - XL
• Price: $5,999 (revised up since recording the video)
• .polygonbikes.com
Suspension Design
Although the suspension looks wild, it’s similar to the six-bar designs seen on the Specialized Enduro or Canyon Sender CF in that there’s a four-bar linkage which just dictates the axle path, anti-squat and anti-rise, then the other two links can be deigned independently to optimize the leverage curve (the progression). With a four-bar design, one of the frame members which dictates the axle path also drives the shock, and these separate considerations can lead to compromises, at least according to six-bar proponents.
Polygon's six-bar linkage is made up of a pair of counter-rotating short links just above the motor, which form a four-bar linkage connecting the mainframe to the swingarm. This determines the axle path all on its own. Meanwhile the seatstay and rocker link are there purely to drive the shock, as well adding stiffness, and bring the number of frame members up to six.
Two short links sit above the motor, leaving plenty of space behind the motor housing.
Most ebikes, like this Merida, have a pivot behind the motor, which limits how close the rear wheel can get.
Most ebikes have a physical pivot somewhere above and behind the motor, which is where it needs to be in order to have a reasonable amount of anti-squat. Polygon's design allows them to have all the pivots further out of the way above (not behind) the motor. This, along with the elevated chainstay, allows them to shorten the chainstay length to 435mm while fitting a 29" wheel with a 2.6" tire. To the best of my knowledge, you won't find a shorter rear-center on a full-suspension ebike. (More on whether this is actually a good thing later.)
Theoretically, you might be able to get the chainstay this short by placing a pivot directly above (rather than slightly behind) the motor, but this would mean very high levels of anti-squat. The Mt Bromo's two short links create an Instant Center ("virtual pivot") which sits inside the motor, somewhere you couldn't place a physical pivot. This results in about 115% anti-squat at sag, depending on what gear you’re in. That's still on the higher side, but nothing out of the ordinary. In my opinion you don’t want any more anti-squat than this on an ebike because being able to pedal smoothly over rough terrain is half the fun. The Instant Center also moves downwards as the suspension compresses. That means the anti-squat drops off steeply later in the travel, which reduces pedal-kickback.
The other part of the "independent" suspension design is the leverage curve, which is moderately progressive with most of the progression happening at the start of the travel. The leverage ratio stays within a typical range too, so Polygon hasn't used the freedom of the six-bar design to do something particularly radical here.
The anti-squat is moderately high in the first part of the travel before dropping off steeply.
The leverage curve offers a modest 15% progression (measured from 0 to 100% travel).
Frame Details
The Mt Bromo's frame is all-alloy and there are no plans for a carbon version. There's no space for a water bottle above the down-tube, into which the battery slots in from below with its own integrated cover. Like many eMTBs, there are rubber bumpers on the downtube to stop the fork crown / bars turning too far, though the crown clears the downtube anyway so you could remove them if you wanted to. The internal cable routing is pretty neat and rattle-free. The paint has an iridescent quality giving it a rainbow reflective look in bright light.
Polygon has gone with the Shimano EP8 motor along with the 504Wh battery, so you won’t get as much range as many of the latest ebikes which often have 630Wh or more. Don’t think that smaller battery makes it lightweight, though. This XL bike weighs 25Kg exactly on my scale, or 55.1lbs. But considering the burly parts, I wouldn’t say that weight is drastically higher than many other ebikes in this category.
Build Kit
I've ridden the top spec model, the N8, which costs $5,999. For that you get a four-way-adjustable Fox Float X2 shock and a Fox 38 fork with the Grip damper. The drivetrain and brakes are Shimano XT, with a Deore cassette and chain. The hubs are Shimano XT too – you don’t see too many of those these days. The N7 ($4,399) uses a Deore-based group, Fox DPX2 shock and SR Suntour Durolux fork. You also get Shimano's E7000 motor instead of the more punchy EP8. With either bike, the wheels have a 35mm internal width and are shod with 2.6” Schwalbe Magic Mary tires in the SuperGravity casing. That’s a big thumbs up form me. Too many ebikes come with lightweight tire casings which just aren’t strong enough, and I like the extra suppleness and grip of the 2.6” over the 2.35” version, whether on an ebike or not. The short 160mm crank arms also help to reduce pedal strikes when pedaling over rocky terrain.
Geometry
The MT Bromo is up to date but nothing too radical. You get a 64.5 degree head angle and a fairly steep 77-degree effective seat angle, which helps offset that short rear center when climbing. The reach figure changes by 20mm between frame sizes, from 450mm for the small to 510mm for the XL. The wheelbase in this XL is 1280mm - not outrageously long but not short either. The short 435mm rear-center is what sets this bike apart from the competition.
Ride Impressions
The short chainstay is something Polygon have clearly worked hard to achieve and it has a noticeable effect on the ride. While I'm a fan of long chainstays on regular mountain bikes, an eMTB is a different ballgame. The extra weight in the downtube can make them a lump to manual or hop over obstacles, especially when combined with chainstay lengths which are often north of 450mm. And while front-end traction can be an issue with short chainstays, the substantial sprung mass of the battery, plus burly tires and supple fork ensured I had no such issues here. The steep seat angle means it's no drama keeping the front wheel from lifting on steep climbs either. The position feels nice and upright and the suspension stays high in its travel. At the same time, there's plenty of suppleness and traction when pedaling over bumpy ground; there's none of the choppy ride feel which can occur with very high anti-squat suspension.
And although it's undeniably heavy to lift over fences or into a car, it's surprisingly easy to loft the front wheel and hop over obstacles while riding. Okay, it's no BMX, but relative to most other full-power ebikes it's very easy to manual. At 190cm (6'3"), this is something I particularly enjoyed when riding the Mt Bromo, so I imagine the short chainstay will be even more beneficial to shorter riders.
As for the six-bar suspension, it works well enough. It's not the most progressive out of the box but it doesn't bottom-out harshly, thanks in part to the X2's large bottom-out bumper. I've far from optimized the setup of the X2 shock, but its rebound needs to be run close to fully open to get the most from it - I'm currently fully open on HSR and four clicks from open on LSR. Set up like this, it's very supple but still predictable, with no odd quirks to tune-out or get used to. Still, I don't think the six frame members and seven pivot locations provide a tangible advantage over more conventional designs in terms of suspension performance. The main benefit is that it allows for those short chainstays.
With an inseam of 93cm, I had to run the seatpost at the minimum insert line and the bars are a few centimetres lower than I'd like, so the XL isn't going to work for very tall riders without some modifications.
Nevertheless, I got to grips with the MT Bromo quickly and found that I could ride it pretty fast within the first ride. The geometry, suspension and tires work well together, providing impressive grip and stability to truck on without any nasty surprises or weak links. My biggest complaint is the 504Wh battery, which stops the fun too soon when compared to bigger-battery ebikes.
Photos: Andy Lloyd
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I think its death was cause it was Friggen hideous.
I only know one guy who would ride it, and he has one of those praying mantis looking bikes now (linkage front end).
I dunno, think its just you can only step so far out of the box and still be acceptable - but the REACT seemed pretty legit to me.
is it? The Enduro is a Horst link bike, and isn't this a linkage driven single pivot? It has more in common with a Kona I would have thought, although with substantially different main pivot location and the small links to control leverage curve. I should have paid more attention in Physics...
It is definitely not a 6 bar.
Looks closer to a DW or VPP with 2 linkages on the rear triangle. The seat stay here acts as an additional linkage and the chain stays are basically just slimmed down rear triangle.
Why isn’t there a video of the suspension in action?
If you try to think of this as a four-bar with a reeeaally long drop-out (like longer than most chainstays), it might help to visualize how its similar. Can do the same with VPP, dw-link, or other short-link designs: the rear triangle is like a huge drop-out and the links are like mini chainstays and seatstays.
There are FOUR lower pivots, that's the key. They make one virtual pivot that does, as you noted, move relative to the BB. Those four pivots together define the axle path, same as the entirety of a four-bar system, and just as a single-pivot defines it's axle path completely with the main pivot.
Difference is that a single-pivot can only have a single arc for an axle path, while the others can modify the axle-path. Similarity is that single pivots also completely isolate the axle path (is always one arc) from the leverage curve (defined by the seatstay and rocker combo)
Polygon is trying for the best of both those worlds (tunable axle-path and independent leverage curve), while also getting a nice way to have super short chainstays even with a motor there.
In this case you will have a 100% defined axle path. The upper bars have nothing to do with the axle path so it is, by definition, a four bar design.
Other thing about the pivot point movint forwards, is that there will be way less pedal kickback relatively to a equivalent 4 bar or single pivot design.
Just because you don't know or understand the true definition, which is well known in the transportation world, doesn't mean you can redefine it.
Happy Riding
I understand training well enough that if you 'hammer' you are not saving your legs. Also cross-training on a moped is simply not what riders do. But training is a personal thing and if what you say works for you all the more power (batteries) to you.
BTW you have earned some personal notoriety for sledding out of bounds of Brohm Ridge in Garibaldi Park.
If I suddenly had some way to go more miles in the few hours I carve out each week for riding, I wouldn't just stop when I hit some mile number and leave some hours on the table, I'd just ride more miles in the given hours.
To me, that's a super backwards way of thinking. If you want to, and get to, spend 3 hours riding, you should spend 3 hours riding, regardless of how many miles it is. An e-bike isn't going to make those hours longer or shorter...
Although in some situations I can see how it can help people get slightly more enjoyment out of those few hours, but that's pretty much only self-shuttling up long [fire] roads, IMO.
Smiles not miles.
No. I don't care about miles. I care about going down when I'm alone. Only. So 2 big downhills in the Alps is what I'm there for. In E-Bike I'm doing it much faster. Because going up takes me a fraction of the normal time.
But you do care about the distance over the time. You want 2 "big" downhills, and you're ok with doing it taking less time. That's literally exactly what I was saying. If you had X hours and could do 2 big downhills without e-power, then why wouldn't you do 3 big downhills in the same X hours with e-power, since it makes it faster?
For example, a Kona has always been a single pivot with a linkage driving the shock (linkage driven single pivot).
This Polygon being a 6 bar design is clearly fake news. IMO Pinkbike has a duty to call out the BS that marketing is spewing but they often perpetuate the fake news.
@seb-stott: back to polygon, if the rear pivot is on the chainstay instead of seatstay, only then can their new design be called as "true-six-bar"?
I’m a fan of short chainstays on the ebikes I’ve tried also.
The rocky mountain altitude mk1, canyon sprectral on mk1 and the focus jam2 (with flip chip in short position) all have slightly shorter stays.
Those _intertwined_ considerations of a four-bar's axle path vs leverage curve DO lead to compromises. However, those compromises CAN be acceptable for a the performance target of a given design.
Can same the same thing about single-pivot vs four-bar. Single pivot means the center of rotation is fixed, and that IS a compromise vs the complexity but axle path potential of a four-bar, but that also CAN be perfectly acceptable for a specific design target.
For those complaining about the 504W battery, I have a 540W battery and typically my legs will go before the battery does. I can even get out after work for a 2 hour blast on boost for the climbs (eco for the DH), and still have 40% battery left.
Polygon: All of them.