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.
Polygon Mt Bromo Details
- Wheelsize: 29" only
- 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)
I've also had time to grab a few rides on the Mt Bromo since recording the video so scroll down for a first impression of how it rides.
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).
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.
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.
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.
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
See the original article here.