Inverted 4-Bar Suspension
The Solstice 160mm travel, inverted 4-bar suspension chassis needed only a 300-yard section of trail—one that resembled class V rapids frozen in a river of stone—to prove its merit. The handling was precise, even while under severe stress, and the rear end tracked through the worst of it with the accuracy of a laser-guided sledge hammer: Pedaling through rock gardens was luxurious, and 4-foot drops left us confidently seeking out ever more pounding terrain.
Built as a rugged all-mountain bike, the Solstice (no model name necessary: the small company run by one man in Manitou Springs, Colo. makes only one model) is solid from tip to tail with progressive features like a tapered, zero stack (lower cup) head tube, curved top tube for low stand-over height, and stout 6061 aluminum tubes with ample bracing at key junctures. It’s built to take many seasons of demanding riding. But the heart and soul of the bike lies in owner/designer Chuck Dunlap’s inverted 4-bar suspension chassis.
Like an apparition—a vision through the mind’s-eye of the engineer who created it—the main suspension chassis stands out firmly from the rest of the bike; one’s eyes are immediately drawn to it. It’s artful, in an I-beam, industrial sort of way. The Solstice website describes it as a first-of-its-kind design (and the U.S. patent office agreed).
The modular rear swingarm components are cold forged from 6061 aluminum and the chassis is bound together, along with the floating rear air-shock, using sealed cartridge bearings at the two main frame pivot points and rear pivot, and needle bearings at the lower swingarm pivot. It’s all put together with precision hardware: It’s ultra stiff and allows for frictionless motion through the suspension travel. The 12x135mm, pinch-bolt thru axle adds considerable stability to the rear end as well.
The small lower swingarm link is protected from impacts with a smash-resistant plastic coating and is meant to come into contact with occasional trail objects, which in turn activates the suspension and helps the rear wheel over whatever rock the rider failed to clear. In addition, the swingarm link is replaceable and available in two lengths to allow the rider to modify the bike’s geometry for different types of riding. You can adjust the bottom bracket height from 13.6 to 14.1 inches and thus change the head tube angle from 66 to 67 degrees, respectively.
The inverted 4-bar chassis is designed to be lively while eliminating pedal feedback and brake-jack (that awful feedback you get when braking momentarily locks up the suspension), because the wheel is mounted on a floating chainstay. The design is a variation on what has become known as a short dual link, or SDL, (except the Solstice uses a single, stout link under the bottom bracket rather than two.)
SDL is the basis for a host of successful virtual pivot point designs like the DW link (used by Ibis, Pivot, and others) and Giant’s Maestro suspension, in which the wheel is mounted on a floating link, allowing for the wheel path and shock rate to be tuned almost infinitely to the engineer’s desires. The astute designer can create specific wheel path curves to eliminate squat at very specific points in the travel, creating a bike that pedals well while still offering a supple, active feeling at other points in the travel.
The SDL does have its drawbacks. With the immense tuneability options come plenty of room for error. Very small changes in the pivot positions can change the ride characteristics dramatically. Harmonizing the shock rate, chain growth and wheel path is very tricky as they all affect one another. It’s a balancing act. There is no consensus in the bike design community as to what suspension design is best (and there never will be). The ultimate location—the “sweet spot”—for the virtual pivot point is elusive and debatable depending on the desired effects, drive train choice, and type of bike. In the end it’s the rider’s perception that matters most.
The key element to consider with the Solstice design is that when the suspension actuates, the wheel follows what engineering types dryly call a “forward migrating instant center virtual pivot point.” The wheel path and chassis follow an instant center, or a single dynamic pivot point, around which all the other points on the suspension chassis are rotating at that given instant in time.
In the case of the Solstice, that instant center virtual pivot point is located at an intersection off a projection from the lower link and upper seat stay pivots: It falls in front of the seat tube and about 3 inches above the bottom bracket. The location of that pivot point is proprietary and a critical point that can make or break the design.
As the chassis swings during suspension travel, that virtual point begins to move forward and the rear wheel axle approaches a path that is nearly vertical. This, in effect, creates more usable travel as the wheel moves upward at the same trajectory as the force of the impact. The bike’s geometry stays true throughout the compression, so the bike does not feel like it’s squatting or rotating backward at the end of the travel.
Since the air shock is floating (mounted between the swingarms) and moves in relation to the wheel path, it acquires unique characteristics for the shock rate. According to Dunlap, his goal was to make the rate supple off the top with a high leverage ratio. The ratio then levels out mid-stroke, creating a lively, fully active feel through the middle of the travel, and finally, as the wheel path reaches the end of its travel, the air shock progressively ramps up to handle the big hits without bottoming out.
Wheel path and shock rate do have an effect on one another, but as a design philosophy I think it’s important to keep those coincidental effects as separate as possible,” Dunlap says. “With the inverted 4-bar, there was a focus for the wheel path to control the pedal bob and brake-jack while using the shock rate to keep the wheels grounded and give a good feel.”
Done right, this adds up to a bike with little to no pedal bob, great small bump compliance at speed, a predictable, reactive feel mid-stroke and the ability to send it off of big hits.
Clearly, it’s complicated. Clearly, Dunlap’s humble little bike company has serious technological experience behind it. If one searched the archives of bike industry design, Dunlap’s name would appear in more than one file. In fact, the mechanical engineering grad—with additional study in vehicle dynamics—has amassed more than 12 years of design experience including developing Titus’ custom frame program and working as Yeti’s lead designer from 1999 to 2003.
That experience, and the typical engineer’s curiosity, led him to establish Solstice Cycles. Currently, the company produces frames in very small batches and sells them mostly by word of mouth; growth charts and aspirations to make the big bucks don’t appear to be part of the inspiration.
Solstice is not out to grab market share or maximize profitability but rather fund Dunlap’s curiosity and passion and make a great-riding bike. That could be a good thing for his customers. The Solstice design was more than four years in the making and included four iterations of the current bike. The result is a technically advanced bike with an inspiring ride.
It’s also been documented that gear ratios have a distinct effect on the 4-bar SDL designs. Depending on the virtual pivot location, an SDL bike will tend to pedal well in certain gear combinations but not in others. The Solstice is optimized for a 2x10 or 1x10 drive train; the front derailleur is attached to the swingarm using braze-on mounts, making the bike easy to convert to a 1x10 using an MRP chain guide in place of the front derailleur.
The medium frame has a 23.3-inch top tube, 17.1-inch chainstays, and a nice, compact 4.5-inch head tube, and, with the aforementioned swingarm link options, the head tube angle can be adjusted from 66 to 67 degrees. The frame weighs 7.25 pounds, so, depending on component and fork choice, the bike can be built up to weigh 27 to 29 pounds. The suspension chassis is optimized to work with a Rock Shox Monarch air shock, but other options are available.
Engineering aside, the Solstice and its inverted 4-bar suspension behaved awesomely on the trail. It’s a very capable bike. The chassis design puts all the major suspension components very low, near the bottom bracket, keeping the bike’s center of gravity low and tight: It handles fantastically in slow technical terrain (climbing or descending), takes high-speed big hits with amazing control and even climbs reasonably well on long, fire road climbs.
With some understanding of the dynamics behind the inverted 4-bar suspension chassis, the perceptive rider may even be able to feel the forward migration of the instant center virtual pivot point in relation to the vertical wheel path while floating the bike through a virtual river of rock drops and debris … but if the design is doing its job, you probably won’t be thinking about that, you’ll just be enjoying the ride.
Price: $1,700 (frame only)
Weight: 7.25 lbs. (frame only)