BMW K75 shocks
Some enthusiasts might scoff at the notion of including BMW’s 1985-1995 K75 triple in a classic motorcycle magazine, but the truth is, it has become something of a classic, and rightly so.
Introduced for the 1985 model year, the K75 was a smaller version of the revolutionary 4-cylinder K100 introduced just two years before, which featured a fuel injected, double overhead cam, water-cooled inline four – lying on its side. Except for its shaft drive, it was unlike any motorcycle BMW had ever made, and rumors quickly circulated that BMW had a smaller companion to the big four in the works. Nobody expected a triple, however, yet for all its visual and technical similarities to the bigger K100, the K75 proved to have its own distinct character.
The K75 range was successful, too, with almost 68,000 manufactured in K75, K75C, K75RT and K75S variants during the model’s 10-year production run. Built with typical BMW quality, they are long-lasting, high-mileage machines, a fact that means there are tens of thousands of them still running around. And that brings us to their “classic” status. While traditional riders may not see them in the same light as, say, a Sixties Triumph or a Seventies Norton, their ready availability — and the ready availability of parts for them – makes them strong contenders as everyday mounts. Straddling something of a line between old and new, they are an excellent option for commuting and touring. And with almost 35 years between introduction and today, their mid-1980s styling has acquired a dated yet comfortable old school look.
Every bike has its failings of course, and one of the K75’s biggest was the rear Boge monoshock. Softly sprung and under-damped, it was considered a poor performer when new, and today, any owner with a K75 still wearing its original shock should seriously think about replacing it. And while we’re big on OEM parts, this is one case where we wouldn’t even consider OEM as an option. At just over $500 from BMW, the stock shock is hardly cheap, and adding insult to injury, they don’t work any better than they did 30 years ago. Fortunately, there are excellent options on the market, three of which we tested: the Hyperpro 360, the Ikon 3610 and the YSS MZ366.
The stock Boge has 3-position adjustable preload and non-adjustable rebound damping, typical of the era. In addition to adjustable preload, all three of the tested shocks have adjustable rebound damping, a desirable feature on any bike and essential in our opinion for proper performance on K bikes owing both to their weight – our 1995 K75 test bike comes in at around 515 pounds – and their shaft final drive. Shaft drive can induce so-called “shaft effect,” where hard acceleration causes the bike to rise and deceleration makes it fall, upsetting handling. The K bikes (even pre-Paralever models, a feature K75s never got) don’t exhibit the effect as badly as some, but the issue becomes particularly evident running poorly damped and under-sprung rear shocks. Fitted to our 1995 K75, the three shocks tested allow fine-tuning preload as well as rebound, mitigating any shaft effect.
Rebound adjustment on all three is a simple matter of turning a dial, located at the bottom of the Hyperpro and YSS shocks and at the top of the Ikon. Of the three, the Ikon’s was harder to adjust, but only because the shock mounting bracket on the K75’s frame partially obscures the top of the shock, making it hard to access the adjustment dial. This wouldn’t be an issue on many bikes, but it is a minor pain on the K75. At the same time, the Ikon was also simpler to tune, owing to its four-position rebound adjustment versus the Hyperpro’s 50-position and the YSS’ 35-position adjustment, which allow finer tuning while also requiring more experimentation to find that “just right” setting.
Although our tests were literally a seat of the pants assessment, we sampled each shock riding the same 60-mile loop of mixed condition, two-lane roads. In each case, we rode the first half with rebound set as delivered. Then, depending upon our assessment, we adjusted the rebound to tailor it to the K75 and the roads we were riding. Not surprisingly, all three of the tested shocks performed light-years better than stock.
First up for our test was the Hyperpro 360. A relative newcomer to the category, Netherlands-based Hyperpro has been making shocks since the early 2000s. Initially focused on high-performance race and sport bikes, the company’s offerings have broadened substantially over the ensuing years, with applications for a wide variety of 1970s to the present European and Japanese motorcycles, and Harley-Davidson.
Weighing in at 4 pounds, 8 ounces on our scale, the gas-charged Hyperpro 360 is about 3/4-pound lighter than the stock Boge. Fully rebuildable, its features include a stout 16mm shaft diameter, a progressively wound spring with a screw-collar for preload adjustment, and 50-position rebound damping adjustment. It comes with a five-year warranty.
As delivered (packed in a cool reusable plastic tote), our Hyperpro was sprung for an average 180-200-pound rider, with the rebound set in the middle, 25 clicks from the bottom (fastest rebound). A check of rear suspension sag showed the preload was properly set.
Ride quality on the first half of our loop was generally excellent, yet there was no question the rebound needed to be slowed, the rear wheel tending to bounce over washboards and road irregularities rather than roll over them. With a wide adjustment range as on the Hyperpro, it’s a good idea to make broad rather than fine initial adjustments to get in the ballpark; you’re more likely to feel the change and know if you’re going the right way. For the return ride, I slowed rebound by five clicks, which made a marked difference in ride quality. The rear wheel tracked cleanly and easily over washboards and road irregularities, greatly enhancing ride quality and control. A bit more experimentation showed that 20-click setting to be ideal.
Remember Koni shocks? They’re still available, they’re just called Ikon, a slight-of-hand name change effected when Australian shock absorber reseller Proven Products acquired the rights to Koni’s motorcycle shocks in 2000. In the years since, Ikon has continuously upgraded and expanded its line of shocks, with applications for everything from Aprilias to Zündapps.
The 3610 series was designed specifically for single-shock applications like the K75. Weighing in at 7 pounds, 4.6 ounces – almost 2 pounds more than the stock Boge – the fully rebuildable, gas-charged 3610-1009 is a stout piece, and looks it. Features include a 14mm shaft diameter, a progressively wound spring with a screw-collar for preload adjustment, and four-position rebound damping adjustment. It comes with a one-year warranty.
The Ikon came sprung for an average 180-200-pound rider, and with the rebound set at 1, its fastest setting. Checking rear suspension sag showed preload was properly set.
Ride quality was very good right out of the box, the rear tracking cleanly and delivering an almost plush ride, with little evidence of wanting to jump across washboards. Setting the dial to 3 for the second leg of the ride returned a firmer, more controlled ride, and without any loss in tire contact rolling over rough surfaces.
Based in Bang Phli, Thailand, YSS Suspension began building motorcycle shocks in 1983. Twenty years later, YSS was supplying shocks for more than 15 motorcycle brands, and is now one of the largest motorcycle shock manufacturers in the world.
The MZ366 slots into YSS’ 4G line of gas-charged, rebound adjustable shocks. Specified for monoshock applications, it weighed in at 4 pounds, 5.2 ounces, the lightest in our group. Fully rebuildable, its features include a 16mm shaft diameter, a progressively wound spring with a screw-collar for preload adjustment, and 35-position rebound damping adjustment. It comes with a two-year warranty.
As with the Hyperpro and Ikon, the MZ366 was delivered sprung for an average 180-200-pound rider, and with the rebound set roughly in the middle at 15 clicks from the bottom (fastest rebound). Checking rear suspension sag showed preload was properly set.
Heading out for the first leg of our route it was immediately apparent the MZ366 needed slower rebound to provide the kind of controlled ride we were looking for. On the first leg of our 60-mile loop, the YSS returned a soft, under-damped ride that had the K75 bouncing on the worst washboards.
For the return ride, we dialed the MZ366 up five clicks, putting us at 20 of the 35 available. This provided a much improved ride, the back end feeling firmly planted and comfortable, smoothly tracking over washboards. At the end of the return loop, we experimented with slowing rebound just a bit more, settling in at 22 clicks from the bottom. This further improved the ride, suggesting the YSS has its most nuanced rebound control at the upper end of its adjustment.
All things considered
We’re not suspension specialists, so we didn’t go into this looking for an outright winner. Rather, we wanted to educate ourselves about available quality shock options and the differences we could perceive between them. The tested shocks are all high-quality units; much better than OEM, and in the case of the YSS, cheaper, too.
We did come away with some conclusions that underscore the individual nature of shock manufacturers and their specific products. Namely, for two-up riding on the K75, and/or with luggage, the $600 Ikon 3610 would be our top choice. Built like a tank, our experience suggests it would accommodate increased loads well after fine-tuning both preload and rebound. If most of our riding was sport/solo in nature, we’d definitely lean toward the $569 Hyperpro 360, which we found easiest to dial in owing to its surprisingly sensitive response to changes in rebound. And if a good ride on a tighter budget was our main criteria, we’d look at the $389 YSS. Although rebound adjustment sensitivity wasn’t as good as on the Hyperpro, it’s a solidly built unit that seems more than capable of dealing with the K75’s heft, and like the rest of the tested units, miles better than the original.
As a final note, proper spring preload adjustment is critical to good shock performance. To learn more about setting sag, go to MotorcycleClassics.com/sag.