Down The Trail: ProFlex 4000

Fondly remembering MBA’s past

The ProFlex 4000

Any mountain biker who’s been into the sport for more than a couple decades remembers ProFlex full-suspension bikes. They were the iconic, bullet-proof, elastomer-sprung dual squishers and can occasionally still be seen on trails today. Back in 1998, the successful ski company K2 bought the up-and-coming bike brand and made its own version of the bike. The big bike brand buyout resulted in a bold new line of full-suspension machines that we had the duty to put through the test.

What MBA thought of the K2 4000: We thought it was a stellar trailbike for the time “in the ultimate sense of the term.” As a sport-level racer or a weekend expert, the 4000 could be competitive. The 4000 was lightweight and solid, and the suspension fork worked well. We gave this bike two thumbs up. My, how things have changed.

The Noleen fork: Noleen suspension was one of the first to bring electronics to the mountain bike world. The new F-Smart fork used what they called “piezoelectric-controlled” damping to give the widest range of impact control on the market. The valving allowed the fork to run with essentially no compression damping over small impacts, yet allowed the electronics to turn on the damping on the big hits. The system was controlled with a 9-volt battery that would last 24 hours of ride time.

“K2 Bike doesn’t mess with success”: That was the line we wrote about the new K2, which was essentially a re-branded ProFlex. The bike weighed a claimed 27 pounds with 4 inches of travel in the rear and 3 inches of travel in the front via a Noleen linkage fork. The new bike got rid of the elastomers the previous generation ProFlex relied on in favor of a coil-over shock. The Easton aluminum frame came with a 70.5-degree head angle, a 12-inch bottom bracket and a 72.5-degree seat angle. If those numbers seem out of whack, you never rode mountain bikes in the ’90s. Those were pretty aggressive back in the day.


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  1. Don Paul says

    I rode my “Way Big” (XL) 1998 K2 ProFlex 5000 frame for 20 years and XX,XXX miles all over the U.S. and loved it. Replaced the rear Noleen Smart Shock within the first couple of years after it kept breaking with a Fox Float Vanilla shoc. Also replaced the K2 ProFlex Girvin front “Over The Bars” inducing front shock with a Rock Shok Recon front fork. . Huge improvement.

    Broke both original rims (probably from wearing down their sides with the original rim brakes) and replaced them with much sturdier Mavics when I found a custom fabricated Shark(something) disc brake adapter from England that fit perfectly on the rear carbon fiber swingarm. Replaced scary crappy rim brakes with SRAM BB5 mechanical disc brakes. HUGE improvement in braking. Wore out the drivetrain and replaced it with a 3×9 Shimano XT. Replaced the handlebars & seat post with super lightweight Easton carbon fiber bits. Replaced the worn out headset + F&R hubs with Chris King bits. My wife, kids and bike mechanic called it “FrankenBike” since it was so freaky looking and such a quirky bunch of parts. Final bike weight ~ 32 lbs.

    Finally replaced my K2 ProFlex in 2011 with an XXL Santa Cruz Tallboy c 29er with 2×10 SRAM drivetrain and Roval carbon fiber wheels. XXL Tallboy cockpit fit my 6′-3″ body a lot better, weighed 6.5 POUNDS less, and SO much easier to pedal and ride through rocky terrain. Recently upgraded my Tallboy c to SRAM 1×12 Eagle to save some more weight… and free up left grip control space for my seat dropper.

    Gave my K2 ProFlex 5000 “FrankenBike” to my son and it got stolen by some low life scumbag A$$HOLE. Fortunately his AAA renter’s insurance gave him a nice settlement… and he bought me a Path Bike Shop demo XXL 2017 Santa Cruz Hightower 27.5+ with SRAM 1×12 Eagle for my 36rd birthday. The Hightower is a plusher ride which is easier on my 63-year old / 28 year mountain biking joints… and XXL Hightower cockpit fits even better than my XXL 2011 Tallboy c cockpit. Love having both… but still miss my K2 ProFlex 5000 “Frankenbike”. Loved hearing stunned MTBers asking “Is that a ProFlex?” especially when I passed them on their 20 year younger MTBs. 😎

  2. Dave says

    Had a pro flex 855/6 (frame crack warranty) up until 2008. A couple changes here and there. Yup, bikes are better now but I was living the dream back then.

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