Chumba Racing F4 Review - FARKIN.NET Australian Mountain Biking

Chumba Racing (originally Chumbawumba Bikes) are a brand with a rich racing heritage in the United States that have only just begun to grace our trails here in Australia thanks to their new importer, Plush Rides. Chumba are a limited-production brand that still builds everything in house and yet manages to compete in a domestic U.S. market that is now dominated by big brands with big-dollar R&D budgets and massive offshore manufacturing facilities.

Over the years, Chumba have sponsored some of the fastest Downhill riders to come out of the U.S.A including Duncan Riffle, Lisa Sher, Shaums March, Lars Tribus and ‘Pistol’ Pete Loncarevich as well as hooking Brisbane-based pinner Craig ‘Woody’ Woodhead up for his assault on the NORBA Series in 2006. Couple this wealth of input from genuinely fast racers with full in-house manufacturing facilities at their HQ in California and a commitment to refining their designs rather than re-inventing the wheel every few years and you should have frames that ride well, built with quality and attention to detail that’s second-to-none. We took delivery of the smooth looking Chumba Racing F4 Downhill frame to see how it stocks up against some of the other heavy-hitters more commonly seen on our trails.

The Frame

Check out ChumbaRacing.com for the full geometry specs

The first word that will come to mind when you clap eyes on the Chumba F4 is “Braaaaaaaapt!” Guaranteed. With a low-slung, heavily gusseted front-end, massive simple swingarm and a linkage activated shock, this puppy looks for all the world like your favourite 450R just got a schmick white paint job and had the motor ripped out. If having people ask you “HURRR, where’s the engine?!” tends to upset you, it might be worth crossing it off your new bikes list straight away.

Profile shot of the F4's Moto Stylings

The swingarm itself is a work of CNC-machined art, relieved of excess material at both the dropouts and around the pivot / linkage then bolted to the mainframe with oversized, sealed bearings. The main pivot is directly above and slightly forward of the bottom bracket, suggesting a highly active ride but at the expense of pedalling efficiency. Rear hub is a 12mm x 150mm job, secured with a single pinch bolt on the rear dropout. Our test bike came specced with a Hadley rear hub for extra bling and buzz when freewheeling.

Top view of the link and the dogbone, showing the 6mm bolts

The swingarm activates a link that pushes the rear shock, giving the rear-suspension a rising rate that is plush in the initial stages of the travel, before ramping up to control bottoming. The linkage attaches to a small dog-bone link that connects behind the bottom bracket, adding torsional stiffness to the whole arrangement. This link is the first area on the bike that gave me concern as it exclusively utilises M6 bolts that are, in my experience, way undersized for this application. This seems like a pretty simple problem to fix however, it looks as if Chumba are sticking with M6 bolts in this location for the next version of the frame.

The optional rear-floater setup

Our test bike came equipped with the optional floating disc arrangement which is also CNC machined and hangs off the left-hand-side of your bike about as low as a short-cage rear derailleur would on the drive side. This is one of those floating disc arrangements that, due to it's location would unfortunately be somewhat vulnerable to crash-damage, so if you’re someone who likes plowing through rock-gardens and has been known to tear-off derailleurs in the past, this is definitely worth considering. For serious weight-weenie racer-heads and straight-out bike wreckers the bike can also be run without the floating brake but obviously, at the expense of braking performance.

Who needs braking performance anyway?

The front-end of the Chumba is a serious piece of work, with a heavily manipulated top-tube giving excellent standover clearance and a huge, wraparound gusset enveloping the top and down-tubes, giving the front triangle a monocoque appearance without the filing-cabinet acoustics associated with such designs. There are gussets all-over the place on the front-end, giving the impression that this bike was built to last with little thought given to how much it weighed. Impressions can be deceiving though, as our complete bike tipped the scales at a shade over 19kg with nary a light-weight part hanging from it.

Monster wrap around head-tube gusset, 1.125" head-tube

The front-end of the bike is also, however, something of an area of concern for me. I’ve gotten used, it seems, to bikes manufactured in South-East Asia that come stock with welds that look like a row of 5-cent pieces stacked next to each-other and I guess I’ve come to expect that quality of welding on all frames, particularly those that promote themselves on the basis of being “Hand-Made in the USA” as if that label somehow makes the workmanship magically superior to the work that Taiwanese welders can produce.

The Chumba however, does not exhibit this high quality of welding, with numerous flat-spots and in the welds, particularly on some of the ‘tricky’ looking gusset work. Anyone who knows welds will tell you that “looks aren’t everything” and that these daggy looking welds are probably just as strong as the ones that do look like a stack of 5-cent coins but for this sort of money I expect attention to detail, particularly when then frame is coming from a low-volume builder.

The Bits

While this isn’t a review of the complete build kit, as most people at this end of the market are building their own bike to a custom spec, the bike as reviewed did come with a solid build which would be a shame not to mention. Up front suspension duties were handled by an ‘06 Rock Shox Boxxer Team with Motion Control damping while out back, bounce was provided by Fox Shox superlative DHX 5.0 unit. Sun MTX rims wrapped in the ubiquitous dual-ply Maxxis Minion DHF’s and rolling on Hadley Hubs front and rear made for a strong, light and tight wheelset that was brought to a halt by Avid Juicy 7’s with 8” rotors. Shifting duties were covered by Shimano XT gear while an MRP chain device spinning on Holzfeller cranks kept the chain on. From this year the bike comes specced with an 83mm wide bottom bracket. Easton provided the cockpit in the form of an EA70 bar and stem while a Thomson layback seatpost gave some much-needed room in the top-tube department.

All up, this kit built the bike up to a quite respectable 19kgs (give or take a few, weighed on bathroom scales) without compromising on strength anywhere, so some lighter wheels, air-sprung Boxxers, lighter tubes and tyres and so-on you could happily take it down to the mid-to-low 18kg mark without too much hassle. The importer tells me that with the DHX shock the frame weighs around 10.3lbs, definitely on the "light" end of the scale for a downhill frame. If the bike was my own the only changes I'd realistically make would be SRAM running gear and possibly Mavic rims, as overall this was solid, sensible gear that didn't give me a single hassle over the course of the test while still keeping the whole bike reasonably light.

The Ride

Chumba F4 is an out-and-out downhill race bike and this becomes more than apparent the second you sling a leg over it and start pedalling. Off the line, it lacks the snap of higher-pivot designs that utilise chain-torque to partially lock out the suspension under acceleration but correspondingly, the suspension is a lot more active under power, meaning you can keep pedalling for longer on rougher terrain.

The Pro-Pedal on the Fox rear damper can be tuned to dial out most of the pedal-bob but this comes at the cost of some small bump sensitivity. Like everything on every downhill bike, compromises have been made in this design; pedalling efficiency has been traded off for better small-bump compliance and suspension that remains more active under power, you have to ride it for yourself and decide whether these compromises provide the sort of ride you’re looking for.

Matt railing a berm on the Chumba, yesterday...

Muscle it up to speed and you begin to see what the Chumba F4 is all about. With the extremely low centre-of-gravity coupled with a low bottom bracket, long chainstays and a shorter-than-average front-end your immediately find yourself set up in the “attack” position; with your elbows bent, hips low and weight centred. I mentioned a shorter-than-average front-end and I wasn’t kidding; our Medium sized frame with it’s 23” effective top-tube was only just comfortable for my 175cm frame with a layback seatpost on it and with a straight post it was uncomfortably cramped.

So it’s low in the bottom-bracket, long in the wheelbase, has a low centre of gravity and a short top-tube with excellent standover height and a rising-rate rear suspension design. Got it? Point it down a run however and all these technicalities are replaced by one, overriding feeling; “FUN!”

Matt smashes a berm at Dayboro

The low bottom bracket and centre-of-gravity combine to produce a ride with seemingly contradictory characteristics; extremely plush and stable through the roughest of terrain while being nimble and responsive to rider input and never feeling “stuck” to the ground in the way that some big, virtual pivot bikes tend to feel. You can point this bike down the gnarliest rock-garden imaginable totally confident that it will hold the line you put it on but also aware that you could, with just a flick of the body, switch lines at a moments notice without upsetting the bikes poise.

The 216mm/8.5" of rising-rate rear travel is extremely compliant and responds to small hits and chatter-bumps flawlessly, controlling middle-to-large sized hits at speed with considerable aplomb with an extremely plush mid-range before ramping up nicely on high speed impacts and big landings to control bottoming. The rising-rate of this design is so effective in fact, that the myriad of settings provided on the DHX 5.0 and similar dampers (5^th Element, Swinger) to make them more progressive, are pretty much un-necessary.

While I appreciated the added complexity in the form of Pro-Pedal to tame the F4’s bob-happy pedalling characteristics, I found myself running its air chamber as large as possible, with only 125lbs of pressure and 4 or 5 clicks in on the Pro-Pedal dial to get the most out of the bike. With this set-up I never bottomed harshly and the suspension felt smooth and controlled throughout the bikes travel. Too much pressure in too small an air chamber however and the progressiveness of the shock combined with the rear suspension to make the rear end feel like it was spiking on big hits or repeated high-speed impacts, which was no fun at all, unsettling the bike through high speed sections as the rear end skipped around fighting for traction.

The F4 isn't designed to run deep in it's sag like some other big bikes, so it's easy to pop off rocks and kickers to float over the terrain and pump for speed. With the steep head angle and low center-of-gravity the front end of the bike always feels light and responsive to your input, willing to pull off ridiculous line changes at just about any speed and carving in to corners with confidence with plenty of weight naturally transerred to the front wheel.

Matt launches the Chumba at Dayboro

Under brakes the floating rear-unit did it’s best to minimise the effect of the brakes on the suspension, with some success. While there was noticeable stiffening of the rear suspension under brakes, it was certainly less than what I’ve felt on similar suspension designs without a floater. The floater also makes noises that make you wish you were deaf, squealing, shuddering and generally getting loud nearly every time the brakes were applied. Still, the improvement to the ride quality of the F4 was probably worth the noise and this is just speculation, but I imagine that you’d have to be a committed “off-the-brakes” style rider to get the most out of this bike without the rear floater, as I can imagine the brake-jack getting pretty fierce without one.

Cornering, ohhh-ho-ho, cornering! Did I mention that this bike was fun? With the short front-end and steeper than normal head-angle, your weight is naturally shifted onto the front wheel when you dive into corners and this leads to a bike that loves, absolutely LOVES to drift like few frames I’ve ever ridden. I mentioned that you’d be thinking “Braaaapt!” when you first slapped eyes on this bike well, after your first ride, I challenge you not to be yelling it at the top of your lungs when the rear-end steps out half-way through a corner like you just grabbed a big fist-full of throttle.

The F4 will drift all day if you want it to...

Problem is, drifting arse-backwards through every corner isn’t necessarily the fastest way to get down a hill (unless you’re Sam Hill) so if you want to ride the Chumba quickly, you’re going to have to make a few adjustments to your riding style.

First off, the bike, as mentioned, has a steeper head angle than normal for a DH frame, which gives it amazing manoeuvrability in tight, technical sections but makes it less stable at high speeds. At full speed you need to have a steady hand with your direction changes until you’re used to this or you’ll find yourself spearing off the inside of turns and generally flipping your guts out. Secondly, the bike positions you with your weight well forward, so if you actually want traction in corners, you’ll need to ride with your arse hanging off the back of the seat. It feels ungainly at first but as you adjust to it you realise just how much time you can pick up when your rear wheel isn’t trying to beat your front through corners. Thirdly, said short-cockpit and steep head-angle mean you have to get way, way, way off the back on really steep technical sections lest you flip yourself over the bars and again, this feels ungainly at first but quickly becomes second nature. Overall, it’s not a bike you can jump on and immediately ride at top speed, it’s a rig that requires a bit of patience and a few adjustments to technique to get the most out of.

The Verdict

The Chumba is a big bike that can plow down the nastiest, roughest, rockiest terrain you can find while still maintaining a lively, controllable ride that’s responsive to rider input. It's low center of gravity makes it extremely stable while the short cockpit, steep head angle allow you to ride the bike on the forks and make quick direction changes when needed. In that respect, I’d say it’s more of a “riders” bike, than a “pilots” bike, it will plow down trails all day if you want it to, but you’ll get the most out of it if you’re the sort of rider who likes to pick lines, pump and jump the terrain and generally give the bike some input rather than just pointing it down a trail and letting it do all the work. If you’re in the market for a high-end downhill race frame the Chumba should definitely be on your short list if you can get a ride on one to see if the unique geometry and design produces a result that suits your riding style.

Finally, a review of the Chumba wouldn’t be complete without mentioning one major concern; durability. Try as I might to ignore it, when I questioned two people who have owned and ridden F4's in the past I got almost identical summaries of the bike that went along the lines of "Great bikes to ride, break heaps of linkage bolts though." This isn't necessarily a huge issue and your local bolt shop should be able to hook you up with a packet of the correct length, high tensile bolts without any trouble but for a frame at this price-point it's something that can make or break your decision to buy and I feel I'd be irresponsible if I didn't mention it. Our test bike did not break any linkage bolts in the 3 weeks it was in our hands, for the record.

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