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BMC Fourstroke 2010

Today’s rare lycra clad adventure is over. With a coffee in hand now, it’s time to pen the review of the BMC Fourstroke FS01 frame.

Bike setup

It has been a while since a dedicated XC bike came to my hands. Much less a souped up lightweight carbon frame with top end parts.Though the review is for the frame but it would be hard to not to take other components into account as everything work together as a whole. The objective is to isolate any component weakness and focus on the characteristics on the performance of the frame itself. Namely the working of the suspension system tweaked in the context of a racy xc bike and the overall stiffness, construction and sensibilities in the numbers and geometry of the frame.

Except for some Crankbrother eggbeaters pedals that’s swapped to pair of Xpedo Ti-Ti, the setup really couldn’t be faulted. Full XTR for drivetrain duty save for a front XT deeraileur and a pair of Formula RX for stopping power.

The rest of the contact points were a mix of lightweight Ritchey Superlogic low rise 620mm carbon bar with a 110mm WCS stem, BMC’s own carbon post and a racy but rather comfy Selle Italia SLR.

A Fox F100 up front completes the lightweight package. The only thing that caused a wince was the rather anorexic looking UST Racing Ralphs on an otherwise nice set of Mavic Crossmax SL hoops.

Overall the setup would do justice for the review of the frame set

The Frame

The BMC fourstroke FS01 came with a FOX RP2 rear shock. It has been said that with a good suspension design, initial compression platform in shock technology, in this case, the Propedal (for Fox) is not really necessary. A “locked up” suspension equals no suspension and thus loss of traction, the primary function of any suspension system. It remains to be tested on the trail to see if the claimed APS, Advance Pivot System can really negate the use of the propedal and yet not suffer in the climbing department.

One thing that I come to appreciate on BMC bikes is the stiffness of the BB area emphasized as “core stiffness”. More can be read at the official BMC website, http://www.bmc-racing.com/int-en/technologie/precision-engineering/ on the drop down list under technologies.

Glancing at the Fourstroke did not disappoint. Classic BMC signature of a stiff looking boxy downtube with a width that’s flush with the bottom bracket This stiffness is further reinforced when one zooms in to the first of two pivot point of the main link just above the BB and more or less directly aligned perpendicularly with the seat tube. The entire main pivot is short and beefy leaving hardly room for flexy issues.

APS System— While rocker arms are two separate triangular pieces in this design, the minimal length and boxy alloy links (together with the main pivot piece, the only non carbon parts of the frame, other than shock) ensure another minimal flex area. Big seal bearings with nice anodized covers ensures things will hold up with repeated stress of suspension movements.

A nice touch comes in the form of a sag indicator over the top of the right side rocker arm that is “calibrated” with a line marked on the seat tube. Nice but my preference still goes to measuring physical sag with a metal ruler on the shock itself though.

As far as XC bikes goes, usually most rear suspension design are rather progressive due to the kind of riding as compared to slacker geometry bikes. The RP2 is positioned perpendicular to the ground. The bottom mounting point is yet another point which has a secondary function of increasing stiffness in the BB area, a short no nonsense section that holds the mounting hole and also wedges itself between the down and seat tube. Clean design that will also make washing and cleaning after muddy rides a breeze.

Yet another point in their whole pursuit of “stiffness” can be seen in the seat clamp area named the “skeleton concept” that has undergone some changes over time.

The earlier design on this “concept” had been less than desirable in some cases especially on the aluminium models where if memory served well, was named the “crossloc”. This time around, it seems that more thoughts has been put into this area. Resulting in what looks to be a functional design. This together with the bracing under the top tube completes the entire mid section of the frame’s stiffness.

Quick calculation gives a leverage ratio of 1.778 (4 inch travel on a 2.25 inch stroke) which is pretty low. First thing that came to mind was the loss of suppleness in initial small bumps and with a progressive nature. It would probably be rather hard as the bike works itself further into the range of rear wheel travel. Hence the initial dialling on the test ride in started with 25% sag. A hefty amount of sag in xc bikes but which later proves to be insufficient and a trail fix got it to 30%. More on that later…

Specification for this med 17.5″ frame states a 69.5 degree head angle that’s likely based on a 100mm travel fork. Over a longish 23.4″ effective top tube. Together with a seat tube angle of 73.5 degrees and a 17″ chain stay length, these numbers have obviously changed from the earlier fourstroke which was considered a little too upright as a race xc machine.

The 2010 makes for a more racy stretched out top tube (23.4″ TT on a 2010 Med vs a 23.5″ on older 2008 model, Large). As far as head angle goes I would like to believe the 1.5 degree reduction would make the descents a tad more manageable.

The boxy flat top tube is another thing that can be appreciated. Unlike many models nowadays where the top bottom and headtube is fused into one mass to achieve further stiffness of the front end, the top and bottom tube is not fused here. This makes for easy mounting of things like battery packs on night rides. Both tubes are massive and the whole front end with the headtube looks pretty stiff.

An observation that proves true later in the ride as all flexing were narrowed down to the fork and 9mm skewered wheels. The front end of the frame yielded nothing in the way of being flexy.

Routing of cables is not common by today’s standard for mountain bikes. With the routing going to the bottom end of the down tube, that is a trend that was much more common in early mountain bikes that were still very much influenced by road bikes. The problem has always been mud and grit over time which affected shifting. However rear dee cable guide on the Fourstoke is meant for full length cabling which makes it a non issue. Examining the exposed cable for the front shows its one long section that will be little affected by mud and grime.

The only point left to ponder was whether running in such configurations will affect shifting as the rear suspension starts working and stretches out the cable in the process.

It proves to be an unnecessary worry as there was no ghost shifting nor weird unexplained shifting issues. The routing and the XTR drivetrain works flawlessly during the entire test.

Tire clearance from the stays are narrow. The position of the bridge on the seat stays might even have prevented other more true to size and higher center knobbed 2.1 tires (The 2.1 Racing Ralphs used here are really more like 1.95 tires).

Rear brake mounting was well thought out BMC call it the “nbp” aka “natural born postmount”.

A direct post mount with replaceable inserts (should you somehow manage to mess up the threadings). It claims “The brake mount is directly bolted to the frame without the loss of power through the need for adaptors.”

True or not but an adapter-mounted front brake on this bike that was less then perfect for one reason or another, clearly made the rear brake feels more powerful and less mushy during the ride. Much of the test ride was centered around feathering the rear to regulate speed coming into turns which was more than sufficient in most cases.

All in, one would be looking at a race bred carbon xc frame that screams “stiff and fast” every inch of the way from headtube to dropouts. With all the menacing growling stiffness staring back and challenging me to put a leg over the saddle, it remains to be seen if their TCC, Tuned Compliance Concept of carbon molding really works to “optimize overall ride quality through carefully selected combinations of shapes and materials. Applying stepped profiles, specific carbon fibre alignments and different material compositions in key areas of our frames, forks and seat posts enables us to increase desirable vertical compliance while maintaining high levels of lateral and torsional stiffness. TCC improves the cycling experience by simultaneously providing precise control, reduced rider fatigue, confidence-inspiring handling and efficient power transfer.”…. as taken from BMC description of this technology.

Preparing

For all the dialling its important to dial in more than just the setup. Thhe right mindset for the type of bike in question. Meant to be a fast racy xc rig, expecting the same comfort level of a 6 inch trail bike would not be a fair review of it’s capabilities. This will be a frame that is valued more for its climbing prowess and flat out speed on the straights than the gravity oriented sections. Things started off by tuning in to a 25% sagged rear and ~17-18% front sag to let the suspension sync. together with 35psi in the tires…..

The Ride

Out on the short tarmac stretch leading into the trail, the bike surged like a bat of of hell on the first pedal stroke. Quick, responsive and very much on the big ring up front that feels light on the pedaling stroke, the feeling of speed was written everywhere. However it soon proved too much, once into the trail proper every piece of root and rock was a kicker that tries to get the saddle to high five with my lycra rear end. Travel on the shock registers hardly more than 60% (bearing in mind that includes the initial 25% sag).

Preconceived ideas and pure eyeballing has under estimated the stiff nature of the bike. Less than 1 km in, it was back on the trailside tweaking the rear sag to 30% while dropping tire pressure to 25 psi. Second lap onwards, all the fiddling was done, with just one click off the rebound to reduce the rear kicking up during successive step-downs on descents. This also allows the rear travel to be used more fully.

From here on the overall traction was much better but yet retains the earlier firmness without any feeling of loss of speed or pedaling efficiency.

Due to the progressive nature, there is no worry of the shock packing down or bottoming out. If there had been more time to spend with this bike, I am inclined to believe that a good setting on the rear would run between 32-35% sag for 65-75kg riders.

Ups

If you are looking for the special kind of ride quality in steel and titanium then this is not the frame to get. But as a climbing rig, frankly it will be hard to find something to match. At times I forgot its actually a full suspension frame beneath me until hitting a square edge or finding a wrong turn into a bank during a climb.

On a hardtail many a time the kick over such obstacles results in a loss in momentum and would find one foot down signaling a premature end of a climb that we are all so familiar with.

In or out of the saddle, any wallowing of the suspension could hardly be felt. Once the sag and rebound was dialled in properly, when hitting those aforementioned edges and banks, the suspension kicks into action nicely yet remains firm with the next forward pedal stroke to propel the whole bike forward and continue climbing away.

Euphoric with the speed on ascending, more than a few times, I took the wrong line and found myself working up a section of rocks and gutters. There was just enough traction on the rear that finds a grip and allows pedaling forward. Its not the tush friendly soft feel like a coil on a bigger bike but a traction that hardly registers pedal feedback (in all three rings up front) and no perceptible squat that takes away power.

Lack of engagement with the Mavic SL was no issue as the whole bike was light enough to carry sufficient momentum together with the pedaling efficiency. At no point of time was there even a hint of a snaking front end even on the steepest climb which would have slowed things down.

Downs

What goes up must come down. Realizing the 110mm stem is probably a tad too long with the longish top tube, the down sections was initially handled with a little more caution than the reckless abandon associated with bigger and slacker bikes.

The first couple of descent was taken in slow deliberate fashion wriggling down and regulating brakes along the way. Dredging up the memories of xc riding from the back of the mind with constantly weight shifting the descents soon got faster and faster as the rear behaves in a much more controllable manner and not kicking up evey time it hits a drop or root after the last trail side tweak.

The test came when attacking multiple foot high drop in quick succession. After the efficiency on the climbs, it would be natural to expect that when bouncing down, finding traction would be a little less than stellar. Bearing in mind the steeper head and seat tube angles, by pulling the weight back appropriately, the bike holds down sufficiently well and that’s also the realization that slower rebound tends to work better with the very suspension design between my legs.

Already impressed with the point and shoot capability on the climb and ability to change lines quickly, the same nimbleness was a pleasant surprise when pointed down despite a slightly stretched out cockpit of the longish stem mated to the head tube. Turn the front and the rear follows predictably. At high speed, the rear starts to drift , catching up with the front but with a sensible tire pressure, its all still very predictable and controlled.

Speed on the Flats

Out of a turn from a descending stretch, immediate response to acceleration back into the flats will be another thing that puts a smile on the face. It instantly goes back into hardtail pedaling efficiency as the bike hardly squats into the travel leaving lag time in between while you try to pedal.

Weather was hot and dry today but the recent rain has left lots of muddy patches. Not wanting to be slowed by the sucking mud, I launched into a series of speed hops before each one that can be cleared. Preloading and air over longer distances rather than height. Another pleasant feel, the bike lands with a firm feel that is not too hard on the feet and let one immediately continue pedaling. After awhile the same move was taken over all the perpendicular logs and roots I could find on the trail. It sure beats unweighting the front all the time to clear at higher speed. Being light becomes a bonus as you can repeatedly fly over stuff without tiring out easily.

Was something amiss? Yes, the use of the Propedal. Truth is it got fogotten totally. With the kind of firmness and ability the bike already provided going up and out of the saddle charges without feeling the loss of efficiency with a wallowing suspension, it simply did not register in my mind to use the propedal.

Therefore in my books, yes, the APS design has succeeded in having a suspension system that does negate the use of a initial compression platform. Would climbs and flat out speed have gotten even faster with the Pro pedal on? I doubt.

The lack of pedal feedback and pedal induced bobbing is what makes the whole ride stands out. Propedal would only have been counterproductive in making the bike skip when hitting the rough uneven parts on the way up or on the flats.

Conclusion

Skepticism was the word when I was first presented with a full carbon xc bike to try. The mind conjures up an image of bone shattering bouncing and an aching weekend but what I rediscovered was the fun in xc riding

The test ride was at least twice if not more longer in terms of distances I usually ride at one of our local trail. All done in yesterday’s blistering mid day sun, I have to admit its not something that was attempted in a long time nor do I find the energy to do so with many other bike.

It is hard to determine if the TCC technology actually has a positive effect in reducing rider’s fatigue. As far as could be observed, the combination of the lightweight setup and a stiff well designed frame tamed by a functional suspension design leaves a lot more energy to ride longer and harder.

A nicely tuned drivetrain with flawless shifting was much appreciated. It allows the full focus to be channelled on the characteristics of the frame and the way it works during the ride. Other less than stellar performance from thin tires and flexiness of smaller forks should not be factored in here as the review is on the frame itself.

A slightly less aggressive xc cockpit set up to ease pressure off the palm plus beefier tires would have left me wanting for more and probably riding into the dusk.

Everything said so far has been positive, therefore what are the cons?

First its a hollow feel which amplifies every little creak and groan on the bike. While it doesnt point to any mal-alignment of the pivot or worn bearings, the sound does get a bit annoying until you are well into the riding to stop noticing as the focus is on attacking the trail ahead.

Part of the TTC technology sees the application of stepped profile which means there are likely thinned out sections while beefing up the high stressed area on the frame. Together with a lack of a thick layer of lacquer over the frame which all no doubt serves to reduce weight, any flying chips hitting the tubes producing a whip cracking sound could put a wince on the face. Thankfully the test rig came with protection in the right places. Still I did notice there were quite a few minor dings on the carbon surface which though do not seem to affect its integrity but wouldn’t be too aesthetically pleasing over time.

Tire clearance could be improved. Not so much to plonk a big 2.35 tires in place but maybe to accommodate some sensible 2.25 tires on day long epic rides would be nice. However this could a design challenge without affecting the chain stay and main pivot measurements which would certainly have contributed to much of the positive ride qualities described.

Frankly there is not very much to nit pick. Its a frame that does well for its intended function. At the end of the day, if you are a xc racer or just simply looking for speed and climbing efficiency and don’t see yourself belonging to the long travel tush cushy crowd, this looks to be a fine choice.