Once we got back home and the Africa Cycle Fair dust settled, I got in touch with David again to talk bike. As per my previous post, I considered a custom geometry, but soon realised that what I felt at the Fair was what I've been looking for and there was no reason to toy with a winning formula.
David was kind enough to share some details on the Hungry Monkey frame and his frame building process with us below:
I've had a primate obsession all my life although I mostly manage to keep it under wraps. As a youngster I thought it was terribly unfair that humans didn't have tails - I mean, how cool would it be to have a tail? These days I still find myself wishing for a tail at times - it could be mighty handy in the workshop!
While my monkey madness has cooled down here in these southern latitudes I can't help but poke fun at my past and so all of my own frames have primate inspired names. When I came to name my very first handmade mountain bike frame I couldn't think of anything more appropriate than Hungry Monkey - what could be more manic, willing to take more risks and generally more mischievous than a truly hungry monkey? Hungry Monkey II takes this theme and expands on it - I saw no reason to change the name and I hope others like it too. The only question is: are you ready to feed the Monkey?
Why build a long travel, steel hardtail?
Hungry Monkey II is a very versatile AM inspired lightweight bruiser. Very long travel forks haven't really been sensible options on hardtails until the introduction of Rock Shox' new Pike. Great support from a truly luscious travel stroke ensures that every inch of travel is used but without the wallow and dive traditionally associated with such long travel. Couple this with relatively lightweight construction, sensible through axles and there really hasn't been a better time for long travel hardtails.
In designing the Hungry Monkey II I wanted to create a bike that would be most at home on technical singletrack descents. Primarily though it needed to be fun to ride. It also needed to be able to get to the singletrack capably. I design my MTB frames around fork lengths with sag dialled in. For a 160mm fork that meant that 25-30% sag could be dialled in to give maximum plushness and to allow the fork to drop into small holes as it travels along the trail.
When deciding on the head angle I look at what would happen at maximum compression: for every inch of suspension travel the head angle will change by approximately 1 degree. At full compression it's safe to assume that the rider is taxed to the maximum and so having a bottom out head angle that is too steep will result in a bike that is liable to throw you over the front when you least want to be. In my experience anything steeper than 72 or 73 degrees makes it difficult to recover the the front end in these G-out situations. If we start with a 73 degree head angle and progressively add travel by the time we've added 5 inches (roughly 160mm MINUS 25% sag) the head angle is sitting at 68 degrees.
The seat angle does less to the handling of the frame but does influence it's maneuverability. Slacker seat angles make for bikes that manual easily and endow the frame with a playful feel as it's easy to get your weight back and over the rear wheel. The compromise here is that slack seat angles reduce your reach for a given top tube length. A bike with a very short reach may still feel stretched out when seated but as soon as the rider stands up and is positioned over the BB it can suddenly become very cramped - this hinders climbing performance. Very slack seat angles may also make a bike difficult to pilot up steep terrain as the front end continually wants to loft and wander as the riders weight is so far back.
I gave the Hungry Monkey a reach of 428mm in the large size. A 73 degree seat angle is slacker than a lot of other newer bikes on the market but I feel it gives a good balance between playfulness and sensibility. Using these figures it then follows that the frame's theoretical top tube length works out at an even 620mm. If a large (19") frame is your usual size then you should be perfectly comfortable aboard a Hungry Monkey with a shortish stem. Seat angles are actually easily adjusted through pushing the seat forward or back and by changing to an inline or set-back seat post.
The rear end of the Hungry Monkey II contains some mild trickery. It utilises sliding dropouts so that the rider can customise the rear centre length of the bike depending on their riding style. Very short rear ends favour drifting and aggressive over-the-front type riding but can be a bit of a handful for longer rides.
Long rear ends add stability and encourage carving of corners - they do better when traction is good or when the rider wants a predictable feel that doesn't depend on their initiating a rear slide. Longer rear ends also beat you up less - the bumps transfer into the frame from further back and there's a longer rear triangle to help absorb shock - they also tend to climb better. On the frames I've owned with adjustable dropouts I've enjoyed the opportunity to play with these parameters depending on how I feel on the day and where I am riding. I hope this makes the Monkey as capable on gnarly trail days as on long rough endurance rides. As a happy aside it also caters nicely for single speeders. The rear end on the Hungry Monkey II can grow by 25mm from a short 420mm to a relatively long 445mm. In it's shortest setting there isn't room for a front derailleur - this is a 1 x specific bike.
The BB is relatively low. With sag it sits 45mm below the axle line. This keeps the riders weight lower in the frame and makes cornering more stable. The disadvantage is that the pedals are closer to the ground and initially you may encounter more pedal strike than you are used to - most people adapt very quickly to this and really enjoy the cornering benefits and high speed stability it gives. Downhill bikes often have extremely low BB's and yet you never hear DH riders complaining!
I gave a lot of thought to the tubing I used. I didn't want to make the frame unduly expensive but also wanted to take advantage of some of the metallurgical advancements steel has been through in recent years. Most of the frame is constructed with Dedacciai COM tubing. This is a Nivacrom type steel. Nivacrom was introduced by Columbus in the 80's and for a long time was the highest performing bicycle steel available. Nivacrom contains small amounts of Niobium and Vanadium alloyed with Chromoly steel. Vanadium and Niobium precipitate in the metal matrix when heat is applied and this blocks grain growth. The fineness of a steel's grain is responsible for it's resilience and resistance to fatigue. The finer the grain the better a steel will withstand fatigue. Think about mud pies - mud pies made of fine grained clay will hold together far better than mudpies made of coarse sand. In a standard steel alloy the addition of heat causes the grain structure to grow - this happens right at the weld site where you least want it and causes brittleness and loss of yield strength.
Dedacciai is an Italian tubing manufacturer - they aren't as storied as their far older and more famous cousins, Columbus. As a result their tubing is a little cheaper. Another factor I considered was the length of the butted sections of the tubes. Dedacciai has quite long butts on one end of the tubes and this allows the framebuilder to use a longer butted section at the head tube junction where most of the forces are going to be acting. The walls are still relatively thin though and vary from 0.9mm at the butted ends to 0.6mm in the thinner walled centres. Butting removes material where it isn't needed but also gives high quality steel tubing another property: zing.
Much is made of steel's vaunted reputation for ride quality but this cannot happen if the tube walls are too thick. Thinner walled tubing is able to vibrate and flex and gives a frame zing and zest. That 'feel of steel' will be absent in a frame built of plumbers pipe - I didn't want it to be absent in my Monkeys. To avoid any wet noodle syndrome I chose oversized tubing - the down tube is a manly 38mm diameter while the top tube is 31.7mm. The seat tube (a Columbus Zona offering) is a 33mm diameter that accepts modern 31.6mm dropper posts. Large diameter 30 x 17mm chainstays taper to a delicate 12mm at the dropout and 16mm tapered s-bend seat stays help the rear wheel to track better. I also just like the way the bends look. Up front a 44mm machined head tube will accept any new tapered steerer fork. The BB shell is 73mm wide and threaded. Press fit designs haven't really impressed me.
Putting it all together
Everything is fillet brazed together - brass is melted into and around the join to fuse the tubes to one another. On a microscopic level some interesting things happen and there's a transition zone of bronze that forms through the interaction of steel and brass. The fillets are then smoothed out with a file and emery cloth to form smooth radii at each juncture - this helps to dissipate stresses at the join as there are no sharp transitions to form stress risers. Since brass is also more ductile than steel there's a microscopic amount of flex that occurs at the joins. Traditional frame builders believe that this ductility further enhances resilience.
I don't use gussets on the front triangle as I feel that this just concentrates stress around the ends of the gusset. Where I do use gussets is to increase the wall thickness of the chain stays where they are joined to the BB. I do this because I don't use chainstay bridges. By leaving out the bridge there's a bit more clearance for larger tires and mud. Since a chainstay bridge effectively shortens the chainstays one needs a little additional material to offset the stress. Chainstay gussets do this and... I just think they look cool.
It's important to realise that as far as AM rigs go, the Hungry Monkey is a relatively lightweight machine. A bare 19" (large) frame weighs 2.14kg. Complete build comes in around 12.5kg. As such it's a tool for experienced riders who can read the terrain and pick a line. Hammer it into the face of doubles or flat land too many roof drops and the lightweight tubing will complain. In the right hands I believe this will be a very rewarding bike to ride and it'll beg you to push it further and faster.
-- David Mercer
Once my order was placed, all that was left to decided was the build kit and whether to keep the frame raw or to give it a lick of paint. More on that in Part 3.
For more information on Mercer Bikes and to contact David visit http://mercerbikes.co.za
For more information on Mercer Bikes and to contact David visit http://mercerbikes.co.za