Thursday, September 4, 2008

RANS Crank Forward Mountain Biking

Photo: David Friedman

Several readers of this blog have asked me about using RANS crank forward bikes for mountain biking and unpaved touring. I had some ideas, but haven't used a RANS CF for that mission so I wasn't 100% on how they would perform. David Friedman posted some great info on BROL about using a RANS CF for mountain biking and he kindly consented to let me reproduce his thoughts here. Thanks David! I know some folks will find his experiences very informative.

"As someone who has benefited from BRO but not previously posted, I thought I would share my experiences with using a CF Dynamik as a mountain bike. There have been a few threads regarding this topic in the past, and I hope my thoughts may help those who may be thinking about this option.

About a year ago, hand, knee and neck issues made using conventional bikes too painful. For road purposes, I use a Corsa, which I think is a terrific ride. For mountain riding, I have used a Turner 5-Spot for several years. The 5-Spot is generally considered to be one of the better full suspension frames. However, hand and saddle issues in particular made the Turner too painful on all but the shortest ride.

Although I was tempted to try a Rans Enduro or similar “true” recumbent as a potential mountain biking replacement, the Dynamik seemed like it was worth a try as a kind of “tweener” option. I bought a stock Dynamic, and moved all of the Turner parts over, including a Fox Talas 100-120-140mm adjustable front fork, WTB 2.4” Mutanoraptor race tires (actually about 2.2-2.3” in width), Avid mech discs, a Shimano XT crank with outboard bearings, a SRAM X.9-X.0 drive train, and an 11-34 Ti cassette. Equipped this way, the Dynamic weighed just over 27 lbs, about the same as the 5-Spot. The attached picture shows the bike with the Talas at the fully extended 140mm position.

I live in the Los Angeles area. State parks and national forests in the area offer numerous fire and jeep road and single track rides. I’m 51, and a typical ride includes about 2000-3000 feet of climbing over 15-20 miles (I have one of the Cateye altimeters, which works surprisingly well), a few significantly steep (15%+) climbs and descents, minor “stunts” such as smaller logs, rocks or steep “U” drop-offs and rises, and steep, loose, rocky, rutted singletrack. I don’t do jumps and try to avoid significant (OK, any) “air.”

I’ve taken the Dynamic on several rides, and my impression so far is very good. As others have noted, getting the seat and cockpit set up takes time. Early use generated some upper thigh and butt pain that I think was caused by the having the seat pan too flat. This has largely vanished over time. The bike brakes exceptionally well, which is a real confidence-booster on steep drops. I’ve found I increase speed compared to the Turner on the same smoother downhill runs (where the 5-spot’s full suspension would offer less significant benefits). The Talas makes the Dynamik quite versatile. At 100mm, the geometry is pretty much as spec’d with the stock Trail and Pro models that use an 80mm SID fork. This position allows for better climbing and more nimble handling. At 140mm, the front end comes up and the bike feels a bit more like a LWB ride. The mechanical (and psychological) benefits of this geometry allow for rapid descents and confidence on steep drops. I believe the Talas is a more solid fork than the SID and may enhance climbing; I would think that it performs better in rough terrain and on downhills.

Thoughts on the Dynamik for use as a mountain bike include the following:

1. On flats, downhills, and modest (maybe 5% or lower) climbs, the Dynamik seems to perform at least as well as the 5-Spot. On flats and climbing, where momentum can be maintained, I often find myself using a higher gear than with the Turner. The Dynamik’s braking and handling—especially at speed—are extremely good and comparable, if not better than the Turner. This performance helps maintain the “fun” part of mountain biking—i.e., the few minutes of downhill after the many minutes of uphill.

2. On rougher downhills and drops, the bike requires more thought and strategic riding to compensate for the lack of rear suspension. Weight shifting on and off the seat is required to take up the shock from bottoming out at the end of a drop or riding over rocks, ridges and trenches at speed. I find that the CF position makes such adjustment a little more challenging because my knees are lifting from a more angled initial position than on a conventional mountain bike. The need to adjust and think about how to react rather than let the full suspension deal with obstacles makes the rides a bit more involving, but also less forgiving. Like any unsuspended frame, the Dynamik will let you know when a poor line is chosen or body weight is not sufficiently adjusted to handle an obstacle.

3. On steeper terrain, I tend to use a lower gear than used with the Turner. As many have noted, on steeps the CF tendency is to pull on the handlebars to stay more forward and to maintain power to the pedals. This can put some strain on the fingers and biceps during long climbs. I view the additional arm workout as a plus, but it is a noticeable difference. The CF climbing hand pressure is the opposite of what occurs on a conventional bike which related to downward stress (rather than pulling) on the handlebars. I have not found the need to downshift a bit and pull on the bars to be a significant climbing negative in most uphill circumstances.

4. In my experience, the bike performs less reliably on steep singletrack climbs where it is important to maintain a steady line to avoid loose stones, dirt, ruts or vegetation on either side of a narrow path (e.g., one that can be only a few inches wide). I found that the bike would sometimes drift into looser stuff or a rut that I wanted to circumvent. In one case, where the trail is extremely steep and strewn with ruts and broken rock, I skillfully managed to drive the bike into a manzanita treelet that mangled the rear DR and dropout. This section, and others like it, were difficult but almost always makeable on the Turner. They are more of a challenge, at least for me, on the Dynamik.

5. On twisty singletrack, the bike seems to do relatively well. Sections where very sharp turns are combined with deep ruts, off-camber topography or strategically placed rock gardens are generally as makeable as with the Turner. I have found that proactive weight shifting and gear management are more important in these sections than with the Turner.

6. As discussed at Spincyclz dot com, rear chainsuck can occur with a CF running larger rear tires. This problem seems most likely to occur when: (a) the smallest cog in front and the larger cogs in the rear are engaged; and (b) the rider is not pedaling and the chain is not tensioned. Under these conditions, the chain can encounter the outer knobs on the rear tire and be sucked into the rear chainstay gap. The solution suggested at Spincyclz dot com is to place a retainer to catch the chain before it is pulled into the stays. I adjusted the XT cranks as far to right as the chainline would allow to increase the gap between the chain and the outer wheel knobs. By shifting to the middle or outer ring front prior to downhills, the problem has been pretty much a nonisssue for me. I found that chainsuck could occur, however, when riding through a series of short ups and downs when it is desirable to keep the bike in a very low gear for an upcoming steep bit. If this kind of terrain is regularly encountered, it may be necessary to install a physical retainer to preclude chainsuck risks. It appears that chainsuck is related to the longer CF chain which may flex more from side to side than on a conventional bike and potentially hit the rear wheel knobs when untensioned.

7. The Dynamik has proven to be significantly more comfortable on my butt, seat area and neck. It also seems to reduce back strain. The bike appears to reduce, but does not fully avoid, downward hand pressure. Long downhill runs still result in some “traditional” hand strain. The bike also generates more “pull” pressure on the hands and arms for climbing. In general, my hands feel better after a comparable ride on the Dynamik than on a conventional mountain bike. Weight management requirements can stress my knees to a greater extent than on the Turner, possibly related to lifting off the seat with the knees in a more forward angle compared with a conventional bike. I may be more sensitive to potential knee stress than other riders, and varying pedal positions during downhill runs seem to reduce this concern.

The Dynamik, at least as equipped with a 100-140mm Talas, seems to offer a very reasonable mountain biking option for the fire road and most SoCal-style singlerack that I routinely ride. It is not as nimble as the 5-Spot steep on more technical, narrow uphill sections. It seems like the longer wheelbase and lack of rear suspension would also underperform on more twisty trails, but this has not yet proved to be the case. I would be disinclined to use the bike on trails with rocky drop-offs, substantial rock gardens, lots of larger rock fins (or logs) and other conditions where full suspension and a higher bottom bracket seem obviously required. A lightweight suspension seatpost with a lockout for climbing might improve the bike’s overall downhill utility, but so far the lack of rear suspension has not been a significant problem on the routes that I ride.

I hope this summary is useful. My thanks to this website for all of the great information it provides!"

No comments: