The Mountain Bike Life

I think I am the last person on the planet to try out a 29er.  I have ridden a couple around a parking lot, but that isn’t quite the same as getting on the trail and spending some quality time on one.  With all the great press and good things I have heard about the riding experience you would think that I would already be sold on the concept. So…my goal is to finish my 29er by spring and put it into my rotation for the year.  FYI, I am building a Soma Juice which is a lightweight steel frame.  Remember steel is real 🙂  After a dozen or so rides I can look at some Strava times and evaluate my experiences before I decide if I will keep a 29er in my garage or break it down and sell it off.  Either way it will be fun!

 
If you have been reading some of my posts you know I spend as much time on cars as I do on bikes. Naturally the two sports have a lot in common.  Both waste incredible amounts of time and money…….Wait this is about the physics of the wheel.  Cars and bikes both share them right?  Yeah that was my point! Both also have an engine.  The one in a car is a bit noisier, takes gas, and is very heavy.  On my bike the engine does get a bit whiny at times, passes gas and could use to lose a couple of pounds.  Cool thing about a car is we can very accurately measure the horsepower it generates on a dyno.  We can’t get the exact power from the engine, but how much is transferred to the road. On a bike we do the opposite.  We can calculate the human power, but have to estimate the wheel power.  So where am I going with this?

Over the years I have raced cars and spent a lot of time on the dyno.  Racing a car is all about maximizing what you have within the rules.  Being the weirdo that I am I chose to race a 1979 VW Scirocco with about 80WHP.  One of the easiest ways to maximize that number is to take off weight.  The lighter the car or bike the faster you go….in theory anyway.  I have dynoed cars back to back changing only wheel size and weight to see the effect on HP.  Add as little as 2-3 lbs per wheel and an inch in diameter on a car and it can cost as high as 5-10% off your wheel HP.  That can be the difference between getting to the finish line .001 seconds faster.  Ever wonder why they don’t run 19s on a formula one car?  The extra weight and loss of wheel HP can make all the difference in the world on those high tech road machines.

The wheel size in road auto racing is usually determined by the smallest diameter you can fit over your brakes and then adding as much rubber as will fit in the wheel well.  This does 3 things, reduces rotational mass (covered already), lowers center of gravity for handling and increases your effective final drive gear ratio.  Higher final drive gives more power, but can result in loss of traction on a small wheel.  This is the variable the 29ner bike depends on.  With a higher ratio gears , but larger wheels you can maintain traction better.  In theory this helps on steep uphills and over obstacles.  What keeps bothering me though is the extra effort on flat ground from the extra weight and effective higher gear ratio.  No matter what you do the 26er wheel with same components will always weigh less than the 29er because it just takes less material to make it.  So in effect the rider should have more wheel HP.  No getting around that. But wait aren’t we talking about off road and not smooth tracks?

Just to confuse everyone a bit more consider off road rally, truck and Jeep races.  Depending on the terrain the wheel/tire combo size can get pretty big.  Well over 30 inches in diameter on a Jeep for rock climbing.  Yet if you watch a WRC rally race they actually use a fairly small overall diameter wheels and tires.  Those car travel in excess of 100mph and go over some pretty gnarly stuff.  So the bigger the wheel tire size the slower the vehicles seem to travel down the trail.  Can we conclude there is some inverse correlation between terrain, wheel size and speed.   The rougher the terrain the larger diameter you will need for a better angle of attack.  (Didn’t think I would miss that 29er cliche did you?)   Then your overall velocity will drop.  So then why do I keep hearing the 29er is the fastest way down the trail?  I am guessing the answer doesn’t lie in the bike, but the trail itself.  This is the variable to consider. How rough and challenging is the trail you are riding?  If the obstacles overwhelm the smaller diameter then the extra weight is worth the effort to stay moving, even if it is a touch slower.  Overall the effect over distance should be a shorter overall duration because you maintain a more consistant pace.  Did I loose you yet?

The questions that ring in my mind are these.  Was the 26 inch wheel a mistake in the evolution of mountain biking or does it still have it’s place?  Is the 29 inch wheel a size that should only be suited to certain conditions?  Is possibly the 27.5 inch wheel the answer to all this?  Like how many licks does it take to get to the center of a Tootsie Roll, we may never know.  The cool thing is there are a lot of options right now for bikers.  I know it may seem a bit frustrating to decide what you should be riding, but it beats having no options at all.  As I contemplate the biking industry more and more I think choices are a good thing as we all benefit from being able to pick what is right for us as an individual.

So what are my thoughts on riding a 29er so far?   Well I did manage to cobble the bike together enough to take it for a ride around my neighborhood.  There are some empty lots still down the street from me which are overgrown and fairly hilly.  The 29er plowed through them really well and I was impressed.  I am not sure I would have gotten through on a 26er.  It did feel a little sluggish accelerating and turning, but overall had a very good feel to it.  So far I like it, very different. My plan when it warms back up a little is to compare Strava times on the same trail under similar conditions to one of my 26ers.  Then also see how I feel after rides.  Was I more comfortable, less tired, fewer touches to the ground?  Everyone is different so my conclusion may not agree with someone elses, but I am going to approach this as objectively as possible and look at all the data.  Stay tuned…

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