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Boulder Bicycle Lugged

Tuesday, July 16, 2013

Should I Choose 650b or 700c?

650b or 700c?
By Michael Kone

Customers and friends ask us this question constantly.  For many riders, the answer is not a simple decision.  Like with most things, there are tradeoffs.  When trying to answer this question, we like to make two statements.

Statement One:
High quality 700c tires, including the wider models, seem quicker on acceleration, and many riders find that they feel a smidge faster, than a high quality wide 650b tire such as the Grand Bois Hetre.  The 700c tire also provides handling that feels a bit more sporting than 650b.

Statement Two:
It is amazing how little one gives up when they go from a nice wide 700c tire (30mm or so) to the even wider high quality 650b tires such as the Grand Bois Hetre.  And the cushy ride of a high quality 650b tire is so nice!  On a 650b tire, you feel like a kid as you feel able to go anywhere yet it’s exceptionally stable and fast.  Fatigue is lessened on a cushy 650b, and hitting a bump late at night when you’re tired is less likely to cause mayhem if you’re on a wide 650b tire.

So these statements outline the dilemma;  650b tires do seem to give up something, but they don’t give up much, and they have important advantages.

Some of the fastest long distance brevet riders believe that 650b tires are their best choice.  If you do the math, this makes a good bit of sense.  Very fast riders spend a relatively small portion of their energy overcoming rolling resistance but a great portion of their energy fighting wind resistance.  So if the rolling resistance is greater on a 650b tire, the percentage decrease in their speed is slight.  But a slower rider is using more of their watts for overcoming rolling resistance and its increase will decrease that rider’s speed by a greater percentage.  It almost seems unfair – a rocket of a rider can use comfy tires and pay a relatively small speed penalty for that comfort gain.  Slower riders like myself will pay a higher penalty.

Of course, for the slower rider, more time is spent on the bicycle for a given distance.  The comfort benefit (and resultant rider energy savings to the rider of not getting bumped around so much) of the sweet 650b wide tire may be far more important than any speed loss.

There is some question as to why a wide 650b tire with its smaller diameter and therefore reduced inertia would be slower than a 700c.  The math, according to my conversations with Jan Heine of Bicycle Quarterly, indicates that 700c wheels should not be faster.  One argument I make is that the debate on hysteresis losses (losses from tire casing deflection under load such as when climbing out of the saddle) have not been accurately measured.  Bicycle Quarterly tests have attempted to measure hysteresis losses by analyzing rolling resistance at low tire pressures when a rider is pedaling smoothly.  With relatively unsmooth pedaling (hard climbing or sprinting) the losses associated with a larger tire casing may be greater.  Testing this assertion, though, is very difficult. 

It also is possible that at least a significant reason 700c seems faster is psychological.  Even wide 700c tires (such as a Grand Bois Cypress or Challenge Strada Bianco/Eroica) seem to handle in a more sporting fashion and seem to accelerate quicker than wide 650b tires such as the Grand Bois Hetre (once you ride a Hetre equipped bike for a long time, this difference may be forgotten).   But this difference may not be real.  But for many riders, does that matter?  With enough horsepower and proper tires, one can possibly accelerate a large SUV through a curve at a faster speed then some lightweight old British sports cars.  But which vehicle puts a smile on your face and makes you want to drive the twisty roads more?  If you like the feel of a bike, even if it is not measurably faster, you may ride more and ride happier.  Isn’t that what it’s all about?

Even slight differences in tire speed, though, can make a huge difference on some rides.  Sometimes just a little difference in acceleration can determine whether a rider catches another rider’s wheel when the tempo ramps up.  By not hanging onto a group, the overall difference in time on a ride may become much greater than the slight difference in speed one would calculate from slight differences in rolling resistance.

Without a doubt, a rider’s typical choice of terrain tempers the 650b versus 700c debate.  If a rider spends many miles on gravel or dirt, or very rough roads, then the optimal choice swings towards 650b.  If a rider does rough stuff only on rare occasions, and likes to do go-fast lunchtime hammer fests, then the more perky nature of 700c might make it the preferred choice. 

Aside from these big picture considerations, equipment availability, lack of availability, and recently introduced products also influence the 650b versus 700c decision.  What follows is a list of products and product issues that should help influence a rider’s choice.

Limited availability of 650b tires and rims
If you destroy a wheel or tire, you may not find a replacement easily.  Also, almost all 650b tires are made by Panaracer (Grand Bois, Rivendell, Soma, Pacenti).  If the Panaracer factory was destroyed, 650b tire availability would evaporate.  That said, if that did occur, there are other makers that would jump into the market.  The wide array of Panaracer tires in this size is actually a detriment to other potential entrants wanting to enter the market.  It’s the right size for one major player, but not for many manufacturers.

Lack of an optimal 650b rim – no longer an issue!
We now are offering for late July or early August delivery the Velocity A23 OC (offset) rim in 650b. But previous to this introduction, there has been no perfect 650b rim that we totally love.  We use Velocity Synergy rims, but the deep bead wells make for a tire that needs to be carefully seated.  We use two layers of rim tape and have virtually no issues.  But others for some reason appear to have issues.  Also, for high-dish wheels an asymmetric (offset rim) makes tremendous sense.  But Velocity Synergy rims must not be tensioned too high or there is risk of rim cracking by the eyelets.  This rarely is a big deal, but it doesn’t give anyone a warm feeling.  The Velocity A23 rim does not suffer from eyelet issues.

A number of folks seem to like the Pacenti rim.  But we refuse to use it for a high dish wheel on a wheel we would send to a customer.  Box section rims are notorious for eyelet failure, and until there are riders with many thousands of miles on these, we aren’t sure there will not be eyelet issues.  We could be wrong.  But the high dish wheels we’ve seen built with these rims had very low tension on the non-drive side.  And even those wheels had enough tension on them to distort the rim around the eyelets.  Now, for a rider who is experienced with wheels, the Pacenti rim could certainly be built to its appropriate tension, and then re-tensioned as needed should the wheel’s tension fall below threshold.  But this is not a plan for wheels that go out to customers who want to simply ride and not futz with their wheels.  But for some, it is fine.  And for a low dish application, the Pacenti rim is great. 

As we said, the rim issue is now solved.  But remember, there are still many more 700c rim options out there compared with 650b!

The Challenge Eroica/Strada Bianca 700c x 30mm and the Grand Bois 700c Exta Leger
These tires really messed with our thinking.  They are each so fast and so comfortable that riding a 700c bike (specifically my old 1970’s Cinelli) with these really diminished our enthusiasm for 650b a bit.  The velvety ride and speed and perky 700c handling has been intoxicating. 

The Eroica arrived in late September 2012 and I couldn’t get off the Cinelli once they were installed.  But Challenge for legal reasons needed to drop the Eroica name and the new tire is supposed to be the same but is called Strada Bianca.  Note that later Eroica tires and the new Strada Bianca tire have an additional anti-puncture strip.  This does diminish the tire’s performance.  I actually removed this strip (it was on the inside of the tire) by carefully prying and peeling it off but we’re not sure it is ok to do this (perhaps one could damage the tire’s casing).  For some rider’s, the puncture strip might be very welcome in helping against the battle against flats.  But for me, it was an unwelcome solution to a problem I didn’t want to solve. 

The Grand Bois Cypress Extra Leger is in very tight supply.  But it is super comfortable and seemingly very fast.  I find that the Grand Bois tires do not feel as “alive” as the Challenge tires.  Even the Grand Bois Cypres Extra Leger does not have overly high TIP counts compared with Challenge.  But for smoothness and comfort, the Grand Bois tires are amazing.  It appears, though, that the Cypress in general and the Extra Leger in particular, may have a casing that is a tad delicate for much off road use. Our shop “testers” are seeing some casing cut issues.  The Challenge Eroica/Strada Bianca as we see it, is probably going to be the go-to tire for hardcore 700c off-road performance riding.         

Grand Bois Hetre Extra Leger 650b x 42
So we’re thinking that maybe with the fabulous 700c Challenge Eroica that 650b isn’t as compelling.  But then Grand Bois upped the game with their Hetre Extra Leger.  This is an important tire.  While it may be pushing the weight/durability limit a bit for more aggressive off road riding, for on-road riding the measurable performance gap between 650b and 700c has to be getting mighty small.  So in terms of speed, one can get mighty close to mainstream 700c performance (if not better) with the Extra Leger, and then use the heavier casing Hetre (or perhaps a Soma B-line) for more aggressive dirty stuff.  A downside is that currently this tire is in very short supply.

In summary, there is no perfect answer to the 700c versus 650b choice.  That is because each person has their own preferences as to which attributes are most important to optimize.  Many of our customers have opted for both a 700c and also a 650b machine.  For others, that is not a feasible luxury.  The good news is that with current tire and rim availability it is a tough decision because great performing bicycles can be made in each of these sizes.  This is a nice problem to have!