A while back I received a hostile diatribe in my comment queue about my Meral 650b conversion. I spammed the comment, but then thought it was potentially illustrative, albeit rude and obnoxious. The moron’s comment appears at the end of this post, and because it is full of misconceptions and mythologies couched as “expertise”, I’d like to thank him for inspiring this post.
I purchased my 1980 Meral 700c sport touring frame after researching the ideal geometry and clearance requirements for a wheel size conversion. I consulted Sheldon Brown’s 650B conversion guide, as well as resources available from many other cyclists, mechanics, and frame builders. A particularly easy to read guide is available at Rivendell’s site. Since that time, I have done a number of other wheel size conversions, from 650c to 26 inch, and from 700c to 650c.
Those of us who have undertaken 650B conversions understand the brake reach, tire clearance and other considerations that must be explored when contemplating whether to convert a bike to 650B. My spammer, however, believes that one can alter the geometry of a frame by changing the wheel size. Without a blow torch, that would not be possible.
The outer diameter of a 38 mm 650B tire is only a bit smaller than that of a 700c x 20 tire. The effect of the 650B conversion is to give one a chance to ride on wider tires, making the bike more useful and comfortable, and to provide for fender clearance that didn’t exist with the larger wheel size. And, as I have stated in past posts, you don’t want to convert a frame to a smaller wheel size if your frame has a lot of bottom bracket drop. Rivendell recommends no more than 70 mm, but you may be able to get away with a bit more drop if you are using shorter cranks. Many bicycles from the 1970’s on have way too high bottom brackets to begin with, so lowering the bottom bracket a bit will improve the bike’s handling and cause no negative side effects.
My own Meral has very little bottom bracket drop, so the conversion improved the handling, by dropping the bottom bracket height DOWN to 27.5 cm, still very high, and much higher than recommended by some frame builders. My trail went from 43 mm to 41, and my wheel flop stayed the same at 12. The world did not stop spinning due to my audacious acts. What are the factors at work that cause these changes?
When you install smaller diameter wheels, there will be a change in the distance from the center of the axle to the ground, thus reducing the distance of the horizontal line between a straight line following the fork/headtube angle, and a vertical line from the wheel axis to the ground. Smaller wheel diameter = less trail. More rake also = less trail, which you can determine from the above drawing by imagining the vertical line through the axis, moving forward, reducing the trail line. Less trail almost always equals less wheel flop, which can provide improved handling for carrying front end loads. Wheel flop is a function of head angle and trail, so you can alter wheel flop also by changing to a longer or shorter fork, and/or to a fork with less or more rake. But in a well thought out 650B conversion, there’s no need to worry about changing the fork.
Another misconception is that a 650B conversion causes fork shimmy. Even though no one seems to know what does cause shimmy, I think we can say for certain that it is not caused by the “wrong combination of rake, trail and head angle.” which of course a 650B conversion does not affect anyway (rake and head angle being impossible to change without changing the fork). Fortunately, I eliminated the shimmy on my initial build by using different tires – I switched from the ponderous Panasonic Col de la Vie tires to the comfortable, delightful and fast Loup Loup Pass tires from Compass.
Finally, here is the comment which inspired this post, in full and unedited, with misspellings and grammatical errors intact:
“As you state, the frames is well designed and its construction very well crafted; it was mostly likely built by Francis Quillon the head framebuilder at Meral..and he would be proud of it. However..he designed the bike around 700c wheels and would be astonished that you have fitted 650B, thereby upsetting all the correct design features that he had used in the frames constructions ie head angle, front end clearance, fork rake and trail…all those important features that govern how a bike handles..OH! not forgetting the height of the bracket.
Shimmy is often a result of the wrong combination of fork rake, trail and head angle..compounded, without doubt by using the wrong wheel size. So what you have managed to do is to take a delightful frame that was intended for fast road riding ie sportif use, and try to turn it into a type of randonneur…which it was never designed from the box of frame tubes , lugs etc to be.
As for the massive amount of handlebar stem quill that protrudes dangerously out of the fork column, Quillon would be alarmed at the thought ..and the sight it presents. The least you could do would be to buy one of those elegant Stronglight extra long headset lock-nuts that would both add about 30mms of extra grip to the quill while at the same time making the bike look less ridiculous than it does now…
Never mind the chrome hilights, the wonderful deep purple flamboyant paintwork..you have turned the bike into a travesty of what the designer/framebuilder intended and,
in doing so, insulted his skills.
If you really need so much seat pillar projecting from the seat cluster and such a high riding position, I suggest you get a frame that is more appropriate to your inner leg and body length.
Just a footnote…no French builder, large or small would ever let a bike with toeclip overlap leave their workshop or factory.”
This diatribe points out how narrow minded some cyclists are – adhering to the idea that if they do not personally experience something, then it must not exist. One of the reasons the seat post and stem are tall is because I am using 160mm cranks, which help to eliminate toe overlap. Shorter cranks means a taller seat post, which in turn means a taller stem. And yes, this frame had toe overlap with the larger 700c wheels, and it was indeed designed that way – something that happens when small and even medium sized frames are built around 700c wheels. Whether the builder considered this a necessary compromise to please a particular customer, we will never know. Most disturbing about this rant is the ridiculous concept that style trumps comfort when setting up a bike for a particular rider. Many riders know that taller stems mean more hours of comfortable riding.
Apparently, the original Shimano 600 French headset is an absolute eyesore, when paired to the tall Nitto stem.
So hideous is the bike that it is now a “travesty”. Well, me and my travesty will see you out on the road. Happy riding!