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Technical FAQ: Oversized helmets, bottom bracket drop, and more

This week, Lennard Zinn answers questions about oversized helmets, bottom bracket drop, and more

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Oversized helmets

Dear Lennard,
I am a guy with a big head (7 3/4 hat) and I currently wear the Uvex Magnum helmet. Alas, it appears to be discontinued and most helmets I’ve tried to replace it with feel either snug or fit like a Styrofoam fez.
— Brandon

Dear Brandon,
Uvex offers the X-Fit (or X-Ride) helmet, and it is made for large heads. It is huge. It goes up to size 65, and there is an incredible amount of adjustment range on the dial tensioner in the back — enough that you can probably tighten it down onto a cantaloupe. It would look like an umbrella sitting on the melon!

The nice thing about this helmet, in my opinion, is that it is a nice-looking helmet with technology in it like an in-mold shell and a nice dial adjustment system. Many really huge helmets are only low-end models.

Although it is not in the Uvex catalog of Magura Direct, the importer, the company does import it and says the MSRP is $139. There is some question about the name, however. It says X-Ride inside the helmet I saw at a Uvex product introduction in Sedona, Arizona, last week, and that’s the way it was introduced, but Jeff Enlow, the general manager of Magura Direct, called it X-Fit. Internet searches for Uvex X-Ride generally come up with ski helmets; I found the link above after searching for X-Fit. Keep searching; it’s out there!
— Lennard

Swapping out chainrings

Dear Lennard,
Can you tell me where I can buy a 36-tooth inner chainring to fit on a standard 130mm Shimano Ultegra 10-speed crank? Does Ultegra make a 36-tooth or will it be a mountain bike part?
— Gary

Dear Gary,
No, I cannot, because if you have a standard 130mm bolt circle diameter (BCD) crank, you cannot install a 36-tooth chainring on it. You would see why if you were to hold the chainring up to the spider arms — the chainring bolts would be right where the teeth are! You need a compact crank (110mm BCD chainring spider arms) to use a 36-tooth chainring.
— Lennard

Bottom bracket height

Dear Lennard,
How do you figure bottom bracket height? If you can throw some examples down that would be great. Also, what does +40 BB mean? If you could explain this, that would be great. I ride trials, and on some of the frame descriptions, it’ll say +20, +40, etc. Can you explain to me why this makes a huge difference? I don’t like a really high bottom bracket height, like [on] the Beast of the East.
— Bobby

Dear Bobby,
Bottom bracket height, as you know, is the distance from the ground to the center of the bottom bracket when the bike is standing up vertically. The hitch comes, particularly in your application, in that this measurement is generally taken with no weight on the bike, and it is obviously dependent on tire choice. Since you run very low tire pressures in trials, and you’re interested in the height of the pedals off the ground when you’re on the bike, this static measurement is of limited value to you.

The +20 or +40 designation means that the center of the bottom bracket is 20mm or 40mm above the line connecting the centers of the hubs. This measurement is called “bottom bracket drop,” and it is a much more useful measurement for your purposes because it is independent of tire or wheel diameter, tire inflation pressure, or of whether there is weight on the bike or not.

Trials bikes generally have the bottom bracket higher than the hubs. As the drop measurement increases, so does the bottom bracket height. Hence the “+” in the measurements for the frames you saw. (And I don’t know how high Cannondale made it on the Beast of the East trials bike.)

This is in contrast to road bikes, where the bottom bracket is below the height of the hubs. On a road bike, a larger bottom bracket drop value indicates a lower bottom bracket height, not a higher one (the drop is a negative number, and the bottom bracket height is a positive number).

Given this confusion that increasing the drop decreases the height, road riders don’t generally think in terms of bottom bracket drop. And for road bikes, bottom bracket height is a useful measurement because tire size on racing-style road bikes is generally within a small range, and the tires are pumped up so hard that the bottom bracket doesn’t get much lower when the rider sits on the bike.

For full-suspension mountain bikes, bottom bracket height and drop are both constantly changing, and riders will probably be most interested in them when the bike is at full height (indicating how high the saddle and top tube will be when jumping back onto it), and at full compression (for how much pedal clearance they’ll have when pedaling through a rocky section).

I’ve never ridden trials and have no idea how you would choose what height to make it for that. As for how I choose it when building a frame, I consider what kind of riding the rider will do and how high I would want the pedal spindle centers to be off the ground. Then I add the crank length to that and subtract half of the tire diameter, and I get the bottom bracket drop I want.

For the examples you requested, for a rider using his bike in criteriums, I would want his pedal spindles (this will vary depending on pedal type) to be around 100mm off the ground. So with 170mm cranks, this will be a 270mm bottom bracket height. Subtracting the wheel radius, which for many 23mm tires will be half of 672mm total tire diameter, or 336mm, gives a drop of 66mm. Now if I had another (much taller!) rider using 200mm cranks and wanting to ride similar races, it would be 100 + 200 = 300mm bottom bracket height, or 300 – 336 = -36mm bottom bracket drop.
— Lennard