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Discussion Starter · #1 ·
Going to build up a custom steel frame later this year and have not thought about BB height before. Most manufacturers publish BB drop but not height so is it correct to assume less drop = more height across the board? If true why the massive difference in drop and therefore height from one company to the next? For example: Ritchey SwissX size 53 = 63, Felt F1X size 53 = 65, Cannondale Hi-mod 52 = 67 and Specialized Crux size 54 is 69 and size 52 =71.

I'll be riding down here in the desert SW so mostly dry conditions fwiw.
 

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Several issues come in to play with BB hight. Following are what happens when the BB is lowered...

1, Center of gravity is lower.

2, Clearance of pedals and shoes are lessened.

3, The whole geometry of the frame is changed.

4, Since everything is lowered, wind resistance will also be lower.

That's my 2 cents in a nut shell and I'll let you folks who have time to type out every detail go at it. :D
 

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Not sure I agree with the wind resistance comment above (not seeing the relationship between BB height and the size of the "aero" footprint the rider creates).

I will say there are basically 2 schools of thought on BB height as it relates to CX. Higher is better or lower is better. And ultimately they come from different eras.

Higher is better is more euro, where the courses are very techincal and much more traditional. Back in the day, having a higher BB with clips and straps hanging off your pedal meant less of a chance of clipping a pedal over uneven ground with your foot out, and more height for clearance (hence why mountain bikes have higher BB's compared to road bikes).

Lower is better (within reason), is more of a modern take and more centered around US courses. They tend to be less technical (all racers ride the same course, not so in EU), so the course needs to have something for everyone (more or less). Lower BB's feel more stable, especially in corners, which US style courses tend to have alot of, since they are often set up in a relatively small area. The down side is the aforementioned clipping of pedals and reduced clearance over obstacles.

My take, as a US based rider is, the sweet spot is around 68-70 of BB drop. I believe CXM did an analysis a few years back and came to a similar conclusion. But again, they are a US centric magazine.

If you are having a custom steel frame made for you, my suggestion would be to speak with your builder about it and discuss what the pros and cons are of the options available to you. As long as the bike isn't lugged, you should be able to balance out all the attributes to come to a drop that produces the desired results.
 

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If I were having a frame built, I'd defer to the builder.
^^^This. I've had lots of custom frames built - for mtb, road, track and cx and this is one dimension I've always left to the frame builder - that and chainstay length. Any frame builder worth the title will find out how you ride and what you ride and use their experience.
 

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Discussion Starter · #6 ·
Thanks all. Will definitely defer to builder but I thought it was interesting as I never really considered it very important before (road or mountain). I'm probably reading too much into what I've read so far.
 

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While it makes perfect sense, the important part of the process is to provide good input to the builder. I read a review in Bicycling magazine a few months back for a 3D Racing custom build and thought there might've been a little disconnect.

For our test, I asked Herting to build the Crossfire, his aluminum ’cross model, with geometry optimized for the fast, dry courses* that are common in Colorado, where I live. He lowered the bottom bracket, which made the bike stable but also increased the number of times my pedals struck the dirt when pedaling through ruts and *off-camber turns. He also gave the bike relatively slack angles, which made it less manueverable on tight courses, but calmed the handling everywhere else, something I really appreciated during long winter training rides on gravel roads.

I'm new to this forum so can't post the link but the whole review can be found on Bicycling dot com
 

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I recently built up a Guerciotti for my partner. My main concern about fitting her to the bike was that the bottom bracket was quite a bit higher than she was used to. She raced most of last season on it and loved the higher bb as it allows her to clear stuff more easily and pedal off camber sections. Now I'm wondering if I should try a higher bb...
 

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I think that some people just have a preference of one over the other due to riding style. Previous to my current bike I rode a Crosshairs for a few years. The BB drop was 70 compared to the drop on my current Spooky which is 58. Mickey at Spooky told me that the geo on their CX frames is very similar to Ridley. I much prefer the higher drop. There was nothing wrong with the Gunnar. It's a great bike but as soon as I did a ride on the Spooky I was like "whoa!" Big difference.

Jeff
 

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I've had both and prefer the a lower BB. I feel more grounded through turns and remounts are slightly easier with a lower bb. I agree with the poster above that 6.8 to 7 is about right. To me the Cannondale SuperX has about the perfect geometry.
 

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That is crazy. My Blue Norcross has a 6cm drop on the ML frame.
Specialized and Cannondale both run a BB drop in the 68-70 range. Not that I would necessarily use them as the staples, but if it were "crazy" neither of those volume manufacturers would be doing it.

The CX bike I am having built right now is designed around a 68mm BB drop. It suits my terrain and intended use.
 

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The diff between a 58mm drop (common euro standard) and a 68 drop (common american) is a hair more than one third of an inch. 58 = higher bike for re-mounts but more clearance underneath. If you race that can be critical. If you don't race, not so much. Go with lower, in that case.

One reason that drop is advertised but not height from the ground is that height is affected by the tires mounted, but drop is part of the built-in geometry.
 

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I have 2 bikes that are vastly different. Race bike is a redline carbon with 70mm drop, 177.5mm cranks. Pit/gravel/B bike is Ridley with 63mm drop, 175 cranks. Yea, they ride differently, the Redline had occasional pedal strikes when I first started riding it... got used to it, and I love the lower center of gravity.

I do sit pretty high from the ground on my bike tho. 85cm BB to saddle (on the 175mm cranks) so I need all the lowering I can get.

Lower BB also lets you put the HT higher relative to bottom bracket for taller front end if you need more comfort/upright position without as many spacers
 
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