Source
r/BicycleEngineering•
Fil_Kay

Determining bottom bracket drop on bare frame?

I'm looking at building a Frankenbike for myself, and was wondering if there's an easy way to determine the bottom bracket drop on a bare frame (without a fork). I've got access to a number of inexpensive frames that I can pick through to find the right one for my needs, but several of them don't have forks and I don't want to go through the hassle of tossing a fork onto them one-by-one to take measurements, or trying to use an angle finder to level the frame prior to measurement.

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I'm mostly looking for something that has no or negative BB drop, so even being able to eyeball whether the BB is above or below the axles would be sufficient for creating a short list, at which point I can take more precise measurements as needed. As far as I know, trials-style bikes can have a BB above the axles, although I don't know if there's other generic "styles" (other than a BMX) that would have a similar setup.

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If anyone can make a recommendation on how to easily gauge BB drop on a bare frame (preferably without needing to measure head tube angle or install a fork), or give any pointers on which frame styles have the BB level or above the axles, it would be much appreciated.

Posted May 23, 2019, 1:11 PM UTC

210
tuctrohs•

A given frame could be set up with different forks to get a range of different BB drop and head angle combinations. No measurement will tell you the original design intent. But you could decide on a head angle you want, put a inclinometer on the head tube, adjust until you get there and then measure bb drop in that position.

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Fil_KayOP•

Yeah, that was my thinking; if I measure the head tube angle and length of the chainstays with the chainstays flat on the ground, I could figure out the BB drop at the angle I want. I may actually have to do that with the frames I'm interested in.

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tuctrohs•

That would work too. With my method you directly measure the drop. With your method, the measurement is a little bit easier, but you still have to do a calculation afterwards.

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HuntBoston1508•

You cant get an accurate measurement because BB height depends on fork length and tire size and without those you're SOL. On top of that, frame makers will then have BB offset to suit their taste so the actual measurement could be anything. Not only that, but different styles of bike have different typical BB heighs - 26ers are commonly close to 0 offset because theyre already low to the ground, but 29ers are known to have lots of BB drop.

That being said there are ways to get an approximation. Place the frame on the ground with the BB and rear stays flat and you can see how high the headtube is from the ground, just size a fork into that gap and you have little to no BB drop. Or measure the length from the rear dropouts to the farthest forward point by the BB to figure out the approximate tire size to size a fork off.

Really it just comes down to tire size and knowing if the frame is suspension corrected or not.

2
asad137•

You cant get an accurate measurement because BB height depends on fork length and tire size and without those you're SOL.

OP wants BB drop, not height above the ground, so the tire size isn't needed, just the fork.

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HuntBoston1508•

Well it depends on the framebuilder, i guess. Some people that build frames take in to account the difference in the diameter of tires, so that when a person changes tires they don't start pedal striking left right and center.

Or if they have a certain amount of ground clearance they have decided is appropriate, bikes with different tires would need a different BB drop to have the same clearance.

So the type of tire this bike was designed around could definitely have an impact on the BB drop measurement.

Anyways, I intended tire size to be a blanket statement covering wheel size as well, since 26ers and 29ers and 650c and 700c such all use different sized tires.

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dock_boy•

Drop will only change on a given bike if you change one wheel or the other, or swap out the fork/headset. If the frame was built around a matched set of tires, changing both of them - pretty common - will not change the bb drop at all, since it is measured from the axle line height.

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HuntBoston1508•

bikes with different tires would need a different BB drop to have the same clearance

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Fil_KayOP•

I appreciate the input.

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With regards to BB height, I'm aware that it fluctuates based on tire/wheel size and fork length, which is why I'm trying to determine BB drop specifically, as my understanding is that's a fixed characteristic based on frame design. Being a Frankenbike, I'm planning on swapping out the fork and tires/wheels to suit my needs, but being able to gauge BB drop will give me some numbers to work with to make sure I have enough clearance for the cranks/pedals in the final product.

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Having said that, my (limited) understanding is that bikes have a particular range of head tube angles, and that seems to be about 60 - 76 degrees (dependent on use). I know that's a pretty big range, but my aim is to put together something very compact, so I don't want to end up with an extremely shallow head angle just to get the BB up high enough. At the same time, it seems like most MTBs have more of a slack head angle, so using a shorter fork (to increase the angle) would effectively drop the BB anyway. Either way, it seems like the only real way to figure this the geometry is to put the bike with the chainstays flat on the ground, then measure the length of the chainstays and the head tube angle at that point, then do a bit of math to figure out the BB drop at the head angle I want.

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Anyway, continued research seems to indicate that something trials-oriented (although with a seat) would be the best fit for me, as it looks like they use 70 - 74 degree head angles with high BBs. The other option would be to use a bike with rear suspension and adjust that as necessary, although I'd like to avoid the added weight of a setup like that.

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[deleted]•

[deleted]

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wrongwayup•

thanks for all your help

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