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The bottom bracket height on some bikes can be adjusted 7.0mm by flipping a "chip" in the shock mount linkage. This also effects head tube angle by 0.5 degrees.

Theoretically speaking, What are the advantages and disadvantages of having a lower bottom bracket setting vs a higher setting?

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2  
Adjusted by 0.7mm? Even if you meant 0.7cm, that's hardly enough to sense. –  Daniel R Hicks Feb 25 at 23:58
    
ahh good eye! 7.0 mm –  SoilSciGuy Feb 26 at 1:22
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An adjustment of that magnitude is mainly there so you can believe you're adjusting something. –  Daniel R Hicks Feb 26 at 1:44
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The only thing I can think of is that it wouldn't really affect the feel of the bike so much as it would reduce pedal strikes, but I still don't think that 0.7cm is enough to make a difference. –  Kibbee Feb 26 at 13:38

2 Answers 2

up vote 5 down vote accepted

in my experience. BB height affects stability of your ride, but mostly while standing on the pedals since you weight is then directly on the BB. but raising it 7mm may or may not make a difference in your ride. however, raising it ABOVE the axle height will make a dramatic difference in stability. BMX bikes are incredibly nimble (aka unstable) due to them having a BB higher than their axles, where as 29'ers will feel solid as a rock since they have a dropped BB height relative to their axles. it has to do with torque applied while standing. (i know wheel size also plays a factor but we're assuming spherical cow here). so considering that the DROP or RISE from the axle height affects the ride more than height, a 13in BB height on a BMX will feel much different than a 13 in BB height on a 29'er.

my track bike has a BB height equal to its axles, and its nimble and fun to ride, where as my road bike has a BB drop height relative to the axles and feels like a cruiser in comparison (both on 700c rims).

TL;DR higher BB, in relation to axle height, means a more nimble bike, lower BB is a more stable bike. especially while standing.

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The bike may ride differently. The only way to find out whether it does anything for you is to buy the bike and try it. Ideally by running the same course repeatedly and writing down how you think the bike performs (and the lap times), while someone takes the bike off you, hands it to a mechanic who never sees you, and the mechanic swaps or doesn't swap the part in question. That way you have a double blind test.

One well-known framebuilder in the USA (whose name escapes me) did a very limited set of experiments with small variations in frame geometry in the 1990's, and concluded that no rider could tell the difference from less than half a degree change in headset angle. Unfortunately those experiments were fairly expensive to conduct because he had to make 3 or 4 "identical" bikes and find a set of riders who were all the same size. Basically he let the normal variation in a hand-built bike create the variations, and measured the bike after they were built to find out what he had. Then the riders rode the bikes and rated them against each other. The conclusion was that a millimetre variation in tube lengths and half a degree in angles did not cause any repeatable difference in rider evaluation. But with sample sizes of 3 frames and five riders it's not statistically significant.

I suspect Trek has similar research (as a result of employing Mike Burrows if nothing else) and would not be at all surprised to find that all major manufacturers have done similar things and specify the tolerances in their manufacturing accordingly.

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Better stability, probably more manoeuverable, less pedal/chainring ground clearance. Polo bikes often use short cranks so they can drop the BB, for example. But that's for a 30-50mm drop, not 1-2mm. For 0.7mm difference in a blind test I'd expect hundredths of a second over a 1-2 minute test circuit, with an uncertainty of seconds. –  Mσᶎ Feb 26 at 0:01

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