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Is it possible by viewing a bike frame to tell roughly what the handling of the bike is going to be? This current question is about bottom bracket height in the above mentioned respect.

I have noticed that some bikes have parallel to ground chainstay, while others have angled. Assuming 26" wheels, is this angle enough to judge BB height, e.g. "low", "medium","high". If so, are those categories enough to provide some concrete information about the handling of the bike (e.h. "a lot of clearance", "aggressive turning"), considering constant wheelbase and constant headtube angle?

Vorac
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  • Head angle has far more to do with handling than does BB height. And on virtually all bikes you cannot expect to pedal on a sharp turn -- the inside crank must be up to avoid ground interference. – Daniel R Hicks Aug 15 '13 at 11:27
  • @Daniel R Hicks, I am asking only about BB height, all else being equal. There is already a general question about frame geometry. Also, I do not mention pedalling (but the other question does). – Vorac Aug 15 '13 at 11:48

2 Answers2

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BB height is something that is very important in mountain biking, as well as other disciplines. It effects where the riders center of gravity is in relation to the bike and to the ground. Generally speaking, a higher BB height will lead to a higher center of gravity making the bike harder to turn when the rider leans into the turn. A higher BB can help you clear obstacles though, especially when you consider how far down a big ring may hang from the BB.

Judging BB height visually is something you'll have to learn from experience or by putting a couple bikes next to each other. How good are you at judging a 5mm difference? If you can't then judging BB height is pretty much useless unless you're comparing a hybrid touring bike to a full downhill bike.

Another important point is that not all chainstays will be straight. What about full suspension bikes? Some of those chainstays arc up and not tell you anything about BB height. Suspension also means that as you put weight and/or pressure on the bike the BB height will lower as the suspension moves through it's travel.

Aaron
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This isn't a situation you're going to run into, but let's run with it. All else being equal with a given set of bikes, visually you could probably just barely tell that one bike had a higher or lower BB than the next. You'd probably need a tape measure. In general, your bike with lower bb is going to transition from turn to turn ever so slightly faster, probably an amount unperceivable to most. It's also going to have pedal strikes more often. The bike with the taller bb height is going to be just the opposite in both respects. Keep in mind that without rider weight, bb height is deceptive on any bike with suspension. Its worse with full suspension vs hardtails. Proper suspension sag is typically taken into account in geometry specs, but on the showroom floor suspension bikes will have exaggerated bb heights.

Now let's talk real world.

We're really only talking about a few cm of difference in bb height even in extreme case comparisons. For instance, looking at Trek's xc race hardtails and comparing them to their downhill bikes, you've got less than 6cm of bb height difference. That's not insignificant, but we're basically comparing apples to oranges at that point and looking at 2.5 inches of difference. Amongst a given category of bikes you're going to have very little variance- probably less than a centimeter.

It's important to keep in mind that bb height is just a small piece of the puzzle in terms of handling dynamics for a bike. There are other factors of frame geometry that I would take into consideration first, such as effective top tube length (the bike needs to fit) headtube angle (how quick will the bike initiate a turn) and wheelbase (how quick will the back of the bike follow the front of the bike). When those are equal I would start considering chainstay length, headtube length, and bottom bracket height. Those are all pretty much in order.

joelmdev
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