Now, I I keep hearing terms like “this bike is awesome because the bottom bracket is so stiff” and stuff like that. Everyone tries to compare the stiffness of one bike to another bike. However, is there a quantitative way to measure bottom bracket stiffness and objectively compare bikes to each other? This is really important to me because I’m a bigger guy and even on bikes that people say are pretty stiff (Cervelo Dual), I still notice a decent amount of bottom bracket sway. I’ve test ridden bikes like the P3C and R3 at Austin Tricyclist and they are obviously stiffer. Yes, they cost 3x as my bike, but I was amazed at the stiffness of them. So, instead of saying an R3 is stiffer than a Dual, is there way a determine to say an R3 is 25% stiffer than a Dual?
I would try a dial indicator at the seat tube . Measuring the chain ring deflection from the seat tube. Use the left pedal at the bottom stroke with your body weight on the one pedal ( better yet a barbell weight 150 lbs on a wire at the same point on the pedal)
There are a ton of possible variables , torque of installation on the bracket ( maybe ride and re-torque ), level of the bike frame ,
Sure, but I am not sure who is going to let you test a frame that you are thinking about buying. Basically, what you do is you put the frame into a jig horizontally (so the opposite of how you ride) that holds the fork and rear dropouts rigidly. Then you hang a weight from the bottom bracket and use a dial indicator to measure the deflection. It’s pretty basic if you have a good machining table. But I am not sure what you will really get out of it. If you knew the R3 was, for example, 25% stiffer than the Dual, what would that tell you? Quantitatively, you have nothing to compare the Dual to. I think, to some extent, you are trying to create and then solve a problem that doesn’t really exist. The real question is “how much faster will I be on an R3 than a Dual because of increased stiffness in the BB,” and there is not really a defintive answer for that one…
I am sure Cervelo has all the data on their bikes; I am also fairly sure they wouldn’t make that data public, but it can’t hurt to ask.
As for noticing bottom bracket sway, every bike is going to sway on the trainer. And on the road, I seriously doubt you would notice any sway in a quality bike, because the bike itself can (and does) move naturally during pedalling.
It is pretty interesting to simply ride various bikes on trainers and visually note the different bb deflections. It is quite an eye opener how much they bend down there. It isn’t terribly scientific, but it is informative.
I’ve ridden the P3C, the R3, and I own a Dual. Those other two bikes are way stiffer than my Dual.
As for the “trying to create and then solve. . .” comment, you cannot deny that there isn’t a quantitative method to measure bottom bracket stiffness. It is very qualitative and subjective. There is a problem out there before I said anything about it, and I’m just trying to find a quantitative (hard numbers) method for comparing stiffness in bike frames instead of qualitative (opinion).
I said there is a quantitative method; I never denied that. I’ve seen it done. My point was, if I tell you frame X deflects x amount, and frame Y deflects y amount, the only conclusion is that Frame X is x/y stiffer in the BB than Frame Y. Now, the question is, what does that really tell you? Unless EVERY bike manufacturer used the exact same standards, you could only compare among manufacturer models. And beyond that, you still don’t really have any REAL information. If the R3 is 25% stiffer in the BB than the Dual, what does that mean? What does that actually do for you? The hard numbers are really no more informative than the qualitative analysis, because you have no baseline for comparison.
And then, because of differences in geometry, you are not neccessarily even making accurate comparisons within models. Look at the case of aerodynamics with Cervelos. The P3C is more aerodynamic, as a standalone frame, than the P2C. But the P2C has a taller headtube in the equivalent size, so that is factor, because a P3C with 2cm of spacers in NOT necessarily more aerodynamic than a P2C with none.
Your position on the bike, stem length, number of spacers (if any), seatpost length, whether you ride seated or stand a lot. All of those things will affect how a bike rides and how the stiffness of the BB manifests itself in real applications.
At the end of the day, you are asking for a quantitative number because it will make you feel better about spending 3x more (or 3x less) on a bike. But that number isn’t really valuable without context, and giving that number context is almost impossible.
The P3C and R3 are stiffer than the Dual. But is the Dual slower as a result? And if so, how much slower? That is the real question you want to answer, and knowing the difference in BB stiffness quantitatively is NOT going to answer that particular question.