Low BB, Short Crank MTB Build Report

From my understanding there are a couple of things at play when considering the cornering confidence a lower BB gives.

Benefit #1
The lower center of gravity enabled by lowering the bottom bracket makes the centripetal force and centrifugal effect feel lower on the body and more manageable. This makes the bike and rider together feel less top heavy.

Benefit #2
Similar to the fulcrum effect is the concept of levers. A longer lever (higher BB) will move an object more with the same amount of movement. This means a lower BB makes the bike feel more agile.
LeverMotion

Benefit #3
Referencing levers again, the load (outward force on the tire) should be less due to less mechanical advantage with a shorter distance between the fulcrum and effort. This generates less outward force on the tire keeping it lower within its friction limit.
CornerForces

Overview
All of these benefits and concepts have a specific role and work together to create these effects. I tried my best to lay out how this all works in the two PDFā€™s attached below. They give better definitions and examples of what I am referencing here. Feel free to check them out and critique if something is off or my work is wrong. It would be for the benefit of everyone, including myself.

Files
If you are going to check out the PDFā€™s, read ā€œLeverDynamicsā€ first. It gives a definition and image explaining the basics. The ā€œCornerForcesā€ PDF takes what ā€œLeverDynamicsā€ started and defines other elements as well.
LeverDynamics.pdf (388.4 KB)
CornerForces.pdf (387.4 KB)

Thanks!

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Nice!

This is the one I am unsure of. Shifting the center of mass is how you control the bike. Is the bike leveraging you with cornering forces, or are you leveraging the bike against the cornering forces? I guess it depends on how you frame the analysis.

Anecdotally, higher BB feels more nimble to me. On the road bike, you can link up chicanes more easily. On the MTB, the bike changes direction quickly.

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Thank you for poking some holes in this! Makes me think and revise my idea.

Solid question, I am operating under the assumption the bike is being leveraged against the lower cornering forces. The upper forces (centripetal and centrifugal) will be felt more by the rider than the bike. The lower forces (friction and load) will be felt from the bike by the rider, rather than on the rider themselves.

I probably should have been more specific in my wording here. Technically, a higher BB will be more ā€œagileā€ as it can move more based on the distance the rider leans the bike over. However, a lower BB should be more ā€œstableā€ since the angles leaned at will result in smaller adjustments to the distance of the wheel from the center of the corner.

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Thanks for this thinking and explanation. Iā€™ve been trying to wrap my head around all the forces at work, and how to translate forces into actual trail feel.

Is the fulcrum in your diagram the hub, or the bars?

I feel like the relationship between the hub height and the BB height is significant, and a big reason why 29ers feel so much more confident and stable than 26ers. The BB height stayed about the same, and the hub height moved up significantly with the advent of the 29er.

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Well Iā€™m oddly happy to say I went too minimal on this rear end, and broke it after 80 hard miles.

I really liked the way the rear end felt- so much torsional flex made it really float in rough stuff.

Though the breaking point was shearing the wishbone connector, the rear end was also actually deforming from riding. Iā€™d like to understand if the yield was a matter of material choose (SS vs 4130), diameter, wall thickness, too many bends, or just a little of everything.

Definitely a fun experiment. Iā€™m going to cut the rear end off, and build a new one with 1/2ā€ stays instead of 3/8ā€. Iā€™d love to find a way to get that torsional flex in a durable solution.

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I apologize!! I never saw this, snuck right past me!

The fulcrum is an arbitrary point where the center of gravity resides. It should be in between the bottom bracket and the bars, closer to the bottom bracket.

This is a really good point that I did not think of! The hub height being higher on a 29" wheel vs a 27.5" wheel allows for more BB drop. The lower the top of the inverted triangle (BB is the ā€œtopā€ point and the hubs are the ā€œleftā€ and ā€œrightā€ points) is, the the more nimble the bike may feel.

Additionally the bike may feel more capable because the height between the BB and ground is higher, allowing for better clearance over rocks and such.

Once again apologies for the late response @gschwell, I would love to hear what you think about the statement above!

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Iā€™m reluctant to critique things online like this without seeing in person but I wouldnā€™t rule out the possibility that the failure was caused by poor quality welding. Again difficult to diagnose remotely but what I see of the weld (and the weld joining to the ST) is that it could be overheated, or you have some inclusions or porosity that has resulted in a stress riser leading to the failure. Basically, I wouldnā€™t immediately jump to conclusions on material choice just yet.

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Critique away- welding problems are very likely.

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Iā€™m with Shand. I see two areas that could hint at not-so-great technique contributing to the failure. Take that with a grain of salt, though ā€“ Weā€™ve all seen stellar looking welds that are purely cosmetic and solid welds that look icky.

This is stainless tubing, right? It might be worth confirming that your purge set-up is adequate for this joint. Iā€™d probably make 3-4 attempts with the same set-up and cut all of those up to confirm that youā€™re not getting any sugaring on the inside of the tubes.

Additionally, consider making an argon dam out of aluminum foil to help keep the argon in place on the outside (I ran into an issue making a rack a few years ago. The narrow tubing didnā€™t get the coverage it needed).

Iā€™d also seriously consider replicating this joint with 4130 and welding it up a dozen times. Then rebuild your rear end. At worst, you spend money, get better at technique and find out that the material spec is still inadequate.

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So youā€™re telling me I have to practice my welding???

Thank you for the insight and tips. My lack of skill is definitely shining through here. I feel like most other joints have some redundancy that let me get away with crappy welds.

Is stainless more sensitive to welding conditions than 4130? If so, rebuilding with 4130 probably makes sense.

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I hope I didnā€™t come off as rude.

Welding is one of those skills that I definitely need to practice frequently otherwise I take about 32 steps backward. Since I donā€™t weld every day, I find myself practicing a joint or two before I start working on a bike.

I donā€™t find welding stainless to be much more difficult than welding 4130 but itā€™s more temperamental for sure.

I clean inside and out with acetone, then use abrasive, then I clean with denatured alcohol.

In addition, stainless needs an argon atmosphere on the outside and inside of the tube. If you donā€™t you will see some evidence of ā€œsugaring.ā€ Thereā€™s plenty of videos on youtube covering this and explaining it better than I ever could. If youā€™re not familiar with it, Iā€™d start there.

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Not rude at all, I totally appreciate the insight!

Iā€™m looking forward to cutting all the joints and seeing what I can see.

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I donā€™t think typical 304 (for example) stainless deals with the flex fatigue as well as 4130. Iā€™m not a materials engineer so Iā€™m not going to speculate on which property causes it. I have seen this with racks as well.

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