• Why does a lower bottom bracket equate to a more responsive drivetrain

    Quote Originally Posted by Bob Ross View Post
    Could you elaborate on that last sentence a bit? How or why does a lower bottom bracket equate to a more responsive drivetrain?
    Sure, at least I will try to. First, drivetrain responsiveness. What I am talking about is the perceived difference in the way that two, otherwise identical bikes will accelerate when the only difference between the two bikes is the BB drop difference of, say 6mm. The lower BB bike will feel as though it is "more willing" to accelerate when the rider is standing. This applies to situations where a rider is jumping out a corner, leaving an intersection or climbing out of the saddle. The perception pretty much goes away once the rider is back in the saddle. What is going on here are a couple of things.

    First, the obvious one; when a bottom bracket is lower, the entire bike is lower, everything. Not just the BB and seat tube, but top tube, saddle, bars ... all of the weight of the bike is lower. Since a bike "rocks" around the fulcrum point of the contact patch of the tires, the closer the structure of the bike is to that point, the less force is required to rock the bike, it feels as though there is less bike fighting the rider.

    Second, and this one is hard for me to communicate, since I really don't understand fully how it works from a bio-mechanical perspective. When a rider stands up to accelerate, usually, there is some degree of rocking motion that the bike goes through. The lower the BB, the less lateral movement the BB and cranks make during a rocking cycle. Bikes with very high bottom brackets keep the riders pedals farther away from the ground and the rocking fulcrum, therefore making for more lateral movement of the BB during rocking. At the same time, downward pedaling forces during rocking have a tenancy to add to the rocking motion. But the higher the BB, the more those forces will add to that motion. Lower bottom brackets have less effect on the natural motion since those BBs are closer to the rocking fulcrum and don't "rock" as far. With the higher bottom brackets, the rider needs to compensate more for the pedaling forces and the bike seems to take more upper body effort to ride naturally, or neutrally.

    Yikes! That wasn't very clear. OK, a few related thoughts. There are some interesting instances where frame builders really need to pay attention to this stuff. Picture a full-on track sprint bike. This frame will need a maximum BB drop of about 57mm if that much, for two reasons both related to pedal clearance. First, on shallow tracks, pedals can hit on the inside during sprint speeds. Second, on steeper tracks, pedals can hit on the outside at slow speeds. So the issue here is just how much drop to use. The trick is to determine a drop which will keep the rider safe, but only just safe. For those of you who have not seen international level sprinting up close, I will also point out that very few sprinters do much rocking as all. The main reason is that at the power levels that those folks have, a rocking (road) type of action would be hugely inefficient and slow. When they are using ALL of their muscle groups, full on, the bike hardly moves at all since they are not dancing on the pedals at all.

    Another area where builders need to find a happy medium is with any type of off-road bike. This will include Cross, Mountain Bikes, BMX, trials, etc. The considerations are obvious; ground (log, barrier and rock) clearance versus center of gravity. So for most off road bikes, the compromise point is really determined by the rider. Riding style, components (crank length, pedals) and terrain will determine the drop.

    Now back to road bikes. The drop considerations are really the same here as well. Pedals clearance versus drivetrain response. I try to keep our BBs as low as possible while at the same time, safe. OK? Hope I didn't get too far off course. I can be pretty long winded. Thanks for the follow-up question.
    This article was originally published in forum thread: Spectrum Cycles started by Tom Kellogg View original post
    Comments 5 Comments
    1. innc's Avatar
      innc -
      Hi TomWow! This popped up and I actually can elaborate a little. What this is centered on is called "arc of motion" which i studied in Kinesiology years ago. Think of someone running. The hip joint is the pivot. When a runner strides, he bends his leg at the knee and makes each stride. The point of the knee travels through an arc from back to front. If the runner keeps his leg straight, the point you measure from is the ankle. The ankle travels the same arc, but it has to go a greater distance, and has to travel faster to cover the arc at the same speed.So the runner bends his knee to move it through the arc faster then extends his leg like a landing gear. Very fast.Now flip it and put the pivot on the road, and instead of a leg, you have a bike moving through the "arc of motion" If the bike has a Low BB, as Tom said, the entire bike is lower. So it is moving through a shorter arc of motion, and the bike feels handier, crisper, when out of the saddle. Same thing with a sloping top tube. The top tube is hitting near your knees instead of near your gentleman's hardware. Shorter arc again, and a change in feel. Now think about handle bars. The pivot is at the fork, so if a rider has a too small frame and a very long stem ,like 140mm ,to make the frame "fit" , then when he turns the bars, they go through a greater arc of motion and it throws the steering off.Back to Tom's example of a track bike for a sprinter. The BB goes up to make it safer on the steepest bank, Mr Sprinter sprints sitting down. But as the BB goes up the chainstays get shorter so the bike gets stiffer. AND the arc of motion of the handle bars should be neutral ( thank you Dazza for this) because when you jockey for position early in the match, if Mr Sprinter's stem is to long, it will have sluggish steering, and if it is to short, it will be to nervous and easier to make mistakes.Thats not getting into center of gravity, just the old text book stuff coming back.dave
    1. Too Tall's Avatar
      Too Tall -
      Thanks Dave. I thought Tom's words and your expansion of this subject surrounding bike design elements contribute and detract from how it is used and ridden are subjects that are a black box mystery for many. When it is explained by someone who races bikes and understands the physics, that is a perfect storm :) Thanks.

      Tom talked about road and track bikes I'd love to hear similar thoughts about something completely different. Bikes designed for urban commuting, light loads and the less common bikes such as one designed for slower riding thru sand and snow. Typically these bikes have shallow head tube angles and relatively shorter stems.
    1. lex's Avatar
      lex -
      Ill take a short crack at it TT, Iv done a ton of experimentation with ridged SS Trail Bikes. Ill break my info into three scenarios. Here are what we determined to be the sweet spots for each scenario based on root clearance, acceleration, cornering and over all performance.- Fixed gear on east coast trails (lots of roots)=20mm drop 175 cranks*-Average skills on the same trails with a freewheel=25.4mm drop with 175 cranks -Highly skilled rider on the same trails= 52mm maximum drop 175 crankThis seems to be the sweet spot but dose require a rider that wants to be more acutely aware of the trail conditions and can ruin the ride for someone that is just out for fun.anything lower than 52 on a mountain bike seems to become laborious.*My fix gear and SS Trail bikes use a 120mm R axle spacing. Shortening the R axle length eliminates unnecessary weight in the R hub while at the same time increasing clearance between the crank and chain stays allowing for a shorter BB further reducing weight and making it easier to get a strait chain line. (say it all in one breath)I should also noat that haveing the absolute minimum tire clearance possible on the fork positively affected the ride qualites of the test bikes for what I would imagine to be similar reasons.On the street side; I recently finalized a new trick track design that we've been working on for two years that has a plus 10mm BB height and 26" wheels as well but thats a whole other story and Im goin on a ride.
    1. delacroix's Avatar
      delacroix -
      I think you complicate things too much. It's simple the bike with the lower BB does not have the chain in the section that is doing to driving. ie. the top, being pulled uphill so much.
    1. Freddy's Avatar
      Freddy -
      Freddy has been on the fence about drop for decades.
      Let me explain a paradox?
      I began only building within current designs and offerings around from the 1940's on and found myself that "I" could get fine performance and "I" won a lot.
      By the end of the 1950's my "Super" engineering father and his fellows began telling me I should begin experimenting with other newer materials and designs.. and I did?
      Well all went well and after returning from several years of helicopter ops in the US Army, well between jobs I began to build again.
      And all went beyond well, lots of avid followers "Until" one fateful day a Lad came into my life that was a monster talent.
      Now he won and won and pestered me to build rides for him that were beyond stiff and I went off the pier obtaining O/S tubes and lighter one in O/S-then tapering and so on.
      The "Drop" kept getting higher to satisfy him and the higher drop also resulted in more track and crit wins?
      SO I tried this design path for a stranger that I had no advance knowledge about and made the first "Big Mac" and guess what? He rode it with the paint still not fully dry and won the Famous "Westwood Criterium" in an amazing "Lone Break Away" .
      The balance and CG is better lower, but speed? I really don't know for sure, but I think lower has a better "Sensation" but speed is not effected much.
      Does that any vision?