Viewing 14 posts - 41 through 54 (of 54 total)
  • LLS becomes LHS
  • chiefgrooveguru
    Full Member

    “ According to the the ‘Inverted Pendulum Theory’ in physics, a higher bottom-bracket height raises the center of gravity of the rider and causes the bike to unbalance more slowly.”

    I don’t think the main effect of a higher bottom bracket is on raising the rider’s centre of mass, I think what’s dominating is that you increase the leverage of the BB around the contact patches. The higher the BB, the greater the horizontal displacement of the BB vs the contact patches for a given lean angle. It’s easier to initiate a change in direction with a higher and less stable BB, whilst a low BB means the bike is better at continuing on the same path.

    Over the last 5 years or so I’ve had bikes with some pretty big differences in BB height and adjustable geometry and fork lengths etc that’s let me investigate how it changes the feel.

    For general riding I’ve found I like a pretty low BB, just under 300mm at sag, but I think for racing a little bit higher helps with quicker turns as well as letting you getting pedal strokes in more often.

    mudrider
    Free Member

    Hi chiefgrooveguru, Your suggestion that by applying a sideways force at the bottom bracket a rider could make the bike lean faster is very interesting. I can think of at least two ways that a rider could apply such a force, and if it were big enough it could in theory influence the angle of lean of the bike. Also, in such a case a higher bottom bracket could result in a larger lateral force then being applied to the tyre contact patches.

    I think this is worthy of further investigation but is complicated because every force applied will have an equal and opposite reaction force. It is highly likely that in order to apply a force that leans the bike to the left the rider will need to apply an initial force to the right. It is also possible for a rider to apply a torque to the bottom bracket via the cranks. I need to try these ideas out on a bike so that I can better understand the possibilities better.

    One thing that is overlooked in discussions about bicycle stability is the difference between
    Longitudinal stability (front to back stability) and lateral stability (side to side stability).
    The first is about the probability of going over the handlebars or doing a wheelie, the second is about how easily the rider to keep the bike from loosing equilibrium and falling over sideways.

    These are two different things and it is possible for have a bike like a unicycle that is longitudinally less stable but more laterally more stable. Alternatively you can have a long wheelbase recumbent bicycle that is very longitudinally stable whilst also being laterally unstable at low speed. I don’t believe you can simply comment about the stability of a bicycle without being specific about which sort of stability you are talking about.

    A bicycle that is not moving is longitudinally stable, but rapid changes in velocity when moving can make a bicycle longitudinally unstable. A bicycle can become laterally unstable at any speed but lateral stability is more difficult to control at low speeds. The difference between the types of stability can produce counter-intuitive results. For instance an old Penny Farthing bicycle is more laterally stable than a safety bicycle, whilst also being very longitudinally unstable.

    chiefgrooveguru
    Full Member

    “Your suggestion that by applying a sideways force at the bottom bracket a rider could make the bike lean faster is very interesting.”

    Sorry I think you’ve misunderstood me! I was referring to the downwards force caused by your weight on the pedals causing the BB to rotate around the contact patches, due to the torque generated by the weight multiplied by the horizontal offset between forces and fulcrum.

    I know from experience that it’s easier to two-wheel drift a bike with a lower BB height. Longer wheelbases help too if the chainstays are also proportionally long. So does having the weight of a motor and battery down low. But nothing helps as much as that low BB.

    My last full-sus bike had adjustable geometry and on the same trails it was better in the wet/mud when in the lower position and better in the dry in the higher position – when it’s dry and grippy the ability to change direction fast is critical whilst in the wet it’s about staying balanced when sliding.

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    mudrider
    Free Member

    Hi chiefgrooveguru, You said:
    “I was referring to the downwards force caused by your weight on the pedals causing the BB to rotate around the contact patches, due to the torque generated by the weight multiplied by the horizontal offset between forces and fulcrum.”

    Are you referring to a situation where you are riding out of the saddle? I ask because there are three points where the riders weight can be applied to the bicycle; the handlebars, the saddle and the pedals.

    Your scenario only makes sense when the rider is applying nearly all his weight via the pedals. In which case I would suggest that the handlebars are at one end of the second order lever, but effort in the form of the riders weight can also be applied to the bottom bracket via the pedals. You have already said the fulcrums are the tyre contact patches.

    If this is correct it raises the question of what is the load that this lever is moving?

    Would I be right in thinking that the load to which the mechanical advantage produced by the lever is mainly being applied against the rotational inertia of the bicycle? In saying that I am assuming that the rotation of the rider is caused by gravity acting downwards when the tilted bicycle is no longer directly underneath him or her. In other words, the function of the lever is only rotate/angle the bicycle and once the bicycle is leaning, gravity will do the rest.

    If the above is correct it implies that there is a marked difference between the physics of how a bicycle corners when ridden in the saddle, compared to that of a skilled rider riding out of the saddle.

    mudrider
    Free Member

    A far simpler example of this principle is that of someone balancing on a pogo-stick. If they unbalance to their right side they will slowly accelerate in that direction as gravity pulls them down. However, if someone balancing in the same way moves the top of the pogo-stick to their right, this will move their weight being applied to the footrests to the right of the foot of the stick. This will then introduce an additional leverage/torque that will cause them them fall over more quickly.

    The further away the footrests are from the bottom of the pogo-stick the greater the resulting torque induced by angling the stick and so the more quickly the fall accelerates. In the case of a bicycle, the additional centripetal force generated by the turn should prevent the bike from crashing to the ground.

    The principle that in the two situations are very similar in that; the higher the bottom-bracket relative to the ground the more torque will be generated, and the higher the pogo-stick’s footrests the greater the resulting torque.

    It would be interesting to find some video of bicycle slalom’s to see what techniques the rider’s use. I do know that Penny-farthing riders used to compete in slaloms and those bikes have very high bottom-brackets.

    chiefgrooveguru
    Full Member

    “ Are you referring to a situation where you are riding out of the saddle?”

    Absolutely, I don’t ride anything downhill without the saddle dropped (usually by the max 185mm that my dropper allows). Likewise any flatter stuff with turns. Or technical uphills. To me, and many riders nowadays, the saddle is pretty much for pedalling in straight lines only.

    There is absolutely no way that I could ride the trails I do at the speed I do with the skills I have whilst staying in the saddle.

    chiefgrooveguru
    Full Member

    “ It would be interesting to find some video of bicycle slalom’s to see what techniques the rider’s use.”

    Dual slalom bikes tended to be full-sus 4X bikes, with very low BB heights.

    greyspoke
    Free Member

    Well here’s another theory…

    Doing really quick turns in the woods (slower and tighter than on a slalom track) the bike and rider don’t behave like one thing. Initially the rider rotates the bike (about a longitudinal axis) and moves it under them, staying relatively still themself until the bike is set up for the corner and starts to bite. This is achieved by a combination of counter-steering, unweighting, jumping if you are that way inclined. Possibly preceeded by a Scandinavian flick. So approaching a right hand corner, the objective is to get the tyre contact patch off to the left of the line of travel (and the rider’s cg) so that a turn can happen.

    Let us assume that even the rider’s feet don’t move much in this process (probably they do move a bit more than the rider’s bum does but let’s just assume). The longitudinal axis of rotation (of the bike, not rider) I just mentioned will go through the bb (where the feet are). The more bike there is below the bb, the greater lateral displacement of the tyre contact patch for a given angle of rotation of the bike. This will make a higher bb bike feel easier to flick through this type of corner. Or to think of it another way, trailside trees permitting the rider’s (higher) body can straighten out the curves more than with a lower bb bike.

    mudrider
    Free Member

    You can see the technique that chiefgrooveguru is talking about between nine and fifteen seconds into this video. How to corner fast video

    I tried my own method of leaning the bike into a turn on a Geoff Apps’ bike with a fifteen and three quarter inch high bottom bracket. However, I was put off by the nose of the saddle hitting the back of my leg every time I lent the bike. I need to put on a short nosed saddle and try again.

    With the dropper post down it was still possible to lean the bike a fair angle whilst remaining in the saddle. This cured the problem of the saddle hitting my legs and might be a lazy man’s way of cornering faster.

    greyspoke
    Free Member

    ^That demonstrates the thing I was trying to describe – the bike moves a lot, flicking from one lean angle to the opposite, the rider much less. Complete with little jumps.

    I also agree with the @chief’s point about controlling drift. Once relatively settled in a turn, altering the lean angle of the bike relative to the body can move the tyre contact patch more or less underneath the rider’s cg, and bb height will affect how that works.

    Rickos
    Free Member

    Slap in a normal headset and that Pole is ripe for a mullet.

    jameso
    Full Member

    Well here’s another theory…

    Exactly, and what I was getting at with my poorly worded comment here earlier on,

    Yet having seen how fast some riders get through closely linked corners ..
    With the BB higher, for how much the tyres move more side to side underneath you, your body weight has to move less. From the BB to the contact patch has increased radius. Gets complex with grip and balance as the speed goes up. Seems you could get weight inside in a corner a bit more easily at a lower lean angle – how/if a skilled rider uses that for speed, grip or agility I’m not so sure. There’s a limit on all these things but in slalom-like turns or similar rapid edge to edge riding I can see a theoretical advantage to a higher BB bike.

    But a lower does BB seem to help catching slides more quickly, both the MTB and gravel bike I have with low BBs are good in that situation.

    greyspoke
    Free Member

    Sorry I managed to miss your comment jameso. Exactly. I guess there are a number of things going on, some favouring higher some lower bbs.

    mudrider
    Free Member

    I asked Geoff Apps about the cornering technique shown about nine seconds into this video.
    Faster cornering video
    He said that this was why from 1977 onward all his bike designs had had sloping top tubes. He said that it was essential that the rider can easily and quickly move their body-weight around. In comparison the early US mountain bikes had relatively high top tubes that were parallel to the ground.

    He then went on to talk about the wide variety of cornering techniques that motocross, speedway and motorbike-trials riders use.

    Geoff’s designs are based on decades of experimentation with different geometries and what feels best. Give him any style of bicycle and he will take it off road to find out how it handles. By doing this for the best part of six decades, he came to the opinion that large wheels, high bottom bracket, steep-steering-angled and short wheelbase bikes work best for his purposes. He believes that shorter bikes are work best because it takes less time for the rider to move his body’s centre-of gravity relative to the tyre contact, patches than with longer wheelbase designs. So his design’s principly focuses on it being as easy and quick as possible for the rider to move his weight around in all directions. As I have said before, Apps bikes are fundamentally big wheeled BMX bikes. For him, speed is far less important than technical capability and machine reliability.

Viewing 14 posts - 41 through 54 (of 54 total)

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