Discussion in 'Fixed Gear and Single Speed' started by Blonde, 28 Sep 2007.

  1. Crackle

    Crackle Squatter

    If it flexed on the chainwheel side and you are a heavy rider, it might be enough, in the right gear, to make the chain rub. I know I had a BB on the way out which did this. Replaced BB and rub stopped. I would say it's more likely to be that area than your frame flexing.

    Try it! Stick your foot on the pedal and shove the bike sideways, see what moves. On mine everything flexes and the rear tyre deforms so it's hard to tell what's frame flex, what's wheel flex and what's tyre flex (jig required). But, if you look carefully at the crank, it 'seems' to bend as well.
  2. Brock

    Brock Senior Member

    Interesting... I'll investigate.
  3. Graham O

    Graham O New Member

    I find that when I put pressure on the pedals to slow down, the chain noise increases as the chain line is changed slightly.
  4. starseven

    starseven Guest

    This debate is only still active because its been bonj'd.

    I think we all know frames flex , you adjust your front der when it rubs on the chain on each down stroke, its only a few inches from BB but there is enough flex to cause the rub, There are pages all over the web about deflection of different frames, cannondale have even designed there own BB to reduce the flex. On steel frames you can lean the bike to one side apply weight to the BB and flex it enough to see it visualy.Steel is also by nature such a different product from Alu that it rides with less jarring or the zing that can come through a aluminuim frame.

    Der gears can loose about 5% efficiency when they are tip top condition and more when not, you can tell yourself when you are just in that sweet gear that is . Get rid of the der and you are always in that "sweet" gear.

    Anyone reading who hasn't tried a single or fixed (including you bonj) should try it , do like you had to as a kid when that devils devise the "deraileur" gets out of shape, you take it off, take some links out of the chain and let it fall in the best gear. Pedalling becomes more efficient and best of all you dont have to constantly think "shall I change gear", just pedal.

    The down side is , if your fittest years are behind you(like me), it may put a strain on the knees so gears may be more practical but without a doubt the nicest bikes I have ever ridden are steel frames with one gear.
  5. bonj2

    bonj2 Guest

    I don't know whether I can or not.

    You can make it flex voluntarily when sitting on it, or it does flex whether you like it or not when you sit on it?

    In what way does it flex, i.e. how is the shape that the diamond frame (I assume it is diamond frame:?:;)) deforms to different from the shape it is normally?

    Is it made purely of the pure element scandium, or of some alloy of scandium? Why scandium, and not titanium?

    pah! :becool: tosh. Don't you mean 0.5%? :smile:
    Another myth the fixie brigade feel the need to propogate in order to convince themselves their machines are better.
    If it's bent, 5% maybe. But when set up correctly on a non-bent hanger, not 5%. Sorry.

    If I lived in holland, or lincolnshire, or even london, then maybe I would. But to try one, I'd have to either buy or build one. That would cost money. To spend all that money just to try it would be a waste, when it's not going to be any use to me - a bike without gears just isn't going to cut it round here.
  6. Graham O

    Graham O New Member

    I can't be bothered to re-read all of this thread, but for those who doubt frame flex happens. Saturday was a wet ride on muddy roads in Cheshire. On the flat, tyre clearance was about 3mm from the chain stays on both sides. Honking up a hill, there was a rubbing sound as pressure was put on the right crank. On inspection, there was mud gathering on the left chainstay where it had been rubbed off the tyre. Could it be the tyre changing shape? I tried to reproduce it on the right chainstay, but couldn't.

    Proof enough?
  7. bonj2

    bonj2 Guest

    Why would it only flex in one direction due to honking up a hill?
  8. rustychisel

    rustychisel Well-Known Member

    It wouldn't, but as usual, you're only seeing half the picture, and I don't have time to explain it to you right now. To put it bluntly, greater torrque comes from the drive side - flexing the wheel and frame and distoring the tyre to its left. Hence mud on inside of chainstay. They make left and right chainstays on some bikes for just such a disparity: freaky coincidence I know, but true.
  9. Graham O

    Graham O New Member

    No idea. But since I am stronger on my right side than my left side, it could be that I'm just a uneven cyclist. With this thread in mind, I did try to get it to flex the other way, but couldn't. Which probably rules out the weak left side arguement, i.e. I was deliberately trying to make it happen and it didn't.

    But if it is only flexing in one direction, what explanation can you give for the wheel rubbing?

    PS Wheels are good quality, handmade and very true. Tyres were pumped up to 90psi (Ultra Gatorskins, 700*28)
  10. peejay78

    peejay78 Well-Known Member

    maybe graham only has one leg.
  11. smiorgan

    smiorgan New Member

  12. Graham O

    Graham O New Member

    Thanks Smiorgan, I wasn't doubting that it does flex, just that if as some people claim that it doesn't flex, what is their explanation for wheel rub.

    I'll have a look at those links.
  13. skwerl

    skwerl New Member

    I used to get flex on my IRO frame. I spent ages looking for a clicking sound that I only got, honking up hills, on the right hand downstroke. I checked and greased everything to no avail until, one day, I noticed a small nick in the right chain stay. Every time I hit the bottom on the right-hand crank the chain-ring clipped the stay.

    Now, you could say that was crank flex but I have the same crank/BB combo on my current frame and this contact no longer happens. Only difference between the two is the frame. Chain-ring:stay gap is the same. Only thing I can attribute this to is there's less twisting under load in the BB area.
    IRO was a cheap no-name steel frame. Current frame is Dedacciai Zero replica.
  14. Crackle

    Crackle Squatter

    Skwerl, I must admit if the flame flexed that much I'd be surprised. I've just been across to my bike to shove it and push it and see again what happens. On mine it would take a fair amount of flex to get the inner chainwheel to touch, and down in the BB area, the whole bike seems to move equally including the rear chain stay.

    Could it have been your chainwheel wasn't true due to the crank being badly fitted on the taper or wrong chainweel/BB combination?
  15. starseven

    starseven Guest

    Bonj if I had a spare, I'd lend you it.

    A little reading for you here, its a wiki but theres loads on the web and although more efficient I find the enjoyment comes from not constantly thinking about which gear you are in and just riding

    Go on you really owe to yourself to find one and give it a go before your knees make it difficult.

    Derailleur gears
    External gearing utilizes derailleurs, which can be placed on both the front chainring and on the rear cluster or cassette, to push the chain to either side, derailing it from one sprocket to a neighboring sprocket. The sides of the sprockets may be sculpted to help catch the chain, pulling it up onto their teeth to change gears. There may be 1 to 3 chainrings, and 5 to 10 sprockets on the cassette or freewheel. Derailleur type mechanisms of a typical mid-range product (of the sort used by serious amateurs) achieve between 88% and 99% mechanical efficiency at 100W. In derailleur mechanisms the highest efficiency is achieved by the larger cogs. Efficiency generally decreases with smaller cog and chainwheel sizes.[1] Derailleur efficiency is also compromised with cross-chaining, or running large-ring to large-cog or small-ring to small-cog. This also results in increased wear because of the lateral deflection of the chain.

    Hub gear
    Internal hub gearing works by planetary, or epicyclic, gearing, in which the outer case of the hub gear unit turns at a different speed relative to the rear axle depending on which gear is selected. Rear hub gears may offer 2, 3, 4, 5, 6, 7, 8, 9, 12, or 14 speeds. Bottom bracket fittings offer a choice of 2 speeds, and are generally foot-operated. Internal hub gears are immune to adverse weather conditions that affect derailleurs, and often last longer and require less maintenance. However, they may be heavier and/or more expensive, and often do not offer the same range or number of gears. Internal hub gearing still predominates in some regions, particularly on utility bikes, whereas in other regions, such as the USA, external derailleur systems predominate. In a typical hub gear mechanism the mechanical efficiency will be between 82% and 92% depending on the ratio selected. Which ratios are best and worst depends on the specific model of hub gear.

    Fixed-gear track racing bikes can achieve transmission efficiencies of over 99% (nearly all the energy put in at the pedals ends up at the wheel). Biomechanical factors however determine that a human can deliver maximum power only over a narrow range of crank rotational speed or cadence. To match the power source with the load under varying conditions, a variable gear ratio is needed, and they work very well, though at the expense of mechanical efficiency. The efficiency varies considerably with the gear ratio being used.
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