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unsprung weight and swingarms

Discussion in 'Downhill & Freeride' started by demo 9, Apr 15, 2008.

  1. demo 9

    demo 9 Turbo Monkey

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    asuming unsprung weight makes a bike feel light and respond to impacts sooner.

    wouldnt this matter almost as much as wheel path

    asuming that; how come manufacturors have not made bikes with titanuim or carbon swingarms


    *i know that frames are getting lighter and some have carbon links but why not a full carbon or ti swingarm?

    *do not flame me*
    *yes i know im an idiot*
     

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  2. xy9ine

    xy9ine Turbo Monkey

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    i like the theory. i do believe lahar is on the right track in this regard. the wheel / swingarm assembly is definitely one of the lightest - swingarm is 850g, hub is 157, & no derailleur hanging out there. it's the most supple, knarl eating suspension i've ridden (though of course this is a combination of factors, but the feathery rear can't hurt). i kinda like the inboard brake concept a couple guys have played around with as well. of course wheel weight is a much larger percentage of unsprung mass & i believe there's still a good bit of weight that can be shed with tire solutions.
     
  3. norbar

    norbar Turbo Monkey

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    Light bikes in most cases are just fine. Plus you cant go to thin on the swingarm as they like to snap in many bikes.
    I also think that you feel the unsprung mass more in your arms because legs are stronger so fewer ppl can feel the differance
     
  4. ridiculous

    ridiculous Turbo Monkey

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    Yeah what he said^^ . Rather than switching to exotic materials I think bike co. are trying to transfer alot of the weight to sprung mass. A car example would be solid beam axles vs wishbone. Where your diff is mounted to the chassis and therefore sprung mass. In the case of a dh bike thats like having a gear box or something to that effect rather than your derailluer, cassette, chain etc.

    Carbon and ti are expensive and expensive to work with as you know. I would imagine that would be why you dont see them. However, there is that new yeti seven....
     
  5. - seb

    - seb Turbo Monkey

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    850g swingarm is VERY light - my DHi one is 1400g (single piece of metal only, 2200g once you add the eccentric/axle/lowershock shuttle etc etc :()

    Unsprung weight is definitely worth saving. When I first got my DHi it had 24" doublewides and gazza tyres on, and the suspension was noticably less effective than my 222. As soon as I changed the wheels out to something sensible I saw a very noticable improvement, though obviously going from 24" to 26" clouds that vision a bit.
     
  6. stinky6

    stinky6 Monkey

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    Feel free to correct me if I'm wrong, but I believe that Ti needs more gusseting when welded to be as strong as aluminum. Thats why you see Ti hardtails, but not many full suspension bikes because when its all said and done the bike doesn't weight that much less. Its also expensive.

    I think as carbon fiber becomes more common place the cost of it will go down and we will start to see more carbon on DH bikes. Also it seems like the swing arm gets hit with rocks and what not from the tires which could damage carbon, but that seems like a small issue.
     
  7. - seb

    - seb Turbo Monkey

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    Agreed.

    Certainly aluminum XC frames are lighter than ti ones generally. Ti is used for it's feel, not for it's lightweight qualities (and feel isn't important on a full-sus bike, hence why you see very few ti ones). The lightest XC frames (predictably) are carbon fiber, so that's where we should be looking for our swingarms of the future too :)
     
  8. Curious_George

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    I known that swingarms are the main aspect in sprung mass...but why cant we refer to wheels as being sprung???....to a certain extent they are....
     
  9. - seb

    - seb Turbo Monkey

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    Just to be clear, you say, sprung, but I know you mean unsprung. You've got your terminology mixed up.

    And yes, wheels are definitely 100% part of your unsprung mass, I don't think anyone here is debating that.

    You say swingarms are the main part, they're not at all - your wheel & tyre is the heaviest thing, and much further from the shock so more inertia.

    Equally far from the shock are your cassette/mech/caliper/rotor. All of these play more of a part in your unsprung mass than your swingarm, because they're all right at the end of the lever. This is why I run a 7" rear rotor and a 6-speed 100g cassette, not to mention a tuned Dura-ace mech :)
     
  10. buildyourown

    buildyourown Turbo Monkey

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    You are a bit off on the gusseting issue. Ti is much stronger and stiffer than aluminum for a given weight.
    There is 2 big reasons why 99% of full suspension bikes are aluminum. #1 is obvious. Most are made in asia and asia is tooled to make aluminum bikes.
    #2 relates back to material density. When you are designing a FS bike, you end up with areas around the pivots and shock mounts that are easiest to make out of a block of material. In order to make a Ti bike weight competive, every last bit of unessesary material must be removed. This is very hard and expensive. With alum, if a little bit of material is left in there, it doesn't add a lot of weight.
    Removing all that material is easy on a hardtail. Hense why steel and Ti road bikes are weight competitive.
     
  11. Curious_George

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    changing the terminology around then, what would be the most weight consuming sprung mass
     
  12. ridiculous

    ridiculous Turbo Monkey

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    Agreed, infact the swing arm is actually a hybrid piece between sprung and unsprung. Alot of this is determined from pivot location and cg.

    I have almost the same setup as you, I did it to save unsprung weight/rotating mass.
     
  13. ridiculous

    ridiculous Turbo Monkey

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    sprung mass would be you and the frame. Way easier to take 10 lbs off your body than your frame.
     
  14. - seb

    - seb Turbo Monkey

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    But your suspension will actually work better if you have MORE sprung mass. So if you're all about the suspension action then go eat some pies, get yourself some stronger springs, and (in the theory in my head at least) the better sprung/unsprung ratio will make your suspension more effective.

    (Maybe, I think?) :p
     
  15. karpi

    karpi Monkey

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    I believe the new scott dh frame uses a part carbon swingarm in their higher model to lighten things up and keep it buff. I guess lowering the weight on your swing arm is pretty important but not as much as lowering the weight on your rear wheel, since this is rotating mass. Apart from lowering un-sprung or sprung (at this stage Im pretty confused as to which is which...) and making the suspension more suple, it's good cause it centers the weight of the bicicycle further towards the middle (center of gravity for the bike) making it, I believe, more stable and handable. This is achieved really well with gearboxes, its all pretty logical really.
     
  16. - seb

    - seb Turbo Monkey

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    Think of it this way - your frame is suspended by springs between the wheels. Thus, it is "sprung".
     
  17. SPINTECK

    SPINTECK Turbo Monkey

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    I've always thought unsprung weight was bad, but if that was true, why don't we see more inverted front shocks- suspend those big hefty lowers?!
     
  18. ridiculous

    ridiculous Turbo Monkey

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    So many variables here but it could be one of the reasons why peaty was so good on that single pivot of his.
     
  19. karpi

    karpi Monkey

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    thus frame would be the unsprung mass and swingarm and wheels sprung correct? Is it okay to assume that pieces on the bike that have some kind of displacement in an area or space are sprung?
     
  20. ridiculous

    ridiculous Turbo Monkey

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    no you have that backwards. Wheels are unsprung, swing arm is hybrid, frame is sprung.
     
  21. Curious_George

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    thats what im thinking...theoretically the true definition of sprung/unsprung would lie in their mouvement...objects that have a considerable amount of movement are sprung...to me...or maybe its my misconception
     
  22. - seb

    - seb Turbo Monkey

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    Because lowers actually weigh feck all, stanchions are a lot weightier.
     
  23. SPINTECK

    SPINTECK Turbo Monkey

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    gotcha, thanks for an answer.
     
  24. - seb

    - seb Turbo Monkey

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    Plus upside downs have one less clamping point (no brace) plus they get damaged more, plus they look ghey.... in short I don't like 'em ;)
     
  25. stinky6

    stinky6 Monkey

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    Because to make an inverted fork as stiff as a non-inverted fork it would require a larger thru axle hub and most people don't want to a specific part. I personally think manufactures should do that; if its better people will buy it and it will catch on, 1.5 head tube for example. Sure everyone would complain when it came out and then five to ten years from when it came out everyone would say "remember those flexy old 20mm thru axle forks?"
     
  26. ridiculous

    ridiculous Turbo Monkey

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    ...seems to be the case with this new 15mm stuff. Foes at least took a stab at the strong inverted fork.
     
  27. karpi

    karpi Monkey

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    okay, so what I'm thinking is that, moving parts such as swing arms, wheels, pedals, cranks, bars etc also have to some degree, I mean, are, to some degree sprung mass right? they have movement so... that should give them some sort of "sprung property"? Am I just going nuts with this?
     
  28. DirtyMike

    DirtyMike Turbo Fluffer

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    Think of sprung and unsprung like this.....Does a spring hold it Up off the ground, or does its weight push directly into the ground. The frame some of the swingarm, some of the fork and your body/gear, are all held up with springs..... Hence Sprung weight, where as your wheels, tires, others parts of swingarm/fork rear cassette brake rotors are all directly pushing on the ground and not through a "Spring" making it Un srpung


    Just to clarify some parts that can be confused, and I am not going to try to list every single part on a bike....

    Sprung weight.......
    Cranks, Stem, Bars, Frame, Seat, Seat post, Pedals Brake levers, Shift pods.

    Unsprung weight.......

    Wheels, Tires Cassette, Brake Rotors, Fork lowers.

    Hybrid parts.....

    Swingarms, brake lines, forks,
     
  29. freakrock

    freakrock Monkey

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    unsprung mass is everything that is attached to the moving parts of the suspension. swing arms, brakes, part of the dampers, wheels and so on
     
  30. - seb

    - seb Turbo Monkey

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    The concept of "unsprung mass" is about terrain. Imagine you're going in a straight line, not pedalling, and there's a bump in the trail. If your suspension works perfectly your body won't deviate from a straight line. You wont feel a bump, you wont go up, you wont go down, you'll just carry on as if cruising along a freshly-surfaced road.

    Since you don't want to move, you dont want your bars/saddle/pedals to move either, since they're your points of contact. All of those are joined together by your frame, and there's some other gubbins too like gear shifters and brake levers, chain device, that sort of thing. All of this is "sprung" mass.

    The bits which DO move, are your "unsprung mass". So think about everything that has to move as you go over that bump to keep you and your bike level and straight.

    Your wheels hit the bump first, they move up out of it's way. With them they take your rotors, your cassette, your calipers, your mech, your lower legs on your forks, some of the internals in your forks, your swingarm, your shock. All of that has to move. The lighter those bits are, the quicker they can move out of the way. Hence, low unsprung mass is good.

    I hope that's cleared some of your questions up?

    "True" unsprung mass are items where the entire object moves. Let's say you hit a 4" bump. Your entire wheel needs to move 4" up. As does your cassette, etc. The lower end of your shock on the other hand, probably only has to move an inch or two. But the lower end of your shock is still unsprung mass, since all of it is having to move. For example, this is why (if possible) you should mount your shock with the reservoir on the sprung end, because it weighs more than the non-resi end.

    "Hybrid" objects are bits which don't all move uniformly. For instance, your shock spring. One end of it moves up, the other end stays put. Your swingarm is another example, it's moving, but not uniformly - obviously the rear of it moves 4", but the front portion does not as it's pivoting.


    Hope some of that makes sense and has clarified the concepts for you, and I've not just confused you further :) (And I hope I've not spoken too much crap! :))
     
  31. DirtyMike

    DirtyMike Turbo Fluffer

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    This is kinda tough to explain without having it drawn out, Seb your doing pretty damn good at it. That last explanation is really accurate. I likes
     
  32. Curious_George

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    hold up, i dont see how having the reservoir point upwards [top right position] would actually change anything
     
  33. William42

    William42 fork ways

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    because its weight (like the rest of the unsprung mass, wheels, etc) is resting on the spring, rather then on the frame
     
  34. karpi

    karpi Monkey

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    wow! that clarifys everthing... I got kinda lost, but re read it and was okay! I hope this helps the guys who didn't understand. Will this all contributes to the idea of a gearbox bike being for suspension effecient plus concentrating the mass where it should always be, smack in the middle!
     
  35. - seb

    - seb Turbo Monkey

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    Definitely, I'm all over the gearbox idea for exactly this reason. Though to be fair it wont make a MASSIVE differnce, my mech + cassette comes to less than 300g. I suppose you can have a lighter hub too, but not much, maybe 50g.

    So in total a max saving of 350g unsprung. Obviously good, but my wheel, tyre, rotor and caliper comes to 2400+...

    I don't know if there's scope for moving the brake inboard as well and transmitting the torque via the chain, sounds a bit dubious?
     
  36. no skid marks

    no skid marks Monkey

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    The whole swingarm accept for a tiny bit around the bearings is unsprung weight.
    If you remove wheel and unbolt shock,the swingarm would fall to the ground(bike in stand),it has weight, that weight has to be controlled once momentum is added.
    Lahar has it nailed. My Lahar with 823 and tubeless set up,with XTR caliper is so flickable it aint funny,and that's with a long chainstay/wheelbase. Lahar would probably have the least rear un sprung weight.