Yeah, that all makes perfect sense. But we are talking about a conventional bike application where the axle-path must move a great deal in the Y direction. So moving the axle-path rearward along the x axis, then forward, then rearward is not practical at all. Neither is an axle-path that moves diagonally in a straight line forward. I was only thinking about examples that would actually be used in real life. But it's good to see this thread finally back on track!!!dw said:
WheelieMan said:
I don't think anyone said they aren't related, they just aren't exactly the same thing as you seemed to say earlier.WheelieMan said:
Now this is getting into something I've been wondering about. The effects of rider mass seem to be mentioned more and more recently. As a non-engineer I can understand how shifting weight around under braking and acceleration (and just body english) will effect suspension, but what I can't seem to wrap my head around is how you can do much about it through frame design. (I can see how platform damping and other valving tricks could help.) I know some of it is probably top secret and you will have to kill me...but can anyone describe how those that don't "completely ignore" rider mass attack the issue? Can of worms or what?ohio said:
Ian Collins said:
Yeah thanks Dave! :eviltongudw said:
Nope, I'm not finished yet. That horizontal movement (assuming the BB is at exactly the same height as the axle, anyway) is, as Ohio said, RADIAL to the BB. If you were to change the angle so that the axle was out at 15 degrees (from horizontal) relative to the BB, but it still moved ONLY in and out related specifically to the BB (as in, directly towards the BB, then directly away again etc), it now has an X (horizontal) and Y (vertical) component on an absolute scale, but the chaingrowth is still identical to before. Now think about it this way - if you had a circular axle path with the centre of curvature at the BB, it would generate no chain growth right? If you were to take that path, but play with the radius of curvature so that it tightened and widened at various points, you will still get an axle path that is still roughly a C shape, but it's not circular (a good way to visualise it would be a BB-centric pivot, with horizontal chainstays with the axle moving in and out at certain points). However, the distance from the BB to the axle will change (shorten and lengthen), and you will get an S-shaped chaingrowth curve.WheelieMan said:
What does this have to do with anything? I never said it was impossible to get an s-shaped chaingrowth curve.thaflyinfatman said:
I'm gonna take a stab at it, Dave feel free to correct me if/when I go wrong:OGRipper said:
) that will be equal in magnitude to the compressive force caused by having more weight on the rear wheel, then you can get a perfect balance going (ie no "bob" whatsoever). You do have to take into account the fact that the rider's mass moves around when pedaling and stuff, as well as the fact that gearing/chainline changes, and I'm not really sure but I'd hazard a guess that there would need to be some sort of "fudge factor" to account for that. I have NFI just how much that would be though.And I never said you said it was impossible, so what?WheelieMan said:
Haha. It's funny. I always tried to consider the two factors (chain interactions and rider mass transfer) seperately, until dave pointed out that I already read a book that explained it perfectly and simply.dw said:
No, an s-shaped axle-path would cause an s-shaped chaingrowth curve. But you are correct, an s-shaped chaingrowth curve does not require an s-shaped axle-path.thaflyinfatman said:
Not necessarily. It wouldn't necessarily have any reduction in bb-axle distance.WheelieMan said:
There doesn't have to be a reduction in bb-axle distance for the chaingrowth curve to be s-shaped. It would just be a stretched out s-shape instead. We are arguing about something that is so meaningless, makes me mad.thaflyinfatman said:
Actually I think hypothetical examples are helpful, if not needed, to illustrate certain concepts, especially if you have a small brain like me. On that note, I am going to get another scotch and try to digest thatflyinfatman's mass transfer explanation...see ya on the other side.WheelieMan said:
What sort of shops will stock that book? (I'm in Australia btw so I need a description rather than a name)ohio said:
I bought it direct from Tony when he first published the new edition in 2002... printed by him in Spain with no ISBN number. I don't know where you'd get it now. Try www.tonyfoale.com.thaflyinfatman said:
Cheers, I'll get my hands on a copy of that at some stage.ohio said:
Thanks for taking the time to do that, it makes sense that you can counteract mass movement in the way you describe. How to actually accomplish it while addressing all the other issues is the trick I guess. No wonder lots of companies just punt.thaflyinfatman said:
haha yep, there's a fair bit of stuff to cover. Fortunately you can kind of separate the factors, because pedalling reactions within the frame (ie not mass shifts) are determined (for a given gearing/chainline) only relative to axle path (insofar as I understand, I could be wrong), whereas braking takes into account both axle path and swingarm/rear triangle rotation. Pedalling doesn't worry about swingarm rotation because the only place the internal pedalling reactions act on the swingarm is at the axle - and the axle can only provide point forces to the swingarm (in the 2d plane, ie forgetting about sideways flex etc) because the wheel is free to rotate around it (and both the chain tension and the wheel's driving force act directly on the wheel). In that respect, you can relatively easily create two setups that will pedal identically (well other than whatever minor changes in CoM/moments of inertia that rearranging a linkage might make) but brake differently or vice versa. A good example is a typical FSR setup - they generally can be pretty closely approximated by a singlepivot in terms of axle path (as in they have a constant centre of curvature), but you can play with the braking reactions somewhat separately (if you know how to) by f'ing round with the instant centre position. I really have no idea how many bike designers actually take this stuff into account or know how to do it though.OGRipper said:Thanks for taking the time to do that, it makes sense that you can counteract mass movement in the way you describe. How to actually accomplish it while addressing all the other issues is the trick I guess. No wonder lots of companies just punt.
Understatement of the week: With pedaling forces, braking forces, mass shifting around, etc., there's a lot to cover eh?
My 2c - having zero chaingrowth is good for pedalling, and you are correct that the chain tension (on its own) will have no extension/compression effect on the axle. However you have to take into account the driving force (which you kinda alluded to, but I'm not sure exactly how you meant that sentence), which is basically a horizontal force on the tyre that is transferred directly to the axle. You can work out how the forces balance - neglecting the wheel's own mass moment of inertia - in a simple FBD of the wheel taking into account tyre traction force, chain tension force, and the subsequent axle reactions (assuming other forces are at equilibrium). Trying to balance that with *calculated* weight shifts I imagine will be pretty tricky, however my experience with BB7s etc (high pivot bikes with pulleys) would lead me to believe that a typical "high pivot" (as in 7-9 inches directly above the BB) would probably pedal pretty well. So - sounds good, can't wait to see itrbx said:
yeah my custom high pivot does as well, but more for the flats. Once you get into a climbing grade theres way too much extension (or anti-squat)thaflyinfatman said:... however my experience with BB7s etc (high pivot bikes with pulleys) would lead me to believe that a typical "high pivot" (as in 7-9 inches directly above the BB) would probably pedal pretty well. So - sounds good, can't wait to see it
Yeah but who the hell tries to pedal a ~50(?)lb DH bike uphill? My SGS weighs 46lbs and that won't go up anything steeper than dead flat.zedro said:
Do you mean once you need to pedal that hard? Or are you referring to weight shifting or some other effect from the grade? I would think you could generate the same pedal force in a sprint on the flats as you could trying to pedal that beast uphill no? And anyway doesn't the dual chain/jackshaft isolate pedalling forces from the suspension? Can I ask more dumb questions?zedro said:
(Don't answer that last one.)Me! v-10, 46lbs, 34 tooth ring and 12-34 cassett.thaflyinfatman said:
the grade shifts the center of gravity which messes things up, plus theres a tendency to pull on the bars more which really upsets it. Its kinda odd when you're hammering uphill with it and it has like a subtle inchworm effect. Climbing with the low gearing generates alot more torque at the wheel than you could with taller gearing, plus pedal strokes tend to be more hacky when climbing.OGRipper said:Do you mean once you need to pedal that hard? Or are you referring to weight shifting or some other effect from the grade? I would think you could generate the same pedal force in a sprint on the flats as you could trying to pedal that beast uphill no? And anyway doesn't the dual chain/jackshaft isolate pedalling forces from the suspension? Can I ask more dumb questions?
Interesting to think of how acceleration can have an impact even though the modified chainline minimizes pedal feedback, thanks.zedro said:
yup good call, the multiple mounts on the frame and adjustable dogbones let me change the rate without changing the leverage ratio, although i can vary that slightly too (as well as geometry)OGRipper said:
That is pretty nifty, nice work. I'm trying to wrap my brain around how you can change rate without changing the leverage ratio. I've always thought they were so related that you couldn't change one without changing the other...? I probably need a better understanding of the terms.zedro said:
Yeah I can see how an acceleration force from the rear wheel on a squishy bike could really screw with things. I'm picturing a rear wheel chasing a front wheel, giving an inchworm effect if it's not factored in how the design responds to acceleration. How you deal with it is the trick eh?dw said:
