|   some weird observations regarding linkages | ccm
Mar 10, 2003 4:53 PM | | I've spent some time on a RM Element and a Specialized FSR, and a little time on a RM ETSX, in the past week.
Some weird observations.
I removed the shock from the Element and cycled the swing arm through complete arcs, the distance from the chain contact point, at the top of the chain ring, to the chain contact point, at the top of the rear cog, shortened by approximately one chain link, as the swing arm arc'ed up (shock compression). The distance the rear wheel rotates for every link of chain rotation depends on gear ratio, more in higher gears.
But the weird thing was that this loss of one link of drive under compression was returned at the bottom of the pedal stroke, as the swing arm arc'ed down (shock extension) the chain pulled and rotated the rear wheel forward. (This took some technique of holding the crank locked to one position, ratcheted backward at end of arc to regain chain tension, and again holding the crank locked, while pedalling the crank would be rotating to retain chain tension, so maybe there is some advantage to Chris King hubs and/or ETSX coasting over rough ground?)
The ETSX is suposed to do the opposite chain growth thing, as the swing arm(s)arcs up the chain is pulled and rotates the rear wheel forward. Therefore, does it lose one link of chain drive as the swing arm arcs down? I did not remove the shock to test this out as it wasn't my bike.
I've always wondered why under seated pedaling on pavement I've preferred to not lock out my Element (locked out seemed to make my pedal stroke more choppy under extreme high cadence sprints). Locked out and standing, you definitedly feel a surge with each downstroke of the crank, whereas standing and unlocked the rear end seemed to mush and lose drive, but the overall speed didn't change. Chain tension and corresponding drive seems to be more evenly distributed unlocked (great for poor traction conditions).
On the ETSX with 1/3 shock sag, it bobbed like crazy, even seated. With more shock air pressure, it was quite firm under standing pedalling. Seems very sensitive to shock pressure. Further investigation is warranted to figure out the reported advantages of an ETSX.
The FSR with 1/3 shock sag, bobbed very little for standing pedalling. It should theortically have very little chain growth or shrinkage. weird thing was that most of the hard tail like climbing I noticed on the FSR was attributable to a very stiff front shock, so I locked out my Elements front shock and it felt more like the FSR. I couldn't change the spring rate of the FSR's front shock because it was a very stiff coil.
The plushest bike with acceptable bob was the element, the one that seemed to mush the least and still be active was the FSR, but I need more time on the ETSX. |
| re: some weird observations regarding linkages | Dougal
Mar 10, 2003 5:28 PM | | The reason your chain shortens on the top is because the pivot is much lower than the chainline. If the pivot was up near the top of the ring the chainline wouldn't change at all.
Unfortunately the current drivetrain setup doesn't allow that.
Try my site below for more information on the differences in linkages.
Dougal.co.nz |
| are you going to discuss ETSX on your site? | ccm
Mar 10, 2003 6:24 PM | | any thoughts on smoother pedalling, unlocked, on a seat stay pivot bike?
anyway, I already know why bob occurs, I just want to know more about how the ETSX chain growth under compression is supposedly superior to the Element chain growth under extension, or almost no chain growth with the FSR. |
| Not specifically. | Dougal
Mar 10, 2003 8:48 PM | | My website is in a constant state of revision, but I don't think I'll be adding anything specifically on the ETS. It's just another variant of the four bar family, the different traits don't make it any different in analysis or prediction.
Something that'll be very useful to you is Gergely Kovac's linkage software. Have a hunt around here for it.
I'm running the ETS on it right now, the EP on it appears to move fore and aft in a slight arc of 6-7 inches.
Chain Growth is a term I don't like using at all, because almost all bikes have circular wheel paths (the VPP family offer some interesting exceptions), so "chain growth" isn't linear. The axle path can be best described by the centre of curvature (I call it the Effective Pivot, EP). Where the chain is pulling with regard to the EP gives you an excellent idea on how the bike will pedal.
Where the EP is in regard to the COM gives you an excellent idea on how the bike will squat under climbing and acceleration loads.
Dougal.co.nz |
| what do you think of the biopacing? | ccm
Mar 11, 2003 8:16 PM | | with the seat stay pivot bikes, a small amount of drive is lost during the power stoke of the pedal cycle (1 to 5 o'clock) and regained during the top and bottom of the pedal cycle, this redistribution of power was what biopace chain rings were intended to do back in the 1980s. I do find that I out climb my stronger mates on technical sections, but suck on pavement. Do I just have better technique or is my Element helping. I think the Element is using the energy stored in the spring to help propell the bike forward during the dead spot of the pedal stroke, and I seem to lose traction less and not get stalled as much on small obstructions.
anyone else notice this biopace effect? |
| Biopacing is always bad. | Tscheezy
Mar 11, 2003 10:29 PM | | Biopace was designed to make the radius of the chain ring larger when the crank arm was approximately between 3 and 5 o'clock (the point at which the speed of the pedal revolution was generally greatest and gave the most power, and the point biomechanically where you are strongest). This "tapping" of pedaling momentum was why biopace shot your spin all to hell.
Non Horst-link bikes behave as single pivots with the rocker and linkage driving the shock. This we understand. Most of these bikes have low and rearward mounted main pivots. They should compress somewhat under chain tension depending on the gear (bigger ones). This energy is not really "returned" to you but rather it will be absorbed by the extending shock's dampener, or it will be expressed at the weak point of your pedal stroke (noon to 2 o'clock) at which point it would be felt as a tiny "pedal kickback". It is returned at that point because you are not strong enough between noon and 2 o'clock to compress the shock anymore!
The chain length is trying to "grow" a tiny amount as the shock extends, but the added energy being applied to the chain is simply energy you already used to compress the shock earlier in the pedal stroke. This energy is being "returned" at the weak point in your stroke, and without your legs working to counteract this "pedal kickback" there is very little propulsion developed. The big issue here is that some of the energy stored in the compressed shock will be absorbed by the rebound dampener during this extension. Bobbing or biopacing warms shocks, it does not drive you forward in any meaningful way. The best you can hope for is a suspension which is not influenced by pedaling forces.
The situation you are describing with your friends sounds like a tire selection issue or a hard tail verses fully scenario imo.
tscheezy |
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