Mirage drive adjustment - comment and questions

The FAQ (and perhaps the Mirage Drive manual, too?) specifies adjusting the idler cable FIRST, and then the other two cables. I don't understand that. Since the sole purpose of the idler cable is to merely create a difference between the two other cable tensions, wouldn't it make more sense to adjust the other two cables first, and then the idler cable?

It seems to me that the only "adjustment" possible to the idler cable, if done BEFORE the other two cables, would be to initially just set the idler cable to some middle of the road random adjustment, because how are you going to adjust the above-mentioned idler cable differential adjustment properly BEFORE the other two cables have been adjusted?

Personally, this is how I adjust my drives:

After lubricating the drive properly and shoving a Round Bic pen into the pedal adjustment holes to keep the pedal arms parallel to each other...I first disengage or loosen the idler cable, then I initially "adjust" the front and rear cables so that all four nuts are as close to being in the middle of their travel as possible, then tweak the 4 front and rear nuts as equally as possible until the masts are centered and aligned and the proper tension exists on the front and rear chains. My definition of "proper tension" is as tight as possible WITHOUT CAUSING ANY INCREASE IN PEDAL EFFORT. In other words, tighten the chains in increments (use smaller and smaller increments as you get closer to ideal tension) and work the pedals in between adjustments until you notice an increase in pedal effort, and then back off the nuts just enought so that the extra effort goes away. Obviously, the drive MUST be properly LUBRICATED prior to doing this, or you will not be able to properly "feel" the point at which the pedal effort increases. The theory behind all this is that the drive sprockets and sprocket shafts will survive longest with neither too much chain slack (which would cause lash/slapping to the sprocket teeth) nor too little (which would put too much stress on the sprocket body and sprocket shaft).

Once the above has been completed, I DO then adjust the idler cable so that the front chain is SLIGHTLY tighter than the rear chain...but Matt had mentioned in the FAQ that this was far more important with the older steel sprockets than with the newer plastic ones...and so I suspect that having the front chain a little tighter than the rear one on a plastic-sprocketed drive is just "insurance" so that the chain doesn't jump sprocket teeth if the drive becomes grossly out of adjustment. I suspect that this just doesn't happen on plastic-sprocketed drives as long as the drive's front and rear chains are properly in adjustment. In 3 years of heavy use of my plastic-sprocketed drives, I've never had any of my chains ever even come close to jumping sprocket teeth, but then again, I only use the original smallest fins...someone with Turbo Fins has, maybe???

My concern about using the idler chain to create the tension differential between the front and rear cables is that doing so guarantees that either the front or rear cable or both will then not be optimally adjusted (because they can't BOTH be at the proper tension if they are at DIFFERENT tensions), but perhaps these plastic drives are more "bullet-proof" than I am giving them credit for being, and "perfect" adjustment just really isn't that critical to drive life?

Jerry

Jerry, it's great to see your experiments with tuning.

Not everyone is in agreement as to the function of the idler cable. The newer drives can actually run without it, so what does it do? I see it as serving two purposes:

1. It helps distribute the load from the "driven" pedal / drum / fin to the "idle" side.

2. It helps keep the drums from sliding out along the drumshaft (which pedaling action has a tendency to induce).

Without it, the Drive has more slack. In order to get rid of the slack, the other cables have to be adjusted tighter, causing more friction and a net loss of power (IMO).

I don't see the idler as having any deliberate purpose in creating a differential tension between the front and rear chain / cables -- each has its own tension adjustment.

I think it's important to tighten the idler first since it pre-loads the drums, which alters the tensions of the other cables; you can't get a stable adjustment otherwise. Lots of times if the front cable is a little loose and the idler is also, you can bring both into spec by simply resetting the idler.

Your method of pre-tensioning, centering and fine tuning the cables sounds great. Finding the optimum balance between too much friction and too much slack should be the ultimate goal regardless of method. I would just add that frequent adjustment wears out the Nyloc nuts and (especially with the '08 and newer Drives), they should be stabilized with a dab of Locktite Blue.

You may be right about Hobie's original thinking regarding the different tension settings between front and rear cables. Your experimentation in that regard seems to suggest that it is not really necessary to set them differently, at least with the plastic sprockets. Sounds great!

Jerry, I think of it this way:

Without the idler cable, there is a single closed loop formed by the front cable, the rear cable and the two drums. Because it is a loop, there is equal tension between the front and rear cables when the drive is at rest. So you only need to turn one nut to tighten the front and rear cables equally. When you tighten one of the front nuts, you shorten the front cable, pulling the pedals forward. Likewise, tightening a rear nut shortens the rear cable and pulls the pedals back. However the front and rear cables will have equal tension as long as the drive is at rest.

In operation though, when you stomp on a pedal, you remove any slack from the rear cable and put it all in the front cable. That makes the front chain more susceptible to skipping than the rear. Without the idler cable, you'd have to tighten both the front and rear cables to higher than ideal tension to reduce the risk of skipping the front chain.

With the idler cable, you can leave the rear chain at its ideal tension while tightening only the front one just a bit tighter than ideal. Then when you stomp a pedal, any slight slack gets pulled out of the idler cable and put into the front cable, reducing the tension there to a hopefully more ideal level. The interaction between the three cables does make adjusting them a bit more difficult.

_________________
Frank
2007 Oasis

Excellent explanation...now I get it. Apparently my brain was originally assuming a perfect, completely non-elastic loop with zero play in it. After reading your explanation, I realized my error. Now when I adjust that idler cable I will no longer be wondering why I am doing it!

Thanks,



Jerry