The i-Drive is produced by the GT bicycle company (Linkage data).
Information on the i-Drive, directly from high-level personnel at GT, has been extremely difficult to come by. Efforts to contact an authority from the GT product development department meet with no success and GT offered no useful information on its web site when we last checked.
However, we have spoken to both representatives of GT's tech support line and the director of race support recently. All explanations from these sources were consistent in asserting that the purpose of the mechanism is to keep the BB static with respect to the main triangle. Mountain Bike Action magazine (MBA) also has given this explanation in an article by Richard Cunningham [Page 83, Mountain Bike Action, June 2001], saying “The i-Drive eccentric allows the cranks to remain fixed in space as if they were bolted to the main frame as the rear suspension cycles.” Indeed, this seems to be very close to the case by examination of the mechanism, so we feel reasonably confident about this much.
All explanations from GT personnel also asserted that chain tension was eliminated as a consideration, in the same way believed for a URT mono-pivot, because the BB is on the swing arm. MBA again echoes this [Page 83, Mountain Bike Action, June 2001]. Cunningham states, “The high pivot position adds big-hit compliance to the suspension. If the GT was a monoshock suspension, chain tension would lock out the suspension under power. Because the crank axle... is attached to the swing arm, this cannot occur.” (Monoshock is an unfortunate name for a type of mono-pivot borrowed from motorcycle jargon).
Taking the i-Drive objective as keeping the BB static relative to the main triangle, we see that the i-Drive mechanism is all for nothing. To the extent that the i-Drive achieves this objective, its component paths are the same as those created by a simple non-URT mono-pivot with the main pivot at the same place as that of the i-Drive. As we did in the “The Natural Mirror Bike.” section, we can create a mirror bike, this time with both an i-Drive side and a mono-pivot side. Neither of the mechanisms will interfere with the other. The movement of mass in the i-Drive is almost identical to a mono-pivot, the only (insignificant) differences being the movement of the eccentric on the swing arm and of the “dogbone”.
It is very easy to see that, given the high main pivot position, the distance between the bottom bracket and the cranks will increase as the suspension goes through its travel. This means that pedaling will cause an extending force on the suspension in most (if not all) gearing, and there will be bump feedback to the pedals, just as in a mono-pivot. One wonders how Cunningham envisions these effects not to occur with a lengthening chain line between the cogs merely because the bottom bracket is on the swingarm.
Some time ago, I was talking to a physics professor who told me that a problem has developed in the United States Patent and Trademark Office (USPTO). Apparently the USPTO now has a problem in recruiting qualified people to examine and award patents. The professor told me that one of the clearest signs that this problem is very serious is that there is now a culture in the USPTO that espouses the viability of “Free Energy Devices”, the most commonly known of which is the “Perpetual Motion Machine”. Apparently numerous patents have been awarded for free energy devices.
Now although the i-Drive claims do not involve perpetual motion explicitly, if the i-Drive could indeed do as is claimed, then one could easily use it to construct a perpetual motion machine.
How to construct a perpetual motion machine:
The i-Drive is claimed to be “unaffected under power” [Richard Cunningham, page 83, Mountain Bike Action, June 2001]. This means that the suspension will not activate when the pedals are pushed, if the frame is part of a bicycle.
Now if we take an i-Drive frame and fix the main frame member (the one that defines the cockpit) to the ground, then the forces on the rear suspension when the pedals are pushed will be different then when the frame is part of a bicycle. In particular, this is true at the rear axle dropout.
As we learned from the “Center of Mass” (CM). and “An Intuitive Look at Forces and Torques.” sections, the tension in the chain and the force on the pedals from the rider's pedal stroke will be felt at the crank axle as parallel forces. These are the forces that act on the rear triangle, at the bottom bracket. This means that the i-Drive rear suspension will activate when the crank axle is pushed if the main frame member is fixed to the ground. But the bottom bracket will still not move with respect to the main frame member and thus the earth.
Energy is equal to force over a distance, or E = F*d. When we apply a force to the crank axle and the axle does not move, we are doing no work on the mechanism, since the distance is zero. But if we attach an appropriate mechanism to the i-Drive rear dropout, the i-Drive rear triangle will produce a force over a non-zero distance and thus do work on that mechanism, when we push on the crank axle.
Voila! A free energy device! From this we can create perpetual motion by feeding the energy from the i-Drive rear dropout, back through the attached mechanism, to produce more force at the crank axle.
All of this, as well as what we demonstrate in the “Ellsworth's “Instant Center Tracking” (ICT).” section, shows that a patent is no guarantee that a device will do what is claimed. As we have noted, a patent only requires a new idea, not that the idea actually work (to say nothing of overworked or inadequate patent examiners).
We have sent copies of this work with additional emphasis on the i-Drive to both GT and Richard Cunningham. We have also made numerous efforts to contact both parties in an effort to clear up their confusion. Neither party seems interested in a thorough examination of the problems in their theories.
Now, it may be that GT got the main pivot point just right and that this is why some people seem to like it. But in any case, to the extent that the i-Drive achieves its objectives, the same results could have been obtained using a much lighter, simpler mono-pivot, with the main pivot in the same place as that on the i-Drive.
A footnote:
Ray Scruggs, an avid mountain biker, has done some measurements on a GT i-Drive. He says that the BB actually drops somewhat with respect to the main triangle, as the suspension compresses. This would smooth the pedal stroke for the forward or driving pedal, while increasing kickback for the non-driving pedal. All in all, this may tend to make the suspension feel as if the rider were pedaling a bike with a lower pivot and less kickback. But this also would reduce the anti-squat from the chain. So the result is still no net advantage over conventional mono-pivot designs, for the dilemma of kickback verses anti-squat, with the complicated i-Drive mechanism. And the suspension is certainly not “unaffected” by pedaling, as GT and Cunningham have claimed.
However, very interestingly, if Mr. Scruggs' observations are correct, they may entail some significance for the i-Drive. With the i-Drive's very rearward tilting wheel path, there may be a significant increase in bump performance, but with possibly a less obtrusive kickback then what is normally associated with such a rear tilting wheel path.
So in the end, there may be some significance to the i-Drive mechanism, though it has nothing to do with what the manufacturer and magazines have claimed.
We have, as of yet, not duplicated Mr. Scruggs' measurements, though we know him to be a fairly careful man. Analysis of Linkage data indicates that the bottom bracket does drop just slightly, which would tend to support Mr. Scruggs' claim. However, the vertical movement is slight. There is actually much greater horizontal movement.
One certainly would not expect the mechanism to achieve its results perfectly, so we are not terribly surprised that there should be some movement of the bottom bracket. As is always the case, each person should decide for himself or herself whether or not the deviation is significant.
At this time, we think it best to keep the main expose on the i-Drive as it has been, based on the manufacturer's claims, while noting the above deviations.