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Discussion Starter · #1 ·
I suppose this has probably been asked before, but i've searched the forums quite thoroughly, and on the web too, and i can't find in depth info on how this design works, only what it does, and basic principles behind it. I'd like to know what actually goes on, in detail.

Eg. In all the aspects of the performance of a rear suspension, what takes place in effort to:

- minimize pedal bob/squat, kick back
- minimize brake jack/squat or stiffening due to braking
- minimize chain growth, chain pull
- maintain suspension activity/bump compliance during various conditions (eg. under pedaling, over bumps, pedaling while going over bumps, in/out of saddle)

And the reasoning (benefits and drawbacks) behind:

- the placements of the pivots
- the axle path
- leverage ratio

- is there, and how much is chain tension is at play here

And anything else i've missed.

Can someone please explain? Thanks a bunch.
 

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"Eccentric"

I'm don't know it all, but my feeling after thousands of miles on iDRIVE's since around 2000 (Santa Ana, made in U.S.A. GT's) is the eccentric (which keeps distance between crank and seat equal) eliminates other counter forces, which would otherwise make a more po-going, rougher ride.

Of course, there are other good bikes. As humans, we learn to compensate for whatever we're using/ doing, but the iDRIVE is exceptionally smooth when set-up properly and the right cadence/ rhythm achieved by the rider.

Visit below link to view some graphics. There's also a lot of research info online and here on Mtbr by keying "GT iDRIVE Eccentric":

http://www.gtbicycles.com/usa/eng/tech
 

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Discussion Starter · #4 ·
raycharlesonacid said:
There's also a lot of research info online and here on Mtbr by keying "GT iDRIVE Eccentric"
Thanks. But i also (and more particularly) would like info on the newer refinement of the iDrive system which is no longer eccentric, found on the force and sanctions.
 

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Brake jack occurs when braking forces affect the suspension action by pulling the swingarm down (so it feels like the back end stiffens). This mainly occurs on traditional four bar linkage system's (not FSR) or on URT's where the main pivot is in the wrong
spot (i think it's when it's too high, could be wrong). The placement of the i-Drives upper
pivot allows the swingarm's movement to be un-effected by braking forces. So no Jack.

The lower pivot is placed where it is to mimic the action of the old eccentric. The old
system worked great by isolating forces put on the pedals from the rear shock, the new
system does the same job, but is lighter. Couldn't tell you what the actual chain growth
number is but it's not much.

Axel path is dictated by the upper pivot, and is as vertical as possible while keeping the upper pivot where it needs to be.

Leverage ratio i dunno. Anyone know if i-drive is a linear rate or a falling rate? It'd be linear wouldn't it?
 

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Eatyapeas said:
This mainly occurs on traditional four bar linkage system's (not FSR) or on URT's where the main pivot is in the wrong spot (i think it's when it's too high, could be wrong). The placement of the i-Drives upper pivot allows the swingarm's movement to be un-effected by braking forces. So no Jack.
Umm, the last time I looked at my buddy's iDrive 5 it sure looked like a modified URT rear suspension design to me. They added a linkage system to hang the bottom bracket down from the swingarm with a smaller linkage to control its motion relative to the frame.

My buddy is going to be replacing the rear shock this coming weekend so I'll see if I can remember to try cycling the rear suspension through its full travel to see what the bottom bracket does relative to the rest of the frame. If I remember to bring a camera and tripod then I'll post pictures of it here.
 

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Discussion Starter · #10 ·
I think i have worked out how the ID system works in some key areas...

I understand that there are two ways squatting can occur when accelerating a bike, the driving force of the chain which can compress the rear suspension, and the load transfer of the rider's mass.

On the ID system, the main pivot is located high above the BB, above the chainline, which when you pedal, creates a counter clockwise torque on the swing arm (anti-squat?). So because the driving force here is actually decompressing the rear suspension, this cancels out the first cause of squatting (because there is no driving force compressing the rear), and prevents squatting from the second cause (because the decompressive motion to whatever degree cancels out the load transfer).

So this explains how the ID is to minimize 'rider induced bobbing' when pedaling/accelerating on a flat surface. Now how does the ID remain active while pedaling? Here's how i understand it...

Well, i would think that the anti-squat would cause the suspension to stiffen when going over bumps, while pedaling. This is because the driving force decompressing the rear is restricting the suspension from compressing when hitting a bump. This is where the floating BB attached to the i-Link comes into play. (I'll try my best to explain this part.) The motion of the BB moves in such a way that allows the cranks to move somewhat freely from the rear axle, and vice versa, despite that they are connected by a chain. So the moving BB cancels out excess driving force (by preventing chain growth thus preventing excess chain tension) that would otherwise stiffen the suspension when pedaling over a bump if the BB was in a static position. (If the BB was in a static position on the front triangle, then the rear axle would move away from the BB as it goes into its travel creating excess tension on the chain). Additionally, the floating BB keeps the pedals in a same position relative to the riders seating position.

So anyway, this is my understanding of how the system works. I would appreciated any comments, constructive criticism or correction if i have made any errors.
 

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The system is designed to reduce pedal kickback which would be found on a system with a similarly placed main (mono) pivot. It does this by suspending the BB off a short link from the swingarm, which is then tethered to the main frame via a short dog bone link - this controls the movement of the bottom bracket.

As the axle moves back and up, the BB moves back slightly and rotates: the chain length will not increase as much as it would with the monopivot, and the pedals stay pretty much in the same position. This allows smoother pedaling over rocky sections.

The anti squat is achieved with that fairly high main pivot position. They can still be prone to some brake induced effects though.

Overall you get a system with many of the benefits of a high monopivot (backwards arc, high antisquat) but without the main drawback of pedal kickback. However suspending some of your mass on the swingarm can reduce the effectiveness of the suspension when standing.
 

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Anti squat is achieved by 2 factors: higher main pivot (already explained) and moving forward BB

When the suspension compresses it also extends the chain-stay.
When you pedal, the force on the chain tries to compress the chain-stay and decompress the suspension.

In other words, when the suspension compresses the chain-stay decompresses. So when you pedal both are partially canceling each other.
Also note - the smaller the front chain ring the higher the contra-reacting effect and lesser the bob (because of higher chain tension).

Interesting thing about this design is that it "turns-off" when you don't pedal (since there's no force on the chain and chain-stay)
 
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